Variable ultrasound trigger delay for improved magnetic resonance acoustic radiation force imaging

Reduced-field of view three-dimensional MR acoustic radiation force imaging with a low-rank reconstruction for targeting transcranial focused ultrasound

PURPOSE

To rapidly image and localize the focus in MR-guided focused ultrasound (FUS) while maintaining a low ultrasound duty cycle to minimize tissue effects.

METHODS

MR-acoustic radiation force imaging (ARFI) is key to targeting FUS procedures such as neuromodulation, and works by encoding ultrasound-induced displacements into the phase of MR images. However, it can require long scan times to cover a volume of tissue, especially when minimizing the FUS dose during targeting is paramount. To simultaneously minimize scan time and the FUS duty cycle, a 2-min three-dimensional (3D) reduced-FOV spin echo ARFI scan with two-dimensional undersampling was implemented at 3T with a FUS duty cycle of 0.85%. The 3D k-space sampling scheme incorporated uniform undersampling in one phase-encoded axis and partial Fourier (PF) sampling in the other. The scan interleaved FUS-on and FUS-off data collection to improve displacement map quality via a joint low-rank image reconstruction. Experiments in agarose and graphite phantoms and living macaque brains for neuromodulation and blood-brain barrier opening studied the effects of the sampling and reconstruction strategy on the acquisition, and evaluated its repeatability and accuracy.

RESULTS

In the phantom, the distances between displacement centroids of 10 prospective reconstructions and a fully sampled reference were below 1 mm. In in vivo brain, the distances between centroids ranged from 1.3 to 2.1 mm. Results in phantom and in vivo brain both showed that the proposed method can recover the FUS focus compared to slower fully sampled scans.

CONCLUSION

The proposed 3D MR-ARFI reduced-FOV method enables rapid imaging of the FUS focus while maintaining a low FUS duty cycle.

A robotic MR-guided high-intensity focused ultrasound platform for intraventricular hemorrhage: assessment of clot lysis efficacy in a brain phantom

OBJECTIVE

Intraventricular hemorrhage (IVH) is a neurovascular complication due to premature birth that results in blood clots forming within the ventricles. Magnetic resonance-guided high-intensity focused ultrasound (MRgHIFU) has been investigated as a noninvasive treatment to lyse clots. The authors designed and constructed a robotic MRgHIFU platform to treat the neonatal brain that facilitates ergonomic patient positioning. The clot lysis efficacy of the platform is quantified using a brain phantom and clinical MRI system.

METHODS

A thermosensitive brain-mimicking phantom with ventricular cavities was developed to test the clot lysis efficacy of the robotic MRgHIFU platform. Whole porcine blood was clotted within the phantom's cavities. Using the MRgHIFU platform and a boiling histotripsy treatment procedure (500 W, 10-msec pulse duration, 1.0% duty cycle, and 40-second duration), the clots were lysed inside the phantom. The contents of the cavities were vacuum filtered, and the remaining mass of the solid clot particles was used to quantify the percentage of clot lysis. The interior of the phantom's cavities was inspected for any collateral damage during treatment.

RESULTS

A total of 9 phantoms were sonicated, yielding an average (± SD) clot lysis of 97.0% ± 2.57%. Treatment resulted in substantial clot lysis within the brain-mimicking phantoms that were apparent on postsonication T2-weighted MR images. No apparent collateral damage was observed within the phantom after treatment. The results from the study showed the MRgHIFU platform was successful at lysing more than 90% of a blood clot at a statistically significant level.

CONCLUSIONS

The robotic MRgHIFU platform was shown to lyse a large percentage of a blood clot with no observable collateral damage. These results demonstrate the platform's ability to induce clot lysis when targeting through simulated brain matter and show promise toward the final application in neonatal patients.

A reduced aperture allows for transcranial focus localization at lower pressure

Localizing the focus during transcranial focused ultrasound procedures is important to ensure accurate targeting of specific brain regions and interpretation of results. Magnetic resonance acoustic radiation force imaging uses the displacement induced by the ultrasound focus in the brain to localize the beam, but the high pressure required to displace brain tissue may cause damage or confounds during subsequent neuromodulatory experiments. Here, reduced apertures were applied to a phased array transducer to generate comparable displacement to the full aperture but with 20% lower free field pressure.

