CEAs without shunting [1]. No correlation was found between rCBF measurements and neurological morbidity or overall complication rate.

A second method of measuring CBF relies on near-infrared spectroscopy to assess cerebral oxygenation status. Also known as "cerebral oximetry", this technique measures changes in the levels of oxygenated, deoxygenated and total hemoglobin as well as oxidized cytochrome in the local cerebral blood supply. The advantage of using the near-infrared spectrum is that this wavelength of light passes easily through the extracranial tissues, allowing for non-invasive monitoring. However, variations in anatomy or extra- to intracranial collateral blood supply can make interpretation of the results somewhat difficult. The sensor patch is placed over the forehead on the ipsilateral side and continuous measurements of regional cerebral oxygen saturation (rSO2) are obtained. Several studies have assessed the utility of cerebral oximetry as a monitoring technique during CEA; results have been mixed and it is clear that significant improvements need to be made to this technique before it can be used with any consistency for intraoperative monitoring.

Intraoperative Ultrasound

B-mode ultrasound began to be used by neurosurgeons soon after it became available in the early 1980s. It quickly proved its value for localizing lesions, delineating normal and pathological anatomy, guiding instrumentation, and identifying residual tumor following resection. It can be particularly useful for localizing intramedullary spinal cord pathology. More recently, stereotactic intraoperative ultrasound has been employed as an adjunct during surgical procedures using image guidance. As intraoperative "shift" can instill significant error into these systems, real-time ultrasound imaging can be compared with the pre-operative scans used for image guidance, and appropriate corrections can be made.

Transcranial Doppler (TCD) ultrasound has been used for the intraoperative assessment of flow velocities and detection of embolic events during CEA by insonation of the terminal ICA, MCA or ACA through a temporal window. A

large study (n = 1058) of patients undergoing CEA with intraoperative TCD monitoring concluded that microemboli detected during dissection/wound closure, decreases of MCA velocities equal to or greater than 90%, and increases of pulsatility index of 100% or more were significantly associated with post-operative stroke [23].

Microvascular Doppler has several intraoperative uses, including evaluation of flow in the carotid artery following CEA, documentation of graft patency in cases of EC-IC bypass, and assessment of flow in an aneurysm and adjacent vessels before and after clip application.

Intraoperative Angiography

Intraoperative angiography is used during a wide range of neurovascular procedures including aneurysm clipping, AVM resection, and EC-IC bypass. The imaging procedure itself is identical to non-operative angiography; however, patient positioning and preparation and the use of radiolucent stabilizing equipment are crucial for the successful use of this technique in the operating room. The potential complications are similar to those seen during nonoperative cerebral angiography, namely groin hematoma, femoral artery thrombosis, stroke and vasospasm.

The utility of intraoperative angiography has been documented by several studies. In a series of 115 patients undergoing various neurovascu-lar procedures with angiography, the operative procedure was altered in 19 of these cases owing to concerns raised by the intraoperative angiogram, while only 2 of the 115 patients had a post-operative complication that could potentially have been related to angiography [24].

Peripheral Nerve Monitoring

Peripheral nerve monitoring relies primarily on EMG recordings, nerve conduction velocity (NCV), nerve action potentials (NAPs) and SSEPs, all of which are performed in the same fashion as during routine non-operative evalu

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