Principles of diffusion MRI.

<p>(A) When water is free to diffuse, as in a glass or in brain ventricles containing cerebrospinal fluid (CSF), random water molecular displacement obeys a Gaussian distribution, the width at half height of which gives the diffusion coefficient. In tissues, diffusion is constrained by the presence of molecular and cellular obstacles, so the displacement distribution becomes sharper, especially in tumors where cell density is high. As a result, the diffusion coefficient (width at half height of the displacement distribution) appears reduced compared with free diffusion. (B) In the presence of a magnetic field gradient, the MRI resonant frequency will vary along the direction of the gradient. As a result, the phase of the radiowaves emitted by the magnetized hydrogen nuclei of water molecules contained in a voxel (box representing the image elementary volume) will vary (red arrows) compared to otherwise static nuclei (blue arrow), depending on their displacement behavior. For the diffusion-driven random displacements, the average phase shift is zero but exhibits a distribution that is wider for water nuclei experiencing large displacements (fast diffusion, as in CSF, top) than for those experiencing small displacements (slow diffusion, as in white matter brain tissue, bottom). Considering the very large number of water molecules present in each image voxel, each with its own random displacement history, this phase distribution results in an attenuation of the MRI signal amplitude due to phase interference, and the MRI signal (red curve) decays faster than in the absence of diffusion (blue curve). This attenuation is larger in voxels where water movement is fast, and hence where diffusion is high, and vice versa. The MRI images obtained at a given time (yellow triangle) are then “diffusion weighted”: regions of slow diffusion appear in “white” and those with fast diffusion in “black.” Quantitative maps of the apparent diffusion coefficient can be calculated based on this differential signal attenuation.</p

Main applications of water diffusion MRI.

<p>(A) Acute stroke. The diffusion-weighted image (bottom) clearly shows a bright signal corresponding to a drop in water diffusion resulting from cell swelling (cytotoxic edema) in the tissue undergoing acute ischemia. The conventional MRI image (top) shows no abnormal feature. (B) Pelvic cancer. Water diffusion is usually reduced in malignant tissues compared to normal tissues because of the underlying cell proliferation in the tumor. Areas with reduced diffusion are shown in pink (a primary cancer in the rectum with several metastases). (C) Main applications of water diffusion MRI: rat brain 9L glioma model. Diffusion MRI is widely used for preclinical research, for instance, to evaluate the effects of new therapies on cancers. Here, in a composite image of the ADC and the kurtosis parameter (arbitrary color scale from blue for normal tissue to red for highly proliferating tissue), the developing tumor appears in red as an area of low diffusion and high kurtosis, reflecting diffusion hindrance from cell proliferation. (D) Main applications of water diffusion MRI: brain connectivity. Water diffusion in brain white matter fibers is anisotropic, i.e., faster in the direction of the fibers. By measuring water diffusion in many directions, the orientations of the whiter matter bundles can be determined at each brain location. Algorithms then identify bundles, which are represented with arbitrary colors (courtesy of C. Poupon, CONNECT/NeuroSpin). (E) Main applications of water diffusion MRI: diffusion functional neuroimaging. Water diffusion decreases during activation of neural tissue (here the primary visual cortex was stimulated by a flickering checkerboard for 10 seconds). The time course of the diffusion MRI responses (blue) appears much faster than the usual blood oxygenation level-dependent (BOLD) response (red) both at onset and offset. The BOLD response results from a local increase in blood flow. The diffusion response might reflect more directly cellular events occurring in the neural tissue upon activation, such as cellular swelling.</p

Diffusion MRI in single neurons and buccal ganglia of <i>Aplysia californica</i> obtained with a 17.2 tesla MRI system.

<p>(A) Image of a single neuron at 25 μm resolution. Left: the approximate region of cell bodies chosen for the diffusion measurements are indicated by the red outline on a fixed and immunostained ganglion slice (nuclei are labeled in blue and the cytoskeleton and neurites in orange). Middle: MRI image of this selected region; right: MRI image of a single neuron within the selected region (see reference [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002203#pbio.1002203.ref053" target="_blank">53</a>] for details). (B) ADC measurements in the soma (left) and in the ganglia (right), pre- and poststimulation with oubain, an inhibitor of the plasma membrane Na+/K+ ATPase, which causes cell swelling. The ADC increases in the soma by 30% and decreases in the ganglion tissue by 18% upon treatment with the inhibitor. This discrepancy in diffusion behavior suggests the importance of cell membranes for the ADC measured at tissue level (where a hypothetical layer of slowly diffusing water molecules bound to membranes would increase in size upon cell swelling and membrane surface expansion). Courtesy of I. Jelescu and L. Ciobanu, NeuroSpin.</p

Water diffusion closely reveals neural activity status in rat brain loci affected by anesthesia

