Patent Description:
In medical imaging, areas of subjects are imaged by medical imaging devices. In some cases, such as in cardiological diagnosis, the same areas are mapped with location tracking systems. The resulting images and maps may be presented in the coordinate system of the imaging or mapping device, as well as in standardized formats and coordinate systems, such as DICOM (Digital Imaging and Communications in Medicine). Various third-party programs are available for further analysis of the medical images.

Various methods of comparing images and maps from different devices are described in the patent literature. For example, <CIT> describes a method for sending from a first medical device to a second medical device a request for data using a communication protocol that includes messages for conveying medical measurement results.

As a further example, <CIT> describes a method for registering a measured MRI volume image with appropriate anatomical and blood supply territory Atlases to enable Atlas information to be mapped onto the measured MRI volume image.

As a yet further example, <CIT> describes a system and method for automatically registering a three dimensional (3D) pre-operative image of an anatomical structure with intra-operative electrophysiological (EP) points of a 3D electro-anatomical (EA) image map of the anatomical structure.

As another example, <CIT> describes a system and method that relates to enhanced medical workflows.

<CIT> describes a location pad that includes a housing having a flat surface and multiple field generators. The multiple field generators are fixed to the housing and are configured to generate respective magnetic fields having respective axes that are perpendicular to the flat surface.

<CIT> describes an apparatus that includes a detector assembly, a positioning unit, and interface circuitry. The detector assembly includes an array of multiple magnetic field detectors. The positioning unit is configured to fix the detector assembly at one or more known positions relative to a location pad, which generates magnetic fields for performing position measurements on an intra-body magnetic field detector using a positioning system. The interface circuitry is configured to output electrical signals that are produced by the magnetic field detectors of the detector assembly when the detector assembly is fixed at the known positions, so as to calibrate the position measurements performed by the positioning system.

In <CIT> there is described real-time tracking of surgical tools relative to a pre-operative surgical plan and intra-operative images which involves an image-based registration and tool tracking registration.

Embodiments of the present invention that are described hereinbelow provide improved methods for analyzing medical images.

There is therefore provided, in accordance with an embodiment of the invention, a method for registering images. The method includes installing a location tracking system, which is configured to map anatomical structures in a first coordinate system, in a fixed position within a medical imaging system, which captures three-dimensional (3D) images of the anatomical structures in a second coordinate system. The 3D images are converted to and stored in a standardized format in a third coordinate system in accordance with a first coordinate transformation between the second coordinate system and the third coordinate system. A first 3D image captured by the imaging system is registered with the first coordinate system so as to produce a second coordinate transformation between the first coordinate system and the second coordinate system. The first and second coordinate transformations are combined so as to derive a third coordinate transformation between the first coordinate system and the third coordinate system. A second 3D image of a body of a subject, captured by the imaging system, is processed in order to extract image features in the third coordinate system. The extracted image features are joined with location data captured by the location tracking system by applying the third coordinate transformation.

In a disclosed embodiment, the location tracking system comprises a magnetic tracking system, and registering the first 3D image with the first coordinate system includes inserting a jig including calibration targets into the medical imaging system, capturing the jig in the first 3D image, and measuring locations of the calibration targets.

In some embodiments, the medical imaging system includes a magnetic resonance imaging (MRI) system or a computerized tomography (CT) system.

In a disclosed embodiment, the third coordinate system is defined according to a Digital Imaging and Communications in Medicine (DICOM) protocol, and processing the second 3D image includes reading and processing the second 3D image by a software application complying with the DICOM protocol
In some embodiments, processing the second 3D image includes at least one of rotating and segmenting the image.

Additionally or alternatively, the location data captured by the location tracking system includes a location of a distal end of a catheter within an anatomical structure in the body.

Further additionally or alternatively, joining the extracted image features with location data includes displaying the extracted image features and the location data concurrently on a display.

