CT extremity scanner

[0001] This application claims priority to U.S. Provisional Application Ser. No. 60/493,405 filed Aug. 7, 2003.

[0002] This invention relates generally to a CT scanner and more particularly to a small CT scanner that is particularly adapted for scanning the extremities of a patient.

[0003] CT scanners are generally very large, occupying an entire dedicated room. Thus, doctors must refer patients to hospitals for CT scanning. Also, because of the inconvenience and expense, CT scans are not used for extremities in some cases where a CT scan might be helpful.

[0004] A CT scanner according to the present invention is particularly adapted for scanning the extremities of a patient. The CT scanner is small and easy to use. The CT scanner scans only the desired extremity, not unnecessarily exposing the rest of the patient to x-rays.

[0005] The CT scanner includes an x-ray source and x-ray detector mounted for rotation and translation along an axis parallel to a table for supporting the patient's extremity. The source and detector are mounted opposite one another on an inner circumference of an inner ring. The inner ring is rotatable about the axis within an outer ring mounted on at least one carriage. The carriage is movable parallel to the axis. During scanning, the inner ring rotates within the outer ring while the rings and carriage move along the axis, thereby producing a helical scan of the extremity.

[0011] A CT scanner 20 according to a first embodiment of the present invention is shown in FIG. 1. The CT scanner 20 is particularly useful for scanning extremities, such as knees, ankles, arms, etc. The CT scanner 20 includes an x-ray source 22 and detector 24 that are mounted on diametrically opposing inner surfaces of an inner ring 26, rotatably mounted within an outer ring 28. Rotation of the inner ring 26 may be accomplished by one or more motors 29 in the outer ring 28. The inner ring 26 has an axis, x, at its center and encircles a radiolucent platform 30 for supporting patients' extremities, such as legs, feet, arms and hands. In this example, the platform 30 supports a leg 32. The source 22 is preferably a cone-beam x-ray source 22 and is directed toward the detector 24 through the axis, x. The outer ring 28 is fixed between a pair of carriages 34, which are movable parallel to the x-axis along a pair of parallel rails 36 supported by opposite stands 38, shown in FIG. 2. The rails 36 threadably engage the carriages 34. Translation along the x-axis is controlled by one or more motors 40 in the stands that rotatably drive one or both of the rails 36. Thus, the source 22 and detector 24 are able to orbit about the x-axis while translating parallel to the x-axis. Alternatively, translation of the outer ring 28, inner ring 26, source 22 and detector 24 along the rails 36 may be accomplished by motors in the carriages 34.

[0012] A computer 50 is suitably programmed to control the functions of all of the devices described herein and to perform the image-processing described herein. The computer 50 controls all of the motors in the manner described and receives all of the x-ray images from the detector 24. The computer 50 then generates 3D images of the scanned patient based upon the x-ray images and the known positions of the source 22 and detector 24 at which each x-ray image was taken. Suitable reconstruction algorithms are known and one could be adapted for the present invention by those of skill in the art.

[0013] The inner ring 26 may be capable of unlimited rotations about the x-axis if provided with a rotatable electrical coupling (or wireless connection). Alternatively, the inner ring 26 or may alternate between rotating one direction through some fixed angle of rotation and then in the opposite direction through the same fixed angle of rotation, all as the inner ring 26 also translates along the x-axis. For example, the inner ring 26 may rotate 270 degrees clockwise, then 270 degrees counterclockwise, then 270 degrees clockwise, etc. as the inner ring 26 translates along the x-axis. It is not necessary for the inner ring 26 to rotate 360 degrees, only that the computer 50 algorithm that transforms the plurality of images taken by the CT scanner 20 into a three dimensional model knows the angle and the position on the x-axis for each image.

[0014] The CT scanner 20 is particularly useful for scanning extremities, such as knees, ankles, arms, etc. Therefore, the inner diameter of the inner ring 26 is preferably between approximately 18″ and approximately 24″ and more preferably about 18″.

[0015] FIG. 3 illustrates an alternate extremity CT scanner 20a including a source 22a, similar to the source 22 of the previous embodiment, and a pair of generally perpendicular detectors 24a, 25a. The source 22a is mounted to move generally along an arc such that it initially directs all of its beam onto the horizontal detector 24a, then directs its beam onto both detectors 24a, 25a (shown in position 22a′), then directs all of its beam onto the vertical detector 25a (source shown in position 22a″). The source 22a may be mechanically constrained to move along the arc, such as by a four-bar linkage 52 (shown schematically), and its position controlled by a computer-controlled motor 54. The embodiment of FIG. 3 may also be used effectively for scanning hips.

[0016] Alternatively, as shown in FIG. 4, a single detector 24b can be mounted to move along a complementary arc as the source 22b moves along its arcuate path. Again, the detector 24b may be mechanically constrained to move along the arc, such as by a four-bar linkage 56 (shown schematically), and its position controlled by a computer-controlled motor 58.

[0017] Any of the previous three embodiments may be used to scan a joint, such as a knee, that is under consideration for replacement surgery. In a method according to the present invention, a full CT scan of the knee (for example) is first taken in one or two predetermined stationary positions, such as fully extended to 180 degrees and bent to 90 degrees. Then, one or more full CT scans are taken of the knee while moving, so that the kinematics of the knee can be fully modeled. Based upon this complete model, the replacement knee can be placed, oriented and optimized in the computer model prior to surgery. Then, using image-guided surgery, the replacement knee can be implanted in the optimal position and orientation, as determined in the computer model.

[0018] Post-operatively, the replacement joint (or other implant) and surrounding tissue can be monitored with any of the above CT scanners. Dual energy sources 22 can be used to distinguish soft-tissue from bone, determine bone density, provide a quantitative measure of bone mass and detect potential infection. The bone growth can be tracked and monitored by comparing the three-dimensional images of the replacement joint and surrounding tissue over time. For example, the bone mass can be shown quantitatively and the new growth can be shown in a different color. It can also be determined whether the bone is growing into pores (or other areas) in the implant. Tracking the three dimensional images of the replacement joint and surrounding tissue over time can also assist doctors in determining if and when the replacement joint needs to be replaced again.