Improved MRI methods to quantify retinal and choroidal blood flow applied to a model of glaucoma.

Purpose: Blood flow (BF) of the retinal and choroidal vasculatures can be quantitatively imaged using MRI. This study sought to improve methods of data acquisition and analysis for MRI of layer-specific retinal and choroidal BF and then applied this approach to detect reduced ocular BF in a well-established mouse model of glaucoma from both eyes. Methods: Quantitative BF magnetic resonance imaging (MRI) was performed on glaucomatous DBA/2J and normal C57BL/6J mice. Arterial spin labeling MRI was applied to image retinal and choroidal BF using custom-made dual eye coils that could image both eyes during the same scan. Statistics using data from a single eye or two eyes were compared. BF values were calculated using two approaches. The BF rate per quantity of tissue was calculated as commonly done, and the peak BF values of the retinal and choroidal vasculatures were taken. Additionally, the BF rate per retinal surface area was calculated using a new analysis approach to attempt to reduce partial volume and variability by integrating BF over the retinal and choroidal depths. Results: Ocular BF of both eyes could be imaged using the dual coil setup without effecting scan time. Intraocular pressure was significantly elevated in DBA/2J mice compared to C57BL/6J mice (P<0.01). Both retinal and choroidal BF were significantly decreased in DBA/2J mice in comparison to the age-matched normal C57BL/6J mice across all measurements (P < 0.01). From simulations, the values from the integrated BF analysis method had less partial volume effect, and from in vivo scans, this analysis approach also improved power. Conclusion: The dual eye coil setup allows bilateral eye data acquisition, increasing the amount of data acquired without increasing acquisition times in vivo. The reduced ocular BF found using the improved acquisition and analysis approaches replicated the results of previous studies on DBA/2J mice. The ocular hypertensive stress-induced BF reduction found within these mice may represent changes associated with glaucomatous progression.

Connexin 50 Influences the Physiological Optics of the In Vivo Mouse Lens.

Purpose: The purpose of this study was to utilize multi-parametric magnetic resonance imaging (MRI) to investigate in vivo age-related changes in the physiology and optics of mouse lenses where Connexin 50 has been deleted (Cx50KO) or replaced by Connexin 46 (Cx50KI46). Methods: The lenses of transgenic Cx50KO and Cx50KI46 mice were imaged between 3 weeks and 6 months of age using a 7T MRI. Measurements of lens geometry, the T2 (water-bound protein ratios), the refractive index (n), and T1 (free water content) values were calculated by processing the acquired images. The lens power was calculated from an optical model that combined the geometry and the n. All transgenic mice were compared with control mice at the same age. Results: Cx50KO and Cx50KI46 mice developed smaller lenses compared with control mice. The lens thickness, volume, and surface radii of curvatures all increased with age but were limited to the size of the lenses. Cx50KO lenses exhibited higher lens power than Cx50KI46 lenses at all ages, and this was correlated with significantly lower water content in these lenses, which was probably modulated by the gap junction coupling. The refractive power tended to a steady state with age, similar to the control mice. Conclusions: The modification of Cx50 gap junctions significantly impacted lens growth and physiological optics as the mouse aged. The lenses showed delayed development growth, and altered optics governed by different lens physiology. This research provides new insights into how gap junctions regulate the development of the lens's physiological optics.

Functional Magnetic Resonance Imaging (fMRI) of the Visual Cortex with Wide-View Retinotopic Stimulation.

High-resolution retinotopic blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) with a wide-view presentation can be used to functionally map the peripheral and central visual cortex. This method for measuring functional changes of the visual brain allows for functional mapping of the occipital lobe, stimulating >100° (±50°) or more of the visual field, compared to standard fMRI visual presentation setups which usually cover <30° of the visual field. A simple wide-view stimulation system for BOLD fMRI can be set up using common MR-compatible projectors by placing a large mirror or screen close to the subject's face and using only the posterior half of a standard head coil to provide a wide-viewing angle without obstructing their vision. The wide-view retinotopic fMRI map can then be imaged using various retinotopic stimulation paradigms, and the data can be analyzed to determine the functional activity of visual cortical regions corresponding to central and peripheral vision. This method provides a practical, easy-to-implement visual presentation system that can be used to evaluate changes in the peripheral and central visual cortex due to eye diseases such as glaucoma and the vision loss that may accompany them.

