2. Background
Susceptibility weighted imaging (SWI) is emerging as a useful technique in a wide variety of intracranial pathologies.
The SWI sequences demonstrate high sensitivity to magnetic susceptibility differences of various tissues in particular blood,
calcification and haemosiderin.
Technical aspects of susceptibility sequences.
SWI is a fully velocity-compensated,
three-dimensional,
gradient-echo sequence that uses both phase and magnitude data to achieve exquisite sensitivity to tissue magnetic susceptibility effects.
The phase and magnitude data are acquired separately.
The combination of magnitude and phase data produces an enhanced contrast magnitude image that is particularly sensitive to some molecules.
Minimum intensity projections image (mIP) can further demonstrate the continuity of tortuous vascular structures thus differentiating them from focal lesions.
Fig. 2
Susceptibility sequences include T2 * and SWI that differs because this one is three dimensional gradient echo sequence and has a high sensitivity.
To understand the SWI sequences we have to review some basic concepts.
The first one is the magnetic susceptibility which is defined as the ability of magnetization of a material in response to an applied magnetic field.
In this way there are
- Diamagnetic materials: that have only weak local magnetic effects (no unpaired electrons).
- Paramagnetic materials: that generates magnetic fields that additively are going to combine with the external magnetic field.
The second basic concept is the susceptibility effects of haemoglobin products.
Haemoglobin is the transporter of oxygen in the blood.
It has four globin subunits that each has a haeme molecule.
The haeme molecule is composed of an iron atom (Fe2+) and a porphyrin ring.
When oxygen binds to the iron atom the molecule is term OXYHAEMOGLOBIN, which is diamagnetic and has a weak local magnetic field.
DEOXYHAEMOGLOBIN is formed when oxygen dissociates from the iron atom.
It is paramagnetic and causes alterations in local magnetic field.
DEOXYHAEMOGLOBIN can be further oxidized to METHAEMOGLOBIN that appears bright in all MRI sequences and has few susceptibility effects.
Finally,
HAEMOSIDERIN is a heavily iron laden protein and is strongly paramagnetic and produce alteration in local magnetic field.
Fig. 3
An interesting observation is seen in patients undergoing general anesthesia for MRI.
The cortical veins appear attenuated.
This might be due to an increased rate of oxygen.
Fig. 4
Calcium presents diamagnetic susceptibility effects.
Although it induces less phase effects than blood products,
it still leads a dephasing of signal that is detected on SWI.
Calcium is commonly found in the brain and may be physiologic or present in a wide variety of conditions.
Iron is paramagnetic in nature and produces strong susceptibility effects.
Iron accumulation increases with age and is also observed in various neurodegenerative diseases.
Fig. 5
The most common SWI artifact is at air tissue interface manifests as concentric hypointensities,
limiting the investigation of areas next to paranasal sinuses and temporal bones.
Fig. 6
Another limitation is the sequence acquisition time that ranges from 5 to 8 minutes,
leading to increased incidence of movement’s artefacts.
We also have to be aware of the possible limitation of this technique in differentiating calcium and blood.