Magnetic resonance imaging of blood and clots in vitro.

The effects of variations in blood clot composition on magnetic relaxation rates and magnetic resonance (MR) image have been characterized in vitro. Both 1/T1 and 1/T2 were found to be linear functions of hematocrit for blood and clot, with increases in hematocrit resulting in progressive decreases in image signal intensity. Clot formation in fully oxygenated samples produced no change in relaxation rates or MR images compared with unclotted blood, but clot retraction was associated with a significant increase in 1/T2 that resulted in a decreased signal. Retraction resulted in a heterogenous image with appearance of a hypointense peripheral rim; differences in the method of clot preparation resulted in significant image inhomogeneity. The pattern of fibrinolysis was found to depend on the type of plasminogen activator used and its site of initial application. Injection of tissue plasminogen activator into the clot resulted in lysis, primarily in the clot interior, whereas placing the enzyme in the surrounding serum caused degradation from the outside of the clot. Both observations are consistent with the high binding affinity of tissue plasminogen activator for fibrin. By comparison, streptokinase, with low fibrin binding affinity, dissolved thrombi in a peripheral pattern whether injected into the thrombus or introduced in the serum. These findings identify several variables of clot composition and structure that influence MR images of thrombi and should be considered in their interpretation.