Patent Description:
Surgical tracking systems are configured to track a surgical object such as a surgical instrument or a patient during a surgical procedure. To this end, one or more trackers are attached to the object that is to be tracked. Each tracker comprises one or more markers configured to be detected by a tracking sensor of the tracking system. Optical tracking systems, for example, use a tracking camera to capture image data representative of optically detectable markers. Based on positions of the markers in the image data, information about one or both of a position and an orientation of the tracker and, thus, of the tracked object can be determined.

The positions of the markers of a given tracker define a marker arrangement. Information indicative of these positions is typically required for the tracking system to identify the markers (and, thus, the tracker) in the image data.

Conventional trackers are mass-produced at high precision to ensure that they all have an identical marker arrangement. Therefore, all the trackers of a given tracker type have exactly the same relative positions between the markers. The tracking system thus only requires knowledge about these relative positions for a given tracker type, but not for each individual tracker. It has, however, been found that such pre-defined tracker types are not suitable for all use cases. For example, they may block a surgeon's activities or result in loss-of-sight issues.

In a surgical tracking context, the tracking camera requires a continuous line-of-sight towards the tracker that has previously been registered relative to a camera coordinate system. In case the line-of-sight is lost, the tracker can no longer be tracked and procedures relying on the tracker, such a navigation procedures, have to be paused or aborted.

A particular object may carry multiple trackers to allow for a continuous tracking even if the tracking camera loses its line-of-sight to one of the trackers. However, using multiple trackers may hinder a surgeon, i.e., may block surgical access or the surgeon's line-of-sight to an anatomical region of interest.

A technique for planning a tracker location to reduce a line-of-sight loss is discussed in <CIT>. The tracker location is determined based on at least one of pre- and inter-operative information regarding predetermined or tracked positions of objects located in an operating room.

Document <CIT> discloses devices, methods and systems for frameless guidance of image-based medical procedures such as stereotactic radiotherapy and surgery. Devices including custom fitting subject-specific articles include contoured surfaces that provide spatial reference to the location of target regions within the subject. The devices are attached exclusive of fasteners anchored to the tissue of the subject and reside on the front of a head of the subject when attached. The devices and articles can be fabricated using computer-directed fabrication technology.

Document <CIT> discloses systems and methods for stabilizing a target location within a human body. The system provides a tissue anchor for holding a tissue mass within a human body. The tissue anchor may include a lead; a tissue fastener coupled to the lead; and a marker which can be detected by a position detection system to facilitate placement of the anchor. The tissue anchor can be used to help stabilize tissue in a surgical procedure, e.g., in excising a lesion in amorphous, pliable tissue (e.g., breast tissue) or other body parts.

There is a need for a marker arrangement for tracker that avoids one or more of the above or other problems.

According to the present disclosure, there is provided a computer-implemented method, a computer program product, an apparatus and a system according to the independent claims. Developments are set forth in the dependent claims.

According to a first aspect, a method for determining a patient-specific marker arrangement for a tracker of a surgical tracking system is provided. The marker arrangement is defined by marker positions. The method comprises receiving three-dimensional image data of a patient, obtaining, based on the three-dimensional image data, planning data for a surgical intervention, and determining the marker arrangement for the tracker based at least on the obtained planning data.

The marker arrangement may be defined in various ways. In some cases, the marker arrangement is defined by relative (e.g., Euclidean distances) between the marker positions. In other cases, the marker arrangement is defined by marker positions in a tracker coordinate system.

The three-dimensional image data may have been acquired using a three-dimensional medical imaging technology, such as computed tomography (CT) imaging, magneto-resonance imaging (MRI), or a two-dimensional medical imaging technology that permits a three-dimensional image reconstruction (e.g., by taking two-dimensional X-ray images from different orientations of a C-arm).

The marker arrangement may be a (planar or non-planar) two-dimensional or a three-dimensional arrangement. In certain variants, the method may comprise generating manufacturing instructions for manufacturing the marker arrangement as determined based at least on the obtained planning data. Thus, a customized tracker comprising an optimized marker arrangement may be manufactured (e.g., optimized from an obstruction perspective). The manufacturing instructions may comprise at least one of printing instructions and instructions for additive manufacturing (e.g., for three-dimensional printing). The manufacturing instructions may be machine-readable instructions (e.g., to control a printer or additive manufacturing device) or instructions addressed to a user (e.g., how to cut out the marker arrangement from a larger sheet on which multiple markers are printed or otherwise realized).

