The role of dopamine in monitoring negative action outcomes and feedback-based learning was tested in a neuroimaging study in humans grouped according to the dopamine D2 receptor gene polymorphism ...DRD2-TAQ-IA. In a probabilistic learning task, A1-allele carriers with reduced dopamine D2 receptor densities learned to avoid actions with negative consequences less efficiently. Their posterior medial frontal cortex (pMFC), involved in feedback monitoring, responded less to negative feedback than others' did. Dynamically changing interactions between pMFC and hippocampus found to underlie feedback-based learning were reduced in A1-allele carriers. This demonstrates that learning from errors requires dopaminergic signaling. Dopamine D2 receptor reduction seems to decrease sensitivity to negative action consequences, which may explain an increased risk of developing addictive behaviors in A1-allele carriers.
Purpose
To investigate the effects of prospective motion correction on turbo spin echo sequences and optimize motion correction approaches, mitigating signal dropout artifacts caused by the ...imperfections of motion tracking data.
Methods
Signal dropout artifacts caused by undesired phase deviations introduced by tracking errors are analyzed theoretically. To reduce the adverse effect of such deviations, two approaches are proposed: (1) freezing the correction for example, for even‐numbered or higher number of echoes and (2) shifting the correction event prior to the left crusher gradient preceding the refocusing pulse. A comprehensive analysis is presented, including both signal simulations and experimental verifications in phantoms and in vivo. Performance of the proposed approach is validated in two healthy volunteers imaged under two types of motion conditions simulating inadvertent fast motions associated with discomfort and continuous large motions.
Results
The results show that the proposed optimization is able to efficiently correct for the motion artifacts and at the same time avoid signal dropout artifacts. Specifically, performing correction every 4th echo prior to the left crusher gradient was shown to improve image quality.
Conclusion
An optimization approach is proposed to exploit the potential of external tracking for intra‐echo‐train motion artifact correction for turbo spin echo sequences.
Purpose
Spin‐echo (SE) functional MRI (fMRI) can be highly advantageous compared to gradient‐echo (GE) fMRI with respect to magnetic field‐inhomogeneity artifacts. However, at 3T, the majority of ...blood oxygenation level‐dependent (BOLD) fMRI experiments are performed using T2∗‐weighted GE sequences because of their superior sensitivity compared to SE‐fMRI. The presented SE implementation of a highly accelerated GE pulse sequence therefore aims to improve the sensitivity of SE‐fMRI while profiting from a reduction of susceptibility‐induced signal dropout.
Methods
Spin‐echo MR encephalography (SE‐MREG) is compared with the more conventionally used spin‐echo echo‐planar imaging (SE‐EPI) and spin‐echo simultaneous multislice (SE‐SMS) at 3T in terms of capability to detect neuronal activations and resting‐state functional connectivity. For activation analysis, healthy subjects underwent consecutive SE‐MREG (pulse repetition time TR = 0.25 seconds), SE‐SMS (TR = 1.3 seconds), and SE‐EPI (TR = 4.4 seconds) scans in pseudorandomized order applied to a visual block design paradigm for generation of t‐statistics maps. For the investigation of functional connectivity, additional resting‐state data were acquired for 5 minutes and a seed‐based correlation analysis using Stanford’s FIND (Functional Imaging in Neuropsychiatric Disorders) atlas was performed.
Results
The increased sampling rate of SE‐MREG relative to SE‐SMS and SE‐EPI improves the sensitivity to detect BOLD activation by 33% and 54%, respectively, and increases the capability to extract resting‐state networks. Compared with a brain region that is not affected by magnetic field inhomogeneities, SE‐MREG shows 2.5 times higher relative signal strength than GE‐MREG in mesial temporal structures.
Conclusion
SE‐MREG offers a viable possibility for whole‐brain fMRI with consideration of brain regions that are affected by strong susceptibility‐induced magnetic field gradients.
Purpose
Highly undersampled acquisitions have been proposed to push the limits of temporal resolution in functional MRI. This contribution is aimed at identifying parameter sets that let the user ...trade‐off between ultra‐high temporal resolution and spatial signal quality by varying the sampling densities. The proposed method maintains the synergies of a temporal resolution that enables direct filtering of physiological artifacts for highest statistical power, and 3D read‐outs with optimal use of encoding capabilities of multi‐coil arrays for efficient sampling and high signal‐to‐noise ratio (SNR).
