WO2008121578A2

WO2008121578A2 - Intervention applications of real time x-ray computed tomography

Info

Publication number: WO2008121578A2
Application number: PCT/US2008/057764
Authority: WO
Inventors: Timm-Michael L. Dickfeld; Thorsten Fleiter
Original assignee: University Of Maryland, Baltimore
Priority date: 2007-03-30
Filing date: 2008-03-20
Publication date: 2008-10-09
Also published as: WO2008121578A3

Abstract

Techniques for positioning a probe in a living subject include inserting the probe into the subject and then exposing the subject to a first X-ray beam. First scan data that consists of a plurality of volume elements that each indicates X-ray absorption at a corresponding volume inside the living subject is received based on computer tomography and the first X-ray beam. A first image based on the first scan data and a position of the probe is presented in real time, shorter than 0.5 seconds after exposing the living subject to the first X-ray beam. These techniques allow the probe to be operated in real time relative to features of an anatomy of the subject, which are evident in the first image.

Description

INTERVENTION APPLICATIONS OF REAL TIME X-RAY COMPUTED

TOMOGRAPHY

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of United States Provisional Appln.

60/921,026, filed March 30, 2007, under 35 U.S.C. §119(e).

[0002] This application claims benefit of United States Provisional Appln.

60/949,237, filed July 11, 2007, under 35 U.S.C. §119(e).

[0003] This application claims benefit of United States Provisional Appln.

60/970,520, filed September 6, 2007, the entire contents of which are hereby incorporated by reference as if fully set forth herein, under 35 U.S.C. §119(e).

STATEMENT OF GOVERNMENTAL INTEREST

[0004] This invention was made with Government support to one of the inventors provided by the Department of Veterans Affairs. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0005] The present invention relates to applications for real time X-ray computer tomography (CT) involving a probe that is inserted into a living subject.

2. Description of the Related Art

[0006] Real time X-ray computer tomography (CT) has been developed but has not previously been applied to interventions, such as electrophysiological intervention. Special problems and opportunities are involved in such applications. [0007] Computed tomography (CT), is a medical imaging technology in which digital geometric processing is used to generate a three-dimensional scan of the internals of an object from a large series of two-dimensional X-ray projections taken around a single axis of rotation. CT produces digital data which can be manipulated in order to demonstrate various structures based on differences in those structures' capacity to block at least a portion of an X-ray beam. Although historically the images generated were in the axial or transverse plane (orthogonal to the long axis of the body), modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.

[0008] In 3rd and 4th generation CT scanners, a pyramidal- shaped X-ray beam is able to cover an entire two-dimensional (2D) field of view. This avoids the need for any horizontal motion of the patient or scanner to cover multiple scan lines; an array of lines can be captured in an instant. This allows simplified motion confined to rotation of the X- ray source. Third and fourth generation designs differ in the arrangement of the detectors. In 3rd generation, the detector array is only as wide as the beam, and must therefore rotate as the source rotates. In 4th generation, an entire ring of stationary detectors is used. [0009] In helical CT technology, the X-ray source (and detectors in 3rd generation designs) are attached to a freely rotating gantry. During a scan, the table moves the patient smoothly through the scanner. As a result, the X-ray source traces a helical path relative to the subject, such as a patient. In a traditional shoot and step approach, one rotational slice is captured on a stationary subject then the scanner or subject is stepped with a translation motion in an axial direction. A major advantage of the helical CT technology over the shoot-and-step technology is speed. A large volume can be scanned in 20-60 seconds. This is advantageous for a number or reasons: 1) often the patient can hold their breath for the entire study, reducing motion artifacts, 2) it allows for more optimal use of intravenous contrast enhancement, and 3) the study is quicker than the equivalent conventional CT- permitting the use of higher resolution acquisitions in the same study time. The data obtained from helical CT is often well-suited for 3D imaging because of the lack of motion miss-registration and because of an increased out of plane resolution. These major advantages led to the rapid rise of helical CT as the most popular type of CT technology.

[0010] Multi-slice CT scanners are similar in concept to the helical CT but each includes more than one detector ring. Multi-slice CT scanners with 4, 8, 16, 32, 40 and 64 detector rings (also called rows of detectors), and with increasing rotation speeds, have been introduced. Current models have up to 3 rotations per second, resolution of 0.33 mm (millimeters, 1 mm=10^"3 meters) x 0.33mm x 0.33 mm volume elements (voxels) with z- axis scan speed of up to 18 centimeters per second (cm/s, 1 cm = 10^"2 meters). Such scanners permit the study of the heart and coronary arteries with stunning detail, yet have sufficient flexibility to scan the entire chest, abdomen and pelvis within a single breath hold.

[0011] A CT model with dual X-ray tubes and dual arrays of 64 slice detectors has also been introduced. Dual sources increase the temporal resolution by reducing the rotation angle required to acquire a complete image, thus permitting cardiac studies without the use of heart rate lowering medication. Dual X-ray sources also allow the use of two different X-ray wavelengths (with different photon energies) which allows an estimate of the average atomic number in a voxel, as well as the total attenuation. This permits automatic differentiation of atomic content, such as calcium (e.g. in bone, or diseased arteries) from iodine (in contrast medium) or titanium (in stents), which might otherwise be impossible to differentiate. It is expected that dual wavelength X-ray sources also improve the characterization of tissues and thus allow better tumor differentiation during diagnosis.

[0012] Modern computer power allows increased post measurement processing (called post-processing). Bone suppression, volume rendering in real time, with a natural visualization of internal organs and structures, and automated volume reconstruction, and images in any arbitrary plane orientation are available in ever shorter times after measurement.

[0013] To date these advances in real time CT technology have been applied to provide improved and speedier medical diagnoses of phenomena inside a living subject.

SUMMARY OF THE INVENTION

[0014] Techniques are provided for positioning a probe in a living subject based on CT technology. In a first set of embodiments, a method includes inserting a probe into a living subject and then exposing the subject to a first X-ray beam. First scan data that consists of a plurality of volume elements that each indicates X-ray absorption at a corresponding volume inside the subject is received based on computer tomography and the first X-ray beam. A first image based on the first scan data and a position of the probe is presented in real time, shorter than 0.5 seconds after exposing the living subject to the first X-ray beam.

[0015] In other sets of embodiments, an apparatus or a computer-readable medium is configured to perform one or more steps of the above method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

[0017] FIG. 1 is a block diagram that illustrates an example system for positioning a probe in a living subject.

[0018] FIG. 2A is a block diagram that illustrates scan elements in a 2D scan;

[0019] FIG. 2B is a block diagram that illustrates scan elements in a 3D scan;

[0020] FIG. 3 is a flow diagram that illustrates at a high level an example method of using CT in real time to position a probe.

[0021] FIG. 4 is a flow diagram that illustrates an example method for performing a step depicted in FIG. 3;

[0022] FIG. 5 is a flow diagram that illustrates an example method for performing a different step depicted in FIG. 3;

[0023] FIG. 6 is a flow diagram that illustrates an example method for performing yet another different step depicted in FIG. 3;

[0024] FIG. 7 is an example 2D fluoroscope scan that depicts an electrophysiological catheter and ablating catheter tip that serves as a probe in a living body of a swine;

[0025] FIG. 8 is an example electro-cardiograph that depicts different features in an electrocardiogram that trigger an X-ray source in an example CT system;

[0026] FIG. 9A is an example slice from a CT scan with voxels that depict a tip of an ablation catheter;

[0027] FIG. 9B is a photograph that illustrates a subcutaneous needle with lead assembly used to implant an electrical contact and lead in heart tissue;

[0028] FIG. 9C is an example high resolution CT scan used to plan access for the subcutaneous needle with lead assembly;

[0029] FIG. 9D is an example real time CT scan used to guide the subcutaneous needle with lead assembly to a desired site in the heart tissue;

[0030] FIG. 1OA and FIG. 1OB simulate the removal of the sunburst artifact around probe voxels from presentation data;

[0031] FIG. 11 is an example real time quartile view showing a probe relative to anatomy based on CT data; [0032] FIG. 12 is an example real time intersecting planes view;

[0033] FIG. 13 is an example real time 3D rendering ;

[0034] FIG. 14A is a simulated real time image with probe measurements;

[0035] FIG. 14B is a 2D rendering with anatomical features from CT data and colored voxels based on probe measurements.

[0036] FIG. 14C is a 3D rendering with anatomical features from CT data and colored voxels based on probe measurements.

[0037] FIG. 15 is an example real time image showing a probe fly through view inside a previously mapped endocardial surface;

[0038] FIG. 16 is an example 3D dynamic heart with real time catheter and probe; and

[0039] FIG. 17 is a block diagram that illustrates a computer system upon which an embodiment of the invention may be implemented.

DETAILED DESCRIPTION

[0040] Techniques are described for using real time X-ray computer tomography (CT) for procedures involving a probe that is inserted into a living subject. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention.

