Transformation selection method based on tracker movement

Abstract

To provide a method for selecting a transformation between a pose of a tracker and a coordinate system associated with image data acquired by a medical imaging apparatus.SOLUTION: A method is executed by a processor and includes: acquiring tracking information showing a plurality of different poses of a tracker that is fixedly arranged to a movable part of a medical imaging apparatus; determining movement of the tracker on the basis of the acquired tracking information; and selecting one of a plurality of transformations between a pose of the tracker and a coordinate system associated with image data acquired by the medical imaging apparatus on the basis of the determined movement of the tracker. A corresponding system and non-transitory computer-readable storage medium are also provided.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates generally to a method for selecting a transformation between a pose of a tracker and a coordinate system associated with image data acquired by a medical imaging device, as well as a system, computer program, and carrier such as a non-transitory computer storage medium. [Background technology]

[0002] In a surgical navigation procedure, it may be necessary to align image data acquired by a medical imaging device, such as an X-ray device with a C-arm, to a tracker tracked by a tracking system. Such alignment may be based on a transformation between the pose of the tracker and a coordinate system associated with the image data.

[0003] In current solutions, the tracker can be fixedly positioned relative to the medical imaging device, which means that the transformation can be the same as long as the tracker is positioned at the same pose relative to the medical imaging device. In other words, the tracker can only be positioned at a single predetermined pose relative to the medical imaging device, and the transformation that can be used to align the image data to the tracker is the single predetermined transformation between the single predetermined pose of the tracker and the coordinate system of the image data acquired by the medical imaging device.

[0004] In some surgical scenarios, the visibility of a tracker placed on a medical imaging device by the tracking system may be limited, for example due to a medical instrument being placed between the camera of the tracking system and the medical imaging device. For this and other reasons, it may be desirable to place the tracker in a pose different from the single predefined pose mentioned above. In such a placement, the transformation between the different poses of the tracker and the coordinate system of the image data may be different. As a result, it may not be possible to always use the same single predefined transformation to align the image data to the tracker. Summary of the Invention

[0005] There is a need in the art for a technique that addresses one or more of the above-referenced or other problems.

[0006] In a first aspect, a method is provided for selecting a transformation between a pose of a tracker and a coordinate system associated with image data acquired by a medical imaging device. The method is executed by at least one processor and includes acquiring tracking information indicative of a plurality of different poses of a tracker disposed at a fixed position relative to a movable part of the medical imaging device. The method includes determining a movement of the tracker based on the acquired tracking information. The method further includes selecting one of a plurality of transformations between the pose of the tracker and the coordinate system associated with image data acquired by the medical imaging device based on the determined movement of the tracker.

[0007] The method may further include determining a spatial relationship between the tracker and the movable part based on the determined movement of the tracker.

[0008] A transformation may be selected from among the multiple transformations based on the determined spatial relationships.

[0009] The spatial relationship can include the side of the moving part on which the tracker is located.

[0010] The determined movement of the tracker may indicate a direction of movement of the tracker.

[0011] The side of the moving part can be determined based on the direction of movement of the tracker.

[0012] The side of the movable part can be selected from a first side of the movable part associated with a clockwise direction of movement of the tracker and a second side of the movable part associated with a counterclockwise direction of movement of the tracker.

[0013] The spatial relationship can include the position of the moving part on which the tracker is positioned.

[0014] The determined movement of the tracker may indicate a path of movement of the tracker.

[0015] The position of the movable part can be determined based on the path of movement of the tracker.

[0016] The movable part may be configured to move along a circle or a spiral.

[0017] The path of movement of the tracker may include at least one segment of a circle, an ellipse, or a spiral.

[0018] The position of the movable part can be determined based on at least one section of a path of movement of the tracker.

[0019] The spatial relationship can include an orientation of the tracker relative to the moving part.

[0020] The method may further include obtaining gravity information indicative of a direction of gravity.

[0021] The transformation can be selected based on gravity information.

[0022] The movement of the tracker can be determined relative to the direction of gravity as indicated by the gravity information.

[0023] The transformation can be selected based on the determined movement of the tracker relative to the direction of gravity.

[0024] The method further includes obtaining at least one setup of the medical imaging device selected from start and end poses of the movable part of the medical imaging device relative to a stationary part of the medical imaging device or a component part that is in a fixed spatial relationship to the stationary part of the medical imaging device.

[0025] The transformation may be selected based on at least one setup of the medical imaging device.

[0026] The method may further include determining and obtaining at least one setup of the medical imaging device based on the tracking information, the gravity information, and the determined movement of the tracker.

[0027] The side of the movable part can be determined based on a direction of movement of the tracker and at least one acquired setup of the medical imaging device.

[0028] The side of the movable part can be determined based on the determined movement direction of the tracker and at least one obtained setup of the medical imaging device by selecting a side of the movable part from: (i) a first side of the movable part associated with a clockwise movement direction of the tracker and a first setup of the medical imaging device, and (ii) a counterclockwise movement direction of the tracker and a second setup of the medical imaging device, and a second side of the movable part associated with (i) a clockwise movement direction of the tracker and a second setup of the medical imaging device, and (ii) a counterclockwise movement direction of the tracker and a first setup of the medical imaging device.

[0029] At least two or all of the multiple different poses of the tracker can be associated with different times during an image acquisition process performed by the medical imaging device to acquire image data, during which a movable part of the medical imaging device is moving.

[0030] The medical imaging device may be configured to acquire computed tomography (CT) image data.

[0031] The movable part of the medical imaging device may be part of a C-arm.

[0032] A second aspect provides a system, comprising at least one processor configured to perform the method of the first aspect. The at least one processor may be configured to perform a method of selecting a transformation between a pose of a tracker and a coordinate system associated with image data acquired by a medical imaging device, the method including acquiring tracking information indicative of a plurality of different poses of a tracker disposed at a fixed position relative to a movable part of the medical imaging device, determining a movement of the tracker based on the acquired tracking information, and selecting one of a plurality of transformations between the pose of the tracker and the coordinate system associated with image data acquired by the medical imaging device based on the determined movement of the tracker.

