Elena Kaye

Real‐time MR thermometry for monitoring HIFU ablations of the liver

A high‐resolution and high‐speed pulse sequence is presented for monitoring high‐intensity focused ultrasound ablations in the liver in the presence of motion. The sequence utilizes polynomial‐order phase saturation bands to perform outer volume suppression, followed by spatial‐spectral excitation and three readout segmented echo‐planar imaging interleaves. Images are processed with referenceless thermometry to create temperature‐rise images every frame. The sequence and reconstruction were implemented in RTHawk and used to image stationary and moving sonications in a polyacrylamide gel phantom (62.4 acoustic W, 50 sec, 550 kHz). Temperature‐rise images were compared between moving and stationary experiments. Heating spots and corresponding temperature‐rise plots matched very well. The stationary sonication had a temperature standard deviation of 0.15° C compared to values of 0.28° C and 0.43° C measured for two manually moved sonications at different velocities. Moving the phantom (while not heating) with respect to the transducer did not cause false temperature rises, despite susceptibility changes. The system was tested on nonheated livers of five normal volunteers. The mean temperature rise was − 0.05° C, with a standard deviation of 1.48° C. This standard deviation is acceptable for monitoring high‐intensity focused ultrasound ablations, suggesting real‐time imaging of moving high‐intensity focused ultrasound sonications can be clinically possible. Magn Reson Med, 2010.

Rapid MR-ARFI Method for Focal Spot Localization during Focused Ultrasound Therapy

MR‐guided focused ultrasound (FUS) is a noninvasive therapy for treating various pathologies. MR‐based acoustic radiation force imaging (MR‐ARFI) measures tissue displacement in the focal spot due to acoustic radiation force. MR‐ARFI also provides feedback for adaptive focusing algorithms that could correct for phase aberrations caused by the skull during brain treatments. This work developed a single‐shot echo‐planar imaging–based MR‐ARFI method that reduces scan time and ultrasound energy deposition. The new method was implemented and tested in a phantom and ex vivo brain tissue. The effect of the phase aberrations on the ultrasound focusing was studied using displacement maps obtained with echo‐planar imaging and two‐dimensional spin‐warp MR‐ARFI. The results show that displacement in the focal spot can be rapidly imaged using echo‐planar imaging–based MR‐ARFI with high signal‐to‐noise ratio efficiency and without any measurable tissue heating. Echo‐planar imaging–based displacement images also demonstrate sufficient sensitivity to phase aberrations and can serve as rapid feedback for adaptive focusing in brain treatments and other applications. Magn Reson Med, 2011.

Adapting MRI acoustic radiation force imaging for in vivo human brain focused ultrasound applications

A variety of magnetic resonance imaging acoustic radiation force imaging (MR‐ARFI) pulse sequences as the means for image guidance of focused ultrasound therapy have been recently developed and tested ex vivo and in animal models. To successfully translate MR‐ARFI guidance into human applications, ensuring that MR‐ARFI provides satisfactory image quality in the presence of patient motion and deposits safe amount of ultrasound energy during image acquisition is necessary. The first aim of this work was to study the effect of motion on in vivo displacement images of the brain obtained with 2D Fourier transform spin echo MR‐ARFI. Repeated bipolar displacement encoding configuration was shown less sensitive to organ motion. The optimal signal‐to‐noise ratio of displacement images was found for the duration of encoding gradients of 12 ms. The second aim was to further optimize the displacement signal‐to‐noise ratio for a particular tissue type by setting the time offset between the ultrasound emission and encoding based on the tissue response to acoustic radiation force. A method for measuring tissue response noninvasively was demonstrated. Finally, a new method for simultaneous monitoring of tissue heating during MR‐ARFI acquisition was presented to enable timely adjustment of the ultrasound energy aimed at ensuring the safety of the MR‐ARFI acquisition. Magn Reson Med, 2013.

Improved half RF slice selectivity in the presence of eddy currents with out-of-slice saturation.

Ultrashort echo time imaging with half RF pulse excitation is sensitive to eddy currents induced by the slice‐select gradient that distorts the half pulse slice profile. This work demonstrates improvements in the half pulse profile by using spatial saturation on both sides of the imaged slice to suppress the out‐of‐slice magnetization. This effectively improves the selectivity of the half pulse excitation profile. A quadratic phase RF pulse with high bandwidth and selectivity was used to achieve a wide saturation band with sharp edges. Experimental results demonstrate substantially improved slice selectivity and R  2* quantitation accuracy obtained with the out‐of‐slice saturation. This approach is effective in making short T2 imaging and quantitation with half pulses less sensitive to eddy currents. Magn Reson Med, 2009.

Consistency of signal intensity and T2* in frozen ex vivo heart muscle, kidney, and liver tissue.

To investigate tissue dependence of the MRI‐based thermometry in frozen tissue by quantification and comparison of signal intensity and T2* of ex vivo frozen tissue of three different types: heart muscle, kidney, and liver.

Toward MR-guided high intensity focused ultrasound for presurgical localization: focused ultrasound lesions in cadaveric breast tissue.

To investigate magnetic resonance image‐guided high intensity focused ultrasound (MR‐HIFU) as a surgical guide for nonpalpable breast tumors by assessing the palpability of MR‐HIFU‐created lesions in ex vivo cadaveric breast tissue.

MRI of frozen tissue demonstrates a phase shift.

While temperature mapping is desired during cryosurgery for prostate cancer treatment, an effective approach for this purpose is still needed. We have demonstrated a phase shift with temperature in our in vivo canine experiments and ex vivo tissue sample experiments within the frozen tissue. The phase shift is much larger (∼0.7 °/°C with an echo time of 0.1 ms at 0.5 T) in magnitude than that predicted by conventional proton resonant frequency shift (0.008 °/°C). It shows little dependence on the echo times used and thus is not due to a frequency change, although frequency‐dependent phase shift has been observed near the frozen tissue. This phase shift varies monotonically with temperature within the frozen tissue and therefore may be potentially used as a novel temperature mapping approach in cryoablation applications. Magn Reson Med, 2011.

Transcranial phase aberration correction using beam simulations and MR-ARFI

In this paper, we propose a technique to achieve phase aberration correction for transcranial MR-guided Focused Ultrasound Surgery. The technique uses ultrasound beam propagation simulations with MR Acoustic Radiation Force Impulse (MR-ARFI) imaging to correct skull caused phase aberrations. This technique resulted in a 10% improvement of the focal intensity using only one MR-ARFI image.

MR imaging-guided cryoablation for the treatment of benign prostatic hyperplasia.

A patient with benign prostatic hyperplasia presented with chronic lower urinary tract symptoms despite prior surgery and continued medical therapy. Using a magnetic resonance imaging-guided transperineal approach, two cryoprobes were placed into the transition zone of the prostate gland, and two cryoablation freeze-thaw cycles were performed. At 10 weeks after treatment, the frequency of nocturia had decreased from once every 1.5 hours to once per night, urinary peak flow rates had increased from 5.1 mL/s to 10.3 mL/s, and postvoid residual urinary bladder volume had decreased from 187 mL to 58 mL. Improved flow rates and symptoms remained stable 16 weeks after treatment.

Rapid HIFU autofocusing using the entire MR-ARFI image

Phase aberrations and attenuations caused by bone can defocus HIFU in the brain and organs behind the ribcage. To refocus the beam, MR-ARFI can be used to measure tissue displacements created by each element in the transducer, and optimize driving signal delays and amplitudes. We introduce a new MR-ARFI-based autofocusing method that requires many fewer image acquisitions than current methods. The method is validated in simulations of bone and brain HIFU transducers, and compared to a conventional method.