[0041] Some embodiments of the invention are described below in the context of electrophysiological intervention with electrical, thermal or chemical ablation based on real time CT technology. However, the invention is not limited to this context. In other embodiments other probes inserted into the living body are operated based on real time CT technology. For example, in some embodiments electrophysiological measurements are made with an electrophysiological catheter absent ablation activities. In various embodiments the probe is any diagnostic or therapeutic tool, such as catheters, needles for injection of bio-active agents, or electrical contacts or optical scopes for diagnosis or recording, whether attached to a catheter or other tether, or freely moving through a body lumen. In various embodiments, the probe is used to monitor the application of therapy, or monitor therapy success, or to provide any diagnosis, guidance, therapy, and follow-up of therapies, including any interventional or cardiac therapies, or some combination. In some embodiments the probe is one or more parts or all of a pacemaker system or of a defibrillator system, such as an electrical lead or a needle for placing an electrical lead. [0042] Increasing clinical data suggests that accurate anatomic guidance can result in safer, faster, and more successful interventional procedures. For example, in electrophysiology, the advent of the anatomically guided ablation approach for typical, isthmus-dependent right sided flutter decreased procedure times and treatment failures and made atrial flutter ablation the "first-line" treatment approach. Similar observations have been made for even more complex ablation procedures such as for atrial fibrillation or non-idiopathic ventricular tachycardia. However, the available technology to obtain this anatomic information during the procedure is limited. For example, in electrophysiology, the two most widely used catheter guiding modalities are fluoroscopy and three-dimensional mapping system. However, fluoroscopy is very limited in visualizing the myocardial soft tissue structures and cannot accurately assess the correct catheter position, catheter-tissue contact, or assess ablation success. Three-dimensional mapping systems either use a mathematical reconstruction of the cardiac chambers or use a registered, surface-reconstruction (mostly from pre-operative magnetic resonance imaging, MRI, or CT scans) to display the cardiac anatomy. However, these reconstructions cannot reflect interval changes (e.g. chamber size change due to volume changes) or acute complications (e.g. cardiac effusion/tamponade) or the effects of ablations executed during the operation. Additionally, these images have to be registered to the catheter mapping space, which introduces significant position errors.

1. Structural Overview

1.1 System structures

[0043] FIG. 1 is a block diagram that illustrates an example system 100 for positioning a probe in a living subject based on CT technology. The system 100 includes a CT system 120, a probe system 140 and a computer system 150. The system 100 operates on a patient 190, who is a living subject, such as an animal or human. Although depicted for purposes of illustration, the patient 190 is not part of the system 100. [0044] Like most CT systems, CT system 120 includes an X-ray source 122, an X-ray detector array 124, a CT gantry 128 and a translating table 110. In the illustrated embodiment, the CT system 120 includes CT data transfer point 160 and CT process 152 on computer system 150.

[0045] The X-ray source 122 generates an X-ray beam 123 with a particular shape in a particular portion of the X-ray spectrum. The X-ray spectrum is a portion of the electromagnetic spectrum with wavelengths in the range from 10 to 0.01 nanometer, (1 nanometer, nm, = 10^"9 meters), corresponding to frequencies in the range 30 to 30,000 PetaHertz (PHz, 1 PHz = 10¹⁵ Hertz, 1 Hertz = 1 cycle per second). The product of the wavelength and the frequency of an electromagnetic wave is the speed of light, a constant, so each different wavelength has a different frequency associated with it. To avoid confusion with other uses of the term frequency, used below, the X-ray spectrum is described herein in terms of its wavelength. The energy of a photon of electromagnetic energy, often expressed in thousands of electron volts (kilo-electron volts, keV) for X- rays, is proportional to the square of the inverse of the wavelength. The shorter wavelengths have the higher photon energies. In the illustrated embodiment, the X-ray source is a dual source that emits X-rays in two distinct portions of the X-ray spectrum. In other embodiments, a single band X-ray source or a spectral X-ray source with more than two portions of the X-ray spectrum is used. In the illustrated embodiment, each X-ray beam is pyramidal shaped, like a thick fan, with a rectangle-like cross sectional area. In other embodiments, electron beams of other shapes are used, such as a thin beam, a thin fan, and a cone.

[0046] The X-ray detector array 124 is an array of one or more X-ray detectors sensitive in at least one portion of the X-ray spectral bands emitted by the X-ray source 122. In the illustrated embodiments, the X-ray detector array 124 includes hundreds or thousands of X-ray detectors arranged as a two-dimensional array of rows and columns of detectors. For example, in some embodiments, the X-ray detector array has 256 columns of detectors in one row. A row is in the plane of FIG. 1, and the columns are in the z dimension perpendicular to the plane of FIG. 1. In other embodiments the X-ray detector has 64 columns of detectors in 16 rows of detectors.

[0047] The CT gantry 128 provides a rotational support structure for the X-ray source 122 and X-ray detector array 124 so that an un-diverted X-ray beam 123 from X-ray source 122 impinges on all the detectors in the X-ray detector array 124. In the illustrated embodiment, the gantry 128 rotates in the direction of rotation 127 indicated by a curved arrow at a rate of about three rotations per second. In some embodiments, the X-ray source 122 emits the X-ray beam 123 continuously while rotation and in some embodiments, the X-ray source pulses the beam 123 so that the beam is off until the detector array 124 has moved to a new position that does not overlap a previous position on the same rotation.

[0048] The translating table 110 supports the patient 190 at a center of the gantry 128 so that the X-ray beam 123 intersects a volume of interest in the patient 190. The translating table also moves the patient through the gantry in both directions perpendicular to the plane of FIG. 1, so that a different volume of interest in the patient 190 may be positioned to intersect the X-ray beam 123. In some embodiments, the patient 190 is supported by a stationary table, and the gantry 128 moves relative to the patient in a direction perpendicular to the plane of FIG. 1. In various other embodiments, the translating table also moves the patient in a horizontal or vertical direction, or both, in the plane of FIG. 1, so that the correct portion of the patient 190 can be illuminated by every beam emitted by the X-ray source during each rotation of the gantry 128. [0049] Based on X-ray intensity (related to the number of X-ray photons that are received) measured at each detector as the detector array completes a circuit of the gantry 128, CT process 152 executing on computer system 150, determines the X-ray absorption properties of each of thousands of volume elements in the volume of the patient 190 that intersects the X-ray beam 123. X-ray absorption is conventionally expressed in units of Hounsfield (H), named for an early researcher in the field. The size of the smallest volume element (voxel) for which the system can derive an X-ray absorption value is called the spatial resolution of the system. The spatial resolution of modern helical systems is about 0.3 millimeter (mm, 1 mm = 10^"3 meters) by 0.3 mm by .3 mm, for a voxel size of about 0.03 mm³. The temporal resolution is based on the time for one revolution of the gantry, which is about 0.3 seconds, yielding about 3 new scans every second, where a scan includes millions of 0.03 mm³ voxels.

[0050] The CT transfer point 160 is any mechanism, such as data slip rings, used to transfer CT data from the rotating gantry to computer system 150 for use by CT process 152.

[0051] Probe system 140 includes a probe 142, a catheter 143 and a probe controller 144. In the illustrated embodiment, the probe system 140 includes probe position sensor 146a and probe position sensor 146b (collectively referenced hereinafter as probe positions sensors 146), and probe measurement process 154 on computer system 150. [0052] The probe 142 is any device that is inserted into a living body for any reason, such as an ablating electrophysiological tip, well known in the art, for measuring voltage in the heart and generating scar tissue in the heart to change electrical conductance associated with arrhythmia. In some embodiments, also illustrated below, the probe is a needle for guiding the PM contact for a lead to a position on the heart of the patient 190.. [0053] The probe controller is any device that is used to control operation of the probe, such as hand held manipulators that control the movement of the probe and control probe operations, such as measurement and ablation. In some embodiments, the probe controller is simply manual controls for a surgeon, such as to insert a needle straight into the heart for placement of drugs or an electrical lead. [0054] The catheter 143 is a tube inserted into a lumen of the living subject, such as a blood vessel, through which the probe is passed to a particular location in the patient. Inside the catheter 143 are one or more control lines for connecting the probe to the probe controller 144. In other embodiments, the catheter is replaced by any tether that ties the probe to a device located outside the living subject and used to control the probe. In some embodiments the catheter is replaced by a wireless communication link between the probe 142 inside the patient and the probe controller 144 outside the patient. [0055] In some embodiments, the probe system includes one or more probe positioning sensors, such as probe positioning sensors 146. Probe positioning sensors 146 determine the three dimensional position of probe 142 using any method known in the art, such as measuring strength of electromagnetic induction from an electrical source in the probe 142. A probe positioning process, such as a process executing on probe controller 144 or computer system 150, uses triangulation or other algorithms to deduce probe position from the measurements made at position sensors 146. Well known probe positioning systems for an electrophysiological catheter tip include CARTO™ and CARTOMERGE™ provided by Biosense Webster, Inc. of Diamond Bar, California. [0056] A probe measurement process, such as probe measurement process 154 on computer system 150, determines conditions in patient 190 based on measurements made by probe 142. In some embodiments, probe measurement process 154 includes the probe positioning process, described above. In example measurements in some embodiments, probe measurement process 154 determines the action potential on an inner surface of the heart based on voltage measurements made over one or more heart cycles at probe 142, a probe position determined based on sensors 146, patient position (e.g., based on markers attached to the patient) and a model of the heart of patient 190 based on generic features or pre-operative internal scans of the patient. Such action potential is presented as a colored area on a cartoon representation of a heart in a two dimensional screen image displayed to a human operator of probe controller 144. The probe position relative to the model heart is estimated using any of several estimation processes that are well known in the art.

[0057] According to an illustrated embodiment, a combined presentation process 156 executing on computer system 150 combines information about current probe position and probe measurements, if any, from probe measurement process 154 with real time CT scan data from CT process 152 to present to a human operator of probe controller 144 presentation data and imagery that simultaneously indicates real time probe position and real time anatomy. In some embodiments, real time probe position and real time anatomy are based only on measurements from CT system 120 (not on probe position sensors 146) and presented to the human operator of probe controller 144.

1.2 Data structures

[0058] FIG. 2A is a block diagram that illustrates scan elements in a 2D scan 210, such as one slice from a CT scanner with a one dimensional detector array The two dimensions of the scan 210 are represented by the x direction arrow 202 and the y direction arrow 204. The scan 210 consists of a two dimensional array of 2D scan elements 212 each with an associated position. A value at each scan element position represents a measured or computed intensity that represents a physical property (e.g., X- ray absorption) at a corresponding position in at least a portion of the spatial arrangement of the living subject 190. Although a particular number and arrangement of equal sized circular scan elements 212 are shown for purposes of illustration, in other embodiments, more elements in the same or different arrangement with the same or different sizes and shapes are included in a 2D scan.

