Validation of High-Resolution Tractography Against In Vivo Tracing in the Macaque Visual Cortex.

Diffusion magnetic resonance imaging (MRI) allows for the noninvasive in vivo examination of anatomical connections in the human brain, which has an important role in understanding brain function. Validation of this technique is vital, but has proved difficult due to the lack of an adequate gold standard. In this work, the macaque visual system was used as a model as an extensive body of literature of in vivo and postmortem tracer studies has established a detailed understanding of the underlying connections. We performed probabilistic tractography on high angular resolution diffusion imaging data of 2 ex vivo, in vitro macaque brains. Comparisons were made between identified connections at different thresholds of probabilistic connection "strength," and with various tracking optimization strategies previously proposed in the literature, and known connections from the detailed visual system wiring map described by Felleman and Van Essen (1991; FVE91). On average, 74% of connections that were identified by FVE91 were reproduced by performing the most successfully optimized probabilistic diffusion MRI tractography. Further comparison with the results of a more recent tracer study ( Markov et al. 2012) suggests that the fidelity of tractography in estimating the presence or absence of interareal connections may be greater than this.

Magnetic resonance imaging-based cerebral tissue classification reveals distinct spatiotemporal patterns of changes after stroke in non-human primates.

BACKGROUND: Spatial and temporal changes in brain tissue after acute ischemic stroke are still poorly understood. Aims of this study were three-fold: (1) to determine unique temporal magnetic resonance imaging (MRI) patterns at the acute, subacute and chronic stages after stroke in macaques by combining quantitative T2 and diffusion MRI indices into MRI 'tissue signatures', (2) to evaluate temporal differences in these signatures between transient (n = 2) and permanent (n = 2) middle cerebral artery occlusion, and (3) to correlate histopathology findings in the chronic stroke period to the acute and subacute MRI derived tissue signatures. RESULTS: An improved iterative self-organizing data analysis algorithm was used to combine T2, apparent diffusion coefficient (ADC), and fractional anisotropy (FA) maps across seven successive timepoints (1, 2, 3, 24, 72, 144, 240 h) which revealed five temporal MRI signatures, that were different from the normal tissue pattern (P < 0.001). The distribution of signatures between brains with permanent and transient occlusions varied significantly between groups (P < 0.001). Qualitative comparisons with histopathology revealed that these signatures represented regions with different histopathology. Two signatures identified areas of progressive injury marked by severe necrosis and the presence of gitter cells. Another signature identified less severe but pronounced neuronal and axonal degeneration, while the other signatures depicted tissue remodeling with vascular proliferation and astrogliosis. CONCLUSION: These exploratory results demonstrate the potential of temporally and spatially combined voxel-based methods to generate tissue signatures that may correlate with distinct histopathological features. The identification of distinct ischemic MRI signatures associated with specific tissue fates may further aid in assessing and monitoring the efficacy of novel pharmaceutical treatments for stroke in a pre-clinical and clinical setting.

Cortical depth-specific microvascular dilation underlies laminar differences in blood oxygenation level-dependent functional MRI signal.

Changes in neuronal activity are accompanied by the release of vasoactive mediators that cause microscopic dilation and constriction of the cerebral microvasculature and are manifested in macroscopic blood oxygenation level-dependent (BOLD) functional MRI (fMRI) signals. We used two-photon microscopy to measure the diameters of single arterioles and capillaries at different depths within the rat primary somatosensory cortex. These measurements were compared with cortical depth-resolved fMRI signal changes. Our microscopic results demonstrate a spatial gradient of dilation onset and peak times consistent with "upstream" propagation of vasodilation toward the cortical surface along the diving arterioles and "downstream" propagation into local capillary beds. The observed BOLD response exhibited the fastest onset in deep layers, and the "initial dip" was most pronounced in layer I. The present results indicate that both the onset of the BOLD response and the initial dip depend on cortical depth and can be explained, at least in part, by the spatial gradient of delays in microvascular dilation, the fastest response being in the deep layers and the most delayed response in the capillary bed of layer I.

3D micro-CT imaging of the postmortem brain.

