SWI is a relatively new developed technique in MRI increasingly used as routine in image services and helping in the diagnosis of many CNS disorders.
It has superior sensibility compared with conventional T2*GRE as it is a high resolution 3D technique with flow compensation.
It uses the magnetic susceptibility of different compounds like deoxygenated blood,
blood degradation products,
iron or calcium as a source of contrast in MR.
There are some substances with paramagnetic,
diamagnetic and ferromagnetic properties resulting in local heterogeneity of the local magnetic field producing signal changes.
Diamagnetic materials are repelled by the magnetic field but paramagnetic and ferromagnetic materials are attracted by the magnetic field.
Regarding different blood states there is oxyhemoglobin (arteries) that is diamagnetic in contrast to deoxyhemoglobin (veins) that is paramagnetic [1].
- Diamagnetic substances: water,
bone minerals,
calcifications and stable salts.
Their susceptibility is weak.
- Paramagnetic substances: include iron,
gadolinium,
manganese,
copper.
Paramagnetic susceptibility is much stronger than the diamagnetic substances.
- Ferromagnetic substances: their strong susceptibility remains even after the external magnetic field is removed,
rare in human tissue.
There are no endogenous ferromagnetic substances in the human body.
Magnitude and phase data are processed to obtain SWI reconstruction (Fig. 1) ,
(Fig. 2).
The original magnitude image is a 3D long TE gradient echo fully compensated in all three directions.
It is similar to GRE conventional sequence except that it is more sensitive to susceptibility artifacts.
The phase image is filtered of not necessary inhomogeneities of the magnetic field and interface effects between air and tissue,
data are processed to obtain the maximum information in each voxel.
Both magnitude and phase data are essential to create the SWI image and tissue characterization that is very sensitive to compounds that distort the magnetic field.
It is useful in the detection of blood products or calcium and to visualize venous structures and iron.
We can also obtain thick reconstruction in minimum intensity projection (MinIP) for a more anatomic view.
These reconstructed images attenuate the signal from the brain tissue enhancing the continuity of the veins through the slices.
Fig. 1: Combination of Phase mask and Magnitude Image to obtain SWI and MinIP reconstruction.
Fig. 2: Combination of Phase mask and Magnitude Image to obtain SWI and MinIP reconstruction.
There are useful differences between blood and calcium [2,3].
Both paramagnetic substances (iron/blood) and diamagnetic (calcium) appear dark on SWI images.
They will have opposite signal intensities on filtered phase images.
In practice the particular color dark or bright depends on the brand of the MR system.
Siemens and Canon use so-called "left-handed" coordinate systems reference schemes where blood products appear bright and calcium dark on phase images.
GE and Philips use a "right-handed" reference where blood products appear dark and calcium bright on phase images (Fig. 3),
(Fig. 4).
All images on this poster are acquired on a right handed system (GE 3.0T).
Iron produces strong susceptibility effects unlike calcium that produces lower susceptibility.
Fig. 3: Differences between blood and calcium.
Fig. 4: Differences between blood and calcium.