[0059] FIG. 2B is a block diagram that illustrates scan elements in a 3D scan 220, such as stacked multiple slices from a CT scanner with a one dimensional detector array or scan data from one rotation of the CT system of FIG. 1 with a rectangular cross sectional X-ray beam 123 and a two-dimensional detector array 124. The three dimensions of the scan are represented by the x direction arrow 202, the y direction arrow 204, and the z direction arrow 206. The scan 220 consists of a three dimensional array of 3D scan elements (voxels) 222 each with an associated position. A value at each scan element position represents a measured or computed intensity that represents a physical property (e.g., X-ray absorption or acoustic reflectivity) at a corresponding position in at least a portion of the spatial arrangement of the living subject 190. Although a particular number and arrangement of equal sized spherical scan elements 222 are shown for purposes of illustration, in other embodiments, more elements in the same or different arrangement with the same or different sizes and shapes are included in a 3D scan 220. [0060] In the following, the term voxels is used interchangeably with the term scan elements to mean both 3D scan elements and 2D scan elements that represent measured output from a CT system. A collection of scan elements generated in a particular time interval, such as one revolution of gantry 128, is a scan.

[0061] The term image is used herein to indicate a two dimensional set of picture elements (pixels) that are presented on a two dimensional display, such as a cathode ray tube (CRT) display screen or plasma screen or liquid crystal display (LCD) screen, for use by a human viewer, such as the human operator of probe controller 144. In some embodiments, each of one or more image pixels represents data from one or more scan elements (voxels). In some embodiments, the presentation image comprises a holograph of 3D presentation elements that are each based on one or more scan elements (voxels). [0062] Certain voxels in the scan data are associated with a particular object inside the patient 190, such as the probe, a type of tissue that is a target of the intervention using the probe, and altered tissue that is a result of the intervention using the probe. The spatial arrangement of the particular object is represented by the set of voxels that are associated with the particular object, or by a boundary between such voxels and surrounding voxels. Such spatial arrangements can be displayed as an image in any number of ways known in the art, such as by segmented cross sections through the object on planes of arbitrary location and orientation, by a 3D rendering of a surface of the object absent other objects, or by the surface of the adjacent tissues absent the particular object. [0063] Various data structures employed in various embodiments include data structures that store: 1] a time series of probe 142 measured values, if any; 2] a time series of probe 3D positions based on probe position sensors 146, if any; 3] a time series of one or more 2D voxels of X-ray intensity received at one instant on detector array 124 (also called herein an X-ray projection); and 4] a time series of one or more volume scans of X- ray absorption values interior to a living body 190 based on CT processing of multiple X- ray projections.

2. Method

[0064] According to an example embodiment, a method involves using the probe 142 in concert with a real time image based on CT data. In this context, real time images based on CT data includes any image generated within 0.5 seconds of the time an X-ray beam 123 used to generate the CT data is emitted by X-ray source 122. It is understood that CT scan data is generated based upon a large number of X-ray beams, or a continuous X-ray beam, emitted over one complete or partial rotation of gantry 128, which occurs in about 0.3 seconds or less in current and evolving CT systems. Thus the real time image is presented within 0.5 seconds of the start of the 0.3 second rotation by the gantry. A CT image can be produced with data received in half a gantry rotation. This means that the CT scan is produced and a presentation image is derived from the scan data within 0.35 seconds after the end of a half gantry rotation. In emerging CT systems with faster rotations, or partial rotations, more time is available for producing the scan data and the presentation image.

2.1 Method Overview

[0065] FIG. 3 is a flow diagram that illustrates at a high level an example method 300 of using CT in real time to position a probe. Although steps in FIG. 3 and subsequent flow diagrams (FIG. 4, FIG. 5, FIG. 6), are shown in a particular order for purposes of illustration, in other embodiments, one or more steps may be performed in a different order or overlapping in time, in series or in parallel, or one or more steps may be omitted or added, or changed in some combination of ways.

[0066] In step 302, pre-operations data is collected. Pre-operations data is any data used during operation of a probe (e.g., probe 142) that is not processed within 0.5 seconds of collection. Such data includes patient history, and one or more scans collected, registered with respect to each other and segmented into tissues of different types and used to diagnose the patient or plan the intervention involving the probe before the probe is inserted into the patient. For example, during step 302, multiple CT scans or magnetic resonance imaging (MRI) scans or both are collected, analyzed, aligned with each other and segmented to produce a 3D model of the heart of the patient before an ablating catheter tip is inserted into the patient. In some embodiments, data about the operation of the CT system 120 and options for presenting the data, described in more detail below with reference to FIG. 4, FIG. 5 and FIG. 6, are also received during step 302. [0067] Any method may be used to receive this data. For example, in various embodiments, the data is included as a default value in software instructions, is received as manual input from a care giver or system administrator on a local or a remote node of a network, is retrieved from a local file or database, or is sent from a different node on the network, either in response to a query or unsolicited, or the data is received using some combination of these methods. [0068] In step 320, a probe is selected that is visible in CT scans. Step 320 includes the development and production of interventional equipment that is specifically designed to be used and worked in the CT environment, such as catheters, sheaths, needles, recording systems, wires, which are produced from metallic and non-metallic material which optimizes the visibility of the probe during CT scanning. For example, iron is opaque to X-rays at wavelengths used in most CT scanners and is highly visible in CT scans. However, even a small circular cross section of an iron wire in a CT scan plane causes an artifact that appears as a sun burst pattern emanating from that circular cross section, as described in more detail below with reference to FIG. 9 A and FIG. 1OA. Metals of other types are expected to be visible with less distracting artifacts. Different shapes of the same material are also expected to produce different effects in a CT scan and are considered in determining the type of probe to use. For example, the artifacts are reduced using a spherical shape for the probe tip. The spherical shape allows homogenous absorption of the x-ray photons regardless of the orientation of the probe in the scan field. The resulting artifacts can be reduced further through iterative reconstruction and consecutive correction of the missing scan data depending on the given shape and known position of the probe and predicted voxels of the probe artifacts. [0069] In step 322, the selected probe, e.g., probe 142, is inserted into the living subject, e.g., patient 190. For example, during step 322, a conventional metallic electrophysiological ablating catheter tip, as probe 142, is inserted into a heart of patient 190 through a catheter 143 inside a major vein leading to the heart of patient 190. As another example, during step 322, a PM lead guiding needle, as probe 142, is inserted directly into a heart of patient 190 through the skin of patient 190. [0070] In step 330 any contrast agents that highlight a target tissue or intervention result or both are inserted into the living subject, e.g., patient 190. For example, in ablations of atrial fibrillation or ischemic ventricular tachycardia, the cardiac anatomy such as the left atrium, pulmonary veins, myocardial scar are target tissues and are desirably visualized in the CT scan data. After an ablation agent is delivered, it is desirable to visualize lesion formation, transmural extent and potential gaps with CT scans to guide further ablation actions. In some embodiments, neural structures such as ganglionated plexi, nerve strands, and terminal nerve endings are additional targets for ablation therapy. In various embodiments, contrast agents are administered during step 330, which enhance any of these tissues in CT scans for single or multiple X-ray spectral bands. For example, Iodine-containing contrast materials demonstrate different wash-in and wash-out kinetics in ablated than in non-ablation tissue. This results in an increase in signal intensity at the ablated tissue, which can be observed in the first 30 minutes after injection of the contrast. In another application, contrast material is combined with markers (such as antibodies) for sub-cellular structures found in cells participating in electrical conduction (such as connexin 41 and 43). This allows a selective imaging of tissue displaying certain electrophysiological properties. In another embodiment, contrast material is combined with markers (such as antibodies) that bind to neurotransmitters in cardiac structures such as acetylcholine. This allows a selective imaging of tissue participating in the innervation of the heart. In various embodiments, step 330 is performed before step 322 or both before and after step 322.

[0071] In step 340, the position of the probe in the living body, e.g., patient 190, is determined. Any method may be used to determine the position of the probe, including using conventional systems that rely on probe position sensors 146. In some embodiments, described in more detail below with reference to FIG. 4, step 340 includes using 2D X-ray projections from at least one spectral band of X-ray source 122, as measured at X-ray detector array 124, in CT system 120 to determine the position of the probe in the living subject. Thus, fast capture of CT scan data is combined with real-time catheter tracking technology, which enables the system 100 to automatically find the correct imaging plane and update it during the catheter navigation and ablation. In some embodiments, the fast CT scan capture is combined with a remote catheter control system 144, in which the information about the catheter movement is forwarded to the CT scanner, which allows the automatic navigation and visualization of the catheter and catheter tip.

[0072] In step 350, it is determined whether the probe is outside the desired scan volume based on the position of the probe determined in step 340. If so, then control passes to step 352. In step 352, the scan volume is changed to include the probe by moving the table or rotating the gantry with source 122 and detector array 124 so that the beam 123 intersects the patient 190 nearer to the vicinity of the probe's position. In some embodiments, the probe is moved to lie within the scan volume for the current position of the table and the gantry. In some embodiments, some combination of the probe and the table and the gantry is moved to bring the probe within the scan volume. In some embodiments, step 350 and step 352 are omitted. For example, in some embodiments, the probe is always within the field of view of source 122 and detector array 124, so step 350 and step 352 are omitted.

[0073] After step 352, control passes to step 360, described below. [0074] If it is determined, in step 350, that the probe is inside the desired scan volume, then control passes to step 354. In step 354, it is determined whether it is time for another scan of the volume. If so, then control passes to step 360. In some embodiments, each scan is made as the gantry completes each rotation and step 354 is omitted and control passes to step 360 directly. However, each scan involves exposing the patient to ionizing radiation and it is desirable to limit the patient's exposure by skipping some scans some of the time. In some embodiments, radiation exposure is limited within step 360, as described below with reference to FIG. 5. In some embodiments it is determined in step 354 whether it is time for another scan. Any method may be used to determine whether another scan is warranted. In some embodiments, scans are taken repeatedly during probe movement, but intermittently during therapeutic operations, such as during ablation activities, until those activities are completed. In some embodiments, scans are suspended while a human operator of probe controller releases a trigger to indicate suspension of probe activities. In some embodiments, scans are taken on a schedule based on input received during step 302.

