Diffusion weighted imaging
Diffusion weighted imaging (DWI) shows whether the movement of water is restricted or free. There are two types of images we gain from a DWI sequence: DWI and ADC map.
When scrolling through the DWI sequence you might see two sets of images called the B0 and B1000 (some centres will also display a B500). All you need to know is that it is the sequence in which the CSF in the ventricles is dark that we look at. Water that freely moves is dark (as is the fluid in the CSF spaces) and water that has restricted movement is bright. We use the terms true restricted diffusion when high DWI signal is actually due to restricted diffusion, and false restricted diffusion when high DWI signal is due to something else.
- Step 1
- Look for bright signal on the B1000 DWI map. This usually indicates pathology. Now, this bright signal can either be due to true restricted diffusion or due to artifact. To determine which we need to look at the ADC
- Step 2
- Dark signal on ADC = true restricted diffusion
- Bright signal on ADC = false restricted diffusion
False restricted diffusion
The things that cause false restricted diffusion are:
T2 shine through
DWI has some T2 weighting. If something has very bright T2 signal it will appear bright on DWI. This is why when we see bright signal on DWI we look at the ADC map. ADC is calculated from the DWI sequence. It has no T2 component and the signal is the inverse of DWI i.e. the CSF will be bright and true restricted diffusion will be dark.
Artifact
DWI is very sensitive to susceptibility effect at air / bone interfaces and you will often see high signal adjacent to the frontal sinuses, middle cranial fossa and petrous bones.
DWI is also very sensitive to susceptibility from hemorrhage and metal. Metal causes a large area of signal dropout, obscuring the surrounding tissue. Artefact from haemorrhage makes the DWI signal uninterpretable.
True restricted diffusion
There are two main things that cause true restricted diffusion:
- swollen cells in cytotoxic oedema e.g. acute infarcts
- lots of densely packed cells in hypercellular lesions like tumours
Tip
Look at the ventricles. CSF within the ventricles is freely moving. If a lesion is of the opposite signal to the CSF on DWI and ADC then you can say that it is restricted diffusion.