Diffusion-weighted Imaging

Abstract

Diffusion-weighted imaging (DWI) is an important tool in imaging of the pediatric brain, especially in the neonatal brain which presents challenges in lesion visualization on conventional sequences due to incomplete myelination and a relatively higher water content. It is now a routine sequence that is added to nearly all pediatric neurological disorders. The clinical utility of DWI is in early diagnosis of pathologies, aiding characterization of lesions and in prognostication. DWI is able to provide timely diagnosis compared to conventional sequences primarily because of its ability to detect cytotoxic edema and excitotoxic injury, an early event which can be seen in multiple neurological disorders including hypoxic-ischemic encephalopathy, pediatric and neonatal arterial ischemic strokes, non-accidental injury and diffuse axonal injury, neonatal hypoglycemic encephalopathy, and some viral infections and metabolic disorders. When evaluated with conventional T2W sequences, DWI is able to aid lesion characterization by differentiating cytotoxic from vasogenic edema, e.g. in differentiating ADEM from ischemic lesions. Also, it is effective in characterizing the content of lesions, e.g. in differentiating posterior fossa arachnoid cysts from epidermoids. In the evaluation of posterior fossa brain tumors and brainstem tumors, quantitation of ADC can be useful to differentiate high grade (e.g. medulloblastoma) from low grade (e.g. pilocytic astrocytoma) tumors as ADC can serve as a biomarker for brain tumor cellularity and tumor grade. Diffusion is restricted in densely packed tumors with high cellularity, small extracellular space, and high nuclear-to-cytoplasmic ratio. However, studies have also found overlap in values, which may limit its clinical utility. There is on-going research in the use of ADC for assessment of treatment response and tumor progression.