Deuterium metabolic imaging (DMI) is a novel MR-based method to spatially map metabolism of deuterated substrates such as [6,6'-2 H 2 ]-glucose in vivo. Compared with traditional 13 C-MR-based metabolic studies, the MR sensitivity of DMI is high due to the larger 2 H magnetic moment and favorable T 1 and T 2 relaxation times.Here, the magnetic field dependence of DMI sensitivity and transmit efficiency is studied on phantoms and rat brain postmortem at 4, 9.4 and 11.7 T. The sensitivity and spectral resolution on human brain in vivo are investigated at 4 and 7 T before and after an oral dose of [6,6'-2 H 2 ]-glucose. For small animal surface coils (Ø 30 mm), the experimentally measured sensitivity and transmit efficiency scale with the magnetic field to a power of +1.75 and −0.30, respectively. These are in excellent agreement with theoretical predictions made from the principle of reciprocity for a coil noise-dominant regime. For larger human surface coils (Ø 80 mm), the sensitivity scales as a +1.65 power. The spectral resolution increases linearly due to nearconstant linewidths. With optimal multireceiver arrays the acquisition of DMI at a nominal 1 mL spatial resolution is feasible at 7 T. K E Y W O R D Sdeuterium metabolic imaging, magnetic field dependence, resolution, sensitivity | INTRODUCTIONDeuterium metabolic imaging (DMI) is a novel MR-based method to spatially map metabolism. 1 DMI lies in the category of stable isotope methods, in which an enriched substrate isotope is followed over time as it appears in downstream metabolic products. Common stable isotope methods include 13 C MR spectroscopy (MRS), 2 inverse 1 H-[ 13 C] MRS 3 and hyperpolarized 13 C MRS. 4,5 DMI is characterized by its technical simplicity and robustness as well as the relatively high sensitivity due to the larger magnetic moment and short T 1 relaxation time constants. The low natural abundance of 2 H of 0.0115% 6 leads to low-intensity water and lipid signals, thus eliminating the need for water and lipid suppression. To maximize sensitivity, the 2 H signal is excited by a single RF pulse, after which spatial localization is achieved with short 3D phase-encoding blips before FID acquisition. The robustness of DMI is further enhanced by the low sensitivity to magnetic field inhomogeneity due to the low 2 H Larmor frequency.

In this article, we present a first-pass perfusion imaging protocol to determine quantitative regional perfusion values (in mL min(-1) g(-1)) of the mouse myocardium. Perfusion was quantified using a Fermi-constrained deconvolution of the myocardial tissue response with the arterial input function. A dual-bolus approach was implemented. Experimental evidence is presented for the linearity of signal intensity in the left-ventricular lumen during the prebolus (r=0.99, P<0.001) and in the myocardium during the full-bolus injection (r=0.99, P<0.01) as function of Gd(DTPA)2- injection concentration used. The prebolus was used to reconstruct a nonsaturated arterial input function. Regional perfusion values proved repeatable in a cohort of nine healthy C57BL/6 mice. The perfusion values over two measurements with a 1-week interval were 7.3±0.9 and 7.2±0.6 mL min(-1) g(-1), respectively. No effects of time (P>0.05) and myocardial region (P>0.05) were observed. The between-session coefficient of variation was only 6%, whereas the inter-animal coefficient of variation was 11 and 8% for the separate experiments. We expect that the first-pass perfusion method here presented will be useful in preclinical studies of myocardial perfusion deficits and valuable to assess the impact of pro-angiogenic therapy after myocardial infarction.

