Tissue mechanical properties can be interrogated using focused ultrasound (FUS) and MR acoustic radiation force imaging (ARFI). This abstract presents a new method for acquiring ARFI simultaneously with proton resonance frequency (PRF) shift thermometry. Temporally constrained reconstruction was used to retrospectively increase the temporal resolution of the measurements.

A GRE segmented EPI pulse sequence (Figure 1) was modified to include a bipolar motion encoding gradient (MEG). The sequence emits an optical pulse to trigger short ultrasound (US) bursts (10 ms, 175W) synchronized with the second MEG lobe. Two image sets are acquired: (1) OFF images with no ultrasound, and (2) ON images with US bursts. Tissue displacement is encoded into the phase of the ON images and a complex subtraction between ON and OFF images reveals the phase change. However, tissue temperature changes due to the US bursts will cause additional phase accumulations due to the PRF shift phenomenon. Prior attempts to eliminate temperature effects from the displacement measurement include using spin echo sequences, which are not sensitive to PRF shift, using multi-contrast acquisitions¹, and alternating the MEG polarity². This abstract presents a novel acquisition method that interleaves ON and OFF image data acquisition every TR (Figure 1). The phase of the OFF image will be due solely to the temperature rise caused by the US bursts. The phase difference between the ON/OFF pair yields the displacement-phase, which can be converted to displacement in µm by: $${\Delta}D_eff=Δϕ_D⁄2πγ∫_0^{t_{US}}G_{MEG}(t)δ$$

The phase difference between an OFF measurement and a baseline prior to heating gives the temperature-phase, which can be converted to temperature rise in °C by: $$ΔT=-Δϕ_T/{γB_0TEα}$$

This new acquisition scheme was tested in a gelatin phantom³ with a 1 MHz, 256-channel phased array transducer (Imasonic and IGT). Six interleaved measurement pairs (12 total measurements) were acquired on a Siemens 3T Trio MRI. Image pairs were acquired in 3D and in the following order: {OFF,OFF}{OFF,ON}{OFF,ON}{OFF,ON}{OFF,OFF}{OFF,OFF}. See Table 1 for acquisition parameters. Displacement measurements were compared to a 3D segmented EPI spin echo ARFI sequence⁴, and temperature was compared to a regular 3D segmented EPI PRF thermometry sequence where the US generator replicated the US burst pattern generated by the interleaved ARFI/PRF sequence. Temporally constrained reconstruction⁵ with a sub-sampling factor of 5 was performed to retrospectively improve temporal resolution of the simultaneous ARFI/PRF sequence from 86.4 s to 17.3 s. Temperature and displacement with and without TCR were compared.

Figures 2a and 2b compare the displacement maps measured by the simultaneous ARFI/PRF (Sim-ARFI) sequence and the 3D SE-ARFI sequence. Figure 2c shows a line through the maximum displacement for the Sim-ARFI case, with and without TCR, and SE-ARFI. Maximum displacement for this time frame was 4.9 / 5.7 / 4.6 µm for Sim-ARFI, Sim-ARFI with TCR, and SE-ARFI, respectively. Figures 3a and 3b show temperature maps acquired by the simultaneous ARFI/PRF (Sim-PRF) sequence and a 3D PRF thermometry sequence. Figure 3c shows a line through the maximum temperature measured by the Sim-PRF sequence, with and without TCR, and the standard PRF sequence. Maximum temperature for this time frame was 4.9 / 2.9 / 4.2 °C for Sim-ARFI, Sim-ARFI with TCR, and SE-ARFI, respectively. The time evolution of displacement and temperature are shown in Figure 4 for the maximal voxels plotted in Figure 2c and 3c. After repeated measurements, the location of peak Sim-ARFI displacement moved and peak displacement increased from 4.9 to 7.5 µm as the phantom was heated by 7.6 °C. A peak temperature rise of 6.8 °C and 6.1°C was measured by standard PRF thermometry and Sim-PRF with TCR, respectively.The Sim-ARFI/PRF sequence provided similar measurements of temperature and displacement compared to standard methods. The Sim-PRF temperature measurement exceeded the standard PRF measurement, and thus provided a conservative estimate of temperature rise suitable for safety monitoring. Employing TCR allowed for improved temporal resolution of both ARFI and temperature and followed the temperature measurements of standard PRF sequence and the displacement of SE-ARFI. Compared to sequentially acquired ARFI measurement, this interleaved acquisition strategy reduces the US duty cycle by 50% for a constant TR. Thus, the TR can be halved for faster imaging or kept the same for reduced temperature rise. This method can also be used in 2D mode to provide faster temperature/displacement feedback.This novel method of measuring ARFI and temperature simultaneously makes it possible to continuously monitor changing mechanical properties of tissue as a function of temperature. This could be an additional metric for evaluating the success of thermal therapies.