| Background:The proton resonance frequency shift (PRFS) method is by far the most common MR temperature-monitoring technique used in the clinic because of the temperature linearity and sensitivity of this method. However, PRFS thermometry is also sensitive to magnetic field changes, respiration and cardiac-associated motion and ultrasound transducer movement, which sometimes result in incorrect temperature readings. In this study, we demonstrate the use of a T1 magnitude sequence to monitor a HIFU ablation procedure in ex vivo and in vivo experiments to assess the relationship between changes in the signal magnitude and areas of necrosis and to investigate the feasibility of T1 fast low-angle shot two-dimensional (FLASH 2D)-based thermometry.Purpose:To evaluate the feasibility of using a T1 magnitude sequence to monitor a high-intensity focused ultrasound (HIFU) ablation procedure in ex vivo/in vivo experiments.Material and Methods:Ex vivoTwenty pieces of fresh cow’s liver,8 cm×8 cm×5 cm, the acoustic power was 400 W, and the sonication durations were 5 s and 8 s. The distance from the bottom of the liver to the focus spot was 20 mm, and the space between the two sonication spots was at least 30 mm. The boundary of the region where the signal changed was outlined on the MR image immediately after the sonication, in addition, the same region in the reference image was outlined, and the average signal magnitude was calculated automatically by a computer.The changes in the signal magnitude were calculated and divided into five ranges:15%-20%,20%-25%,25%-30%,30%-35%, and 35%-80%. The five ranges in signal changes corresponded to the following five areas: V1 (signal changes greater than 15%), V2 (signal changes greater than 20%), V3 (signal changes greater than 25%), V4 (signal changes greater than 30%), and V5 (signal changes greater than 35%), and the areas were calculated automatically. In vivoExperiments were performed in ten New Zealand White rabbits with an average weight of 3 kg. The same procedure used with the ex vivo experiments was performed, and immediately after the last sonication, T1-weighted fast spin-echo contrast-enhanced MR images were then obtained. A result was expected from the ex vivo experiment, i.e., signal changes greater than x% would indicate a necrotic area; based on these results, the area with signal changes greater than x% in vivo would be obtained. Results: Ex vivoFrom a total of 55 sonicated sites, valid data were obtained from 49 (89%). After sonication, the average signal magnitude of the visible area was (101.83±11.51), and the average signal magnitude of the reference image was (150.97±6.52); a paired t-test showed a significant difference between these two values (P<0.001), indicating that the signal decreased after sonication. A comparison of the actual area of coagulation necrosis with the area of the regions in different ranges of signal variation indicated a positive correlation. When the signal variation rate was greater than 25%, there was no significant difference between the corresponding average area of 70.37±32.25 mm2 and the actual area of coagulation necrosis of 74.53±40.31 mm2 (P=0.36)(Table 1). A high degree of correlation was observed with the univariate analysis (r=0.99; <0.001).In vivoValid data were obtained from 15 (75%) of 20 sonicated sites (in total). The average area with no diffusion on the enhanced image was 80.05±28.12 mm2, and the average area with a signal variation rate greater than 25% was 75.92±22.92 mm2; there was a significant congruence between the two areas (P=0.43).ConclusionsThe use of a T1 magnitude sequence to monitor HIFU ablation procedures in ex vivo/in vivo experiments is feasible. |
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