Monitoring of laser and freezing-induced ablation in the liver with T1-weighted MR imaging.

During both interstitial laser ablation therapy and cryoablation therapy for liver tumors, real-time monitoring is necessary for assessment of ongoing thermal effects in tissue. With single-section images obtained every 30 seconds with a T1-weighted RARE (rapid acquisition with relaxation enhancement) sequence, signal intensity changes in both ex vivo and in vivo animal liver were readily seen. The reversible loss of signal intensity that took place during laser irradiation and the increased intensity at the beginning of cooling can be explained mainly by altered T1 due to temperature change. The frozen area was seen as a sudden decrease in signal intensity at 0 degrees C due to a T2 decrease. This preliminary work showed that the protocol provides enough temporal and temperature resolution to accurately depict the extent of thermal damage, as confirmed at histologic examination. Signal intensity decreased linearly with temperature in the range 10 degrees C-50 degrees C, yielding a pixel-to-pixel temperature resolution of 5.37 degrees C.