that is at a second angle 162 relative to the axis 140. As illustrated in FIG. 2A, the first angle 154 at the position 134i and the angle 162 at the second position 134n may differ. Accordingly, as discussed further herein, image projections acquired at the first position 134i may differ, such as not being aligned along the Z-axis, from those acquired at the second position 134n.

Additionally, with reference to FIG. 2B, the gantry 134 may also be at other orientations at the first position 134i and the final position 134n. The console 130 and the gantry 134 may be viewed from a top view, as illustrated in FIG. 2B, and viewed relative to an axis 168 that extends perpendicular to the axis 140 through the subject, but parallel to the base or floor 146. The gantry 134 in the first position 134i may extend along an axis 172 that forms a first angle 176 relative to the axis 168 and the gantry 134 at the second position 134n may extend along or oriented along an axis 180 that extends at a second angle 184 relative to the axis 168. As illustrated in FIG. 2B, the angle 176 may differ from the angle 184 and/or position the gantry 134 at a different orientation relative to the subject 28, such as the axis 168.

As illustrated in FIGS. 2A and 2B, therefore, it is understood that the gantry 134 may be at a different positions relative to the subject 28 to acquire a plurality of image projections of the subject 28. The different projections according to the different orientations may, therefore, differ and not be initially aligned relative to each other, as discussed further herein.

It is also understood, however, that various other movements of the gantry 134 may occur during an acquisition of projections at a plurality of positions. For example, the gantry may move, such as “wobble” due to a weight distribution of the gantry 134 relative to the based 130. Further, the gantry 134 may move in other ways due to movement of the gantry 134 to acquire the plurality of projections along a length or along the axis 140 of the subject 28. Accordingly, the movement of the gantry 134 as illustrated in FIGS. 2A and 2B is merely exemplary of a process discussed further herein.

With continued reference to FIGS. 1, 2A, and 2B, the subject or patient tracker (DRF) 98 may be tracked relative to the subject 28 for a selected period with the navigation system 29 including the relevant or appropriate tracking system. The tracking system including the respective localizers 50, 100 may be used to track the DRF 98 during a selected procedure and/or for a period of time. Further, the tracking systems may track other appropriate tracking devices, such as the tracking device 40 associated with the instrument 20. In addition, a tracking device, also referred herein to as an imaging tracking device 190, may be associated with the gantry 134. The tracking device 190 may be tracked with the respective tracking system included with the navigation system 24. Therefore, the position of the gantry 134 may be determined with the navigation system 24, similar to determining a position of the various other instruments or portions with respective tracking devices. In various embodiments, for example, the tracking device 190 may be tracked relative to the subject tracking device 98 during the selected procedure, such as acquiring image data of the subject 28. This allows the position of the gantry 134 to be determined relative to the subject 28, where the position of the subject 28 may be determined based upon the position of the patient tracker 98.

The position of the gantry 134, therefore, may be determined at a plurality of positions relative to the subject 28. As discussed above, the gantry 134 may move between at least the positions 134i and 134n and/or other selected or determined intermediate positions. The position of the gantry 134 may be tracked at both the gantry positions 134i and 134n. Thus, the position, which includes location and orientation of the gantry 134, may be determined at a plurality of positions relative to the subject 28. As discussed above, image data may be acquired of the subject with the imaging system 31 at the plurality of positions of the gantry 134. The positions of the gantry may change, such as by the various angle and/or translational position relative to the subject 28.

With continued reference to FIGS. 1-2B and additional reference to FIG. 3, the imaging system 31 may acquire a plurality of projections through the subject 28. In various embodiments, the imaging system 31 may include a x-ray imaging system that may acquire a plurality of projections of the subject 28. The imaging device 31 may acquire a plurality of projections that may be two-dimensional projections and/or three-dimensional image or image data. For example, image data at each position may include a three-dimensional image projection or reconstruction based on a plurality of two-dimensional image data.

Further, the plurality of projections may be combined in a selected manner, as discussed further herein, to generate a selected image such as a three-dimensional image, a long film image, or the like. In various embodiments, a long film image may be an image that is longer than what is acquired with a single acquisition projection through the subject 28 with the imaging system 31. Accordingly, a plurality of projections or images may be combined to form a combination image in a selected manner, such as “stitched” together to form a long film.