[0075] In step 360 CT scan data is collected in the vicinity of the probe. Any method may be used to collect the scan data, as long as the collection is made in a time short relative to real time, e.g., short compared to 0.5 seconds. In some embodiments, described in more detail below with reference to FIG. 5, the scan is collected under particular conditions to optimize usefulness for the purposes of the intervention by probe 142 or to limit patient exposure to ionizing radiation or both. Control then passes to step 370, described below.

[0076] After a scan is completed in step 360, or after it is determined, in step 354, that it is not time for another scan, then control passes to step 370.

[0077] In step 370, an image based at least in part on the scan data is presented in real time, e.g., within 0.5 sec of the start of the scan taken in step 360. In some embodiments, the image presented shows both the probe and anatomy of the subject, such as a target tissue or an altered tissue or both. Any method may be used to determine the image presented. In many embodiments, the image presented is at an oblique angle to the plane of gantry rotation in order to include both the probe and the target or altered tissue. Several different presentation images are available at the discretion of a human operator according to a particular embodiment described in more detail below with reference to FIG. 6. In some embodiments, the probe or target tissue or altered tissue, or some combination, is automatically identified in the scan data and displayed as one or more 3D objects. Control then passes to step 380.

[0078] In step 380, the probe is operated based on the image presented in step 380. For example, a measurement is made with the probe at the current probe position, a therapy is applied with the probe (such as releasing ablating energy) at the current probe position, or a new target is identified by the operator and the probe is moved toward the target from the current probe location. In some embodiments, during step 380, the probe is operated automatically based on an automatically determined current probe position and an automatically determined target tissue object or altered tissue object or both. Control then passes to step 390.

[0079] In step 390, it is determined whether the probe has moved during the operations of step 380. Any method may be used to determine whether the probe moved. In some embodiments, signals sent from probe controller 144 are intercepted to determine not only whether the probe is moved, but also to determine the direction and speed of movement in order to predict the next probe position.

[0080] If it is determined, during step 390, that the probe has not moved, then control passes back to step 354 to determine whether it is time for another scan and eventually to present another image during step 370.

[0081] If it is determined, during step 390, that the probe has moved, then control passes back to step 340 and following to determine the new position of the probe and perform a new scan or present a new image based on the new position of the probe. [0082] Using the steps of method 300 in ablations of atrial fibrillation or ischemic ventricular tachycardia, real-time display of imagery based on CT scans allows visualization of the cardiac anatomy such as the left atrium, pulmonary veins, myocardial scar. This then allows the accurate and detailed navigation of ablation catheters to target ablation sites. Tissue contact can be directly monitored. When ablation energy is delivered, lesion formation, transmural extent and potential gaps can be monitored and used to guide further probe movement or ablation energy application or both. [0083] In another embodiment, right atrial or right ventricular leads, or both, are placed. Currently, such leads are placed using standard fluoroscopy. Similarly, coronary sinus (CS) leads are placed currently using standard fluoroscopy. CS leads are used to indicate severe heart failure and wide QRS features and have been shown to reduce morbidity and mortality. In 10% of the attempts using current approaches, a CS lead cannot be placed due to the inability of the operator to obtain access to a suitable position inside the CS or one of the side branches. In these cases, patients have to undergo a more invasive surgical placement that is associated with increased morbidity and risk of mortality. Even after successful placement about 1/3 of patients do not achieve an improvement in symptoms. This failure is possibly due to limitations to placement via CS or side branches only. Location seems to be important for the clinical response. [0084] In various embodiments using real-time CT, a right atrial or right ventricular or CS lead, or some combination, is placed with better knowledge of the cardiac and surrounding structures, which allows an immediate check for possible complications. Real-time CT even allows placement of percutaneous leads and placement of an epicardial or mid-myocardial left ventricular lead. We have evaluated the use of real time CT data placement of leads in 4 pigs and shown the feasibility of such an approach. The real-time placed CT leads gave good pacing and sensing parameters. We determined a safe access route with standard CT. Then, for the next steps, used only real-time CT. A percutaneous needle and pacer lead were advanced based on the real time CT images presented to the operator along a planned trajectory. After a target position had been reached, the pacemaker lead was deployed. The needle was then retracted and no complication observed in nearly all placements. The single complication of a pericardial effusion was immediately detected. At necropsy, imaged location and lead placement correlated well with the locations derived from the real-time CT presentation images.

2.2 Method for Probe Detection in CT Data

[0085] In many embodiments, a probe or tether/catheter or both is made of material that is readily evident in 2D X-ray projections (equivalent to 2D fluoroscopy scans). A single X-ray beam 123 from X-ray source 122 received in one instant of time at an X-ray detector array 124 that is arranged as a 2D array is equivalent to a singe 2D fluoroscope scan, and should display a projection of the probe or tether/catheter or both. FIG. 7 is an example 2D fluoroscope projection 700 that depicts an electrophysiological catheter 710 and also depicts the ablating catheter tip 712 that serves as a probe in a living body of a swine. This is a negative image in which high intensity (low absorption) voxels are dark or black and low intensity (high absorption) pixels are light or white. Simply selecting 2D voxels with intensity values below a threshold value indicates those 2D voxels associated with the probe/tether.

[0086] A second 2D projection in a second plane can be combined with the first 2D projection to provide a matched pair of stereoscopic images for presentation to the intervening practitioner. The offset of the two projected images can be adjusted to the typical view angle of the intervening practitioner and used to orient the practitioner' s perception of the probe position, rather than produce a precise position. [0087] In some embodiments, the probe position is determined in real time during step 340 based on two, near-orthogonal 2D projections measured using the X-ray source 122 and detector array 124 of CT system 120. In such embodiments, data from probe position sensors 146 can be ignored; and in some embodiments probe position sensors 146 are omitted from system 100. FIG. 4 is a flow diagram that illustrates an example method 400 for performing step 340 depicted in FIG. 3 based on data from CT system 120. Method 400 is a particular embodiment of step 340; control passes to method 400 from step 330 and passes to step 350 after method 400.

[0088] In step 402, default data is received indicating the angles and fields of view of the CT system 120 to produce the 2D projections to use to solve for probe/tether 3D positions. In some embodiments, the default data also indicates one or more X-ray spectral bands that produce 2D projections in which the probe/tether voxels are most easily detected automatically. In some embodiments, the default data indicates the two projections are measured repeatedly at low dose X-ray beams rather with high resolution, high dose X-ray beams. In some embodiments, the default data received during step 402 is collected with other pre-operations data during step 302.

[0089] In step 410, it is determined whether manual data is received to augment or override some or all of the default data received during step 402. If so, control passes to step 412 to add to or replace the previous data about angles and doses based on the manual data received. Control then passes to step 420. If it is determined in step 410 that manual data is not received, then control passes to step 420 directly without passing first to step 412.

[0090] In step 420, it is determined whether a prior CT scan is available for determining the present position of probe or tether or both. Any method may be used. For example, in some embodiments, step 420 includes determining whether a CT scan with multiple measurements over a complete or partial gantry rotation has been performed and whether the probe has not been moved or performed ablations since that scan. If both conditions are met, then a previous CT scan is considered to be available. [0091] If it is determined that a previous CT scan is available, then control passes to step 422. In step 422, the default or manually updated angles and fields of view and X- ray spectral bands are collected from the projection data structures used to derive the previous CT scan, and control passes to step 440, described below, skipping step 430. [0092] If it is determined that a previous CT scan is not available, then control passes to step 430. In step 430 the X-ray source is pulsed at low dose and one or more spectral bands at two or more angles, or angle ranges, of gantry rotation to provide two, near- orthogonal equivalent 2D fluoroscope projections. Low dose is used because it is sufficient to capture the probe/tether in the projections and it exposes the subject to less ionizing radiation. Control then passes to step 440.

[0093] In step 440, probe position is automatically determined in multiple 2D projections at the selected angles and corresponding fields of view. For example, the voxels below the threshold value are determined and a voxel at an end of the line of voxels below the threshold and away from the edge of the field of view and away from the outer skin of the living body, if any, is determined to be the probe position voxel in each 2D projection. Control then passes to step 450.

[0094] In step 450, it is determined whether the probe is in the field of view of both (or all) of the fields of view at the two (or more) angles of rotation of the gantry. If not, the probe is out of the field of view. Control passes to step 452 to move the translating table in a direction to bring the probe into the field of view. For example, the table is sent instructions to move in a direction toward a boundary of the field of view where the line of voxels below the threshold value (which indicates the tether) intersects that boundary. Control then passes back to step 430 to take another measurement. [0095] If it is determined in step 450 that the probe is in the field of view of both of the fields of view at the two angles of rotation, then control passes to step 460. In step 460, the 3D position of the probe (and, in some embodiments, the tether) is determined based on the positions of the probe (and tether) in the two or more 2D projections. Control then passes to step 350, described above.

2.3 Method for Collecting CT Scan Data

[0096] In some embodiments, the CT system 120 is operated to collect scan data during step 360 in any way known in the art, e.g. in helical mode with continuous X-ray beams for one or more spectral bands from X-ray source 122 while gantry 128 rotates at a constant rate, with or without constant movement of translating table 110. [0097] However, in some embodiments, the CT scan data is collected during step 360 in a special way so as to be especially suited for a particular purpose, or to reduce radiation exposure of patient 190 during an interventional procedure with probe 142, or both. FIG. 5 is a flow diagram that illustrates an example method 500 for performing step 360 depicted in FIG. 3. Method 500 is a particular embodiment of step 360; control passes to method 500 from step 352 or step 354 and control passes to step 370 after method 500.

