Technique and Normal Findings
Patient motion causes severe degradation of the MRI signals; therefore, general anesthesia is required.14 All equipment brought inside the MRI room should be approved MRI-compliant equipment because metal objects attracted to the strong magnetic field may cause injury to the patient.15 Generation of a complete MRI study takes longer than the equivalent CT examination because several different sequences are produced to highlight tissues of different characteristics.
A typical examination may last between 60 to 90 minutes, depending on the number of sequences and the number of planes generated. Transverse sequences known as T1-weighted spin-echo, T2-weighted spin-echo, fluid-attenuation inversion recovery (FLAIR), and fluidattenuation spoiled gradient echo (FLASH) can be performed for the examination of the middle ear. Additionally, T1-weighted sequences after injection of contrast material in transverse, sagittal, and dorsal planes are also generated. The contrast material used is gadopentetate dimeglumine, a paramagnetic agent that appears hyperintense (white) on the T1-weighted sequence. Obtaining 5-mm transverse slices, as commonly performed for the brain, may obscure the fine structures of the membranous labyrinth due to partial volume averaging with surrounding bone.7 Thin slices of 1 to 2 mm thickness, generated by volume acquisition protocols, have been used to detect the intralabyrinthine fluid.16On T1- and T2-weighted images the air and walls of the bullae and the petrosal portion of the temporal bone are represented as black areas (signal void) and cannot be evaluated17 (see Figure 4-20, A). The intralabyrinthine fluid is isointense tissue (to brain) on the T1-weighted sequence and hyperintense on the T2-weighted sequence (see Figure 4-20, B). The signal resembles the lateral silhouette of a duct18 and represents the fluid (endolymph) associated with the semicircular canals and cochlea. The appearance of the fluid has not been studied with the FLASH and FLAIR protocols. This author has noted a hyperintense (white) signal on the FLASH sequence and speculates that a low-intensity signal exists on the FLAIR protocol.