Magnetic resonance microscopy (microMRI) is becoming an important tool for non-destructive analysis of fixed brain tissue. However, unlike MRI, X-ray computed tomography (CT) scans show little native soft tissue contrast. In this paper, we explored the use of contrast enhanced (brains immersion stained in iodinated CT contrast media) micro-CT (microCT) for high resolution 3D imaging of fixed normal and pathological brains, compared to microMRI and standard histopathology. An optimum iodine concentration of 0.27 M resulted in excellent contrast between gray and white matter in normal brain and a wide range of anatomical structures were identified. In glioma bearing mouse brains, there was clear deliniation of tumor margin which closely matched that seen on histopathology sections. microCT tumor volume was strongly correlated with histopathology volume. Our data suggests that microCT image contrast in the immersion-stained brains is related to axonal density and myelin content. Compared to traditional histopathology, our microCT approach is relatively rapid and less labor intensive. In addition, compared to microMRI, microCT is robust and requires much lower equipment and maintenance costs. For simple measurements, such as tumor volume and non-destructive postmortem brain screening, microCT may prove to be a valuable alternative to standard histopathology or microMRI.

Association fibre pathways of the brain: parallel observations from diffusion spectrum imaging and autoradiography.

Understanding the long association pathways that convey cortical connections is a critical step in exploring the anatomic substrates of cognition in health and disease. Diffusion tensor imaging (DTI) is able to demonstrate fibre tracts non-invasively, but present approaches have been hampered by the inability to visualize fibres that have intersecting trajectories (crossing fibres), and by the lack of a detailed map of the origins, course and terminations of the white matter pathways. We therefore used diffusion spectrum imaging (DSI) that has the ability to resolve crossing fibres at the scale of single MRI voxels, and identified the long association tracts in the monkey brain. We then compared the results with available expositions of white matter pathways in the monkey using autoradiographic histological tract tracing. We identified 10 long association fibre bundles with DSI that match the observations in the isotope material: emanating from the parietal lobe, the superior longitudinal fasciculus subcomponents I, II and III; from the occipital-parietal region, the fronto-occipital fasciculus; from the temporal lobe, the middle longitudinal fasciculus and from rostral to caudal, the uncinate fasciculus, extreme capsule and arcuate fasciculus; from the occipital-temporal region, the inferior longitudinal fasciculus; and from the cingulate gyrus, the cingulum bundle. We suggest new interpretations of the putative functions of these fibre bundles based on the cortical areas that they link. These findings using DSI and validated with reference to autoradiographic tract tracing in the monkey represent a considerable advance in the understanding of the fibre pathways in the cerebral white matter. By replicating the major features of these tracts identified by histological techniques in monkey, we show that DSI has the potential to cast new light on the organization of the human brain in the normal state and in clinical disorders.

Serial diffusion tensor MRI after transient and permanent cerebral ischemia in nonhuman primates.

BACKGROUND AND PURPOSE: We measured the temporal evolution of the T2 and diffusion tensor imaging parameters after transient and permanent cerebral middle cerebral artery occlusion (MCAo) in macaques, and compared it to standard histological analysis at the study end point. METHODS: Stroke was created in adult male macaques by occluding a middle cerebral artery branch for 3 hours (transient MCAo, n=4 or permanent occlusion, n=3). Conventional MRI and diffusion tensor imaging scans were performed 0 (acute day), 1, 3, 7, 10, 17, and 30 days after MCAo. Animals were euthanized after the final scan and the brains removed for histological analysis. RESULTS: Apparent diffusion coefficient in the lesion was decreased acutely, fractional anisotropy was elevated, and T2 remained normal. Thereafter, apparent diffusion coefficient increased above normal, fractional anisotropy decreased to below normal, T2 increased to a maximum and then declined. Reperfusion at 3 hours accelerated these MRI changes. Only the fractional anisotropy value was significantly different between transient and permanent groups at 30 days. Final MRI-defined fractional lesion volumes were well correlated with corresponding histological lesion volumes. Permanent MCAO animals showed more severe histological damage than their transient MCAO counterparts, especially myelin damage and axonal swelling. CONCLUSIONS: Overall, the MRI evolution of stroke in macaques was closer to what has been observed in humans than in rodent models. This work supports the use of serial MRI in stroke studies in nonhuman primates.

Acute studies of a new primate model of reversible middle cerebral artery occlusion.