There is also provided, in accordance with an embodiment of the invention, an apparatus for displaying registered images. The apparatus includes a location tracking system, which is configured to map anatomical structures in a first coordinate system. A medical imaging system, within which the location tracking system is installed in a fixed position, is configured to capture three-dimensional (3D) images of the anatomical structures in a second coordinate system. The 3D images are converted to and stored in a standardized format in a third coordinate system in accordance with a first coordinate transformation between the second coordinate system and the third coordinate system.

A processor is configured to register a first 3D image captured by the imaging system with the first coordinate system so as to produce a second coordinate transformation between the first coordinate system and the second coordinate system. The processor combines the first and second coordinate transformations so as to derive a third coordinate transformation between the first coordinate system and the third coordinate system and processes a second 3D image of a body of a subject captured by the imaging system in order to extract image features in the third coordinate system. It joins the extracted image features with location data captured by the location tracking system after applying the third coordinate transformation to the location data.

Intra-body probes, such as catheters, are used in various therapeutic and diagnostic medical procedures. The catheter is inserted into the living body of a patient and navigated to the target region in a body cavity in order to perform the medical procedure. In magnetic field-based location tracking systems, an external magnetic field is applied to the patient's body. The magnetic field is produced by multiple magnetic field generators, e.g., field generating coils, typically fixed in a location pad in the vicinity of the patient. A sensor installed in the distal end of the catheter responds to the field by producing an electric signal. The signal is then used by the tracking system to locate the position and orientation of the catheter in the patient's body.

Magnetic location tracking of the catheter may be performed in or near a medical imaging system, as described, for example, in the references cited above in the Background section. Collocation of these two systems enables, inter alia, joint display of magnetic position tracking and medical imaging data.

DICOM (Digital Imaging and Communications in Medicine) is a standard protocol for, among other things, storing medical images according to the protocol. Numerous third-party applications that allow the stored medical image to be manipulated and/or analyzed are available, typically operating in the Visualization Toolkit (VTK) format. For example, such applications can be used to rotate and segment the image. Displaying the magnetic location tracking data, such as a location of a catheter, jointly with the rotated and segmented image would be very useful for the medical professional manipulating the catheter. In general, however, this sort of joint display can be performed only off-line, since the manipulated and/or analyzed image is stored in a DICOM coordinate system, whereas the location tracking system operates in a coordinate system typically defined by a location pad of that system.

The embodiments of the present invention that are described herein address the problem described above by providing simple and seamless registration between the location tracking system and the standard coordinate systems used in medical image processing applications, such as the DICOM coordinate system. The registration is performed initially between the location tracking system and the medical imaging system (such as an MRI scanner), and is then applied in deriving a coordinate transformation between the location tracking system and the image processing coordinate system. The initial registration can generally be performed only once, but the transformation that is derived can be used repeatedly thereafter in enabling location tracking results to be integrated with a variety of different applications that use the standard image processing coordinate system, in both real-time and off-line applications.

In the disclosed embodiments, a location tracking system, which maps anatomical structures in a locator coordinate system, is installed in a fixed position within a medical imaging system, which captures three-dimensional (3D) images of the anatomical structures in a device coordinate system. The 3D images are converted to and stored in a standardized format, such as DICOM, in a patient coordinate system in accordance with a first coordinate transformation between the device coordinate system and the patient coordinate system. A second coordinate transformation, between the locator coordinate system and the device coordinate system, is derived by registering a 3D image captured by the imaging system with the locator coordinate system. The first and second coordinate transformations are then combined to derive a third coordinate transformation between the locator coordinate system and the patient coordinate system.

Subsequently, when 3D images of a body of a subject are captured by the imaging system, and are then converted to and processed in the standardized patient coordinated system, for example in order to extract image features, the extracted image features can joined with location data created by the location tracking system in the subject's body by applying this same (third) coordinate transformation. The initial, one-time derivation of the transformation thus enables the use of any one of a variety of third-party applications to display a manipulated and/or processed medical image, while simultaneously viewing in real-time information provided by the location tracking system. This capability can be used, for example, to superimpose on the medical image an icon representing a catheter tracked by the location tracking system or electrophysiological mapping data gathered by the catheter.

