5.1\ Intraoperative MRI

5.1.1\ Discussion

The main goal of using MRI during brain tumor resection is to safely maximize the extent of tumor resection. In particular, imaging during surgery can also help compensate for the brain shift, which represents surgically induced volumetric deformation of the intracranial contents attributable to resection of the tumor, as well as intracranial pressure changes that result from craniotomy and cerebrospinal fluid (Fig. 5.1). Intraoperative MRI is also useful for identifying complications during surgery that might require intervention, such as hyperacute intracranial hemorrhage. Hyperacute

on T2-weighted sequences due to the presence

D.T. Ginat, M.D., M.S. (*)

University of Chicago, Pritzker School of Medicine, Chicago, IL, USA

e-mail: dtg1@uchicago.edu

P.W. Schaefer, M.D.

Department of Neuroradiology, Harvard Medical School, Massachusetts General Hospital, Boston, MA, USA

M.D. Moisi, M.D., M.S.

Department of Neurosurgery, Swedish Neuroscience Institute, Seattle, WA, USA

of ­oxyhemoglobin. On susceptibility-­weighted sequences, hyperacute intraparenchymal hematomas tend to appear hyperintense centrally with a thin rim of dark signal. Extraparenchymal hyperacute hemorrhage tends to be of intermediate signal on T1-weighted sequences, but conspicuously hyperintense on T2-FLAIR MRI (Fig. 5.2). Hemostatic agents can mimic hemorrhage or residual enhancing tumor due to the rapid T1 shortening effect on blood products. Serial postcontrast T1-weighted images can be useful for depicting residual enhancing tumor. A potential confounder is the presence of contrast leakage at the margins of the resection cavity, which tends to appear more diffuse than nodular (Fig. 5.3).

Laser interstitial thermal therapy comprises various minimally invasive procedures that are increasingly used to treat selected brain tumors, neuropsychiatric disorders, and epileptogenic foci. MRI is also useful for real-time monitoring of these thermal ablation procedures. In particular, MR thermography, which can exploit phase shifts of protons at different temperatures, can provide a temperature map during ablation and from which an irreversible damage model can be derived based on the treatment duration (Fig. 5.4). Besides coagulative necrosis, a small amount of hemorrhage and transient swelling commonly result from thermal ablation (Fig. 5.5). However, an increase in lesion size, heterogeneity, ­peripheral nodular enhancement, restricted diffusion, elevated blood volume,

and surrounding edema that persists or develops after one or 2 months following treatment should raise the suspicion for tumor recurrence (Fig. 5.6). Otherwise, thermal ablation results in a predictable progression of signal changes on MRI. In particular, MRI of recently thermally ablated lesions displays a marginal zone with low T1 and high T2 signal due to edema with rim enhancement, which eventually transforms­

into gliosis; a peripheral zone with low T1 and high T2 signal due to additional edema, which ultimately resolves; and a central zone surrounding the probe tract with high T1 and low T2 signal due to the presence of blood products and coagulative necrosis, which persists amidst encephalomalacia. Some of these findings are exemplified in subsequent sections of this chapter.

Fig. 5.1  Brain shift. Preoperative FLAIR image (a) shows a hyperintense lesion in the right frontal lobe. Intraoperative FLAIR image (b) shows partial resection of

the lesion and a change in the overall morphology of the surrounding right frontal lobe parenchyma

Fig. 5.2  Enhancing tumor resection and contrast leakage. Initial axial post-contrast T1-weighted image (a) shows a peripherally enhancing left temporal lobe glioblastoma. Axial post-contrast T1-weigthted MRI (b) obtained after the first resection attempt shows a punctate focus of nodular enhancement in the medial resection bed

(arrow), which represented residual tumor. Axial post-­ contrast T1-weigthted MRI (c) obtained after further resection shows that there are no longer residual enhancing tumor components. Faint enhancement along the margins of the resection cavity represents contrast leakage (arrowheads)

Fig. 5.3  Hyperacute hemorrhage and hemostatic material. Axial T1- (a) and T2-FLAIR (b) intraoperative MR images obtained at the end of right frontal lobe tumor resection show a small left parietal convexity subdural

hematoma with intermediate T1 and high T2 signal (arrows). The hemostatic agent in the extradural space along the right frontal convexity surgical bed displays high T1 and T2 signal (arrowheads)

Fig. 5.5  Transient tumor swelling after laser ablation. Preoperative coronal T2-weighted MRI (a) shows a hyperintense hypothalamic tumor, which proved to be a pilocytic astrocytoma (arrow). The coronal T2-weighted MRI (b) obtained 1 week after laser ablation of the tumor

when the patient developed memory formation difficulties shows increase in size of the tumor (arrows) and lateral ventricles. Follow-up coronal T2-weighted MRI (c) after steroid taper shows interval decrease in size of the tumor (arrow) and lateral ventricles

Fig. 5.6  Tumor progression after laser ablation. Preoperative axial T1-weighted MRI (a) shows a homogeneously enhancing right midbrain tumor. Axial

T1-weighted MRI (b) obtained over 1 month after laser ablation shows central necrosis, but overall increase in size of the enhancing tumor