3D medical images such as CT (Computed Tomography) scans or MR (Magnetic Resonance) scans are being used to an increasing extent for the planning of difficult surgeries. These images, which are acquired prior to surgery, contain precise quantitative information which a surgeon can use to build a surgical plan. Examples include planning the location of an implant or the trajectory of a probe. However, this quantitative information is known only relative to the medical image as opposed to the patient. As soon as the patient leaves the radiology department, the relationship between patient and the image is lost.
In order to exploit this information during surgery, the surgical plan must be "registered" with the actual location of the relevant anatomical structures of the patient. One method which is currently used in neurosurgery consists of fixating a metallic "stereotactic" frame to the patient before CT-scan acquisition. The frame, which is easy to locate in the CT image, stays in place during surgery. Since the surgical plan is specified with reference to the frame, it is easily transfered from the CT image to the patient in the operating room. However, this method, which requires an additional surgical intervention to affix the frame, causes significant patient discomfort. Furthermore, its use is limited to a very few types of surgeries.
IBM Research is developing a method of performing registration that works without a steretactic frame. The registration method, called "anatomy based registration" uses radiographs (X-ray images) taken during surgery in place of the stereotactic frame. The preoperative CT-scan is digitally positioned such that the simulated radiograph taken through the CT would match the radiograph actually taken in the operating room. Once the CT position has been found, the CT-scan is said to be "registered" to the radiograph. That is, the position of the source and plate of the X-ray machine is known relative to the CT-scan. If the position of the surgical instruments is then measured relative to the same radiographic source and plate, the surgical plan can be executed.
The following illustration, which was produced using Visualization Data Explorer* shows the result of the registration process. The last two figures are images produced as part of the registration process.
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Shown is a 3D CT data set registered to an accuracy of 2mm using radiographs.
The blue lines represent X-ray beams passing through the CT volume, forming the
simulated radiograph shown in black. Only the beams tangent to the surface
of the bone are represented. The white disks are the projection
of calibration beads which are not relevant to this process.
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Digitized radiograph of a femur with intensity correction. The white
disks are calibration beads.
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Contours extracted from the digitized radiograph shown above.
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