<FIG> is a schematic, pictorial illustration of a system <NUM> comprising a location tracking system collocated with a medical imaging system, in accordance with an embodiment of the present invention. <FIG> and parts of the description that follows are based on the above-mentioned <CIT>.

In the pictured embodiment, the location tracking system comprises a magnetic location tracking system <NUM>, and the imaging system comprises a magnetic resonance imaging (MRI) scanner <NUM>. The principles of the present invention, however, may similarly be applied to other sorts of medical imaging systems, such as coaxial tomography (CT) scanners, and other types of location tracking systems, such as impedance-based and ultrasonic tracking systems, as will be apparent to those skilled in the art. All such alternative embodiments are considered to be within the scope of the present invention.

Magnetic tracking system <NUM> can be realized as, for example, the Carto®<NUM> system, produced by Biosense Webster, of <NUM> Technology Drive, Irvine, CA <NUM> USA. MRI scanner <NUM> can be realized as, for example, the MAGNETOM Aera, produced by Siemens Healthcare GmbH, of Henkestrasse <NUM>, <NUM> Erlangen, Germany.

Magnetic tracking system <NUM> comprises an intra-body probe <NUM>, such as a catheter, and a control console <NUM>. An operator <NUM>, such as a cardiologist, percutaneously navigates catheter <NUM> through the vascular system of a patient <NUM> so that a distal end <NUM> of the catheter <NUM> enters a body cavity, herein assumed to be the cardiac chamber. Catheter <NUM> may be used, for example, for mapping electrical potentials in a chamber of a heart <NUM> of patient <NUM> with multiple electrodes disposed near distal end <NUM> of catheter <NUM> that contact the tissue of the heart cavity at multiple points. In alternative embodiments, catheter <NUM> may be used, mutatis mutandis, for other therapeutic and/or diagnostic functions in the heart or other body organs.

Console <NUM> uses magnetic position sensing to determine the orientation and position coordinates of distal end <NUM> of catheter <NUM> inside heart <NUM>. Console <NUM> operates a driver circuit <NUM>, which drives one or more magnetic field generators <NUM> in a location pad <NUM> below the patient's torso on a table <NUM> as shown in a dotted inset <NUM> in the upper right hand corner of <FIG>. Alternatively, location pad <NUM> may be have a different shape and be positioned in a different location, for example above patient <NUM>, in order to comply with the space requirements of a specific MRI scanner <NUM>.

A position sensor installed in distal end <NUM> generates electrical signals in response to the magnetic fields generated by location pad <NUM>, thereby enabling console <NUM> to determine the position and orientation of the distal end with respect to the location pad, and thus, the position and orientation within heart <NUM> of patient <NUM>.

MRI scanner <NUM> comprises magnetic field coils <NUM>, including field gradient coils, which together generate a spatially variant magnetic field. The spatially variant magnetic field provides spatial localization for radio frequency (RF) signals generated by the scanner. In addition, the scanner comprises transmit/receive coils <NUM>. In a transmit mode, coils <NUM> radiate RF energy to patient <NUM>, the RF energy interacting with the nuclear spins of the patient's tissue and thereby realigning the magnetic moments of the nuclei away from their equilibrium positions. In a receive mode, coils <NUM> detect RF signals received from the patient's tissue as the tissue nuclei relax to their equilibrium state.

MRI scanner <NUM> depicted in <FIG> comprises a structure that is open along one side of patient <NUM>. Alternatively, MRI scanner <NUM> may have a different, tubelike structure, such as for example the Siemens MAGNETOM Aera scanner previously referred to.

Table <NUM> in MRI scanner <NUM> normally supports patient <NUM>, as shown in inset <NUM>. In the pictured embodiment, however, a registration jig <NUM> is placed on table <NUM> in order to register the coordinate system of MRI scanner <NUM> with the coordinate system of the magnetic catheter tracking system, as is described in the above-mentioned <CIT>. The details of the registration process will be described further below. Jig <NUM> is placed on table <NUM> above location pad <NUM> within MRI scanner <NUM> in the same region where the torso of patient <NUM> would normally be positioned on table <NUM>.

