Color-coded facial map of ENT bone distances

Abstract

One method includes receiving computed tomography (CT) images comprising voxels of body parts of a subject, segmenting the images to identify skin and bone surfaces in the images, measuring corresponding minimum distances to the bone from multiple points on the skin surface, and rendering an image of the skin surface while visually encoding the rendered image to indicate the corresponding minimum distances.

Description

Technical Field

[0001] The present invention relates generally to image guided surgery and, more particularly, to registration between magnetic tracking instruments and computed tomography (CT) images. Background Art

[0002] In image-guided surgery (IGS), medical personnel use instruments that are tracked in real time so that the position and / or orientation of the instrument can be presented on an image of the patient's anatomy during a surgical procedure. In some cases, both tracking and imaging of the patient's anatomy can be accomplished with one modality, such as fluoroscopy. However, because fluoroscopy uses ionizing radiation, its use should be minimized. Therefore, in many cases, images of the patient are prepared in one modality (e.g., magnetic resonance imaging (MRI) or computed tomography (CT) fluoroscopy), and instrument tracking uses a different modality, such as electromagnetic tracking.

[0003] US Patent 5,335,173 describes a medical diagnostic image display method comprising the steps of converting corresponding three-dimensional image information on the skull and the skin covering the outer surface of the skull of a subject to be examined and diagnosed.

[0004] U.S. Patent 6,081,739 describes a conventional digital panoramic radiography unit including an acoustic or optical three-dimensional scanning detector and a color video detector so that when panoramic X-ray data is obtained, a three-dimensional contour of the subject's skin surface and the outward visual appearance of the subject's skin are also obtained as related data sets.

[0005] US Patent 5,813,984 describes a method and apparatus for generating a forensic skull and soft tissue database for use in online facial reconstruction of victims and age-progressed portrait rendering of missing children by utilizing pre-diagnostic radiological modalities.

[0006] European patent EP0581704B1 describes a method for determining the position of an organ of a subject relative to at least two imaging devices.

[0007] US Patent 6,524,250 describes a device that can be easily used by a surgeon to measure and monitor changes before, during and after liposuction surgery and help create a symmetrical body contour. Summary of the invention

[0008] The embodiments of the invention described below provide a method for improving the registration between a magnetically tracked surgical instrument and a CT image.

[0009] Therefore, according to an embodiment of the present invention, a method is provided, comprising receiving a computed tomography (CT) image comprising voxels of a body part of a subject, segmenting the image so as to identify a skin surface and a bone surface in the image, measuring corresponding minimum distances to the bone from multiple points on the skin surface, and rendering an image of the skin surface while visually encoding the rendered image so as to indicate the corresponding minimum distances.

[0010] In an embodiment, visually encoding the rendered image includes applying a first image characteristic to a first area of ​​skin where the minimum distance does not exceed a predetermined threshold, and applying a second image characteristic to a second area of ​​skin where the minimum distance exceeds the predetermined threshold.

[0011] In another embodiment, the first image characteristic and the second image characteristic are two distinguishable colors. Alternatively, the first image characteristic and the second image characteristic are two distinguishable patterns. Alternatively, the first image characteristic and the second image characteristic are two distinguishable gray levels.

[0012] In another embodiment, visually encoding the rendered image includes applying a first image characteristic to a first region of the skin surface image where the minimum distance exceeds a predetermined first threshold, applying a second image characteristic to a second region of the skin surface image where the minimum distance does not exceed a predetermined second threshold, and applying a third image characteristic to a third region of the skin surface image where the minimum distance does not exceed the first threshold but exceeds the second threshold.

[0013] In another embodiment, the first image characteristic, the second image characteristic, and the third image characteristic are three distinguishable colors.