Age-Dependent Changes in the Water Content and Optical Power of the In Vivo Mouse Lens Revealed by Multi-Parametric MRI and Optical Modeling.

Purpose: The purpose of this study was to utilize in vivo magnetic resonance imaging (MRI) and optical modeling to investigate how changes in water transport, lens curvature, and gradient refractive index (GRIN) alter the power of the mouse lens as a function of age. Methods: Lenses of male C57BL/6 wild-type mice aged between 3 weeks and 12 months (N = 4 mice per age group) were imaged using a 7T MRI scanner. Measurements of lens shape and the distribution of T2 (water-bound protein ratios) and T1 (free water content) values were extracted from MRI images. T2 values were converted into the refractive index (n) using an age-corrected calibration equation to calculate the GRIN at different ages. GRIN maps and shape parameters were inputted into an optical model to determine ageing effects on lens power and spherical aberration. Results: The mouse lens showed two growth phases. From 3 weeks to 3 months, T2 decreased, GRIN increased, and T1 decreased. This was accompanied by increased lens thickness, volume, and surface radii of curvatures. The refractive power of the lens also increased significantly, and a negative spherical aberration was developed and maintained. Between 6 and 12 months of age, all physiological, geometrical, and optical parameters remained constant, although the lens continued to grow. Conclusions: In the first 3 months, the mouse lens power increased as a result of changes in shape and in the GRIN, the latter driven by the decreased water content of the lens nucleus. Further research into the mechanisms regulating this decrease in mouse lens water could improve our understanding of how lens power changes during emmetropization in the developing human lens.

Imaging ocular water inflow in the mouse with deuterium oxide MRI.

Abnormal intraocular fluid flow or clearance is involved with a variety of eye diseases such as glaucoma and diabetic retinopathy, but measurement of water exchange dynamics in the vitreous and aqueous remain challenging. 2H MRI can be used to image deuterium oxide (D2O) as a tracer, but the signal-to-noise ratio for deuterium is low due to its low concentration, which has hampered its application to imaging the eye. To overcome this challenge, we investigated the feasibility of direct D2O MRI to measure water dynamics in the mouse eye. The balanced steady-state free precession (bSSFP) sequence provided substantially higher signal-to-noise ratio for imaging D2O in fluid compared to standard gradient echo and spin echo sequences. bSSFP allowed dynamic imaging of intraocular water inflow in the mouse with 41 s temporal resolution. The inflow rate in the vitreous was found to be faster than in the aqueous. These studies demonstrate the feasibility of in vivo imaging of water inflow dynamics into the both the vitreous and aqueous in mice, which could be useful in studies of abnormal fluid exchange in rodent models of eye disease.

Effects of chronic mild hyperoxia on retinal and choroidal blood flow and retinal function in the DBA/2J mouse model of glaucoma.

PURPOSE: To test the hypothesis that mild chronic hyperoxia treatment would improve retinal function despite a progressive decline in ocular blood flow in the DBA/2J mouse model of glaucoma. MATERIALS AND METHODS: DBA/2J mice were treated with chronic mild hyperoxia (30% O2) beginning at 4.5 months of age or were untreated by giving normal room air. Retinal and choroidal blood flow (RBF and ChBF, respectively) were measured at 4, 6, and 9 months of age by MRI. Blood flow was additionally measured under hypercapnia challenge (5% CO2 inhalation) to assess vascular reactivity. Intraocular pressure (IOP) was measured using a rebound tonometer at the same time points. Scotopic flash electroretinograms (ERGs) were recorded at 9 months of age. RESULTS: Both ChBF and RBF were reduced and significantly affected by age (p < 0.01), but neither were significantly affected by O2-treatment (p > 0.05). ChBF significantly increased in response to hypercapnia (p < 0.01), which was also unaffected by O2-treatment. Significant effects of age (p < 0.001) and of the interaction of age with treatment (p = 0.028) were found on IOP. IOP significantly decreased in O2-treated mice at 6 months compared to 4 months of age (p < 0.001), while IOP trended to increase with age in untreated mice. The amplitude of the b-wave from ERG was significantly increased in O2-treated DBA/2J compared to the untreated mice (p = 0.012), while the a-wave and oscillatory potentials were not significantly affected (p > 0.05). CONCLUSION: This study investigated the effects of chronic mild hyperoxia on retinal function and on retinal and choroidal blood flow in a mouse model of glaucoma. Retinal function was improved in the O2-treated mice at late stage, despite a progressive decline of RBF and ChBF with age that was comparable to untreated mice.