[0033] The system may further comprise at least one of the following entities: (i) a medical imaging device having a movable part and configured to acquire image data; (ii) a tracking system, such as an optical tracking system comprising a camera, configured to provide tracking information; (iii) a gravity sensor configured to provide gravity information, optionally affixed to the tracking system and / or the camera; (iv) Tracker

[0034] In a third aspect, a non-transitory computer-readable storage medium is provided, the storage medium storing a computer program including instructions that, when executed on at least one processor, cause the at least one processor to perform the method of the first aspect. The storage medium can store a computer program including instructions that, when executed on at least one processor, cause the at least one processor to perform a method for selecting one of the transformations between a pose of a tracker and a coordinate system associated with image data acquired by a medical imaging device, the method including acquiring tracking information indicative of a plurality of different poses of a tracker fixedly positioned with respect to a movable part of a medical imaging device, determining a movement of the tracker based on the acquired tracking information, and selecting one of a plurality of transformations between the pose of the tracker and a coordinate system associated with image data acquired by the medical imaging device based on the determined movement of the tracker.

[0035] In a fourth aspect, a computer program is provided, the computer program comprising instructions which, when executed by at least one processor, cause the at least one processor to perform the method of the first aspect. The computer program can be transmitted by a data stream or stored on a (e.g. non-transitory computer readable) storage medium.

[0036] In a fifth aspect, there is provided a carrier carrying a computer program according to the fourth aspect. The carrier may be a data stream or a (for example non-transitory computer readable) storage medium. [Brief description of the drawings]

[0037] Further details, advantages, and aspects of the present disclosure will become apparent from the embodiments referenced in conjunction with the drawings. [Figure 1] 1 illustrates a system according to the present disclosure. [Diagram 2]1 illustrates a method according to the present disclosure. [Diagram 3] 1 shows different mounting arrangements of the tracker on the C-arm. [Figure 4a] 4 shows a side view of the different mounting arrangement of FIG. 3; [Figure 4b] 4 shows a side view of the different mounting arrangement of FIG. 3; [Figure 5a] 1 shows a medical imaging device with a tracker. [Figure 5b] 1 shows a medical imaging device with a tracker. [Figure 5c] 1 shows a medical imaging device with a tracker. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Exemplary embodiments of the present technology will now be described with reference to the drawings, in which, unless otherwise stated, the same reference numbers refer to the same or similar structural features.

[0039] 1 illustrates a system 100 according to the present disclosure. System 100 may be a surgical navigation system.

[0040] The system 100 comprises a processing device 2 including at least one processor 4. The at least one processor 4 is communicatively connected to a memory 6, which is an example of a non-transitory computer-readable storage medium. The at least one processor 4 may further be communicatively coupled to an interface 8 of the device 2. The device 2 may also be implemented as a spatially distributed processing device 2, for example by implementing the at least one processor 4, the memory 6, and / or the interface (such as, for example, a virtual processor, a virtual memory, or a memory) on different servers. In other words, at least a part of the device 2 may be realized by a cloud computing platform. Alternatively, the device 2 may be implemented as a local processing device 2, for example located in a scanning room or an operating room of a hospital.

[0041] The system 100 further comprises a tracking system 10. In the illustrated example, the tracking system 10 is an optical tracking system comprising a stereo camera 12. A gravity sensor 14 (e.g., a three-axis accelerometer) may be disposed fixedly relative to the stereo camera 12 to obtain a gravity direction 16. The tracking system 10 may be communicatively coupled to at least one processor 4 via an interface 8 and may be configured to provide tracking data indicative of a pose of the tracked tracker to the at least one processor 4. The term "pose" as used herein should be understood to mean "at least one of position and orientation".

[0042] The system 100 further comprises a medical imaging device 18 configured to acquire image data. The image data may include one or more medical images of the patient's body 20. The medical imaging device 18 comprises a stationary part (e.g., fixed) 22 and a movable part 24. When the image data is acquired by the medical imaging device 18, the movable part 24 is configured to move relative to the stationary part 22. The stationary part 22 is configured to remain stationary (e.g., in an operating room) when the image data is acquired. The medical imaging device 18 is communicatively connected to at least one processor 4 via an interface 8 and may be configured to provide acquired image data to the at least one processor 4. In the illustrated example, the medical imaging device 18 is configured to acquire computed tomography, CT, images of the patient's body 20 as image data, and comprises a C-arm having an X-ray source 26 and an X-ray detector 28 as the movable part 24, and a cart as the stationary part 22. The C-arm is configured to rotate about a central point 29, also called the isocenter, when acquiring a CT image, also called a CT scan.

[0043] The system 100 further comprises one or more trackers 30-34. The tracker 30 is fixedly arranged with respect to the moving part 24 of the medical imaging device 18, and in the illustrated example is directly attached to the X-ray detector 28, and may therefore be referred to as a medical imaging device tracker. The tracker 32 is spatially fixed with respect to the patient's body 20, e.g. glued to the patient's skin, and may therefore be referred to as a patient tracker. The tracker 34 is attached to a medical device 36 that is handled by a surgeon or a surgical robot, and may therefore be referred to as a medical device tracker. Each of the trackers 30-34 can be tracked by the tracking system 10, which means that the tracking system 10 can determine the pose of each of the trackers 30-34 in a (e.g., real-world) tracking coordinate system. The tracking coordinate system (e.g., origin and orientation of the coordinate system) may be defined by one of the trackers, e.g., by the patient tracker 32. The tracking system 10 can be configured to provide information indicative of the tracked poses of the trackers 30-34, e.g., in a tracking coordinate system, to the at least one processor 4. In the illustrated example, each of the trackers 30-34 includes a plurality of optical tracking markers detectable by the stereo camera 12, although the disclosure is not limited thereto.

[0044] The system 100 further comprises a display unit 38. The display unit 38 may be communicatively coupled to the at least one processor 4 via the interface 8. A navigation view may be displayed by the display unit 38, in which the tracked pose of the medical device 36 is shown relative to a representation of the patient's body 20. For example, the trajectory 40 is shown superimposed on a medical image 42 of the patient's body 20 contained in the image data acquired by the imaging device 18.