[0014] In another embodiment, the color of the third image characteristic is a combination of the colors of the first image characteristic and the second image characteristic. The relative weights of the first color and the second color in the combination are determined based on a ratio of a first difference to a second difference, wherein the first difference is the difference between the first threshold and the minimum distance, and the second difference is the difference between the minimum distance and the second threshold. Alternatively, the first image characteristic and the second image characteristic are two distinguishable gray levels, and the third image characteristic is a third gray level. The third gray level is an interpolated gray level between the gray levels of the first image characteristic and the second image characteristic, wherein the interpolated gray level is determined by the difference between the minimum distance and the first threshold and the second threshold, respectively.

[0015] In another embodiment, the body part is the head.

[0016] In further embodiments, the minimum distance for each of the plurality of points is established along a normal to the bone surface at each of the plurality of points.

[0017] According to an embodiment of the present invention, there is also provided an apparatus comprising a display device and a processor, wherein the processor is configured to receive a computed tomography (CT) image comprising voxels of a body part of a subject to segment the image so as to identify a skin surface and a bone surface in the image, measure corresponding minimum distances to the bone from multiple points on the skin surface, and render an image of the skin surface on the display device while visually encoding the rendered image so as to indicate the corresponding minimum distance.

[0018] In an embodiment, the visually encoded image includes applying a first image characteristic to a first area of ​​skin where the minimum distance does not exceed a predetermined threshold, and applying a second image characteristic to a second area of ​​skin where the minimum distance exceeds the predetermined threshold.

[0019] In another embodiment, the first image characteristic and the second image characteristic are two distinguishable colors. Alternatively, the first image characteristic and the second image characteristic are two distinguishable patterns. Alternatively, the first image characteristic and the second image characteristic are two distinguishable gray levels.

[0020] In another embodiment, the visually encoded image includes applying a first image characteristic to a first region of the skin surface image where the minimum distance exceeds a predetermined first threshold, applying a second image characteristic to a second region of the skin surface image where the minimum distance does not exceed a predetermined second threshold, and applying a third image characteristic to a third region of the skin surface image where the minimum distance does not exceed the first threshold but exceeds the second threshold.

[0021] In another embodiment, the first image characteristic, the second image characteristic, and the third image characteristic are three distinguishable colors.

[0022] In another embodiment, the color of the third image characteristic is a combination of the colors of the first image characteristic and the second image characteristic. The relative weights of the first color and the second color in the combination are determined based on a ratio of a first difference to a second difference, wherein the first difference is the difference between the first threshold and the minimum distance, and the second difference is the difference between the minimum distance and the second threshold. Alternatively, the first image characteristic and the second image characteristic are two distinguishable gray levels, and the third image characteristic is a third gray level. The third gray level is an interpolated gray level between the gray levels of the first image characteristic and the second image characteristic, wherein the interpolated gray level is determined by the difference between the minimum distance and the first threshold and the second threshold, respectively.

[0023] In another embodiment, the minimum distance for each of the plurality of points is established along a normal to the bone surface at each of the plurality of points.

[0024] According to an embodiment of the present invention, a computer software product including a non-transitory computer-readable medium storing program instructions is also provided. When read by a computer, the instructions cause the computer to receive a computer tomography (CT) image including voxels of a body part of a subject to segment the image so as to identify a skin surface and a bone surface in the image, measure corresponding minimum distances to the bone from a plurality of points on the skin surface, and render an image of the skin surface while visually encoding the rendered image so as to indicate the corresponding minimum distance.

[0025] The present disclosure will be more fully understood through the following detailed description of embodiments of the present invention in conjunction with the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a schematic diagram of a surface registration system according to an embodiment of the present invention;

[0027] Figure 2 is a flow chart of a registration process according to an embodiment of the present invention;

[0028] Figure 3 is a flowchart of a process for rendering an image in an image rendering step according to an embodiment of the present invention;

[0029] Figure 4 A map indicating thickness of soft tissue on the face of a subject is shown according to an embodiment of the present invention;

[0030] Figure 5 shows a view on a screen during preliminary registration according to an embodiment of the present invention;

[0031] Figure 6 shows a view on a screen at the start of final registration according to an embodiment of the present invention;

