Axon diameter inferences in the human corpus callosum using oscillating gradient spin echo sequences.

Previous methods used to infer axon diameter distributions using magnetic resonance imaging (MRI) primarily use single diffusion encoding sequences such as pulsed gradient spin echo (PGSE) and are thus sensitive to axons of diameters >5 µm. We applied oscillating gradient spin echo (OGSE) sequences to study human axons in the 1-2 µm range in the corpus callosum, which include the majority of axons constituting cortical connections. The ActiveAx model was applied to calculate the fitted mean effective diameter for axons (AxD) and was compared with values found using histology. Axon diameters from histological data were calculated using three different datasets; true diameters (minimum diameter), a combination of minimum and maximum diameters, and diameters measured across a consistent diffusion direction. The AxD estimates from MRI were 1.8 ± 0.1 µm to 2.34 ± 0.04 µm with an average of 2.0 ± 0.2 µm for the ActiveAx model. The histology AxD values were 1.43 ± 0.02 µm when using the true minimum axon diameters, 5.52 ± 0.02 µm when using the combination of minimum and maximum axon diameters, and 2.20 ± 0.02 µm when collecting measurements across a consistent diffusion direction. This experiment demonstrates the first known usage of OGSE to calculate axon diameters in the human corpus callosum on a 1-2 µm scale. The importance for the model to account for axonal orientation dispersion is indicated by histological results which more closely match the MRI model results depending on the direction of axon diameter measurements. These initial steps using this non-invasive imaging method can be applied to future methodology to develop in vivo axon diameter measurements in human brain tissue.

Inferring diameters of spheres and cylinders using interstitial water.

OBJECT: Most early methods to infer axon diameter distributions using magnetic resonance imaging (MRI) used single diffusion encoding sequences such as pulsed gradient spin echo (SE) and are thus sensitive to axons of diameters > 5 µm. We previously simulated oscillating gradient (OG) SE sequences for diffusion spectroscopy to study smaller axons including the majority constituting cortical connections. That study suggested the model of constant extra-axonal diffusion breaks down at OG accessible frequencies. In this study we present data from phantoms to test a time-varying interstitial apparent diffusion coefficient. MATERIALS AND METHODS: Diffusion spectra were measured in four samples from water packed around beads of diameters 3, 6 and 10 µm; and 151 µm diameter tubes. Surface-to-volume ratios, and diameters were inferred. RESULTS: The bead pore radii estimates were 0.60±0.08 µm, 0.54±0.06 µm and 1.0±0.1 µm corresponding to bead diameters ranging from 2.9±0.4 µm to 5.3±0.7 µm, 2.6±0.3 µm to 4.8±0.6 µm, and 4.9±0.7 µm to 9±1 µm. The tube surface-to-volume ratio estimate was 0.06±0.02 µm-1 corresponding to a tube diameter of 180±70 µm. CONCLUSION: Interstitial models with OG inferred 3-10 µm bead diameters from 0.54±0.06 µm to 1.0±0.1 µm pore radii and 151 µm tube diameters from 0.06±0.02 µm-1 surface-to-volume ratios.

T1, diffusion tensor, and quantitative magnetization transfer imaging of the hippocampus in an Alzheimer's disease mouse model.

Alzheimer's disease (AD) pathology causes microstructural changes in the brain. These changes, if quantified with magnetic resonance imaging (MRI), could be studied for use as an early biomarker for AD. The aim of our study was to determine if T1 relaxation, diffusion tensor imaging (DTI), and quantitative magnetization transfer imaging (qMTI) metrics could reveal changes within the hippocampus and surrounding white matter structures in ex vivo transgenic mouse brains overexpressing human amyloid precursor protein with the Swedish mutation. Delineation of hippocampal cell layers using DTI color maps allows more detailed analysis of T1-weighted imaging, DTI, and qMTI metrics, compared with segmentation of gross anatomy based on relaxation images, and with analysis of DTI or qMTI metrics alone. These alterations are observed in the absence of robust intracellular Aß accumulation or plaque deposition as revealed by histology. This work demonstrates that multiparametric quantitative MRI methods are useful for characterizing changes within the hippocampal substructures and surrounding white matter tracts of mouse models of AD.