Resting-State Functional Magnetic Resonance Imaging of Interhemispheric Functional Connectivity in Experimental Traumatic Brain Injury.

Although resting-state functional magnetic resonance imaging (rsfMRI) has the potential to offer insights into changes in functional connectivity networks after traumatic brain injury (TBI), there are few studies that examine the effects of moderate TBI for monitoring functional recovery in experimental TBI, and thus the neural correlates of brain recovery from moderate TBI remain incompletely understood. Non-invasive rsfMRI was used to longitudinally investigate changes in interhemispheric functional connectivity (IFC) after a moderate TBI to the unilateral sensorimotor cortex in rats (n = 9) up to 14 days. Independent component analysis of the rsfMRI data was performed. Correlations of rsfMRI sensorimotor networks were made with changes in behavioral scores, lesion volume, and T2- and diffusion-weighted images across time. TBI animals showed less localized rsfMRI patterns in the sensorimotor network compared to sham (n = 6) and normal (n = 5) animals. rsfMRI clusters in the sensorimotor network showed less bilateral symmetry compared to sham and normal animals, indicative of IFC disruption. With time after injury, many of the rsfMRI patterns in the sensorimotor network showed more bilateral symmetry, indicative of IFC recovery. The disrupted IFC in the sensorimotor and subsequent partial recovery showed a positive correlation with changes in behavioral scores. Overall, rsfMRI detected widespread disruption and subsequent recovery of IFC within the sensorimotor networks post-TBI, which correlated with behavioral changes. Therefore, rsfMRI offers the means to probe functional brain reorganization and thus has the potential to serve as an imaging marker to longitudinally stage TBI and monitor for novel treatments.

Diabetic mice have retinal and choroidal blood flow deficits and electroretinogram deficits with impaired responses to hypercapnia.

PURPOSE: The purpose of this study was to investigate neuronal and vascular functional deficits in the retina and their association in a diabetic mouse model. We measured electroretinography (ERG) responses and choroidal and retinal blood flow (ChBF, RBF) with magnetic resonance imaging (MRI) in healthy and diabetic mice under basal conditions and under hypercapnic challenge. METHODS: Ins2Akita diabetic (Diab, n = 8) and age-matched, wild-type C57BL/6J mice (Ctrl, n = 8) were studied under room air and moderate hypercapnia (5% CO2). Dark-adapted ERG a-wave, b-wave, and oscillatory potentials (OPs) were measured for a series of flashes. Regional ChBF and RBF under air and hypercapnia were measured using MRI in the same mice. RESULTS: Under room air, Diab mice had compromised ERG b-wave and OPs (e.g., b-wave amplitude was 422.2±10.7 µV in Diab vs. 600.1±13.9 µV in Ctrl, p < 0.001). Under hypercapnia, OPs and b-wave amplitudes were significantly reduced in Diab (OPs by 30.3±3.0% in Diab vs. -3.0±3.6% in Ctrl, b-wave by 17.9±1.4% in Diab vs. 1.3±0.5% in Ctrl). Both ChBF and RBF had significant differences in regional blood flow, with Diab mice having substantially lower blood flow in the nasal region (ChBF was 5.4±1.0 ml/g/min in Diab vs. 8.6±1.0 ml/g/min in Ctrl, RBF was 0.91±0.10 ml/g/min in Diab vs. 1.52±0.24 ml/g/min in Ctrl). Under hypercapnia, ChBF increased in both Ctrl and Diab without significant group difference (31±7% in Diab vs. 17±7% in Ctrl, p > 0.05), but an increase in RBF was not detected for either group. CONCLUSIONS: Inner retinal neuronal function and both retinal and choroidal blood flow were impaired in Diab mice. Hypercapnia further compromised inner retinal neuronal function in diabetes, while the blood flow response was not affected, suggesting that the diabetic retina has difficulty adapting to metabolic challenges due to factors other than impaired blood flow regulation.