To be able to examine dynamic and detailed brain functions, the spatial and temporal resolution of 7 T MRI needs to improve. In this study, it was investigated whether submillimeter multishot 3D EPI fMRI scans, acquired with high-density receive arrays, can benefit from a 2D CAIPIRINHA sampling pattern, in terms of noise amplification (g-factor), temporal SNR and fMRI sensitivity. High-density receive arrays were combined with a shot-selective 2D CAIPIRINHA implementation for multishot 3D EPI sequences at 7 T. In this implementation, in contrast to conventional inclusion of extra k z gradient blips, specific EPI shots are left out to create a CAIPIRINHA shift and reduction of scan time. First, the implementation of the CAIPIRINHA sequence was evaluated with a standard receive setup by acquiring submillimeter whole brain T 2 *-weighted anatomy images. Second, the CAIPIRINHA sequence was combined with high-density receive arrays to push the temporal resolution of submillimeter 3D EPI fMRI scans of the visual cortex. Results show that the shot-selective 2D CAIPIRI-NHA sequence enables a reduction in scan time for 0.5 mm isotropic 3D EPI T 2 *weighted anatomy scans by a factor of 4 compared with earlier reports. The use of the 2D CAIPIRINHA implementation in combination with high-density receive arrays, enhances the image quality of submillimeter 3D EPI scans of the visual cortex at high acceleration as compared to conventional SENSE. Both the g-factor and temporal SNR improved, resulting in a method that is more sensitive to the fMRI signal. Using this method, it is possible to acquire submillimeter single volume 3D EPI scans of the visual cortex in a subsecond timeframe. Overall, high-density receive arrays in combination with shot-selective 2D CAIPIRINHA for 3D EPI scans prove to be valuable for reducing the scan time of submillimeter MRI acquisitions. K E Y W O R D S3D EPI, 7T, CAIPI, high resolution, receive arrays, shot selective, visual cortex Abbreviations used: BOLD, blood oxygenation level-dependent; CAIPIRINHA, controlled aliasing in parallel imaging; EPI, echo-planar imaging; FOV, field of view; MB, multiband; RF, radio frequency; SENSE, sensitivity encoding; tSNR, temporal signal-to-noise ratio.

Purpose Assess the potential gain in acceleration performance of a 256‐channel versus 32‐channel receive coil array at 7 T in combination with a 2D CAIPIRINHA sequence for 3D data sets. Methods A 256‐channel receive setup was simulated by placing 2 small 16‐channel high‐density receive arrays at 2 8 different locations on the head of healthy participants. Multiple consecutive measurements were performed and coil sensitivity maps were combined to form a complete 256‐channel data set. This setup was compared with a standard 32‐channel head coil, in terms of SNR, noise correlation, and acceleration performance (g‐factor). Results In the periphery of the brain, the receive SNR was on average a factor 1.5 higher (ranging up to a factor 2.7 higher) than the 32‐channel coil; in the center of the brain the SNR was comparable or lower, depending on the size of the region of interest, with a factor 1.0 on average (ranging from 0.7 up to a factor of 1.6). The average noise correlation between coil elements was 3% for the 256‐channel coil, and 5% for the 32‐channel coil. At acceptable g‐factors (< 2), the achievable acceleration factor using SENSE and 2D CAIPIRINHA was 24 and 28, respectively, versus 9 and 12 for the 32‐channel coil. Conclusion The receive performance of the simulated 256 channel array was better than the 32‐channel reference. Combined with 2D CAIPIRINHA, a peak acceleration factor of 28 was assessed, showing great potential for high‐density receive arrays.

The goal of this study was to introduce and evaluate the performance of a lightweight, high-performance, single-axis (z-axis) gradient insert design primarily intended for high-resolution functional magnetic resonance imaging, and aimed at providing both ease of use and a boost in spatiotemporal resolution. The optimal winding positions of the coil were obtained using a genetic algorithm with a cost function that balanced gradient performance (minimum 0.30 mT/m/A) and field linearity (≥16 cm linear region). These parameters were verified using field distribution measurements by B 0 -mapping. The correction of geometrical distortions was performed using theoretical field distribution of the coil. Simulations and measurements were performed to investigate the echo planar imaging echo-spacing reduction due to the improved gradient performance. The resulting coil featured a 16-cm linear region, a weight of 45 kg, an installation time of 15 min, and a maximum gradient strength and slew rate of 200 mT/m and 1300 T/m/s, respectively, when paired with a commercially available gradient amplifier (940 V/630 A). The field distribution measurements matched the theoretically expected field. By utilizing the theoretical field distribution, geometrical distortions were corrected to within 6% of the whole-body gradient reference image in the target region. Compared with a whole-body gradient set, a maximum reduction in echo-spacing of a factor of 2.3 was found, translating to a 344 μs echo-spacing, for a field of view of 192 mm, a receiver bandwidth of 920 kHz and a gradient amplitude of 112 mT/m. We present a lightweight, single-axis gradient insert design that can provide high gradient performance and an increase in spatiotemporal

