Endogenous Contrast Mechanisms: MT & Relaxometry
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Tuesday May 10th
Room 518-A-C  10:30 - 12:30 Moderators: Karla Miller and Bruce Pike

10:30 224.   Fast Bound Pool Fraction Imaging of the In Vivo Rat Brain: Assocation with Myelin Content and Validation in the C6 Glioma Model  
Hunter R Underhill1, Robert C Rostomily1, Andrei M Mikheev1, Chun Yuan1, and Vasily L Yarnykh1
1University of Washington, Seattle, WA, United States

Cross-relaxation imaging is a quantitative MRI technique that measures the kinetic parameters of magnetization transfer between protons bound to water and protons bound to macromolecules (i.e. bound pool fraction, f). A new time-efficient approach for solely capturing the in vivo whole-brain parametric map of f was proposed, validated with histology, and used to estimate myelin density in the normal rat brain on a 3.0 T clinical scanner. In both whole-brain f maps and myelin density maps, replacement of normal gray matter and white matter by proliferating and invading tumor cells was readily identified in vivo and confirmed with histology.

10:42 225.   Modeling and Measuring the Myelin g-ratio 
Nikola Stikov1, Lee M Perry2, Aviv Mezer2, John M Pauly3, Brian A Wandell2, and Robert F Dougherty2
1Montreal Neurological Institute, McGill University, Montreal, QC, Canada, 2Psychology, Stanford University, Stanford, CA, United States, 3Electrical Engineering, Stanford University, Stanford, CA, United States

In myelinated axons, the ratio between the axon caliber (diameter) and the total caliber of the axon plus its myelin sheath (i.e., the fiber caliber) is relatively constant and is observed to be near the theoretically optimal value of 0.6. Recently, variations in this axon-to-fiber ratio (the "g-ratio") have been proposed to be associated with differences in brain development. Here we describe a method to estimate the g-ratio in-vivo by combining diffusion imaging and quantitative magnetization transfer. The methods described here form a novel MR contrast mechanism that may be useful for quantifying the development of white matter and the deterioration of white matter in demyelinating diseases.

10:54 226.   Paradoxical changes in magnetization transfer ratio and susceptibility contrast in the motor cortex 
Olivier E Mougin1, Samuel J Wharton1, Rosa M Sanchez Panchuelo1, Richard W Bowtell1, and Penny A Gowland1
1Sir Peter Mansfield Magnetic Resonance Centre, University of Nottingham, Nottingham, Nottinghamshire, United Kingdom

Myelination and iron storage is variable across the brain, creating specific cortical architecture for different functional areas. We used magnetization transfer imaging as well as susceptibility mapping to look at in-vivo variation across the cortex of healthy volunteers. Variation between areas such as the motor cortex, the visual cortex, the occipital cortex and the frontal lobe shows that contrast in the grey matter is determined not only by iron, but also by myelin.

11:06 227.   Fast macromolecular proton fraction mapping from a single off-resonance magnetization transfer measurement 
Vasily L. Yarnykh1
1Radiology, University of Washington, Seattle, WA, United States

This study demonstrates the feasibility of fast and accurate macromolecular proton fraction (MPF) mapping using only a single-offset MT-weighted image. This approach is the fastest to date quantitative MT (qMT) method, which requires no extra time compared to traditional magnetization transfer ratio (MTR) mapping, if a complementary T1 map is available. Since the proposed method rapidly determines MPF with high accuracy, it provides a viable alternative to both time-consuming multi-parameter qMT techniques and fast but poorly-interpretable MTR measurements.

11:18 228.   Origins of the Ultrashort-T2 1H NMR Signals in Myelinated Nerve: A Direct Measure of Myelin Content? 
Robert Adam Horch1,2, John C Gore2,3, and Mark D Does1,2
1Biomedical Engineering, Vanderbilt University, Nashville, TN, United States, 2Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, TN, United States, 3Radiology and Radiological Sciences, Vanderbilt University, Nashville, TN, United States

Ultra-short echo time (uTE) MRI of the brain has demonstrated white matter-specific signal enhancement from short T2 (<1 ms) signals suspected to arise from myelin. However, the T2 characteristics of these signals, as well as their origins and possible relationship to myelin, remain uncharacterized. Herein, we perform T2characterizations and isotopic 1H perturbations on myelin phantoms and myelinated central and peripheral nerves to determine the origins of short-lived T2s relevant to uTE MRI. We find sizeable signals in the ≈ 50 µs – 1 ms T2 domain and provide compelling evidence that they arise from myelin membrane methylene 1H.