An exemplary manufacturing approach may comprise the steps of providing a substrate with at least one reflective surface configured to reflect electromagnetic radiation, and printing an absorbent layer configured to absorb electromagnetic radiation over the substrate in such a way that the absorbent layer covers less than the entire reflective surface. The substrate and the absorbent layer may form a layer stack. The absorbent layer may be printed to realize the marker arrangement as determined based at least on the obtained planning data. The marker arrangement may additionally be determined based on at least one of a surgery setup and tracker design rules (e.g., regarding a minimum and/or maximum number of markers, minimum and/or maximum distances between markers, etc.). The marker arrangement may take the form of a pattern.

At least one of the planning data (e.g., in the form of a surgery plan) and the surgery setup may define at least one of: a field of view of a signal detector (e.g., a tracking camera), a position and/or orientation of a patient or a body part of the patient, and a position and/or orientation of a surgeon. The marker arrangement may be dimensioned to scale with the viewing angle, or a dimension (e.g., size) of the patient. The marker arrangement may be printed to fit onto a surface of the patient covered by the field of view of the tracking camera and not blocked by the surgeon.

The planning data (e.g., in the form of a surgery plan) may define at least one of an area of interest of the patient, a dedicated surgical procedure, and a trajectory of a surgical instrument. The marker arrangement may be printed to fit onto a surface of a body part that includes the area of interest. The marker arrangement may be printed to fit onto a surface of the patient that is not covered by (e.g., intersects with) the trajectory. The surface of the patient may be covered by the trajectory if a projection of the trajectory perpendicular the patient surface or along the field of view coincides with said surface of the patient.

The tracker design rules may define at least one marker or marker arrangement where the absorbent layer has an opening that does not cover the reflective surface. The tracker design rules may define at least one of a size and a shape of the at least one marker or the marker arrangement. The tracker design rules may define a minimum distance between markers. The minimum distance between markers may depend on a distance of the tracker to the tracking camera (e.g., the minimum marker distance may increase with increasing distance to the tracking camera, and vice versa). Additionally, or in the alternative, the minimum distance between markers may depend on a distance of the tracker to the anatomy that is to be tracked (e.g., the minimum marker distance may decrease with decreasing distance to the anatomy to be tracked, and vice versa).

In some variants, the method may comprise determining a surface of the patient based on the three-dimensional image data. Conventional image processing techniques such as thresholding can be used in this regard (e.g., thresholding of Hounsfield values obtained by a CT scan). In such an implementation, the marker arrangement may be determined also based on the determined surface of the patient. Determining the marker arrangement also based on the patient surface may comprise virtually arranging one or more markers on the determined surface of the patient. The marker arrangement may be determined based on the virtually arranged markers (e.g., in accordance with their virtually arranged pattern and/or distances).

Taking the surface of a patient into account may facilitate positioning of the tracker on the patient. As an example, the marker arrangement may be determined to not include markers on a protruding body part of the patient (e.g., a patient' nose) as derived from the determined surface. As a result, the tracker may be configured to have an opening or indentation to accommodate the protruding body part. In another example, the marker arrangement may be determined taking into account a surface curvature at the position where the tracker is to be placed on the patient.

In some variants, the planning data may be indicative of an anatomical region of interest of the patient. Determining the marker arrangement may comprise virtually arranging one or more markers relative to the anatomical region of interest. The markers may, for example, be arranged to surround the region of interest. The marker arrangement may be determined based on the virtually arranged markers (e.g., in accordance with their virtually arranged pattern and/or distances). Based at least in part on the region of interest, a size of the tracker and thus at least one of a size, a number and relative positions of the markers to each other may be determined. For example, the further the tracker is away from the region of interest, the larger it must be designed in size to achieve a desired accuracy. The desired accuracy may be predefined, e.g., according to a specific medical field and use-case.

