Improved Slice Selection for R2* Mapping During Cryoablation with Eddy Current Compensation

Purpose: To improve the slice profile and image quality of R2* mapping in the iceball during cryoablation with ultrashort echo time (UTE) imaging by compensating for eddy currents induced by the selective gradient when half-pulse radiofrequency (RF) excitation is employed to achieve UTEs.

Materials And Methods: A method to measure both B0 and linear eddy currents simultaneously is first presented. This is done with a least-square fitting process on calibration data collected on a phantom. Eddy currents during excitation are compensated by redesigning the RF pulse and the selective gradient accordingly, while that resultant from the readout gradient are compensated for during image reconstruction. In vivo data were obtained continuously during the cryoablation experiments to calculate the R2* values in the iceball and to correlate them with the freezing process.

Results: Image quality degradation due to eddy currents is significantly reduced with the proposed approaches. R2* maps of iceball throughout the cryoablation experiments were achieved with improved quality.

Conclusion: The proposed approaches are effective for compensating eddy currents during half-pulse RF excitation as well as readout. TEs as short as 100 microsec were obtained, allowing R2* maps to be obtained from frozen tissues with improved quality.