With reference to FIG. 3, for example, the imaging system 31 may acquire a first image projection 200 and a second image projection 204. Both of the image projections may have a selected dimension 208. The dimension 208 may be about 8 centimeters (cm) to about 20 cm, including about 12 cm to about 16 cm, and further including about 15 cm. The selected projections 200, 204 may have the dimension 208 that is determined or a maximum dimension based upon the imaging system 31. For example, a detector of the imaging system 31 may have a maximum dimension that is the dimension 208 or allows a maximum projection dimension of 208. Any single one image projection of the subject 28 may be limited by the physical dimensions of the imaging system 31.

As illustrated in FIG. 3, the two projections 200, 204 may include selected portions of the subject, such as vertebrae 36. For example, the first projection 200 may be acquired at the position 134i. The projection 200 may include selected vertebrae such as a first vertebrae 361, 362, and 363. The three vertebrae 361, 362, and 363 may be viewed in the projection 200 that is acquired with image data that may be acquired with the gantry 134 in a selected first position, for example the position 134i. The second projection 204 may include a selected number of vertebrae, such as the vertebrae 362 and 363. The second projection 204 may further include the vertebrae 364. Accordingly, the second projection 204 may include additional image data that is then included in the first image 200. Further, the second image 204 may not include image data that is in the first projection 200, such as a portion of a sacrum S of the subject 28. As discussed further herein, the two images 200, 204 may be stitched together in a selected manner, as discussed further herein, to obtain an image that includes a dimension greater than the dimension 208.

The two images 200, 204 are acquired with the gantry 134 that may be initially selected to have only be linearly displaced from each other, such as along the axis 140 (Z-axis). Movement of the gantry 134, however, may create the projections through the subject 28 at slightly different positions that are not only due to a direct translation or straight translation along the axis 140. As discussed above, the gantry 134 may include a plurality of positions relative to the subject 28, such as due to a movement at an angle relative to the axis 140 and/or the surface 146. As discussed above, the gantry 134 may be moved to selected positions including the first position 134i and the second position 134n that are not a straight linear translation along the axis 140. Therefore, while the two projections 200, 204 may initially include image portions that are along the axis 140 they may differ from one another, such as due to or by a distance 220 exemplary illustrated between a first line or plane 224 and a second line or plane 228. It is understood that the distance 220 is merely exemplary, and that the difference between the two projections 200, 204 may be due to any movement of the gantry 134 including a vertical translation away from or toward the surface 146, a rotation or angle change relative to the axis 140 and/or the surface 146, or other appropriate difference. Therefore, the gantry 134 may not only move in a linear translational along the axis 140 to acquire the multiple projections 200, 204, but may include other movement relative to the axis 140. The relative movements may generate projections that include the spatial difference 220 between the two projections 200, 204.

It is understood that the number of projections may be any appropriate number of rejections such as one or at least one projection acquired at each position of the gantry 134. Therefore, the illustration of two projections 200, 204 is merely exemplary for the current discussion. Further the type of movement may be a complex movement that includes both a rotational movement and a translational movement other than along the axis 140. For example, the gantry 134 may both rotate relative to the axis 140 and include a vertical movement relative to the surface 146 and the axis 140. Accordingly a complex or plurality of movements may occur during the acquisition of a plurality of image projections of the subject 28 with the imaging system 31.

In various embodiments, however, the navigation system 29 may track or determine a position of the gantry 134 at each or during each image acquisition or image data acquisition. During the acquisition of the first projection 200, the gantry tracking device 190 may be tracked and the position determinative of the navigation system 29. Similarly during the acquisition of the second projection 204, the gantry tracking device 190 may be tracked to determine a second position of the gantry 134 during the acquisition of the second projection 240. The position of the imaging portion of the gantry 134 may be known relative to the position of the gantry 134, therefore the position of the gantry that is tracked may be used to determine the position at various acquisitions.

As noted above, the image projection 200 or image data 200 may be acquired with the imaging system 31. The projection 200 may be acquired with a gantry at the initial position 134i. As discussed above the imaging system 31 may include the imaging system tracker 190. The imaging system tracker may be tracked with a navigation system 29 to allow for a determination of a pose or position of the gantry 134 relative to the patient tracking device 98 during acquisition of the projection 200. The position during the acquisition of the projection 200 may be noted and saved with the navigation system 29 and/or appropriate system, such as with a memory associated with a processor system 70, such as a memory 71. Therefore, the image projection 200 may have a determined and stored pose of the gantry 134 during the acquisition thereof. Similarly, the tracking system may track a position of the gantry 134 during the acquisition of the second projection 204, such as in the position 134n. The position of the gantry 134 during the acquisition of the second projection 204 may also be stored with the imaging system or the navigation system 24, as discussed above.

