MRI pearls

1. FLAIR - fluid attenuated inversion recovery sequence eliminates the signal from CSF. it is sensitive to oedema and inflammation. Axial T2 FLAIR: nulls the normally bright T2 signal of CSF (becomes black), while other parenchymal fluid appears bright (as in oedema or tumour).


2. Diffusion Weighted Imaging (DWI) DWI assesses the movement of protons due to diffusion over a short time. If there is restricted diffusion the signal is of high intensity. If there is unrestricted diffusion (e.g. CSF), there is low intensity signal. Unfortunately the signal on DWI is not just produced by the diffusion characteristics but also changes in parallel with the T2 and PD of the tissue. To eliminate these cofounders an apparent diffusion coefficient (ADC) map is generated. The ADC signal is low with true restricted diffusion. Using a combination of these sequences vasogenic oedema (intensity high on T2, high on DWI and high on ADC map) may be distinguished from cytotoxic oedema (intensity high on T2, high on DWI and reduced on ADC map).


3. Gadolinium contrast: Gd is a paramagnetic substance that acts as a contrast agent by markedly shortening T1 when it is present. Gd based contrast distributes throughout the extracellular fluid, and does not cross the normal BBB. In pathological conditions in which the BBB breaks down, there is marked hyper-intensity (enhancement) of affected areas on T1 weithted images.