[0045] For this purpose, a transformation may be required between the tracker coordinate system and a coordinate system associated with the image data (e.g., associated with or defined by one or more images contained in the image data, such as medical image 42). The coordinate system associated with the image data may be defined by an isocenter 29 in a fixed spatial relationship with respect to the x-ray detector 28. More generally, the coordinate system associated with the image data may have a (e.g., predefined) relative spatial relationship with respect to the movable part 24 of the medical imaging device 18.

[0046] In the present disclosure, the tracker 30 can be attached to the movable part 24 in two or more predefined poses. This means that the relative pose of the tracker 30 and the movable part 24 can be different for each image acquisition process, and therefore the respective transformations between the pose of the tracker 30 and the coordinate system associated with the respective image data will be different for different instances of acquired image data. The techniques disclosed herein allow for the automatic selection of an appropriate transformation among multiple transformations. In the following, this approach will be described in detail. In the following, the reference numbers used in FIG. 1 will be used, but this should not be understood as necessarily limiting the features described below to the respective features shown and described in FIG. 1.

[0047] 2 illustrates a method according to the present disclosure, which can be executed by at least one processor 4. The memory 6 can store a computer program comprising instructions that, when executed by the at least one processor 4, cause the at least one processor 4 to execute the method of FIG.

[0048] In optional step 202, the medical imaging device 18 is initiated to acquire image data.

[0049] Step 202 may include acquiring image data by starting the medical imaging device 18 and performing an image acquisition process in which the movable part 24 moves.

[0050] At least steps 202-214 of the methods disclosed herein may be performed prior to scanning the patient's body 20. However, in some scenarios, it may be desirable for the image data acquired by the medical imaging device 18 initiated in step 202 to include a depiction of at least a portion of the patient's body 20.

[0051] The image data may include or be medical image data. The image data may include one or more medical images (e.g., images acquired using medical imaging techniques). The image data may include three-dimensional image data. The image data is acquired by the medical imaging device 18 while the movable part 24 moves relative to the stationary part 22. The image data may include computed tomography (CT) images, also referred to as CT scans. The image data may include cone-beam CT (CBCT) scans. The image data may include spiral CT scans or multi-slice CT scans. The image data may include one or more single-spectral or multi-spectral images. The image data may include one or more three-dimensional images. The image data may include one or more images acquired by the medical imaging device 18 while the movable part 24 moves relative to the stationary part 22.

[0052] In step 204, tracking information is obtained.

[0053] The tracking information indicates a number of different poses of a tracker 30 that is fixedly disposed relative to a movable part 24 of the medical imaging device 18. The tracker 30 may be attached directly to the movable part 24 or may be attached to the movable part 24 via a mounting assembly disposed on the movable part 24. In at least one of the movable parts 24, the mounting assembly and tracker 30 may be configured such that the tracker 30 is fixedly disposed in a position relative to the movable part 24 in two or more (e.g., predetermined) poses. The movable part 24 may have a number of (e.g., opposing) sides (e.g., two or more of a left side, a right side, a front side, and a back side) on which the tracker 30 may be disposed.

[0054] The tracking information may further indicate one or more tracked poses of trackers 32 and / or 34. The one or more poses indicated by the tracking information may be indicated relative to a similar coordinate system, such as a tracking coordinate system.

[0055] At least two or all of the different poses of the tracker 30 can be associated with different points on a time axis (e.g., one time point per pose). The different time points can be part of an image acquisition process performed by the medical imaging device 18 to acquire image data. The different poses of the tracker 30 can be acquired by the tracking system 10 during the image acquisition process. During that image acquisition process, the movable part 24 of the medical imaging device 18 can move relative to the stationary part 22. As a result, the tracker 30 can also move in space as it acquires the image data, resulting in different tracked poses of the tracker 30.

[0056] In optional step 206, gravity information is obtained.

[0057] Gravity information may be obtained from gravity sensor 14. The gravity information may, for example, be indicative of a gravity direction 16 for one or more of the tracked poses of trackers 30-34 (e.g., for one or more of a plurality of different poses of tracker 30). A tracking coordinate system (e.g., its orientation) may be defined based on the gravity direction 16. One or more, for example, all, of the tracked poses of trackers 30-34 may be defined relative to the gravity direction.

[0058] In step 208, the movement of the tracker 30 is determined.

[0059] The movement of tracker 30 is determined based at least on the obtained tracking information.

[0060] The movement of the tracker 30 can be determined relative to the gravity direction 16 indicated by the gravity information.

[0061] The determined movement of tracker 30 may indicate a direction of movement of tracker 30, such as a clockwise or counterclockwise direction of movement (e.g., as observed by tracking system 10). The movable part 24 and tracker 30 may move in that direction of movement during an image acquisition process (e.g., initiated in step 202) to acquire image data.

[0062] The determined movement of the tracker 30 may indicate a movement path of the tracker 30. The movable part 24 and the tracker 30 may move along the movement path during an image acquisition process (e.g., initiated in step 202) to acquire image data. For example, the movable part 24 may be configured to move along (e.g., at least a portion of) a circle or a spiral. In this case, the movement path of the tracker 30 may include at least one segment of a circle, an ellipse, or a spiral.

[0063] The determined movement of tracker 30 may indicate a rotation of tracker 30, such as a clockwise or counterclockwise rotation (e.g., as observed by tracking system 10). The movable part 24 may move during an image acquisition process (e.g., initiated in step 202) to acquire image data. For example, tracker 30 rotates and, optionally, moves in space (e.g., along a movement path).

[0064] Determining the movement of the tracker 30 may include virtually connecting multiple different poses of the tracker 30 as indicated by the tracking information. The direction of the movement may be determined based on different time points associated with the multiple different poses of the tracker 30. The movement of the tracker 30 may be determined by a machine vision algorithm based at least on the tracking information. The machine vision algorithm may be configured to separate the pose of the tracker 30 from poses of other tracked trackers 32, 34, may be configured to remove outlier poses of the tracker 30 (e.g., due to tracking errors), and / or may be configured to interpolate, average, and / or extrapolate between two or more of the multiple different poses of the tracker 30.

[0065] In optional step 210, at least one setup of the medical imaging device 18 is obtained.

