The evolution of subarachnoid hemorrhage (SAH) on MRI differs significantly from hemorrhages confined to the brain parenchyma. Clots in the subarachnoid space are usually small, multiple, and diffuse. They are exposed to, and mixed with, cerebrospinal fluid, which has high pO2 values. High ambient oxygen levels mean that SAH will evolve more slowly than parenchymal hematomas on MRI.
T2/T2*-shortening occurs with transition to deoxyhemoglobin, but decreased signal may be difficult to appreciate on T2-weighted images unless the hemorrhage is massive or collects in the ventricular system. Susceptibility-weighted (SW) or gradient echo (GRE) images may be better for SAH detection during this phase, although T2-FLAIR remains an important diagnostic sequence at all stages.
Transition to methemoglobin with shortening on T1-weighted images is often delayed to late in the first week after SAH. In mild SAH this phase may never be seen as the hemoglobin may be diluted, resorbed, or dispersed before methemoglobin formation occurs. If present, however, the T1-shortening from methemoglobin may allow diagnosis of SAH at a time when CT images have returned to normal. Even though T1 shortening occurs, T2-FLAIR may still be superior for detection even in this later period.
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