Wednesday, 29 July 2015

xrayoftheweek 30: the start of a series on diffusion weighted imaging (DWI)

One of the most interesting and stimulating parts of being a doctor in general, and a radiologist in particular, is keeping up with new technology and techniques and being involved in their evolution and application. A technique that has risen to prominence in abdominal imaging in this early part of my career is diffusion weighted imaging (DWI) - an MRI sequence that essentially interrogates how freely water molecules can move in the extracellular space. This sequence is quick to acquire, doesn't require contrast, and provides valuable functional information. This video by Frank Gaillard of Radiopaedia does an amazing job of explaining what DWI is and how it works.

For the next few weeks, the #xrayoftheweek will feature DWI - and I'll describe how I have introduced it and adapted it for various abdominal applications in my Consultant post in Leicester. 

The first of these is this panel of selected images from a liver MRI:

a: diffusion weighted image, b=1000
c: apparent diffusion coefficient map
d: calculated ADC map (takes into account T2 shine through)

The lesion in the right lobe of the liver is seen on the STIR image as having central hyperintensity, with a rim of intermediate signal tissue. This rim is bright on the DWI image and darker on the ADC map, indicating restricted diffusion. Bear in mind that this is a subjective assessment, and that proper reading of these images necessitates using ROIs to measure the ADC value. The calculated ADC map shows the rim as bright - confirming restricted diffusion. 

Conversely, the centre of the lesion is mildly hyperintense on both the DWI and ADC, in keeping with T2 shine through - confirmed on the calculated ADC map.

This turned out to be a metastatic deposit with central necrosis. The active tumour (with restricted diffusion) is the rim of the lesion, with the tumour growing radially outwards and leaving necrosis in the centre. Abscesses usually have restricted diffusion throughout.

This chart is a very simplistic illustration of how signal change is classified on DWI images, and once again, this subjective assessment should be backed up by an objective assessment of the ADC values using ROI tools.

One of the applications of this technique under research is using the ADC value of lesions to monitor the response to therapy, as we move away from using simply 2 dimensional measurements to a multiparametric assessment of tumours. For detailed reading, the following references will provide a lot of relevant background information, and are useful for an overview of use of DWI in abdominal imaging:

Sinha et al in Radiographics and Sandrasegaran in Radiology Clinics of North America.


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