Simultaneous acoustic radiation force imaging and MR thermometry based on a coherent echo-shifted sequence

Background

Simultaneous magnetic resonance (MR) acoustic radiation force imaging (ARFI) and MR thermometry (MRT) (STARFI) based on coherent echo-shifted (cES) sequence was proposed and comprehensively compared to radiofrequency (RF)-spoiled gradient echo (spGRE) STARFI.

Methods

Through use of delicately designed gradients, a collection of echoes was delayed by one repetition time (TR) cycle. The crusher gradient after readout (RO) was used as the displacement encoding gradient (DEG). The sequence was intrinsically sensitive to temperature. High-intensity focused ultrasound (HIFU) pulses were interleaved ON/OFF in successive TRs to separate the phase changes induced by displacement due to acoustic radiation force (ARF) impulses and temperature. Bloch simulation was performed to study the phase sensitivity to displacement of the proposed cES STARFI and spGRE STARFI. The proposed cES sequence was evaluated and compared to spGRE STARFI in ex vivo porcine muscle and ex vivo porcine brain.

Results

The minimally achievable TR of cES STARFI was shorter than that of spGRE STARFI, indicating that the cES sequence was more time efficient. It was verified through Bloch simulation and ex vivo experiments that the phase sensitivity to displacement of cES STARFI was higher than that of spGRE STARFI. The optimal trigger delays of cES STARFI and spGRE STARFI in ex vivo porcine muscle were t_offset =-2 and -1 ms, respectively. The displacement-induced phase change to acoustic pressure slopes of cES STARFI were 0.079, 0.079, and 0.047 rad/Mpa across the three muscle samples, while the slopes of spGRE STARFI were only 0.047, 0.052, and 0.027 rad/Mpa. The maximum temperature difference between cES STARFI and spGRE STARFI was 1.1 °C. In ex vivo porcine brain, both the displacement-induced phase-to-noise ratio (PNRd) and the temperature uncertainty of cES STARFI were better than those of spGRE STARFI (P<0.05). The temperature and displacement-induced phase change maps of cES STARFI and spGRE STARFI during HIFU treatment were in good accordance in time and spatial location.

Conclusions

The cES STARFI sequence can provide simultaneous MR-ARFI and temperature measurements during pulsed HIFU applications. Though the exact displacement cannot be quantified directly, the sequence showed increased phase sensitivity compared with the spGRE sequence and provided efficient visualization of the focal spot. cES STARFI could therefore be a desirable alternative to spGRE STARFI in practical applications.

Considerations for ultrasound exposure during transcranial MR acoustic radiation force imaging

The aim of this study was to improve the sensitivity of magnetic resonance-acoustic radiation force imaging (MR-ARFI) to minimize pressures required to localize focused ultrasound (FUS) beams, and to establish safe FUS localization parameters for ongoing ultrasound neuromodulation experiments in living non-human primates. We developed an optical tracking method to ensure that the MR-ARFI motion-encoding gradients (MEGs) were aligned with a single-element FUS transducer and that the imaged slice was prescribed at the optically tracked location of the acoustic focus. This method was validated in phantoms, which showed that MR-ARFI-derived displacement sensitivity is maximized when the MR-ARFI MEGs were maximally aligned with the FUS propagation direction. The method was then applied in vivo to acquire displacement images in two healthy macaque monkeys (M fascicularis) which showed the FUS beam within the brain. Temperature images were acquired using MR thermometry to provide an estimate of in vivo brain temperature changes during MR-ARFI, and pressure and thermal simulations of the acoustic pulses were performed using the k-Wave package which showed no significant heating at the focus of the FUS beam. The methods presented here will benefit the multitude of transcranial FUS applications as well as future human applications.

