Magnetization Transfer Contrast (MTC) and Chemical Exchange Saturation Transfer (CEST) experiments measure the transfer of magnetization from molecular protons to the solvent water protons, an effect that becomes apparent as an MRI signal loss ("saturation"). This allows molecular information to be accessed with the enhanced sensitivity of MRI. In analogy to Magnetic Resonance Spectroscopy (MRS), these saturation data are presented as a function of the chemical shift of participating proton groups, e.g. OH, NH, NH, which is called a Z-spectrum. In tissue, these Z-spectra contain the convolution of multiple saturation transfer effects, including nuclear Overhauser enhancements (NOEs) and chemical exchange contributions from protons in semi-solid and mobile macromolecules or tissue metabolites. As a consequence, their appearance depends on the magnetic field strength (B) and pulse sequence parameters such as B strength, pulse shape and length, and interpulse delay, which presents a major problem for quantification and reproducibility of MTC and CEST effects. The use of higher B can bring several advantages. In addition to higher detection sensitivity (signal-to-noise ratio, SNR), both MTC and CEST studies benefit from longer water T allowing the saturation transferred to water to be retained longer. While MTC studies are non-specific at any field strength, CEST specificity is expected to increase at higher field because of a larger chemical shift dispersion of the resonances of interest (similar to MRS). In addition, shifting to a slower exchange regime at higher B facilitates improved detection of the guanidinium protons of creatine and the inherently broad resonances of the amine protons in glutamate and the hydroxyl protons in myoinositol, glycogen, and glucosaminoglycans. Finally, due to the higher mobility of the contributing protons in CEST versus MTC, many new pulse sequences can be designed to more specifically edit for CEST signals and to remove MTC contributions.

Hyperpolarization of spins via dynamic nuclear polarization (DNP) has been explored as a method to non-invasively study real-time metabolic rocesses occurring in vivo using 13 C-labeled substrates. Recently, hyperpolarized 13 C pyruvate has been used to characterize in vivo cardiac metabolism in the rat and pig. Conventional 3D spectroscopic imaging methods require in excess of 100 excitations, making it challenging to acquire a full cardiac-gated, breath-held, whole-heart volume. In this article, the development of a rapid multislice cardiac-gated spiral 13 C imaging pulse sequence consisting of a large flip-angle spectral-spatial excitation RF pulse combined with a singleshot spiral k-space trajectory for rapid imaging of cardiac metabolism is described. This sequence permits whole-heart coverage (6 slices, 8.

ADC values for emphysematous lungs were significantly increased compared with healthy lungs in age-matched subjects, and all values were comparable to those reported previously at 1.5 Tesla. Ventilation defect score and ventilation defect volume results were also comparable to results previously reported in COPD subjects Reproducibility of ADC for same-day scan-rescan and 7-day rescan was high and similar to previously reported results.

Abstract13 C MR spectroscopy studies performed on hearts ex vivo and in vivo following perfusion of prepolarized [1-13 C]pyruvate have shown that changes in pyruvate dehydrogenase (PDH) flux may be monitored non-invasively. However, to allow investigation of Krebs cycle metabolism, the 13 C label must be placed on the C2 position of pyruvate. Thus the utilization of either C1 or C2 labeled pre-polarized pyruvate as a tracer can only afford a partial view of cardiac pyruvate metabolism in health and disease. If the pre-polarized pyruvate molecules were labeled at both C1 and C2 position, then it would be possible to observe the downstream metabolites that were the results of both PDH flux ( 13 CO 2 and H 13 CO 3 − ) and Krebs cycle flux ([5-13 C]glutamate) with a single dose of the agent. Cardiac pH could also be monitored in the same experiment, but adequate SNR of the 13 CO 2 resonance may be difficult to obtain in vivo. Using an interleaved selective RF pulse acquisition scheme to improve 13 CO 2 detection, the feasibility of using dual-labeled hyperpolarized [1,2-13 C 2 ]pyruvate as a substrate for dynamic cardiac metabolic MRS studies, to allow simultaneous investigation of PDH flux, Krebs cycle flux, and pH was demonstrated in vivo.

