WO2008146273A1

WO2008146273A1 - Method for imaging during invasive procedures performed on organs and tissues moving in a rhythmic fashion

Info

Publication number: WO2008146273A1
Application number: PCT/IL2008/000697
Authority: WO
Inventors: Morris Mosseri; Yoav Smith
Original assignee: Yissum Research Development Company Of The Hebrew University Of Jerusalem; Hadasit Ltd.
Priority date: 2007-05-25
Filing date: 2008-05-22
Publication date: 2008-12-04

Abstract

A method of virtually freezing the movement of the heart and its structures during invasive procedures such as catheterization, which is visualized using x-ray fluoroscopy (10), to yield anatomical information. The invention relies on the concept of comparing at least two images taken at different moments. After one of the images is defined as being a reference image (18), the processing software of the invention is capable of altering the second image by shifting any structures that have moved from their original position in the reference image, back to their original location as they appeared in the reference image (2S). Alignment of two images having similar content is termed 'image registration'. Thus, in images recorded successively, movement of the vasculature and the heart occurring during the cardiac cycle is eliminated on screen. The operator is provided with an additional screen with a 'virtually frozen' heart and 'frozen' arteries, to assist in diagnosis and in catheter navigation of the heart and vasculature.

Description

METHOD FOR IMAGING DURING INVASIVE PROCEDURES PERFORMED ON ORGANS AND TISSUES MOVING IN A RHYTHMIC FASHION

FIELD OF THE INVENTION

The present invention relates to medical imaging, and more specifically, to processing of images obtained from tissues and organs that undergo repetitive movement within the body.

BACKGROUND OF THE INVENTION

The heart chambers contract and relax in a timely and successive fashion, termed a "cardiac cycle". The cycle is triggered by electrical stimuli, which are measurable using an electrocardiogram (ECG). The coronary arteries lie on the heart surface and inside the heart muscle and undergo considerable movement within the thoracic cavity in response to the cardiac cycle. Blood is pumped within the arteries at significant pressure so that imaging of the arteries reveals considerable movement of the arteries at any given time. This movement complicates a catheterization operator's task of navigating a wire or a catheter within the highly branched arteries or within the cardiac chambers during a catheterization procedure in order to reach a specific artery or chamber that requires treatment.

During catheterization, a radio-opaque contrast medium is injected into the coronary arteries and into the heart chambers, and visualized using x-ray fluoroscopy, to yield anatomical information. The images containing this anatomical information are projected on a cathode-type screen, as continuous video or directly digitized feed, which the operator must evaluate. The constant movement of the anatomical structures in response to contraction of the heart during the cardiac cycle makes the evaluation more difficult. The operator in effect views a video or digitized feed loop where the anatomical structures continuously appear to travel across the screen, then return to their original position when the heart briefly rests between beats.

During therapeutic procedures such as angioplasty (the process of dilating the arteries with balloon catheters, stents and other equipment), valvuloplasty (dilation of narrowed valves), correction of valve regurgitation and other procedures, the operator has to introduce medical equipment such as wires, balloon catheters, stents and clippers inside these moving tissues, chambers and arteries. Operators gain experience in following the images and navigating the catheter and associated equipment inside these moving structures by subconsciously translating the images into frozen 2-D frames. It would be highly advantageous if the anatomical structures would remain stationary or appear to remain stationary during crucial moments in heart procedures.

It is possible to physically prevent a heartbeat for several seconds without endangering the patient, using drugs or rapid pacing that renders the heart contraction ineffective. However, it would be preferable to virtually prevent movement of the anatomic structures on screen alone, without administration of chemical means for physically stopping the heart; these cannot be administered continuously (which would mean stopping the heart beat for several minutes), and cannot safely be administered repeatedly during the procedure.

Merely pausing the video image at any given time would not be sufficient, since the operator navigates a guide wire under fluoroscopic imaging, through a moving artery, and the wire present within an artery is often situated at a curved segment or a junction of two to five arteries, through which the operator must navigate to reach an area of interest. The operator applies gentle pressure and rotates a guide wire having a J shaped tip in a specific direction, in order to aim it towards the correct artery within the junction.