[0032] Figure 7 shows a view on a screen during a final iterative registration according to an embodiment of the present invention;

[0033] Figure 8 is a flowchart of a process for rendering an image in an image rendering step according to another embodiment of the present invention; and

[0034] Fig. 9 An image indicating the thickness of soft tissue on a subject's face is shown according to an alternative embodiment of the present invention. DETAILED DESCRIPTION

[0035] Overview

[0036] The position and orientation of surgical instruments in a subject can be determined by utilizing magnetic tracking. In ear, nose and throat (ENT) surgery, a magnetic tracking system is first registered to the subject's head. The registration procedure typically utilizes a pre-recorded CT image of the subject's head. In the preliminary stages of the registration procedure, the surgeon touches the subject's face at several (usually four) points using a rod-shaped probe assembly carrying a registration probe. The position and orientation of the registration probe are identified by the magnetic tracking system. Based on these four points, the processor performs an initial registration by fitting the positions of the points in the magnetic tracking system to their positions in the CT image using a registration algorithm.

[0037] The resulting registration is often not sufficient to accurately determine the position and orientation of the surgical instruments. In the second stage of the registration procedure, the surgeon touches the subject's face with a wand in several points. During this process, a processor uses a registration algorithm to iterate and improve the registration based on these additional points on the face.

[0038] Since the registration algorithm is based on fitting additional points to the outer (skin) contour of the CT image, any error introduced by the surgeon by pressing the rod into the soft tissue will result in an error in the registration. The surgeon can then detect the error in the registration only after he inserts a guidewire into the subject's sinus and finds a discrepancy between the guidewire position given by the registration algorithm and, for example, his tactile sense of the position.

[0039] In such cases, the surgeon typically extracts the guidewire and re-performs a second stage of registration, resulting in a delay in the surgical procedure.

[0040] Embodiments of the invention described herein address this problem by rendering an image of a subject's face in which the thickness of the soft tissue overlying the facial bones is represented by a visual encoding, such as different colors.

[0041] In an embodiment of the present invention, a processor receives a computed tomography (CT) image of a voxel comprising a body part of a subject. The processor segments the image to identify the skin surface and the bone surface in the image and measures the respective minimum distances from a plurality of points on the skin surface to the bone. The minimum distance corresponds to the distance along the normal of the bone surface. The processor then renders an image of the skin surface and visually encodes the image to indicate the corresponding minimum distance. The image can be presented on a screen to a surgeon operating on the subject.

[0042] Visual encoding of an image typically includes applying a first image characteristic such as a color or shade to a first area of ​​the skin where a minimum distance does not exceed a predetermined threshold, and applying a second image characteristic such as another color or shade to a second area of ​​the skin where the minimum distance exceeds the predetermined threshold.

[0043] Alternatively, the visual encoding of the image includes applying a first image characteristic, such as a first color or shade, to a first area of ​​skin where the minimum distance exceeds a predetermined first threshold, applying a second image characteristic, such as a second color or shade, to a second area of ​​skin where the minimum distance does not exceed a predetermined second threshold, and applying a third image characteristic to a third area of ​​skin. In one embodiment, the third image characteristic typically includes a combination of the first image characteristic and the second image characteristic, wherein the relative weights of the first image characteristic and the second image characteristic are determined by interpolation.

[0044] While the above embodiments describe ENT procedures associated with a subject's head, other embodiments include medical procedures applied to other parts of a subject's body.

[0045] System Description

[0046] Figure 1 1 is a schematic diagram of a surface registration system 10 according to an embodiment of the present invention. The system 10 is used to register a magnetic tracking system 12 with an image, which is assumed to be a computed tomography (CT) image of a subject 14 by way of example in this article. The tracking system 12 is used to track the position and orientation of one or more instruments, such as a catheter or a guidewire, which is inserted into the subject 14 during a medical procedure performed on the subject. As described below, the tracking system 12 is also capable of tracking the position and orientation of a registration probe 16 outside the subject. The probe 16 is fixedly connected to a handle 18, which can be held by a professional 20 (usually a surgeon) during use of the system 10. The combination of the probe 16 and the handle 18 forms a rigid probe assembly 22, which is conducive to positioning the probe to a desired position by the professional 20.

