Purpose: To determine whether signal variations and subtle anatomic deformities observed in high-resolution MR studies of temporal bones were caused by the large susceptibility differences at air-fluid interfaces near the round and oval window.
Methods: A systematic study of healthy subjects and plastic phantoms was conducted. The phantom consisted of a series of cylindrical holes of various small sizes within a solid block of plastic. These holes were partially filled with water and then covered with a reservoir of gelatin to simulate the otic capsule air-water interfaces. On a 1.5-T system, T2-weighted fast spin-echo images and three-dimensional Fourier transform gradient acquisition in steady state images were obtained using dedicated phased-array radio frequency coils. The directions of the frequency and in-plane phase-encoding gradients were swapped, and the receiver bandwidth was changed to demonstrate the dependence of the artifacts on these parameters.
Results: The phantom images confirmed and characterized artifacts consistent with magnetic susceptibility differences at the air-water interfaces. There is a combination of signal loss, misregistration in the frequency-encoding direction, and high signal foci related to the air-water interfaces. Furthermore, the artifacts were worse with narrower receiver bandwidth. Similar consistent artifact patterns were seen near the oval and round windows in studies of healthy subjects.
Conclusions: In high-resolution MR imaging there are significant deformities in the display of the normal anatomy because of magnetic susceptibility.