[0066] At least one setup includes a start pose and / or an end pose of the movable part 24 of the medical imaging device 18. The start pose and / or the end pose can be defined relative to an operating room, a stationary part 22 of the medical imaging device 18, and / or a component having a fixed spatial relationship to the stationary part 22 of the medical imaging device 18. The start pose can correspond to a pose of the movable part 24 at the beginning of an image acquisition process, and the end pose can correspond to a pose of the movable part 24 at the end of an image acquisition process. Examples of start and end poses include a pose in which the detector 28 of the medical imaging device 18 is at the topmost ("12 o'clock") position, a pose in which the detector 28 of the medical imaging device 18 is at the leftmost ("9 o'clock") position, a pose in which the detector 28 of the medical imaging device 18 is at the bottommost ("6 o'clock") position, and a pose in which the detector 28 of the medical imaging device 18 is at the rightmost ("3 o'clock") position.

[0067] At least one setup may be obtained by determining at least one setup based on tracking information, and optionally based on gravity information and / or a determined movement of the tracker 30. For example, a start pose may be determined by comparing an earliest tracked pose of the tracker 30 with one or more later poses of a plurality of different poses of the tracker 30. As another example, an end pose may be determined by comparing a latest tracked pose of the tracker 30 with one or more earlier poses of a plurality of different poses of the tracker 30. Gravity information may be used to avoid ambiguity (e.g., distinguish a top position from a bottom position). This is particularly useful when the tracking system 10 (e.g., camera 12) may be positioned in different orientations relative to the gravity direction 16. The start pose and / or end pose may be derived from the determined movement.

[0068] In optional step 212, the spatial relationship between tracker 30 and movable part 24 is determined.

[0069] A spatial relationship between the tracker 30 and the movable part 24 is determined based at least on the determined movement of the tracker.

[0070] The spatial relationship between the tracker 30 and the movable part 24 may include the side (e.g., one of multiple sides) of the movable part 24 on which the tracker 30 was and / or is positioned (e.g., while the tracker 30 is moving through multiple different poses and / or during an image acquisition process to acquire image data).

[0071] As mentioned above, the determined movement of the tracker may indicate the direction of movement of the tracker. In this case, the side of the movable part 24 may be determined based on the direction of movement of the tracker 30. In one variant, the side of the movable part 24 is determined by selecting between a first side (e.g., the right side) of the movable part 24 where the direction of movement of the tracker 30 is clockwise, and a second (e.g., opposite) side (e.g., the left side) of the movable part 24 where the direction of movement of the tracker 30 is counterclockwise. The association of each side with the direction of movement of the tracker 30 may be predefined, obtained from the medical imaging device 18, and / or input by a user. These associations may be specific to the medical imaging device 18.

[0072] The side of the movable part 24 can be determined based on the movement direction of the tracker 30 and at least one acquired setup of the medical imaging device 18. The side of the movable part 24 can be determined by selecting a side of the movable part 24 from a first side (e.g., a right side) and a second side (e.g., a left side) based on the determined movement direction of the tracker 30 and at least one acquired setup of the medical imaging device 18. The first side of the movable part 24 can be associated with (i) a clockwise movement direction of the tracker 30 and a first setup of the medical imaging device 18, and / or (ii) a counterclockwise movement direction of the tracker 30 and a second setup of the medical imaging device 18. The second side of the movable part 24 can be associated with (i) a clockwise movement direction of the tracker 30 and a second setup of the medical imaging device 18, and / or (ii) a counterclockwise movement direction of the tracker 30 and a first setup of the medical imaging device 18. The associations between each side, the direction of movement and the setup of the tracker 30 may be predefined, obtained from the medical imaging device 18, and / or input by the user. These associations may be specific to the medical imaging device 18.

[0073] The spatial relationship may include a location of the movable part 24 where the tracker 30 was and / or is located (e.g., while the tracker 30 moves through a number of different poses and / or during an image acquisition process to acquire image data). As mentioned above, the determined movement of the tracker 30 may indicate a path of movement of the tracker 30. In this case, the position of the movable part 24 may be determined based on the path of movement of the tracker 30.

[0074] For example, the movement path of the tracker 30 can be compared with a number of predefined movement paths associated with different positions of the movable part 24 where the tracker 30 can be located to determine the position of the movable part 24 where the tracker 30 was and / or is located. If the movement path of the tracker 30 includes at least one section of a circle, an ellipse, or a spiral, the position of the movable part 24 can be determined based on at least one section of the movement path of the tracker 30 (e.g., by comparing the at least one section with the multiple predefined movement paths). The movement path or at least one section can be compared with the multiple predefined movement paths by a curve matching algorithm such as the Affine Curve Matching Algorithm (ACMA) and / or the Fast Marching Method (FMM). It is also possible to determine the position without comparing with a predefined movement path, for example, by making the movement path or at least one section correspond to a predefined geometric model of the movable part 24 or the entire medical imaging device 18.

[0075] The spatial relationship may include an orientation of the tracker 30 relative to the movable part 24. As mentioned above, the determined movement of the tracker 30 may indicate a rotation of the tracker 30. In this case, the orientation of the tracker 30 relative to the movable part 24 may be determined based on the rotation. It may be desirable to further determine the orientation of the tracker 30 based on at least one setup of the medical imaging device 18.

[0076] In step 214, one (e.g., only one) of multiple transformations between the pose of the tracker 30 and the coordinate system associated with the image data acquired by the medical imaging device 18 is selected.

[0077] One of the multiple transformations is selected based at least on the determined movement of tracker 30.

[0078] One or more (e.g., each) of the plurality of transformations may be predefined. One or more (e.g., each) of the plurality of transformations may be specific to one or more of the medical imaging device 18, the tracker 30, the mounting assembly, and combinations thereof. One or more (e.g., each) of the plurality of transformations may be stored in the memory 6.

[0079] Each of the multiple transformations can be associated with a different predetermined (e.g., possible) movement of the tracker 30. One of the multiple transformations can be associated with a predetermined movement of the tracker 30 that matches (e.g., at least by a predetermined amount) the determined movement of the tracker 30.

