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 ra pidly monitor temperature during MR-guided radiofrequency (RF) interve ntional procedures on a .2-T open configuration scanner. Temperature c alibration was performed on thermally controlled gel phantom and ex vi vo bovine liver samples. Keyholing methods were implemented for rapid imaging and tested both in simulation experiments and in the gel phant om. Phase drifts from extraneous sources were monitored and compensate d for using reference phantoms. Sequence parameters TE, TR, and flip a ngle (FA) were optimized for maximum temperature sensitivity and minim um noise., Reduction of phase noise from coupling of the magnetic fiel d to external perturbations using navigator-echo-based correction sche mes were also investigated. The extraneous phase drifts from the magne t could be minimized by keeping the electromagnet on continuously. Nav igator echo corrected keyholed FLASH sequences (TE = 30 msec, TR = 60 msec, FA = 40 degrees, 64 x 128 matrix) were used to monitor the RF le sioning process in gel phantoms yielding images every 4 seconds with a temperature sensitivity of .015 ppm/degrees C. RF ablation in the bov ine 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 a nd a temperature sensitivity of .007 ppm/degrees C. The results indica te 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.