Gunther Helms^1,2, Lenka Vaculčiaková², Kerrin J Pine², Harald E Möller³, and Nikolaus Weiskopf^2,4
1Clinical Sciences, Medical Radiation Physics, Lund University, Lund, Sweden, ²Neurophysics, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, ³NMR Methods & Development Group, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, ⁴Felix Bloch Institute for Solid State Physics, Leipzig University, Leipzig, Germany

Inhomogenous Magnetization Transfer (ihMT), the differential response to irradiation at single and dual frequency offsets, is more complex than MT approaches. Expressing ihMT in terms of MT-saturation (ihMTsat) is a first step to quantification as it corrects for underlying T₁ and B₁⁺. Larger ihMTsat was observed for smaller frequency offsets, which is explained using MTsat as proxy for bound pool saturation. For longer TR, ihMTsat increased faster than MTsat indicating recovery of dipolar order as ihMTsat increased super-linearly to 1.1pu in WM and 0.2pu in GM at TR=52ms. ihMTsat mapping in vivo was performed at 1.3mm isotropic resolution.

Christopher D Rowley^1,2, Ilana R. Leppert¹, Jennifer S.W. Campbell¹, G Bruce Pike³, and Christine L Tardif^1,2,4
1McConnell Brain Imaging Centre, McGill University, Montreal, QC, Canada, ²Neurology and Neurosurgery, McGill University, Montreal, QC, Canada, ³Hotchkiss Brain Institute and Departments of Radiology and Clinical Neuroscience, University of Calgary, Calgary, AB, Canada, ⁴Biomedical Engineering, McGill University, Montreal, QC, Canada

Sequence simulations are used to compare different conventional and boosted saturation schemes and imaging protocols for the purpose of high-resolution ihMT_sat cortical imaging. The simulations are constrained to a maximum of four minutes per volume. We find an optimal region for conventional saturation that produces the greatest ihMT_sat. For the boosted approach, larger ihMT_sat values are found with respect to the conventional approach, and ihMT_sat increases with longer TRs. However, the practical utility of the boosted approach for cortical imaging is hampered by a wider PSF associated with higher turbo factors. Simulated ihMT_sat findings are supported by in vivo data.

Jakob Assländer^1,2, Cem Gultekin³, Sebastian Flassbeck^1,2, Steffen J Glaser⁴, and Daniel K Sodickson^1,2
1Center for Biomedical Imaging, New York University School of Medicine, New York, NY, United States, ²Center for Advanced Imaging Innovation and Research, New York University School of Medicine, New York, NY, United States, ³Courant Institute of Mathematical Sciences, New York University, New York, NY, United States, ⁴Department of Chemistry, Technische Universität München, München, Germany

We introduce a classical model to describe magnetization transfer (MT). Like a quantum-mechanical description of spin dynamics and like the original Bloch equations, but unlike existing MT models, the proposed model is based on the algebra of angular momentum in the sense that it explicitly models the rotations induced by radio-frequency (RF) pulses. It unifies the original Bloch model, Henkelman's steady-state theory for magnetization transfer, and the commonly assumed rotation induced by hard, i.e. short, pulses, and describes experimental data better than previous models.

Sebastian Flassbeck^1,2 and Jakob Assländer^1,2
1Center for Biomedical Imaging, New York University School of Medicine, New York, NY, United States, ²Center for Advanced Imaging Innovation and Research, New York University School of Medicine, New York, NY, United States

Most magnetization transfer experiments are insensitive to the longitudinal relaxation rate of the invisible semi-solid spin pool and it has become common practice to heuristically assume 1/second. This assumption has, however, been challenged recently. Because this parameter is notoriously hard to estimate, previous studies have relied on the analysis of large white matter regions or the entire brain. Here, we explore the ability to estimate this value on a voxel-by-voxel basis with an MR-Fingerprinting-like pulse sequence. 

