The invention relates generally to systems which generate images during medical and surgical procedures, and in particular, a system for generating images during medical and surgical procedures based on a scan taken prior to the procedure.
Image guided medical and surgical procedures comprise a technology by which images, obtained either preprocedurally or intraprocedurally (i.e., prior to or during a medical or surgical procedure), are used to guide a doctor during the procedure. The recent increase in interest in this field is a direct result of the recent advances in imaging technology, especially in devices using computers to generate three dimensional images of parts of the body, such as computed tomography (CT) or magnetic resonance imaging (MRI).
The majority of the advances in imaging involve devices which tend to be large, encircle the body part being imaged, and are expensive. Although the images produced by these devices depict the body part under investigation with high resolution and good spatial fidelity, their cost usually precludes the dedication of a unit to the performance of procedures. Therefore, image guided surgery is usually performed using images taken preoperatively.
The reliance upon preoperative images has focused image guidance largely to the cranium. The skull, by encasing the brain, serves as a vessel which inhibits changes in anatomy between imaging and surgery. The skull also provides a relatively easy point of reference to which a localization system may be attached so that registration of preprocedural images to the procedural work space can be done simply at the beginning of the procedure. Registration is defined as the process of relating preprocedural images of anatomy to the surgical or medical position of the corresponding anatomy. For example, see Ser. No. 07/909,097, now U.S. Pat. No. 5,383,454, and PCT application PCT/US95/12894 filed on Oct. 5, 1995, entitled "Surgical Navigation Systems Including Reference and Localization Frames," the entire disclosures of which are incorporated herein by reference.
This situation of rigid fixation and absence of anatomical movement between imaging and surgery is unique to the skull and intracranial contents and permits a one-to-one registration process as shown in FIG. 1. The position during a medical procedure or surgery is in registration with the preprocedural image data set because of the absence of anatomical movement from the time of the scan until the time of the procedure. In almost every other part of the body there is ample opportunity for movement which degrades the fidelity of the pre-procedural images in depicting the intraprocedural anatomy. Therefore, additional innovations are needed to bring image guidance to the rest of the body beyond the cranium.
The accuracy of image guided surgery is based on the identification of structures within the body that do not change shape, do not compress, nor deform between the process of imaging and surgery. Such structures are termed "rigid bodies," and the bones of the skeleton satisfy this definition for a rigid body. Bones are commonly a target for medical or surgical procedures either for repair, fusion, or biopsy. Therefore, a technique is needed whereby registration can be performed between the bones or bone fragments (skeletal elements) as depicted pre-procedurally on scans and the position of these same skeletal elements as detected intraprocedurally. This technique must take into account that movement can occur between portions of the skeleton which are not rigidly joined, such as bones connected by a joint, or fragments of a broken bone.