MRI study of cerebral blood flow, vascular reactivity, and vascular coupling in systemic hypertension.

Chronic hypertension alters cerebrovascular function, which can lead to neurovascular pathologies and increased susceptibility to neurological disorders. The purpose of this study was to utilize in vivo MRI methods with corroborating immunohistology to evaluate neurovascular dysfunction due to progressive chronic hypertension. The spontaneously hypertensive rat (SHR) model at different stages of hypertension was studied to evaluate: i) basal cerebral blood flow (CBF), ii) cerebrovascular reactivity (CVR) assessed by CBF and blood-oxygenation level dependent (BOLD) signal changes to hypercapnia, iii) neurovascular coupling from CBF and BOLD changes to forepaw stimulation, and iv) damage of neurovascular unit (NVU) components (microvascular, astrocyte and neuron densities). Comparisons were made with age-matched normotensive Wistar Kyoto (WKY) rats. In 10-week SHR (mild hypertension), basal CBF was higher (p < 0.05), CVR trended higher, and neurovascular coupling response was higher (p < 0.05), compared to normotensive rats. In 40-week SHR (severe hypertension), basal CBF, CVR, and neurovascular coupling response were reversed to similar or below normotensive rats, and were significantly different from 10-week SHR (p < 0.05). Immunohistological analysis found significantly reduced microvascular density, increased astrocytes, and reduced neuronal density in SHR at 40 weeks (p < 0.05) but not at 10 weeks (p > 0.05) in comparison to age-matched controls. In conclusion, we observed a bi-phasic basal CBF, CVR and neurovascular coupling response from early to late hypertension using in vivo MRI, with significant changes prior to changes in the NVU components from histology. MRI provides clinically relevant data that might be useful to characterize neurovascular pathogenesis on the brain in hypertension.

Longitudinal multiparametric MRI study of hydrogen-enriched water with minocycline combination therapy in experimental ischemic stroke in rats.

Free radicals are downstream mediators of several cytotoxic cascades contributing to ischemic brain injury. Molecular hydrogen (H2) is an antioxidant potentially useful in the treatment of stroke. Hydrogen is easy to deliver, biologically non-toxic and diffuses freely through all biological structures including the blood-brain barrier and cellular membranes. This study evaluated the efficacy of hydrogen treatments in a rat stroke model compared to vehicle-treated controls using multiparametric MRI and neurological tests. Additionally, comparison of H2 treatment alone was made with H2 combined with minocycline (H2M) treatment (12 rats per group). The primary findings were: i) H2 therapy reduced infarct volume in both H2 and H2M groups compared to controls at 1 and 7 days after stroke, and ii) both H2 and H2M improved neurologic functional recovery on day 7. The secondary outcomes were: iii) H2M treatment attenuated post-stroke hyperperfusion in the hyperacute phase, and iv) H2M markedly minimized white matter injury. In conclusion, this is the first study to use MRI to longitudinally study H2 and H2M treatment on ischemic stroke and the first study to compare H2 treatment combined with another potential stroke therapeutic (H2M).

Multi-parametric MRI of the physiology and optics of the in-vivo mouse lens.

