Magnetic resonance characterization of non-flowing intravascular blood.

We eliminated the effects of flow on the magnetic resonance (MR) appearance of intraluminal blood by imaging the superficial veins in the arms and legs of 3 volunteers during venous occlusion with tourniquets. T1, intermediate and T2 weighted transaxial images were obtained using a multisection spin-echo technique at 1.5 T. The images were analyzed by visual inspection and by region of interest pixel intensity measurements. The intensity of non-flowing blood was compared with muscle, fat and a tube of water taped to the skin. We determined that non-flowing, unclotted blood has intermediate T1 characteristics and long T2 characteristics. On T1 weighted images, non-flowing blood is of greater signal intensity than water or muscle, but less than fat. On T2 weighted images, non-flowing blood is of greater signal intensity than fat and muscle, but slightly less than water. These results may help explain the signal observed with very slow intravascular flow and hyperacute intracranial hematomas, where intermediate T1 and long T2 relaxation characteristics are reported.