Birdcage volume coils and magnetic resonance imaging: a simple experiment for students.

BACKGROUND: This article explains some simple experiments that can be used in undergraduate or graduate physics or biomedical engineering laboratory classes to learn how birdcage volume radiofrequency (RF) coils and magnetic resonance imaging (MRI) work. For a clear picture, and to do any quantitative MRI analysis, acquiring images with a high signal-to-noise ratio (SNR) is required. With a given MRI system at a given field strength, the only means to change the SNR using hardware is to change the RF coil used to collect the image. RF coils can be designed in many different ways including birdcage volume RF coil designs. The choice of RF coil to give the best SNR for any MRI study is based on the sample being imaged. RESULTS: The data collected in the simple experiments show that the SNR varies as inverse diameter for the birdcage volume RF coils used in these experiments. The experiments were easily performed by a high school student, an undergraduate student, and a graduate student, in less than 3 h, the time typically allotted for a university laboratory course. CONCLUSIONS: The article describes experiments that students in undergraduate or graduate laboratories can perform to observe how birdcage volume RF coils influence MRI measurements. It is designed for students interested in pursuing careers in the imaging field.

Ex vivo tissue imaging for radiology-pathology correlation: a pilot study with a small bore 7-T MRI in a rare pigmented ganglioglioma exhibiting complex MR signal characteristics associated with melanin and hemosiderin.

To advance magnetic resonance imaging (MRI) technologies further for in vivo tissue characterization with histopathologic validation, we investigated the feasibility of ex vivo tissue imaging of a surgically removed human brain tumor as a comprehensive approach for radiology-pathology correlation in histoanatomically identical fashion in a rare case of pigmented ganglioglioma with complex paramagnetic properties. Pieces of surgically removed ganglioglioma, containing melanin and hemosiderin pigments, were imaged with a small bore 7-T MRI scanner to obtain T1-, T2-, and T2*-weighted image and diffusion tensor imaging (DTI). Corresponding histopathological slides were prepared for routine hematoxylin and eosin stain and special stains for melanin and iron/hemosiderin to correlate with MRI signal characteristics. Furthermore, mean diffusivity (MD) maps were generated from DTI data and correlated with cellularity using image analysis. While the presence of melanin was difficult to interpret in in vivo MRI with certainty due to concomitant hemosiderin pigments and calcium depositions, ex vivo tissue imaging clearly demonstrated pieces of tissue exhibiting the characteristic MR signal pattern for melanin with pathologic confirmation in a histoanatomically identical location. There was also concordant correlation between MD and cellularity. Although it is still in an initial phase of development, ex vivo tissue imaging is a promising approach, which offers radiology-pathology correlation in a straightforward and comprehensive manner.

In Vivo Detection of Gray Matter Neuropathology in the 3xTg Mouse Model of Alzheimer's Disease with Diffusion Tensor Imaging.

A diagnosis of Alzheimer's disease (AD), a neurodegenerative disorder accompanied by severe functional and cognitive decline, is based on clinical findings, with final confirmation of the disease at autopsy by the presence of amyloid-ß (Aß) plaques and neurofibrillary tangles. Given that microstructural brain alterations occur years prior to clinical symptoms, efforts to detect brain changes early could significantly enhance our ability to diagnose AD sooner. Diffusion tensor imaging (DTI), a type of MRI that characterizes the magnitude, orientation, and anisotropy of the diffusion of water in tissues, has been used to infer neuropathological changes in vivo. Its utility in AD, however, is still under investigation. The current study used DTI to examine brain regions susceptible to AD-related pathology; the cerebral cortex, entorhinal cortex, and hippocampus, in 12-14-month-old 3xTg AD mice that possess both Aß plaques and neurofibrillary tangles. Mean diffusivity did not differ between 3xTg and control mice in any region. Decreased fractional anisotropy (p < 0.01) and axial diffusivity (p < 0.05) were detected only in the hippocampus, in which both congophilic Aß plaques and hyperphosphorylated tau accumulation, consistent with neurofibrillary tangle formation, were detected. Pathological tau accumulation was seen in the cortex. The entorhinal cortex was largely spared from AD-related neuropathology. This is the first study to demonstrate DTI abnormalities in gray matter in a mouse model of AD in which both pathological hallmarks are present, suggesting the feasibility of DTI as a non-invasive means of detecting brain pathology in vivo in early-stage AD.

