Optimization of acoustic radiation force imaging: Influence of timing parameters on sensitivity

Purpose: Optimization of timing parameters for MR-guided ARFI to achieve the highest displacement signal-to-noise ratio (SNR_d ).

Theory and methods: In MR-guided ARFI the phase change induced by motion encoding gradients (MEGs) is measured to assess tissue displacement. The sensitivity of this encoding procedure depends on several timing parameters, such as the MEG duration and the offset time between ultrasound (US) and MEG. Furthermore, mechanical and MR tissue constants and MEG schemes (bipolar or three-lobed) influence SNR_d . Optimal timing parameters were determined in simulations for bipolar and three-lobed MEGs, and the results were compared with measurements. To provide clinically usable timing parameters, physiologically relevant ranges of tissue constants were considered.

Results: For the considered ranges of tissue constants, optimal timing parameters provide only 6% higher SNR_d for bipolar than for three-lobed MEG. Three-lobed MEG is less sensitive to motion as confirmed in phantom experiments. Bipolar MEG can use approximately 1.5-fold shorter MEG durations.

Conclusion: Both bipolar and three-lobed MEGs can yield approximately the same SNR_d if the optimal timing parameters are chosen. Bipolar MEG allows for shorter durations, which is preferable if deposition of US energy needs to be minimized, and three-lobed MEG is more suitable when residual motion compensation is necessary. Magn Reson Med 79:981-986, 2018. © 2017 International Society for Magnetic Resonance in Medicine.