[0098] In step 502, default data is received, which indicates any angles and fields of view for any 2D projections. In some embodiments, the default data also indicates one or more X-ray spectral bands that produce 2D projections or 3D CT scans that reveal particular features of interest, such as target tissues or tissues altered by intervention of the probe or the probe itself or the probe's tether. In some embodiments, the default data also indicates one or more conditional triggers for selecting times at which a 3D CT scan or 2D projection is desired. For example, in some embodiments the default data indicates a particular point in an electrocardiogram trace at which a 2D projection or a CT scan is desired.

[0099] In some embodiments, the default data indicates time intervals between high resolution CT scans, which use a large number of X-ray beams during each rotation of the gantry 128. High resolution CT scans are not measured within this interval in order to reduce the exposure of the patient 190 to ionizing radiation. In some embodiments, the interval is filled with one or more low resolution CT scans that pulse the X-ray source 122 at fewer angles during rotation of the gantry 128, and thus reduces the exposure of the patient 190 to ionizing radiation compared to a high resolution CT scan. [0100] In some embodiments, the default data received during step 502 is collected with other pre-operations data during step 302.

[0101] In step 510, it is determined whether manual data is received to augment or override some or all of the default data received during step 502. If so, control passes to step 512 to add to or replace the previous data about 2D angles, triggers and intervals for high and low resolution CT scans based on the manual data received. Control then passes to step 520. If it is determined, in step 510, that manual data is not received, then control passes to step 520 directly without passing first to step 512.

[0102] In step 520, it is determined whether the EKG trigger has occurred within a field of view. In other embodiments, one or more other trigger conditions are tested. In an illustrated embodiment, to minimize heart wall motion artifacts during ablation in an invasive electrophysiological procedure, the CT scan data or 2D projections are synchronized with a particular cardiac phase that is most relevant for the intended procedure.

[0103] FIG. 8 is an example electro-cardiograph 800 that depicts different features in an electrocardiogram 810 that trigger an X-ray source in an example CT system. The horizontal axis 802 indicates elapsed time with a scale of 0.04 seconds given by interval 803. The vertical axis 804 indicates measured voltage with a scale of 0.1 milliVolts (mV, 1 mV = 10^"3 Volts) given by interval 805. The EKG trace 810 begins at time R 811, includes a P peak 812, a QRS peak 814 at time R' 813, a T peak 816 and a U peak 818, that define PR segment 822, PR interval 823, QT interval 824, ST segment 826 and R interval 828 starting at time R 811. These features define time point of the cardiac cycle describing the atrial and ventricular depolarization and repolarization, i.e. how the membranes of the cardiac cells get charged and discharged during the heart beats. P reflects the electrical depolarization of the atria. QRS represents the electrical depolarization of the ventricles. T reflects the electrical depolarization of the ventricles; and U represents an abnormal late depolarization or after depolarization. PR represents a time it takes for the electrical wave from the beginning of the atrial depolarization to the beginning of the ventricular depolarization. ST represents a time from the end of the electrical ventricular depolarization to the end of the ventricular repolarization. QT represents a time from the beginning of the electrical ventricular depolarization to the end of the ventricular repolarization. R R' represents a time from one cardiac cycle to the next, a measure of the heart rate.

[0104] An X-ray exposure can be gated to one or more of these features to take an image of the probe relative to the heart wall during a particular phase, such as the systolic phase or the diastolic phase of the heart cycle. For example, during a pulmonary vein ablation it is beneficial to image the heart triggered to the end-diastolic phase. This corresponds in FIG. 8 to a time point just before the first dotted vertical line at the end of R interval 828, which presents the beginning of the QRS complex. An ablation would be timed at that time point, and images synchronized with this point in the cycle are most useful to the intervening practitioner. In other embodiments, such as for other applications, the X-ray pulse is gated at a different time.

[0105] In some embodiments, the gantry rotation rate is synchronized with the heart beats to allow consistent 2D projections at each cardiac cycle. A synchronized gantry rotation rate also allows consistent 3D scans. Cardiac pacing with a constant rate is used in some embodiments in order to achieve the highest possible temporal resolution by maintaining a specific ratio between the heart rate and the rotation rate of the CT gantry. In some embodiments, the ratio varies with the heart rate.

[0106] If it is determined in step 520 that the EKG trigger fires while the gantry has the correct field of view, control passes to step 522. In step 522, the X-ray source 122 in at least one spectral band is pulsed at a high dose to produce a high resolution projection. In other embodiments, the high resolution is continued for a complete or partial rotation of the gantry to produce a high resolution scan that is synchronized with the particular phase of the cardiac cycle. Control then passes to step 370 described above with reference to FIG. 3. If it is determined, in step 520, that the EKG trigger does not fire while the gantry has the correct field of view, then control then passes to step 530. [0107] In step 530, it is determined whether the time interval has passed for another high resolution scan. If so, control passes to step 532 to perform one rotation of the gantry with a large number of X-ray beams to produce a high resolution CT scan. Control then passes to step 370, described above with reference to FIG. 3. If it is determined, in step 530, that the time interval has not yet passed for another high resolution scan, then control passes to step 540. [0108] In step 540, it is determined whether the time interval has passed for another low resolution scan. If so, control passes to step 542 to perform one rotation or partial rotation of the gantry with relatively few X-ray beams to produce a low resolution CT scan, which exposes the patient to reduced ionizing radiation. Control then passes to step 370, described above with reference to FIG. 3. If it is determined, in step 540, that the time interval has not yet passed for another low resolution scan, then no scan is initiated; and control passes back to step 520 and following until the time for the next scan of any type is reached.

2.4 Method for Presenting Real-Time Imagery Based on CT Scan Data [0109] At least a portion of the scan data is presented to the operator of the probe during step 370, within real time, as described above with reference to FIG. 3. Any method may be used to present the scan data in real time.

[0110] According to the illustrated embodiments, in order to respond to the real-time position of the probe with respect to anatomy evident in the CT data, including target tissue, or the altered tissue already affected by the probe or both, it is desirable to present to the operator of the probe not all the scan data or the raw scan data, but, instead, that portion of the scan data, or data derived therefrom, that reveals the target tissue or altered tissue or both relative to the probe position. Thus, in the illustrated embodiment, during step 370, an image is generated and presented that shows some anatomy of the living subjects, such as the target tissue or the altered tissue or both in the vicinity of the probe. Stated another way, an image is presented that is made up of pixels that include data from at least one voxel that represents some anatomical feature of the living body in the vicinity of the probe. In some embodiments, the image is presented that includes pixels that include at least one voxel that represents the probe, as well. FIG. 6 is a flow diagram that illustrates an example method 600 for performing step 370 depicted in FIG. 3. Method 600 is a particular embodiment of step 370; control passes to method 600 from step 360 or step 354, and passes to step 380 after method 600. [0111] In step 602, default data is received, which indicates default presentation options, such as the type of image (3D rendering or planar views), the angles relative to the axis of rotation of the gantry for any planar views, the number of images, the type of anatomy to show relative to the probe, and the type of non-CT data, if any, to show in the presentation image. In some embodiments, the default data received during step 602 is collected with other pre-operations data during step 302.

[0112] In step 610, it is determined whether manual data is received to augment or override some or all of the default data received during step 602. If so, control passes to step 612 to add to or replace the previous data about presentation options. Control then passes to step 620. If it is determined in step 610 that manual data is not received, then control passes to step 620 directly without passing first to step 612. [0113] In step 620, it is determined whether a plane view is to be presented. If so, control passes to step 622. In step 622 an image is formed and presented to the operator of the probe controller, which represents a plane through the scan data that includes at least one voxel that shows the probe, along with one or more voxels that show some of the anatomy of the living subject, such as the target tissue or the altered tissue. In some embodiments, step 622 includes presenting an image based on a 2D projection taken with the X-ray source (e.g., source 122) and detector array (e.g., array 124) of the CT system (e.g., system 120).

[0114] Because the materials of many probes are already visible in CT scan data, or can be selected, during step 320, to be visible in CT scan data at one or more spectral bands, it is assumed for purposes of illustration that at least a portion of the probe can be automatically detected in scan data. This information is used to force the presentation plane through the scan data to include a voxel automatically identified as a probe voxel that is coincident with a position of the probe in the living subject. [0115] In embodiments using probes with iron components, such as conventional electrophysiological catheter tips, a CT scan that includes the probe exhibits a sun burst artifact. In the illustrated embodiment, such artifacts are minimized in the presentation image produced during step 622.

[0116] FIG. 9A is an example slice from a CT scan with voxels that depict a tip of an ablation catheter. The ablation catheter tip 910 is shown at the center of an artifact 912 that looks like rays radiating from a central core in a pattern called herein a "sunburst." The artifact 912 is pronounced, having a pure white pattern because the catheter tip contains substantial iron used to conduct electrical current for voltage measurements and ablation. The artifact 912 makes it simple to deduce the presence of the catheter tip 910 automatically in the scan data, and hence select an image plane that includes the tip. [0117] In some embodiments the presentation image plane through the scan data is determined by an angle expressed in the default or manual data and a probe voxel, such as the probe voxel at the tip of the probe or at the center of the probe. The probe position and orientation is either determined by image parameters or external tracking system. [0118] For example, in some embodiments the presentation image plane is determined to include the length of a percutaneous needle used to place an electrical lead for a pacemaker or defibrillator. FIG. 9B is a photograph that illustrates an example subcutaneous needle 920 with lead assembly 930 used to implant an electrical contact and lead in heart tissue. The depicted needle and lead assembly is for a 3.5F active fixation pacemaker lead from MEDTRONIC of Minneapolis, Minnesota. The lead assembly includes a lead anchor 932 and a lead electrical contact 934,and is connected to an external device by a lead wire 936. The needle is positioned in the target tissues, the lead assembly is attached to the tissue using the anchor 932 (such as a corkscrew-shaped anchor), and the needle 920 is withdrawn, leaving the lead assembly 930 in place. [0119] FIG. 9C is an example high resolution CT scan 940 used to plan access for the subcutaneous needle with lead assembly. The scan 940 shows in high resolution the right ventrical (RV) 942, the left ventricle (LV) 944 and the myocardium (MY) 946, in which is the target for the pacemaker (PM) lead. In the illustrated embodiment, the scan 940 is a pre-operative high resolution axial scan taken during step 302, before the probe is inserted into the body. The access to the myocardium of the left ventricle is planned along this axial plane.

