Simultaneous bilateral hip joint imaging at 7 Tesla using fast transmit B₁ shimming methods and multichannel transmission - a feasibility study.

Abstract

The objective of this study was to demonstrate the feasibility of simultaneous bilateral hip imaging at 7 Tesla. Hip joint MRI becomes clinically critical since recent advances have made hip arthroscopy an efficacious approach to treat a variety of early hip diseases. The success of these treatments requires a reliable and accurate diagnosis of intraarticular abnormalities at an early stage. Articular cartilage assessment is especially important to guide surgical decisions but is difficult to achieve with current MR methods. Because of gains in tissue contrast and spatial resolution reported at ultra high magnetic fields, there are strong expectations that imaging the hip joint at 7 Tesla will improve diagnostic accuracy. Furthermore, there is growing evidence that the majority of these hip abnormalities occur bilaterally, emphasizing the need for bilateral imaging. However, obtaining high quality images in the human torso, in particular of both hips simultaneously, must overcome a major challenge arising from the damped traveling wave behaviour of RF waves at 7 Tesla that leads to severe inhomogeneities in transmit B1 (B(1) (+) ) phase and magnitude, typically resulting in areas of low signal and contrast, and consequently impairing use for clinical applications. To overcome this problem, a 16-channel stripline transceiver RF coil was used, together with a B1 shimming algorithm aiming at maximizing B(1) (+) in six regions of interest over the hips that were identified on axial scout images. Our successful results demonstrate that this approach effectively reduces inhomogeneities observed before B1 shimming and provides high joint tissue contrast in both hips while reducing the required RF power. Critical to this success was a fast small flip angle B(1) (+) calibration scan that permitted the computation of subject-specific B1 shimming solutions, a necessary step to account for large spatial variations in B(1) (+) phase observed in different subjects.