As is understood by one skilled in the art, the imaging system tracking device 190 may be tracked as may be the patient tracking device 98. Accordingly, a position or pose of the gantry 134 may be determined relative to the patient tracker when in the first position, such as schematically illustrated by the line 240 and a position when the gantry is in the second position 134n may be determined such as schematically illustrated by the line 244. Thus, the position of the gantry 134 during the acquisition of the plurality of projections through the subject 28 may be known or determined during the acquisition of the projections. Each of the projections, such that the projections 200, 204 may include the position information regarding the gantry 134, as noted above. It is understood, as discussed above, that the number of acquisitions and/or positions of the gantry may be any appropriate number and two is illustrated merely for the current discussion. Further, it is understood that any appropriate amount of position data and projections may be acquired at any one of the positions of the gantry relative to the subject 28.

With reference to FIG. 2A and FIG. 2B, at each position of the gantry for acquisition of image data 134i, 134n the position of the patient tracker 98 may be determined and the tracked position of the imaging system tracker 190 may be determined. Accordingly, the schematic line 240 may be understood to be a transformation or a correlation of a tracked position of the patient and a tracked position of the imaging system at the first position 134i. Similarly the schematic line 244 may be understood to be a tracked position of the patient relative to or transformed to a tracked position of the imaging system at the second position 134n. Accordingly, a determination of a position of the imaging system relative to a reference, such as the axis 140, and the subject 28, may also be determined at each of the gantry positions. This also allows for each of the projections 200, 204 to be registered to the subject 28, as noted above, at least because the position of the imaging system and the subject 28 are determined and known for each projection 200, 204.

The position of the gantry 134 at the first position 134i may include or be determined relative to the axis 150 and may be an orientation of the gantry 134 relative to the axis 140 at the angle 154. The axis 150 of the gantry 134 may be tracked to determine with the navigation system 29 while acquiring the projection 200. Similarly, the axis 158 may be determined relative to the axis 140 by tracking the gantry 134 by acquiring the second projection 204. The angle 162 may include or illustrate a portion of the position information relative to the axis 140 and may identify a difference of position between the first position of the gantry 134i and the second position of the gantry 134n. It is understood, however, that the gantry tracking device 190 may allow for tracking of the gantry 134 and all position information.

At each of the positions, therefore, a transformation of the tracked position of the patient with the patient tracker 98 to the tracked position of the gantry 190 may be made. As illustrated in FIGS. 2A and 2B, therefore, two transformation or correlations of the position of the patient and the position of the gantry 134 may be determined. Further, at each of the positions of the gantry 134i, 134n a transformation or correlation of a position of the gantry 134 relative to a reference, such as the axis 140 may be determined therefore a first transformation or position may be determined at the first gantry position 134i and a second transformation or position may be determined at the second gantry position 134n.

According to various embodiments, therefore, a transformation of the position of the gantry during the acquisition of the projections may be made. The projection 200 may be transformed to the position of the image 204, or vice versa. Such a process allows for combining various images and correcting for or removing distortion or error due to a non-linearity of the acquisition of the images. The transformation allow for aligning the two projections, 200, 204 as discussed further herein.

For example, a transformation of the first position of the gantry to the second position of the gantry may be determined such as using auto-registration systems or techniques such as those included with the StealthStation® S8 navigation system, sold by Medtronic, Inc. having a place of business in Minnesota. For example, with reference to Equation 1:

$\begin{matrix} T_{ISiISn} = T_{ISRi} * {(T_{ISPi})}^{- 1} * (T_{ISPn}) * {(T_{ISRn})}^{- 1} & Eq . 1 \end{matrix}$

- Where T_ISiISnrepresents a transformation of the position of the gantry at the first position 134i (ISi) and the position of the gantry of the second position 134n (ISn). This may be determined by combining the various correlations or transformations of the patient to the imaging system. This may include a product of T_ISRithat is a determination or transformation of the tracked first position of the gantry and the determined position of the gantry, an inverse of T_ISPithat is a transformation of a tracked position of the patient and a tracked position of the imaging system at a first position, a transformation T_ISpnof a tracked position of the patient at the second position to a tracked position of the imaging system at the second position, and an inverse of T_ISRnwhich is a position of the imaging system at the second position transformed to a tracked position of the imaging system.

