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large explorations toward approaches with limited access and restricted visibility. These new applications increase the demand for image guidance and compel the use of the most advanced imaging methods. It is expected that allowing medical imaging systems to play a more direct role in interventional and therapeutic procedures—thus enabling greater precision, increasing foreknowledge, and facilitating even less invasive surgical access—will reduce the cost of patient management and improve the quality of patient outcomes. Image-guided therapy is a new, emerging field that has close relationships to interventional radiology, minimally invasive surgery, computer-assisted visualization, and robot-assisted surgery. 12.1 Therapeutic Intervention Experience with Different Imaging Modalities12.1.1 X-Ray ImagingStatic x-ray imaging has been used to allow the therapist to plan radiation therapy and, to a lesser extent, surgery; however, the projection nature of the image limits its value, particularly when tomographic images can be available for this purpose. For guidance, control, and monitoring operations during a procedure, fluoroscopic images are used rather than static images. Fluoroscopy has the advantage of presenting real-time images. Although it is not tomographic, overlying structures are eliminated by using contrast media to accentuate the specific anatomy (usually blood vessels) to be studied. A major disadvantage of fluoroscopic methods is that the x-ray dose delivered to the patient and to the interventionalist can be high. However, x-ray fluoroscopy is the key imaging technique used in interventional procedures. 12.1.2 Computed TomographyComputed tomography (CT) imaging has been used in interventional applications in basically two ways: (1) to provide information for planning radiation therapy and for surgery—including in particular stereotactic biopsy, stereotactic craniotomy, and modern stereotactic radiosurgery—as well as for planning conventional radiation therapy; and (2) to provide guidance for image-guided biopsy of various body parts. The advantage of CT over projection x-ray imaging is that CT is tomographic, presenting the anatomy on a slice-by-slice basis for more exact localization. It is not preferred, at this time, over standard angiographic
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