Susceptibility weighted imaging

Susceptibility weighted imaging (SWI) is an MRI sequence that 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. Unlike most other conventional sequences, SWI takes advantage of the effect on phase as well as magnitude ⁴. 

Compounds that have paramagnetic, diamagnetic, and ferromagnetic properties all interact with the local magnetic field distorting it and thus altering the phase of local tissue which, in turn, results in a change of signal ². 

Paramagnetic compounds include deoxyhaemoglobin, ferritin and hemosiderinhaemosiderin ¹. 

Diamagnetic compounds include bone minerals and dystrophic calcifications ¹. 

Following the acquisition, post-processing takes place which includes a high-pass filter, to remove background inhomogeneity of the magnetic field, and the application of a phase map to accentuate the directly observed signal loss ^2,4. 

Typically the images presented are: 

• magnitude 
• filtered phase
• SWI (combined post-processed magnitude and phase)

Often a fourth set of images is provided, minimum intensity projection (minIP) which is just a thick slab of the conventional SWI images and is better able to demonstrate venous anatomy. 

Clinical use

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

They are also well suited to assess veins as deoxyhaemoglobin results in both a loss in magnitude and a shift in phase ⁴. 

Distinguishing between calcification (made up primarily of calcium phosphate, but also containing very small amounts 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 differently (i.e. veins/haemorrhage and calcification will appear of opposite signal intensity) ³. 

Pitfalls

Handedness

Unfortunately, how phase information is shown is not uniform and varies from vendor to vendor and sequence ⁵. This is referred to as 'handedness' depending on the direction a positive change in phase is shown. If clockwise, this is referred to as a left-handed system because when you curl your left hand into a fist the fingers curl in a clockwise direction. This will in turn affect whether paramagnetic and diamagnetic materials will appear as white or black. 

Windowing and greyscale inversion

FilteredAs if the variability in handedness was not sufficiently annoying, how phase images are not uniformly windowed or presented by all manufacturers and as such care must be takenis also variable. And, to ensure correct interpretationmake matters worse, if filtered phase images are grey-scale inverted they will look very similar. This can cause confusion of mistakes when interpreting phase images. 

A simple step to make sure you avoid this is to first check the signal intensity of a known structure e.g. the internal cerebral veins. This will tell you what paramagnetic compounds should look like. 

If you want to, you can also invert the image if need be so that you always view thelook a phase 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 greyscale)(e.g. Then window the image narrowly such that the image appears a little reminiscent of aveins appearing dark CT of the brain. Calcifications will now appear bright (white). 

Aliasing

Although filtered phase images are probably more sensitive to minimal amounts of calcium than CT ³, they perform poorly and can be confusing when larger amounts of calcification or hemosiderinhaemosiderin are present.  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. The converse is also seen in large areas of profound hemosiderinhaemosiderin staining. 

Oxygenation

Patients that are receiving substantial supplemental oxygen (e.g. intubation) may have very little deoxyhaemoglobin in venous blood, resulting in limited visualisation of venous structures. This is due to oxygen in haemoglobin shielding the iron from having their usual paramagnetic effect ⁴. The same effect is taken advantage of to perform blood oxygen level dependent (BOLD) functional MRI.