<div><p>Diffusion functional MRI (DfMRI) reveals neuronal activation even when neurovascular coupling is abolished, contrary to blood oxygenation level—dependent (BOLD) functional MRI (fMRI). Here, we show that the water apparent diffusion coefficient (ADC) derived from DfMRI increased in specific rat brain regions under anesthetic conditions, reflecting the decreased neuronal activity observed with local field potentials (LFPs), especially in regions involved in wakefulness. In contrast, BOLD signals showed nonspecific changes, reflecting systemic effects of the anesthesia on overall brain hemodynamics status. Electrical stimulation of the central medial thalamus nucleus (CM) exhibiting this anesthesia-induced ADC increase led the animals to transiently wake up. Infusion in the CM of furosemide, a specific neuronal swelling blocker, led the ADC to increase further locally, although LFP activity remained unchanged, and increased the current threshold awakening the animals under CM electrical stimulation. Oppositely, induction of cell swelling in the CM through infusion of a hypotonic solution (−80 milliosmole [mOsm] artificial cerebrospinal fluid [aCSF]) led to a local ADC decrease and a lower current threshold to wake up the animals. Strikingly, the local ADC changes produced by blocking or enhancing cell swelling in the CM were also mirrored remotely in areas functionally connected to the CM, such as the cingulate and somatosensory cortex. Together, those results strongly suggest that neuronal swelling is a significant mechanism underlying DfMRI.</p></div

Summarized relationship of the Apparent Diffusion Coefficient (ADC) with underlying neural activity status in the Central Medial thalamic nucleus (CM).

<p>Scatter plots between local field potential (LFP) power and ADC (A) or blood oxygenation level—dependent (BOLD) change ratio (B) in the CM under all anesthetic conditions (isoflurane: 1.5%, 2.0%, and 2.5%; medetomidine: 0.1 and 0.3 mg/kg/h). Scatter plots of CM ADC (C) and CM BOLD change ratio (D) with the threshold amplitude to trigger an awake response during CM electrical stimulation under anesthesia with 0.1 and 0.3 mg/kg/h of medetomidine. (E) Scatter plots between CM ADC and the threshold amplitude to trigger an awake response under anesthesia (0.1 mg/kg/h medetomidine) before and after CM infusion of furosemide and H-80. Data with anesthetic dosages for the CM of individual rats can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s009" target="_blank">S1 Data</a> for ADC and <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s010" target="_blank">S2 Data</a> for BOLD. LFP power data with anesthetic dosages for CM of individual rats can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s012" target="_blank">S4 Data</a>. Data for minimum amplitudes of individual rats can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s013" target="_blank">S5 Data</a>. ADC data for pharmacological diffusion functional MRI (DfMRI) of individual rats found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s011" target="_blank">S3 Data</a>.</p

Comparison of Apparent Diffusion Coefficient (ADC) and Blood Oxygenation Level—Dependent (BOLD) changes with Local Field Potentials (LFPs) in the Central Medial (CM) and Ventral Posterolateral (VPL) thalamic nuclei.

<p>BOLD changes under each dosage of isoflurane (A) and medetomidine (B) at the CM and the VPL. Region of interest (ROI) locations for the CM and the VPL are overlaid on structural images. ADC changes under each dosage of isoflurane (C) and medetomidine (D) at the CM and the VPL. (E) Representative LFP signals in a single animal at the CM (upper) and the VPL (below), for each dosage of isoflurane and medetomidine. Total LFP power (frequency range: 1–70 Hz) under each dosage of isoflurane (F; <i>n</i> = 8 for CM, <i>n</i> = 8 for VPL) and medetomidine (G; <i>n</i> = 8 for CM, <i>n</i> = 8 for VPL). Bar plots exhibit mean ± the standard error of the mean (SEM). * <i>p</i> < 0.05, ** <i>p</i> < 0.0, *** <i>p</i> < 0.001 (Paired <i>t</i>-test, versus low dose of each anesthesia). Data for the CM and the VPL of individual rats can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s009" target="_blank">S1 Data</a> for ADC and <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s010" target="_blank">S2 Data</a> for BOLD. LFP power data for the CM and the VPL of individual rats can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s012" target="_blank">S4 Data</a>.</p

Apparent Diffusion Coefficient (ADC) and Blood Oxygenation Level—Dependent (BOLD) changes and time courses for each dosage of both anesthetic conditions.

<p>Averaged ADCs of whole brains for each dosage of isoflurane (A; <i>n</i> = 10) and medetomidine (B; <i>n</i> = 8). Averaged BOLD signal change ratios of whole brain for each dosage of isoflurane (C; <i>n</i> = 6) and medetomidine (D; <i>n</i> = 7). Averaged time courses of ADCs (E, F) and BOLD signal changes ratios (G, H) at whole brains for each dosage of isoflurane (E, G) and medetomidine (F, H), showing the stability of the ADC change while the BOLD signal exhibits a significant negative drift with time. Note that the BOLD change levels for each medetomidine dosage are inverted between E and H. Mean arterial blood pressure (MABP) under each dosage of isoflurane (I; <i>n</i> = 5) and medetomidine (J; <i>n</i> = 5). Bar plots exhibit mean ± standard error of the mean (SEM). * <i>p</i> < 0.05, ** <i>p</i> < 0.01, *** <i>p</i> < 0.001 (Paired <i>t</i>-test, versus low dose of each anesthesia). Data for whole brains of individual rats can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s009" target="_blank">S1 Data</a> for ADC and <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s010" target="_blank">S2 Data</a> for BOLD. MABP data of individual rats can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s012" target="_blank">S4 Data</a>.</p

Effects of H-80 infusion in the Central Medial thalamus nucleus (CM) on the Apparent Diffusion Coefficient (ADC), Local Field Potentials (LFPs) and awake response threshold.