The optics of the ocular lens are determined by its geometry (shape and volume) and its inherent gradient of refractive index (water to protein ratio), which are in turn maintained by unique cellular physiology known as the lens internal microcirculation system. Previously, magnetic resonance imaging (MRI) has been used on ex vivo organ cultured bovine lenses to show that pharmacological perturbations to this microcirculation system disrupt ionic and fluid homeostasis and overall lens optics. In this study, we have optimised in vivo MRI protocols for use on wild-type and transgenic mouse models so that the effects of genetically perturbing the lens microcirculation system on lens properties can be studied. In vivo MRI protocols and post-analysis methods for studying the mouse lens were optimised and used to measure the lens geometry, diffusion, T1 and T2, as well as the refractive index (n) calculated from T2, in wild-type mice and the genetically modified Cx50KI46 mouse. In this animal line, gap junctional coupling in the lens is increased by knocking in the gap junction protein Cx46 into the Cx50 locus. Relative to wild-type mice, Cx50KI46 mice showed significantly reduced lens size and radius of curvature, increased T1 and T2 values, and decreased n in the lens nucleus, which was consistent with the developmental and functional changes characterised previously in this lens model. These proof of principle experiments show that in vivo MRI can be applied to transgenic mouse models to gain mechanistic insights into the relationship between lens physiology and optics, and in the future suggest that longitudinal studies can be performed to determine how this relationship is altered by age in mouse models of cataract.

Retinal Vascular and Anatomical Features in the Spontaneously Hypertensive Rat.

Purpose: To evaluate whether invivo optical imaging methods and histology can detect comparable vascular and neuronal damage in the retina due to the effects of progressive chronic hypertension on the retinal vasculature and neurons using the spontaneously hypertensive rat (SHR) model at young and old ages. Methods: Male SHR and normotensive Wistar Kyoto (WKY) rats were studied at 10 and 40 weeks of age (n = 6 each group). Arterial blood pressure was measured with a tail-cuff. Under anesthesia, fundus photography was used to measure retinal arterial diameters and optical coherence tomography was used to measure retinal layer thicknesses. Histology was then used to measure microvascular and cell density in different retinal layers. Results: Blood pressure was significantly higher in SHR than WKY in both age groups (p < .05). Fundus images showed no gross abnormalities, hemorrhage, or stenosis in all groups. Retinal vessels, however, appeared more tortuous in SHR compared to WKY at both ages. Retinal vessel diameters in SHR were significantly narrower than in WKY at both age groups (p < .05). Microvascular densities at 10 weeks were not significantly different (p > .05) but were markedly reduced in SHR at 40 weeks compared to WKY (p < .05). The outer nuclear layer thickness of SHR was significantly thinner than that of WKY at both ages (p < .05), consistent with histological cell density measurements (p < .05). The ganglion cell layer and inner nuclear layer thicknesses were not significantly different between SHR and WKY (p > .05), consistent with the corresponding histological cell density measurements (p > .05). Conclusion: In vivo optical imaging showed that systemic hypertension progressively reduces retinal arterial diameter and thicknesses of the outer retina in spontaneously hypertensive rats, with consistent vascular and neuronal findings from histology.

Functional MRI of the mouse olfactory system.

Although olfactory dysfunction is an early warning sign of Alzheimer's and Parkinson's diseases, and is commonly present in a range of other neurodegenerative disorders, the mechanisms for its pathogenesis are not yet clear. Since fMRI allows the mapping of spatial and temporal patterns of activity in multiple brain regions simultaneously, it serves as a powerful tool to study olfactory dysfunction in animal models of neurodegenerative diseases. Nonetheless, there have been no reports to date of mapping odor-induced activation patterns beyond the olfactory bulb to the extended networks of olfactory and limbic archicortex, likely due to the small size of the mouse brain. Therefore, using an 11.7 T magnet and a blood volume-weighted fMRI technique, we mapped the functional neuroanatomy of the mouse olfactory system. Consistent with reports on imaging of the much larger human brain, we mapped activity in regions of the olfactory bulb, as well as olfactory and limbic archicortex. By using two distinct odorants, we further demonstrated odorant-specific activation patterns. Our work thus provides a methodological framework for fMRI studies of olfactory dysfunction in mouse models of neurodegeneration.

Preclinical Arterial Spin Labeling Measurement of Cerebral Blood Flow.