Lucas Soustelle^1,2, Thomas Troalen³, Andreea Hertanu^1,2, Maxime Guye^1,2, Jean-Philippe Ranjeva^1,2, Gopal Varma⁴, David C. Alsop⁴, Olivier M. Girard^1,2, and Guillaume Duhamel^1,2
1Aix Marseille Univ, CNRS, CRMBM, Marseille, France, ²APHM, Hôpital Universitaire Timone, CEMEREM, Marseille, France, ³Siemens Healthcare SAS, Saint-Denis, France, Paris, France, ⁴Division of MR Research, Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, United States

Two major biases in the current qMT methodology arise from i) on-resonance saturation effects induced by readout pulses used in variable flip angle SPGR experiments, and ii) dipolar order effects induced by single-frequency off-resonance saturation pulses leading to a reduced saturation efficiency. In this work, we evaluate these two biases by performing experiments and analyses by accounting for on-resonance saturation in the qMT model and using simultaneous symmetric dual-offset frequency saturation pulses to cancel out dipolar order effects. Results show improvement in the fitting procedure and more accurate estimation of T_1,f and MPF values in white and grey matter structures.

Roya Afshari^1,2, Grzegorz Bauman^1,2, and Oliver Bieri^1,2
1Division of Radiological Physics, Department of Radiology, University Hospital Basel, Basel, Switzerland, ²Department of Biomedical Engineering, University of Basel, Basel, Switzerland

Magnetization transfer (MT) imaging has been extensively used to explore microstructural changes in the brain at high fields. In this work, we explore the potential of a 3D half-radial dual-echo balanced steady-state free precession (bSSFP) sequence for fast whole-brain magnetization transfer ratio (MTR) imaging at low-field strength. Our work indicates superiority of MT-sensitized bSSFP against conventional MT-prepared spoiled gradient echo (SPGR) in terms of MT contrast and resolution within similar scan time. In conclusion, MTR imaging with bSSFP offers excellent prospects for broad clinical translation and application at low fields.

Blake Benyard¹, Ravi Prakash Reddy Nanga¹, Neil Wilson¹, Deepa Thakuri¹, Abigail Cember¹, and Ravindeer Reddy^1
1CAMIPM, Department of Radiology, University of Pennsylvania, Philadelphia, PA, United States

Nuclear Overhauser Effect (NOE) is an emerging technique to study mobile macromolecules such as lipids in the gray (GM) and white matter (WM) of the brain. In this work, we optimized the saturation pulse parameters of  NOE Magnetization Transfer Ratio (MTR) MRI and investigated its reproducibility on a healthy human volunteer at 7 Telsa.  We found that NOE-MTR of GM and WM regions of the brain was highly reproducible (COV<10%) over a three year time span.  In addition, a saturation length of 3 to 4 sec provided optimal NOE-MTR contrast from both GM and WM regions of the brain. 

Francisco J. Fritz¹, Gunther Helms^2,3, Lenka Vaculčiaková³, Nikolaus Weiskopf^3,4, and Siawoosh Mohammadi^1,3
1Institut für Systemische Neurowissenschaften, Universitätklinikum Hamburg-Eppendorf, Hamburg, Germany, ²Medical Radiation Physics, IKVL, Lund University, Lund, Sweden, ³Department of Neurophysics, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, ⁴Felix Bloch Institute for Solid State Physics, Faculty of Physics and Earth Sciences, Leipzig University, Leipzig, Germany

Inhomogeneous magnetic transfer (ihMT) is more sensitive to myelin macromolecules than standard MT proxies. Measuring ihMT in the multi-parameter mapping protocol allows calculating ihMT from MT saturation (MTsat) maps and thus inherently correct for the undesired  dependencies on flip angle and the longitudinal relaxation rate. Further validation of this new ihMT metric requires measurement of MPM-based ihMT of human post-mortem material. Here, we showed that ihMT of a whole human post-mortem brain is feasible but can lead to temperature increase in the specimen, which is particularly pronounced in white matter.

Jonah P. W. Weigand¹, Maria Sedykh², Kai Herz^3,4, Jaume Coll-Font^1,5, Christopher Nguyen^1,5,6, Moritz Zaiss^2,3, Christian T. Farrar¹, and Or Perlman^1
1Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Charlestown, MA, United States, ²Department of Neuroradiology, Friedrich-Alexander Universität Erlangen-Nürnberg (FAU), University Hospital Erlangen, Erlangen, Germany, ³Magnetic Resonance Center, Max Planck Institute for Biological Cybernetics, Tubingen, Germany, ⁴Department of Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany, ⁵Cardiovascular Research Center, Cardiology Division, Massachusetts General Hospital, Charlestown, MA, United States, ⁶Health Science Technology, Harvard-MIT, Cambridge, MA, United States