Damage to the optic chiasm in myelin oligodendrocyte glycoprotein-experimental autoimmune encephalomyelitis mice.

Optic chiasm lesions in myelin oligodendrocyte glycoprotein (MOG)-experimental autoimmune encephalomyelitis (EAE) mice were characterized using magnetic resonance imaging (MRI) and validated using electron microscopy (EM). MR images were collected from 3 days after induction to remission, approximately 20 days after induction. Hematoxylin and eosin, solochrome cyanin-stained sections, and EM images were obtained from the optic chiasms of some mice approximately 4 days after disease onset when their scores were thought to be the highest. T2-weighted imaging and apparent diffusion coefficient map hyperintensities corresponded to abnormalities in the optic chiasms of EAE mice. Mixed inflammation was concentrated at the lateral surface. Degeneration of oligodendrocytes, myelin, and early axonal damage were also apparent. A marked increase in chiasm thickness was observed. T2-weighted and diffusion-weighted MRI can detect abnormalities in the optic chiasms of MOG-EAE mice. MRI is an important method in the study of this model toward understanding optic neuritis.

Active inflammation increases the heterogeneity of MRI texture in mice with relapsing experimental allergic encephalomyelitis.

Inflammation modulates tissue damage in relapsing-remitting multiple sclerosis (MS) both acutely and chronically, but its severity is difficult to evaluate with conventional MRI analysis. In mice with experimental allergic encephalomyelitis (EAE, a model of MS), we administered ultra small particles of iron oxide to track macrophage-mediated inflammation during the onset (relapse) and recovery (remission) of disease activity using high field MRI. We performed MRI texture analysis, a sensitive measure of tissue regularity, and T2 assessment both in EAE lesions and the control tissue, and measured spinal cord volume. We found that inflammation was 3 times more remarkable at onset than at recovery of EAE in histology yet demyelination appeared similar across animals and disease course. In MRI, lesion texture was more heterogeneous; T2 was lower; and spinal cord volume was greater in EAE than in controls, but only MRI texture was worse at relapse than at remission of EAE. Moreover, MRI texture correlated with spinal cord volume and tended to correlate with the extent of disability in EAE. While subject to further confirmation, our findings may suggest the sensitivity of MRI texture analysis for accessing inflammation.

Comparison of manual and semi-automated segmentation methods to evaluate hippocampus volume in APP and PS1 transgenic mice obtained via in vivo magnetic resonance imaging.

BACKGROUND: Magnetic resonance imaging (MRI) of transgenic mouse models of Alzheimer's disease is valuable to understand better the structural changes that occur in the brain and could provide a means to test drug treatments. A hallmark pathological feature of Alzheimer's disease is atrophy of the hippocampus, which is an early biomarker of the disease. MRI can be used to detect and monitor this biomarker. METHOD: Repeated measurements using in vivo 3D T2-weighted imaging of mice were used to assess the methods. Each mouse was imaged twice in one week and twice the following week and no changes in volume were expected. The hippocampus was segmented both manually and semi-automatically. Registration was done to gain information on shape changes. The volumes from each mouse were compared intra-mouse, between mice and to hippocampus volume values in the literature. RESULTS: A reliable method was developed which was able to detect difference in volumes of hippocampus between mice when performed by a single individual. The semi-automated segmentation was unable to detect the same level of differences. The semi-automated segmentation method gave larger hippocampus volumes, with 78-87% reliability between the manual and semi-automated segmentation. Although more accurate, the manual segmentation is laborious and suffers from inter- and intra-variability. CONCLUSION: These results suggest that manual segmentation is still considered the most reliable segmentation method for small structures. However, if performing longitudinal studies, where there is at least one year between imaging sessions, the segmentation should be done all at once at the end of all the imaging sessions. If segmentation is done after each imaging session, with at least a year passing between segmentations, very small variations in volumes can be missed. This method provides a means to quantify the volume of the hippocampus in a live mouse using manual segmentation, which is the first step toward studying hippocampus atrophy in a mouse model of Alzheimer's disease.