In some variants, the planning data may be indicative of a planned trajectory of a surgical instrument. Determining the marker arrangement may comprise virtually arranging one or more markers relative to the planned trajectory. The marker arrangement may be determined based on the virtually arranged markers. For example, the marker arrangement may be determined in order to prevent collision of the tracker, i.e., a tracker's carrier substrate and the markers, with a surgical instrument moved along the planned trajectory during surgery. Determining the marker arrangement may comprise taking into account a margin for correction of the planned trajectory of the surgical instrument. The margin for correction may be predefined, e.g., according to a specific medical field and use-case.

In some variants, the planning data may be indicative of a planned incision.

Determining the marker arrangement may comprise virtually arranging one or more markers relative to the planned incision. The marker arrangement may be determined based on the virtually arranged markers. The planned incision may be taking into account to prevent complicating access of a surgeon to the anatomical region of interest. For example, the markers may have a predefined minimum distance to the planned incision. Determining the marker arrangement may comprise taking into account a margin for correction of the planned incision.

The planning data are indicative of a planned position of a signal generator or signal detector of the surgical tracking system relative to a patient anatomy represented in the three-dimensional image data. Such a signal generator or signal detector may be provided for generating or detecting, respectively, an optical signal (e.g., in the infrared or visible spectrum). Taking into account the planned position, a region of the three-dimensional image data that has a line-of-sight to the signal generator or signal detector may be determined based on the planning data. The marker arrangement may then be determined based on the determined region of the three-dimensional image data.

In some variants, the tracker may have a substrate supporting the marker arrangement, and the method may comprise determining a shape of the substrate based on the planning data. The manufacturing instructions described above may comprise trimming instructions, e.g., instructions on using scissors or perforations, in order to obtain a custom-shaped tracker from the substrate. The trimming instructions may comprise trimming markings printed on the tracker substrate, e.g., on a layer stack, that indicate where the substrate is to be trimmed. Alternatively or additionally, the trimming instructions may be provided on a display, e.g., in form of an image of the substrate, e.g., the layer stack, with trimming markings or an animation of how to trim the substrate. The trimming instructions may be determined based on at least one of a surgery setup, the planning data, and tracker design rules as described herein. The marker arrangement may be determined based on the trimmed tracker (e.g., such that it fits on the trimmed tracker).

In some variants, the three-dimensional image data may have been captured by an imaging apparatus (e.g., a medical imaging apparatus configured to image a patient anatomy). In such a scenario, the method may comprise receiving positional data indicative of a position of the imaging apparatus when capturing the three-dimensional image data, wherein the marker arrangement is determined based on the positional data. The position of the imaging apparatus may be determined in a coordinate system of an operating room, i.e., relative to other objects in the operating room like the signal generator or signal detector of the surgical tracking system.

In some variants, determining the marker arrangement may take into account a criterion related to a minimum distance between markers. The minimum distance between markers may depend on a distance of the tracker to the signal detector, e.g., a tracking camera (e.g., the minimum marker distance may increase with increasing distance to the tracking camera, and vice versa). Additionally, or in the alternative, the minimum distance between markers may depend on a distance of the tracker to the anatomy that is to be tracked (e.g., the minimum marker distance may decrease with decreasing distance to the anatomy to be tracked, and vice versa).

According to a second aspect, a computer program product, comprising instructions that, when executed on at least one processor, cause the at least one processor to carry out any of the method steps described herein. The computer program product may be stored on a computer-readable recording medium.

According to a third aspect, an apparatus for determining a patient-specific marker arrangement for a tracker of a surgical tracking system is provided. The marker arrangement is defined by marker positions. The apparatus is configured to receive three-dimensional image data of a patient, and wherein the planning data also are indicative of a planned position of a signal generator or signal detector of the surgical tracking system relative to a patient anatomy represented in the three-dimensional image data; to obtain, based on the three-dimensional image data, planning data for a surgical intervention; and to determine the marker arrangement for the tracker based at least on the obtained planning data.

In some variants, the apparatus may be configured to perform any of the method steps described herein.

According to a fourth aspect, a system is provided. The system comprises the apparatus for determining a patient-specific marker arrangement described herein and a manufacturing device configured to manufacture a tracker with the determined marker arrangement.

In the following description of exemplary embodiments, aspects in the context of determining a patient-specific marker arrangement for a tracker of a surgical tracking system are presented. The tracker with the patient-specific marker arrangement may be configured to be detectable by any tracking technology, in particular involving emission or reflection of electromagnetic radiation (e.g., in the visible or infrared spectrum). The same reference numerals are used to denote the same or similar features.