[0120] FIG. 9D is an example real time CT scan 950 used to guide the subcutaneous needle with lead assembly to a desired site in the heart tissue. The scan 950 shows in lower resolution the right ventricle (RV) 952, the left ventricle (LV) 954 and the myocardium (MY) 956, in which is the target for the PM lead in the same viewing plane as the planning scan 940. The voxels 960 depict the needle in place penetrating to the target MY tissue. The voxels 970 depict the anchor and electrical contact of the lead assembly at one point of its movement into the MY tissue. Thus, real time CT imagery guides the deployment of the pacemaker lead in this example embodiment. [0121] The sunburst artifact depicted in FIG. 9A has been shown to be produced by scattering patterns that are predictable given the spectral band of the X-ray source and the atomic composition, shape and position of the probe. In some embodiments, the effects of the scattering are mathematically reversed, during step 622, to effectively remove the artifact in the scan data from the presentation image. For example, in some embodiments, the probe has a spherical shape and the expected scatter from a spherical object of metal in an x-ray beam is calculated, using simple equations known in the art. The data correction is performed according to the spatial position of the probe. In various embodiments, the correction includes CT value shifting or data interpolation or both. In some embodiments, the energy scattered out of the image according to the calculation is added back in at the appropriate voxels for a limited number of voxels in the vicinity of the tip. In some embodiments, the position of the spherical probe is provided as input from the position tracking system. The number of voxels so corrected is chosen so that the corrections can be computed within real-time.

[0122] The principle limitation is the available hardware - which is of course undergoing changes in spatial resolution, acquisition- and reconstruction speed with each generation of new scanner. In an illustrated embodiment, current speed limits are overcome using the following steps. a) Initial high resolution scan of the heart. b) Fluoroscopic scans with complete acquisition of the data - but only partial reconstruction through the use of a limited number of voxels in the region of interest around the tip of the probe during fluoroscopy c) Full reconstruction of the last acquired dataset whenever the fluoroscopy is not actively used.

These steps b) and c) are used in a cycle: The last reconstructed full dataset is used as the background for the targeted reconstruction during the fluoroscopy. The fast reconstructed block around the tip of the probe includes a multiple of the number of rows used in the CT-scanner. In the case of a 64 row scanner, the number of voxels included may be any of: 64x64x64; 128x128x128; etc.

[0123] In the illustrated embodiment, higher frame-rates are achieved with the available hardware. The reconstruction is targeted to the area of interest instead of encompassing a full image. It is anticipated that the spatial resolution within the small block around the tip of the probe is the same as that in the full image. The block of fast reconstructed voxels then replaces the same block of the original or slow reconstructed voxels in the complete dataset. This dataset is then used to generate rendered images or virtual projections allowing a better orientation for the user through the landmarks in the heart. The resulting frame-rate increase is therefore dependent on the used size of the block around the tip of the probe. A 64x64x64 block is reconstructed 64 times faster than a 512x512x64 block with the actual available hardware. For example, the position of the catheter tip is determined either through an external tracking device - or through internal tracking in the scan data. In some embodiments, the internal tracking based on the scan data includes using a fast full field low matrix reconstruction to identify the position of the catheter tip based on the metal content of the tip. This reconstruction is followed by the focused reconstruction and correction with maximum resolution in a small volume near the catheter tip.

[0124] FIG. 1OA and FIG. 1OB simulate the removal from presentation data of the sunburst artifact around probe voxels. FIG. 1OA is a slice of a CT scan 1000 showing the ablation catheter tip voxels 1010 and the sunburst rays artifact 1012. FIG. 1OB is a slice of a CT scan that intersects the catheter away from the iron catheter tip, and which emulates a presentation image 1001 that includes the ablation catheter tip voxels 1010 but which greatly reduces the sunburst rays artifact by mathematical correction for the expected scattering.

[0125] In some embodiments, during step 622, the position of the probe in the presentation image is automatically highlighted, such as with a colored cross or circle centered on the centermost probe voxel.

[0126] In some embodiments, the voxels associated with target tissue or altered tissue are made evident to a human operator by the contrast agents injected during step 330 or the X-ray spectral bands chosen during step 360. In some embodiments, the voxels in one or more anatomical features are also automatically identified or enhanced, such as with a fast segmentation algorithm available at the time an embodiment is implemented. In some embodiments, the process is not segmentation in the sense of explicit identification and labeling of anatomical structures, but is a more simple image calculation - which can be done at several frames per second. For example, in some embodiments, automatic visual enhancement of the target tissue is provided with a simple substration of consecutive images over a certain period of time during and after the contrast injection (in principle like a digital substration angiography, DSA). Substraction means that the contents of one image are subtracted from the contents of a similar image to produce the difference between the two images, which is normally the structure of interest for that procedure. In some embodiments using a multiple wavelength CT, such as a spectral CT, a substraction of the different energies of consecutive scans is performed.

[0127] Control passes from step 622 to step 630. In step 630 it is determined whether multiple planes images are to be presented to the operator of the probe controller 144. If so, control passes to step 632.

[0128] In step 632 multiple plane images, as determined in step 622, are displayed simultaneously in a single presentation image. For example, in some embodiments four plane images are presented simultaneously in a format called quartile view. FIG. 11 is an example real time quartile view 1100 showing a probe relative to anatomy based on CT data. The quartile view 1100 is a single presentation image that includes four plane images: plane image 1101, plane image 1102, plane image 1103, and plane image 1104 with artifacts removed from each plane image. The plane images represent planes that are positioned in order from most caudal to most cranial in the living subject. The probe (catheter tip) is evident as catheter tip voxels 1110 in plane image 1101 and catheter tip voxels 1112 in plane image 1102. The catheter tip is absent in plane image 1103 and plane image 1104.

[0129] In some embodiments, during step 632, the multiple plane images represent CT scan data from intersecting planes and the presentation image shows the planes intersecting. FIG. 12 is an example real time intersecting planes view 1200. The images depicted in FIG. 12 are based on MRI data to simulate the presentation image showing intersecting planes generated based on CT data. The intersecting planes view 1200 includes three plane images: plane image 1201, plane image 1202 and plane image 1203 with artifacts removed from each.

[0130] After step 632, control passes to step 650, described below. If it is determined, in step 630, that multiple planes images are not to be presented, control passes to step 640. In step 640, it is determined whether a 3D view is to be rendered. If so, control passes to step 642.

[0131] In step 642, the probe is rendered (e.g., without artifacts) in 3D with anatomic information (such as target tissue or altered tissue) also rendered in 3D. 3D rendering programs are widely available and have become close to real time, as evidenced by state of the art video games. FIG. 13 is an example real time 3D rendering 1300. The rendering 1300 shows inferior vena cava (IVC) 1310, catheter tip 1320 and left common inferior pulmonary vein (LCIPV) 1330 and can be viewed in any operator selected angle. FIG. 13 depicts a standard display available on all workstations with frame rates of at least 10-15 frames per second depending on the number of voxels and the projection method as well as the specific hardware. In some embodiments color is used to help distinguish the different anatomical elements. Control then passes to step 650. [0132] If it is determined, during step 640, that a 3D view is not to be rendered, or after step 632 or step 642, control passes to step 650. In step 650, it is determined whether to display probe measurements along with probe position. If not, control passes to step 660, described below. However, if it is determined in step 650 to display probe measurements, then control passes to step 652.

[0133] In step 652, probe measurements are overlaid on the previously selected display based on real-time CT data, such as single plane images, multiple plane images or 3D rendering. For example, measurement made by the probe, such as action potential measured by an electrophysiological catheter tip, are overlaid as colored areas on voxels that correspond to the location of the probe when the measurement was made. FIG. 14A is an example real time image 1400 with probe measurements. In the image 1400, grayscale MRI data simulates an image of heart tissue 1402 based on real time CT data. The action potential phase is overlaid on portions of the image 1400. The pixels of image 1400 that correspond to probe voxels when the probe measured action potential that occurred in a first phase are shown as hatched area 1410. The pixels of image 1400 that correspond to probe voxels when the probe measured action potential that occurred in a second phase are shown as hatched area 1420a and 1420b. The pixels of image 1400 that correspond to probe voxels when the probe measured action potential that occurred in a third phase are shown as hatched area 1430a and 1430b. The hatched areas are shown with smooth oval-shaped borders for purposes of illustration. In actual embodiments, the hatched areas corresponding to each phase are found to have irregular borders. Any method may be used to display the measurements on top of voxels derived from real time CT data. For example, such displays are currently available in the CARTOMERGE™ product; and, thus, in some embodiments a commercial product such as CARTOMERGE is used. In other embodiments, the measurements are displayed on the real time imagery using special purpose processes. In another embodiment, the probe measurement values are shown in color in 2D and 3D renderings.

[0134] FIG. 14B is a 2D rendering 1450 with anatomical features from CT data and colored voxels based on probe measurements. Cardiac anatomy derived from CT is depicted in grey scale regions 1452. Specific measurements such as electrical timing, local myocardial voltage, fractionation of signals etc are displayed directly superimposed on the 2D CT anatomy from the CT scanner using in this example a color scale. The colored voxels are represented in FIG. 14B by the gray area indicated by 1461 for red, 1463 for green, 1465 for light blue and 1467 for deep blue.