Neurofibrillary tangles and plaques are not accompanied by white matter pathology in aged triple transgenic-Alzheimer disease mice.

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is the most common cause of dementia in aging populations. Although senile plaques and neurofibrillary tangles are well-established hallmarks of AD, changes in cerebral white matter correlate with cognitive decline and may increase the risk of the development of dementia. We used the triple transgenic (3xTg)-AD mouse model of AD, previously used to show that white matter changes precede plaque formation, to test the hypothesis that MRI detectable changes occur in the corpus callosum, external capsule and the fornix. T2-weighted and diffusion tensor magnetic resonance imaging and histological stains were employed to assess white matter in older (11-17months) 3xTg-AD mice and controls. We found no statistically significant changes in white matter between 3xTg-AD mice and controls, despite well-developed neurofibrillary tangles and beta amyloid immunoreactive plaques. Myelin staining was normal in affected mice. These data suggest that the 3xTg-AD mouse model does not develop MRI detectable white matter changes at the ages we examined.

Quantitative MRI and ultrastructural examination of the cuprizone mouse model of demyelination.

The cuprizone mouse model of demyelination was used to investigate the influence that white matter changes have on different magnetic resonance imaging results. In vivo T2 -weighted and magnetization transfer images (MTIs) were acquired weekly in control (n = 5) and cuprizone-fed (n = 5) mice, with significant increases in signal intensity in T2 -weighted images (p < 0.001) and lower magnetization transfer ratio (p < 0.001) in the corpus callosum of the cuprizone-fed mice starting at 3 weeks and peaking at 4 and 5 weeks, respectively. Diffusion tensor imaging (DTI), quantitative MTI (qMTI), and T1/T2 measurements were used to analyze freshly excised tissue after 6 weeks of cuprizone administration. In multicomponent T2 analysis with 10 ms echo spacing, there was no visible myelin water component associated with the short T2 value. Quantitative MTI metrics showed significant differences in the corpus callosum and external capsule of the cuprizone-fed mice, similar to previous studies of multiple sclerosis in humans and animal models of demyelination. Fractional anisotropy was significantly lower and mean, axial, and radial diffusivity were significantly higher in the cuprizone-fed mice. Cellular distributions measured in electron micrographs of the corpus callosum correlated strongly to several different quantitative MRI metrics. The largest Spearman correlation coefficient varied depending on cellular type: T1 versus the myelinated axon fraction (ρ = -0.90), the bound pool fraction (ƒ) versus the myelin sheath fraction (ρ = 0.93), and axial diffusivity versus the non-myelinated cell fraction (ρ = 0.92). Using Pearson's correlation coefficient, ƒ was strongly correlated to the myelin sheath fraction (r = 0.98) with a linear equation predicting myelin content (5.37ƒ - 0.25). Of the calculated MRI metrics, ƒ was the strongest indicator of myelin content, while longitudinal relaxation rates and diffusivity measurements were the strongest indicators of changes in tissue structure.

Reduced subventricular zone proliferation and white matter damage in juvenile ferrets with kaolin-induced hydrocephalus.

