Diffusion-Weighted Imaging
Diffusion weighted imaging (DWI) is an MRI sequence that measures the random movement (Brownian motion) of water molecules in tissue. Where diffusion is restricted by cell swelling, high cellularity, viscous fluid or tight structure, the signal stays high on images with strong diffusion weighting.
The sequence uses a rapid echo planar acquisition with two or more b values (the strength of the diffusion weighting, in s/mm2): a low value (0 or 50) and a high value (800 to 1000 for the brain, 800 to 1500 for the body and prostate).
Each acquisition takes seconds, and no contrast is needed. From the images an apparent diffusion coefficient (ADC) map is calculated, in which restricted diffusion appears dark.
DWI is part of nearly every brain MRI, and is also used in the body (liver, pancreas, kidney, pelvis, prostate, breast, lymph nodes, bone marrow) and in whole body surveys.
Diffusion tensor imaging, an extension that measures the direction of diffusion, maps white matter tracts and is described briefly with the interpretation.
The study adds 2 to 5 minutes to an examination. Slices are 3 to 5 mm thick.
Stroke is the second commonest cause of death worldwide. DWI shows acute ischaemic infarction within minutes of onset, with a sensitivity of 88 to 100% and a specificity of 86 to 100%, against about 30 to 40% for CT in the first hours.
In oncology, DWI separates malignant from benign lesions and monitors the response to treatment, without radiation or contrast.
Indications.
- Suspected acute ischaemic stroke or transient ischaemic attack, and assessment of the infarct core.
- Brain abscess versus necrotic tumour, empyema, encephalitis, prion disease and epidermoid cysts.
- Hypoxic ischaemic injury in neonates and adults, and metabolic encephalopathy.
- Demyelination and other white matter disease (acute lesions).
- Tumour characterisation and grading in the brain, and treatment response.
- Body oncology: detection and staging of liver, pancreatic, prostate, breast, pelvic and lymph node tumours, bone marrow disease and treatment response.
- Inflammation in bowel disease, and sacroiliac and spinal inflammation or infection.
- Spinal cord infarction and compressive myelopathy.
- Whole body diffusion for myeloma and metastasis.
- Tractography (diffusion tensor imaging) before surgery.
Not as a stand alone test. It is always read with the anatomical sequences.
Contraindications and precautions.
- The usual MRI safety screen. No contrast is needed, so renal function does not limit the use.
- Metal, air tissue interfaces and the skull base cause distortion and signal loss.
- Claustrophobia and inability to lie still. The sequence is fast and tolerates mild motion better than longer sequences.
- Stroke patients may be unstable, and monitoring is needed. Imaging must not delay emergency treatment.
- Pregnancy: acceptable if the clinical need is clear.
Preparation and technique.
No preparation is needed. The patient lies supine. For brain imaging, axial slices cover the whole brain with 3 to 5 mm thickness and b values of 0 and 1000, with an ADC map. Three directions of diffusion weighting are combined into a trace image.
For body imaging, free breathing or respiratory triggered sequences with multiple b values (0, 50, 400, 800 or higher) are used, with fat suppression. A calculated high b value image can be produced to improve contrast in the prostate and body.
Quantitative values (ADC in 10-3 mm2/s) are measured by placing a region of interest on the lesion, avoiding necrosis and cysts. Whole body studies combine multiple stations, usually with inverted grey scale display for survey reading.