A processor <NUM> has multiple functions in the embodiment shown in <FIG>. First, processor <NUM> is configured to receive electrical signals induced in the position sensor at catheter distal end <NUM> in response to the magnetic field generated by location pad <NUM> via interface circuitry (not shown). Processor <NUM> uses the received electrical signals to locate the catheter in the patient's body.

Secondly, processor <NUM> operates MRI scanner <NUM> by using circuitry to control MRI coils <NUM>, including forming required magnetic field gradients, as well as other circuitry to operate transmit/receive coils <NUM> around patient <NUM>. Processor <NUM> acquires MRI data within a volume of interest <NUM> (shown in <FIG>), as will be described below. Volume of interest <NUM>, for example, may comprise heart <NUM> of patient <NUM>. Using the MRI data, processor <NUM> displays an image <NUM> of heart <NUM> to operator <NUM> on a display <NUM>. The position of catheter <NUM> acquired by magnetic tracking system <NUM> can be superimposed on image <NUM> of heart <NUM> on display <NUM> acquired by MRI scanner <NUM>. As will be further described below, operator <NUM> may process the MRI data using a third-party application, and display a processed medical image <NUM> (<FIG>) with an image of catheter <NUM> superimposed thereon.

Processor <NUM> typically comprises a general-purpose computer, which is programmed in software to carry out the functions that are described herein. The software may be downloaded to processor <NUM> in electronic form, over a network, for example, or it may be provided on non-transitory tangible media, such as optical, magnetic or electronic memory media. Alternatively, some or all of the functions of processor <NUM> may be carried out by dedicated or programmable digital hardware components, or by using a combination of hardware and software elements.

Alternatively, the functions of processor <NUM> may be split between two or more processors, for example, one processor managing the magnetic position tracking system and one managing the MRI scanner. More generally, the embodiment shown in <FIG> is presented merely for conceptual clarity, and not by way of limitation of the embodiments of the present invention. MRI scanner <NUM> and magnetic tracking system <NUM> may have separate processors for each system and not shared as in the embodiment shown in system <NUM>. Single or separate displays may be used for MRI scanner <NUM> and magnetic tracking system <NUM>.

<FIG> is a schematic, pictorial illustration of registration jig <NUM>, in accordance with an embodiment of the present invention. Registration jig <NUM> comprises a positioning unit <NUM> having multiple slots <NUM> that are separated by a fixed, predefined distance between adjacent slots. Jig <NUM> is placed above location pad <NUM>, as shown in <FIG>. For other configurations and locations of location pad <NUM>, jig <NUM> may be placed in a different position. For example, for a location pad <NUM> positioned against the interior ceiling of MRI scanner <NUM>, jig <NUM> may be placed below it.

A registration assembly <NUM>, also referred to as a shelf, comprises an array of cubical receptacles <NUM>. Other forms of receptacles <NUM> are also applicable, as will be shown in <FIG>. Registration assembly <NUM> can be inserted into any of slots <NUM> in positioning unit <NUM>. The multiple slots in positioning unit <NUM> are configured to fix registration assembly <NUM> at one or more known positions relative to location pad <NUM>. In the embodiment shown in <FIG>, the slots <NUM> control the position (e.g., the height) of registration assembly <NUM> with respect to location pad <NUM>.

A baseplate <NUM> of positioning unit <NUM> is connected to a conformal adapter <NUM>, which is configured to fit and conform to the shape of location pad <NUM> such that the array of receptacles <NUM> will be in the X<NUM>-Y<NUM> plane at a fixed distance above the location pad and orthogonal to the Z<NUM>-axis. The X<NUM>-, Y<NUM>-, and Z<NUM>-axes are the coordinate axes of a Cartesian first coordinate system <NUM>. Adapter <NUM> may be machined, or formed, by any suitable process so as to conform to the curvature of location pad <NUM>. Location pad <NUM> is shown here as an example, and is described in more detail in the above-mentioned <CIT>. Alternatively, other sorts of location pads and calibration jigs may be used for the present purposes, as will be apparent to those skilled in the art after reading the present description.