[0047] For the sake of clarity and simplicity, in the following description, it is assumed that the medical procedures mentioned above include invasive procedures on the sinuses of subject 14, so that surface registration system 10 and magnetic tracking system 12 are assumed to be configured to operate in and around the sinus area. However, it should be understood that system 10 and system 12 can be configured to operate in and around other areas of a subject, such as the kidneys or abdomen, and a person of ordinary skill in the art will be able to adapt the description herein for use with such other areas.

[0048] The tracking system 12 is operated by a system processor 24, which includes a processing unit 26 in communication with a probe tracking module 28. The functions of the module 28 are described below. The processor 24 may be mounted in a console 30, which includes operating controls 32, which typically include a pointing device, such as a mouse or trackball. The professional 20 uses the operating controls to interact with the processor 24, which may be used to present the results generated by the systems 10 and 12 to the professional on a display device 34 (also referred to herein as a screen 34) as described below.

[0049] The processor 24 uses software stored in a memory of the processor to operate the system 10. For example, the software may be downloaded to the processor 24 in electronic form over a network, or alternatively or additionally, the software may be provided and / or stored on a non-transitory tangible medium such as magnetic, optical or electronic memory.

[0050] In order to track the above-mentioned instruments in the subject 14, and to track the probe 16, the processing unit 26 uses the probe tracking module 28 to operate a plurality of magnetic field generators 36 (usually coils) via cables 35. In one embodiment, if the subject 14 is anesthetized and has a fixed head 38 lying on a bed 40, it is generally applicable, such as Figure 1 As shown, generator 36 is fixed to a frame 42 that is typically placed on a bed, except for the subject's head. In an alternative embodiment (not shown), applicable if subject 14 is not anesthetized, generator 36 is fixed relative to each other and to a frame attached to head 38. A triaxial reference coil 41 is fixed to head 38 and is connected to processing unit 26 by cable 43.

[0051] Generator 36 radiates alternating magnetic fields into and out of head 38 of subject 14, and these fields generate signals in magnetic detectors in instrument and probe 16. The signals are transmitted back to processing unit 26 and probe tracking module 28, typically in the case of probe 16 via cable 44 connecting the probe to console 30, and the processor and module together analyze the signals to provide the position and orientation of instrument and probe 16 relative to generator 36. It should be understood that magnetic field generator 36 defines a reference coordinate system 46 for magnetic tracking system 12.

[0052] Produced by Biosense Webster, Irvine, CA The system uses a tracking system similar to that described herein to track the position and orientation of the distal tip of a probe inserted into a subject.

[0053] System processor 24 stores a digitized CT image 48 of head 38 of subject 14. Digitized CT image 48 is accessible by processing unit 26 for use in registering system 10, and in particular for generating an image 50 of subject's head 38 on screen 34. During the registration process, probe 16 is brought into contact with a surface 52 of subject 14, i.e., the subject's skin, such that surface 52 is also referred to herein as skin 52.

[0054] Figure 2 FIG. 9 is a flow chart of a registration process according to an embodiment of the present invention. In the data acquisition step 60 and the image rendering step 62, an image 94 ( Figure 4 In a data acquisition step 60, the CT image 48 is read by the system processor 24. Figure 3 Flowchart and Figure 4 In a preliminary registration step 64, the CT image 48 and the tracking system 12 are registered to each other based on a small number (typically four) of points acquired by the practitioner 20 using the rigid probe assembly 22 (also referred to herein as the wand 22), as described in reference to FIG. Figure 5 The final registration includes acquiring a large number of points in a high density sampling step 66, an updating step 68 and a decision step 70. Figure 6 to Figure 7 Steps 66, 68 and 70 are described.