Post-mortem diffusion imaging of whole, human brains has potential to provide data for validation or high-resolution anatomical investigations. Previous work has demonstrated improvements in data acquired with diffusion-weighted steady-state free precession (DW-SSFP) compared with conventional diffusion-weighted spin echo at 3 T. This is due to the ability of DW-SSFP to overcome signal-to-noise and diffusion contrast losses brought about by tissue fixation related decreases in T2 and ADC. In this work, data of four post-mortem human brains were acquired at 3 T and 7 T, using DW-SSFP with similar effective b-values (beff ~ 5150 s/mm2) for inter-field strength comparisons; in addition, DW-SSFP data were acquired at 7 T with higher beff (~ 8550 s/mm2) for intra-field strength comparisons. Results demonstrate that both datasets acquired at 7 T had higher SNR and diffusion contrast than data acquired at 3 T, and data acquired at higher beff had improved diffusion contrast than at lower beff at 7 T. These results translate to improved estimates of secondary fiber orientations leading to higher fidelity tractography results compared with data acquired at 3 T. Specifically, tractography streamlines of cortical projections originating from the corpus callosum, corticospinal tract, and superior longitudinal fasciculus were more successful at crossing the centrum semiovale and projected closer to the cortex. Results suggest that DW-SSFP at 7 T is a preferential method for acquiring diffusion-weighted data of post-mortem human brain, specifically where the primary region of interest involves crossing white matter tracts.

A novel imaging method is presented, Flip Angle Variation for Offset of RF and Relaxation (FAVOR), for rapid and efficient measurement of rat lung ventilation using hyperpolarized helium-3 ( 3 He) gas. The FAVOR technique utilizes variable flip angles to remove the cumulative effect of RF pulses and T 1 relaxation on the hyperpolarized gas signal and thereby eliminates the need for intervening air wash-out breaths and multiple cycles of 3 He wash-in breaths before each image. The former allows an improvement in speed (by a factor of Ϸ30) while the latter reduces the cost of each measurement (by a factor of Ϸ5). MR imaging with hyperpolarized 3 He gas has been proposed for measurement of regional ventilation in the rodent lung (1) and has been recently validated with xenon-enhanced CT imaging (2). It is anticipated that 3 He MR will provide a favorable approach for measurement of ventilation in animal cohorts to track lung disease over time without the complications associated with accumulated x-ray dose (3). This method measures the dynamic change in lung 3 He signal as a function of breath number and extracts the relative refreshment of gas in a given lung voxel per breath. However, the conventional 3 He ventilation measurement requires knowledge of the longitudinal relaxation time, T 1 , of the 3 He gas in the ventilator system and in the lung, the latter requiring knowledge of the alveolar oxygen partial pressure (p A O 2 ).Furthermore, without accurate knowledge of the RF pulse history the method requires multiple 3 He breathing cycles (i.e., 3 He wash-in breaths) with air wash-out breaths between cycles in order to completely clear the lung of 3 He gas, which is time-consuming (i.e., several minutes), an inefficient use of hyperpolarized 3 He gas (i.e., costly), and can lead to imprecision due to variations in tidal volume from the ventilator.Perhaps most important, the conventional technique involving multiple 3 He and air breathing cycles requires several minutes (Ϸ10 min) for a single ventilation map, which is likely too slow to capture rapid changes in ventilation associated with short-duration bronchoconstriction such as a methacholine (MCh) challenge (Ͻ1 min), similar to asthma (4,5). Previous work has been limited to measurement of pre-and postsensitization effects or postlong-term challenge in part due to the time required to measure ventilation using the conventional method. The ability to detect changes in ventilation over time scales of less than 1 min may provide improved sensitivity to shortterm challenge and insight into disease which more closely resembles asthma (6 -9). As well, rapid measurement of ventilation may prove critical for evaluation of fast-acting drug therapies for asthma (10,11).We propose a novel approach, Flip Angle Variation for Offset of RF and Relaxation (FAVOR), for obtaining regional ventilation in a single set of breathing cycles (i.e., only one set of 3 He wash-in breaths and no air wash-out breaths). This approach utilizes variable flip angle (VFA) RF pulses that comp...

In rural US communities, emergency department physician-initiated interhospital transfer of STEMI patients for primary or rescue PCI is feasible and was safely executed with achievement of timely reperfusion when performed within coordinated healthcare networks.

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