Effect of methemoglobin formation on the MR appearance of subarachnoid hemorrhage.

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Abstract
Subarachnoid hemorrhage has a much higher intensity in magnetic resonance (MR) images with the passage of time. Acute subarachnoid hemorrhage is difficult to see; within 1 wk its appearance has been intensified on T1-weighted images. Different concentrations of blood and lysed red blood cells in CSF were examined spectroscopically but did not significantly alter T1 and T2 relaxation of CSF acutely. UV visible spectroscopy of bloody CSF stored hypoxically for 3 days showed the presence of methemoglobin. The Fe in methemoglobin is paramagnetic; in combination with water this facilitates T1 relaxation. Methemoglobin formation with T1 shortening at least partially accounts for the increasing intensity of the MR appearance of subarachnoid hemorrhage over time in the CNS and may also explain the intense appearance of subacute hemorrhage in MR images elsewhere in the body.

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