With continued reference to FIGS. 1-3 and additional reference to FIG. 4, a method 250 of acquiring image data, such as the first projection 200 and the second projection 204 and determining and/or correcting for movement or positions of the gantry 134 that is in addition to a predetermined selected movement, such as a linear or straight translation relative to a reference, such as the translation along the axis 140 (Z-axis). It is understood that the gantry 134 includes various imaging systems, as described above, and therefore reference to movement or the positioning of the gantry may be understood to include and/or refer to movement or positions of the imaging portions that acquire the image data of the subject 28.

The method 250 may be initiated by the user 24 in a start block 254. The start block 254 may include various procedures, such as preparing the subject 28, attaching the patient tracker 98 relative to the subject 28, initiating the navigation system 29, or other appropriate procedures or steps. Nevertheless in the start block 254 the method 250 may be initiated to acquire in a line image data.

The method 250 includes various processes or steps that may be further subdivided such as a first subroutine or process of an initial registration and image acquisition subprocess or subroutine 258. The subprocess 258 may include various portions that may occur in any appropriate manner and/or order is understood by one skilled in the art. The subject 28 may be tracked at the initial gantry position 134i in block 262. The tracking of the subject at the initial position in block 262 may include tracking the subject tracker 98 when the gantry 134 is in the initial position 134i. Additionally, the imaging system may be tracked at the initial position in block 266. Tracking the imaging system (IS) at the initial position may also be performed with the navigation system 29 that includes the various tracking systems and may be used to track the IS tracking device 190. Accordingly, the subject 28 may be tracked at the initial position in block 262 and the imaging system 31 may be tracked in the initial position of block 266. The imaging system 31 may acquire image data at the initial position in block 270. The initial image data projection may include the projection 200 as discussed above. Accordingly, the image data may be acquired at a tracked position of the subject 28 and a tracked position of the imaging system 31, including the gantry 134. Thus, the image data is acquired at the initial position 134i may have associated therewith the tracked position of the subject block 262 and the tracked position of the imaging system in block 266.

As discussed above, the imaging system 31 may be used to acquire a plurality of projections of the subject. As illustrated in FIG. 4 the process 250 exemplary illustrate the acquisition of initial image data in block 258 and a second subprocess to acquire subsequent image data referred to as “n”. It is understood that any appropriate number of projections may be acquired and that a second image acquisition subprocess 274 may refer to a second or any appropriate number of image projections. Accordingly, the process 250 may be used to acquire a selected number of projections to generate a final image volume of a selected dimension, such as length, of the subject 28. In various embodiments, as discussed above, the vertebrae 36 may be imaged and the length of the image may be selected to include a selected number of the vertebrae 36 of the spinal column of the subject 28. It is further understood that an image acquisition may be used to acquire additional image data, such as portions of a pelvis, sacrum S, leg bones, or the like. Nevertheless the subprocess 274 may include the acquisition of any appropriate number of projections in addition to the initial projection.

The subprocess 274 may include tracking the subject at a position n in block 278. The position n may be any appropriate position other than the initial position, such as a position after the initial position and may include the second position, a third position, or any appropriate position. As used herein, therefore, “n” may refer to any appropriate position. The imaging system may be tracked at the position n at block 282. The imaging system 31 may acquire image data of the subject in block 286. The position of the imaging system in block 282 may be the position of the gantry 134n as discussed above. As exemplary illustrated in FIGS. 2A and 2B, the imaging system may have an initial position 134i and a second position or “n” position 134n. The subprocess 274 may include the acquisition of the image data at the 134n position such as the second projection 204, while tracking the subject 278 in block 278 and the gantry in block 282. Accordingly, the imaging system 31 may be used to acquire a plurality of projections such as an initial projection and other, including subsequent, projections with a tracked position of the gantry 134 and the subject 28 during the acquisition of the image data.

As discussed herein, due to the tracking of the imaging system, including the gantry 134 and the subject 28 while acquiring the image data, the acquired image data projections are registered to the subject. Thus, the image data 200, 204 may be used to define a coordinate system that is registered (i.e., transformed) to a subject space defines by the subject 28.

