"increases the T2 relaxation time, slows the rate of decay, and causes these processes to be highlighted on T2-weighted images." This feature of MRI provides a nearly custom-designed camera for studies, not merely of the brain's gross anatomical structure, but of the condition of brain tissue as well.

Because MRI allows physicians to look directly at biochemical effects and cell processes, many strategies have been developed, including new approaches to internal bleeding. Explained Bradley: "Hemorrhage has many different forms. We can be fairly specific about the age of hemorrhage based on its MR appearance. Acute hemorrhage is primarily deoxyhemoglobin, which has a short T2 and thus causes loss of signal. Typically, some days later, methemoglobin will be formed" and emit a different but characteristic signal that has "high intensity on any sequence and is therefore easily detectable."

The short T1 time associated with methemoglobin can be attributed to the fact that it contains iron, which reacts directly with the surrounding water protons in a dipole-dipole interaction. Deoxyhemoglobin can be clearly distinguished from methemoglobin, said Bradley, because its associated "water molecules are unable to approach the heme iron within 3 angstroms for a dipole-dipole interaction," the mechanism whereby T1 is shortened.

Paramagnetic substances act as contrast agents to enhance MR images. One such solid is ferritin, a "first cousin," explained Bradley, to "the hemosiderin that is a breakdown product of hemorrhage. Seeing this, we know that the only way it could have gotten there was from a prior bleed." Both of these show up as a conspicuous blackness due to short T2 relaxation times. "Early in the subacute phase of hemorrhage,'' continued Bradley, methemoglobin within the red cells is magnetically very susceptible when compared to the plasma just outside these cells, leading to a short T2. Later in the subacute stage, lysis of the red cells leads to a long T2 and high signal. Thus variation in the T1 and T2 relaxation times indicates the sequential stages that may develop—and between which it is important to distinguish—in intracranial hemorrhage.

Capturing Fluid Motion with MRI

"Few aspects of MR are as potentially confusing as the effect of motion on the MR image," wrote Bradley (Stark and Bradley, 1988, p. 108). But the detection of motion "has no correlate in CT" scanning, and so pictures of flow represent a fundamental advance in diagnostic imaging. An invasive diagnostic x-ray technique called angiogra-

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