[0080] One of the plurality of transformations can be further selected based on one or more inputs selected from: (i) a determined spatial relationship between the tracker 30 and the movable part 24; (ii) gravity information; (iii) a determined movement of the tracker 30 with respect to the gravity direction 16; and (iv) at least one set-up of the medical imaging device 18. Each of the plurality of transformations can be associated with one or more different (e.g., possible) predefined criteria selected from: (i) a predefined spatial relationship between the tracker 30 and the movable part 24; (ii) predefined gravity information; (iii) a predefined movement of the tracker 30 with respect to the predefined gravity direction; and (iv) at least one predefined set-up of the medical imaging device 18. The selected one of the plurality of transformations can be associated with one or more predefined criteria that match (e.g., at least to a predefined extent) the one or more inputs. Such a selection can be referred to as a "direct" selection based on the one or more inputs. Alternatively, one of the plurality of transformations can be "indirectly" selected based on information derivable from the one or more inputs. For example, the selection may be based on the direction of movement of the tracker 30 and, optionally, on the side of the movable part 24, which is determined based on at least one setup. A combination of direct and indirect inputs is also possible to select one transformation.

[0081] The method may further include determining a relative pose between the coordinate system of the image data and the tracking coordinate system based on the selected one of the transformations and optionally a pose of the tracker in the tracking coordinate system. Alignment between the image data and a real-world coordinate system, such as the tracking coordinate system, may be determined based on the selected one of the transformations. Step 214 may include determining the alignment based on the selected one of the transformations.

[0082] In optional step 216, the display of the navigation view is initiated.

[0083] A navigation view may be initiated for display on the display unit 38. The navigation view is generated based on the one transformation selected in step 214. The navigation view may show a tracked pose of the medical device 36 relative to the medical image 42. For example, the trajectory 40 of the medical device 36 may be overlaid on the medical image 42 to generate the navigation view. The details of how such a navigation view is generated when a transformation between the pose of the tracker 30 and a coordinate system associated with the image data acquired by the medical imaging device 18 is selected will be apparent to one skilled in the art.

[0084] Exemplary details and implementation options of the techniques disclosed herein, including the method of FIG. 2, are described below with reference to FIGS. 3-5c.

[0085] Figure 3 shows different mounting arrangements of the tracker on the C-arm.

[0086] The tracker 30 may be positioned on the left side of the detector 28, as shown at 30a. Alternatively, the tracker 30 may be positioned on the right side of the detector 28, as shown at 30b. As can be seen, different predefined transformations 44a, 44b are required to describe the spatial relationship between each tracker arrangement 30a, 30b and the isocenter 29 between the detector 28 and the x-ray source 26. If only a single pose of the tracker 30 is obtained, it may not be possible to clearly determine which of the transformations 44a, 44b should be used. The present technique relies on determining the movement of the tracker 30 based on tracking information to select the appropriate transformation.

[0087] 4a-4b show side views of different mounting arrangements of FIG. 3. In the illustrated example, detector 28 is in the top 12 o'clock position at the start of the image acquisition process, and the C-arm has a predetermined direction of movement along which movable part 24 moves during the acquisition of a CT scan, as indicated by the large arrow. In this case, if it is determined that tracker 30 is moving in a clockwise direction, it can be derived that the tracker is located on the left side of the C-arm according to arrangement 30a, and thus the correct transformation 44a can be selected. On the other hand, if it is determined that tracker 30 is moving in a counterclockwise direction, it can be derived that the tracker is located on the right side of the C-arm according to arrangement 30b, and thus the correct transformation 44b can be selected.

[0088] The same approach is possible if one considers the direction of rotation of the tracker 30 rather than the direction of translation of the tracker 30, such that a clockwise movement of the movable part 24 also results in a clockwise rotation (and optionally a clockwise translation) of the tracker 30.

[0089] Furthermore, the position on the detector 28 at which the tracker 30 is to be located can be determined according to the radius of the path of movement of the tracker 30 around the isocenter 29. Then, an appropriate transformation can be selected from among a number of predefined transformations associated with various positions at which the tracker 30 can be located on the detector 28.

[0090] Some C-arms are manually reconfigured and the predefined movement directions shown in Figures 4a and 4b may be reversed. In these cases, the predefined movement directions may still be specific for different combinations of the side of the moving part 24 on which the tracker 30 is located and the start and / or end poses of the moving part 24. Thus, even if the C-arm is manually reconfigured, the techniques disclosed herein allow for unambiguous selection of the correct transformation by further considering the start and / or end poses in addition to the movement of the tracker 30.

[0091] 5a, the coordinate system of the image data need not be defined by isocenter 29, but may instead be defined by a spatial location 46 (e.g., arbitrarily selected) that is outside the volume imaged by medical imaging device 18 in acquiring the image data. This spatial location may also have a predetermined spatial relationship to movable part 24. It is also shown that tracker 30 may be located at x-ray source 26 rather than at x-ray detector 28.

[0092] As shown in FIG. 5b, the present disclosure is not limited to the medical imaging device 18 in which the movable part 24 moves along a circular movement path. Instead, the movable part 24 can move along a linear movement path 48 while the medical imaging device 18 is performing the image acquisition process. Furthermore, the tracker 30 can be disposed on an emitter 50 used in the image acquisition process instead of the X-ray detector 28. The detector 52 used in the image acquisition process can be part of the stationary part 22 of the medical imaging device 18. It is also possible that the movable part 24 includes a detector and the stationary part 22 includes a detector, each of which is used in the image acquisition process.

[0093] As shown in Fig. 5c, the movable part 24 can be configured to move along a movement path of any (e.g., two-dimensional and / or three-dimensional) shape that is neither linear nor circular, and the shape may or may not be predefined. Also, as illustrated in Fig. 5c, a transformation can be selected based not only on the determined movement of a single tracker 30, but also on the determined movements of multiple trackers 30, each attached to a different part of the movable part 24. This approach can improve reliability through redundancy, but can also be useful in cases where the shape of the movable part 24 changes during the image acquisition process.

