STIR MR imaging of the orbit

Department of Radiology, Hospital of the University of Pennsylvania, Philadelphia 19104.

Fifteen patients with CT-documented orbital lesions were evaluated with MR imaging at 1.5 T with both conventional spin-echo (SE) and short inversion time inversion recovery (STIR) sequences. Fat signal was reliably nulled at inversion times of approximately 120-200 msec in all cases, thereby allowing clear detection of all retrobulbar lesions and normal structures on STIR images as markedly hyperintense relative to fat. All lesions were also clearly depicted on SE images; in fact, short repetition time/short echo time SE sequences were at least as useful as STIR images for illustrating anatomic structures and mass lesions, and in a much shorter scanning time. Separation of optic nerve from perioptic subarachnoid space was clear on SE images, but often difficult or impossible on STIR images owing to the relatively high intensity of normal optic nerves on STIR images. The synergism of relaxation prolongation with STIR actually resulted in loss of information, as any ability to separate the effects of T1 from T2 on signal intensity was impossible when STIR was the sole pulse sequence. We believe that more information is obtained with standard SE sequences than with STIR sequences, and therefore SE remains the method of choice for orbital MR imaging.
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