Even if the video or digitized image would be paused at an arbitrary time, to allow the operator to study the junction and decide in which direction to send the catheter, in reality the junction continues to move and pulsate in response to the heartbeat. Advancement of the guide wire in a direction which would seem to be the right one based on a paused video screen, would not necessarily direct the catheter to the correct artery within the junction, since the junction is in constant movement, and a different artery within the junction is aligned opposite the catheter at any given time.

Therefore, the need exists for an imaging method that produces a virtual freezing of the heart and arteries, and allows the operator to view them as if they were not in constant movement. The method must take into account the existence of movement, and neutralize it to create a static image that truly reflects the position of the arteries in relation to the catheter. SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the present invention to overcome the imaging problems associated with movement of rhythmically moving organs or tissues such as coronary arteries.

In accordance with a preferred embodiment of the present invention, there is provided a method of virtually freezing the movement of the heart and its structures using computer software. While the heart is beating normally and all its structures are moving regularly with each cycle, the operator is provided with an additional screen with a "virtually frozen" heart and "frozen" arteries which can aid him in diagnosis and in catheter navigation during therapeutic procedures of the heart and vasculature. On this screen, the lungs and diaphragm continue moving with respiration and are not frozen. This provides the operator with the sense of a virtually frozen heart and coronaries and distinguishes the image from a prior art angiography in which the frame sequence is merely paused.

The operator may simultaneously view prior art non-edited screens on which the heart and its structures are depicted in their moving condition. The virtually frozen configuration enables the operator to concentrate better on his performances without devoting part of his attention to interpreting the information relating to a moving target.

The present invention thus provides a method for performing imaging of moving organs and tissues of the human body, useful during invasive procedures, comprising: a) capturing a sequence of several images of the moving organs and tissues; b) dividing the images into frames; c) correlating the timing of the frames with the cyclic movement of the moving organs and tissues; d) selecting a reference frame; e) locating at least one mapping point upon a moving tissue or organ within each frame; f) comparing the location of each mapping point with its location in the reference frame; g) for each frame of interest, identifying and performing a mathematical image registration transformation for shifting the tissue or organ and each mapping point upon said tissue or organ, back to its position and orientation in the reference frame; and h) displaying a plurality of shifted frames upon display means to create a display of non-moving tissue or organ.

According to a certain embodiment, the tissue and organs of the human body are vasculature and the heart, imaged during a catheterization procedure.

According to a preferred embodiment, the images are captured and divided into frames using rapid pulsed fluoroscopy.

According to one embodiment, the method includes the step of locating a plurality of static points within each frame, allowing comparison of the location of the mapping points to said static points for determining the degree of movement and the orientation of said mapping points. In one option, the static points are present upon a radio-opaque grid placed beneath the patient prior to capturing of the images. In a second option, the static points are selected upon the spine.

According to another embodiment, the method comprises the step of performing edge detection and edge enhancement of the image, performed before step (g). In such case, optionally after the edge detection and edge enhancement of the image is performed, the enhanced edges of structures within said image are inserted into an image captured without injection of contrast medium. This allows visualization of the core (center) of the structure, and of surgical implements present within the structure.

According to a preferred embodiment, the method is used repeatedly on a plurality of image sequences captured from a plurality of locations within the human body, allowing imaging of an entire anatomical system (such as the cardiovascular system or the gastrointestinal system). The present invention further provides computer readable storage medium comprising software capable of: a) correlating the timing of a sequence of several image frames of moving organs and tissues with the cyclic movement of the moving organs and tissues; b) selecting a reference frame; c) locating at least one mapping point upon a moving tissue or organ within each frame; d) comparing the location of each mapping point with its location in the reference frame; e) for each frame of interest, identifying and performing a mathematical image registration transformation for shifting the tissue or organ and each mapping point upon said tissue or organ, back to its position and orientation in the reference frame; and f) displaying a plurality of shifted frames upon display means to create a display of non-moving tissue or organ.

Moreover, the present invention further provides a system for performing imaging of moving organs and tissues of the human body, useful for imaging during invasive procedures, comprising: a) display means; and b) processing means comprising a memory device, a driver; said processing means being in communication with said memory device, and being configured to: i) correlate the timing of a sequence of several image frames of moving organs and tissues with the cyclic movement of the moving organs and tissues; ii) select a reference frame; iii) locate at least one mapping point upon a moving tissue or organ within each frame; iv) compare the location of each mapping point with its location in the reference frame; v) for each frame of interest, identify and perform a mathematical image registration transformation for shifting the tissue or organ and each mapping point upon said tissue or organ, back to its position and orientation in the reference frame; and vi) display a plurality of shifted frames upon said display means to create a display of non-moving tissue or organ.

