Gopal Varma¹, Olivier M Girard², Valentin Prévost², Aaron K Grant¹, Guillaume Duhamel², and David C Alsop^1
1Radiology, Division of MR Research, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts, United States, ²CRMBM UMR 7339, CNRS and Aix-Marseille Université, Marseille, France

The inhomogeneous magnetization transfer technique (ihMT) has gained attention for producing myelin selective images, but a model for its mechanism has not been established. This work develops a model based on existing applications of the Redfield-Provotorov theory to regular MT that predicted a dipolar order. The results from model fitting to mouse and human data provide measurable values for a relaxation time of the dipolar order, T_D, consistent with the observable ihMT signal. The variation observed in T_D between different regions of interest might provide further insight into the selectivity and/or optimization of the ihMT technique.

Scott D. Swanson¹, Dariya I. Malyarenko², and Mario L. Fabiilli^2
1Department of Radiology, University of Michigan, Ann Arbor, Michigan, United States, ²Department of Radiology, University of Michigan, Michigan, United States

ihMT theory

Henrik Marschner¹, Riccardo Metere¹, Stefan Geyer¹, André Pampel¹, and Harald E. Möller^1
1Nuclear Magnetic Resonance, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Sachsen, Germany

We examine the apparent dependence of qMT parameters, in particular those of the T2b relaxation time of bound protons, on the white matter fiber orientation in a Post-Mortem Marmoset Brain. The observed angular dependence in the post-mortem data for highly ordered WM supports the hypothesis of orientation dependence of the RF absorption lineshape that was observed in human brain in vivo. Here, variation of the orientation is achieved by rotating the sample in the magnetic field.

Peter van Gelderen¹, Xu Jiang¹, and Jeff H Duyn^1
1AMRI, LFMI, NINDS, National Institutes of Health, Bethesda, MD, United States

Magnetization Transfer (MT) contrast is sensitive to brain myelination but lacks specificity, while quantification is difficult and requires lengthy experiments, including T1 measurement. Pulsed MT experiments may offer a simpler way to obtained quantitative measures, such as bound pool fraction and transfer rates. Here we explore a variation on a binominal pulse design for optimizing the pulsed MT experiment and demonstrate its application to study MT contrast in white matter.

Wolfgang G Rehwald¹, David C Wendell², Elizabeth R Jenista², Han W Kim², Enn-Ling Chen², Igor Klem², and Raymond J Kim^2
1Siemens Healthcare, Durham, NC, United States, ²Cardiology, Duke University Medical School, Durham, North Carolina, United States

Magnetization transfer contrast could be a clinically and technically advantageous alternative to T2-contrast. It would be useful for imaging edema and creating tissue-blood contrast, if its energy and power requirements did not exceed the capabilities of most clinical MRI scanners. To address this issue, we designed an efficient adiabatic on-resonance MT module that imparts significant MT-weighting while remaining within the energy and power constraints of most clinical MRI scanners. In ten cardiac patients, we compared this module to standard high-power off-resonance MT preparation and established its image quality and contrast equivalence while only requiring a fraction of its energy.

Xu Jiang^1,2, Peter van Gelderen¹, Xiaozhen Li¹, Emily Leibovitch³, Pascal Sati⁴, Afonso C. Silva⁵, and Jeff H Duyn^1
1AMRI, LFMI, NINDS, NIH, Bethesda, MD, United States, ²Department of Physics, University of Maryland, College Park, MD, United States, ³Viral Immunology Section, Neuroimmunology Branch, NINDS, NIH, Bethesda, MD, United States, ⁴Translational Neuroradiology Unit, NINDS, NIH, Bethesda, MD, United States, ⁵CMU, LFMI, NINDS, NIH, Bethesda, MD, United States

Accurate knowledge of the T2 of bound (non-water) protons is very essential for optimization and quantification of MT contrast. Using optimized, 2ms long binomial MT pulses with varying amplitudes, and studying delay-dependent saturation of mobile water protons, we determined bound proton T2 to average 27μs, 17.5μs and 13.5μs in white matter, grey matter, and muscle tissue in marmoset in vivo, while the exchange rates between water and non-water protons were 13s-1, 30s-1, and 47s-1 respectively. The pulse-delay approach followed here proved a sensitive way to study MT contrast and is applicable to study of human brain at high field.