This is an open access article under the terms of the Creat ive Commo ns Attri butio n-NonCo mmerc ial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

SummaryEnergy metabolism of the human visual cortex was investigated by performing 31P functional MRS.IntroductionThe human brain is known to be the main glucose demanding organ of the human body and neuronal activity can increase this energy demand. In this study we investigate whether alterations in pH during activation of the brain can be observed with MRS, focusing on the mitochondrial inorganic phosphate (Pi) pool as potential marker of energy demand.MethodsSix participants were scanned with 16 consecutive 31P‐MRSI scans, which were divided in 4 blocks of 8:36 minutes of either rest or visual stimulation. Since the signals from the mitochondrial compartments of Pi are low, multiple approaches to achieve high SNR 31P measurements were combined. This included: a close fitting 31P RF coil, a 7 T‐field strength, Ernst angle acquisitions and a stimulus with a large visual angle allowing large spectroscopy volumes containing activated tissue.ResultsThe targeted resonance downfield of the main Pi peak could be distinguished, indicating the high SNR of the 31P spectra. The peak downfield of the main Pi peak is believed to be connected to mitochondrial performance. In addition, a BOLD effect in the PCr signal was observed as a signal increase of 2–3% during visual stimulation as compared to rest. When averaging data over multiple volunteers, a small subtle shift of about 0.1 ppm of the downfield Pi peak towards the main Pi peak could be observed in the first 4 minutes of visual stimulation, but no longer in the 4 to 8 minute scan window. Indications of a subtle shift during visual stimulation were found, but this effect remains small and should be further validated.ConclusionOverall, the downfield peak of Pi could be observed, revealing opportunities and considerations to measure specific acidity (pH) effects in the human visual cortex.

The increased signal‐to‐noise ratio (SNR) and chemical shift dispersion at high magnetic fields (≥7 T) have enabled neuro‐metabolic imaging at high spatial resolutions. To avoid very long acquisition times with conventional magnetic resonance spectroscopic imaging (MRSI) phase‐encoding schemes, solutions such as pulse‐acquire or free induction decay (FID) sequences with short repetition time and inner volume selection methods with acceleration (echo‐planar spectroscopic imaging [EPSI]), have been proposed. With the inner volume selection methods, limited spatial coverage of the brain and long echo times may still impede clinical implementation. FID‐MRSI sequences benefit from a short echo time and have a high SNR per time unit; however, contamination from strong extra‐cranial lipid signals remains a problem that can hinder correct metabolite quantification. L2‐regularization can be applied to remove lipid signals in cases with high spatial resolution and accurate prior knowledge. In this work, we developed an accelerated two‐dimensional (2D) FID‐MRSI sequence using an echo‐planar readout and investigated the performance of lipid suppression by L2‐regularization, an external crusher coil, and the combination of these two methods to compare the resulting spectral quality in three subjects. The reduction factor of lipid suppression using the crusher coil alone varies from 2 to 7 in the lipid region of the brain boundary. For the combination of the two methods, the average lipid area inside the brain was reduced by 2% to 38% compared with that of unsuppressed lipids, depending on the subject's region of interest. 2D FID‐EPSI with external lipid crushing and L2‐regularization provides high in‐plane coverage and is suitable for investigating brain metabolite distributions at high fields.