Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B34/00—Computer-aided surgery; Manipulators or robots specially adapted for use in surgery
  • A61B34/20—Surgical navigation systems; Devices for tracking or guiding surgical instruments, e.g. for frameless stereotaxis
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B17/00—Surgical instruments, devices or methods, e.g. tourniquets
  • A61B2017/00681—Aspects not otherwise provided for
  • A61B2017/00725—Calibration or performance testing
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B34/00—Computer-aided surgery; Manipulators or robots specially adapted for use in surgery
  • A61B34/20—Surgical navigation systems; Devices for tracking or guiding surgical instruments, e.g. for frameless stereotaxis
  • A61B2034/2046—Tracking techniques
  • A61B2034/2051—Electromagnetic tracking systems
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B34/00—Computer-aided surgery; Manipulators or robots specially adapted for use in surgery
  • A61B34/20—Surgical navigation systems; Devices for tracking or guiding surgical instruments, e.g. for frameless stereotaxis
  • A61B2034/2072—Reference field transducer attached to an instrument or patient
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B90/00—Instruments, implements or accessories specially adapted for surgery or diagnosis and not covered by any of the groups A61B1/00 - A61B50/00, e.g. for luxation treatment or for protecting wound edges
  • A61B90/36—Image-producing devices or illumination devices not otherwise provided for
  • A61B2090/363—Use of fiducial points
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B90/00—Instruments, implements or accessories specially adapted for surgery or diagnosis and not covered by any of the groups A61B1/00 - A61B50/00, e.g. for luxation treatment or for protecting wound edges
  • A61B90/39—Markers, e.g. radio-opaque or breast lesions markers
  • A61B2090/3995—Multi-modality markers

Definitions

• the present application relates to systems and methods for dynamic metal distortion compensation for electromagnetic tracking systems, particularly using active fiducial markers.
• EMTS electromagnetic tracking systems
• An EMTS works by using an electromagnetic (EM) field generator, which creates a local EM field at the site of the procedure, and a medical instrument containing a suitable miniaturized sensor coil.
• EM electromagnetic
• a current is induced in the sensor coil that is a function of the position and orientation of the sensor coil relative to the EM field generator.
• the EMTS can compute the position of the sensor coil, and therefore the position of the medical instrument.
• a particular advantage of EMTS is that line of sight is not required, because the EM field can penetrate the human body mostly undisturbed. Therefore EMTS is especially suitable for tracking needles or catheters inside the anatomy.
• One of the main problems with using EMTS in a medical environment is the presence of metallic conductive or ferromagnetic objects in proximity to the EM field. These objects create distortions, or metal artifacts, which create errors in the position and orientation tracking of the medical instrument.
• the main sources of distortion come from the medical imaging equipment upon which the patient lies (e.g. CT gantry, CT table, X-ray C-arm, etc.).
• Another source of distortions is moveable medical equipment or tools (ECG monitors, metallic tools, etc.) that come within the vicinity of the EMTS.
• US 2005/0107687 to Anderson proposes a system and method for distortion analysis and reduction in an EMTS.
• a tracking modification unit relies on a predetermined distortion model for each specific tool or instrument being tracked.
• the predetermined model is developed through an analysis process including field mapping and/or modeling/simulation, which also takes into account sensor placement and shielding.
• the tracking analysis unit generates a map and/or a model of a distortion characteristic of the instrument, which is essentially a lookup table for the instrument. This system does not attempt to reduce distortions created by static objects within the environment.
• US 2008/0079421 to Jensen proposes a static mapping of distortion fields created by objects within an environment.
• An array of EM sensors is positioned within the volume of interest and the array of sensors is sampled to acquire signals representative of the location of the EM sensors within the array.
• the array includes a fixed, known geometry, the EM field distortion can be calculated. This system cannot be used in real time during a medical procedure, nor can it take into account field distortions created by moving medical instruments and tools within the volume of interest.
• WO 2007/113719 to Shen et al. proposes a system for local metal distortion correction for improving the accuracy of EMTS in a medical environment.
• the system contains an electromagnetic field generator monitoring a medical device having a suitable sensor coil wherein a correction function, derived from an error correction tool, is applied to the position and orientation readings of the sensor coil.
• the error correction tool consists of a number of electromagnetic sensors arranged in a fixed and known geometric configuration and is placed surrounding the site of the medical procedure. Sensor data is displayed on an imaging system.
• a distortion mapping can be undertaken utilizing optical sensors for relative positioning readings along with an electromagnetic tracking system sensor.
• a method for dynamic metal distortion compensation using an Electromagnetic Tracking System includes generating an electromagnetic field from at least one electromagnetic field generator.