Methods
One‐ to four‐shot interleaved spherical stack‐of‐spiral trajectories with repetition times from 96 to 352 ms at a nominal resolution of 3 mm using different sampling densities were compared for image quality and temporal SNR (tSNR). The one‐ and three‐shot trajectories were employed in a resting state study for functional characterization.
Results
Compared to a previously described single‐shot trajectory, denser sampled trajectories of the same type are shown to be less prone to blurring and off‐resonance vulnerability that appear in addition to the variable density artifacts of the point spread function. While the multi‐shot trajectories lead to a decrease in tSNR efficiency, the high SNR due to the 3D read‐out, combined with notable increases in image quality, leads to superior overall results of the three‐shot interleaved stack of spirals. A resting state analysis of 15 subjects shows significantly improved functional sensitivity in areas of high off‐resonance gradients.
Conclusion
Mild variable‐density sampling leads to excellent tSNR behavior and no increased off‐resonance vulnerability, and is suggested unless maximum temporal resolution is sought.
Among the hyperpolarization techniques geared toward in vivo magnetic resonance imaging, parahydrogen-induced polarization (PHIP) shows promise due to its low cost and fast speed of contrast agent ...preparation. The synthesis of 13C-labeled, unsaturated precursors to perform PHIP by side arm hydrogenation has recently opened new possibilities for metabolic imaging owing to the biological compatibility of the reaction products, although the polarization transfer between the parahydrogen-derived protons and the 13C heteronucleus must yet be better understood, characterized, and eventually optimized. In this realm, a new experimental strategy incorporating pulse-programmable magnetic field sweeping and in situ detection has been developed. The approach is evaluated by measuring the 13C polarization of ethyl acetate-1-13C, i.e., the product of pairwise addition of parahydrogen to vinyl acetate-1-13C, resulting from zero-crossing magnetic field ramps of various durations, amplitudes, and step sizes. The results demonstrate (i) the profound effect these parameters have on the 1H to 13C polarization transfer efficiency and (ii) the high reproducibility of the technique.
Purpose
An open‐source spatially resolved phase graph framework is proposed for simulating arbitrary pulse sequences in the presence of piece‐wise constant gradients with arbitrary orientations in ...three dimensions. It generalizes the extended phase graph algorithm for analysis of nonperiodic sequences while preserving its efficiency, and is able to estimate the signal modulation in the 3D spatial domain.
Methods
The framework extends the recursive magnetization‐evolution algorithm to account for anisotropic diffusion and exploits a novel 3D k‐space grid‐merging method to balance the computational effort and memory requirements against acceptable simulation errors. A new postsimulation module is proposed to track and visualize the signal evolution both in the k‐space and in the image domain, which can be used for simulating image artifacts or finding frequency‐response profiles. To illustrate the developed technique, three examples are presented: (1) fast off‐resonance calculation for dictionary building in MR fingerprinting, (2) validation of a steady‐state sequence with quasi‐isotropic diffusion weighting, and (3) investigation of the magnetization evolution in PRESS‐based spectroscopic imaging.
Results
The grid‐merging algorithm of the proposed framework demonstrates high calculation efficiency exemplified by frequency‐response simulation of pseudo steady‐state or diffusion‐weighted steady‐state sequences. It further helps to visualize the signal evolution in PRESS‐based sequences.
Conclusions
The proposed simulation framework has been validated based on several different example applications for analyzing signal evolution in the frequency and spatial domain.
Display omitted
•Direct matching methods for coils and preamplifiers in receiver arrays.•The reactance of the input impedance of preamplifiers is used to decouple the coils.•The coil loop can be ...matched to the transistor in the preamplifier directly.•The new matching networks have fewer noise than the conventional method.
In this paper, direct matching methods for coils and preamplifiers in receiver arrays are presented. Instead of compensating the reactance of the input impedance of preamplifiers, in our method, the reactance was used to resonate with the coil matching networks and thus to decouple the coils. Furthermore, coil matching networks and preamplifier input matching networks were combined, meaning the coil loop can be matched to the transistor in the preamplifier directly. These matching methods and, for comparison, the conventional matching method were implemented with custom-made preamplifiers and coils. Decoupling and noise-matching performance were compared between these three configurations. Phase shifting networks between coils and preamplifiers are not necessary in our matching methods. With fewer components, these matching networks showed lower noise factors, while similar preamplifier-decoupling performance was found for all three methods.
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