High sensitivity MR acoustic radiation force imaging using transition band balanced steady‐state free precession

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MR‐guided focused ultrasound (MRgFUS) is an increasingly important nonsurgical medical intervention technique. By focusing the ultrasound beam, thermal and mechanical effects of ultrasound can be delivered to prescribed targets inside the human body while having minimal effect on tissue in the beam pathway. Thermal ablation with MRgFUS has been used to treat various diseases in different parts of the body 1. One important step before ablative treatments is to visualize the ultrasound focus for accurate targeting, which is typically performed by applying interrogation sonications to raise the temperature by a few degrees at the focal spot and measure the proton resonance frequency (PRF) shift of aqueous tissues 9. However, unwanted thermal dose is still deposited by the interrogation sonications, especially in well‐perfused organs like the liver where a substantial amount of energy is needed to generate a measurable temperature rise 11. Moreover, this method does not work in adipose tissues, for which PRF is not temperature dependent 12.
Another technique that can be used to visualize the ultrasound focal spot is MR acoustic radiation force imaging (MR‐ARFI), in which image contrast is generated by the tissue displacement induced by the ultrasound radiation force 13. This technique does not depend on PRF shift and, therefore, can be used for both aqueous and adipose tissues. When ultrasound is absorbed, a force is exerted on the tissue along the beam direction. This force is strongest at the focus and can cause displacement on the order of micrometers with typical high‐intensity focused ultrasound (HIFU) transducers 13. By synchronizing pulses of focused ultrasound (FUS) with motion‐encoding gradients (MEG), spins at the focal spot experience an additional amount of precession, which shows up as a phase change in MR images. The basic motion‐sensitization method used for MR‐ARFI is identical to that used for MR elastography, which also requires careful synchronization of MEGs with the induced motion 15. FUS pulses of 5–20 ms have been used in MR‐ARFI experiments to generate good contrast 16. Long repetition times (TRs) are usually used to keep the FUS duty cycle low to avoid heating and to achieve high signal‐to‐noise ratio. The total acquisition time can be as long as two minutes with 2DFT readout 17. MR‐ARFI using fast echo‐planar imaging (EPI) readout has also been reported 11. EPI readout greatly shortens the total acquisition time, but by its nature is susceptible to geometric distortions induced by off‐resonance.
Balanced steady‐state free precession has been proposed for functional MRI (fMRI) 20, thermometry 21, as well as motion detection (with alternating steady state) 23. In this work, we have developed a high sensitivity MR‐ARFI pulse sequence by adding MEG to a balanced steady‐state free precession (bSSFP) pulse sequence 25. The phase of transition‐band bSSFP signal is very sensitive to the phase that individual spins accrue during each TR. This unique feature has been used for high sensitivity fMRI 26 and MR‐thermometry 27, because both brain activation and temperature rise can induce a change in the off‐resonance frequency that leads to additional phase accumulation. In the proposed bSSFP‐ARFI technique, spins at the focal spot accrue additional phase through motion encoding, and the image phase changes by an amount that is larger than the motion‐encoded phase itself. We compared MR‐ARFI images acquired using both bSSFP‐ARFI and spoiled gradient echo (spGRE) ARFI pulse sequences in a homemade gel phantom. With identical FUS pulses and MEG, bSSFP‐ARFI showed higher sensitivity and contrast‐to‐noise ratio (CNR) time efficiency.

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