Susceptibility weighted imaging

Susceptibility weighted imaging (SWI) is an MRI sequence which is particularly sensitive to compounds which distort the local magnetic field and as such make it useful in detecting blood products, calcium etc... 

Physics

SWI is a 3D high-spatial resolution fully velocity corrected gradient echo MRI sequence ^1-3. Compounds which have paramagnetic, diamagnetic and ferromagnetic properties all interact with the local magnetic field distorting it and thus altering the phase of local tissue which turn results in loss of signal ². 

Paramagnetic compounds include deoxyhaemoglobin, ferritin and hemosiderin ¹. 

Diamagnetic compounds include bone minerals and dystrophic calcifications ¹. 

Following acquisition post processing takes place which includes application of a phase map to accentuate the directly observed signal loss ². 

Typically the images presented are: 

1. magnitude 
2. filtered phase
3. SWI (combined post processed magnitude and phase)
4. SWI minimum intensity projection (minIP) e.g. 5mm thick

Clinical use

The most common use of susceptibility weighted imaging (SWI) is for the identification of small amounts of haemorrhage / blood product or calcium, both of which may be inapparent on other MRI sequences. 

Distinguishing between calcification (made up primarily of calcium phosphate, but also containing very small amounts of of copper (Cu), manganese (Mn), zinc (Zn), magnesium (Mg), and iron (Fe)) ³ and blood products is not possible on the post processed SWI images as both demonstrate signal drop out and blooming. 

The filtered phase images are however able to (in most cases) distinguish between the two as diamagnetic and paramagnetic compounds will affect phase in opposite directions (i.e veins / haemorrhage and calcification will appear of opposite signal intensity) ³. 

Pitfalls

Widowing and greyscale inversion

Filtered phase images are not uniformly windowed or presented by all manufacturers and as such care must be taken to ensure correct interpretation. As simple step to make sure that you always view the images in the same way is to look at venous structures and make sure they are of low signal (if bright you should invert the grey scale). Then window the image narrowly such that the image appears a little reminiscent of a dark CT of the brain. Calcifications will now appear bright (white). 

Aliasing

Although filtered phase images are probably more sensitive to very small amounts of calcium than CT ³, they perform poorly and can be confusing for larger amounts of calcification. When the field is large enough that the phase exceeds π (pi) radians, it will alias to -π radians and will now appear to be dark rather than bright ³. The net effect is that large regions of calcifications can have areas that appear dark, or be surrounded by dark regions.