[0094] The techniques disclosed herein may be modified in various other ways. For example, the variations described in Figures 5a to 5c may be combined with each other and / or with the configuration shown in Figure 1. The techniques disclosed herein are not limited to optical tracking systems. Instead, other tracking modalities may be used. For example, the tracking system 10 may be an electromagnetic tracking system that includes an electromagnetic field generator and uses one or more electromagnetic field sensors as trackers. The techniques disclosed herein are not limited to medical imaging devices configured to acquire X-ray images. For example, the medical imaging device 18 may be configured to acquire magnetic resonance (MR) image data, ultrasound image data, microscopy image data, microwave image data, or positron emission tomography (PET) image data. The moving part of the medical imaging device may not be part of a C-arm and may include one or both of the emitters and detectors of the medical imaging device, which need not be configured for X-rays. Additionally, the order of steps 202-216 of one or more methods may be different than that shown in Figure 2. For example, gravity information may be used only to determine at least one of the setups of step 210, and step 206 may be performed between steps 206 and 208. Multiple steps may be performed simultaneously, such as data acquisition steps 204, 206. It should be understood that optional steps 202, 206, 210, 212, 216 are not required, but one or more, or all of these steps may be performed.

[0095] Details of the technology disclosed herein, including the method of FIG. 2, are restated and described below with reference to the numerals used in the figures, which are not to be understood as limiting the following disclosure to the features described with respect to each figure.

[0096] Registration of volumetric image data acquired by a 3D C-arm scanner as medical imaging device 18 may require the following prerequisites to be met: 1. The C-arm may need to be trackable and therefore a tracker 30, also called a C-arm tracker, may need to be attached to the C-arm. 2. The spatial relationship between the tracker 30 and a tracking coordinate system defined, for example, by the patient tracker 32, may need to be determined based on the tracking data. 3. A pre-computed calibration matrix, also referred to herein as a predetermined transformation, containing the spatial relationship between the tracker 30 and the volumetric image data may need to be known.

[0097] If these three conditions are met, an automatic image registration is calculated when the acquired volumetric image data is sent to the at least one processor 4. The calibration matrix may define the transformation between the pose of the tracker 30 and the isocenter 29 of the C-arm. The isocenter 29 is the point around which the C-arm rotates and may be the center of the resulting image volume (see, for example, FIG. 1).

[0098] Some C-arm models provide multiple interface points for attaching tracker 30 to the C-arm. These different attachment points result in different calibration matrices (see FIG. 3). In order to select the correct calibration matrix 44a or 44b, it may be desirable to determine which side of the C-arm tracking system 10 is currently tracking.

[0099] Due to the relative nature of navigational tracking techniques and the possibility that the orientation of each tracker is not known in advance, it is not possible to rely on information provided in a static scenario where neither the patient tracker 32 nor the C-arm tracker 30 are moving. Instead, it is proposed to use tracking data acquired during the image data acquisition process. When acquiring 3D volumetric image data, the 3D-C-arm rotates around the isocenter 29 from a start pose to an end pose. The direction in which the C-arm tracker 30 rotates in the tracking coordinate system during the image acquisition process allows to determine the side of the C-arm that is currently shown towards the camera 12. This allows to use one tracker 30 that can be placed in various locations on the moving part 24 (e.g., attached to multiple adapter points of the C-arm) and to select the correct calibration matrix 44a, 44b for the alignment calculation. This allows the customer to remount the same tracker 30 on multiple mounting points depending on the current needs of the surgeon and / or the spatial arrangement of the operating room.

[0100] To determine the direction of C-arm movement, the continuous movement of the C-arm tracker 30 can be tracked over a particular period of time, or a starting point and one or more points during the rotation of the C-arm can be captured. These tracked points can form a segment of a circle or ellipse.

[0101] From the circular or elliptical spatial path formed by the tracking points of the tracker 30, the direction of movement (e.g., clockwise or counterclockwise) can be determined. If the C-arm tracker 30 is fixed to one side of the C-arm, the determined direction of movement of the tracker 30 directly translates into observing a clockwise or counterclockwise rotation of the C-arm. In this case, it can be assumed that the C-arm always rotates in the same way and the direction of rotation of each side is known in advance. When moving and rotating the camera 12, clockwise and counterclockwise always maintain the same relationship to the C-arm side (see, for example, FIG. 4).

[0102] The camera 12 does not have to be positioned so that its field of view is aligned (e.g. parallel or centered) with the rotation axis of the C-arm. When the camera 12 is positioned in front of or behind the C-arm, the C-arm tracker 30 moves towards or away from the C-arm. As long as the angular difference between the camera 12 and the C-arm tracker 30 is other than 0 or 90 degrees, the circular or linear back and forth movement is transformed into an ellipse. Tracking this ellipse not only identifies the attachment side of the C-arm, but also makes it possible to distinguish between different attachment points on the same side.

[0103] In some C-arms, the C-arm is reconfigured (e.g., manually) by rotating the "C" along the axis of rotation. This may result in a reversal of the clockwise / counterclockwise rotation of the C-arm relative to each side of the C-arm. In this case, the gravity direction 16 may be taken into consideration. Based on the gravity direction, it is clear whether the detector 28 starts rotating from the top position or the bottom position, and therefore the starting pose of the imager 18 (e.g., the starting position of the detector 28) can be determined at the beginning of the image acquisition process. Without the gravity direction clearly pointing to an external reference, the camera 12 does not know how it is rotating in space, and may not be able to calculate absolute top and bottom positions unless it is assumed that the camera 12 always faces the same direction relative to the gravity direction 16. The gravity direction combined with the determined movement direction of the tracker 30 allows the determination of various starting and ending poses for the setup of the imager 18.

[0104] The relationship between the rotation directions and the sides of the C-arm to be tracked can be predefined, for example based on a calibration where the user stores the relationship. The user is asked to rotate the C-arm in each direction or to skip one of the rotations if it is not applicable (e.g. if the C-arm does not support rotation along the roll axis). Once the association between the rotation directions and the sides of the C-arm to be tracked (and optionally one or more setups of the C-arm) is defined, the association is stored in the memory 6 and can potentially be used for subsequent image acquisition processes as well.

