Aiming Lu¹, Christopher P Favazza¹, David A Woodrum¹, Joel P Felmlee¹, Jacinta Browne¹, Brian T Welch¹, and Krzysztof R Gorny^1
1Radiology, Mayo Clinic, Rochester, MN, United States

Cryoablation with MRI guidance is desirable is a feasible treatment for localized tumors. There is a MRI-conditional cryoablation system and,  to the best of our knowledge, no previous report of RF heating/burn incidence. However, since the cryoneedles are metallic and the gas lines have metallic components, potential risks of RF heating/burn exists. In fact, an incidence of skin burn recently occurred in our institution during a MRI-guided liver treatment case. In this work, we demonstrated that RF heating could be significant during MRI-guided cryoablation and showed that several strategies could be potentially used to mitigate the risk. 

Le Zhang¹, Tess Armstrong^1,2, and Holden H. Wu^1,2,3
1Radiological Sciences, University of California, Los Angeles, Los Angeles, CA, United States, ²Physics and Biology in Medicine, University of California, Los Angeles, Los Angeles, CA, United States, ³Bioengineering, University of California, Los Angeles, Los Angeles, CA, United States

MR thermometry in the liver is challenged by mismatch between baseline and dynamic images caused by motion, leading to temperature errors. To address motion, previous methods had to compromise spatial coverage to increase temporal resolution. We propose a variable-flip-angle (VFA) 3D stack-of-radial technique for combined proton resonance frequency shift (PRF) and T₁-based MR thermometry with volumetric coverage and high spatiotemporal resolution. Accurate VFA T₁ calculation is achieved by synthesizing  B₁+ maps that match the liver position in dynamic images. A multi-baseline approach is used for accurate dynamic PRF measurements. Results from non-heating scans demonstrate reliable liver T₁ and PRF measurements.

Yangzi Qiao¹, Chao Zou¹, Chuanli Cheng¹, Changjun Tie¹, Xin Liu¹, and Hairong Zheng^1
1Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China

Simultaneous MR acoustic radiation force imaging and MR thermometry (STARFI) based on coherent echo-shifted sequence (cES) was proposed and comprehensively compared to RF spoiled gradient echo (spGRE). The calculated displacement of cES STARFI was always larger than the value of spGRE STARFI through both the simulation and experiments, while the accuracy of the temperature monitoring of cES was maintained. The temperature and displacement map acquired during HIFU heating were in good accordance with each other. The cES STARFI can be an alternative for comprehensively monitoring of HIFU treatment with increased displacement sensitivity and time efficiency compared to spGRE STARFI. 

Sumeeth Jonathan^1,2, M Anthony Phipps², Charles F Caskey², and William A Grissom^2
1Biomedical Engineering, Vanderbilt University, Nashville, TN, United States, ²Vanderbilt University Institute of Imaging Science, Vanderbilt University Medical Center, Nashville, TN, United States

Magnetic resonance-guided focused ultrasound (MRgFUS) has many potential neurological applications, but skull-induced aberrations of the acoustic pressure field limit its specificity and safety. MR-acoustic radiation force imaging (MR-ARFI)-based methods have been proposed to refocus the pressure field in situ. However, they take too long for practical in vivo use. We propose a multi-voxel MR-ARFI-based autofocusing method for rapid aberration correction of MRgFUS acoustic pressure fields. We compare our proposed method to the canonical single-voxel MR-ARFI-based refocusing method and demonstrate that as few as two MR-ARFI acquisitions can be used to refocus a programmatically aberrated pressure field.

Dinank Gupta¹, Ning Lu¹, Jonathan Sukovich¹, Krisanne Litnas², Aditya Pandey³, Badih Junior Daou³, Timothy Hall¹, Zhen Xu¹, Scott Peltier², and Douglas Noll^1
1Biomedical Engineering, University of Michigan, Ann Arbor, MI, United States, ²Functional MRI Laboratory, University of Michigan, Ann Arbor, MI, United States, ³Department of Neurosurgery, University of Michigan, Ann Arbor, MI, United States

Transcranial MR guided cavitation-based Focused Ultrasound (FUS) treatment (histotripsy) is performed in vivo for the first time on a pig brain. Transcranial histotripsy is delivered by an MRI compatible FUS transducer array inside a 3T MRI scanner. Real-time MRI monitoring with 2 second temporal resolution is carried with an intra-voxel incoherent motion (IVIM) pulse sequence synchronized with the FUS array. IVIM images show the histotripsy ablation effect at the intended treatment location in real-time, and the ablation zone was confirmed by post-treatment images. This is the first study to show successful in vivo transcranial histotripsy guided by MRI.

