MRI of the acute oncology patient
MRI, with its unrivalled soft tissue contrast, multiplanar and functional capability, plays an essential role in the imaging and subsequent management of patients with cancer. Among functional imaging techniques, diffusion-weighted imaging (DWI) has significantly impacted oncological imaging over the past two decades. While DWI is primarily the focus of cancer research as a potential biomarker, it also provides qualitative insights into lesion characterisation and disease response assessment.
The most common clinical DWI sequence is a modified spin echo T2-weighted sequence, with a fast readout that ‘freezes’ bulk motion, and can provide a measure of the free diffusivity of water molecules in the intra and extracellular spaces, which gives an indication of tumour cellularity.
This is achieved by the application of two large, symmetric diffusion sensitising gradients, placed either side of the 180-degree refocusing pulse. The degree of diffusion sensitisation is given by the b-value, which varies according to the gradient amplitude, duration of the gradient, and time between the two gradients. At least two b-values are required for calculation of the apparent diffusion coefficient (ADC), which is a quantitative measure of the restriction of water diffusion in tissues.
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