Parallel imaging of head with a dedicated multi-coil on a 0.4T open MRI.

BACKGROUND: Parallel imaging is widely used for cylindrical magnetic resonance imaging (MRI); however, few studies apply parallel imaging to open MRI. We previously developed a parallel method called "RAPID" (rapid acquisition through a parallel imaging design) for imaging the heart on a 0.7T open MRI apparatus, and we have now developed a RAPID head coil and shading correction algorithm for imaging the brain with a 0.4T open MRI apparatus. Images acquired with RAPID were compared with those acquired using a conventional quadrature-detection (QD) head coil. MATERIALS AND METHODS: The images were acquired using a dedicated 4-channel RF receiving coil consisting of a solenoid coil and surface coils. For MRI of the brain, we developed 2 methods to acquire the necessary calibration data: a pre-scan method that acquires the calibration data before the main scans and a self-calibration method that acquires the calibration data and imaging data simultaneously. We also modified the algorithm for calculating the shading distribution so that it only uses acquired image data and then corrects the shading. RESULTS: RAPID was applied for T1-weighted, T2-weighted, fluid-attenuation inversion recovery (FLAIR), time-of-flight (TOF), and diffusion-weighted echo-planar (DW-EPI) imaging. The RAPID images had no visible unfolded artifacts or motion artifacts. Images with the same contrast as that with a conventional QD coil were acquired using the RAPID coil and shading correction. CONCLUSION: These preliminary results show that RAPID can be applied to imaging of the head using a 0.4T open MRI apparatus.

Cardiac cine parallel imaging on a 0.7T open system.

BACKGROUND: Parallel imaging can be applied to cardiac imaging with a cylindrical MRI (magnetic resonance imaging) apparatus. Studies of open MRI, however, are few. This study sought to achieve cardiac cine parallel imaging (or RAPID, for "rapid acquisition through parallel imaging design") with an open 0.7T MRI apparatus. MATERIALS AND METHODS: Imaging time was shortened in all slice directions with the use of a dedicated four-channel RF receiving coil comprising solenoid coils and butterfly coils. Coil shape was designed through an RF-coil simulation that considered biological load. The auto-calibration of a 0.7T open MRI apparatus incorporated a modified image-domain reconstruction algorithm. Cine images were obtained with a BASG, or balanced SARGE (steady-state acquisition with rewound gradient echo), sequence. Image quality was evaluated with cylindrical phantoms and five healthy volunteers. RESULTS: Multi-slice phantom images showed no visible artifacts. Cine images taken under breath-hold with an acceleration factor of two were evaluated carefully. With auto-calibration, the images revealed no visible unfolded artifacts or motion artifacts. RAPID thus improved the acquisition speed, time resolution, and spatial resolution of short-axis, long-axis, and four-chamber images. CONCLUSION: The use of a dedicated RF coil enabled cardiac cine RAPID to be performed with an open MRI apparatus.