Quantitative metabolite concentration and pH biomarker maps, as provided by semisolid MT/CEST-MR-Fingerprinting (MRF), constitute a useful means for determining the molecular origin of pathology. However, the lengthy dictionary generation time and the prolonged 3D acquisition time may hinder clinical dissemination. Here, we developed a generative adversarial network (GAN), aimed to drastically shorten the 3D semisolid MT/CEST-MRF acquisition time and circumvent the need for dictionary generation. In-vitro and in-vivo experiments in 4 volunteers and a patient were conducted at 3 different sites using 3 different scanner models, showing substantial reduction in scan time, while retaining a good agreement with ground-truth reference.

Patrick Schuenke¹, Kerstin Heinecke¹, George Henrik Narvaez¹, Moritz Zaiss², and Christoph Kolbitsch^1
1Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany, ²Department of Neuroradiology, Friedrich- Alexander Universität Erlangen- Nürnberg, University Hospital Erlangen, Erlangen, Germany

Obtaining isolated effects in Chemical Exchange Saturation Transfer (CEST) MRI requires the correction for the influence of B₀- and B₁-inhomogeneities and the T₁ relaxation time during post-processing. We present an extended approach for simultaneous mapping of B₀, B₁ and T₁ using a CEST-like pulse sequence and a regularized neural network-based analysis that provides additional uncertainty estimations. We demonstrate the in vitro and in vivo applicability of the proposed approach, which is trained using simulated data and confirm its accordance with established reference methods.

Pei Han^1,2, Tianle Cao^1,2, Karandeep Cheema^1,2, Hsu-Lei Lee¹, Fei Han³, Nan Wang⁴, Hui Han¹, Yibin Xie¹, Anthony G. Christodoulou^1,2, and Debiao Li^1,2
1Biomedical Imaging Research Institute, Cedars-Sinai Medical Center, Los Angeles, CA, United States, ²Department of Bioengineering, UCLA, Los Angeles, CA, United States, ³Siemens Healthineers, Los Angeles, CA, United States, ⁴Department of Radiology, Stanford University, Stanford, CA, United States

We developed a 3D abdominal CEST MRI technique at 3T using MR Multitasking, which enables entire-liver coverage with free-breathing acquisition. By exploiting the correlation among images throughout the spatial, time, frequency offset, and respiration dimensions, the low-rank tensor framework shows the possibility to acquire the whole Z-spectrum of 53 frequency offsets within 11 min.

Chuyu Liu¹, Zhensen Chen², Yibing Chen³, Xubin Chai¹, Zhongsen Li¹, and Xiaolei Song^1
1Center for Biomedical Imaging Research, Department of Biomedical Engineering, Tsinghua University, Beijing, China, ²Institute of Science and Technology for Brain-Inspired Intelligence, Fudan University, Shanghai, China, ³Xi’an Key Lab of Radiomics and Intelligent Perception, School of Information Sciences and Technology, Northwest University, Xi'an, China

We developed a Fast Acquisition and Reconstruction CEST (FAR-CEST) method at 3T human scanners, based on a deep learning approach. A 10X accelerated acquisition was achieved, which under-sampled K-space using a randomized Cartesian pattern of variable density. To fully utilize the correlation among saturation offset dimension, especially to compensate for sparsely-sampled K-space edge, a 3D-Res-Unet model was trained for reconstruction. Results on healthy adult brain suggested that FAR-CEST can produce high quality saturation-weighted images and Z-spectra，but the CEST contrast slightly altered. The highly-acceleration feature of FAR-CEST has been initially validated, yet still require improvement on reconstruction accuracy.