[0105] There may be some error factors in the rotational information acquired, since there may be jitter in both rotation and tracking, however, the recorded curves are detectable despite the jitter, and this jitter in the recorded positions can be made the same between all C-arm models, since only the clockwise / counterclockwise information (and optionally the C-arm setup) may be relevant for determining the side on the C-arm where the tracker 30 is mounted.

[0106] By implementing side detection, it may be possible to acquire registered, navigable medical images using a single C-arm tracker 30, with the camera 12 being able to be mounted on either side of the C-arm, where the tracker 30 can be switched between different mounting points, each with its own calibration based on a selected single transformation 44a, 44b.

[0107] In summary, the techniques disclosed herein may enable automatic selection of a transformation between a pose of a medical imaging device tracker and a coordinate system associated with image data acquired by a medical imaging device. The transformation can be selected based on different poses of the medical imaging device tracker tracked during an image acquisition process that acquires one or more medical images of a patient's body. The selected transformation enables alignment of the medical images to a real-world coordinate system without the need to image the patient or an alignment phantom. This technique is particularly useful when a surgeon wants to attach the medical imaging device tracker to the device at different attachment poses. No user input regarding the attachment pose is required, allowing for reduced errors in the selection of the transformation. Further advantages of this technique will be apparent to those skilled in the art. EXAMPLES

[0108] The disclosure also provides the following non-limiting examples.

[0109] [Example 1] A method for selecting a transformation between a pose of a tracker (30) and a coordinate system associated with image data acquired by a medical imaging device (18), the method being performed by at least one processor (4), comprising: acquiring (204) tracking information indicative of a plurality of different poses of a tracker (30) fixedly disposed relative to a movable part (24) of a medical imaging device (18); determining (208) a movement of the tracker (30) based on the obtained tracking information; and selecting (214) one of a plurality of transformations between a pose of the tracker (30) and a coordinate system associated with image data acquired by the medical imaging device (18) based on the determined movement of the tracker (30); The method includes:

[0110] [Example 2] determining (212) a spatial relationship between the tracker (30) and the movable part (24) based on the determined movement of the tracker; 2. The method of claim 1, wherein the one transformation is selected from the plurality of transformations based on the determined spatial relationship.

[0111] [Example 3] 3. The method according to any one of the preceding claims, wherein the spatial relationship includes a side of the movable part (24) on which the tracker (30) is disposed.

[0112] [Example 4] 4. The method of claim 3, wherein the determined movement of the tracker indicates a direction of movement of the tracker (30), and the side of the movable part (24) is determined based on the direction of movement of the tracker (30).

[0113] [Example 5] 5. The method of claim 4, wherein the side of the movable part (24) is determined by selecting the side of the movable part (24) from a first side of the movable part (24) associated with a clockwise movement direction of the tracker (30) and a second side of the movable part (24) associated with a counterclockwise movement direction of the tracker (30).

[0114] [Example 6] 6. The method according to any one of claims 2 to 5, wherein the spatial relationship comprises a position of the movable part (24) at which the tracker (30) is disposed.

[0115] [Example 7] 7. The method of embodiment 6, wherein the determined movement of the tracker (30) indicates a path of movement of the tracker (30), and the position of the movable part (24) is determined based on the path of movement of the tracker (30).

[0116] [Example 8] The method of embodiment 7, wherein the movable part (24) is configured to move along a circle or a spiral, the movement path of the tracker (30) includes at least one section of a circle, an ellipse, or a spiral, and the position of the movable part (24) is determined based on the at least one section of the movement path of the tracker (30).

[0117] [Example 9] 9. The method according to any one of claims 2 to 8, wherein the spatial relationship comprises an orientation of the tracker (30) relative to the movable part (24).

[0118] [Example 10] Further, the method includes obtaining gravity information indicating a direction of gravity; 10. The method according to any one of claims 1 to 9, wherein the transformation is selected based on the gravity information.

[0119] [Example 11] 11. The method of claim 10, wherein the movement of the tracker is determined relative to the direction of gravity indicated by the gravity information, and the transformation is selected based on the movement of the tracker determined relative to the direction of gravity.

[0120] [Example 12] further comprising obtaining at least one setup of the medical imaging device selected from a start pose and an end pose of the movable part (24) of the medical imaging device (18) relative to a stationary part (22) of the medical imaging device (18) or a component part having a fixed spatial relationship to the stationary part (22) of the medical imaging device (18); 12. The method according to any one of the preceding claims, wherein the transformation is selected based on at least one setup of the medical imaging device (18).

[0121] [Example 13] The method of any one of Examples 12 and 10 or 11, further comprising determining the at least one setup of the medical imaging device (18) based on the tracking information, the gravity information, and the determined movement of the tracker (30), and obtaining the at least one setup of the medical imaging device (18).

[0122] [Example 14] A method according to claim 12 or 13, dependent on claim 4, wherein the side of the movable part (24) is determined based on the movement direction of the tracker (30) and the acquired at least one setup of the medical imaging device (18).

[0123] [Example 15] adjusting the side of the movable part based on the determined movement direction of the tracker (30) and the acquired at least one setup of the medical imaging device (18); (i) a first side of the movable part associated with a clockwise direction of movement of the tracker and a first setup of the medical imaging device, and (ii) a counterclockwise direction of movement of the tracker and a second setup of the medical imaging device; (i) a clockwise direction of movement of the tracker and the second setup of the medical imaging device, and (ii) a second side of the movable part associated with a counterclockwise direction of movement of the tracker and the first setup of the medical imaging device. The method of claim 14, wherein the side of the movable part (24) is determined by selecting from the

[0124] [Example 16] 16. The method according to any one of claims 1 to 15, wherein at least two or all of the plurality of different poses of the tracker (30) are associated with different points in time of an image acquisition process performed by the medical imaging device (18) to acquire the image data, during which the movable part of the medical imaging device (18) moves.

[0125] [Example 17] The method according to any one of claims 1 to 16, wherein the medical imaging device (18) is configured to acquire image data of a computed tomography (CT), and the movable part (24) of the medical imaging device (18) is part of a C-arm.

[0126] [Example 18] A system comprising at least one processor (4) configured to execute the method according to any one of the preceding embodiments.