<FIG> schematically illustrates the planning of a surgical intervention. Details of the planned surgical intervention are defined in planning data.

In more detail, <FIG> illustrates a visualization, or representation, of three-dimensional image data <NUM> of a patient <NUM>. The three-dimensional image data <NUM> have been captured using a medical imaging technology, e.g., using an x-ray apparatus, a CT apparatus or an MRI apparatus (not shown in <FIG>).

<FIG> indicates a position of a planned incision <NUM> and several planned trajectories <NUM> of one or more surgical instruments (not shown in <FIG>) relative to the three-dimensional image data <NUM>. Corresponding positional and/or orientational information may be comprised by the planning data. Still further, the planning data may also indicate a position and orientation of a surgeon <NUM> and of a signal detector <NUM>, e.g., a tracking camera, of the surgical tracking system relative to the three-dimensional image data <NUM>. The planning data may be generated in an automated manner or responsive to a user input (e.g., by a surgeon). It will be apparent that the scenario of <FIG> is only illustrative. As an example, the planning data may only be indicative of a single planned trajectory or incision, optionally in combination with a position of at least one of the surgeon <NUM> and the signal detector <NUM>.

One or more coordinate systems may be defined for one or more of the three-dimensional image data <NUM>, the surgeon <NUM> and the surgical tracking system (e.g., relative to the tracking camera <NUM>). In some variants, the planning data is represented in such one or more coordinate systems. In particular, the planning data may be represented in a coordinate system of the three-dimensional image data (i.e., the image coordinate system). The coordinate system of the three-dimensional image data <NUM> may be defined by the medical imaging apparatus capturing the three-dimensional image data <NUM>. The planned position of the surgeon <NUM> may also be indicated in the coordinate system of the three-dimensional image data <NUM>. Alternatively, the surgeon <NUM> may be tracked by the surgical tracking system or another tracking system with a known position relative to the surgical tracking system, and the position of the surgeon <NUM> may be determined within the coordinate system of the surgical tracking system. The coordinate system of the surgical tracking system may be defined relative to a signal generator or the signal detector <NUM> of the surgical tracking system.

The individual coordinate systems may be registered with each other or with a global coordinate system, e.g., a coordinate system of an operating room, OR, in which the surgical intervention will take place. The OR may accommodate the patient <NUM>, the surgeon <NUM>, and the surgical tracking system. After one or more coordinate system registrations, the planning data (e.g., in the image coordinate system) may include the positions and/or orientations of the surgeon <NUM> and the signal detector <NUM> relative to the position of the three-dimensional image data <NUM> and relative to each other. In such or other variants, the planning data may indicate one or more of the planned incision <NUM> and at least one planned trajectory <NUM> of one or more surgical instruments relative to the position of the three-dimensional image data <NUM>.

The surgical tracking system may comprise one or more trackers, the signal detector <NUM> and, optionally, the signal generator. The one or more trackers may be attached to the patient <NUM>, and, optionally, to at least one of the surgeon <NUM> and the one or more surgical instruments. The surgical tracking system may be configured for tracking one or more of the trackers (not shown in <FIG>).

In some variants, at least one tracker with active markers (e.g., light emitting diodes) is provided that emit electromagnetic radiation to be captured by the tracking camera <NUM>. In such or other variants, at least one tracker with passive markers can be provided that reflect electromagnetic radiation to be captured by the tracking camera <NUM>. The reflected electromagnetic radiation may be emitted by a signal generator (e.g., a light source) suitably positioned in the OR.

As shown in <FIG>, the surgeon <NUM> may (virtually) be located between the signal detector <NUM> and the three-dimensional image data <NUM> (e.g., of a patient anatomy that is to be treated). Therefore, the surgeon <NUM> may partly block, or shade, a line-of-sight LOS between the signal detector <NUM> and a tracker (not shown in <FIG>) that is to be attached to the patient <NUM>. Further, the tracker that is to be attached to the patient <NUM> may complicate or block the surgeon's <NUM> access to an anatomical region of interest (not shown). For example, the tracker may block access to the planned incision <NUM> and/or at least one of the planned trajectories <NUM> when located between the surgeon <NUM> and the incision <NUM> or between the surgeon <NUM> and one of the trajectories <NUM>.