Christos Papageorgakis¹, Eleni Firippi¹, Benoit Gy¹, Timothé Boutelier¹, Ibrahim Khormi^2,3,4, Oun Al-iedani^2,3, Bryan Paton^3,5, Jeannette Lechner-Scott ^3,6,7, Amir Fazlollahi⁸, Anne-Louise Ponsonby^9,10, Patrick Liebig¹¹, Saadallah Ramadan^2,3, Moritz Zaiss¹², and Stefano Casagranda^1
1Department of Research & Innovation, Olea Medical, La Ciotat, France, ²School of Health Sciences, College of Health, Medicine and Wellbeing,, University of Newcastle, Newcastle, Australia, ³Hunter Medical Research Institute, Newcastle, Australia, ⁴College of Applied Medical Sciences, University of Jeddah, Jeddah, Saudi Arabia, ⁵School of Psychology, College of Engineering, Science and Environment, University of Newcastle, Newcastle, Australia, ⁶Department of Neurology, John Hunter Hospital, New Lambton Heights, Australia, ⁷School of Medicine and Public Health, College of Health, Medicine and Wellbeing, University of Newcastle, Newcastle, Australia, ⁸CSIRO Health and Biosecurity, Brisbane, Australia, ⁹The Florey Institute of Neuroscience and Mental Health, Victoria, Australia, ¹⁰Murdoch Children's Research Institute, Royal Children's Hospital, University of Melbourne, Victoria, Australia, ¹¹Siemens Healthcare GmbH, Erlangen, Germany, ¹²Department of Neuroradiology, University Clinic Erlangen, Friedrich-Alexander Universität Erlangen-Nürnberg (FAU), Erlangen, Germany

This work provides a new method for fast post-processing of MRI data acquired using the WASAB1 sequence for simultaneous B₀ and B₁ mapping, used in CEST imaging for field inhomogeneity corrections. We are proposing a new processing method with outstanding acceleration of the parameter estimation procedure, without compromising the stability of the estimation. The stability of the method is demonstrated on phantom data and in vivo 3 Tesla clinical data.

Thaddée Delebarre ^1,2, Vincent Gras^1,2, Franck Mauconduit^1,2, Alexandre Vignaud^1,2, Nicolas Boulant^1,2, and Luisa Ciobanu^1,2
1NeuroSpin, CEA, Saclay, France, ²Paris-Saclay University, Gif-sur-Yvette, France

Compared to the traditional circular polarization (CP) transmission mode, parallel transmission (pTx) excitation allows significant gains in contrast and homogeneity for Chemical Exchange Saturation Transfer (CEST) imaging. Using an in-house developed CEST-EPI sequence incorporating a pTx optimized CEST saturation block, we show that both tailored and universal pTx pulses provide significant improvement at 7T for APT-CEST, GluCEST and GlucoCEST imaging and enable saturation powers unattainable with CP.

Verena Schirmer¹, Maximilian Gram^1,2, Petra Albertova¹, Simon Mayer^1,3, Martin Blaimer⁴, Peter Michael Jakob¹, and Fabian Tobias Gutjahr^1
1Experimental Physics 5, University of Würzburg, Würzburg, Germany, ²Department of Internal Medicine I, University Hospital Würzburg, Würzburg, Germany, ³Leibniz Institute of Plant Genetics and Crop Plant Research (IPK), Gatersleben, Germany, ⁴Magnetic Resonance and X-ray Imaging Department Fraunhofer IIS, Fraunhofer Institute for Integrated Circuits IIS, Würzburg, Germany

Contrary to the basic principle of CEST, the RACETE technique allows for the direct detection of positive chemical exchange contrast. This method, which was previously demonstrated only under ultra-high field conditions at 7T-17.5T, has now successfully been implemented on a clinical 3T scanner in initial phantom experiments. Furthermore, we present a novel dual-contrast RACETE-technique for simultaneous imaging of the positive RACETE and the negative CEST contrast.

Munendra Singh¹, Babak Moghadas¹, Shanshan Jiang¹, Peter van Zijl¹, Jinyuan Zhou¹, and Hye-Young Heo^1
1Johns Hopkins University, Baltimore, MD, United States

Conventional semisolid magnetization transfer contrast (MTC) and chemical exchange saturation transfer (CEST) MRI studies typically employ acquisition of a series of images at multiple RF saturation frequencies.  Furthermore, quantitative MTC and CEST imaging techniques based on MR fingerprinting (MRF) require a range of values for multiple RF saturation parameters (e.g. B₁, saturation time). These multiple saturation acquisitions lead to a long scan time, which is likely vulnerable to motion during in vivo imaging. Motion correction is hard due to varying image intensity between acquisitions. Herein, we proposed a deep learning-based motion correction technique for conventional Z-spectra and MRF data.