Richard D Bucholz md facs and Lee McDurmont bs Introduction

Surgeons deal with an unavoidable conflict on a daily basis. As physicians, they are bound by the Hippocratic oath to first avoid doing any harm to their patients. However, the very act of surgery implies an exposure of the internal anatomy of the patient and cannot be performed without inflicting harm to tissues superficial to the surgical target. To justify inflicting such harm, a surgeon must balance damage done by the possible good done for the patient and proceed only if the benefit exceeds the damage done. In effect, a surgeon can remain true to the oath if the patient experiences no net harm after the procedure has been performed. Viewed in this fashion, the external tissues are not only a barrier to exposure but also present the rate-limiting step for many interventions, as the harm done to these tissues during exposure may rule out the marginal benefit of many procedures.

In order to increase the number of conditions justifiably treated by surgery, it is to be expected that surgeons actively pursue methods to minimize harm. The development of anesthesia, for example, was a pivotal development, in that the pain and suffering of an awake procedure limited surgery to only the most dire and life-threatening situations; even in those extreme situations, surgery had to be of the most basic sort, as finesse and refinement would only come at an unacceptable cost of prolonging the agony for the patient. The advent of anesthesia was therefore a powerful enabling technology that vastly increased the utilization of surgery in the treatment of disease throughout the body. More recently, the technology of endoscopy has greatly increased the number of procedures performed by diverse surgical specialists, as the diminished invasive-ness of endoscopic procedures justifies surgery in medically fragile patients, or in patients in which the benefit may be questionable, and therefore the prolonged recuperation of a traditional, open procedure is not justified. Many knee arthroscopies are performed, for example, in patients who would never consider undergoing the risk of open knee surgery.

To further the cause of noninvasiveness, one must understand why surgery, by nature, requires invasiveness. Surgery differs from medical interventions in

From: Minimally Invasive Neurosurgery, edited by: M.R. Proctor and P.M. Black © Humana Press Inc., Totowa, NJ

that the pathology is exposed to allow direct intervention or alteration, as opposed to indirect intervention through the use of medication. In order to reach the pathology, the surgeon must recognize, and become oriented to, the patient's anatomy en route to the target structure, so as not to veer off course and cause collateral damage to surrounding tissue. Once at the target pathology, to allow definitive treatment, a specific amount of exposure is needed to treat the lesion. Surgical invasiveness is therefore the result of two needs: orientation with respect to normal anatomy and sufficient exposure of abnormal anatomy so as to ensure definitive treatment. For the purposes of this chapter, the first need will be called visualization and the second therapeutic access.

It is easy to see how image-guided surgery, which employs high-resolution images of a patient's anatomy to navigate the course of a procedure, would prove helpful in decreasing the invasiveness required to obtain visualization during the course of a procedure. Less obvious, but perhaps even more powerful, is the ability of image guidance to permit a therapeutic intervention with minimal invasiveness using a technology that requires less access. To examine how image-guided surgery enhances the cause of minimal invasiveness, it is useful to review the history of the field and reach an understanding of the forces that helped develop instrumentation to its current state.

Somewhat surprisingly, image guidance has its beginnings in the field of stereotaxis, which actually initially increased the invasiveness of surgery through the use of stereotactic frames. These devices, bolted to the head prior to imaging, allowed the use of preoperative images for navigation by registering the images to the surgical field, a process discussed in more detail in the Registration section next. Frames developed specifically for cranial applications with current imaging modalities include the Brown-Roberts-Wells (BRW) system, for use with computed tomography (CT) (1) (Fig. 1), and the Leksell frame as adapted for use with magnetic resonance imaging (MRI) (2). Such frames were painful when applied to the patient, prolonged the interventional procedure by coupling surgery to the act of imaging, and could hardly be considered as anything but invasive. Widespread use of image guidance did not occur until alternative means for registering images to surgery became easily available. These techniques, which can be gathered together under the concept of registration, are a major source of error in image guidance and are key issues when one is considering the overall desirability of image guidance for a particular patient.

This chapter reviews how image guidance can allow minimally invasive techniques to proliferate and also addresses modern methods that reduce the invasiveness of image guidance itself. As the role of image guidance is still evolving, the organization of this chapter reflects current developments that employ this technique in the three surgical necessities that require invasiveness: registration, visualization, and therapeutic access.

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