[0135] FIG. 14C is a 3D rendering 1470 with anatomical features from CT data and colored voxels based on probe measurements. 3D anatomy was derived from CT scan data and then integrated into the CARTO 3D mapping system, which depicts the 3D anatomy in illuminated grayscale perspective 1472. Specific measurements such as electrical timing, local myocardial voltage, fractionation of signals etc are displayed on the integrated 3D CT using in this example a color scale 1480. The colored voxels are represented in FIG. 14C by the gray area indicated by 1481 for red, 1483 for violet, 1485 for blue. In various other embodiments, other 3D rendering processes are used in addition to or instead of CARTO 3D.

[0136] For another example embodiment, a first CT scan is performed to reconstruct the 3D anatomy of the heart and the great cardiac vessels. In various embodiments, this is done either as a surface reconstruction using only the endocardial or epicardial surface or as a volume reconstruction demonstrating the myocardium with both endocardial and epicardial surface. In some embodiments, abnormal conditions or pathology are also displayed. The care giver assesses anatomic and topographic relationships based on this imagery.

[0137] Using real-time CT guidance a mapping and/or ablation catheter is then advanced into the heart. The catheter is navigated to multiple positions at the heart. At each location electrical information such as voltage, waveform, timing, frequency, catheter motion etc. are recorded and a 3D location with x, y, and z location is prescribed. A software algorithm is able to display each of these points with the measurements on the 3D dataset. This can for example be displayed with a color code that covers the range of measurements. In one of these examples, different voltages measured on the myocardial surface as displayed with a color range, which is integrated into the 3D reconstruction of the CT derive myocardium. In another application this is done by displaying in a color coded form the timing of the electrical wavefront. For example, the electrical wavefront starts at a red region and propagates across a yellow region to a green region and then to a blue region over time, as shown in FIG. 14B. In various embodiments, these colors are displayed on the reconstructed heart of the initial CT scan or on the real-time displayed CT images or the real-time 3D reconstructed cardiac anatomy. Such presentation images allow the display of the measured parameters such as voltage, activation and propagation maps, etc. Using real-time guidance enables an operator to return to a location in the heart that would need further mapping or ablation using heat, cold or other energy delivery for therapeutic purposes.

[0138] In various other embodiments, similar superimposition of measured data on CT imagery is used for other organs and disease processes, such as oncological applications

[0139] After step 652 in the illustrated embodiment, control passes to step 660. [0140] If it is determined, during step 650, that probe measurements are not to be overlaid, or after step 652, control passes to step 660. In step 660, it is determined whether to combine the real time CT measurements with measurements from other measurement modalities, such as numerical models, pre-operative MRI or CT scans or echocardiograms or positron emission tomography (PET) scans or real time echocardiographs. If not, control passes to step 380, described above with reference to FIG. 3. However, if it is determined in step 660 to combine the real time CT measurements with measurements from other modalities, then control passes to step 662. [0141] In step 662, the probe and anatomy from the CT system are combined with data from another measurement modality. During step 662, real-time CT scan data is combined with clinical grade mapping technology such as are currently available in several commercial forms (e.g., CARTOMERGE™ , 3D Cardiac Segmentation Software from Endocardial Solutions, Inc. (ESI) of St. Paul, Minnesota, RealTime Position Management (RPM) from Boston Scientific of Natick, Massachusetts, and LOCALISA™ Intracardiac Navigation System from Medtronic, Inc. of Minneapolis, Minnesota, among others) to achieve "modality fusion". In such an application an imaging component of real-time CT is used in combination with the mapping or other measurement abilities of such clinical EP systems. In various embodiments, real time CT data is combined with other imaging technologies such as fluoroscopy (e.g. integrated into a rotational C-arm technology), ultrasound, MRI or nuclear medicine techniques, taken in real time or before invasive operations. From a previously constructed dataset, a three-dimensional model is created in which the updated information of the catheter position is presented in real-time and the pre-acquired model is corrected by means of the actual real-time scanning or catheter tracking system. Control then passes to step 380, described above with reference to FIG. 3.

[0142] For example, based on high resolution CT or MRI data taken prior to invasive operations with the probe, a 3D model of the endocardial surface is determined. A view inside the endocardial surface is rendered in real time based on that model and the current position and direction of the probe. FIG. 15 is an example real time image 1500 showing a probe fly through view inside a previously mapped endocardial surface. The image 1500 is based on MRI data and a simulated position for a probe, such as an ablating catheter tip. Images such as depicted in FIG. 15 are currently available on standard workstation with the frame rate depending on the size of the dataset, the hardware, the algorithm, the number of iterations, the calculation of reflections and number of virtual light sources and the view angle, with frame rates between 1 frame per sec (fps) and 20 or more fps. In example embodiments, the hardware, size of the dataset, algorithm, number of iterations and reflections and light sources are chosen so that the rendering is completed at about 5 or more fps, sufficient for completion within real time. [0143] As a further example, based on a 3D dynamic heart model, real time measured data is shown on a beating heart. FIG. 16 is an example 3D dynamic heart with real time catheter and probe. In FIG. 16, MRI scan data is used to emulate real time dynamic heart data.

3. Hardware Overview

[0144] FIG. 17 is a block diagram that illustrates a computer system 1700 upon which an embodiment of the invention may be implemented. Computer system 1700 includes a communication mechanism such as a bus 1710 for passing information between other internal and external components of the computer system 1700. Information is represented as physical signals of a measurable phenomenon, typically electric voltages, but including, in other embodiments, such phenomena as magnetic, electromagnetic, pressure, chemical, molecular atomic and quantum interactions. For example, north and south magnetic fields, or a zero and non-zero electric voltage, represent two states (0, 1) of a binary digit (bit). A sequence of binary digits constitutes digital data that is used to represent a number or code for a character. A bus 1710 includes many parallel conductors of information so that information is transferred quickly among devices coupled to the bus 1710. One or more processors 1702 for processing information are coupled with the bus 1710. A processor 1702 performs a set of operations on information. The set of operations include bringing information in from the bus 1710 and placing information on the bus 1710. The set of operations also typically include comparing two or more units of information, shifting positions of units of information, and combining two or more units of information, such as by addition or multiplication. A sequence of operations to be executed by the processor 1702 constitute computer instructions.

[0145] Computer system 1700 also includes a memory 1704 coupled to bus 1710. The memory 1704, such as a random access memory (RAM) or other dynamic storage device, stores information including computer instructions. Dynamic memory allows information stored therein to be changed by the computer system 1700. RAM allows a unit of information stored at a location called a memory address to be stored and retrieved independently of information at neighboring addresses. The memory 1704 is also used by the processor 1702 to store temporary values during execution of computer instructions. The computer system 1700 also includes a read only memory (ROM) 1706 or other static storage device coupled to the bus 1710 for storing static information, including instructions, that is not changed by the computer system 1700. Also coupled to bus 1710 is a non-volatile (persistent) storage device 1708, such as a magnetic disk or optical disk, for storing information, including instructions, that persists even when the computer system 1700 is turned off or otherwise loses power.

[0146] Information, including instructions, is provided to the bus 1710 for use by the processor from an external input device 1712, such as a keyboard containing alphanumeric keys operated by a human user, or a sensor. A sensor detects conditions in its vicinity and transforms those detections into signals compatible with the signals used to represent information in computer system 1700. Other external devices coupled to bus 1710, used primarily for interacting with humans, include a display device 1714, such as a cathode ray tube (CRT) or a liquid crystal display (LCD), for presenting images, and a pointing device 1716, such as a mouse or a trackball or cursor direction keys, for controlling a position of a small cursor image presented on the display 1714 and issuing commands associated with graphical elements presented on the display 1714. [0147] In the illustrated embodiment, special purpose hardware, such as an application specific integrated circuit (IC) 1720, is coupled to bus 1710. The special purpose hardware is configured to perform operations not performed by processor 1702 quickly enough for special purposes. Examples of application specific ICs include graphics accelerator cards for generating images for display 1714, cryptographic boards for encrypting and decrypting messages sent over a network, speech recognition, and interfaces to special external devices, such as robotic arms and medical scanning equipment that repeatedly perform some complex sequence of operations that are more efficiently implemented in hardware.

[0148] Computer system 1700 also includes one or more instances of a communications interface 1770 coupled to bus 1710. Communication interface 1770 provides a two-way communication coupling to a variety of external devices that operate with their own processors, such as printers, scanners and external disks. In general the coupling is with a network link 1778 that is connected to a local network 1780 to which a variety of external devices with their own processors are connected. For example, communication interface 1770 may be a parallel port or a serial port or a universal serial bus (USB) port on a personal computer. In some embodiments, communications interface 1770 is an integrated services digital network (ISDN) card or a digital subscriber line (DSL) card or a telephone modem that provides an information communication connection to a corresponding type of telephone line. In some embodiments, a communication interface 1770 is a cable modem that converts signals on bus 1710 into signals for a communication connection over a coaxial cable or into optical signals for a communication connection over a fiber optic cable. As another example, communications interface 1770 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN, such as Ethernet. Wireless links may also be implemented. Carrier waves, such as acoustic waves and electromagnetic waves, including radio, optical and infrared waves travel through space without wires or cables. Signals include man-made variations in amplitude, frequency, phase, polarization or other physical properties of carrier waves. For wireless links, the communications interface 1770 sends and receives electrical, acoustic or electromagnetic signals, including infrared and optical signals, that carry information streams, such as digital data. [0149] The term computer-readable medium is used herein to refer to any medium that participates in providing information to processor 1702, including instructions for execution. Such a medium may take many forms, including, but not limited to, nonvolatile media, volatile media and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1708. Volatile media include, for example, dynamic memory 1704. Transmission media include, for example, coaxial cables, copper wire, fiber optic cables, and waves that travel through space without wires or cables, such as acoustic waves and electromagnetic waves, including radio, optical and infrared waves.

