Marker tracking and accuracy verification for interventional magnetic resonance by measuring magnetic field inhomogeneity and determining a noise figure of merit

A fiducial marker assembly (30) is tracked using a magnetic resonance scanner (10). At the tracked position of the fiducial marker assembly, local B0 magnetic field inhomogeneity is measured. A warning is issued if the measured local B0 magnetic field inhomogeneity satisfies a warning criterion. A noise figure of merit of the tracking is also determined, and the warning is also issued if the noise figure of merit satisfies a noise-based warning criterion.

The following relates to the magnetic resonance arts. It finds particular application in locating or tracking catheters, biopsy probes, and other interventional instruments used in procedures employing interventional magnetic resonance imaging, and will be described with particular reference thereto. It finds application more generally in conjunction with locating micro-coils in the magnetic resonance scanner volume.

Interventional magnetic resonance imaging is used to guide interventional devices for procuring biopsies, locating radiation sources for brachytherapy, for targeted delivery of drugs or gene therapy, or so forth. In procedures employing interventional magnetic resonance imaging, a fiducial marker or fiducial marker assembly is disposed on or in a catheter, biopsy probe, or other interventional instrument. The fiducial marker can include an active micro-coil designed to be detected by the magnetic resonance scanner, or a passive coil or other passive marker having a magnetic susceptibility that shows up in magnetic resonance images.

A single fiducial marker arranged at the operative tip of the interventional instrument provides tip location but does not typically provide information on the orientation of the interventional instrument. Moreover, a fiducial marker positioned at the instrument tip can distort or otherwise interfere with imaging precisely at the point of intervention. Accordingly, a fiducial marker assembly including two or more (usually three or more) fiducial markers is disposed on the instrument at a defined distance from the instrument tip. By locating the markers, the location and orientation of the fiducial marker assembly is determined. As the interventional instrument has fixed orientation and position respective to the fiducial marker assembly, this in turn determines the location, and optionally the orientation, of the instrument tip.

Using such a fiducial marker assembly is complicated by the positioning of the fiducial marker assembly a distance away from the isocenter of the magnetic resonance system. In some cases, the fiducial marker assembly may be located near an edge of the field of view, for example 200 millimeters or more away from the isocenter. At these distances, the tracking accuracy decreases due to B0/B1inhomogeneity, inadequately corrected gradient non-linearities, and so forth. If the fiducial marker assembly moves outside of the field of view, gradient ambiguity can lead to wholly erroneous tracking information. The potential for moving entirely outside of the field of view is greatest in the z-direction, since there is no inherent limit on movement through the bore.

Medical personnel rely upon tracking of the interventional instrument provided by the fiducial marker assembly during the performing of interventional procedures. If the tracking accuracy is questionable, medical personnel should be alerted. However, existing tracking systems do not provide reliable mechanisms for detecting tracking inaccuracies, or for alerting medical personnel of such tracking inaccuracies.

In one approach for ascertaining tracking accuracy, the tracking history is used to determine if and when the fiducial marker assembly leaves the field of view or other reliable tracking range. However, this approach can be compromised by low or variable tracking frame rates, and is not usable for single-shot tracking techniques.

The following contemplates improvements that overcome the aforementioned limitations and others.

According to one aspect, a tracking method is disclosed. Local B0magnetic field inhomogeneity of a B0magnetic field generated by a magnetic resonance scanner is measured at an apparent location of a fiducial marker assembly. A warning is issued if the measured local B0magnetic field inhomogeneity satisfies a warning criterion.

According to another aspect, a tracking system is disclosed. A B0homogeneity tracking checker measures local B0magnetic field inhomogeneity of a B0magnetic field generated by a magnetic resonance scanner at an apparent location of a fiducial marker assembly. A user interface issues a warning if the local B0magnetic field inhomogeneity measured by the B0homogeneity tracking checker satisfies a warning criterion.

According to another aspect, an interventional magnetic resonance system is disclosed, including a magnetic resonance scanner, a fiducial marker assembly, a tracking processor that performs tracking processing to track at least an apparent location of the fiducial marker assembly using the magnetic resonance scanner, and the B0homogeneity tracking checker and user interface as set forth in the preceding paragraph.

One advantage resides in more reliable interventional magnetic resonance tracking.

Another advantage resides in reduced likelihood of medical mistakes caused by inaccurate tracking.

Another advantage resides in providing information to medical personnel in a straightforward format as to the accuracy of tracking.

Numerous additional advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments.

With reference toFIG. 1, a magnetic resonance imaging scanner10performs magnetic resonance imaging in a region of interest12. In the illustrated embodiment, the magnetic resonance imaging scanner10is a Philips Panorama 0.23T scanner available from Philips Medical Systems Nederland B.V. This scanner has an open bore that facilitates interventional medical procedures. It will be appreciated that the scanner10is an example only, and that the instrument tracking methods and apparatuses including monitoring of tracking accuracy described herein are generally applicable in conjunction with substantially any type of magnetic resonance imaging scanner, including but not limited to open bore scanners, closed-bore scanners, vertical bore scanners, and so forth. An imaging subject (not shown), such as a human medical patient, is placed on a subject support14and positioned within the region of interest12of the scanner10.

