A conventional T2-weighted spin echo sequence typically yields one echo for every 90° pulse which is then detected by a receiver coil. The rapid acquisition relaxation-enhanced (RARE) or fast spin echo (FSE) sequence shortens the time needed to acquire T2-weighted scans by obtaining multiple echoes for every 90° pulse. Whereas traditional T2-weighted scans require 6 to 10 minutes for scanning whole brain; T2-weighted RARE or FSE images are often acquired in two to five minutes. Although it was initially felt that such scans were not as sensitive to early pathology as peritumoral edema, in fact this has proven to be a minor disadvantage compared with the gain in speed and RARE and FES are now routine. The increased speed of acquisition can also be employed to generate higher resolution images and thus improve detection of small brain lesions. These sequences have the desirable property of reducing artifacts caused by susceptibility and thus are less influenced by surgical clips or air adjacent to the brain.
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