Temperature measurement using echo‐shifted FLASH at low field for interventional MRI

We investigated the feasibility of using echo‐shifted fast low‐angle shot (FLASH) for temperature‐monitored thermotherapeutic procedures in a 0.2 T interventional magnetic resonance (MR) scanner. Based on the proton resonance frequency shift technique, modified echo‐shifted FLASH has sufficiently high signal‐to‐noise ratio to provide accurate temperature maps with short scan times, i.e., 5 seconds in phantoms (TR = 20.5 msec; effective TE = 30 msec; one echo shift; NSA = 2) and ex vivo experiments (TR = 19.4 msec; effective TE = 28.9 msec; one echo shift; NSA = 2) and 3 seconds (TR = 19.4 msec; effective TE = 28.9 msec, one echo shift; NSA 1) for an in vivo case. The proton resonance frequency shifts with temperature observed in a 0.2 T MR scanner using this sequence were –0.0072 ppm/°C (temperature uncertainty = ±2.5°C) for polyacrylamide phantoms and ‐0.0086 ppm/°C (temperature uncertainty = ±1°C) for ex vivo bovine liver. These experiments demonstrated that echo‐shifted FLASH is a viable method for low‐field temperature monitoring despite the decreased signal and decreased phase sensitivity compared with its counterpart in a 1.5 T MR imaging system. The improved temporal resolution of temperature images, now possible in low‐field interventional MR systems using echo‐shifted FLASH, will allow clinicians more accurate monitoring of interstitial ablation in MR‐guided interventional procedures. J. Magn. Reson. Imaging 1999;9:138–145 © 1999 Wiley‐Liss, Inc.