11:30 229.   New MRI contrasts in experimental stroke: What do we measure with RAFF and ZAPI? 
Johanna Närväinen1, Kimmo Jokivarsi2, Timo Liimatainen3, Olli Gröhn3, and Risto A. Kauppinen4
1A.I.Virtanen Institute, University of Eastern Finland, Kuopio, Finland, 2Massachusetts General Hospital, 3A.I.Virtanen Institute, University of Eastern Finland, 4Dartmouth College, United States

Recently introduced MRI techniques, RAFF and ZAPI, were used to study acute stroke in a rat model. It is concluded that 1) Relaxation and MT are not coupled in acute cerebral stroke, even when clean on-resonance ZAPI-MT is used. 2) RAFF signal is sensitive to ischemia with sensitivity and temporal behavior comparable to T2, but inferior to T1lower case Greek rho. 3) RAFF signal change in ischemic tissue can be predicted by simulations. 4) On resonance-MT differs from off-resonance MT, both in magnitude and in temporal evolution during stroke.

11:42 230.   Observation of Myelin Water at Ultra-Short Echo Time by Longitudinal Relaxographic Imaging with Spin-Echo Center-out EPI (DEPICTING) 
Christian Labadie1,2, William D. Rooney3, Charles S. Springer, Jr.3, Jing-Huei Lee4, Monique Aubert-Frécon2, Stefan Hetzer5, Toralf Mildner1, and Harald E. Möller1
1Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, 2Laboratoire de Spectrométrie Ionique et Moléculaire, Université Claude Bernard Lyon 1, France, 3Oregon Health and Science University, Portland, Oregon, United States, 4Biomedical Engineering, University of Cincinnati, OH, United States, 5Bernstein Center for Computational Neuroscience, Berlin, Germany

We adapted a spin-echo center-out echo planar imaging with ultra-short echo time [TE] (DEPICTING) pulse sequence with an inversion recovery [IR] preparation. Two series of 32 pseudo-randomized TIs were acquired with two TEs and analyzed by cross-regularized inverse Laplace transform. The human white matter water proton T2 value associated with the short component w. small water proton T1 detected by IR relaxographic analysis was determined to be 34.2 ms and supports its the assignment as a myelin water fraction.

11:54 231.   3D Quantitative Imaging of T1rho and T2 
Weitian Chen1, Atsushi Takahashi1, and Eric Han1
1Global Applied Science Laboratory, GE Healthcare, Menlo Park, CA, United States

T1rho and T2 mapping have potential in a number of clinical applications. 3D acquisition is usually desired for these applications due to the geometry of anatomy. In this abstract, we reported our continous development of a highly SNR efficient 3D T1rho/T2 mapping method based on a pseudo steady state 3D FSE acquisition and demonstrated its accuracy in phantom and in-vivo scans.

12:06 232.   Is iron the source of post mortem susceptibility contrast in the brain? 
Ferdinand Schweser1,2, Christian Langkammer3,4, Andreas Deistung1, Nikolaus Krebs4, Walter Goessler5, E Scheurer4, K Yen4, Franz Fazekas3, Jürgen R. Reichenbach1, and Stefan Ropele3
1Medical Physics Group, Dept. of Diagnostic and Interventional Radiology 1, Jena University Hospital, Jena, Germany, 2School of Medicine, Friedrich Schiller University of Jena, Jena, Germany, 3Dept. of Neurology, Medical University of Graz, Graz, Austria, 4Ludwig Boltzmann Institute for Clinical-Forensic Imaging, Graz, Austria, 5Institute of Chemistry, Analytical Chemistry, University of Graz, Graz, Austria

Quantitative information of the regional non-heme tissue iron distribution bears clinical potential in the context of various neurological and psychiatric disorders. The goal of the current study was to investigate and compare the relation between tissue iron concentration and tissue magnetic susceptibility of in situ and fixed post mortem brain tissue.

12:18 233.   Three dimensional T2prep spiral imaging with efficient brain coverage for myelin water quantification: Validation at 1.5 Tesla 
Thanh D Nguyen1, Cynthia Wisnieff2, Mitchell Cooper2, Dushyant Kumar1, Ashish Raj1, Martin R Prince1, Yi Wang1, Tim Vartanian3, and Susan A Gauthier3
1Radiology, Weill Cornell Medical College, New York, NY, United States, 2Biomedical Engineering, Cornell University, Ithaca, NY, United States, 3Neurology, Weill Cornell Medical College, New York, NY, United States

The objective of this study was to develop and optimize an SNR efficient 3D T2prep spiral gradient echo sequence for full brain T2 relaxometry and to validate this sequence using 3D FSE as reference standard at 1.5T. The spiral sequence was found to provide similar T2 on phantom and myelin water fraction for various brain tissues. 28 axial slices and 24 T2prep times can be obtained in 24 min with 2.5-fold higher SNR than conventional 2D FSE approach.