Echo combination to reduce proton resonance frequency (PRF) thermometry errors from fat

Abstract

Purpose: To validate echo combination as a means to reduce errors caused by fat in temperature measurements with the proton resonance frequency (PRF) shift method.

Materials and methods: Computer simulations were performed to study the behavior of temperature measurement errors introduced by fat as a function of echo time. Error reduction by combining temperature images acquired at different echo times was investigated. For experimental verification, three echoes were acquired in a refocused gradient echo acquisition. Temperature images were reconstructed with the PRF shift method for the three echoes and then combined in a weighted average. Temperature measurement errors in the combined image and the individual echoes were compared for pure water and different fractions of fat in a computer simulation and for a phantom containing a homogenous mixture with 20% fat in an MR experiment.

Results: In both simulation and MR measurement, the presence of fat caused severe temperature underestimation or overestimation in the individual echoes. The errors were substantially reduced after echo combination. Residual errors were about 0.3 degrees C for 10% fat and 1 degrees C for 20% fat.

Conclusion: Echo combination substantially reduces temperature measurement errors caused by small fractions of fat. This technique then eliminates the need for fat suppression in tissues such as the liver.

Fig 1

The phase accumulation versus echo time TE in terms of phase angle between water and fat is plotted for pure water (dotted line) and a two-component system consisting of 80% water and 20% fat (solid line) for different temperature changes. Using the phase corresponding to the 0°C temperature change as the baseline phase, the phase difference in the water/fat systems is determined, indicated as the dashed line, which oscillates around the true temperature.

Fig 2

Temperature error for temperature changes from -60 to 60°C as a function of echo time, which is expressed in terms of the phase angle between water and fat. The left image shows the absolute temperature error for a mixture of 90% water and 10% fat, the right image for a mixture of 80% water and 20% fat.

Fig 3

Upper row: Temperature errors as a function of temperature change for three different echo times and fat contents of 10%, 20%, and 30%. The black and gray solid lines correspond to the in-phase echo times of 2π and 4π, respectively, and the dashed line corresponds to the out-of-phase echo time of 3π. Combining the echoes according to Eq. 5 results in dotted line. Lower row: Plots showing error and relative error of temperature measurements after echo combination or a fat content of 10% (solid line), 20% (dotted line), and 30% (dashed line).

Fig 4

MRI temperature measurements during cooling of pure water and a mixture of water and fat. Whereas the measurements in water are very similar at all echo times, measurements in the water/fat mixture are echo time dependent. Combination of the echoes (dotted line) results in temperature measurements very similar to that of water.