Suhyung Park¹, Sugil Kim^1,2, and Jaeseok Park^3
1Center for Neuroscience Imaging Research, Institute for Basic Science (IBS), Suwon, Korea, Republic of, ²Department of Brain and Cognitive Engineering, Korea University, Seoul, Korea, Republic of, ³Department of Biomedical Engineering, Sungkyunkwan University, Suwon, Korea, Republic of

Chemical exchange saturation transfer (CEST) imaging has been introduced as a new contrast mechanism for molecular imaging, and typically requires long saturation preparation and multiple acquisitions of imaging data with varying saturation frequencies (called z-spectrum). Since the z-spectrum acquisition is inherently slow and takes prohibitively long imaging time, it has been very difficult to introduce CEST z-spectrum into a clinical routine. In this work, we propose a novel, simultaneous multi-slice (SMS) Spiral CEST encoding with Hankel subspace learning (HSL) for ultrafast whole-brain z-spectrum acquisition within 2-3 minutes, in which: 1) RF segmented uneven saturation is employed to reduce the duration of saturation preparation, 2) Spiral CEST encoding is employed to acquire SMS signals, and 3) SMS signals are projected onto the subspace spanned by the complementary null space, selectively reconstructing a slice of interest while nulling the other slice signals. 

Iris Yuwen Zhou¹, Jinsuh Kim², Takahiro Igarashi¹, Lingyi Wen¹, and Phillip Zhe Sun^1
1Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Charlestown, MA, United States, ²Department of Radiology, University of Illusions at Chicago, Chicago, IL, United States

To resolve metabolites at different chemical shift offsets, complete Z-spectrum is conventionally obtained by varying saturation offset from scan to scan, which is time consuming and not suitable for studying dynamic changes. To overcome this, we innovatively combined superfast Z spectroscopy with chemical shift imaging (CSI) and developed Superfast Chemical exchange saturation transfer (CEST) Spectral Imaging (SCSI). It provides fast Z-spectral CEST information with spatial resolution. While conventional CSI measures dilute metabolites, the proposed SCSI exploits CEST mechanism to investigate the interaction between metabolites/contrast agents and tissue water, providing sensitivity enhanced measurements of metabolites and pH information.

Robert C. Brand¹, Nicholas P. Blockley¹, Michael Chappell², and Peter Jezzard^1
1FMRIB, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom, ²IBME, University of Oxford, Oxford, United Kingdom

Clinical 3D CEST has been hindered by slow acquisition times and z-spectra artefacts that affect fitting. Here, we demonstrate various sequence improvements, including: 1) hexagonal gradient spoiling that minimises ghosting, shortens TR and reduces confounding T2 sensitivity; 2) low readout flip angles combined with symmetric z-spectrum sampling that better maintains the steady state between samples and eliminates the need for T1-restoration periods; and 3) exchange-rate matched 360° CEST pulses that reduce direct water saturation to minimise T1 sensitivity and increase CNR. Together, these improvements result in high-quality whole-brain 39-offset z-spectra measurements at 3mm isotropic resolution in 2:59 minutes.

Rui Li¹, Bing Wu², Chien-yuan Lin³, Xin Lou¹, YuLin Wang¹, and Lin Ma^1
1Department of Radiology, PLA general Hospital, Beijing, China, People's Republic of, ²GE healthcare MR Research China, Beijing, China, People's Republic of, ³GE heathcare Taiwan, Taipei, Taiwan

Differential diagnosis is challenging due to similar appearance using conventional imaging such as T1 contrast enhanced and DWI. In this work, we use the measure of spatially matching 3D arterial spin labeling (ASL) and 3D amide proton transfer (APT) in the lesion proximal regions to differentiate metastasis and glioblastom, in the hypothesis that glioblastom  infiltrates into sourrounding tissues whereas metastasis tumors have clear biological boundaries.

Hye-Young Heo^1,2, Sampada Bhave³, Mathews Jacob³, and Jinyuan Zhou^1,2
1The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, MD, United States, ²F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD, United States, ³Department of Electrical and Computer Engineering, University of Iowa, Iowa City, IA, United States

CEST imaging, such as amide proton transfer (APT) imaging, is a novel, clinically valuable molecular MRI technique that can give contrast due to a change in water signal caused by chemical exchange with saturated solute protons. However, its clinical translation is still limited by its relatively long scan time because a series of RF saturation frequencies are unavoidably acquired. Here, we present a highly accelerated CEST imaging technique (up to 10-fold) using a novel blind compressed sensing framework.