To optimize the line-of-sight LOS between the signal detector <NUM> and the tracker that is to be attached to the patient <NUM> and/or to optimize access to the region of interest, a patient-specific marker arrangement for the tracker is determined based on the planning data. The planning data will in certain embodiments be obtained based on the three-dimensional image data <NUM> of the patient anatomy and, optionally, the positions of one or more of the surgeon <NUM>, the signal detector <NUM>, the planned incision <NUM> and one or more of the trajectories of <NUM> relative to the three-dimensional image data <NUM>.

A method for determining a patient-specific marker arrangement for a tracker of a surgical tracking system is described in the following with reference to <FIG>.

<FIG> shows a flow diagram of a method <NUM> for determining a marker arrangement for a tracker of a surgical tracking system based on planning data. The planning data may have been derived as discussed above with reference to <FIG> or otherwise.

In a first step <NUM>, three-dimensional image data <NUM> of a patient <NUM> is received. As described herein, the image data <NUM> may be captured via a medical imaging apparatus. The image data <NUM> may be received from the image apparatus or from a memory, e.g., a database storing the image data <NUM>. In many variants, the three-dimensional image data <NUM> of the patient <NUM> will be representative of a patient anatomy, e.g., a patient's spine or one or more vertebrae thereof.

In a second step <NUM>, planning data for a surgical intervention are obtained based on the received image data <NUM>. The planning data may comprise an indication of at least one of i) an anatomical region of interest of a patient <NUM> (e.g., a certain bone such as a dedicated vertebra or portion thereof), ii) one or more planned incisions <NUM>, iii) one or more planned trajectories <NUM> for one or more surgical instruments, iv) a planned position and/or orientation of a surgeon <NUM> relative to the three-dimensional image data <NUM>, v) a planned position and/or orientation of a signal generator and/or a signal detector <NUM> of a surgical tracking system relative to the three-dimensional image data <NUM>. The planning data may be provided in a coordinate system of the three-dimensional image data <NUM>. Different variants of planning data contents are explained in more detail below with reference to <FIG>.

In a third step <NUM>, a marker arrangement for the tracker of the surgical tracking system is determined based at least in part on the obtained planning data. Different variants for determining the marker arrangement are explained in more detail below, with reference to <FIG>, <FIG>, <FIG>, <FIG> and <FIG>.

<FIG> schematically shows the three-dimensional image data <NUM> of the patient <NUM>. The three-dimensional image data <NUM> may be determined via image processing of the image data received in the second step <NUM> of <FIG>. The anatomical region of interest (not shown) may be determined manually by the surgeon <NUM> or automatically, for example via image recognition techniques. The three-dimensional image data <NUM> of the patient <NUM> in the scenario of <FIG> may be representative of at least one of the patient's vertebrae 102A (or any other bone) and/or of a skin surface 102B in a region of these one or more vertebrae 102A. The one or more vertebrae 102A may be in need of a surgical intervention (e.g., placement of one or more pedicle screws.

<FIG> schematically shows planned trajectories <NUM> of a surgical instrument in the three-dimensional image data <NUM> of <FIG>. The trajectories <NUM> may be planned preoperatively, for example to define how pedicle screws are to be inserted into a set of vertebrae 102A. The planning data may define an extension of the trajectories <NUM> in a coordinate system of the three-dimensional image date <NUM>. There exist commercial software tools for such planning procedures.

The trajectories <NUM> may be planned in a patient-specific manner relative to the three-dimensional image date <NUM> of the vertebrae 102A. Additionally, or in the alternative, the trajectories <NUM> may be planned in an indication-specific manner. For example, the trajectories <NUM> may be planned based at least in part on the three-dimensional image data <NUM> of the patient <NUM> and a preferred treatment for the patient <NUM>, e.g., based on the medical history of the patient or a set of patients. The trajectories <NUM> may further be planned based at least in part on the preferences of the surgeon <NUM>.