Additional features and advantages of the invention will become apparent from the following detailed description.

In the context of the present invention, the term "frozen", or "virtually frozen" in relation to an organ or tissue, refers to creation of a sequence of images of that organ, in which the rhythmic movement of the organ or tissue which naturally occurs in the body has been eliminated and the organ has been displayed onscreen as a non-moving organ.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention with regard to the embodiments thereof, reference is made to the accompanying drawings, in which:

Figure 1 is an overview of the image registration method of the invention,

Figure 2 is a schematic representation of several image frames, showing movement of a vessel junction in response to the cardiac cycle,

Figure 3 is a table showing the X/Y axis location of several mapping points as they appear in each frame, and

Figure 4 is a schematic drawing showing the correlation of image frames with the cardiac cycle, as it appears in an electrocardiogram printout.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention relies on the concept of comparing at least two images taken at different moments. After one of the images is defined as being a reference image, the processing software of the invention is capable of altering the second image by shifting any structures that have moved from their original position in the reference image, back to their original location as they appeared in the reference image. Alignment of two images having similar content is termed "image registration". Thus in images recorded successively, movement of the vasculature and the heart occurring during the cardiac cycle is eliminated on screen.

In the following description, reference is made to image processing of heart catheterization images, and the structures that are processed and shifted using the invention are blood vessels. However, there is no intention to limit the invention to heart procedures, rather the method is useful for any rhythmically moving organs and tissues.

Referring to Figure 1, an overview of the image registration process of the invention is described. In block 10, an x-ray fluoroscope is used to capture X-ray images of the chest, after injection of radio-opaque material into the arteries during a catheterization procedure. In block 12, the image sequence is divided into successive images, termed "frames" and a list of the frames is created and saved in the memory. Simultaneously, an electrocardiogram (ECG) is performed (block 14), so that the different segments of the heart cycle can be identified based on the wave pattern recorded.

In block 16, the frames are synchronized, or gated, with the ECG printout so that the operator can request retrieval of a frame originating from a specific segment of the cardiac cycle, and the system will identify and display this image. After this, a reference frame can be selected (block 18), such as a mid-systole frame, in which the arteries have undergone maximal movement before returning to their original position. Alternatively, a mid-diastole frame is also a useful reference frame, during which the arteries have returned to their original resting-point position.

In block 22, several mapping points are selected upon each blood vessel in each frame. Preferably, most of these mapping points are present at vessel junctions, thus they are easily identifiable in each frame. Edge detection is performed using an appropriate algorithm, to enhance the edges of the vessels in each frame. This alleviates the need for additional injections of contrast medium, allowing the operator to leave the vessel lumen (or the core of the anatomical structure) free from contrast medium, in order visualize the guide wire inside the lumen as it is advanced towards its target. The position of each of the mapping points is compared to its position in the reference frame, and the mathematical transformation is identified for shifting each vessel to its original position and orientation (block 24). The transformation is then performed, and in each frame, each vessel is returned to its location and orientation in the reference frame (block 26). Twelve or more mapping points may be selected, since each vessel may move to a different degree and in a different plane compared to other vessels in the image. The vessels tend to move in several planes, with movement including rotation and lengthening and shortening of the semi-elastic vessels. Electronically shifting each of the vessels in a given frame, back to its original position and orientation as it appeared in the reference frame is therefore not a simple matter, since distortion of the image occurs in each frame. The shifted image is then displayed to the operator, so that a complete cardiac cycle, is displayed, having all arteries and heart structures in all frames of the cycle transformed into static "frozen" structures which retain accurate information of the true orientation of the vessels relative to one another, at resting phase.