[0055] Figure 3 According to an embodiment of the present invention, Figure 2 Flowchart of the process of generating image 94 in image rendering step 62 of the present invention. Image 94 is also referred to herein as rendered image 94. System processor 24 allocates a 3-D data array having the same dimensions as digitized CT image 48 in allocation step 76 and transfers the CT image to the data array in transfer step 78. Each point in the data array is referred to as a voxel. In identification step 80, system processor 24 identifies those voxels in the data array that are associated with skin 52 based on the radiation density associated with each voxel and its surrounding voxels; these voxels are referred to as "skin voxels".

[0056] In steps 82 to 92, the system processor 24 loops over all of the skin voxels, determining the distance from the skin voxel to the closest point of the underlying bone in a distance step 86. In a comparison step 88, the system processor 24 compares the determined distance from the skin voxel to the bone to a predetermined threshold, which is selected by the professional 20 to be in the range of 0.5 mm to 3 mm. The threshold is assumed to be the minimum acceptable skin-bone distance. If the distance is less than or equal to the threshold, i.e., less than or equal to the minimum skin-bone distance, a green color is associated with the skin voxel in a green association step 90. If the distance is greater than the threshold, a red color is associated with the skin voxel in a red association step 92. Once the system processor 24 has looped through all of the skin voxels, the process ends by the system processor generating an image of the skin voxels with their associated colors in an image generation step 93.

[0057] Figure 4 An image 94 indicating facial soft tissue thickness of subject 14 according to an embodiment of the present invention is shown. In preparation for registration between magnetic tracking system 12 and digitized CT image 48, system processor 24 presents image 94 indicating soft tissue thickness, as described below. In the embodiment described below, the visual encoding is based on different colors.

[0058] As mentioned above Figure 3 As described in the flowchart of , the system processor 24 identifies skin voxels in the digitized CT image 48. For each skin voxel, the system processor 24 segments the CT image 48 to identify the subject's bone material and measures the distance from the skin 52 to the underlying bone. If the distance does not exceed a predetermined threshold, the image of the skin voxel on the image 94 is colored green. However, if the distance exceeds the predetermined threshold, the image of the skin voxel is colored red. (The colors "green" and "red" are in Figure 4 and subsequent figures are represented by two different shades as region 96 and region 98, respectively.) In the resulting image 94, region 96 is a bone region where the thickness of the soft tissue does not exceed a predetermined threshold, and region 98 is a fleshy region where the thickness of the soft tissue exceeds the threshold.

[0059] although Figure 4 The embodiment described in 94 uses green and red as the colors used to encode the image 94, but other colors may be used. In other embodiments, more than one predetermined threshold may be used, and a different color may be assigned to each thickness interval between two consecutive thresholds. In other embodiments, grayscale may be used to indicate thickness. In other embodiments, patterning, or a combination of patterning, color, and grayscale may be used to indicate thickness.

[0060] Figures 5 to 7 The embodiment according to the present invention is shown in Figure 2The flow chart depicts the view on screen 34 during the registration process.

[0061] Figure 5 The preliminary registration step 64 ( Figure 2 ) during the digitized CT image 48. For the purpose of preliminary registration, the system processor 24 displays a facial image 102 of the subject 14 on the screen 34, wherein the facial image corresponds to the subject's skin 52 extracted from the digitized CT image 48 by identifying skin voxels 81. In addition, the system processor 24 presents a schematic facial representation 104 showing four points 106a through 106d. Points 106a through 106d are locations recommended for preliminary registration, selected for their clear locations on the face and the bone regions typically found at these locations.

[0062] Using probe assembly 22, professional 20 touches skin 52 of the face of subject 14 with registration probe 16 at the four points that, in the professional's judgment, are the closest matching recommended points 106a to 106d. When touching each of the four points, professional 20 uses controls on probe assembly 22 (controls not shown) or operating controls 32 to signal system processor 24 to record the position and orientation of probe 16.