Xiao-Yong Zhang¹, Hua Li¹, Junzhong Xu¹, Jingping Xie¹, John C. Gore¹, and Zhongliang Zu^1
1Institute of Imaging Science, Vanderbilt University, Nashville, TN, United States

Magnetization transfer (MT) provides a unique mechanism for producing contrast and makes MRI sensitive to the presence of metabolites, mobile macromolecules, and semisolid macromolecules through their ‘magnetic coupling’ effects on the water signal. Although the nuclear Overhauser enhancement (NOE)-mediated MT signal at -3.5 ppm has been recently studied, to date no in vivo MT effect at -1.6 ppm has been reported. We report a new MT signal MT at -1.6 ppm, which may reflect the NOE effect between choline headgroup of membrane phospholipids and water protons. The MT signal at -1.6 ppm could be used as a new biomarker in healthy rat brain.

Rongwen Tain^1,2, Michael Abern³, Karen Xie¹, X. Joe Zhou^1,2, and Kejia Cai^1,2
1Radiology, College of Medicine, University of Illinois at Chicago, Chicago, Illinois, United States, ²Center for MR Research, College of Medicine, University of Illinois at Chicago, Chicago, Illinois, United States, ³Urology, College of Medicine, University of Illinois at Chicago, Chicago, Illinois, United States

Reduction-oxidation (Redox) imbalance due to oxidative stress may occur in pathologies including cancer. Developing imaging biomarkers for oxidative stress is a key research area. Recently, we presented the first non-invasive MRI method for mapping the tissue redox state based on the endogenous Chemical Exchange Saturation Transfer (CEST) contrast. It is well known that magnetization transfer (MT) can occur via chemical exchange (CEST) and/or dipole-dipole interactions (Nuclear Overhauser Enhancement or NOE). This study on ex vivo egg white tissues and in vivo prostate cancers demonstrated that MT in the broad definition is sensitive to oxidative stress through animal models.

Jochen Keupp¹ and Osamu Togao^2
1Philips Research, Hamburg, Germany, ²Clinical Radiology, Graduate School of Medical Science, Kyushu University, Fukuoka, Japan

MTR_asym can be robustly and efficiently measured in clinical settings, including 3D acquisitions with large coverage in less than 5 minutes. On the other hand, MTR_asym as a measure for APT signal levels, was previously reported to be biased by Nuclear Overhauser effects (NOE) at opposite frequency offsets. The influence of NOE effects may strongly depend on the main magnetic field (B₀) and on saturation parameters. Most reported studies were performed at ultra-high B₀7T and typically employed short RF saturation (T_sat<1s) and low saturation field amplitudes (e.g. B_1,rms=0.5 µT). In this study, a previously proposed APT protocol (Tsat=2s; B_1,rms=2 µT) on 3.0T MRI systems is applied for human brain tumor cases and analyzed for NOE contributions using improved Z-spectral fitting methods. It was hypothesized, that there is only a small or negligible NOE contrast between NAWM and tumor tissue using long saturation pulses at 3T. The study results indicate that MTR_asym in combination with a specific protocol at 3T could be used to assess APT signal levels in clinical applications without significant interference of NOE effects.

Jimin Ren^1,2, Baolian Yang³, A. Dean Sherry^1,4, and Craig R. Malloy^1,5
1Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, Texas, United States, ²Department of Radiology, University of Texas Southwestern Medical Center, Dallas, Texas, United States, ³Philips Healthcare, Cleveland, Ohio, United States, ⁴Department of Chemistry, University of Texas at Dallas, Richardson, Texas, United States, ⁵VA North Texas Health Care System, Dallas, Texas, United States

31P magnetization transfer studies in animals and humans suggest that transfer occurs not only between ATP and exchanging metabolites such as phosphocreatine and inorganic phosphate, but also among ATP spins. The latter effect, although small, is most consistent with 31P-31P NOE. Here, we explore a strategy to amplify this exchange effect within ATP by using a wideband inversion. A four-fold increase in -ATP signal reduction was observed by simultaneously inverting Pi, PCr, - and -ATP using an adiabatic pulse, as compared to by conventional frequency-selective inversion of only -ATP. This observation is not consistent with known chemical exchange pathways.