In an interventional medical procedure, an interventional instrument20, such as a biopsy needle, a catheter, pointer, or the like, is employed to perform a biopsy, a thermal ablation treatment, brachytherapy, slice selection, targeted drug delivery, or so forth. The magnetic resonance imaging scanner10images the area of the procedure during the interventional medical procedure to provide visual guidance to the surgeon or other medical therapist. In some interventional procedures, an unconstrained interventional instrument is manipulated directly by the surgeon or other medical therapist. However, for delicate or sensitive procedures which call for highly precise manipulation of the interventional instrument20, a mechanical instrument manipulator22supports and manipulates the interventional instrument20, or aids in the positioning of the interventional instrument20, under the direction of the surgeon or other medical therapist. In the illustrated embodiment, the mechanical instrument manipulator22is a multi-jointed mechanical assembly providing multiple degrees of freedom for manipulating the interventional instrument20, and is mounted to the subject support14. In other contemplated embodiments the arm may be supported or mounted on the scanner10or on another associated structure.

In order to locate or track the instrument20during the interventional procedure, a fiducial marker assembly30is disposed on the interventional instrument20at a position expected to be within the field of view of the magnetic resonance imaging scanner10at least when the tip or other operational element of the instrument is at the target location. The fiducial assembly includes one or more fiducial markers to provide apparent location information pertaining to the interventional instrument20. More typically, the fiducial marker assembly30includes three or more fiducial markers so as to provide both location and orientation information pertaining to the interventional instrument20. Three fiducial markers at fixed positions relative to one another and relative to the interventional instrument20, and with sufficient spatial distribution in three-dimensions, is generally sufficient to accurately determine the spatial location and orientation of the interventional instrument20. Additional markers are optionally included to provide redundancy and improved tracking robustness.

In some embodiments, it is contemplated to employ only two fiducial markers in the fiducial marker assembly30, which may be sufficient to provide both apparent location and orientation information if, for example, a rotational position of the interventional instrument20is unimportant. In the illustrated embodiment, the fiducial marker assembly30is spaced apart from an operational tip32of the interventional instrument20. This arrangement advantageously reduces the likelihood that image distortions potentially caused by the fiducial marker assembly30will adversely affect imaging in the vicinity of the operational tip32. However, in some contemplated embodiments the fiducial marker assembly may be positioned at the tip of the interventional instrument. In such embodiments, it is contemplated for the fiducial marker assembly to include only a single fiducial marker that indicates the apparent location of the tip without providing orientation information.

The fiducial marker or markers of the fiducial marker assembly30can take various forms. In some embodiments, active micro-coils serve as the markers. These active micro-coils are selectively energized during tracking portions of the magnetic resonance sequence so as to emit a signal that is tracked by the scanner10. In some embodiments, passive fiducial markers are used, such as passive coils, vials of magnetic material, magnetically susceptible elements, or so forth. The passive marker has a magnetic susceptibility which causes the passive marker to be detected during the magnetic resonance imaging. It is also contemplated to employ a combination of active and passive markers in the fiducial marker assembly30.

During the interventional procedure scanner electronics40control the magnetic resonance imaging scanner10to acquire imaging data, reconstruct the imaging data to generate reconstructed images, and display the reconstructed image, and also control the scanner10to perform tracking of the interventional instrument20via the fiducial marker assembly30. In the illustrated embodiment, the scanner electronics40include a user interface computer42having a graphical display44and at least one input device such as a keyboard46, mouse, trackball, or so forth, a tracking module50that performs tracking processing to track the interventional instrument20via the fiducial marker assembly30, a B0magnetic field homogeneity tracking checker54that verifies accuracy of the tracking based on measurements of local B0magnetic field inhomogeneity, and a tracking noise checker56that verifies accuracy of the tracking based on signal-to-noise ratio (SNR) or another noise figure of the tracking.

In the illustrated embodiment, the user interface computer42includes hardware and/or software components (not illustrated) for controlling the scanner to acquire magnetic resonance data, to generate reconstructed images from spatially encoded magnetic resonance data, and to generate graphical renditions of the reconstructed images that are displayed on the graphical display44. It is to be appreciated that the illustrated scanner electronics40are an example; in other embodiments, the tracking and/or tracking monitoring may be integrated into the computer as hardware and/or software components, or conversely data acquisition, reconstruction, and/or image rendering functionality may be embodied as electronics distinct from the computer. In some contemplated embodiments, the scanner electronics do not include a computer; rather, all data acquisition, reconstruction, image rendering, and tracking functions are performed by electronics distinct from a computer.

