Principles And Practice Of Imageguided Neurosurgery

frames. The frame, however, restricts access to the surgical field, interferes with instruments, and requires immediate pre-operative imaging. It gives no feedback to the surgeon and requires multiple calculations that are not always intuitive and simple. It is also inconvenient to the patient. Stereotactic biopsy in the lateral temporal lobe is contraindicated with some frames, where the needle track is liable to traverse the Sylvian fissure, placing the middle cerebral artery at risk. The development of frameless image-guided systems was an important step in increasing the user friendliness of localization systems. Light-emitting diodes (LEDs) attached to the biopsy needles allow their precise tracking by the camera within the operating space, and holding arms have been developed to maintain biopsy needles rigidly in the correct position. The ideal trajectory to approach and biopsy the lesion can be worked out pre-operatively and stored in the workstation. The development of trajectory and targeting software allows the needle to be advanced according to such a pre-planned pathway, with real-time 3D visualization of the position of the needle tip within the brain.

Brain biopsy procedures require a higher accuracy than is necessary for most other procedures performed under image guidance. The use of scalp-applied or even skull-implantable fiducial markers, as well as the holding of the patient's head in a rigid Mayfield head holder, is important. The accuracy now given by most systems is better than 2 mm, but the limitation imposed by the thickness of the image slices remains.

In a study by Barnett et al. [8], 218 biopsy procedures were performed using scalp-applied fiducial markers. The average minimum lesion diameter was 27.7 mm and the average depth from the scalp was 39.8 mm. Lesions included glial tumors, metastases, lymphomas, menin-giomas and demyelination. The procedure yielded a diagnosis that supported the clinical and radiological findings in 96.3% of cases. This was comparable to the accuracy achieved by frame-based stereotactic systems. The most significant complication was intracerebral hemorrhage, which occurred in five cases, two of which required craniotomy. It was noted that the diagnostic accuracy for posterior fossa biopsy, at 70% (7 out of 10 patients), was much lower than that for supratentorial lesions; it was suggested that scanning the patient in the prone position and the application of skull-implantable fidu-cials would increase the accuracy.

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