The tracked information regarding the subject 28 and the gantry 134 may be used to determine movement of the imaging system, generate a stitched or combined model or image of the subject 28 based on or including a plurality image data acquisition at a plurality of positions of the imaging system including the gantry 134. The tracked information and the acquired image data may then be further analyzed at any appropriate time, such as after each of the individual image data acquisitions. It is further understood, the process 250 need not occur in the specific order of acquiring the image data and the subprocesses 258, 274 prior to any registration and/or transformation, such as generated in a transformation subprocess 290.

The subprocess 290 may be referred to as a transformation or correction process by evaluating the tracking information of the subject and the imaging system of the various subprocesses 258, 274 discussed above. For example, an evaluation of the tracked position of the imaging system at position i may be made, such as its position relative to a selected reference, which may include the axis 140 in block 294. Further, an evaluation of the tracked position of the imaging system may be made relative to the subject at position i in block 298. The evaluation of the tracked position of the imaging system at the initial position i relative to the subject 28, such as with the patient tracker 98, may be used to determine a transformation of the imaging system to the subject T_ISPiin block 302. The transformation T_ISPimay be made to transform a position of the imaging system, such as the gantry 134 to the subject 28. The transformation T_ISPimay optionally be stored for further recall or later recall in block 304. The initial imaging system to patient transformation may be used as a reference for later transformations. In other words, later image data may be aligned to the initial image data for further processing, such as combining the plurality of image data. It is understood, however, that any appropriate transformation may be used as a reference as discussed further herein.

Further in the transformation subprocess 290, an evaluation regarding the tracked position of the subject and the imaging system at the position n may also be made in block 310. An evaluation of a tracked position of the imaging system relative to a reference, such as an imaging plane including the axis 140 may also be made in block 314. A transformation T_ISPnmay be determined of the imaging system (IS) to the subject at the second or other position “n” when the second image data is collect in block 318. The transformation may be similar to that discussed above regarding the initial position, but regarding the later or second positions, as noted above, identified as “n”. After determining the transformation of the imaging system to the subject at the position n, the transformation may be stored, optionally, in block 320. As discussed above, storing various data may include storing in the memory system 71, and/or any appropriate memory system. The selected processing system, such as the processor system 70, may then recall the data from the selected memory for further analysis, such as by executing selected instruction that may be included in various applications, programs, or the like.

With the evaluated and/or stored data regarding the tracked positions of the imaging device and the subject for each of the image data acquisitions, transformations regarding the position of the gantry 134 to the tracked position may be made based upon an evaluation of the position of the imaging system at positions i and n to a selected reference, such as from blocks 294, 314. Thus, a determination of a transformation of the imaging system to the reference axis or portion at the initial position T_ISRimay be made in block 324 and a determined transformation of the imaging system to reference axis or plane at the second or other positions T_ISRnmay be made at block 328. Again, the transformations may be optionally stored in a selected memory system in block 332. In this manner, the transformations of the imaging system, such as the gantry 134, to the subject may be determined and/or recalled to allow for a correction of a unselected misalignment of the plurality of image data at the various positions relative to each other. Thus, transformations of the imaging system to a reference axis at the various positions, such as i and n, (T_ISRiand T_ISRn) may be used for further analysis of the image data.

Based upon the various transformations discussed above, the image data acquired at the various positions may be aligned to allow for a stitching or combination of the various images to a combined or final image or image data that is a long image relative to the individual projections. As discussed above and illustrated in FIG. 3, each of the projections 200, 204 may include a selected length 208 of the subject. The length 208 may be substantially identical to the two images based upon the position of the image or gantry 134 and various geometries thereof. Further, each of the image projections may include a selected overlap 208′ that may overlap between subsequent or selected images. The combination of the images may include a total length that is greater than an initial length, such as a combined or stitched image 340, as illustrated in FIG. 5, and which may also be illustrated as the image 84. The image 340 may include a dimension 344 that is generally along the same axis as the dimension 208 of the projections 200, 204. The dimension 344 may be greater than the dimension 208 and may include all selected portions of the subject 28 as selected by the user 24. For example, the composite of stitched image 340 may include four of the vertebrae 361, 362, 363, and 364 and other portions of the anatomy, such as the sacrum S. Thus, the stitched image 340 may include a greater amount of image data than may be acquired with any one of the projections of the imaging system 31.