The tracking processor50employs substantially any suitable tracking technique. In some embodiments, the tracking processor50employs a single-shot tracking method in which a tracked position and orientation of the interventional instrument20is indicated responsive to medical personnel initiating a tracking frame. In some embodiments, the tracking processor50employs a low frame-rate iterative tracking method in which the tracked position and orientation of the interventional instrument20is updated automatically at a low update rate. In some embodiments, the tracking processor50employs a higher frame-rate iterative tracking method in which the tracked position and orientation of the interventional instrument20is updated automatically at a higher update rate. In some embodiments, the tracking processor50employs a variable frame-rate iterative tracking method in which the tracked position and orientation of the interventional instrument20is updated automatically at a variable update rate that depends for example, upon the last tracked position of the fiducial marker assembly30, or upon the portion of the interventional procedure presently being performed, or so forth.

The tracking processor50provides apparent location information, and optionally also apparent orientation information, for the fiducial marker assembly30and hence also for the interventional instrument20. The modifier “apparent” recognizes that the tracking performed by the tracking processor50may be less accurate than desired due to magnetic field inhomogeneity, inadequately corrected gradient non-linearities, and so forth. The B0magnetic field homogeneity tracking checker54verifies tracking accuracy based on measurements of local B0magnetic field inhomogeneity, and the tracking noise checker56verifies tracking accuracy based on SNR or another noise figure of the tracking. The B0magnetic field homogeneity tracking checker54provides an indication of inaccuracy in the tracking due to magnetic field inhomogeneity, inadequately corrected gradient non-linearities, and so forth. On the other hand, if the fiducial marker assembly30moves outside of the field of view, gradient ambiguity can lead to wholly erroneous tracking information in which the apparent location indicated by the tracking processor50is wholly different from the actual physical location of the interventional instrument20. This erroneous tracking condition is detected by the tracking noise checker56.

With continuing reference toFIG. 1and with further reference toFIG. 2, the B0magnetic field homogeneity tracking checker54suitably determines a measurement60of the modulus of the local B0magnetic field inhomogeneity (ΔB0). Additionally a local dB0/dx measurement62, a local dB0/dy measurement64, and a local dB0/dz measurement66or other spatial derivatives or combinations of spatial derivatives are optionally determined if multiple micro-cols are available. More generally, B0magnetic field inhomogeneity derivatives are suitably measured along three mutually non-parallel directions. The local B0magnetic field homogeneity measurements are local to the tracked location of the fiducial marker assembly30. In the illustrated embodiment, thresholders70,72,74,76compare the measured ΔB0inhomogeneity60, the measured dB0/dx derivative62, the measured dB0/dy derivative64, and the measured dB0/dz derivative66, respectively, with suitable warning threshold values. If any of the measured ΔB0inhomogeneity60, dB0/dx derivative62, the measured dB0/dy derivative64, or the measured dB0/dz derivative66exceed the warning threshold, then a warning is issued, for example by displaying text such as “Warning: Inaccurate tracking detected!!!” on the display44of the user interface computer42. With brief reference back toFIG. 1, in some embodiments the user interface for issuing the warning includes an audible alarm77, a flashing light78mounted on the scanner10, or other warning indicator or combination of warning indicators that is likely to be perceived by medical personnel performing the interventional procedure.

The tracking noise checker56includes a tracking peaks SNR processor80that determines a signal-to-noise ratio of the tracking peaks. A thresholder82compares the determined SNR with a noise threshold value. If the SNR is less than the noise threshold value (smaller SNR implies more noise relative to the signal) then the warning of inaccurate tracking is issued. More generally, the tracking noise checker56determines a noise figure of merit of the tracking, and a warning is issued if the noise figure of merit satisfies a warning criterion.

With reference toFIG. 3, a suitable embodiment of the local ΔB0measurement60is described. In a positive gradient measurement90, a magnetic resonance signal is acquired for a positive applied magnetic field gradient (Gx+) in e.g. the x-direction. In a negative gradient measurement92, a magnetic resonance signal is acquired for a negative applied magnetic field gradient (Gx−) in the x-direction. A suitable value of ΔB0estimated from the two measurements90,92with opposite magnetic field gradients is given by a computation94:

Δ⁢⁢B0=Gx+·x+-x-2,(1)
where x+is the apparent position computed from the magnetic resonance signal90measured with the positive applied magnetic field gradient (Gx+) in the x-direction, and x−is the apparent position computed from the magnetic resonance signal92measured with the negative applied magnetic field gradient (Gx−) in the x-direction. The computation94of Equation (1) outputs an estimated ΔB0value96that is input to the thresholder70ofFIG. 2. Optionally, the value of ΔB0can be analogously estimated using gradients in the y- and/or z-direction. Such measurements are typically done to determine the three-dimensional position of the micro-coil array, and can be used to increase the accuracy of the ΔB0estimate, for example by averaging the values of ΔB0estimated using gradients in the x-, y-, and z-directions. The local dB0/dx62, dB0/dy64and local dB0/dz measurement66can be derived by applying the computation94for multiple markers (for example, four or more markers in a suitable configuration, such as along three pairwise perpendicular lines in space) and using numerical differentiation. Instead of or in addition to measuring the x, y, and z magnetic field inhomogeneity derivatives, other non-parallel magnetic field inhomogeneity derivatives can be measured. In some embodiments, only one magnetic field inhomogeneity derivative, such as the inhomogeneity derivative in the axial or z-direction parallel to the patient, is measured.