Hydrocephalus is a neurological condition characterized by altered cerebrospinal fluid (CSF) flow with enlargement of ventricular cavities in the brain. A reliable model of hydrocephalus in gyrencephalic mammals is necessary to test preclinical hypotheses. Our objective was to characterize the behavioral, structural, and histological changes in juvenile ferrets following induction of hydrocephalus. Fourteen-day old ferrets were given an injection of kaolin (aluminum silicate) into the cisterna magna. Two days later and repeated weekly until 56 days of age, magnetic resonance (MR) imaging was used to assess ventricle size. Behavior was examined thrice weekly. Compared to age-matched saline-injected controls, severely hydrocephalic ferrets weighed significantly less, their postures were impaired, and they were hyperactive prior to extreme debilitation. They developed significant ventriculomegaly and displayed white matter destruction. Reactive astroglia and microglia detected by glial fibrillary acidic protein (GFAP) and Iba-1 immunostaining were apparent in white matter, cortex, and hippocampus. There was a hydrocephalus-related increase in activated caspase 3 labeling of apoptotic cells (7.0 vs. 15.5%) and a reduction in Ki67 labeling of proliferating cells (23.3 vs. 5.9%) in the subventricular zone (SVZ). Reduced Olig2 immunolabeling suggests a depletion of glial precursors. GFAP content was elevated. Myelin basic protein (MBP) quantitation and myelin biochemical enzyme activity showed early maturational increases. Where white matter was not destroyed, the remaining axons developed myelin similar to the controls. In conclusion, the hydrocephalus-induced periventricular disturbances may involve developmental impairments in cell proliferation and glial precursor cell populations. The ferret should prove useful for testing hypotheses about white matter damage and protection in the immature hydrocephalic brain.

Intracranial biomechanics of acute experimental hydrocephalus in live rats.

BACKGROUND: The mechanisms of hydrocephalus formation remain unclear. OBJECTIVE: To measure intracranial biomechanical changes in rats with hydrocephalus. METHODS: Stress-strain relationships were determined by using force-controlled indentation through a craniotomy. Cortical blood flow and intracerebral pressures were monitored. In normal rats, deformability of intracranial contents was examined by applying 100 (20-100 mN) indentation cycles and during a 2-hour stress (100 mN) holding test. Hydrocephalus was induced in 56-day rats by cisternal kaolin injection. Magnetic resonance imaging was used to measure ventricle size and cortical blood flow. RESULTS: Application of a constant small force for 2 hours or 100 cycles of a small indentation caused progressive intracranial deformation. Following kaolin injection, the ventricles of 3- to 4-day, 7- to 9-day, and 12- to 15-day hydrocephalic rats progressively enlarged, the dorsal cerebrum thickness decreased by >40%, and cortical blood flow decreased by ∼20%. After 3 to 4 days, intracranial pressure and intraparenchymal pulse pressure increased significantly by ∼85%, and diminished thereafter. After 7 to 9 days, there was a transient significant increase of the intracranial stiffness (indentation modulus). Viscoelastic strain during application of a constant force significantly increased by >50% at 7 to 9 and 12 to 15 days. CONCLUSION: The observation that very small forces applied exogenously or endogenously (through pulsatile brain micromotions) cause progressive intracranial deformation suggests that the brain behaves in a poroviscoelastic manner. Intracranial pulsatility is increased during the early phases of ventriculomegaly. Small viscoelastic property changes of the intracranial contents accompany the ventriculomegaly. Consolidation of brain tissue by the pulsatile forces likely occurs through displacement of intraparenchymal fluids.

Blood-brain barrier disruption in CCL2 transgenic mice during pertussis toxin-induced brain inflammation.

BACKGROUND: The chemokine CCL2 has an important role in the recruitment of inflammatory cells into the central nervous system (CNS). A transgenic mouse model that overexpresses CCL2 in the CNS shows an accumulation of leukocytes within the perivascular space surrounding vessels, and which infiltrate into the brain parenchyma following the administration of pertussis toxin (PTx). METHODS: This study used contrast-enhanced magnetic resonance imaging (MRI) to quantify the extent of blood-brain barrier (BBB) disruption in this model pre- and post-PTx administration compared to wild-type mice. Contrast-enhanced MR images were obtained before and 1, 3, and 5 days after PTx injection in each animal. After the final imaging session fluorescent dextran tracers were administered intravenously to each mouse and brains were examined histologically for cellular infiltrates, BBB leakage and tight junction protein. RESULTS: BBB breakdown, defined as a disruption of both the endothelium and glia limitans, was found only in CCL2 transgenic mice following PTx administration and seen on MR images as focal areas of contrast enhancement and histologically as dextrans leaking from blood vessels. No evidence of disruption in endothelial tight junctions was observed. CONCLUSION: Genetic and environmental stimuli were needed to disrupt the integrity of the BBB in this model of neuroinflammation.