Referring to Figure 2, a schematic representation is illustrated of several frames, showing movement of a vessel junction in response to the cardiac cycle. Frame 1 is the reference frame. Two mapping points (bolded dots) termed "P₁" and "P₂" have been selected at the vessel junction. Several mapping points upon the vessel are always selected, however for simplicity, only two are shown. In Frame 12, the junction has shifted to the right, moving the vessel junction and the mapping points P₁, P₂ to the right. In Frame 25, the vessel junction has returned to its starting position (as in Frame 1).

Referring to Figure 3, a table has been created showing the X/Y axis location of each of the mapping points selected (P1-P16), in each frame. Frames 1 through 25 represent a single complete cardiac cycle. After the table is created, the mathematical transformation can be identified for shifting each vessel to its original position and orientation.

Referring to Figure 4, each frame is synchronized with the wave pattern of the ECG printout so that different segments of the heart cycle can be identified and linked to specific frames. The movement of vessels within the cardiac cycle is shown schematically to progress such that halfway through the cycle the vessels have moved to their most extreme position, after which the vessels return to their resting position. The images after the midway point represent mirror images of images captured before the midway point. Optionally, for each location of the catheter within the body, analysis is performed of a single cardiac cycle which is displayed in a loop after virtual shifting of the vessels back to their starting location; this loop is sufficient to display all anatomic information of this location. When considerable advancement of the catheter towards a different location occurs, frames relating to the anatomy of this second location are analyzed for the duration of a second cardiac cycle then displayed in a second loop until further advancement of the catheter makes a third analysis and a third display loop necessary.

Alternatively, all frames are continuously analyzed and displayed after shifting the vessels back to their starting location, and display is not merely of a loop of a single cardiac cycle for each location.

Preferably, the invention is carried out in a catheterization laboratory using rapid pulsed fluoroscopy in which each frame is translated to digital information. The technology known as "Flat Detector", for example, translates X-ray signals directly into digital signals without requiring an intermediate phase of video signal. Typical Flat Detectors produce pulsed fluoroscopy in which x-ray pulses of about 25 pulses per second are directed at the chest, and read and transmitted to a display screen, which receives 25 frames per second. The miniscule time span between adjacent frames nevertheless enables processing the image using the invention, before sending it to the screen for viewing.

The operator will then view the frames after improving, analyzing or changing the original X-ray signal. The rapid rate of 25 frames per second does not allow the human brain to note any interruption in flow of the image, and it is important to note that the processed fluoroscopic image appears therefore as an actual real-time process. If it is of interest, images can be recorded for future viewing and analysis, preferably using Dicom format for medical image storage and retrieval. In this format, the image is automatically divided into 25 frames per second, and stored accordingly.

Preferably, computer software including pattern recognition, automatic edge- detection and picture transformation capabilities is used. Preferably, Matlab software is used, and the image shifting is performed using the "cp2 transform" affine function of Matlab™, which corrects for skew and movement of an image and thus allows re-registration of the image. Preferably, an ECG signal is generated from the patient, to allow gating (synchronization) of each fluoroscopy frame with the ECG signal. This allows identification of the time frame of each video frame in relation to the cardiac cycle, so that the operator can link each frame with its placement in the cycle. A single cycle, lasting approximately one second, consists of systolic contraction of both atria and both ventricles, followed by a 3 millisecond isovolumic relaxation period. The arteries and veins move in response to blood flow generated from the systolic contraction, and return to their original position during the diastolic relaxation period.

Once the video images are precisely linked with an ECG signal, the midpoint of the cycle is identified; at this midpoint, the anatomic structures have moved to their most extreme position, after which they will return to their original location. Any images captured after the midpoint of the cycle have mirror images from the time period before the midpoint of the cycle, therefore analysis does not always need to be performed for them. Rather, according to one embodiment the calculations to determine how much and how far to shift the moving structures so that they return to their original position, are extrapolated from the mirror image of each frame.

In one embodiment, after edge detection and enhancement is performed on the structures of interest, such as the vessel walls or heart chambers, the edges of these enhanced and stabilized structures are "pasted" onto a prerecorded heart sequence captured without injection of dye. This enables the operator to navigate within the vessels or heart structures without the need for dye injection. The guide wire, which can become obliterated by the dye, can be more easily visualized.

