3 T MR multiparametric assessment of cerebral gliomas

Cerebral gliomas are the most common primary brain neoplasms in adults and represent one of the leading causes of cancer-related death in the general population. Prognosis and decisions on management depend on the histological type and grade of the lesion 1.2. Histopathological analysis is the "gold standard" in formulating the diagnosis of type and grade, but requires an open or stereotactic biopsy, a procedure which carries an appreciable risk of morbidity and mortality. Furthermore, the diagnostic accuracy of various biopsy procedures depends on the amount of tissue obtained and the accurate targeting of areas of high diagnostic yield. Diagnostic pitfalls are the intrinsic heterogeneity of cerebral tumors and the frequent coexistence of small foci of high grade transformation 2 in the background of a largely low grade lesion. Magnetic resonance imaging (MRI), or CT scanning if MRT is not available or contraindicated, is the method of choice for the preoperative evaluation of cerebral gliomas, but has limited sensitivity and specificity in the definition of tumor type and grade, and sometimes can provide ambiguous or misleading information(1). This may be attributable to the difficulty in discriminating between tumor and edema or nonspecific treatment effects, and to the existence of gadolinium-enhanced necrosis, which may be mistaken for tumor, or high grade tumor that do not enhance. These problems may be overcome by the development of imaging modalities that highlight structural, functional and/or metabolic properties of the tumor. Positron emission tomography and single photon emission tomography may be valid candidates for such analysis(3,4), but there would be considerable savings in cost and patient discomfort if similar information could be defined using an MR methodology. Proton magnetic resonance spectroscopic imaging (H-1-MRSI) is a noninvasive technique that provides metabolic information within tissues. Brain gliomas generally have increased choline (Cho) and decreased N-acetylaspartate (NAA), and variable levels of creatine (Cr), while the presence of lactate and/or lipids (LL) usually indicates necrosis. Several studies have demonstrated that H-1-MRSI can be used to guide surgical resection or biopsies, define radiotherapy planning and monitor treatment effects or progression to higher grade(5). Diffusion-weighted imaging (DWI) is a MR technique that is sensitive to the molecular motion of water, thereby providing information on the structural features of biological tissues. The diffusion parameter called apparent diffusion coefficient (ADC) can play a role in the evaluation of tumors and post-therapy monitoring, giving information that has been linked to cellularity and structural integrity(6,7). However, data regarding the distinction between different tumor types and grades, or between tumor infiltration and vasogenic edema(6,8,9) are somewhat conflicting. Perfusion-weighted imaging (PWI), using an exogenous endovascular tracer such as gadopentetate dimeglumine, provides information on the hemodynamics of tumor tissue. The relative cerebral blood volume (rCBV), a widely used PWI parameter, strictly reflects the tumor microvasculature and angiogenesis, and is useful in grading cerebral gliomas. rCBV maps can guide stereotactic biopsy and monitor response to therapy(10). In recent years, several authors have used a combination of 1H-MRSI, DWI and/or PWI in addition to conventional MRI to enhance its ability to differentiate solid tumor from othr intratumoral or peritumoral components(11-12), radiation necrosis from recurrent tumor(13), stable from progressing tumors(14), and among tumor types and/or grades (11,12,15). All the above MR techniques have been performed mainly at magnetic field strengths of 1.5 Tesla.