Improvement of a 4-Channel Spiral-Loop RF Coil Array for TMJ MR Imaging at 7T

Ultra-high field (UHF) magnetic resonance imaging (MRI) has the advantages of a higher signal-to-noise ratio (SNR) (1, 2) as well as enhanced susceptibility T2* contrast (3, 4). However, it also has drawbacks, such as the inherent inhomogeneous radiofrequency (RF) magnetic (B1) field as well as limited penetration depth. The later is often affected by wave interaction between subject and RF coil and could result in attenuation of RF penetration and asymmetric propagation of the electromagnetic (EM) wave inside the biological subject (5-7). For this reason, RF signal experiences distortion by the superposition of RF waves with different phases, especially at UHF MRI such as 7T (8). In view of these intrinsic problems and in order to meet the specifications required for temporomandibular joint (TMJ) MR imaging at UHF, the optimal coil design is needed to provide an improved penetration depth as well as high SNR in the imaging region. Since all the obstacles to MR imaging at UHF such as 7T are closely related to the operating frequency of 300 MHz, selection of optimal coil configuration and their integration into the actual MR system appear to be the most critical factor (9). For the TMJ application, the most common coils used for TMJ imaging in high-field (HF) MRI were the single-channel circular-loop coil (10, 11) or a 4-channel receive (Rx)-only coil with INTRODUCTION www.ksmrm.org JKSMRM 16(2) : 103-114, 2012