Susceptibility weighted imaging
MRI works by manipulating the small hydrogen atoms using applied magnetic fields and gradients. If the local environment also alters the local magnetic field this can cause susceptibility effects. Sometimes these are unwanted e.g. in metal artefact, but sometimes we can use these to our advantage.
There are two main methods of imaging susceptibility effects:
- gradient echo / T2*
- susceptibility-weighted imaging (SWI)
Things that alter the local magnetic field include haemorrhage and calcium. These will appear low signal on these sequences with “blooming artefact” in which they appear much larger than they actually are. SWI is a more sensitive sequence and can pick up much smaller lesions.
Main uses of susceptibility-weighted imaging:
- microhaemorrhages e.g. amyloidosis
- cavernomas
- superficial siderosis
- mineralisation/calcification e.g. in the basal ganglia
You may be asking, if both haemorrhage and calcium are dark on SWI/GRE how do we know which one it is? On SWI, we can answer this question (sometimes).
When we perform an SWI sequence we acquire several images. One is the magnitude, the second is the phase. These are put together to form the SWI images which are often displayed as minimum intensity projection (mIP).
In short, calcium and haemorrhage cause susceptibility for different reasons and phase will exploit these differences by displaying haemorrhage and calcium as opposite signals i.e. one will be dark and one will be bright. Which is dark is dependent on the machine but a good tip is to compare the signal to something that you know is blood (e.g. the venous sinus).
A caveat to this is that in large lesions it can be difficult to tell as aliasing occurs.