A multi-echo MRT approach is presented for application in RF safety assessment and validation of thermal simulations. By water-fat separation, more accurate determination of the drift field is possible. The method was tested in the thigh at 7T, using multi-transmit coils. Precision and accuracy were improved considerably compared to a previous single-echo fat-referenced method (precision: 0.09 vs 0.19 °C). Comparison of measured temperature distributions to simulated counterparts show good relative agreement in three subjects for multiple RF shim settings. Strikingly, simulated heating magnitudes mostly underestimated the observed heating with varying extent, suggesting a role for subject-specific parameters, such as perfusion.
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Figure 1: Schematic representation of the method to iteratively solve temperature and drift components from the gradient echo signal equation (a). Parameter definitions are provided in (b). Essential adaptation with respect to the original method10 is that Salomir’s algorithm8 is applied during initialization to prevent local minima caused by diffusive RF heating. Experimental imaging setup (c) where the antennas are placed in the plexiglass holder, and corresponding setup in the simulation environment (d) where the patient-specific model is merged with the Duke model.
Figure 2: Spatial temperature maps without heating averaged over 300 dynamics in 3 volunteers (V1, V2 and V3) with various fat layer thicknesses; relative temperature maps were calculated both with the adapted multi-echo and the single-echo model. For each volunteer and model, a corresponding spatial map of temporal standard deviation is provided.
Figure 3: Spatial temperature maps after 319-385 seconds of heating with 5.3 W per channel in 3 volunteers (V1, V2 and V3) (a); relative heating maps were calculated both with the adapted multi-echo and single-echo model. A corresponding time series (b) is provided for the indicated voxel locations. The calculated drift field at the end of the series is indicated in (c).
Figure 4: Measured |B1+| distributions and simulated counterparts in the central slice of 3 volunteers (a). Measured spatial heating distribution after 300 dynamics and their simulated counterparts in the central slice (b). Heating scan in volunteers 2 and 3 were performed with three and two different shim settings, respectively. For volunteer 3, the data of shim 2 was recorded with a multi-slice acquisition, for which simulated heating distributions are also provided in the corresponding slices (c).
Figure 5: Animated gif representing heating distributions over a time period of 340 seconds in five 8 mm slices (slice gap of 10 mm). Heating hotspots are clearly visible in the top right and bottom region of the leg. The central slice shows the most gradual temperature increase, which is where the algorithm is most confident.