[0150] Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, a hard disk, a magnetic tape, or any other magnetic medium, a compact disk ROM (CD-ROM), a digital video disk (DVD) or any other optical medium, punch cards, paper tape, or any other physical medium with patterns of holes, a RAM, a programmable ROM (PROM), an erasable PROM (EPROM), a FLASH-EPROM, or any other memory chip or cartridge, a carrier wave, or any other medium from which a computer can read.

[0151] Network link 1778 typically provides information communication through one or more networks to other devices that use or process the information. For example, network link 1778 may provide a connection through local network 1780 to a host computer 1782 or to equipment 1784 operated by an Internet Service Provider (ISP). ISP equipment 1784 in turn provides data communication services through the public, worldwide packet- switching communication network of networks now commonly referred to as the Internet 1790. A computer called a server 1792 connected to the Internet provides a service in response to information received over the Internet. For example, server 1792 provides information representing video data for presentation at display 1714. [0152] The invention is related to the use of computer system 1700 for implementing the techniques described herein. According to one embodiment of the invention, those techniques are performed by computer system 1700 in response to processor 1702 executing one or more sequences of one or more instructions contained in memory 1704. Such instructions, also called software and program code, may be read into memory 1704 from another computer-readable medium such as storage device 1708. Execution of the sequences of instructions contained in memory 1704 causes processor 1702 to perform the method steps described herein. In alternative embodiments, hardware, such as application specific integrated circuit 1720, may be used in place of or in combination with software to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware and software.

[0153] The signals transmitted over network link 1778 and other networks through communications interface 1770, carry information to and from computer system 1700. Computer system 1700 can send and receive information, including program code, through the networks 1780, 1790 among others, through network link 1778 and communications interface 1770. In an example using the Internet 1790, a server 1792 transmits program code for a particular application, requested by a message sent from computer 1700, through Internet 1790, ISP equipment 1784, local network 1780 and communications interface 1770. The received code may be executed by processor 1702 as it is received, or may be stored in storage device 1708 or other non-volatile storage for later execution, or both. In this manner, computer system 1700 may obtain application program code in the form of a signal on a carrier wave.

[0154] Various forms of computer readable media may be involved in carrying one or more sequence of instructions or data or both to processor 1702 for execution. For example, instructions and data may initially be carried on a magnetic disk of a remote computer such as host 1782. The remote computer loads the instructions and data into its dynamic memory and sends the instructions and data over a telephone line using a modem. A modem local to the computer system 1700 receives the instructions and data on a telephone line and uses an infra-red transmitter to convert the instructions and data to a signal on an infra-red a carrier wave serving as the network link 1778. An infrared detector serving as communications interface 1770 receives the instructions and data carried in the infrared signal and places information representing the instructions and data onto bus 1710. Bus 1710 carries the information to memory 1704 from which processor 1702 retrieves and executes the instructions using some of the data sent with the instructions. The instructions and data received in memory 1704 may optionally be stored on storage device 1708, either before or after execution by the processor 1702. 4. Extensions and Alternatives

[0155] In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

Claims

CLAIMSWhat is claimed is

1. A method for positioning a probe in a living subject, comprising: inserting a probe into a living subject; after inserting the probe into the subject, then exposing the subject to a first X-ray beam; receiving first scan data that consists of a plurality of volume elements that each indicate X-ray absorption at a corresponding volume inside the living subject based on computer tomography and the first X-ray beam; and presenting in real time a first image based on the first scan data and a position of the probe, wherein real-time is a time shorter than 0.5 seconds after exposing the living subject to the first X-ray beam.

2. A method as recited in Claim 1, wherein the first image includes from the first scan data a target volume element that depicts a target tissue of the subject.

3. A method as recited in Claim 2, wherein the target volume element depicts myocardium tissue in a heart of the subject.

4. A method as recited in Claim 2, further comprising, before exposing the subject to the first X-ray beam, inserting into the subject a contrast agent that causes the target tissue to be evident in the target volume element.

5. A method as recited in Claim 2, further comprising automatically determining in real time the target volume element in the first scan data.

6. A method as recited in Claim 1, wherein the first image includes from the first scan data a hit volume element that depicts an intervention result caused by the probe in the subject.

7. A method as recited in Claim 6, wherein. the probe is an ablating catheter; and the first image includes from the first scan data the hit volume element that depicts a lesion in the subject caused by an ablation event at a tip of the ablating catheter.

8. A method as recited in Claim 1, further comprising automatically determining in real time a probe volume element that depicts a portion of the probe.

9. A method as recited in Claim 8, further comprising automatically determining in real time the probe volume element based on a characteristic artifact in the scan data caused by a material from which the probe is constructed.

10. A method as recited in Claim 9, further comprising automatically reducing effects in the first

of the characteristic artifact in the scan data.

11. A method as recited in Claim 8, further comprising, before inserting the probe into the living subject, selecting the probe that is found to be evident in scan data of a type like the first scan data.

12. A method as recited in Claim 8, further comprising, before presenting in real time the first image, enhancing visibility of the probe volume element that depicts a portion of the probe.

13. A method as recited in Claim 1, further comprising, before inserting the probe into the living subject, selecting the probe that is found to be evident in images of a type like the first image.

14. A method as recited in Claim 1, wherein presenting the first image in real time further comprises selecting a plane orientation of at least a portion of the first image based on the position of the probe.

15. A method as recited in Claim 2, wherein presenting the first image in real time further comprises selecting a plane orientation of at least a portion of the first image based on the the position of the probe and the target volume element.

16. A method as recited in Claim 2, wherein presenting the first image in real time further comprises selecting a plane orientation of at least a portion of the first image based on manual data received from a human operator.

17. A method as recited in Claim 1, further comprising causing the probe to be moved in the subject based on the first image.

18. A method as recited in Claim 1, further comprising: receiving probe position data different from a probe volume element that depicts a portion of the probe; and positioning the first X-ray beam relative to the living subject based on the probe position data.

19. A method as recited in Claim 18, wherein receiving probe position data further comprises receiving probe position data that is not based on an X-ray beam from an X-ray source that is a source for the first X-ray beam.

20. A method as recited in Claim 18, wherein receiving probe position data further comprises receiving probe position data based on projection data at a two dimensional detector array from a pair of X-ray beams at a corresponding pair of different projection angles from an X-ray source that is a source for the first X-ray beam.

21. A method as recited in Claim 18, wherein receiving probe position data further comprises: exposing the subject to a second X-ray beam and a third X-ray beam before exposing the subject to the first X-ray beam; and determining a three dimensional probe position based on a projection image from the second X-ray beam and the third X-ray beam to reduce exposure of the subject to X-ray beams.

22. A method as recited in Claim 1, wherein: the method further comprises receiving trigger condition data that indicates conditions that affect the subject; and exposing the subject to the first X-ray beam further comprises exposing the subject to the first X-ray beam at a time based on the trigger condition data.

23. A method as recited in Claim 22, wherein the trigger condition data is heart cycle data that indicates a particular phase in a cardiac cycle in the subject.

24. A method as recited in Claim 1, further comprising exposing the subject to a second X-ray beam having an X-ray wavelength different from the first X-ray beam.

25. A method as recited in Claim 24, wherein the first image includes a tissue image element that indicates a particular type of tissue in the subject based on a difference between scan data based at least in part on the first X-ray beam and scan data based at least in part on the second X-ray beam..

26. A method as recited in Claim 1, wherein the first image includes an image element that indicates data based on a different mode of measuring the subject.

27. A method as recited in Claim 26, wherein the first image includes an image element that indicates data based on a measurement made by the probe.

28. A method as recited in Claim 26, wherein the first image includes an image element that indicates data based on a measurement made before inserting the probe into the living subject.

29. A method as recited in Claim 26, wherein the first image includes an image element that indicates data based on a measurement made in real time by system that is not an X-ray computer tomography system.

30. A method as recited in Claim 1, wherein the first image includes an image element that indicates data based on a mathematical model of an internal organ of subject like the living subject.

31. A computer-readable medium carrying one or more sequences of instructions for positioning a probe in a living subject, wherein execution of the one or more sequences of instructions by one or more processors causes the one or more processors to: receive first scan data that consists of a plurality of volume elements that each indicate X-ray absorption at a corresponding volume inside a living subject based on computer tomography and a first X-ray beam; and present in real time a first image based on the first scan data and a position of a probe inserted into the subject, wherein the subject is exposed to the first X-ray beam after the probe is inserted into the subject, and real-time is a time shorter than 0.5 seconds after the subject is exposed to the first X-ray beam.

32. An apparatus for positioning a probe in a living subject, comprising: means for inserting a probe into a living subject; means for exposing the subject to a first X-ray beam, after inserting the probe into the subject; means for receiving first scan data that consists of a plurality of volume elements that each indicate X-ray absorption at a corresponding volume inside the living subject based on computer tomography and the first X-ray beam; and means for presenting in real time a first image based on the first scan data and a position of the probe, wherein real-time is a time shorter than 0.5 seconds after exposing the living subject to the first X-ray beam.

33. An apparatus for positioning a probe in a living subject, comprising: a memory; a processor; logic encoded in a tangible medium that causes the processor to receive first scan data that consists of a plurality of volume elements that each indicate X-ray absorption at a corresponding volume inside a living subject based on computer tomography and a first X-ray beam; and present in real time a first image based on the first scan data and a position of a probe inserted into the subject, wherein the subject is exposed to the first X-ray beam after the probe is inserted into the subject, and real-time is a time shorter than 0.5 seconds after the subject is exposed to the first X-ray beam.