In implementation of the computerized method, a frozen cardiac cycle is constructed: a. An intermediate frame (frame A) of the cardiac cycle (e.g. mid systole or mid- diastole) is chosen. b. The diagnosed or treated area of interest (AOI), for example a coronary artery, valve or a specific chamber, is identified in frame A. A full heart cycle of the AOI is recorded and each frame is synchronized with the patient's ECG. Optionally, this prerecorded loop can be repeatedly displayed, to limit the number of injections of contrast medium necessary. Alternatively, image registration can be performed for each and every frame recorded, in real-time. Edge detection of the AOI is performed, the blood vessels are split into segments with sequential numbers assigned to each. This stage can be performed manually or automatically. c. Each frame of the cycle is transformed back to frame A (25 per second when a "flat detector" is used). This relies on the fact that the location of all segments of the blood vessels are recorded on each one of the cycle frames and so can be tracked back frame by frame. d. All frames are displayed on a designated screen immediately after processing, giving a "frozen" cycle, whilst other features (such as diaphragms and lungs not in the AOI) are unedited and so appear to continue moving on the fluoroscopy screen. The frozen cycle is easily distinguishable from an unedited still (paused) frame of mid cardiac cycle by the movements of non-target organs. However the frozen cycle is essential for therapeutic procedures as described hereinabove, whereas a still frame would be meaningless for these purposes.

Navigation of equipment can be performed towards or within the target organ, using the invention. To illustrate this stage, passing of a wire inside a coronary artery will now be described. A similar navigation process may be used for passing and deploying balloon catheters and stents, for locating and inflating a balloon catheter in stenosed valves, for stitching valvular cusps, for locating electrodes inside cardiac chambers and vessels, for performing electrical ablation (or the like) and for any other interventional therapeutic maneuver. The implementation of this stage is as follows: a. Each frame in the cardiac cycle is gated to its corresponding part in the ECG signal. b. Suppose it is desired to direct a wire in a branch X of a coronary artery and to avoid it from entering into branch Y. The operator will inspect the special fluoroscopy screen with the virtually frozen target. Using the same screen he will introduce and guide a metal wire (which is radio-opaque for X-rays) into the coronary artery. On each frame of the cardiac cycle, the wire and its exact location (in a coronary branch) will be recognized by the system and transformed into its corresponding position in the mid-cycle frozen frame. This position will be viewed by the operator and will be changed as necessary. The operator will therefore conduct and introduce the wire into the right position on a virtually frozen heart, while in reality the heart will go on beating normally.

The invention is not limited to use with fluoroscopic imaging, rather other imaging modalities can be used (e.g. echocardiography or imaging modalities that use thermal, nuclear, radiofrequency or any other physical energy).

The method relies on the assumption that the heartbeat is regular (or reasonably regular). When the heart rate is irregular, the procedure may be performed but the operator must take into account the existence of a certain degree of inaccuracy in the catheter positioning.

The software and hardware of the invention can interface with standard equipment of a catheterization laboratory. The method may be used in every cardiac catheterization laboratory for diagnostic and various therapeutic procedures.

In one embodiment, several static points are identified in the image in each frame; these points do not undergo movement in response to the cardiac cycle. The static points can be present on the spine, or optionally a metal (or otherwise radio-opaque) grid is placed beneath the patient before the images are captured, so that several intersection points of the grid-wire can be selected as static points. The location of the various arteries in each frame will be compared to these static points, in order to determine the relative movement of the arteries at each instant in the cardiac cycle.

The method of the invention is not limited to heart catheterizations and heart procedures. The method can be used for example, during surgery on the lung, especially when the lungs are artificially respirated at a known rate. This may include lung biopsy, dilatation of narrowed airways or other surgery of the lung performed either under fluoroscopy or with robotics. The method can similarly be used on other organs or tissues undergoing movement, especially cyclic movement such as peristaltic movement or the like.

In summary, the invention enables the surgeon or operator to more easily comprehend the anatomic information of various rhythmically moving structures by eliminating repetitive movements that naturally occur within the human body. It enables the operator to perform various therapeutic procedures on a "frozen" image of the organ or tissue. The virtually frozen configuration enables the operator to concentrate better on his performance without devoting part of his attention to interpreting the information relating to moving targets.

Having described the invention with regard to certain specific embodiments thereof, it is to be understood that the description is not meant as a limitation, since further modifications may now suggest themselves to those skilled in the art, and it is intended to cover such modifications as fall within the scope of the appended claims.

