Temperature Mapping During MR-Guided Cryoablation Using a FLORET UTE Sequence

To implement a three-dimensional FLORET-UTE sequence for rapid MR thermometry during cryoablation and to validate voxel-wise temperature maps against fiber-optic probes in ex vivo bovine liver.

A FLORET-UTE trajectory (TE 50 μs, 1.6 mm isotropic, 18 s/volume) was executed on a 3 T scanner while dry-ice cryoablation was applied to liver phantoms. Four fiber-optic sensors provided reference temperatures. Magnitude signal was normalized to baseline and fitted to a mono-exponential temperature model in the first experiment; the unchanged model was applied prospectively to the second dataset. Images were reconstructed offline with ${\ell }_2$ -regularized non-Cartesian SENSE. Accuracy below 0°C was assessed with root-mean-square error (RMSE) and Bland-Altman analysis.

Calibration yielded a strong signal-temperature relationship ( $R^2=0.95$ ). Prospective validation produced RMSEs of 0.93°C-1.73°C (overall 1.33°C). Bland-Altman bias was $-0.08$ °C with limits of agreement $-2.08$ °C to 2.23°C. Serial temperature maps captured three-dimensional ice-ball dynamics at 18-s temporal resolution-substantially faster than previously reported radial-UTE and STIR-UTE approaches.

FLORET-UTE enables quantitative 3-D MR thermometry of frozen tissue with < 20 s temporal resolution and < 2°C accuracy, overcoming the speed limitations of earlier UTE methods. The technique is a promising candidate for real-time MR-guided cryoablation and warrants in vivo evaluation.