The recent failure of many clinical trials of neuroprotective compounds may be due in part to poor animal models of human stroke. We have developed an endovascular stroke model in nonhuman primates that is compatible with serial magnetic resonance imaging (MRI) monitoring. Using cynomologous macaques (n = 4), a microcatheter was navigated transarterially (under fluoroscopic guidance) from the femoral artery to the middle cerebral artery (MCA). The microcatheter was wedged in a branch of the MCA for 3 hours to cause focal cerebral ischemia, as verified angiographically. During occlusion and/or reperfusion, animals were scanned with MRI, and imaging findings were compared with the stained brain sections. All animals demonstrated small stroke lesions in the expected vascular territory, as seen on diffusion-weighted MRI and confirmed by postmortem examination. Reperfusion after 3 hours was confirmed angiographically (n = 2) and also by MRI (n = 4). The mean initial lesion volume, measured on the postreperfusion MRI scans, was 2.3 +/- 1.3 mL (n = 4). There was good agreement between anatomic location of the lesion on MRI and postmortem histological staining (n = 3). A "minimally invasive" primate model of focal cerebral ischemia was developed that is ideally suited to MRI studies of both acute and chronic stroke. By using serial MRI scans to measure changes in lesion size over time, we will be able to control for variability in lesion size/location. This model should prove useful as a test bed for new stroke therapies, in which noninvasive imaging findings are readily comparable to human stroke.

An MRA study of vascular stenosis in a pig model using CH3-DTPA-Gd (NMS60) and Gd-DTPA.

PURPOSE: This study used an experimental arterial stenosis model in pigs to evaluate the utility of a new medium-weight MRI contrast agent, NMS60 (a synthetic oligomeric Gd complex containing three Gd(3+) atoms, molecular weight of 2158 Da) compared to Gd-DTPA for contrast-enhanced MRA. MATERIALS AND METHODS: We used six male white hybrid pigs. Under anesthesia, one femoral artery was exposed and an inflatable cuff placed around it. The cuff was tightened around the vessel until 80-90% stenosis was achieved using digital subtraction angiography as a guide. Animals were then immediately transferred to the MRI scanner and images acquired pre- and postcontrast injection (0.1 or 0.2 mmol Gd/kg Gd-DTPA or NMS60, as a rapid bolus) using high-resolution and dynamic MRA. RESULTS: The dynamic MRA scans acquired during contrast bolus injection clearly showed the stenosed femoral artery as a segment of close to zero enhancement during the arterial phase of the bolus transit, while on the high-resolution scans the stenosis was difficult to detect due to venous signal contamination. The signal-to-noise at peak enhancement on the dynamic scans was significantly greater with 0.1 mmol Gd/kg NMS60 compared to 0.1 mmol Gd/kg Gd-DTPA (14.6 vs. 9.9, P < .05) and not significantly greater than 0.2 mmol Gd/kg (14.6 vs. 12.8). DISCUSSION AND CONCLUSION: This new medium-weight contrast agent demonstrated significantly greater enhancement than Gd-DTPA and may be valuable to aid detection of vascular stenosis in humans.

Dynamic Diffusion Magnetic Resonance Imaging of Infarct Formation and Peri-infarct Spreading Depression after Middle Cerebral Artery Occlusion (MCAO) in macacca fasicularis.

Dynamic diffusion MRI was used to visualize hyperacute stroke formation in the brain of a cynomolgus macaque. Under fluoroscopic guidance, a microcatheter was placed into the middle cerebral artery (MCA). The animal was immediately transferred to a 1.5T clinical scanner. Dynamic T2-weighted imaging during bolus injection of Oxygen-17 enriched water through the microcatheter mapped out the territory perfused by the MCA segment. Serial diffusion measurements were made using diffusion-weighted echo-planar imaging, with a temporal resolution of 15 seconds, during injection of a glue embolus into the microcatheter. The apparent diffusion coefficient declined within the lesion core. A wave of transient diffusion decline spread through peripheral uninvolved brain immediately following stroke induction. The propagation speed and pattern is consistent with spreading peri-infarct depolarizations (PID). The detection of PIDs following embolic stroke in a higher nonhuman primate brain supports the hypothesis that spreading depressions may occur following occlusive stroke in humans.

An approach to high resolution diffusion tensor imaging in fixed primate brain.