Claims

1. A method for performing imaging of moving organs and tissues of the human body, useful during invasive procedures, comprising: a) capturing a sequence of several images of the moving organs and tissues; b) dividing said images into frames; c) correlating the timing of said frames with the cyclic movement of the moving organs and tissues; d) selecting a reference frame; e) locating at least one mapping point upon a moving tissue or organ within each frame; f) comparing the location of each mapping point with its location in said reference frame; g) for each frame of interest, identifying and performing a mathematical image registration transformation for shifting the tissue or organ and each mapping point upon said tissue or organ, back to its position and orientation in said reference frame; and h) displaying a plurality of shifted frames upon display means to create a display of non-moving tissues or organs.

2. The method of claim 1, wherein the tissue and organs of the human body are vasculature and the heart, imaged during a catheterization procedure.

3. The method of claim 2, wherein said images are captured and divided into frames using rapid pulsed fluoroscopy.

4. The method of claim 2, wherein said step of correlating the timing of said frames with the cyclic movement of the moving organs and tissues is performed by synchronizing said frames with an electrocardiogram wave pattern printout to create a list of gated frames.

5. The method of claim 1, wherein the tissue and organs of the human body are the lungs.

6. The method of claim 1, further comprising the step of locating a plurality of static points within each frame, allowing comparison of the location of said mapping points to said static points for determining the degree of movement and the orientation of said mapping points.

7. The method of claim 6, wherein said static points are present upon the spinal column.

8. The method of claim 6, wherein said static points originate in a radio-opaque grid placed beneath the human body before capturing of the image.

9. The method of claim 1, further comprising the step of performing edge detection and edge enhancement of the image, and said step is performed before step (g).

10. The method of claim 9, wherein after said edge detection and edge enhancement of said image, the enhanced edges of structures within said image are inserted into an image captured without injection of contrast medium, allowing visualization of the core of said structure, and of surgical implements present within said structure.

11. The method of claim 1, wherein said method is used repeatedly on a plurality of image sequences captured from a plurality of locations within the human body, allowing imaging of an entire anatomical system.

12. A computer program product comprising a computer usable medium having computer readable code embodied therein for execution on a general purpose computer, said computer usable medium storing instructions that, when executed by the computer, cause the computer to perform a method for performing imaging of moving organs and tissues of the human body, useful during invasive procedures, said method comprising: a) correlating the timing of a sequence of several image frames of moving organs and tissues with the cyclic movement of the moving organs and tissues; b) selecting a reference frame; c) locating at least one mapping point upon a moving tissue or organ within each frame; d) comparing the location of each mapping point with its location in the reference frame; e) for each frame of interest, identifying and performing a mathematical image registration transformation for shifting the tissue or organ and each mapping point upon said tissue or organ, back to its position and orientation in the reference frame; f) displaying a plurality of shifted frames upon display means to create a display of non-moving tissue or organ.

13. The computer program product of claim 12, further capable of performing edge detection and edge enhancement of the image, and said step is performed before step (e).

14. The computer program product of claim 13, further capable of inserting the enhanced edges of structures within said image into an image captured without injection of contrast medium, allowing visualization of the core of said structure, and of surgical implements present within said structure.

15. The computer program product of claim 12, further capable of locating a plurality of static points present within each frame, and comparing the placement of said mapping points in each frame in relation to said static points.

16. A system for performing imaging of moving organs and tissues of the human body, useful for imaging during invasive procedures, comprising: a) display means; b) processing means comprising a memory device, a driver; said processing means being in communication with said memory device, and being configured to: i) correlate the timing of a sequence of several image frames of moving organs and tissues with the cyclic movement of the moving organs and tissues; ii) select a reference frame; iii) locate at least one mapping point upon a moving tissue or organ within each frame; iv) compare the location of each mapping point with its location in the reference frame; v) for each frame of interest, identify and perform a mathematical image registration transformation for shifting the tissue or organ and each mapping point upon said tissue or organ, back to its position and orientation in the reference frame; and vi) display a plurality of shifted frames upon said display means to create a display of non-moving tissue or organ.

17. The system according to claim 16, further comprising a housing for enclosing said system within said housing.