Phase imaging on a .2-T MR scanner: application to temperature monitoring during ablation procedures.

Proton phase shift imaging methods with keyholing were developed to rapidly monitor temperature during MR-guided radiofrequency (RF) interventional procedures on a .2-T open configuration scanner. Temperature calibration was performed on thermally controlled gel phantom and ex vivo bovine liver samples. Keyholing methods were implemented for rapid imaging and tested both in simulation experiments and in the gel phantom. Phase drifts from extraneous sources were monitored and compensated for using reference phantoms. Sequence parameters TE, TR, and flip angle (FA) were optimized for maximum temperature sensitivity and minimum noise. Reduction of phase noise from coupling of the magnetic field to external perturbations using navigator-echo-based correction schemes were also investigated. The extraneous phase drifts from the magnet could be minimized by keeping the electromagnet on continuously. Navigator echo corrected keyholed FLASH sequences (TE = 30 msec, TR = 60 msec, FA = 40 degrees, 64 x 128 matrix) were used to monitor the RF lesioning process in gel phantoms yielding images every 4 seconds with a temperature sensitivity of .015 ppm/degree C. RF ablation in the bovine tissue was monitored using navigator-echo-corrected keyholed fast low angle shot (FLASH) sequences (TE = 30 msec, TR = 100 msec, FA = 40 degrees, 128 x 256 matrix) with a temporal resolution of 13 seconds and a temperature sensitivity of .007 ppm/degree C. The results indicate that monitoring of an RF ablation procedure by mapping temperature with sufficient temporal resolution is possible using phase images of FLASH sequences on a .2-T open scanner.