• a plurality of fiducial markers are provided, each fiducial marker comprising at least one electromagnetic sensor, the electromagnetic sensors oriented in a plurality of sensor orientations, and at least some of the sensors being located proximal to a volume of interest.
• the fiducial markers are imaged to provide at least a baseline position of the fiducial markers in image space.
• the method further includes monitoring position readings of the plurality of electromagnetic sensors using the EMTS, and calculating a metal distortion correction function by comparing the positions of the fiducial markers in image space to the position readings of the electromagnetic sensors in the electromagnetic field.
• Position readings of a medical device moving through the volume of interest are monitored using the EMTS, the device having at least one electromagnetic sensor.
• the distortion correction function is then applied to the medical device position readings to compensate for said metal distortion.
• the correction is dynamic and in real-time, which allows for compensations for objects/distortions brought into the vicinity during the procedure. [0011]
• the positioning of at least one fiducial marker is alterable during the position monitoring.
• At least some of the fiducial markers are placed on a frame surrounding at least a portion of a patient's body during a medical procedure, and/or at least some of the fiducial markers are placed directly onto a patient's skin during a medical procedure. Alternatively, or in addition, at least one of the fiducial markers may be placed internally in a patient's body during a medical procedure.
• the selection of the electromagnetic sensors can be dynamically based on selection criteria.
• the selection criteria can include selecting sensors with orientations closest to the orientation of the tracked medical device to calculate the compensation.
• the selection criteria can include selecting sensors with spatial locations proximal to the spatial location of the tracked medical device to calculate the compensation.
• the selection criteria can include selecting sensors with spatial locations proximal to a target location within a patient's body to calculate the compensation.
• the selection criteria can include selecting sensors based on the geometry of the relative spatial locations of the tracked medical device and a target location within a patient's body to calculate the compensation.
• the selection criteria can change as at least one of the orientation and spatial location of the tracked medical device changes.
• the method for calculating the metal distortion correction function can be selected based upon the selection of electromagnetic sensors. For example, a global transformation (affine) calculation method can be used, an interpolation calculation can be used, a global transformation calculation method can be used if the tracked medical device lies outside a geometric coverage of the selected sensors, and/or an extrapolation calculation method is used if the tracked medical device lies outside a geometric coverage of the selected sensors.
• the method for calculating the metal distortion correction function can be dynamically changed as the selection of electromagnetic sensors is changed due to movement of the tracked medical device.
• a system for dynamic metal distortion compensation using an Electromagnetic Tracking System includes at least one electromagnetic field generator for generating an electromagnetic field.
• a plurality of fiducial markers comprising at least one electromagnetic sensor, the electromagnetic sensors oriented in a plurality of sensor orientations, and at least some of the sensors being located proximal to a volume of interest, the fiducial markers being visible in image space.
• a processor is included for calculating a metal distortion correction function by comparing positions of the fiducial markers in image space to position readings of the electromagnetic sensors in the electromagnetic field.
• At least one electromagnetic sensor is attached to a medical device.
• the processor applies the calculated distortion correction function to said medical device position readings to compensate for the metal distortion.
• At least some of the fiducial markers are provided on a frame adapted to surround at least a portion of a patient's body during a medical procedure.
• At least some of the fiducial markers comprise a plurality of electromagnetic sensors, and the plurality of electromagnetic sensors in such fiducial markers can have differing sensor orientations.
• Figure 1 is a general arrangement of components of the invention.
• Figure 2 shows an abdominal phantom with active fiducial markers attached to a frame.
• Figure 3 shows image acquisition
• Figure 4 shows path planning.
• Figure 5 shows baseline registration
• Figure 6 shows the identification of active fiducial markers in image space.
• Figure 7 illustrates navigation with EM distortion compensation.
• Figure 8 illustrates an example showing active fiducials in three different orientations, arranged around a target.
• the present disclosure relates to electromagnetic tracking systems (EMTS) for medical devices and other structures.
• EMTS electromagnetic tracking systems
• the exemplary embodiments of the present disclosure can be applied to many types of structures, including, but not limited to use in catheter tracking in cardiac and vascular application, oncology applications such as needle biopsies, radio-frequency ablations, cryoablations, prostate cancer therapies, and the like.
• an electromagnetic tracking system (EMTS) 10 having an electromagnetic (EM) field generator 12 is illustrated.
• the generator 12 can create a local EM field capable of tracking sensor data from EM sensors contained within active fiducial markers 14 and a medical instrument 16.
• the markers 14 are arrayed around the patient's body 18.