[0063] After recording the position and orientation of the probe 16 in the four points, the system processor 24 calculates the coordinate transformation between the four points in the reference coordinate system 46 of the magnetic tracking system 12 and the digitized CT image 48, thereby producing the best spatial fit between the four points and the skin voxels. This coordinate transformation gives a preliminary registration between the magnetic tracking system 12 and the digitized CT image 48.

[0064] Figure 6 A view 110 on the screen 34 is shown at the start of final registration according to an embodiment of the present invention (described above with respect to FIG. Figure 2 For the purpose of final registration, system processor 24 displays two images on screen 34: rendered image 94 and facial image 102. Based on the preliminary registration between magnetic tracking system 12 and digitized CT image 48, an icon 103 representing the position of triaxial reference coil 41 is shown on facial image 102.

[0065] For final registration, the professional 20 touches the registration probe 16 at several points on the face of the subject 14 and signals the system processor 24 to accept these points for subsequent registration calculations. In addition, in order for these coordinates to represent the least distorted surface of the skin 52, in one embodiment, the professional 20 touches the skin at the bone region 96 with the registration probe 16 under the guidance of the image 94.

[0066] Figure 7 A view 120 on screen 34 during a final iterative registration is shown according to an embodiment of the present invention. The final iterative registration corresponds to Figure 2 66, 68 and 70 of the flowchart of FIG.

[0067] Point 122 on skin 52 of face of subject 14 indicates the point at which practitioner 20 touched the face with registration probe 16, generally within region 96 (colored green). A signal representing the coordinates of point 122 is sent to system processor 24. For clarity, Figure 7 94 , the practitioner 20 controls the registration accuracy between the reference coordinate system 46 of the magnetic tracking system 12 and the digitized CT image 48.

[0068] Return to reference Figure 2 , in decision step 70, the professional 20 determines whether the registration is sufficiently accurate. For this purpose, the professional 20 touches the probe 16 at a well-defined location on the subject 14, such as the tip of the subject's nose. Based on his visual observation of the indication of the location of the probe on the image of the subject 14, the professional 20 makes his subjective decision on the registration accuracy achieved.

[0069] Figure 8 For use in Figure 2 A flowchart of the process of generating the image 150 in the image rendering step 62, and Fig. 9 An image according to an alternative embodiment of the present invention is schematically shown, also referred to herein as a rendered image 150. The first three steps in the flow chart are similar to Figure 3 The steps in the above are substantially the same: the system processor 24 allocates a 3D data array having the same size as the digitized CT image 48 in an allocation step 76 and transfers the CT image to the data array in a transfer step 78. Figure 3 As shown, each point in the data array is referred to as a voxel. In an identification step 80, the system processor 24 identifies those voxels in the data array that are associated with the skin 52 based on the radiation density associated with each voxel and its surrounding voxels; Figure 3 As shown, these voxels are referred to as “skin voxels”.

[0070] In steps 130 to 142, the system processor 24 loops over all skin voxels, determining the distance from the skin voxel to the closest point of the underlying bone in a distance step 132. In a first comparison step 134, the system processor 24 compares the determined distance from the skin voxel to the bone to a predetermined first threshold, typically 10 mm selected by the practitioner 20. If the distance is greater than the threshold, a red color is associated with the skin voxel in a red color association step 136. In a second comparison step 138, the distance is compared to a predetermined second threshold, typically between 0 mm and 0.5 mm selected by the practitioner 20. If the distance exceeds the second threshold (but does not exceed the first threshold based on the first comparison step 134), the system processor 24 determines a color based on an interpolated mix of red and green colors in an interpolation step 140 based on the ratio of the distance of the skin voxel to the first and second thresholds, respectively. Also in the interpolation step 140, the resulting mixed color is associated with the skin voxel. If, in the second comparison step 138, the system processor 24 determines that the distance is less than or equal to the second threshold, then the system processor associates the color green with the skin voxel in a green association step 142. Once the system processor 24 has cycled through all of the skin voxels, the process ends with the system processor generating an image of the skin voxels with their associated colors in an image generation step 144.

