Susceptibility-weighted imaging (SWI) is a T2* MRI technique that takes advantage of both magnitude and phase to enhance contrast. The signal changes result from disturbances in a homogeneous magnetic field caused by paramagnetic, ferromagnetic, or diamagnetic substances, said E. Mark Haacke, Ph.D., professor of radiology at Wayne State University, Detroit, and director of the Magnetic Resonance Imaging Institute for Biomedical Research in Detroit.
Application of a magnetic field to the brain generates an induced field that depends on the applied magnetic field and the magnetic susceptibility of molecules within the brain. Signal intensity changes are dependent on several factors, including hematocrit, deoxyhemoglobin concentration, and the presence of hemosiderin and other paramagnetic or diamagnetic substances. The technique is very sensitive in detecting intravascular venous deoxygenated blood and extravascular blood products. Most blood products are paramagnetic (deoxyhemoglobin, intracellular methemoglobin, and hemosiderin), letting SWI take advantage of magnetic susceptibility effects.
In brain trauma, the identification of smaller hemorrhages and their locations provides useful data about the mechanism of injury and potential clinical outcome. In this case, SWI showed several bleeds, including shearing of the confluence of the medullary veins into the septal vein. The bleeds appear as black areas, visibile because of the iron in hemosiderin, which accumulated postbleeding.
The join between veins and venules tends to be an area of weakness susceptible to bleeding in brain trauma. In fact, for this patient, “almost all of these bleeds were on the venous side,” said Dr. Haacke. “It's well known biomechanically that the veins are weaker than the arteries, but usually you end up seeing tearing and shearing of both arteries and veins when you have bad trauma.” One of the veins running to the front of the brain—to nerve-containing tissue—also seems to have bled, which could account for this man's headaches.
Iron deposited from bleeding tends to stay in the brain. “Odds are that if we imaged 5 years later, we'd probably still see the major bleeding,” said Dr. Haacke.
The long-term goal is to correlate the imaging with motor and cognitive effects of trauma. “If you could eventually come up with a treatment that helped resolve some of the bleeding, for example, then you would potentially be able to watch and see with MRI if the person is getting better,” said Dr. Haacke.
Conventional MRI (left) hints at a possible bleed in the left front brain. SWI (right) reveals several bleeds from trauma. Photos courtesy Zahid Latif/Detroit Medical Center/Wayne State University
