DIAGNOSTIC-IMAGING IN NEUROONCOLOGY

Brain tumors are the most common solid tumors in children and account for a high proportion of pediatric cancer death. Histologic diversity, the tendency of malignant tumors to disseminate early, the varied sit e of origin, and the detrimental effects of treatment on the developin g nervous system complicate the treatment of childhood brain tumors.(2 2) Imaging techniques to diagnose, stage, and follow children with bra in tumors are central to their clinical management. Magnetic resonance (MR) imaging has advantages over computed tomography (CT) in lesion d etection, definition of extent, detection of spread, and evaluation of either residual or recurrent disease. This is especially true in the posterior fossa, where 50% of pediatric tumors arise. Until recently, MR imaging was used to characterize cerebral neoplasms by demonstratin g anatomy in various planes, displaying differences in relaxation time s (T1 and T2) between normal and abnormal tissues, and detecting break down in the blood-brain barrier on T1-weighted images with use of intr avenous paramagnetic contrast enhancement. T1 relaxation reflects the time needed for protons to regain their magnetization in the longitudi nal plane; T2 relaxation reflects the time interval during which proto ns retain their phase coherence in the transverse plane. In the past f ew years, technological advances (rapid imaging techniques, novel puls e sequences, improved hardware configuration) have opened new windows in MR imaging techniques: functional and metabolic imaging. Functional MR imaging consists of various techniques that examine the changes in cerebral hemodynamics that accompany brain function or characterize m icroscopic tissue perfusion and vascularity. Metabolic imaging techniq ues aim to identify molecular biologic factors that potentially are ab le to guide clinical management decisions. Long the sole domain of pos itron emission tomography (PET) scanning, novel metabolic imaging tech niques include MR spectroscopy, cerebral blood volume mapping, and, in nuclear medicine, single photon emission computed tomography (SPECT). This article reviews recent advances in imaging of pediatric brain tu mors. The increasing role of functional and metabolic imaging is discu ssed. Brain tumors are reviewed by location. Specific tumor types that have recently been characterized will be emphasized.