Although many HGGs are easily differentiated from surrounding brain, others pose serious problems in this regard. Similarly, resection of tumors that cannot be seen on the cortical surface are best approached with some form(s) of intraoperative navigation (Fig. 10.5). Intraoperative ultrasound (IOUS), which is available at most centers, is helpful when the tumor is not iso-echoic with the brain (Fig. 10.5b). This is often problematic for infiltrating low-grade gliomas, but less often with HGG. IOUS can provide accurate data prior to resection, during resection and at the end of the resection (to determine the completeness of the resection). The accuracy of IOUS is not diminished by brain shifts that occur during resection and subsequent brain relaxation. Frame-based and frameless sterotactic systems are quite useful as well (Fig. 10.5c). They can be used in planning the scalp flap, the craniotomy and the resection. However, the accuracy of these systems relies on concordance between brain position and pre-operative imaging studies. Therefore, alterations in brain volume or shifts of the intracra-nial contents during resection may render these guidance systems less useful as the resection progresses. Intraoperative imaging systems allow the neurosurgical oncologist to bring the post-operative "gold standard" into the operation. Currently these systems are expensive, often cumbersome and prolong the surgery. However, they combine the advantages of IOUS and stereotaxy.
Fig. 10.5. a Sagittal MRI (T1-weighted with gadolinium) showing a large left frontal tumor (T) with a small central cystic component (C). b Intraoperative ultrasound image obtained in the transverse plane, demonstrating the tumor and tumor cyst. cIntraoperative MRI navigational system, demonstrating the posterior extent of the resection (at red crosshair).
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