[0071] Fig. 9 An image 150 is shown indicating the thickness of the soft tissue of the face of subject 14 in accordance with an alternative embodiment of the present invention.

[0072] Similar to Figure 4 In preparation for registration between the magnetic tracking system 12 and the digitized CT image 48, the system processor 24 renders an image 150 indicating soft tissue thickness, as described below. In the embodiment described below, the visual encoding is based on different colors.

[0073] As above for Figure 8 As described in the flowchart of , the system processor 24 identifies skin voxels in the digitized CT image 48. For each skin voxel, the system processor 24 segments the CT image 48 to identify the subject's bone material and measures the distance from the skin 52 to the underlying bone. The system processor 24 determines the color of each skin voxel as follows: If the distance exceeds a predetermined first threshold (typically 10 mm), the skin voxel is colored red. If the distance does not exceed a predetermined second threshold (typically 0 mm to 0.5 mm), the skin voxel is colored green. If the distance exceeds the second threshold but does not exceed the first threshold, the skin voxel is colored with a mixture of green and red, where the relative amounts of red and green are based on the relative distances of the skin voxel from the first threshold and the second threshold, respectively. The green, red, and mixed colors are displayed in the figure. Fig. 9150, the soft tissue thickness is less than or equal to the second threshold, region 152 is a fleshy region where the soft tissue thickness exceeds the first threshold, and region 156 is a region where the soft tissue thickness is between the two thresholds. In region 156, the relative "greenness" and "redness" indicate the relative "distance" of each voxel from the two thresholds.

[0074] exist Figures 8 to 9 In the alternative embodiment described in Figure 6 to Figure 7 94 in. Now, the professional 20 is guided by the image 150 to touch the skin with the registration probe 16 at the bone regions 154, and possibly in those parts of the regions 156 where the color indicates that the soft tissue is thin.

[0075] It should be understood that the above embodiments are cited by way of example, and the present invention is not limited to what is specifically shown and described above. On the contrary, the scope of the present invention includes combinations and sub-combinations of the various features described above, as well as variations and modifications thereof, which will occur to those skilled in the art upon reading the above description, and which are not disclosed in the prior art.

Claims

1. A method for registration between a magnetic tracking instrument and a computed tomography (CT) image, comprising: receiving a computed tomography (CT) image comprising voxels of a body part of a subject; segmenting the image to identify skin surfaces and bone surfaces in the image; measuring corresponding minimum distances from a plurality of points on the skin surface to the bone; rendering an image of the skin surface while visually encoding the rendered image to indicate the corresponding minimum distance; as well as updating the registration by utilizing the rendered image to sample additional points on the skin surface located in bone regions; Wherein the minimum distance at each of the plurality of points is established along a normal to the bone surface at each of the plurality of points.

2. A method according to claim 1, wherein visually encoding the rendered image includes applying a first image characteristic to a first area of ​​the skin, wherein the minimum distance does not exceed a predetermined threshold, and applying a second image characteristic to a second area of ​​the skin, wherein the minimum distance exceeds the predetermined threshold.

3. The method of claim 2, wherein the first image characteristic and the second image characteristic are two distinguishable colors.

4. The method of claim 2, wherein the first image characteristic and the second image characteristic are two distinguishable patterns.

5. The method of claim 2, wherein the first image characteristic and the second image characteristic are two distinguishable gray levels.

6. A method according to claim 1, wherein visually encoding the rendered image includes applying a first image characteristic to a first region of the image of the skin surface, wherein the minimum distance exceeds a predetermined first threshold, applying a second image characteristic to a second region of the image of the skin surface, wherein the minimum distance does not exceed a predetermined second threshold, and applying a third image characteristic to a third region of the image of the skin surface, wherein the minimum distance does not exceed the first threshold but exceeds the second threshold.

7. The method of claim 6, wherein the first image characteristic, the second image characteristic, and the third image characteristic are three distinguishable colors.

