Probing neuronal activation by functional quantitative susceptibility mapping under a visual paradigm: A group level comparison with BOLD fMRI and PET

Dynamic changes of brain-tissue magnetic susceptibility provide the basis for functional MR imaging (fMRI) via T2*-weighted signal-intensity modulations. Promising initial work on a detection of neuronal activity via quantitative susceptibility mapping (fQSM) has been published but consistently reported on ill-understood positive and negative activation patterns (Balla et al., 2014; Chen and Calhoun, 2015a). We set out to (i) demonstrate that fQSM can exploit established fMRI data acquisition and processing methods and to (ii) better describe aspects of the apparent activation patterns using fMRI and PET as standards of reference. Under a standardized visual-stimulation paradigm PET and 3-T gradient-echo EPI-based fQSM, fMRI data from 9 healthy volunteers were acquired and analyzed by means of Independent Component Analysis (ICA) at subject level and, for the first time, at group level. Numbers of activated (z-score>2.0) voxels were counted and their mean z-scores calculated in volumes of interest (occipital lobe (Nocc_lobe), segmented occipital gray-matter (NGM_occ_lobe), large veins (Nveins)), and in occipital-lobe voxels commonly activated in fQSM and fMRI component maps. Common but not entirely congruent regions of apparent activation were found in the occipital lobe in z-score maps from all modalities, fQSM, fMRI and PET, with distinct BOLD-negatively correlated regions in fQSM data. At subject-level, Nocc_lobe, NGM_occ_lobe and their mean z-scores were significantly smaller in fQSM than in fMRI, but their ratio, NGM_occ_lobe/Nocc_lobe, was comparable. Nveins did not statistically differ and the ratio Nveins/NGM_occ_lobe as well as the mean z-scores were higher for fQSM than for fMRI. In veins and immediate vicinity, z-score maps derived from both phase and fQSM-data showed positive and negative lobes resembling dipole shapes in simulated field and phase maps with no correlate in fMRI or PET data. Our results show that standard fMRI tools can directly be used for fQSM processing, and suggest that fQSM may have the potential to detect gray-matter activation distant from large veins, to improve detection of veins with stimulus-induced venous oxygen saturation (SvO2) variations, and to better localize areas of activation. However, our results seem to clearly expose issues that phenomenologically resemble an incomplete dipolar inversion and that need to be subject to further investigation

Enhanced quantitative susceptibility mapping (QSM) using real-time field control

PURPOSE: To assess the potential of a real-time field-control (FC) system for mitigating effects of spatiotemporal field fluctuations in quantitative susceptibility mapping (QSM) at 7 T. METHODS: Magnitude, phase, and QSM images of phantoms and healthy volunteers were acquired under standard conditions and under induced field perturbation (FP) (phantoms: periodic water-bottle displacement; volunteers: deep breathing and forearm movement) with and without FC, which continuously detects and minimizes magnetic-field variations. RESULTS: Field control successfully eliminated FP-induced impairment of phantom image quality and deviations from a linear susceptibility increase for increasing gadolinium concentration in a Gd dilution series (y = 320x - 0.60, R(2)  = 0.93 for the scan with FP and FC versus y = 259x - 0.54, R(2)  = 0.78 for the scan with FP and no FC (slope literature value: 326 ppm L/mol)). Similarly, in volunteers, FC allowed a recovery of a FP-induced loss of identifiable brain structures and reduced the relative change of mean susceptibilities and standard deviations (93 ± 53% to 34 ± 46%) in all regions of interests with respect to the reference scan. CONCLUSIONS: Real-time FC improved the delineation of brain structures and the match of susceptibility values with reference values obtained without FP. Magn Reson Med, 2017. © 2017 International Society for Magnetic Resonance in Medicine

Effect of respiratory hyperoxic challenge on magnetic susceptibility in human brain assessed by quantitative susceptibility mapping (QSM)

The purpose of this study was to measure the regional change of magnetic susceptibility in human brain upon inhalation of 100% oxygen by MRI quantitative susceptibility mapping (QSM). Fourteen healthy volunteers were scanned in a 3 T MR scanner with a 3D multi-gradient-echo sequence while breathing medical air (normoxia) and pure oxygen (hyperoxia). QSM images and R2 * maps were calculated. Mean susceptibility differences versus white matter were measured in regions of interest covering veins, gray matter (GM), and cerebrospinal fluid (CSF) under both conditions. Hyperoxia resulted in a strong susceptibility decrease in large veins (-154.4 ± 65.9 ppb, p < 10(-6) ), in a smaller reduction in GM (-1.3 ± 1 ppb, p < 0.001), and in a susceptibility increase in ventricular CSF (3.8 ± 1.8 ppb, p < 10(-5) ). The susceptibility decrease in veins implied an increase of venous oxygen saturation (SvO2 ) by 10.1 ± 4.0%. Compared with QSM, R2 * was more seriously affected by long-distance effects not related to local tissue oxygenation and increased in cerebral frontal regions (3 ± 2 s(-1) , p < 0.0004) due to paramagnetic molecular oxygen in cavities. The results highlight the potential of QSM to yield region-specific quantitative oxygenation information, and, thus, for applications such as oxygen-therapy monitoring or identification of hypoxic tumor tissue during radiotherapy planning. Copyright © 2015 John Wiley & Sons, Ltd