Deconvolution‐based distortion correction of EPI using analytic single‐voxel point‐spread functions

Purpose To develop a postprocessing algorithm that corrects geometric distortions due to spatial variations of the static magnetic field amplitude, B0, and effects from relaxation during signal acquisition in EPI. Theory and Methods An analytic, complex point‐spread function is deduced for k‐space trajectories of EPI variants and applied to corresponding acquisitions in a resolution phantom and in human volunteers at 3 T. With the analytic point‐spread function and experimental maps of B0 (and, optionally, the effective transverse relaxation time, urn:x-wiley:07403194:media:mrm28591:mrm28591-math-0004) as input, a point‐spread function matrix operator is devised for distortion correction by a Thikonov‐regularized deconvolution in image space. The point‐spread function operator provides additional information for an appropriate correction of the signal intensity distribution. A previous image combination algorithm for acquisitions with opposite phase blip polarities is adapted to the proposed method to recover destructively interfering signal contributions. Results Applications of the proposed deconvolution‐based distortion correction (“DecoDisCo”) algorithm demonstrate excellent distortion corrections and superior performance regarding the recovery of an undistorted intensity distribution in comparison to a multifrequency reconstruction. Examples include full and partial Fourier standard EPI scans as well as double‐shot center‐out trajectories. Compared with other distortion‐correction approaches, DecoDisCo permits additional deblurring to obtain sharper images in cases of significant urn:x-wiley:07403194:media:mrm28591:mrm28591-math-0005 effects. Conclusion Robust distortion corrections in EPI acquisitions are feasible with high quality by regularized deconvolution with an analytic point‐spread function. The general algorithm, which is publicly released on GitHub, can be straightforwardly adapted for specific EPI variants or other acquisition schemes

Regional differences of fMR signal changes induced by hyperventilation: Comparison between SE-EPI and GE-EPI at 3-T

PURPOSE: To evaluate whether reproducible signal change of brain tissues by hyperventilation (HV) can be seen on spin-echo (SE)-echo planar imaging (EPI) at 3-T and to examine the sensitivity of SE-EPI for measuring vascular reactivity in regions of the brain, such as the hippocampal formation, that are difficult to visualize with gradient-echo (GE)-EPI due to susceptibility artifacts. MATERIALS AND METHODS: Six healthy human subjects performed a voluntary HV task. The task design was as follows: two minutes normal breathing (rest) followed by two minutes HV, giving a basic four-minute block that was repeated three times for a total scan time of 12 minutes for one run. Each subject performed the run both for SE-EPI and GE-EPI. Statistical analysis was performed to detect the area with significant cerebrovascular reactivity. The percentage signal change was also obtained for each cerebral region. RESULTS: Both GE-EPI and SE-EPI showed globally significant signal decreases in the cerebral cortex. In GE-EPI, the frontal cortex showed a larger signal decrease than the other gray matter tissues (P < 0.05). In SE-EPI, the differences among gray matter tissues except for the hippocampal formation were not significant. The hippocampal formation showed the largest signal change (P < 0.05) in SE-EPI, but no significant signal change was observed in GE-EPI due to the presence of susceptibility artifacts. CONCLUSION: HV using SE-EPI at 3-T provides robust and reproducible signal decreases and may make the evaluation of the vascular reactivity in hippocampal formation feasible

Hydrogels based on polymerized ionic Liquids as innovative Drug Carriers in controllable and individualized Dosage Forms

Novel Polymerized Ionic Liquids (PILs)-based Hydrogels as Innovative Drug Delivery Systems are presented. The embedding of drugs in hydrogels enables the “smart” delivery of bioactive molecules from drugs for an oral route of administration. Therefore, a high mechanical strength as well as a favorable pH-dependent swelling behavior is required which is shown in this study. A mechanical compression of PILs-based hydrogels up to 98.5% and a high swelling behavior of poly(VEImBr) hydrogels in a solution with a high pH value is achieved. A significant lower swelling is achieved in a solution with a lower pH value

Investigating the post-stimulus undershoot of the BOLD signal – A simultaneous fMRI and fNIRS study

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Investigating the stimulus-dependent temporal dynamics of the BOLD signal using spectral methods

PURPOSE: To compare several spectral parameters using different durations of visual hemifield stimulation in order to explore the different temporal behavior of the blood oxygenation-level dependent (BOLD) signal in various brain regions. MATERIALS AND METHODS: Spectral methods were applied to three different groups of subjects with visual stimulation lasting 6, 12, and 30 seconds. Furthermore, diffusion weighting was applied in an interleaved way. The core of the data processing was the computation of the spectral density matrix using the multidimensional weighted covariance estimate. Spectral parameters of coherence and phase shift were computed. RESULTS: The correlation between signal changes and phase shifts was dependent on the duration of the visual stimulation. The shorter the duration of visual stimulation, the stronger the correlation between percentage signal change and phase shift. CONCLUSION: The experiments with short and long stimuli differed mainly in the distribution of the activated voxels in the plane of percentage signal change and phase shift. It was revealed that the height of the signal change depends on the phase shift, whereas the diffusion weighting has no influence

Multi-echo investigations of positive and negative CBF and concomitant BOLD changes: Positive and negative CBF and BOLD changes

Unlike the positive blood oxygenation level-dependent (BOLD) response (PBR), commonly taken as an indication of an ‘activated’ brain region, the physiological origin of negative BOLD signal changes (i.e. a negative BOLD response, NBR), also referred to as ‘deactivation’ is still being debated. In this work, an attempt was made to gain a better understanding of the underlying mechanism by obtaining a comprehensive measure of the contributing cerebral blood flow (CBF) and its relationship to the NBR in the human visual cortex, in comparison to a simultaneously induced PBR in surrounding visual regions. To overcome the low signal-to-noise ratio (SNR) of CBF measurements, a newly developed multi-echo version of a center-out echo planar-imaging (EPI) readout was employed with pseudo-continuous arterial spin labeling (pCASL). It achieved very short echo and inter-echo times and facilitated a simultaneous detection of functional CBF and BOLD changes at 3 T with improved sensitivity. Evaluations of the absolute and relative changes of CBF and the effective transverse relaxation rate, , the coupling ratios, and their dependence on CBF at rest, , indicated differences between activated and deactivated regions. Analysis of the shape of the respective functional responses also revealed faster negative responses with more pronounced post-stimulus transients. Resulting differences in the flow-metabolism coupling ratios were further examined for potential distinctions in the underlying neuronal contributions