Allison Payne¹, Lorne Hofstetter², Henrik Odéen¹, Erik Dumont³, Dennis L Parker¹, and Jean Palussiere^4
1Radiology and Imaging Sciences, University of Utah, Salt Lake City, UT, United States, ²Biomedical Engineering, University of Utah, Salt Lake City, UT, United States, ³Image Guided Therapy, Pessac, France, ⁴Institut Bergonie, Bordeaux, France

3D MR acoustic radiation force imaging (MR-ARFI) is a useful treatment planning and monitoring tool for magnetic resonance guided focused ultrasound (MRgFUS) treatments in the breast. MR-ARFI displacement is easily visualized in fat, fibroglandular and tumor tissues, allowing for accurate localization of the ultrasound beam and quantitative tissue assessment. The potential formation of standing shear waves in the breast requires careful optimization of the pulse sequence to ensure clear visualization of the radiation force displacement point. This effect is shown in both human breast and phantom data.

Pan Su^1,2, Sijia Guo^1,3, Florian Maier⁴, Steven Roys^1,3, Himanshu Bhat², Elias R. Melhem¹, Dheeraj Gandhi¹, Rao P. Gullapalli^1,3, and Jiachen Zhuo^1,3
1Department of Diagnostic Radiology and Nuclear Medicine, University of Maryland School of Medicine, Baltimore, MD, United States, ²Siemens Medical Solutions USA Inc, Malvern, PA, United States, ³Center for Metabolic Imaging and Therapeutics (CMIT), University of Maryland Medical Center, Baltimore, MD, United States, ⁴Siemens Healthcare GmbH, Erlangen, Germany

Transcranial MRI-guided focused ultrasound (tcMRgFUS) is a promising technique to treat multiple diseases. Here we examined the feasibility of leveraging deep-learning to convert MRI dual echo UTE images directly to synthesized CT skull images. We demonstrated that the derived model is capable of not only segmenting the UTE images to generate synthetic CT skull masks that are highly comparable to true CT skull masks, but is also able to reliably predict the CT skull intensities in Hounsfield units. Furthermore, we demonstrated that synthetic CT skull can be reliably used for skull-density-ratio (SDR) determination and predicting target temperature rise in tcMRgFUS.

Maarten L Terpstra^1,2, Federico d'Agata^1,2,3, Bjorn Stemkens^1,2, Jan JW Lagendijk¹, Cornelis AT van den Berg^1,2, and Rob HN Tijssen^1,2
1Department of Radiotherapy, Division of Imaging & Oncology, University Medical Center Utrecht, Utrecht, Netherlands, ²Computational Imaging Group for MR diagnostics & therapy, Center for Image Sciences, University Medical Center Utrecht, Utrecht, Netherlands, ³Department of Neurosciences, University of Turin, Turin, Italy

MRI-guided radiotherapy (MRgRT) enables new ways to improve dose delivery to moving tumors and the organs-at-risk (e.g. in abdomen) by steering the radiation beam based on real-time MRI. While state-of-the-art techniques (e.g. compressed sensing) can provide the required acquisition speed, the corresponding reconstruction time is too long for real-time processing. In this work, we investigate the use of multiple deep neural networks for image reconstruction and subsequent motion estimation. We show that a single motion estimation network can estimate high-quality 2D deformation vector fields from aliased images, even for high undersampling factors up to R=25.

Korel Dursun Yildirim^1,2, Christopher Bruce¹, Rajiv Ramasawmy¹, Kendall O'Brien¹, Adrienne Campbell-Washburn¹, Daniel Herzka¹, Robert J. Lederman¹, and Ozgur Kocaturk^1,2,3
1Cardiovascular Branch, Division of Intramural Research, National Heart Lung and Blood Institute, National Institutes of Health, Bethesda, MD, United States, ²Institute of Biomedical Engineering, Bogazici University, Istanbul, Turkey, ³Transmural Systems, Andover, MA, United States

A clinical-grade active MRI 0.035” guidewire design with a curved distal tip geometry and continuous shaft signal ensuring the mechanical and electrical safety, was introduced. Proposed design was tested in-vitro  and in-vivo for MRI visibility and mechanical performance, and in-vitro for RF induced heating.

Felipe Godinez¹, Raphael Tomi-Tricot², Marylene Delcey³, Gunthard Lykowsky⁴, Steven E Williams¹, Bruno Quesson³, Joseph V Hajnal¹, and Shaihan J Malik^1
1Biomedical Engineering Department, School of Biomedical Engineering and Imaging Sciences, king's College London, London, United Kingdom, ²Siemens Healthcare Limited, London, United Kingdom, ³Centre de recherche Cardio-Thoracique de Bordeaux, Bordeaux, France, ⁴RAPID Biomedical GmbH, Rimpar, Germany

This paper presents first in vivo results of a parallel transmit (PTx) system adept at regulating radiofrequency induced guidewire heating, during MRI guided interventions in sheep. The PTx system, which is an add-on to an unmodified conventional 1.5T scanner, regulates heating by operating in modes that couple/decouple the guidewire from the radiofrequency transmit array. With an inserted guidewire decoupling modes allow operation with unrestricted B1+ to safely visualize anatomy, while the coupling mode operated at low radiofrequency power provides safe visualization of the guidewire itself. Temperature measurements at the guidewire tip and in vivo images are shown.