Ultrashort Echo Time Imaging of Cortical Bone at 7 Tesla Field Strength

Introduction MRI at ultra-high field (UHF) strength of 7 Tesla has been increasingly used and optimized for in vivo applications over the past few years. In particular, the increased signal to noise ratio (SNR) renders this new modality very attractive for imaging soft tissues in the human body. More solid or semi-solid tissues such as trabecular and cortical bone provide little to no signal in conventional MRI scans. This is because their signal has decayed to almost zero by the time the data is acquired. However, by using advanced MR imaging techniques like short radiofrequency pulses and a radial readout (1), the echo time TE can be considerably shortened. In this study, we have implemented a 3D ultra-short TE sequence on a 7T and 3T MR scanner and compared its performance using mid-diaphysial sections of three fresh cadaveric radii specimen. Cortical bone possesses extremely short transverse relaxation times of less than 500 μsec (2). Previously, ultra-short TE (UTE) of cortical bone was performed at 1.5T (3) and more recently at 3T (4). In this study, we investigated the feasibility of 3D UTE on a 7T human MR scanner. In order to compare the performance objectively, we have also built two small identical RF coils for both field strengths. In this feasibility study, we hypothesized that UTE is possible at 7T field strength and provides a significant increase in SNR and that T2 relaxation time might differ between the field strengths.