A dedicated two-element phased array receiver coil for high resolution MRI of rat knee cartilage at 7T.

Despite that on clinical systems phased array technology is now widely used, the high field MRI experimental systems with multiple receiver channels just became available few years ago. For this reason and due to the large range of magnetic field (frequencies between 200 and 500 MHz for proton resonance), commercial phased arrays implemented in narrow bore for high field applications are rare and relatively expensive. Array coil imaging is an advanced method for acquiring high resolution images with enhanced Signal-to-Noise Ratio (SNR) and/or enlarged Field Of View (FOV) compared for example to single loop surface coil. The volume of interest is then covered by several coil elements and images reconstructed for every single channel are combined afterwards. The goal of this work was to develop a dedicated two-element array coil operating at 300 MHz (7T) for high-resolution imaging of rat knee joint in order to quantify cartilage thickness and volume. A dedicated two-element array coil with two square elements encompassing knee joint was designed and built. Decoupling between elements was achieved with a capacitor inserted on the common leg of the two elements. The average gain in SNR compared to a 15 mm reference single loop coil was 2.2. This SNR gain was used to improve spatial resolution of 3D acquisition by decreasing the voxel size from 59 x 59 x 156 microm(3) to 51 x 51 x 94 microm(3) without time penalty.

Non-invasive in vivo quantification of the medial tibial cartilage thickness progression in an osteoarthritis rabbit model with quantitative 3D high resolution micro-MRI.

OBJECTIVE: To develop a quantitative non-invasive in vivo three-dimensional (3D) high resolution (HR) micro-magnetic resonance imaging (microMRI) protocol to measure the medial tibial cartilage thickness (MT.ThC) in the normal rabbit and in the anterior cruciate ligament transection (ACLT) rabbit model of osteoarthritis and quantify the progression of MT.ThC. METHODS: The left knee of 10 control and 40 operated rabbits was imaged in vivo with a 7T microMRI system at 3 and 5 months after ACLT. A 3D fast low angle short (FLASH) fat-suppressed MRI protocol was implemented leading to 44x176 microm(3) spatial resolution and to 44 microm(3) isotropic voxel after cubic interpolation. Semi-automatic MT.ThC measurements were made in 3D, in four different locations, in vivo and longitudinally in both groups. At 5 months, gross macroscopy, visual analogical evaluation of the cartilage and histology were compared to the MR-based MT.ThC. RESULTS: At 3 and 5 months, the MT.ThC measured in the minimum interbone distance area was the thinnest MR-based MT.ThC. It was significantly lower in the operated group and among the four evaluated MT.ThC, it was the most discriminative between the normal and the operated groups (P<0.05). The MT.ThC measured in the minimum interbone distance area was also the most sensitive to change in the operated group (66.4% MT.ThC loss, P=0.003) while no significant changes were observed in the control group. CONCLUSION: Quantitative 3D HR microMRI allowed for non-invasive longitudinal MT.ThC measurements in four different locations in both the normal and the operated rabbits. We concluded the MT.ThC measured in the minimum interbone distance area reflected the severity of the disease and was the most effective to measure the progression of the medial tibial cartilage destruction.

In vivo colon wall imaging using endoluminal coils: feasibility study on rabbits.

PURPOSE: To assess in vivo distal colon wall magnetic resonance imaging (MRI) feasibility on rabbits using an endoluminal radio frequency (RF) coil on a 1.5-T clinical scanner. MATERIALS AND METHODS: The endoluminal coil signal-to-noise ratio (SNR) was compared to a clinical four-element phased-array body coil. High-resolution (HR) MRI of rabbit colon walls was performed on six rabbits. The imaging protocol combined T1-weighted fast low-angle-shot (FLASH) sequences with and without fat saturation (FS), T2-weighted True-Fast imaging with steady state precession (Fisp), turbo spin-echo (TSE), and T1-weighted FLASH FS after contrast media injection. Images were compared to histological sections. Catheter tracking using an endoluminal coil in addition to external coils was also evaluated on two rabbits. RESULTS: HR images allow visualization and identification of rabbit colon wall layers. Real-time tracking allows a clear visualization and a good positioning of the endoluminal coil within the rabbit. CONCLUSION: Compared to a clinical multielement array coil, a dedicated endoluminal RF coil provides an important SNR increase at the region of interest (ROI). Very HR images of in vivo rabbit colon walls were achieved providing detailed information on the different wall layers. This technique could be considered on humans for accurate tumoral and inflammatory bowel disease diagnosis.