<FIG> schematically shows a patient tracker <NUM> virtually superimposed on the three-dimensional image data <NUM>. The exemplary tracker <NUM> of <FIG> is at least partially flexible and virtually attached to the back of the patient <NUM>. The tracker <NUM> comprises a tracker substrate <NUM> and multiple markers <NUM>. The multiple markers <NUM> form a marker arrangement that has been determined based on the three-dimensional image data <NUM> and the planned trajectories <NUM> of <FIG> (see also step <NUM> in <FIG>). The marker arrangement may be defined by Euclidean distances between positions of pairs of markers <NUM> or by absolute positions in a coordinate system of the tracker <NUM>. This coordinate system may have its origin in one of the markers <NUM>.

For example, the marker arrangement has been determined based on a rule that each marker <NUM> must have a minimum distance from each trajectory <NUM> of the surgical instrument. Additionally, such a rule may also apply to the shape of the tracker substrate <NUM>. The minimum distances may be based at least in part on a margin for correction of the planned trajectories <NUM>. Additionally or alternatively, the markers <NUM> may have a minimum distance from each other. The minimum distance may be based at least in part on a distance and orientation of the tracker <NUM> relative to the anatomical region of interest, e.g., one or more vertebrae 102A in which the pedicle screws are to be placed. For example, the larger the distance between the tracker <NUM> and the anatomical region of interest, the larger the minimum distance between the markers <NUM> may be defined. One or more of the markers <NUM> shown in <FIG> may be virtually arranged relative to at least one of a surface of the patient <NUM>, the planned trajectories <NUM>, the planned incision <NUM> (not shown in <FIG>) and the anatomical region of interest (not shown in <FIG>). Based on such a virtual arrangement, manufacturing (e.g., printing) instructions for manufacturing one or more of the marker arrangement and the tracker substrate <NUM> may be generated. The above considerations also apply to any of the markers <NUM>, <NUM>, <NUM>, <NUM> shown in the <FIG>, <FIG>, <FIG> and <FIG> described below.

<FIG> schematically shows the three-dimensional image data <NUM> of the patient <NUM> and a planned position (denoted by an X) of the surgeon <NUM> relative to the three-dimensional image data <NUM>. This planned surgeon position relative to the three-dimensional image data <NUM> (e.g., relative to a patient bone such as a vertebra 102A to be treated by the surgeon) is defined in planning data (step <NUM> of <FIG>) and forms the basis for determining the marker arrangement (step <NUM> of <FIG>).

<FIG> schematically shows a tracker <NUM> superimposed on the three-dimensional image data <NUM> of <FIG> with the associated marker arrangement as determined based on the planning data. As can be seen in <FIG>, the marker arrangement is located in a portion (here: the skin surface 102B) of the patient anatomy indicated in three-dimensional image data <NUM> defined such that a line-of-sight and access from the planned position of the surgeon <NUM> to the anatomical region of interest is improved, i.e., not blocked by the tracker <NUM>. For example, the markers <NUM> may be configured to be arrangeable (with the tracker <NUM>) on a side of the patient bone to be treated, that faces away from the surgeon so as to avoid obstruction issues.

<FIG> schematically shows the three-dimensional image data <NUM> of the patient <NUM> and a planned position and orientation of the signal detector <NUM> of the surgical tracking system relative to the three-dimensional image data <NUM>. Based on the location and orientation of the signal detector <NUM>, a line-of-sight of the signal detector <NUM> may be determined. A part of the three-dimensional image data <NUM>, e.g., a skin surface 102B of the patient <NUM>, is determined as a suitable marker location based on the line-of-sight of the signal detector <NUM>. Non-shaded regions of the skin surface 102B are illustrated in <FIG> as dotted regions and define suitable locations for markers (e.g., in the planning data). Shaded regions of the skin surface 102B are no suitable locations for markers since the signal detector <NUM> has no line-of-sight so such locations. The marker arrangement may thus be determined based on the planning data such that no markers need to be placed in the shaded regions.

<FIG> schematically shows a tracker <NUM> superimposed on the indicated region of <FIG>. The tracker <NUM> comprises a marker arrangement determined based on the line-of-sight of the signal detector of <FIG> (i.e., the planning data) so that none of the markers <NUM> (on the common substrate <NUM>) needs to be placed in a shaded region. In particular, each marker <NUM> is determined to be located in the non-shaded region of the three-dimensional image data <NUM>.