<p>(A) Percentage of animals exhibiting an awake response during CM electrical stimulation after the CM infusion with −80 milliosmole (mOsm) hypotonic artificial cerebrospinal fluid (aCSF) (H-80; <i>n</i> = 6). (B) Average ADC change in the CM and cingulate cortex (Cg) before and after H-80 or aCSF infusion in the CM (<i>n</i> = 6 for each condition). Average time course of ADC change at the CM (C) and Cg (D) with the infusion of aCSF and H-80. (E) Representative local field potentials (LFP) signals at CM and Cg during CM infusion with aCSF (upper) or H-80 (low). (F) Total LFP power (frequency range: 1–70 Hz) in CM and Cg before and after CM infusion with H-80 (<i>n</i> = 6) or aCSF (<i>n</i> = 6). Time course and bar plots exhibit mean ± the standard error of the mean (SEM). ** <i>p</i> < 0.01 (Paired <i>t</i>-test between pre and post), # <i>p</i> < 0.05, ## <i>p</i> < 0.01 (Fisher’s exact test). Data for minimum amplitudes of individual rats can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s013" target="_blank">S5 Data</a>. ADC data of individual rats found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s011" target="_blank">S3 Data</a>. LFP power data can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s012" target="_blank">S4 Data</a>.</p

Effects of electrical stimulation of the Central Medial thalamus nucleus (CM) on anesthetic status.

<p>(A) Percentage of animals exhibiting an awake response during the electrical stimulation at the CM and VPL under 0.1 mg/kg/h medetomidine condition (<i>n</i> = 6). (B) Representative electromyography (EMG) signals of the stimulation, with 0.8 mA amplitude at the CM and the VPL under 0.1 mg/kg/h medetomidine. (C) Percentage of animals exhibiting an awake response during the electrical stimulation at the CM, with an amplitude of 0.3–1.2 mA under 0.1 and 0.3 mg/kg/h medetomidine (<i>n</i> = 6). (D) Percentage of animals exhibiting an awake response during the electrical stimulation at the six brain locations under 0.1 mg/kg/h medetomidine (<i>n</i> = 4). # <i>p</i> < 0.05, ## <i>p</i> < 0.01 (Fisher’s exact test). Data for minimum amplitudes of individual rats can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001494#pbio.2001494.s013" target="_blank">S5 Data</a>.</p

Sedation Agents Differentially Modulate Cortical and Subcortical Blood Oxygenation: Evidence from Ultra-High Field MRI at 17.2 T

<div><p>Background</p><p>Sedation agents affect brain hemodynamic and metabolism leading to specific modifications of the cerebral blood oxygenation level. We previously demonstrated that ultra-high field (UHF) MRI detects changes in cortical blood oxygenation following the administration of sedation drugs commonly used in animal research. Here we applied the UHF-MRI method to study clinically relevant sedation drugs for their effects on cortical and subcortical (thalamus, striatum) oxygenation levels.</p><p>Methods</p><p>We acquired T2*-weighted images of Sprague-Dawley rat brains at 17.2T <i>in vivo</i>. During each MRI session, rats were first anesthetized with isoflurane, then with a second sedative agent (sevoflurane, propofol, midazolam, medetomidine or ketamine-xylazine) after stopping isoflurane. We computed a T2*-oxygenation-ratio that aimed at estimating cerebral blood oxygenation level for each sedative agent in each region of interest: cortex, hippocampus, thalamus and striatum.</p><p>Results</p><p>The T2*-oxygenation-ratio was consistent across scan sessions. This ratio was higher with inhalational agents than with intravenous agents. Under sevoflurane and medetomidine, T2*-oxygenation-ratio was homogenous across the brain regions. Intravenous agents (except medetomidine) induced a T2*-oxygenation-ratio imbalance between cortex and subcortical regions: T2*-oxygenation-ratio was higher in the cortex than the subcortical areas under ketamine-xylazine; T2*-oxygenation-ratio was higher in subcortical regions than in the cortex under propofol or midazolam.</p><p>Conclusion</p><p>Preclinical UHF MRI is a powerful method to monitor the changes in cerebral blood oxygenation level induced by sedative agents across brain structures. This approach also allows for a classification of sedative agents based on their differential effects on cerebral blood oxygenation level.</p></div