Kevin Ray¹, Gogulan Karunanithy², Andrew Baldwin², Michael Chappell³, and Nicola Sibson^1
1Oxford Institute for Radiation Oncology, University of Oxford, Oxford, United Kingdom, ²Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford, United Kingdom, ³Institute of Biomedical Engineering, University of Oxford, Oxford, United Kingdom

CEST-MRI is an imaging technique which is sensitive to tissue pH, and has generated pH-weighted images in acute stroke patients. One key assumption regarding CEST-MRI is that the signal is predominantly intracellular. This assumption has implications in the application of CEST-MRI for pH measurement of tumours, which are generally associated with extracellular acidosis. This study developed a novel pulse sequence, combining stimulated echo acquisition mode diffusion and CEST imaging. Using this novel pulse sequence, the intracellular and extracellular contributions to the acquired Z-spectrum were isolated in a simple cell system and post-mortem mouse brain.

Xiaolei Song^1,2, Yan Bai^1,3, Meiyun Wang³, and Michael T. McMahon^1,2
1The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, MD, United States, ²F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD, United States, ³Department of Radiology, Henan Provincial People’s Hospital, Zhengzhou, China, People's Republic of

Existing CEST methodologies have difficulties in discriminating agents with small difference in chemical shift. As CEST signal is very sensitive to saturation power (B₁) and length (t_sat), indicating a second route to indentify agents by modulating the saturation conditions. We utilized the Multi-echo Parametric VARiation Saturation (MePaVARS), to separate faster and slower exchanging endogeneous CEST metabolites and molecules according to their differences response to B₁. In simulations and phantoms, MePaVARS allowed extraction of faster-exchanging Glutamate from the slower-exchanging Creatine, based on its oscillation patterns. A preliminary study for mice bearing prostate tumor further validated the feasibility of MePaVARS in vivo. 

Shu Zhang¹, Robert E Lenkinski^1,2, and Elena Vinogradov^1,2
1Radiology, UT Southwestern Medical Center, Dallas, TX, United States, ²Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, TX, United States

Recently properties of bSSFP were explored to detect exchange processes (bSSFPX), similar to CEST or off-resonance T_1ρ experiments. We expand the study and implement a transient bSSFPX experiment that acquires bSSFP spectra continuously as the effective saturation time increases, allowing observation of the approach of magnetization to the steady state in a single shot. The magnetization dynamic is governed by the effective field and relaxation times parallel or perpendicular to it. Work is in progress to derive an exact quantification model. The method leads to fast acquisition of time-dependent data and may speed up QUEST-like quantification of the exchange processes.

Alan P Manning¹, Kimberley L Chang², Alex MacKay^1,3, and Carl A Michal^1
1Physics and Astronomy, University of British Columbia, Vancouver, BC, Canada, ²Department of Neurology, University of British Columbia, Vancouver, BC, Canada, ³UBC MRI Research Centre, Department of Radiology, University of British Columbia, Vancouver, BC, Canada

Inhomogeneous MT (IHMT) shows promise for myelin-selectivity. Images acquired with soft prepulses at positive and negative offsets simultaneously show a reduced intensity compared to images with a single positive or negative offset prepulse. The leading hypothesis is that this works due to inhomogeneous broadening of the lipid proton line. Our results contradict this. We show that IHMT can be explained by a simple spin-1 model of a coupled methylene pair, and that it occurs in homogeneously-broadened systems (hair and wood). We propose the relevant timescales for IHMT are the dipolar coupling correlation time and the prepulse nutation period.

Marco Battiston¹, Francesco Grussu¹, James E. M. Fairney^2,3, Ferran Prados^1,4, Sebastien Ourselin⁴, Mara Cercignani⁵, Claudia Angela Michela Gandini Wheeler-Kingshott^1,6, and Rebecca S Samson^1
1NMR Research Unit, Queen Square MS Centre, Department of Neuroinflammation, UCL Institute of Neurology, University College London, London, United Kingdom, ²UCL Department of Medical Physics and Biomedical Engineering, University College London, London, United Kingdom, ³Department of Brain Repair and Rehabilitation, UCL Institute of Neurology, University College London, London, United Kingdom,⁴Translational Imaging Group, Centre for Medical Image Computing, UCL Department Medical Physics and Bioengineering, University College London, London, United Kingdom, ⁵CISC, Brighton & Sussex Medical School, Brighton, United Kingdom, ⁶Brain Connectivity Center, C. Mondino National Neurological Institute, Pavia, Italy

Quantitative Magnetization Transfer (qMT) Imaging techniques offer the possibility to estimate tissue macromolecular fraction, which has been shown to be specific for myelin in the brain and spinal cord. To date, applications of qMT in the spinal cord have been hampered by prohibitive protocol duration. We propose a novel approach for qMT in the spinal cord based on the combination of off-resonance saturation and small field-of-view imaging, with the potential of reducing the scan time needed to perform qMT in the spinal cord.