• the markers 14 are visible in a medical image space, and also contain a sensor coil to provide position and orientation information in the EM tracking space, such that they are also locatable within the EM tracking space.
• the instrument 16 typically penetrates a patient's body 18 beneath the skin to a target location.
• An EM sensor coil is embedded in the instrument 16, for example, close to the tip if the instrument 16 is or includes a needle.
• a current is induced in the sensor coil that is a function of the position and orientation of the sensor coil relative to the EM field generator 12.
• the EMTS 10 can compute the position of the sensor coil, and therefore the position of the medical instrument 16.
• a particular advantage of EMTS is that line of sight is not required. Therefore EMTS is especially suitable for tracking needles or catheters inside the anatomy.
• the active fiducial markers 14 can be placed on the surface of the patient's skin, or on a fixed frame 20 designed to go around the patient 18 (in the illustrated arrangement, an abdominal phantom 18 is shown in place of a patient, as may be used for testing purposes).
• the markers 14 can be placed to encompass a suitable area in the vicinity of the entry point on the patient's skin or in the vicinity of the target location within the patient's body.
• the markers may be connected to the EMTS 10 by wires 22.
• the patient 18 and frame 20 can be positioned on a table 24, with the EM generator 12 positioned above the table 24, or at any suitable location. Images are acquired of the relevant patient anatomy.
• the active fiducial markers 14 are clearly identifiable in the medical images and the positions of the markers in the image space are determined (via software application). This forms a baseline truth for the positions of the active fiducial markers 14, and position readings for the markers 14 are thus acquired by the EMTS. These positions from the EMTS are used to calculate the compensation by comparing the EMTS positions to the baseline truth image positions. If there is a source of metal distortion, the position of one or more of the active fiducial markers 14 will be distorted, or incorrect.
• fiducial markers 14 with a single sensor orientation such as tangential to the skin result in relatively poor registrations, mainly because ultimately the tracked medical instrument such as a needle is inserted normal to the skin surface, and thus the sensor is normal to the surface of the skin.
• the active fiducial markers 14 can be used to achieve a useable correction or compensation, to a first order. The effectiveness is optimal when the tracked medical instrument 16 is oriented in the same direction as the sensors in the active fiducial markers.
• the disclosed system and method uses multiple arrays of active fiducial markers 14, each array having a different sensor orientation.
• the disclosed system and method can use a plurality of active fiducial markers 14 having varying orientations, which are not necessarily organized into arrays of a specific orientation. A selection is made of either the array of active fiducial markers 14 with the closest sensor orientation to the tracked medical instrument 16, or of the individual active fiducial markers 14 with the closest sensor orientation to the tracked medical instrument 16. The selected array or sensors are then used to calculate the distortion correction.
• the active fiducial markers 14 can be placed around the patient's body without precision, that is, a priori location is not needed, because the markers 14 only need to be identifiable in the medical image for a baseline position used to calculate the compensations. This gives the freedom to reposition the markers if necessary during the medical procedure.
• the effects of respiratory motion can be eliminated. If the active fiducial markers 14 are in constant motion due to respiration, this affects the ability to calculate compensations. If respiratory motion can be estimated, then the active fiducial markers 14 may be placed directly on the patient's skin. Position readings from the active fiducial markers 14 would have to coincide with the inspiration level during the acquisition of the image. This can be accomplished through a gating procedure if the patient is on a respirator, or a bellows device could be used. Alternatively, an internal active fiducial marker, or similar marker could be used to estimate the respiratory state using the EMTS or similar tracking system.
• the patient is first imaged (see Figure 3) using any suitable imaging system.
• the target path for the interventional medical procedure is then identified (see Figure 4).
• These two steps may be carried out immediately prior to the start of the medical procedure, during the procedure, or may be carried out in advance of the procedure.
• a baseline registration between the image space and the EM tracking space can be obtained (see Figure 5).
• the baseline registration is an initial registration between the image space and the EM tracking space.
• This step is optional because the transformation between the two spaces can be calculated using the active fiducial markers 14 during the compensation calculation. However, performing this step provides a baseline transformation between the image and EM tracking spaces in the event that the calculation of the EM compensation fails.
• this step is useful if the software application enables user selection of EM distortion compensation (i.e. to turn it on or off).
• the locations of the active fiducial markers 14 are then identified in the image space (see Figure 6).
• the EMTS position information of the active fiducial markers 14 is then read, and the EMTS calculates the transformation or interpolation between the image space positions and the EMTS position, which implements the distortion compensation.
• the image can be corrected, and corrected images can be provided to the physician so that interventional navigation can be carried out with realtime distortion compensation (see Figure 7). Orientation of the tracked medical instrument 16 is thus monitored in real-time.