8. The method of claim 7 , wherein the color of the third image characteristic is a combination of the color of the first image characteristic and the color of the second image characteristic, and wherein a relative weight of the color of the first image characteristic and the color of the second image characteristic in the combination is determined based on a ratio of a first difference value to a second difference value, wherein the first difference value is a difference between the first threshold value and the minimum distance, and the second difference value is a difference between the minimum distance and the second threshold value.

9. The method according to claim 6, wherein the first image characteristic and the second image characteristic are two distinguishable gray levels, and the third image characteristic is a third gray level, wherein the third gray level is an interpolated gray level between the gray levels of the first image characteristic and the second image characteristic, wherein the interpolated gray level is determined by the difference between the minimum distance and the first threshold and the second threshold, respectively.

10. The method of claim 1, wherein the body part comprises a head.

11. An apparatus for registration between a magnetic tracking instrument and a computed tomography (CT) image, comprising: Display device; and a processor configured to receive a computed tomography (CT) image comprising voxels of a body part of a subject, segment the image to identify a skin surface and a bone surface in the image, measure respective minimum distances to the bone from a plurality of points on the skin surface, render an image of the skin surface on the display device while visually encoding the rendered image to indicate the respective minimum distances, and update the registration by utilizing the rendered image to sample additional points on the skin surface that are in bone regions; Wherein the minimum distance at each of the plurality of points is established along a normal to the bone surface at each of the plurality of points.

12. An apparatus according to claim 11, wherein visually encoding the image comprises applying a first image characteristic to a first area of ​​the skin, wherein the minimum distance does not exceed a predetermined threshold, and applying a second image characteristic to a second area of ​​the skin, wherein the minimum distance exceeds the predetermined threshold.

13. The apparatus of claim 12, wherein the first image characteristic and the second image characteristic are two distinguishable colors.

14. The apparatus of claim 12, wherein the first image characteristic and the second image characteristic are two distinguishable patterns.

15. The apparatus of claim 12, wherein the first image characteristic and the second image characteristic are two distinguishable gray levels.

16. A device according to claim 11, wherein visually encoding the image includes applying a first image characteristic to a first region of the image of the skin surface, wherein the minimum distance exceeds a predetermined first threshold, applying a second image characteristic to a second region of the image of the skin surface, wherein the minimum distance does not exceed a predetermined second threshold, and applying a third image characteristic to a third region of the image of the skin surface, wherein the minimum distance does not exceed the first threshold but exceeds the second threshold.

17. The apparatus of claim 16, wherein the first image characteristic, the second image characteristic, and the third image characteristic are three distinguishable colors.

18. The apparatus of claim 17, wherein the color of the third image characteristic is a combination of the colors of the first image characteristic and the second image characteristic, and wherein a relative weight of the color of the first image characteristic and the color of the second image characteristic in the combination is determined based on a ratio of a first difference value to a second difference value, wherein the first difference value is a difference between the first threshold and the minimum distance, and the second difference value is a difference between the minimum distance and the second threshold value.

19. The device according to claim 16, wherein the first image characteristic and the second image characteristic are two distinguishable gray levels, and the third image characteristic is a third gray level, wherein the third gray level is an interpolated gray level between the gray levels of the first image characteristic and the second image characteristic, wherein the interpolated gray level is determined by the difference between the minimum distance and the first threshold and the second threshold, respectively.

20. The apparatus of claim 11, wherein the body part comprises a head.

21. A computer software product for registration between a magnetic tracking instrument and a computed tomography (CT) image, comprising a non-transitory computer readable medium having program instructions stored therein, which, when read by a computer, cause the computer to receive a computed tomography (CT) image comprising voxels of a body part of a subject, segment the image so as to identify a skin surface and a bone surface in the image, measure corresponding minimum distances to the bone from a plurality of points on the skin surface, render an image of the skin surface while visually encoding the rendered image so as to indicate the corresponding minimum distances, and update the registration by utilizing the rendered image to sample additional points on the skin surface that are located in bone regions; Wherein the minimum distance at each of the plurality of points is established along a normal to the bone surface at each of the plurality of points.

Images