Simultaneous MR thermometry and acoustic radiation force imaging using interleaved acquisition

PURPOSE

A novel and practical method for simultaneously performing MR acoustic radiation force imaging (ARFI) and proton resonance frequency (PRF)-shift thermometry has been developed and tested. This could be an important tool for evaluating the success of MR-guided focused ultrasound procedures for which MR-thermometry measures temperature and thermal dose and MR-ARFI detects changes in tissue mechanical properties.

METHODS

MR imaging was performed using a gradient recalled echo segmented echo-planar imaging pulse sequence with bipolar motion encoding gradients (MEG). Images with ultrasound pulses (ON) and without ultrasound pulses (OFF) during the MEG were interleaved at the repetition time (TR) level. ARFI displacements were calculated by complex subtraction of ON-OFF images, and PRF temperature maps were calculated by baseline subtraction. Evaluations in tissue-mimicking phantoms and ex vivo porcine brain tissue were performed. Constrained reconstruction improved the temporal resolution of dynamic measurements.

RESULTS

Simultaneous maps of displacement and temperature were acquired in 2D and 3D while keeping tissue heating < 1°C. Accuracy of the temperature maps was comparable to the standard PRF sequence. Using constrained reconstruction and subsampled k-space (R = 4.33), 3D simultaneous temperature and displacement maps can be acquired every 4.7 s.

CONCLUSION

This new sequence acquires simultaneous temperature and displacement maps with minimal tissue heating, and can be applied dynamically for monitoring tissue mechanical properties during ablation procedures. Magn Reson Med 79:1515-1524, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Preclinical MRI-Guided Focused Ultrasound: A Review of Systems and Current Practices

Effective preclinical research is a vital component in the development of MRI-guided focused ultrasound (MRgFUS) and its translation to clinic. In this review, we seek to outline the challenges at hand for effective preclinical research, survey different solutions, and underline best practices. Furthermore, we summarize efforts to build and characterize dedicated preclinical MRgFUS equipment, including lab prototypes and available commercial products. Finally, we discuss constraints and considerations specific to using clinical MRgFUS equipment in preclinical research. Specifically, we examine additional hardware that has been used to adapt clinical MRgFUS equipment to better position, constrain, and image preclinical subjects, as well as software solutions that have been used to extend the potential and capabilities of clinical devices.

A rapid magnetic resonance acoustic radiation force imaging sequence for ultrasonic refocusing

Magnetic resonance guided acoustic radiation force imaging (MR-ARFI) is being used to correct for aberrations induced by tissue heterogeneities when using high intensity focusing ultrasound (HIFU). A compromise between published MR-ARFI adaptive solutions is proposed to achieve efficient refocusing of the ultrasound beam in under 10 min. In addition, an ARFI sequence based on an EPI gradient echo sequence was used to simultaneously monitor displacement and temperature with a large SNR and low distortion. This study was conducted inside an Achieva 3T clinical MRI using a Philips Sonalleve MR-HIFU system to emit a 1 ms pulsed sonication with duty cycle of 2.3% at 300 Wac inside a polymer phantom. Virtual elements defined by a Hadamard array with sonication patterns composed of 6 phase steps were used to characterize 64 groups of 4 elements to find the optimal phase of the 256 elements of the transducer. The 384 sonication patterns were acquired in 580 s to identify the set of phases that maximize the displacement at the focal point. Three aberrators (neonatal skull, 8 year old skull and a checkered pattern) were added to each sonication pattern to evaluate the performance of this refocusing algorithm (n  =  4). These aberrators reduced the relative intensities to 95.3%, 69.6% and 25.5% for the neonatal skull, 8 year old skull, and checkered pattern virtual aberrators respectively. Using a 10 min refocusing algorithm, relative intensities of 101.6%, 91.3% and 93.3% were obtained. Better relative intensities of 103.9%, 94.3% and 101% were achieved using a 25 min refocusing algorithm. An average temperature increase of 4.2 °C per refocusing test was induced for the 10 min refocusing algorithm, resulting in a negligible thermal dose of 2 EM. A rapid refocusing of the beam can be achieved while keeping thermal effects to a minimum.