Magnetic resonance imaging has been utilized as a quantitative and noninvasive method to image blood flow. Arterial spin labeling (ASL) is an MRI technique that images blood flow using arterial blood water as an endogenous tracer. Herein we describe the use of ASL to measure cerebral blood flow completely noninvasively in rodents, including methods, analysis, and important considerations when utilizing this technique.

Retinotopic fMRI Reveals Visual Dysfunction and Functional Reorganization in the Visual Cortex of Mild to Moderate Glaucoma Patients.

PURPOSE: To investigate retinotopic functional representation in the visual cortex of mild to moderate primary open-angle glaucoma (POAG) participants and age-matched normal volunteers using high-resolution retinotopic blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI). METHODS: fMRI was performed on 9 POAG participants (61±11 y old) and 9 age-matched controls (58±5 y old) were studied. A wide-view visual presentation (±55 degrees) was used to evaluate central and peripheral vision. Cortical magnification factors and BOLD% changes as a function of eccentricity. Correlation analysis between BOLD% changes and visual field scores, and between BOLD% changes and retinal nerve fiber layer thicknesses was performed. Comparison of BOLD% changes for individual visual field quadrants between POAG subgroups and normal group was performed. RESULTS: BOLD% changes of POAG participants in peripheral visual regions were reduced compared to normals but similar in central visual regions, consistent with the notion of peripheral vision being affected first and more compared to central vision. fMRI retinotopic mapping revealed enlarged representation of the parafovea in the visual cortex of POAG participants compared to normals. Cortical magnification of the central, but not peripheral, visual representation in the visual cortex was larger in POAG participants, suggesting functional remapping. BOLD% changes of individual visual field quadrants were significantly correlated with visual field scores and with retinal nerve fiber layer thickness in the corresponding quadrants. CONCLUSIONS: These results support the hypothesis that there are functional alteration and remapping in the topographic representation of the visual cortex in POAG participants, and these changes are correlated with disease severity.

MRI Study of the Posterior Visual Pathways in Primary Open Angle Glaucoma.

PURPOSE OF THE STUDY: The purpose of the study was to evaluate neurodegeneration along brain visual pathways in primary open angle glaucoma (POAG) using improved analysis methods of volumetric and diffusion tensor magnetic resonance imaging (MRI) data. METHODS: Eleven POAG patients (60.0±9.2 y) with primarily mild to moderate POAG and 11 age-matched controls (55.9±7.5 y) were studied using structural and diffusion tensor MRI. Surface-based segmentation was applied to structural MRI to obtain visual cortical area and volume. Fiber tracking was applied to diffusion tensor data to obtain diffusion parameters along the optic tract and optic radiation. MRI parameters in glaucoma patients were compared with the corresponding left and right visual fields and retinal nerve fiber layer thicknesses, instead of with the left and right eye. RESULTS: Area and volume of the primary visual cortex were significantly reduced in POAG patients compared with controls (P<0.05) but did not correlate with visual field loss. Fractional anisotropy was reduced at multiple locations along the optic tracts and optic radiations in POAG patients compared with controls. Axial and radial diffusivity along the fiber tracts showed trends but were not significantly different between POAG patients and controls when averaged over the whole structures. Only fractional anisotropy (P<0.05) of the optic radiations was significantly correlated with visual field loss. No MRI parameters were correlated with retinal nerve fiber layer thickness. CONCLUSIONS: Improved analysis techniques of MRI data improves delineation of degeneration in the brain visual pathways and further supports the notion that neurodegeneration is involved with glaucoma pathogenesis.

MRI of cerebral blood flow under hyperbaric conditions in rats.

