3-DIMENSIONAL MR GRADIENT RECALLED ECHO IMAGING OF THE INNER-EAR - COMPARISON OF FID AND ECHO IMAGING TECHNIQUES

The detailed structures of the inner ear make this region a diagnostic challenge for radiologists. Thin section high resolution CT is the '' gold standard'' for studies of the fine bony detail of the inner ear. Although CT can delineate bony structures, fine soft tissue details su rrounded by CSF/endolymph (such as nerves in the internal auditory can al) are not easily identified. Conventional MR spin-echo T2-weighted i mages provide good image contrast for such structures, but the current commercially available minimum slice thickness of approximately 2-3 m m is too thick for the inner ear. Volume gradient recalled echo (GRE) MR imaging techniques can be used to achieve thin slices (< 2 mm) whil e maintaining adequate contrast f or detailed examination. In the work reported here a volume GRE sequence that images the echo formed in a steady-state-free-precession (termed ''CE-FAST'' or ''SSFP'' on variou s commercial MRI systems and called SSFP-echo in this work) was used t o image inner ear structures. This technique was compared with images generated using conventional volume GRE techniques (GRASS). While smal l flip angle volume GRE imaging has been used for inner ear imaging pr eviously, the low contrast typical of such density weighting makes it difficult to distinguish soft tissue structures f rom surrounding CSF/ endolymph. In this work, contrast-to-noise ratios (CNR) between CSF/en dolymph and brain parenchyma were compared between the sequences at 15 -degrees, 30-degrees, 60-degrees, and 90-degrees flip angles. The SSFP -echo sequence produced higher CNR for such structures and consistentl y outperformed GRASS sequences at flip angles of 30-degrees, 60-degree s, and 90-degrees. Thus, it is concluded that SSFP-echo techniques may be useful for imaging the fine structural anatomy of the inner ear.