• the compensation is calculated using position readings from sensors in the active fiducial markers 14 with the same (within a threshold) orientation as the tracked medical instrument 16.
• the orientation of the tracked medical instrument 16 is not fixed, because it changes dynamically as the medical instrument 16 is repositioned.
• the use of a plurality of sensors with different orientations allows sensors with orientations closest to the orientation of the tracked medical instrument 16 to be selected to calculate the compensation.
• the sensors with the closest proximity to the medical instrument 16 can be selected, or the selection may be based on the geometric position of the sensors to the medical instrument 16. As the orientation of the tracked medical instrument s) 16 changes, the appropriate sensors can be selected dynamically to calculate the compensation.
• a minimum number of sensors must be used to calculate the correction, although the actual minimum number will depend on the method used to perform the calculation.
• the selection of the sensors that are used for the calculation may be used to determine the compensation method employed for the correction. For example, if only a few sensors meet the selection criteria, the compensation might be implemented by a global affine transformation. However, if a sufficient number of sensors are selected with the appropriate geometrical coverage, an interpolation approach may be used. In some arrangements, when the orientation of the tracked medical device does not correspond exactly to the orientation of a minimum number of active fiducial sensors, an interpolation can be calculated from the sensors most closely matching in orientation.
• the speed or frequency of the compensation is only limited by two events, acquiring the position readings from the active fiducial markers 14, and the calculation of the compensation. Depending on the number of active fiducial markers used, the speed of the EMTS, and the compensation algorithm used, one compensation can be done in fractions of a second. True continuous real-time compensation may or may not be necessary in a clinical environment.
• One approach to sensor selection for use in the compensation calculation is to use a plurality of arrays of sensors where each array contains a number of sensors of substantially the same orientation, or which are closest in proximity to the medical instrument 16. Based on the orientation of the tracked medical instrument 16, the appropriate array of sensors is selected to calculate the compensation. The selection can be done via software identification of the appropriate array, or by a hardware multiplexer / selector. Each individual array of sensors has its sensors spaced appropriately around the target site.
• a second approach is to use a number of individual sensors (not grouped into arrays) with different orientations. Based on the orientation of the tracked medical instrument 16, the sensors having orientations closest to the orientation of the tracked medical instrument 16 are selected to calculate the compensation. Selection would most likely be done via software identification of the appropriate sensors.
• FIG. 8 An example simplified illustration of a sensor arrangement is given in Figure 8 with groups of sensors 26 in three different orientations (the sensors are represented by lines indicative of their orientation). Each group of sensors 26 can be located within one active fiducial marker 14, or each active fiducial marker 14 can contain one sensor, and be grouped together in an array. The number of orientations need not be restricted to three, for example two could be used, or many more than three.
• At least one fiducial marker 14 can be temporarily placed internally in the patient during the procedure, for example, close to the medical instrument 16 that is being tracked, close to the target, or in any suitable position to improve the accuracy of the compensation.
• Electromagnetic tracking is a means of improving medical procedures including catheter tracking in cardiac and vascular applications, oncologic applications such as needle biopsies, radio-frequency ablations, cryoablations, prostate cancer therapies, etc.
• the errors induced by metal interference can affect the accuracy of medical procedures using electromagnetic tracking systems.
• the disclosed method and system improve the accuracy of EM tracked medical procedures, and makes the use of EMTS more realistic and practical, in turn creating many opportunities for integrating medical imaging with medical device tracking in minimally invasive applications.
• These medical applications include the use of CT systems, X-ray systems, ultrasound systems - and the technology is generically applicable to almost any situation where a physician needs to guide a medical device to a location within the anatomy.
• the invention can be realized in hardware, software, or a combination of hardware and software.
• the invention can be realized in a centralized fashion in one computer system, or in a distributed fashion where different elements are spread across several interconnected computer systems. Any kind of computer system or other apparatus adapted for carrying out the methods described herein is suited.
• a typical combination of hardware and software can be a general purpose computer system with a computer program that, when being loaded and executed, controls the computer system such that it carries out the methods described herein.
• the invention can be embedded in a computer program product.
• the computer program product can comprise a computer-readable storage medium in which is embedded a computer program comprising computer-executable code for directing a computing device or computer-based system to perform the various procedures, processes and methods described herein.
• Computer program in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) reproduction in a different material form.

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• 2009
  • 2009-11-10 EP EP09756357A patent/EP2384158A1/en not_active Withdrawn
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  • 2009-11-10 WO PCT/IB2009/054997 patent/WO2010076676A1/en active Application Filing
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