The size of the markers <NUM> and/or the distance between the markers <NUM> may be determined based at least in part on a distance between the signal detector <NUM> and the three-dimensional image data <NUM> (e.g., a skin surface 102B of the patient where the markers <NUM> are eventually to be placed). Larger distances between the signal detector <NUM> and the three-dimensional image data <NUM> may require larger sizes of the markers <NUM> and/or larger distances between the markers <NUM> (and vice versa).

Further, the marker arrangement may be determined based on an anatomical region of interest of the patient <NUM>. In the example of <FIG>, the anatomical region of interest may be a vertebra 102A of the patient <NUM>, and the marker arrangement is thus located on the back of the patient <NUM> to be located on both sides of the patient's spine but not above one of the vertebrae 102A to be treated. The size of the markers <NUM> and/or the distance between the markers <NUM> may be determined based at least in part on a distance between the tracker <NUM> and the anatomical region of interest (e.g., the distance between the tracker <NUM> and the vertebrae to be treated). Larger distances between the tracker <NUM> and the anatomical region of interest may require larger sizes of the markers <NUM> and/or larger distances between the markers <NUM> (and vice versa).

<FIG> schematically shows the three-dimensional image data <NUM> of the patient <NUM> with planned positions of both the surgeon <NUM> and the signal detector <NUM> (as defined in the planning data). Analogously to <FIG>, <FIG> schematically shows the three-dimensional image data <NUM> of <FIG> with an indication of a region suitable for arranging markers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> therein, i.e., a non-shaded region in the line-of-sight of the signal detector <NUM> based on its planned position and orientation relative to the three-dimensional image data <NUM>. In comparison to <FIG>, the position of the surgeon <NUM> relative to the three-dimensional image data <NUM> (and thus also to the signal detector <NUM>) has also been taken into account for determining the line-of-sight of the signal detector <NUM> (i.e., shaded locations resulting from the surgeon's planned position). In particular, the line-of-sight from the signal detector <NUM> to the side of the three-dimensional image data <NUM> closer to the position of the surgeon <NUM> is blocked by the surgeon in the example of <FIG>.

<FIG> schematically shows a tracker <NUM> superimposed on the indicated region of <FIG>. The tracker <NUM> comprises a marker arrangement determined based on the line-of-sight of the signal detector <NUM> of <FIG> so that the markers <NUM> on the substrate <NUM> can be located in non-shaded locations.

<FIG> schematically shows the three-dimensional image data <NUM> of the patient <NUM> with an indicated region suitable for arranging markers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> therein. The indicated region shown in the example of <FIG> has been determined based on the three-dimensional image data <NUM> alone, with no restrictions taken into account.

<FIG> schematically shows the three-dimensional image data <NUM> of <FIG>, with the indicated region being restricted based on planned trajectories <NUM> of the surgical instrument. In this example, an circular area around each of the trajectories <NUM> is to be kept free, i.e., not indicated as a suitable region for arranging markers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> therein. The size of the area that is to be kept free may be determined based on at least one of the surgical use-case, patient data, preferences of the surgeon <NUM>, and a margin for correction of the planned trajectories <NUM>. The trajectories <NUM> relative to the three-dimensional image data <NUM> may be comprised by the planning data.

<FIG> schematically shows the three-dimensional image data <NUM> of <FIG>, with the indicated region being further restricted based on the planned position of the surgeon <NUM> relative to the three-dimensional image data <NUM>. Analogously to <FIG> and <FIG>, the side of the three-dimensional image data <NUM> located closer to the position of the surgeon <NUM> is not suited for a marker position, since the tracker <NUM>, <NUM>, <NUM>, <NUM>, <NUM> then may block the line-of-sight of the surgeon <NUM> (or may block access to the anatomical region of interest).

<FIG> schematically shows the three-dimensional image data <NUM> of <FIG>, with the indicated region being further restricted based on the planned position and orientation of the signal detector <NUM>. In the example shown in <FIG>, analogously to the example shown in <FIG>, the line-of-sight of the signal detector <NUM> is partly blocked by the surgeon <NUM> and the region of the three-dimensional image data <NUM> that is thus not in the line-of-sight of the signal detector <NUM>, is not suited as a position for a marker <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

<FIG> schematically shows a tracker <NUM> virtually superimposed on the indicated region of <FIG>. The tracker <NUM> comprises a marker arrangement <NUM> determined based on the line-of-sight of the signal detector <NUM> of <FIG> and on the planned trajectories <NUM>.

