Functional quantitative susceptibility mapping (fQSM)

Blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) is a powerful technique, typically based on the statistical analysis of the magnitude component of the complex time-series. Here, we additionally interrogated the phase data of the fMRI time-series and used quantitative susceptibility mapping (QSM) in order to investigate the potential of functional QSM (fQSM) relative to standard magnitude BOLD fMRI. High spatial resolution data (1 mm isotropic) were acquired every 3 seconds using zoomed multi-slice gradient-echo EPI collected at 7 T in single orientation (SO) and multiple orientation (MO) experiments, the latter involving 4 repetitions with the subject's head rotated relative to B0. Statistical parametric maps (SPM) were reconstructed for magnitude, phase and QSM time-series and each was subjected to detailed analysis. Several fQSM pipelines were evaluated and compared based on the relative number of voxels that were coincidentally found to be significant in QSM and magnitude SPMs (common voxels). We found that sensitivity and spatial reliability of fQSM relative to the magnitude data depended strongly on the arbitrary significance threshold defining “activated” voxels in SPMs, and on the efficiency of spatio-temporal filtering of the phase time-series. Sensitivity and spatial reliability depended slightly on whether MO or SO fQSM was performed and on the QSM calculation approach used for SO data. Our results present the potential of fQSM as a quantitative method of mapping BOLD changes. We also critically discuss the technical challenges and issues linked to this intriguing new technique

The three-prong method: a novel assessment of residual stress in laser powder bed fusion

<p><b>Boxplots of quantitative parameters</b> included <b>a)</b> ratio of N-acetylaspartate and N-acetylaspartylglutamate (NAA) to creatine and phosphocreatine (Cr) both for chemical shift imaging (CSI) and single voxel (SV) measurements, <b>b)</b> ratio of choline containing compounds (Cho) to Cr both for CSI and SV, <b>c)</b> myelin water fraction (MWF), <b>d)</b> magnetization transfer ratio (MTR), <b>e)</b> quantitative susceptibility mapping (QSM), and <b>f)</b> R2*. Parameters were measured in frontal white matter (WM) and two parameters within the cortico-spinal tract (CST): at the level of the posterior limb of internal capsule (PLIC) and at the level of the centrum semiovale (CS), see also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0167274#pone.0167274.g001" target="_blank">Fig 1</a>.</p

Rat brain MRI at 16.4T using a capacitively tunable patch antenna in combination with a receive array

For MRI at 16.4T, with a proton Larmor frequency of 698 MHz, one of the principal RF engineering challenges is to generate a spatially homogeneous transmit field over a larger volume of interest for spin excitation. Constructing volume coils large enough to house a receive array along with the subject and to maintain the quadrature symmetry for different loading conditions is difficult at this frequency. This calls for new approaches to RF coil design for ultra-high field MR systems. A remotely placed capacitively tunable patch antenna, which can easily be adjusted to different loading conditions, was used to generate a relatively homogeneous excitation field covering a large imaging volume with a transversal profile similar to that of a birdcage coil. Since it was placed in front of the animal, this created valuable free space in the narrow magnet bore around the subject for additional hardware. To enhance the reception sensitivity, the patch antenna was combined with an actively detunable 3-channel receive coil array. In addition to increased SNR compared to a quadrature transceive surface coil, we were able to get high quality gradient echo and spin-echo images covering the whole rat brain

17O relaxation times in the rat brain at 16.4 tesla

Purpose: Measurement of the cerebral metabolic rate of oxygen (CMRO2) by means of direct imaging of the 17O signal can be a valuable tool in neuroscientific research. However, knowledge of the longitudinal and transverse relaxation times of different brain tissue types is required, which is difficult to obtain because of the low sensitivity of natural abundance H217O measurements. Methods: Using the improved sensitivity at a field strength of 16.4 Tesla, relaxation time measurements in the rat brain were performed in vivo and postmortem with relatively high spatial resolutions, using a chemical shift imaging sequence. Results: In vivo relaxation times of rat brain were found to be T1 = 6.84 ± 0.67 ms and T2* = 1.77 ± 0.04 ms. Postmortem H217O relaxometry at enriched concentrations after inhalation of 17O2 showed similar T2* values for gray matter (1.87 ± 0.04 ms) and white matter, significantly longer than muscle (1.27 ± 0.05 ms) and shorter than cerebrospinal fluid (2.30 ± 0.16 ms). Conclusion: Relaxation times of brain H217O were measured for the first time in vivo in different types of tissues with high spatial resolution. Because the relaxation times of H217O are expected to be independent of field strength, our results should help in optimizing the acquisition parameters for experiments also at other MRI field strengths