<FIG> is a schematic, pictorial illustration of a registration assembly <NUM>, in accordance with an embodiment of the present invention. Registration assembly <NUM> serves the same purpose as registration assembly <NUM> of <FIG> and can similarly be used in jig <NUM>.

Registration assembly <NUM> comprises an <NUM>×<NUM> array of receptacles <NUM>, with a cone-shaped protrusion <NUM> in the center of each receptacle. Receptacles <NUM> are filled with an MRI-detectible fluid, such as water, and serve as MRI image reference markers. Either all or a subset of receptacles <NUM> are filled with the fluid. Filling an asymmetrical subset of receptacles <NUM> will facilitate a unique determination of the orientation of the subsequent MRI image. The fluid may be sealed in the volume of the receptacles by any suitable procedure. Alternatively, receptacles <NUM> may be left unsealed. Further alternatively, the reference markers may comprise MRI-detectible fluid-filled spheres with known radii, as described in the above-mentioned <CIT>.

In alternative embodiments, comprising different types of medical imaging systems, registration assembly <NUM> may comprise reference markers of a different substance. For example, for an embodiment wherein medical imaging system comprises an x-ray based imager, such as a computerized tomography (CT) system, the reference markers may be filled with a substance opaque or partially opaque to x-rays, such as calcium.

When registration assembly <NUM> is placed in positioning unit <NUM>, MRI scanner <NUM> images the fluid-filled registration array of receptacles <NUM>, and processor <NUM> registers the known positions of these receptacles in system <NUM> relative to location pad <NUM>. The known positions of fluid-filled receptacles <NUM>, acting as MRI reference markers, are then used to register the coordinate systems of MRI scanner <NUM> and magnetic tracking system <NUM>. Stated differently, processor <NUM> uses the array of multiple MRI reference markers <NUM> fixed by positioning unit <NUM> in at least one known position relative to location pad <NUM> for registering the coordinate systems.

To improve the resolution, the positioning unit can be configured to continuously vary the known position of the assembly within the separation distance between adjacent slots <NUM> so as to continuously fine tune the height in the Z<NUM>-direction as shown in <FIG>, after the assembly is fixed in a particular slot. For example, one or more turn screws <NUM> can be embedded in conformal adaptor <NUM> and oriented in the Z<NUM>-direction such that rotating the turn screws <NUM> moves, or jacks up, baseplate <NUM>, and thus, adjusts the Z<NUM>-position of unit <NUM> relative to conformal adapter <NUM>.

The embodiments shown in <FIG> are depicted merely for conceptual clarity, as examples of devices and methods that can be used in registering tracking system <NUM> and MRI scanner <NUM>, and not by way of limitation of the embodiments of the present invention. Other sorts of registration devices and methods will be apparent to those skilled in the art after reading the present description and are considered to be within the scope of the present invention.

<FIG> is a schematic pictorial view of three coordinate systems, which undergo registration and transformation in accordance with an embodiment of the present invention. Three right-handed Cartesian coordinate systems are shown in the figure, as they relate to table <NUM> of MRI scanner <NUM> and to patient <NUM>:.

The three coordinate systems are summarized in Table <NUM>, below.

<FIG> is a block diagram that schematically illustrates coordinate transformations between the coordinate systems described above, in accordance with an embodiment of the present invention.

A block <NUM> refers to magnetic tracking system <NUM> with its first coordinate system <NUM>. A block <NUM> refers to MRI scanner <NUM> with its two coordinate systems: second coordinate system <NUM> and third coordinate system <NUM>. An arrow <NUM> represents the definition of third coordinate system <NUM>, according to the DICOM-protocol, by volume of interest <NUM>. The spatial relationship between second and third coordinate systems <NUM> and <NUM>, respectively, is known to processor <NUM>, and thus the processor calculates a first coordinate transformation TX<NUM>, shown as a double arrow <NUM>, which is the transformation between the second and third coordinate systems.

