A retraced spiral strategy with semi-automatic deblurring for volumetric thermometry.

Purpose: To develop a 3D MRI-thermometry technique for transcranial MR-guided focused ultrasound (MRgFUS).

Methods: A stack of retraced in-out (RIO) spirals was incorporated into a 3D, RF-spoiled, gradient recalled echo (GRE) sequence with a minimized energy deblurring strategy. Bloch simulations examined isochromat precession during RIO readout under temperature profiles matching those encountered during transcranial MRgFUS and measured the resulting effects on observed peak temperatures and FWHM of the heating patterns. The sequence then monitored temperatures in a phantom undergoing insonation from an MRgFUS device. Finally, the sequence monitored temperatures, without insonation, in four patients immediately after MRgFUS thalamotomy. For comparison, temperatures were also monitored using a frequently used, 2D, Cartesian, multi-echo, spoiled GRE sequence.

Results: The RIO sequence removed under-and over-estimation of peak temperatures produced by spiral-in and spiral-out portions, respectively, of the RIO readout. In phantoms, peak temperatures from the RIO sequence were statistically indistinguishable from those from the Cartesian sequence (p > 0.05). While, in silico, the RIO sequence accurately estimated heating FWHM (error <1 mm), in phantoms, the RIO sequence overestimated FWHM (error ˜ 1 mm, p < 0.05), when compared to the Cartesian sequence. In patients, the RIO sequence had a 40% improvement in temperature efficiency compared to the Cartesian sequence, with average temperature uncertainties of 1.32 and 1.75°C, respectively. The RIO sequence also produced residual blur artifacts at tissue interfaces.

Conclusions: The 3D RIO strategy with deblurring monitored focal heating under transcranial MRgFUS conditions with similar accuracy and precision as an existing 2D Cartesian method.