Advanced Magnetic Resonance Imaging Techniques in Brain Tumors Surgical Planning

Abstract

Magnetic Resonance Imaging (MRI) is still the workhorse of tumor detection and diagnosis. In particular, MRI provides detailed information about cerebral tumor anatomy, cellular metabolism and hemodynamic features, making it a fundamental tool for a correct diagnosis, treatment and monitoring of the disease. Various new functional imaging modalities assessing tissue microstructure and physiology have increased the scope of neuro imaging and raised expectations among clinicians. This study provides an overview of the most advanced MR imaging techniques (functional MRI, perfusion-weighted imaging, diffusion-weighted imaging and MR spectroscopy) now available for neurosurgical planning and their role in brain tumors assessment. Their pros and cons are analyzed in order to find out which one may be chosen as best diagnostic pre-surgical protocol. At the moment none of the single techniques can be considered the golden standard; only the integration of advanced and conventional MR imaging proves to be a reliable tool in the hands of the neuroradiologist and neurosurgeon, thus maximizing tumor resection and function preservation. Surgery is the treatment of choice for accessible brain tumors. Accessible tumours are those which can be surgically removed without causing severe neurological impairment. The goal of surgery is gross total resection with functional preservation, providing a chance for other treatments, such as newer forms of radiation therapy.

MRI is used for 1) detection and characterization of the lesions (diagnosis); 2) assessment of local involvement and/or distant extension of the disease (staging); 3) evaluation of morphological/biological changes over time and response to treatment (follow-up). It also offers invaluable information concerning brain anatomy, tumoral growth pattern and in the selection of patients for intra- operative cortical stimulation, in functional neuro-navigation and in the feasibility of surgical treatment. fMRI is primarily used for neurosurgical planning for preoperative risk assessment, especially in lesions located in close proximity to the eloquent cortex. There are three main goals of presurgical functional MRI: 1) determine the risk for eventual neurological deficits, by identifying the distance between the margin of planned tumour resection and the eloquent/essential functional areas. It seems that the risk for postoperative loss of function, as tested using functional MRI, is significantly lower when the distance between tumour periphery and BOLD activity is 10 mm or more; 2) select patients for intraoperative cortical stimulation; 3) provide guidance for functional neuro-navigation, based on preoperatively acquired structural information. Apparent Diffusion Coefficient (ADC) may be used to target the sites of highest cellularity within heterogeneous tumours; identifying these areas may be important for diagnosis and prognosis and can be useful to characterize highly cellular versus low cellular components. DWI therefore represents a promising tool for preoperative grading, postoperative assessment of glial tumours, differential diagnosis between recurrence and radio necrosis or in the assessment of therapeutical response injury. Tractography has allowed visualization of white matter tracts and is becoming an essential tool for neurosurgical planning and postoperative follow-up of surgically treated brain tumours and vascular malformations. White-matter tract visualization by means of diffusion anisotropy maps has had a major impact on neurosurgical case management. Knowledge of the topography, integrity, and degree of involvement of the white matter fibers plays an important role in pre-operative evaluation. DTI provides information about the integrity, displacement and/or interruption of white matter tracts in and around a tumour; it may also detect widening of fiber bundles due to edema or tumour infiltration. Perfusion Weighted Imaging (PWI) PWI permits no invasive in vivo study of tumour vascular hyperplasia and capillary permeability, which are important biological markers of malignancy, grade and prognosis, particularly in gliomas. PWI could be used to more accurately delineate tumor margins, in order to better plan resection and radiation treatment of brain neoplasms. MR perfusion imaging may also be valuable in distinguishing radiation necrosis from tumor recurrence, thus sparing patients from unnecessary treatment. MRS displays brain metabolites in the form of spectra whose shape basically depends on metabolite concentration. Metabolite concentration has been used to measure chemical markers of neoplastic activity. MRS imaging with MRS-guided stereotactic biopsies reported good results for preoperative grading of gliomas. Accordingly, MR imaging of brain tumours has shown substantial improvements during the past few years, both in terms of high quality morphological imaging (with the use of high-field magnets) and in the shift towards functional and molecular imaging. The integration of functional, metabolic and structural MRI information offers standardized preoperative maps of multiple critical functions to facilitate assessment of surgical risk, planning of surgical routes and direction of conventional, intraoperative electrophysiological procedures. These new imaging applications add greater specificity to a no invasive approach of cerebral tumours and help the neurosurgeon in the assessment of resection and function preservation.