High resolution ex vivo diffusion tensor imaging (DTI) studies of neural tissues can improve our understanding of brain structure. In these studies we can modify the tissue relaxation properties of the fixed tissues to better suite the scanner hardware. We investigated the use of Gd-DTPA contrast agent to provide the optimum signal-to-noise (SNR) ratio in 3D DTI scans of formalin fixed nonhuman primate brains at 4.7 T. Relaxivity measurements in gray and white matter allowed us to optimize the Gd concentration for soaking the brains, resulting in a 2 fold improvement in SNR for the 3D scans. FA changed little with Gd concentrations up to 10 mM although ADC was reduced at 5 and 10 mM. Comparison of in vivo, fresh ex vivo and fixed brains showed no significant FA changes but reductions in ADC of about 50% in fresh ex vivo, and 64% and 80% in fixed gray and white matter respectively. Studies of the temperature dependence of diffusion in these tissues suggested that a 30 degrees increase in sample temperature may yield an improvement of up to 55% in SNR-efficiency for a given diffusion weighting. Our Gd soaking regimen appeared to have no detrimental effect on standard histology of the fixed brain sections. Our methods yield both high SNR and spatial resolution DTI data in fixed primate brains, allowing us to perform high resolution tractography which will facilitate the process of 'validation' of DTI fiber tracts against traditional measures of brain fiber architecture.

Middle cerebral artery occlusion in Macaca fascicularis: acute and chronic stroke evolution.

BACKGROUND: An intravascular stroke model designed for magnetic resonance imaging was developed in Macaca fascicularis (M. fascicularis) to characterize serial stroke lesion evolution. This model produces a range of stroke lesion sizes which closely mimics human stroke evolution. This paper describes the care of animals undergoing this stroke procedure, the range of outcomes we experienced and the cause of mortality in this model. METHODS: Anesthesia was induced with atropine and ketamine and maintained with isoflurane or propofol. Non-invasive blood pressure, oxygen saturation, heart rate, respiration rate, temperature and end tidal CO2 were monitored continuously. The stroke was created by occluding a distal branch of the middle cerebral artery. During catheter placement animals were heparinized and vasospasm was minimized using verapamil. RESULTS: Anesthetic induction and maintenance were smooth. Animals with small strokes showed very rapid recovery, were able to ambulate and self-feed within 2 hours of recovery. Animals with strokes of >or=4% of the hemispheric volume required lengthy observation during recovery and parenteral nutrition. Large strokes resulted in significant brain edema, herniation and brainstem compression. CONCLUSIONS: Intracerebral hemorrhage and or subarachnoid hemorrhage coupled with a stroke of any size was acutely fatal. In the absence of an effective acute stroke therapy, the spectrum of outcomes seen in our primate model is very similar to that observed in human stroke patients.

Direct CSF injection of MnCl(2) for dynamic manganese-enhanced MRI.

MnCl(2) was injected intrathecally through the cisterna magna in rats, allowing infusion of divalent manganese ions (Mn(++)) into the CSF space and thence into the brain, without breaking the blood-brain barrier (BBB). Mn(++) uptake and washout dynamics in the brain were measured by serial T(1)-weighted MRI and EPI T(1) and T(2) mapping for up to 3 weeks after injection. Observations within the first 6 hr after injection demonstrated anterograde and bilateral distribution of the Mn(++) within the CSF space, from the olfactory bulb and frontal cortex to the brain stem. Enhancement increased in most brain areas up to 4 days after injection, and then slowly decreased. Relaxation maps at each time point demonstrated higher concentrations of Mn in basal ganglia. Residual concentrations were still observable after 3 weeks in all brain regions. With the use of MnCl(2) calibration phantoms, the maximum Mn concentration in the brain was estimated to be approximately 27 +/- 16 microM, corresponding to changes in relaxation rates of 0.49 +/- 0.30 s(-1) for R(1) and 3.9 +/- 2.4 s(-1) for R(2). For comparison, an intrathecal GdDTPA injection was performed. This injection showed different distribution dynamics: it remained chiefly within the CSF spaces, and was largely washed out after 1 day. This method shows promise as a means of supplying Mn(++) uniformly to the whole brain for a variety of chronic functional activation studies.