OPTIMIZATION OF ENCODING GRADIENTS FOR MR-ARFI

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. For high intensity focused ultrasound (HIFU) treatments without significant heating, MR monitoring could be done by acoustic radiation force imaging (MR-ARFI). MR-ARFI measures small displacement induced by HIFU, which might be useful for many novel HIFU applications, e.g. temporary blood-brain barrier disruption and intracellular molecular delivery. Currently, MR-ARFI uses large single-lobe gradients to encode the displacement, which may render the image sensitive to motion, low SNR from the large b-value, and susceptible to artifacts in the phase map. In this study, MR-ARFI encoding gradients are developed to account for the above problems. A repeated bipolars gradient set and an inverted bipolars gradient set, which consisted of two identical or opposite-polarity bipolar gradients around the refocusing pulse, were compared to the conventional two single lobes gradient set with the same TE/TR and motion encoding pulse width. The HIFU system was triggered by the MR sequence to emit ultrasound synchronized with the encoding gradients with a duty cycle less than 5%.