The imaging system, as noted above, may acquire a plurality of projections, such as the projections 200, 204. These may be stitched together to form the stitched image or model 340. It is understood, that the stitched image 340 and the individual selected projections may be two-dimensional, three-dimensional, or otherwise reconstructed into a three-dimension from a plurality of two-dimensional projections. In various embodiments, for example, the imaging system 31 may acquire a plurality of projections at each of the positions, such as the initial position 134i and the second position 134n and generate a three-dimensional model regarding the subject at each position. The individual projections may be two-dimensions and/or the three-dimensional models that may then be stitched to form the final or selected to stitch image or model 340, as illustrated in FIG. 5. Thus, it is understood that the stitched image 340 may be two-dimensional, three-dimensional, or any of the appropriate dimensions. The various transformations determined in the process 250 may be used to transform the various image data, including reconstructions based thereon at each of the selected positions, to a selected reference frame to allow for generation of the stitched model 340 that does not include artifacts or errors due to the movement of the imaging system 31 which generates the projections at different positions of the imaging system 134 relative to the subject 28, as illustrated in FIG. 3.

With continuing reference to FIG. 4, an alignment of the image data at position n to the image data at position i may be made based upon a transformation matrix as defined by Eq. 2 to determine an orientation or change of the gantry position during the acquisition between the initial position i and the second or subsequent position n in block 350.

$\begin{matrix} T_{ISn ’ -> n} = T_{ISn ’ -> i} * T_{ISi -> n} & Eq . 2 \end{matrix}$

where T_ISn′→nis the transformation of the corrected position imaging system (IS) to the non-aligned IS position as a product of a transformation of the corrected IS position to the initial IS position and the initial IS positon to the second IS position. The transformation allows for a determination of a transformation matrix that may be determined in block 354 as a correction matrix for the current or second image data at position n to the image data at the position i. The correction transformation is illustrated in Eq. 3 and accounts for a corrected position of the gantry n′ that may also be referred as a virtual corrected positon as illustrated in FIG. 6 that allows for a transformation of the image at position n. Eq. 3:

$\begin{matrix} {{{TISn ’}_{-> i} = {RISn ’}_{-> i}, TrISn ’}_{-> i}} & Eq . 3 \end{matrix}$

with RISn′_→I, defining the rotation transform matching the initial and second image data coordinate systems orientation; and TISn′_→Idefining the translation transform matching the initial and second image data coordinate systems origin. The transformation allows for an alignment of the two image data sets based upon a tracked position of the gantry 134 during the acquisition of the image data. With reference to FIG. 6, for example, the gantry at the position 134i includes the orientation or position relative to the axis 140, as discussed above. At the second position 134n, the gantry may have a different orientation relative to the axis 140. Although the gantry 134 may move generally along the axis in the direction that may also be referred to as the z-axis, the gantry may not maintain a substantially identical orientation relative to the axis 140 between the initial position 134i and the second position 134n. Nevertheless, the transformation, as discussed above in the process 250, may allow for a determination of a coordinate system that may be used to transform an orientation of the gantry 134 in the same orientation and only translated in the z-direction. The corrected or virtual position may be referred to as 134n′ and have the angle 154′, as illustrated in FIG. 6. Thus, the image data, such as the second projection 204, may be aligned to the image data acquired at the initial position 134i. Once the image data is corrected or a determination of a correction matrix is made in block 354 a combination of the image data may be made in block 360.

The combination of the image data at the initial position i and the image data at the second position n may be performed in appropriate manner. In various embodiments, for example, the image data may be stitched together by identifying appropriate or identical portions in stitching the related portions together. Such stitching techniques may include estimating the transformation by registering intrinsic features in both image data (which may include image data volumes) by a method such as, but not limited to, non-parametric, block matching image registration algorithms, fiducial- or image-based 3D image registration, as generally understood in the art. In various embodiments, for example, a weighted stitching process may occur and weights may be determined, optionally, in block 358. As discussed above, the various projections may be acquired with a selected amount of overlap. The amount of overlap may be known due to the known translation along the z-axis. Accordingly, the overlap amount 208′ may be known for each of the projections. Along a distance of the overlap a selected weight may be provided or determined for each position along the image. For example, as illustrated in FIG. 7, based upon a position of the gantry 134 that moves along the axis 140, as discussed above which may refer to a z position or translation, a weight factor may be determined based upon the image data 200, 204. As illustrated in FIG. 7, for example, a weight factor 204w for voxels/pixels may increase as the z-position of the overlap and stitching moves closer to the second projection 204 and finally includes only voxels/pixels the second projection 204. Conversely a weight for the first projection 200 may decrease, as illustrated by wave line 200w, as the z-position moves away from the position of the initial projection 200. Accordingly a weight may be determined in block 358 to assist in the blending or stitching to perform or generate a combined image in block 360. It is understood, however, that weighting may not be required and that a selected position may be identified or determined to create a stitching between the two images.

