Fast fat-suppressed reduced field-of-view temperature mapping using 2DRF excitation pulses

Abstract

The purpose of this study is to develop a fast and accurate temperature mapping method capable of both fat suppression and reduced field-of-view (rFOV) imaging, using a two-dimensional spatially-selective RF (2DRF) pulse. Temperature measurement errors caused by fat signals were assessed, through simulations. An 11×1140μs echo-planar 2DRF pulse was developed and incorporated into a gradient-echo sequence. Temperature measurements were obtained during focused ultrasound (FUS) heating of a fat-water phantom. Experiments both with and without the use of a 2DRF pulse were performed at 3T, and the accuracy of the resulting temperature measurements were compared over a range of TE values. Significant inconsistencies in terms of measured temperature values were observed when using a regular slice-selective RF excitation pulse. In contrast, the proposed 2DRF excitation pulse suppressed fat signals by more than 90%, allowing good temperature consistency regardless of TE settings. Temporal resolution was also improved, from 12 frames per minute (fpm) with the regular pulse to 28 frames per minute with the rFOV excitation. This technique appears promising toward the MR monitoring of temperature in moving adipose organs, during thermal therapies.

Fig. 1

(a) The waveform of the echo-planar 2DRF pulse used for fat suppression and reduced field-of-view imaging. The 2DRF pulse consists of 11 SINC sub-pulses and the sub-pulse duration is 1140μs. The amplitude of the sub-pulses is modulated by a Gaussian envelope. (b) The excitation profile obtained using the echo-planar 2DRF pulse features periodic replica along the PE direction.

Fig. 2

The experimental setting for the rFOV temperature mapping in a cheese phantom.

Fig. 3

Simulation of the measured temperature deviation obtained using the PRF model with different TE values. The simulation assumes the presence of 20% fat at various magnetic field strengths, B₀, and for a temperature rise of 10°C.

Fig. 4

(a) Dependence of the measured temperature error on the fat percentage for different temperature changes (dT); (b) The measured temperature deviations for various fat percentages. In order to keep measurement error within 1°C for practical clinical use, the fat component must be less than 5%. For (a) and (b), TE=10ms and B₀ =3T are assumed.

Fig. 5

Fat suppression and rFOV imaging in a phantom consisting of vegetable oil and water when using the 2DRF pulse. (a) The reference image produced by the normal FGRE sequence with the original slice-selective RF pulse. Note the chemical shift along the frequency encoding direction (vertical) due to the narrow receiver bandwidth of 8.06KHz; (b–d) 2DRF pulse excitation profiles with different FOE values. The center of the water passband coincides with the center of the fat stopband to maximize the FOV reduction factor. In (d), only a fraction of water at the FOV center is excited, so rFOV imaging with fewer phase encodes can be applied to accelerate the acquisition without causing aliasing.

Fig. 6

The focal point temperature curves without and with 2DRF pulse excitation with identical FUS heating in a mixed fat-water phantom. The temperature measurements using a normal slice-selective RF pulse (black curves) show a strong dependence on TE value in spite of the identical sonication parameters used. The inconsistency of the temperature measurements is significantly improved due to the fat suppression using the 2DRF excitation (blue curves). By employing a fractional phase FOV factor of 0.4 for rFOV imaging (red curve), the temporal resolution was enhanced from 12fpm to 28fpm without a significant sacrifice in temperature accuracy.