[0127] [Example 19] The system of Example 18 further comprises at least one of the following entities: i) the medical imaging device (18) comprising the movable part (24) and configured to acquire the image data; ii) a tracking system (10), such as an optical tracking system comprising a camera, configured to provide said tracking information; iii) a gravity sensor configured to provide said gravity information, optionally fixed to said tracking system (10) and / or said camera (12); iv) said tracker

[0128] [Example 20] A computer program comprising instructions which, when executed by at least one processor (4), cause the at least one processor (4) to perform a method as described in any one of Examples 1 to 17, said computer program optionally being transmitted in a data stream or stored on a (e.g. non-transitory, computer-readable) storage medium (6).

Claims

1. 1. A method for selecting a transformation between a pose of a tracker and a coordinate system associated with image data acquired by a medical imaging device, the method being performed by at least one processor, the method comprising: acquiring tracking information indicative of a plurality of different poses of a tracker disposed fixedly relative to a movable part of a medical imaging device; determining a movement of the tracker based on the obtained tracking information; selecting one of a plurality of transformations between a pose of the tracker and a coordinate system associated with image data acquired by the medical imaging device based on the determined movement of the tracker; The method includes:

2. determining a spatial relationship between the tracker and the movable part based on the determined movement of the tracker; The method of claim 1 , wherein the one transformation is selected from the plurality of transformations based on the determined spatial relationship.

3. The method of claim 1 , wherein the spatial relationship includes a side of the movable part on which the tracker is disposed.

4. The method of claim 3 , wherein the determined movement of the tracker indicates a direction of movement of the tracker, and the side of the movable part is determined based on the direction of movement of the tracker.

5. 5. The method of claim 4, wherein the side of the movable part is determined by selecting the side of the movable part from a first side of the movable part associated with a clockwise direction of movement of the tracker and a second side of the movable part associated with a counterclockwise direction of movement of the tracker.

6. The method of claim 2 , wherein the spatial relationship includes a position of the moving part at which the tracker is located.

7. 7. The method of claim 6, wherein the determined movement of the tracker indicates a path of movement of the tracker, and the position of the movable part is determined based on the path of movement of the tracker.

8. 8. The method of claim 7, wherein the movable part is configured to move along a circle or a spiral, the path of movement of the tracker including at least a segment of a circle, an ellipse, or a spiral, and the position of the movable part is determined based on the at least a segment of the path of movement of the tracker.

9. The method of claim 2 , wherein the spatial relationship includes an orientation of the tracker relative to the moving part.

10. Further, the method includes obtaining gravity information indicating a direction of gravity; The method of claim 1 , wherein the transformation is selected based on the gravity information.

11. 11. The method of claim 10, wherein the movement of the tracker is determined relative to the direction of gravity indicated by the gravity information, and the transformation is selected based on the movement of the tracker determined relative to the direction of gravity.

12. and obtaining at least one setup of the medical imaging device selected from a start pose and an end pose of the movable part of the medical imaging device relative to a stationary part of the medical imaging device or a component part having a fixed spatial relationship to the stationary part of the medical imaging device; The method of claim 1 , wherein the transformation is selected based on at least one setup of the medical imaging device.

13. further acquiring gravity information indicative of a direction of gravity for one or more of a plurality of different poses of the tracker; determining the at least one setup of the medical imaging device based on the tracking information, the gravity information, and the determined movement of the tracker, and acquiring the at least one setup of the medical imaging device; 13. The method of claim 12, comprising:

14. the spatial relationship includes a side of the movable part on which the tracker is disposed, the determined movement of the tracker indicates the direction of movement of the tracker; the side of the movable part is adjusted based on the direction of movement of the tracker and the acquired at least one setup of the medical imaging device; The method of claim 12 wherein

15. adjusting the side of the movable part based on the determined movement direction of the tracker and the acquired at least one setup of the medical imaging device; (i) a first side of the movable part associated with a clockwise direction of movement of the tracker and a first setup of the medical imaging device, and (ii) a counterclockwise direction of movement of the tracker and a second setup of the medical imaging device; (i) a clockwise direction of movement of the tracker and the second set-up of the medical imaging device, and (ii) a second side of the movable part associated with a counterclockwise direction of movement of the tracker and the first set-up of the medical imaging device. The method of claim 14, wherein the determining step is performed by selecting the side of the movable part from

16. 2. The method of claim 1 , wherein at least two or all of the plurality of different poses of the tracker are associated with different points in an image acquisition process performed by the medical imaging device to acquire the image data, during which the movable part of the medical imaging device moves.

17. The method of claim 1 , wherein the medical imaging device is configured to acquire computed tomography, CT, image data, and the movable part of the medical imaging device is part of a C-arm.

18. 1. A system comprising at least one processor configured to execute a method for selecting a transformation between a pose of a tracker and a coordinate system associated with image data acquired by a medical imaging device, the method comprising: acquiring tracking information indicative of a plurality of different poses of a tracker disposed at a fixed position relative to a movable part of a medical imaging device; determining a movement of the tracker based on the obtained tracking information; selecting one of a plurality of transformations between a pose of the tracker and a coordinate system associated with image data acquired by the medical imaging device based on the determined movement of the tracker; A system including:

19. 20. The system of claim 18, further comprising at least one of the following entities: i) the medical imaging device comprising the movable part and configured to acquire the image data. ii) a tracking system, such as an optical tracking system comprising a camera, configured to provide said tracking information; iii) a gravity sensor configured to provide said gravity information, optionally fixed to said tracking system and / or said camera; iv) the tracker

20. 1. A non-transitory computer-readable storage medium storing a computer program comprising instructions that, when executed on at least one processor, cause the at least one processor to perform a method for selecting a transformation between a pose of a tracker and a coordinate system associated with image data acquired by a medical imaging device, the method comprising: The method acquiring tracking information indicative of a plurality of different poses of a tracker disposed at a fixed position relative to a movable part of a medical imaging device; determining a movement of the tracker based on the obtained tracking information; selecting one of a plurality of transformations between a pose of the tracker and a coordinate system associated with image data acquired by the medical imaging device based on the determined movement of the tracker; 1. A non-transitory computer readable storage medium comprising:

Images