The combined image may then be output in block 364. As discussed above a combined image may include the image 340, which may be displayed as the display image 120. The combined image 340 may registered to the subject 28 due to the tracking of the subject 28 with the subject tracker 98 and the tracking of the image system with the imaging system tracker 190 during image projection acquisition. Therefore, the image data 200, 204 may be individually registered to the subject 28 and the combination image or model 340 may also, therefore, be registered due to the inherent or included registration of the initial image data and the transformation in light of the navigation or tracked data as discussed above. This allows the combined image to include the dimension 344 that may include a selected dimension, such as by the user 24 and input to the imaging system 31, of the subject 28. The image 340 may be used for a selected procedure, such as a navigated procedure, that may allow for an illustration of the graphical representation 124 of the tracked instrument 20 relative to the subject 28 for the selected procedure. Therefore, an image that have a dimension, e.g., that is longer, than one that may be created with a single projection with the single projection with the imaging system 31 may be created and movement of the imaging system other than along a straight line, such as along the axis 140 that may also be referred to as the z-axis may be accounted for to create a selected or combined stitched image.

As discussed above, image data acquired of the subject may be corrected due to movement of the gantry 134 that is not only in a straight line and along an axis or plane, such as the axis 140. In various embodiments, movement of the gantry may be identified as movement along a z-axis which may be parallel and/or collinear to the axis 140.

In various embodiments, the movement of the gantry may not be only in the z-axis during the acquisition of a plurality of projections through the subject 28. In various embodiments, one or more portions may not be tracked. For example, tracking devices, including one or more of the imaging device tracking device 190 and/or the subject tracking device 98 may not be present. Further, the navigation systems 29 may not be present. It is understood that the image data may, however, be aligned according to various techniques, such as those discussed further herein. It is also understood that the various techniques may be combined, such as combining the navigation as discussed above to assist in aligning a plurality of projections to account for or correct for movement of the imaging device that is not only in a z-axis. Accordingly the navigation process, as discussed above, may or may not be used alone and may be used in combination with other processes, such as those discussed further herein.

Turning reference to FIG. 8, two projections, such as a first projection 1200 and a second 1204 may be acquired with the imaging system 31. The projections 1200, 1204 may be similar or identical to the projections 200, 204, as discussed above. The projections 1200, 1204 may be acquired without the imaging system being tracked, the subject being tracked, or the use of the navigation system 29. The image projections 1200, 1204 may, therefore, include a misalignment distance 1200 between lines 1224 and 1228. In each of the projections 1200, 1204 selected vertebrae may be imaged which may be the same or different vertebrae, including the vertebrae 361, 362, 363, 364. In addition, each of the image projections may include selected features that may be selectively or optionally positioned relative to the subject 28, such as a fiducial member, such as a rod or member 1231.

Each of the projections 1200, 1204 may include a selected dimension, such as a length or dimension along the z-axis 1208. As discussed above, the dimension 1208 may be based on or limited on the dimensions of selected portions of the imaging system 31, such as a dimension of the detector, a dimension of the source, or other dimensions of the imaging device 31. Accordingly, the dimension 1208 may be a dimension that is a maximum dimension of any single projection based upon the imaging system 31. Further, as discussed above, when acquiring the projections a certain amount of overlap, such as an overlap distance 1208′ may be included in each of the image projections 1200, 1204. The overlap 1280′ may include portions that are imaged of the subject 28 that are identical or substantially identical between each of the projections. For example, the vertebrae 362 and 363 may be included entirely in both projections. In addition or alternatively a member, such as the rod 1231 may be included or imaged in both projections. The rod 1231 may include selected dimensions and geometry that may be predetermined and stored, such as in the memory 71. The imaging system or other selected processing systems, such as the processor 70, may recall the dimensions to assist in further procedures, such as in registration of the two projections 1200, 1204.

