Advances in CNS tumour imaging

Abstract

Brain tumours are the second most common group of paediatric malignancies after leukaemia. Unlike those in adults, most paediatric brain tumours are primitive lesions. The most common location is the posterior cranial fossa, where tumours prevail between ages 1 and 11 years. Conversely, tumours occurring in children younger than 1 year and older than 12 years are located more commonly in the supratentorial compartment. Magnetic resonance imaging (MRI) is an invaluable tool for identifying and characterising brain tumours, presurgical planning and evaluating the effectiveness of surgery, chemotherapy and radiotherapy. Advanced MRI modalities, including diffusion-weighted imaging (DWI), perfusion-weighted imaging (PWI) and spectroscopy, provide significant additional information regarding quantities, such as tumour cellularity, haemodynamics and metabolism. DWI provides information regarding diffusion of water molecules from which quantitative values, the apparent diffusion coefficient (ADC), can be calculated. The most important application of DWI in the field of brain tumour imaging is in estimation of cellularity, which correlates grossly with tumour grade. Diffusion tensor imaging (DTI) and fibre tractography are important tools for preoperative mapping of brain tumours, including the identification of the anatomical relationship between tumour and functional areas, such as major fibre tracts. PWI measures cerebral haemodynamics at the microcirculation level. The cerebral blood volume (CBV), defined as the volume of blood per unit of brain tissue and expressed in ml/100 g, is the parameter most commonly evaluated in the field of brain tumour imaging. The average relative CBV of high-grade lesions is significantly higher than that of low-grade neoplasms—a direct function of neovascularity, regardless of the integrity of the blood-brain barrier. There are several techniques for performing PWI. The most widely available is T2*-weighted dynamic susceptibility contrast (DSC) imaging, which requires intravenous administration of a bolus of paramagnetic contrast material and rapid acquisition of images over time during the first pass of contrast material through the capillary bed. DSC-PWI poses a number of technical challenges in the paediatric age group, including the need for constant, high-flow contrast material injection by power injectors, and stable, sufficiently sized intravenous access catheters, which may be problematic in the newborn and infant age groups. Arterial spin labelling (ASL), a perfusion technique that does not use an intravenous tracer, while promising, is less widely available in the clinical arena. Its use is currently limited to a few centres where high fieldstrength (i.e. 3 T and higher) magnets are available. Magnetic resonance spectroscopy (MRS) allows noninvasive detection and measurement of normal and abnormal metabolites and plays an important role in the diagnostic work-up of brain tumours in children. Available 1H-MRS techniques include single-voxel and multi-voxel (chemical shift imaging) methods. Acquisition is possible at short (20– 30 ms), intermediate (135–144 ms), and long (270–280 ms) echo times, yielding different possibilities for metabolite identification and peak analysis. In most instances of brain tumours, a pathological spectrum (non-specific for tumour type) consisting of increased choline (Cho)/creatine (Cr) and Disclaimer Dr. Rossi has no financial interests, investigational or offlabel uses to disclose.