A second coordinate transformation TX<NUM>, shown as a double arrow <NUM>, is the transformation between first and second coordinate systems <NUM> and <NUM>. It is calculated by processor <NUM> based on the registration procedure described above.

A third coordinate transformation TX<NUM>, shown as a double arrow <NUM>, is calculated by processor <NUM> as a product between first and second coordinate transformations TX<NUM> and TX<NUM>. Third coordinate transformation TX<NUM> conveys the transformation between first coordinate system <NUM> and third coordinate system <NUM>, i.e. between the coordinates of magnetic tracking system <NUM> in LCS and the coordinates in PCS according to the DICOM-protocol.

The algorithm applied by processor <NUM> for the coordinate transformations is based on <NUM>×<NUM> matrices, wherein each matrix implements one specific action. The individual <NUM>×<NUM> matrices and their effects are listed in Table <NUM>, below.

The individual coordinate transformation matrices of Table <NUM> or their products effect a coordinate transformation by multiplying a <NUM>×<NUM> vector <MAT>, wherein the first three elements are the xyz-coordinates, and the fourth element is required for the translation operation. Thus coordinate transformations TX<NUM> and TX<NUM> are <NUM>×<NUM> matrices, and coordinate transformation TX<NUM> is a matrix product of the former two matrices.

<FIG> is a block diagram that schematically illustrates a process for joining of a medical image <NUM>, processed by a third-party application in DICOM coordinates, for example, and location data <NUM>, in accordance with an embodiment of the present invention.

MRI scanner <NUM> produces a medical image <NUM> of patient <NUM> in third coordinate system <NUM>, i.e., in the DICOM coordinate system. Medical image <NUM> is processed by processor <NUM> using a third-party application, indicated by an arrow <NUM>, giving a processed medical image <NUM>, which is also represented in the DICOM coordinates. The processing of medical image <NUM> comprises, for example, rotating and/or segmenting the image. Magnetic tracking system <NUM> produces location data <NUM> of catheter <NUM> in first coordinate system <NUM>. Third coordinate transformation TX<NUM> is applied by processor <NUM> to location data <NUM>, thus transforming the location data from first coordinate system <NUM> into third coordinate system <NUM>, i.e., into the DICOM coordinate system.

Processor <NUM> then presents on display <NUM> a joint image <NUM> of catheter <NUM> superimposed on processed medical image <NUM>. No recalibration or re-registration is needed in order to register coordinate systems <NUM> and <NUM>, since the transformation that was previously computed can be used for this purpose. The same method can be used to display other sorts of data provided by magnetic tracking system <NUM>, such as marking or coloring processed medical image <NUM> to show electrophysiological data collected by the catheter.

<FIG> is a flowchart <NUM> that schematically illustrates a process for combining and displaying medical image and location data, in accordance with an embodiment of the invention.

A preparatory stage <NUM> starts with a registration step <NUM>, wherein MRI scanner <NUM> and magnetic tracking system <NUM> are registered with each other, as described above. In a second coordinate transformation step <NUM>, processor <NUM> calculates a second coordinate transformation TX<NUM> between second coordinate system <NUM> (device coordinate system, DCS) and first coordinate system <NUM> (location coordinate system, LCS).

In a patient insertion step <NUM>, patient <NUM> is inserted into MRI scanner <NUM>. In a volume of interest step <NUM>, a low-resolution scan of patient <NUM> is performed with MRI scanner <NUM>, and operator <NUM> defines, using control console <NUM>, volume of interest <NUM> as the volume for subsequent high-resolution MRI scans. In a first coordinate transformation step <NUM>, first coordinate transformation TX<NUM> is calculated between second coordinate system <NUM> (DCS) and third coordinate system <NUM> (patient coordinate system, PCS, in DICOM coordinates).