Quantitative and simultaneous measurement of oxygen consumption rates in rat brain and skeletal muscle using 17O MRS imaging at 16.4T

Purpose: Oxygen-17 (17O) MRS imaging, successfully used in the brain, is extended by imaging the oxygen metabolic rate in the resting skeletal muscle and used to determine the total whole-body oxygen metabolic rate in the rat. Methods: During and after inhalations of 17O2 gas, dynamic 17O MRSI was performed in rats (n = 8) ventilated with N2O or N2 at 16.4 T. Time courses of the H217O concentration from regions of interest located in brain and muscle tissue were examined and used to fit an animal-adapted 3-phase metabolic model of oxygen consumption. CBF was determined with an independent washout method. Finally, body oxygen metabolic rate was calculated using a global steady-state approach. Results: Cerebral metabolic rate of oxygen consumption was 1.97 ± 0.19 μmol/g/min on average. The resting metabolic rate of oxygen consumption in skeletal muscle was 0.32 ± 0.12 μmol/g/min and >6 times lower than cerebral metabolic rate of oxygen consumption. Global oxygen consumed by the body was 24.2 ± 3.6 mL O2/kg body weight/min. CBF was estimated to be 0.28 ± 0.02 mL/g/min and 0.34 ± 0.06 mL/g/min for the N2 and N2O ventilation condition, respectively. Conclusion: We have evaluated the feasibility of 17O MRSI for imaging and quantifying the oxygen consumption rate in low metabolizing organs such as the skeletal muscle at rest. Additionally, we have shown that CBF is slightly increased in the case of ventilation with N2O. We expect this study to be beneficial to the application of 17O MRSI to a wider range of organs, although further validation is advised.11Nsciescopu

MRI protocol for this study.

<p>T1-weighted MPRAGE image (T1, left) in two different axial planes of a 17y-old typically developing subject indicating the measurement ROIs guided by the cortico-spinal tract fibre s outlined in red: frontal WM (in blue), cortico-spinal tract at the level of the centrum semiovale (in green) and at the level of the posterior limb of the internal capsule (in yellow). Other image parameters included in the protocol were T2-weighted axial TSE (T2), mean diffusivity (MD), fractional anisotropy (FA), intracellular volume fraction (ICVF), orientation dispersion (ODI), Magnetization Transfer Ratio (MTR), Myelin Water Fraction (MWF), Chemical Shift Imaging (CSI)–MR Spectroscopy, and quantitative susceptibility mapping (QSM, the white matter fibers have been highlighted by rendering diamagnetic effects bright, and paramagnetic dark).</p

Measurement results in the three regions of interest: the frontal WM, the CST at the level of the centrum semiovale (CST-CS), and at the level of the posterior limb of the internal capsule (CST-PLIC).

<p>Measurement results in the three regions of interest: the frontal WM, the CST at the level of the centrum semiovale (CST-CS), and at the level of the posterior limb of the internal capsule (CST-PLIC).</p

Illustration of different underlying fibre architecture in the three measurement regions as found by histology (e.g. [58–61]: Cortico-spinal tract (CST) fibres include those with larger axon diameter and thicker myelin sheaths, which show more parallel arrangements at the level of the posterior limb of internal capsule (PLIC) and more crossing fibres at the level of the centrum semiovale (CS).

<p>On the other hand, the frontal white matter (WM) region also contains crossing fibres, however, with thinner myelin sheaths and lesser axon diameter.</p

Widespread and Opponent fMRI Signals Represent Sound Location in Macaque Auditory Cortex

In primates, posterior auditory cortical areas are thought to be part of a dorsal auditory pathway that processes spatial information. But how posterior (and other) auditory areas represent acoustic space remains a matter of debate. Here we provide new evidence based on functional magnetic resonance imaging (fMRI) of the macaque indicating that space is predominantly represented by a distributed hemifield code rather than by a local spatial topography. Hemifield tuning in cortical and subcortical regions emerges from an opponent hemispheric pattern of activation and deactivation that depends on the availability of interaural delay cues. Importantly, these opponent signals allow responses in posterior regions to segregate space similarly to a hemifield code representation. Taken together, our results reconcile seemingly contradictory views by showing that the representation of space follows closely a hemifield code and suggest that enhanced posterior-dorsal spatial specificity in primates might emerge from this form of coding