When determining the marker arrangement, a minimum number of markers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> comprised by the marker arrangement may be taken into account. This minimum number may be defined in tracker design rules. For example, the minimum number may be <NUM>, <NUM>, <NUM>, <NUM> or higher.

In some of the above examples, it has been explained how a marker arrangement can be determined in view of certain constraints, with the objective to avoid obstructions for either the surgeon <NUM> or the signal detector <NUM>. Once the marker arrangement has been determined, it may become necessary to define a suitably shaped tracker substrate <NUM>, <NUM>, <NUM>, <NUM>, <NUM> configured to accommodate the tracker arrangement. The shape of the tracker substrate <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be determined based on the marker arrangement (e.g., so as to accommodate each marker of the marker arrangement). There may exist additional constraints for the substrate <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, such as defining an opening or indentation in locations defined by the trajectories <NUM> (see <FIG> and <FIG>) or the incision <NUM> (see <FIG>) or relative to an anatomical region of interest (e.g., a patient bone such as a vertebra 102A). Such and further constraints may be defined in tracker design rules).

Once the marker arrangement and, optionally, the shape of the tracker substrate have been determined, instructions may be provided to the surgeon <NUM> regarding proper placement of the marker arrangement relative to the patient <NUM> (see <FIG>, <FIG>, <FIG>, <FIG>, <FIG>). The instructions may be visually output relative to the three-dimensional image data of the patient <NUM> (see, e.g., <FIG>).

The method <NUM> for determining a patient-specific marker arrangement <NUM>, <NUM>, <NUM>, <NUM>, <NUM> for a tracker <NUM>, <NUM>, <NUM>, <NUM>, <NUM> of the surgical tracking system described herein may further comprise a step of generating manufacturing instructions for manufacturing the determined marker arrangement <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In some variants, the manufacturing instructions may comprise at least one of printing instructions and instructions for additive manufacturing. For example, at least a part of the tracker may be manufactured via 3D printing. The 3D-printed tracker may define positions (e.g., comprise support pins) for accommodating the markers (e.g., with the markers being configured as reflecting spheres).

In some variants, the tracker <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, in particular the tracker substrate <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, may comprise a layer stack that may be printed based at least in part on the determined marker arrangement <NUM>, <NUM>, <NUM>, <NUM>, <NUM> (e.g., to directly print a custom shaped tracker <NUM>, <NUM>, <NUM>, <NUM>, <NUM>). In some variants a standardized layer stack may be printed and the manufacturing instructions may comprise trimming instructions to obtain a custom-shaped tracker <NUM>, <NUM>, <NUM>, <NUM>, <NUM> with the determined marker arrangement <NUM>, <NUM>, <NUM>, <NUM>, <NUM> by trimming the printed layer stack.

In the variant shown in <FIG>, the manufacturing instructions comprise trimming, e.g., cutting, instructions for a user to indicate how to obtain the custom-shaped tracker <NUM> with the determined marker arrangement <NUM> from the substrate <NUM>, e.g., a standardized printed layer stack. In particular, <FIG> shows trimming instructions in the form of trimming markings printed on the substrate <NUM>, that indicate where the substrate <NUM> is to be trimmed to obtain the tracker <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. Alternatively or additionally, the trimming instructions may be provided on a display, e.g., in form of an image of the layer stack with trimming markings or an animation of how to trim the substrate.

Claim 1:
A computer-implemented method (<NUM>) for determining a patient-specific marker arrangement (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) for a tracker (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) of a surgical tracking system, wherein the marker arrangement is defined by marker positions, the method comprising
receiving (<NUM>) three-dimensional image data (<NUM>) of a patient;
obtaining (<NUM>), based on the three-dimensional image data, planning data for a surgical intervention; and
determining (<NUM>) the marker arrangement for the tracker based at least on the obtained planning data;
characterized in that the planning data also are indicative of a planned position of a signal generator or signal detector (<NUM>) of the surgical tracking system relative to a patient anatomy represented in the three dimensional image data (<NUM>).