In a third coordinate transformation step <NUM>, third coordinate transformation TX<NUM> is calculated between first coordinate system <NUM> (LCS) and third coordinate system <NUM> (PCS) as a product of TX<NUM> and TX<NUM>. Step <NUM> completes preparatory stage <NUM>.

In a start step <NUM>, imaging of patient <NUM> and location tracking are started, although not necessarily concurrently, as is detailed below. In an MRI scan step <NUM>, volume of interest <NUM> is scanned by MRI scanner with a high-resolution scan, generating medical image <NUM> in third coordinate system <NUM> in an image generation step <NUM>. In a read step <NUM>, medical image <NUM> is read by a processor running a third-party software application, and further processed by the application in a processing step <NUM> to produce processed image <NUM>.

Location tracking of, for example, catheter <NUM> by magnetic tracking system <NUM> is started in a tracking step <NUM>. Location tracking can be delayed until after MRI scan step <NUM> is completed, in order to avoid interference with the location tracking by the magnetic fields of MRI scanner <NUM>. In a location data step <NUM>, the location tracking produces location data in first coordinate system <NUM>. In a transformation application step <NUM>, coordinate transformation TX<NUM> is applied to location data from location data step <NUM>, thereby transforming the location data into DICOM-coordinates.

In a combination step <NUM>, processed image <NUM> from processing step <NUM> and the location data in DICOM-coordinates from transformation application step <NUM> are combined to create joint image <NUM>, which may be shown on display <NUM> and viewed by operator <NUM>. In all of steps <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, both the medical image data and location data are expressed in DICOM-coordinates in order to enable the use of a third-party application and joint display of the image data and the location data.

In a decision step <NUM>, operator <NUM> decides whether he/she desires to modify processed image <NUM> by the third-party application by, for example, rotating the image or segmenting it. In decision step <NUM>, operator <NUM> may also decide to move catheter <NUM> and thus acquire new location data. Both modifying processed image <NUM> and moving catheter <NUM> activate again steps <NUM> and <NUM>, respectively. The loops from decision step <NUM> through steps <NUM> and <NUM> may happen in a continuous fashion. For example, operator <NUM> may move catheter <NUM> continuously and observe the changing location of the catheter on joint image <NUM>. Operator <NUM> may also, for example, rotate processed image <NUM> continuously while observing the location of catheter <NUM> in the rotating image. Operator <NUM> may further continuously change both processed image <NUM> and location of catheter <NUM> and observe these changes in a dynamically changing joint image <NUM>.

The medical procedure that operator <NUM> is performing may change the shape and dimensions of the anatomical details observed within volume of interest <NUM>, for example by surgically operating on the organ in the volume of interest. Based on the operator's experience and observations, he/she may decide that an updated medical image is required. In such a case, operator <NUM> may initiate a new MRI scan as shown by a dotted arrow <NUM>. Location tracking can be stopped while MRI scanner <NUM> is acquiring a new scan in MRI scan step <NUM>. Once the scan is completed, the process returns to the two paths starting with steps <NUM> and <NUM>.

The procedure ends in an end step <NUM>.

Claim 1:
A method for registering images, the method comprising:
installing a location tracking system (<NUM>), which is configured to map anatomical structures in a first coordinate system (<NUM>), in a fixed position within a medical imaging system (<NUM>), which captures three-dimensional (3D) images of the anatomical structures in a second coordinate system (<NUM>), wherein the 3D images are converted to and stored in a third coordinate system (<NUM>) in accordance with a first coordinate transformation (TX1) between the second coordinate system and the third coordinate system;
registering, by a processor, a first 3D image captured by the imaging system with the first coordinate system so as to produce a second coordinate transformation (TX2) between the first coordinate system and the second coordinate system;
combining, by the processor, the first and second coordinate transformations so as to derive a third coordinate transformation (TX3) between the first coordinate system and the third coordinate system;
processing, by the processor, a second 3D image of a body of a subject captured by the imaging system in order to extract image features in the third coordinate system; and
joining, by the processor, the extracted image features with location data captured by the location tracking system by applying the third coordinate transformation.