Variation correction algorithm: Analysis of phase suppression and thermal profile fidelity for proton resonance frequency magnetic resonance thermometry at 0.2 T

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Abstract
Purpose: To develop and analyze the performance of the variation correction algorithm (VCA), a phase correction technique that mitigates the contribution of background phase variations by combining accurate alignment of echoes, K‐space‐based phase correction (as opposed to spatial polynomials), and extraction of alias‐free phase difference images.Materials and Methods: A series of echo‐shifted gradient‐recalled echo (GRE) images was processed with K‐space alignment and phase corrected with increasing sizes of M × M masks of central K‐space coefficients. The extent of background phase variation suppression due to magnet field drift was assessed. Further, a simulated thermal profile was superimposed on the same data in a related experiment. Residual errors in reconstructed simulated thermal profiles were quantitatively characterized to estimate algorithm performance.Results: Using a 3 × 3 K‐space mask, the VCA was able to 1) maintain the typical mean backround error in a 35 × 35 pixel region of interest (ROI) at −0.1°C; and 2) reconstruct, relative to the applied thermal profile, a phase‐corrected profile that typically contains a 1.7°C underestimation of peak temperature difference and a mean error along the 60°C line of −0.8°C.Conclusion: The results suggest that thermal profiles can be accurately reconstructed at 0.2 T using the VCA, even in the presence of over 1 ppm spatially and temporally dependent field drift over a 1‐hour time frame. J. Magn. Reson. Imaging 2003;17:227–240.

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