Hyperbaric oxygen (HBO) therapy has a number of clinical applications. However, the effects of acute HBO on basal cerebral blood flow (CBF) and neurovascular coupling are not well understood. This study explored the use of arterial spin labeling MRI to evaluate changes in baseline and forepaw stimulus-evoked CBF responses in rats (n = 8) during normobaric air (NB), normobaric oxygen (NBO) (100% O2 ), 3 atm absolute (ATA) hyperbaric air (HB) and 3 ATA HBO conditions. T1 was also measured, and the effects of changes in T1 caused by increasing oxygen on the CBF calculation were investigated. The major findings were as follows: (i) increased inhaled oxygen concentrations led to a reduced respiration rate; (ii) increased dissolved paramagnetic oxygen had significant effects on blood and tissue T1 , which affected the CBF calculation using the arterial spin labeling method; (iii) the differences in blood T1 had a larger effect than the differences in tissue T1 on CBF calculation; (iv) if oxygen-induced changes in blood and tissue T1 were not taken into account, CBF was underestimated by 33% at 3 ATA HBO, 10% at NBO and <5% at HB; (v) with correction, CBF values under HBO, HB and NBO were similar (p > 0.05) and all were higher than CBF under NB by ~40% (p < 0.05), indicating that hypercapnia from the reduced respiration rate masks oxygen-induced vasoconstriction, although blood gas was not measured; and (vi) substantial stimulus-evoked CBF increases were detected under HBO, similar to NB, supporting the notion that activation-induced CBF regulation in the brain does not operate through an oxygen-sensing mechanism. CBF MRI provides valuable insights into the effects of oxygen on basal CBF and neurovascular coupling under hyperbaric conditions. Copyright © 2016 John Wiley & Sons, Ltd.

MRI of brain tissue oxygen tension under hyperbaric conditions.

The brain depends on a continuous supply of oxygen to maintain its structural and functional integrity. This study measured T1 from MRI under normobaric air, normobaric oxygen, hyperbaric air, and hyperbaric oxygen (HBO) conditions as a marker of tissue pO2 since dissolved molecular oxygen acts as an endogenous contrast agent. Brain tissue T1 decreased corresponding to increased pO2 with increasing inhaled oxygen concentrations, and tissue oxygenation was estimated from the T1 changes between different inhaled oxygen levels. Tissue pO2 difference maps between different oxygen conditions showed heterogeneous pO2 changes in the brain. MRI-derived tissue pO2 was markedly lower than the arterial pO2 but was slightly higher than venous pO2. Additionally, for comparison with published extracellular tissue pO2 data obtained using oxygen electrodes and other invasive techniques, a model was used to estimate extracellular and intracellular pO2 from the MRI-derived mean tissue pO2. This required multiple assumptions, and so the effects of the assumptions and parameters used in modeling brain pO2 were evaluated. MRI-derived pO2 values were strongly dependent on assumptions about the extra- and intracellular compartments but were relatively less sensitive to variations in the relaxivity constant of oxygen and contribution from oxygen in the cerebral blood compartment. This approach may prove useful in evaluating tissue oxygenation in disease states such as stroke.

Multimodal MRI Evaluation of the MitoPark Mouse Model of Parkinson's Disease.

The MitoPark mouse, a relatively new genetic model of Parkinson's disease (PD), has a dopaminergic neuron-specific knock-out that inactivates the mitochondrial transcription factor A (Tfam), a protein essential for mitochondrial DNA expression and maintenance. This study used multimodal MRI to characterize the neuroanatomical correlates of PD-related deficits in MitoPark mice, along with functional behavioral tests. Compared with age-matched wild-type animals, MitoPark mice at 30 weeks showed: i) reduced whole-brain volume and increased ventricular volume, indicative of brain atrophy, ii) reduced transverse relaxation time (T2*) of the substantia nigra and striatum, suggestive of abnormal iron accumulation, iii) reduced apparent diffusion coefficient in the substantia nigra, suggestive of neuronal loss, iv) reduced fractional anisotropy in the corpus callosum and substantia nigra, indicative of white-matter damages, v) cerebral blood flow was not significantly affected, and vi) reduced motor activity in open-field tests, reduced memory in novel object recognition tests, as well as decreased mobility in tail suspension tests, an indication of depression. In sum, MitoPark mice recapitulate changes in many MRI parameters reported in PD patients. Multimodal MRI may prove useful for evaluating neuroanatomical correlates of PD pathophysiology in MitoPark mice, and for longitudinally monitoring disease progression and therapeutic interventions for PD.