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- Open Access
Improved k-space-based MR thermometry by joint PRF phase shift and T1/T2* attenuation estimation
© Gaur and Grissom; licensee BioMed Central Ltd. 2015
Published: 30 June 2015
MR temperature mapping based on the proton resonance frequency (PRF) shift is used in MR-guided focused ultrasound procedures for dosimetry and safety monitoring. While conventional PRF-shift thermometry is based on calculating a phase difference between two reconstructed MR images, Gaur et al [1, 2] have recently described two algorithms that estimate temperature-induced phase shifts directly from MR k-space data, prior to image reconstruction. The approach enables large dynamic scan acceleration factors and the correction of chemical-shift (CS) effects that geometrically distort the temperature maps. However, that work neglected image attenuation that accompanies the PRF phase shift and is primarily caused by increasing T1 with temperature. Here it is shown that attenuation degrades the accuracy of k-space-based reconstructions, but that it can be accounted for in the reconstructions.
Simulations and experiments were performed using gradient-recalled echo scans at 3 Tesla (Philips Achieva with Sonalleve HIFU) with 16 ms echo time and 44 Hz bandwidth. A phantom was simulated with Gaussian-shaped hot spots ranging from 0 to pi and exponential attenuation factors ranging from 0 to 0.8. A tissue-mimicking gel phantom was imaged and sonicated for 41 s with a 4 mm diameter treatment cell at 110 W and 1.2 MHz. Temperature maps were reconstructed using image-domain hybrid, CS-compensated, and proposed joint attenuation- and CS-compensated approaches. The latter reconstructions were implemented as a refinement stage after a hybrid reconstruction. Gradient descent was used to iteratively update the temperature phase shift and exponential attenuation maps to minimize the error between the measured k-space data and the treatment k-space signal model.
Results and conclusions
This work was supported by the Focused Ultrasound Foundation, the HHMI/VUMC Certificate Program in Molecular Medicine, NIH R25CA136440, and DoD W81XWH-12-BCRP-IDEA
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