To assist in aligning the projections and to correct for the spatial difference 1220 that may be called due to movement of the imaging device, such as the gantry 134, various portions in the respective images 1200, 1204 may be identified and registered. For example the rod 1231 may be imaged and occur in both projections 1200, 1204, as illustrated in FIG. 8. Further, anatomical portions, such as the vertebrae 362 may occur in both of the images 1200, 1204. These similar or identical portions may identified in both projections. Identification of the portions in both of the projections 1200, 1204 may be made manually, such as by identification by the user 24. In addition or alternatively, automatic identification may occur such as through selected automatic segmentation features or programs, such as model based anatomical segmentation algorithms such as, but not limited to, like deep learning image segmentation or supervised min-cut/max-flow image segmentation algorithms. Also, there may be combinations including the user identifying one or more voxels or pixels and a selected segmentation program identifying identical or similar (e.g., color, contrast gradient, etc.) voxels and/or edges relative to the voxels or selected portions.

In various embodiments, various high contrast features, such as an image of the rod 1231 and/or other selected features, such as the vertebrae 362 may be identified and segmented in each of the projections 1200, 1204. Once the selected identical or common features are identified in each of the projections 1200, 1204, the projections may be registered to one another. The registration may include a transformation of pixels or voxels to align the respective projections 1200, 1204. The overlap region 1208′ in each of the image projections 1200, 1204 may be used to perform the alignment. For example, if the rod 1231 is identified and segmented in both of the two projections 1200, 1204 these pixels or voxels may be aligned and the other pixels and voxels in the projection 1200, 1204 may be transformed or moved in a similar or identical manner to achieve alignment of the entire projection such that the first projection 1200 can be aligned with the second projection 1204. Selected transformations can include non-rigid, affine, and/or rigid transformations. In various embodiments, an entire object need not be imaged in both projections. For example, given a known geometry of the rod 1231, only a portion of the rod 1231 need be imaged in both projections to allow a registration. Further, as discussed above, registration between more than two projections, such as three, four, or more projections, may occur with the similar process.

It is further understood that the projections 1200, 1204 may include two-dimensional data, three-dimensional data, or any appropriate data type. For example, the projection 1200 may be a two-dimensional image data projection generated by the imaging system 31 at a first position, such as the position 134i of the gantry and the second projection 1204 may also be a two-dimensional at the second position 134n of the gantry. It is further understood, however, that each of the projections 1200, 1204 may be three-dimensional projections or models generated with image data at each of the respective positions 134i, 134n. In various embodiments, the imaging system 31 may generate three-dimensional image data by acquiring a plurality of projections relative to the subject 28. Therefore, the projections 1200, 1204 may be three-dimensional and the alignment may be a registration of three-dimensional image data or model between the projection 1200 and the second projection 1204.

With continuing reference to FIG. 8 and additional reference to FIG. 9, a process 1300 may be used to access and align and combine the projections 1200, 1204. The process 1300 may begin in start block 1304 which may be similar to the start block 254, as discussed above. In various embodiments, the subject 28 may be positioned relative to the imaging system 31, such as in preparation for procedure, preparation to acquire image data for various purposes including planning a selected procedure, performing a selected procedure, or other appropriate processes. The start block 1304, therefore, may include various actions that may include preparing the imaging system 31, preparing the subject 28, or other appropriate steps to acquire or access image data.

After starting in block 1304, image data may be accessed or acquired at imaging system position i. As discussed above, the gantry 134 may be positioned at an initial or selected position 134i. The image data accessed or acquired in block 1308 may be image data that is acquired in real time of the subject 28, pre-acquired image data that is accessed, or any appropriate image data at a first or initial position. Image data may also be accessed or acquired at imaging system position n. As also discussed above, the imaging system 31 may be moved to a second or additional position including the gantry position 134n. Again the image data accessed or acquired in block 1312 may be image data that is acquired in real time, image data that is accessed that is pre-acquired, or any appropriate image data. In various embodiments, image data acquired at the initial position may include the projection 1200 and image data acquired or accessed at the second position 1312 may be the projection 1204.

In both of the image data, a similar or identical feature, such as the same rod 1231 and/or vertebrae 363 may be identified in block 1320. As discussed above, for example, the rod 1231 and/or a portion of the rod 1231 may be identified in both of the projections 1200, 1204. The identification of the feature may be manual, automatic, or a combination of manual and automatic techniques. In various embodiments, the feature may be identified by use of edge detection or edge identification programs, such as segmentation, that are executed by a processor, such as the processor 70. Further, contrast gradients may be identified in the respective images that may be identified as the same or similar feature. Nevertheless the same or similar feature may be identified in both of the image data acquired or accessed in the respective blocks 1308, 1312. It is understood, therefore, that the identification may include various processes or steps that are carried out automatically, such by execution of program instructions with a processor, such as the processor