Intra-operative imaging has attracted a lot of interest in last decade. The use of the optic microscopy in the operating room has revolutionized neurosurgery in general and in particular microdissection of gliomas. Notwithstanding, intra-operative objective determination of the extent of resection of gliomas remains a big challenge.

Recently the issue of complete resection as a causal, not only prognostic factor for overall survival in patients with GBM has been readdressed in a randomized phase III 5-aminolevulinic acid (ALA) study. This study investigated 5-ALA-induced fluorescence as a tool for improving EOR and provided a very high fraction of patients with postoperative MR imaging data acquired within 72 hours. Residual tumor in postoperative MR imaging was defined as tissue with a volume of contrast enhancement greater than 0.175 cm3. Of the 243 GBM patients included in the ALA study 121 (49%) had incomplete resection and 122 (50.2%) had complete resection: the median overall survival was 11.8 months in the former and 16.9 months in the latter group (Stummer et al., 2008). Long-term survivors (> 24 months) were almost exclusively among patients of the complete resection group.

The EOR not only influences survival, but also the efficacy of adjuvant therapies. The ALA cohort study provided for the first time Level 2b evidence that in GBM as a single factor survival depends on complete resection of the enhancing tumor. This level of evidence is inferior to randomized studies (Level 1) yet superior to case-control studies (Level 3), case series (Level 4), or expert opinions (Level 5).

In a more recent study (Stummer et al., 2011) it was reported that extended resections performed using 5-ALA carries a greater risk of temporary impairment of neurological function; patients with a greater risk of developing permanent postoperative deficits were those with preoperative symptoms such as aphasia unresponsive to steroids. The reason for preoperative neurological deficits in those patients was more likely infiltration and destruction of eloquent brain, rather than vasogenic edema. Thus, permanent neurological deficits in patients may have resulted from resection of fluorescence-marked tumor intermingled with functional eloquent brain tissue. This emphasizes again the importance of identifying the anatomic boundaries of the lesion with presurgical MR tractography and ultimately the functional limits with subcortical IES. EOR has been shown to be an important predictor of overall survival also in a series of 107 patients with recurrent GBM: if gross-total (> 95% by volume) resection is achieved at recurrence, overall survival is maximized regardless of initial EOR, suggesting that patients with initial subtotal resection may benefit from additional surgery (Bloch et al., 2012).

Detection of functional boundaries during surgery should be achieved with the aid of intraoperative neurophysiology and supported by presurgical fMRI and MR diffusion tractography (L Bello, Fava, Carrabba, Papagno, & Gaini, 2010). When a temporary deficit is repeatedly elicited with direct subcortical intraoperative electrical stimulation (IES) in the proximity of the wall of the surgical cavity, the functional limits of the resection are reached and the resection in that part of the tumor should be stopped. Identification of the functional limits is critical especially in gliomas infiltrating the motor system, in particular when the tumor involves the corticospinal tract (CST).

Intra-operative imaging with Magnetic Resonance or Ultrasound (Prada F et al., 2014) has the ability to determine if a resection is incomplete (Nimsky et al., Neurosurgery 2004). When this is the case, it is possible to extend tumor resection during the same surgical procedure. However, extention of resection may result in increased postoperative neurologic deficits due to damage to eloquent brain structures. Damage to the primary motor cortex or to the pyramidal tract may concern motor function; damage to the ventral precentral gyrus (BA6) or to the arcuate fasciculus may concern language function. Integration of functional presurgical data obtained with magnetoencephalography and functional MR imaging into a intra-operative navigational device for identification of eloquent brain areas has been shown to reduce postoperative neurologic deficits (Nimsky C et al., Neurosurgery 2001) and surgery time (Petrella J et al., Radiology 2006). Display of MR tractograms on the neuronavigational device in the operating room is valuable when the neurosurgeon is evaluating the distance of a specific tract from the surgical cavity and he uses subcortical ESM to test its functional relevance (Bello L et al., Neuroimage 2008).

Intraoperative acquisition and real-time display of the results of diffusion tensor imaging is feasible and it has been shown that it can depict online shifting of the corticospinal tract that is caused by surgical intervention (Nimsky et al., Radiology 2005).