Intracortical injection of endothelin-1 induces cortical infarcts in mice: effect of neuronal expression of an adenosine transporter.

BACKGROUND: Activation of adenosine A1 receptors has neuroprotective effects in animal stroke models. Adenosine levels are regulated by nucleoside transporters. In vitro studies showed that neuron-specific expression of human equilibrative nucleoside transporter 1 (hENT1) decreases extracellular adenosine levels and adenosine A1 receptor activity. In this study, we tested the effect of hENT1 expression on cortical infarct size following intracerebral injection of the vasoconstrictor endothelin-1 (ET-1) or saline. METHODS: Mice underwent stereotaxic intracortical injection of ET-1 (1 µl; 400 pmol) or saline (1 µl). Some mice received the adenosine receptor antagonist caffeine (25 mg/kg, intraperitoneal) 30 minutes prior to ET-1. Perfusion and T2-weighted magnetic resonance imaging (MRI) were used to measure cerebral blood flow (CBF) and subsequent infarct size, respectively. RESULTS: ET-1 reduced CBF at the injection site to 7.3 ± 1.3% (n = 12) in hENT1 transgenic (Tg) and 12.5 ± 2.0% (n = 13) in wild type (Wt) mice. At 48 hours following ET-1 injection, CBF was partially restored to 35.8 ± 4.5% in Tg and to 45.2 ± 6.3% in Wt mice; infarct sizes were significantly greater in Tg (9 ± 1.1 mm3) than Wt (5.4 ± 0.8 mm3) mice. Saline-treated Tg and Wt mice had modest decreases in CBF and infarcts were less than 1 mm3. For mice treated with caffeine, CBF values and infarct sizes were not significantly different between Tg and Wt mice. CONCLUSIONS: ET-1 produced greater ischemic injury in hENT1 Tg than in Wt mice. This genotype difference was not observed in mice that had received caffeine. These data indicate that hENT1 Tg mice have reduced ischemia-evoked increases in adenosine receptor activity compared to Wt mice.

Magnetic resonance imaging indicators of blood-brain barrier and brain water changes in young rats with kaolin-induced hydrocephalus.

BACKGROUND: Hydrocephalus is associated with enlargement of cerebral ventricles. We hypothesized that magnetic resonance (MR) imaging parameters known to be influenced by tissue water content would change in parallel with ventricle size in young rats and that changes in blood-brain barrier (BBB) permeability would be detected. METHODS: Hydrocephalus was induced by injection of kaolin into the cisterna magna of 4-week-old rats, which were studied 1 or 3 weeks later. MR was used to measure longitudinal and transverse relaxation times (T1 and T2) and apparent diffusion coefficients in several regions. Brain tissue water content was measured by the wet-dry weight method, and tissue density was measured in Percoll gradient columns. BBB permeability was measured by quantitative imaging of changes on T1-weighted images following injection of gadolinium diethylenetriamine penta-acetate (Gd-DTPA) tracer and microscopically by detection of fluorescent dextran conjugates. RESULTS: In nonhydrocephalic rats, water content decreased progressively from age 3 to 7 weeks. T1 and T2 and apparent diffusion coefficients did not exhibit parallel changes and there was no evidence of BBB permeability to tracers. The cerebral ventricles enlarged progressively in the weeks following kaolin injection. In hydrocephalic rats, the dorsal cortex was more dense and the white matter less so, indicating that the increased water content was largely confined to white matter. Hydrocephalus was associated with transient elevation of T1 in gray and white matter and persistent elevation of T2 in white matter. Changes in the apparent diffusion coefficients were significant only in white matter. Ventricle size correlated significantly with dorsal water content, T1, T2, and apparent diffusion coefficients. MR imaging showed evidence of Gd-DTPA leakage in periventricular tissue foci but not diffusely. These correlated with microscopic leak of larger dextran tracers. CONCLUSIONS: MR characteristics cannot be used as direct surrogates for water content in the immature rat model of hydrocephalus, probably because they are also influenced by other changes in tissue composition that occur during brain maturation. There is no evidence for widespread persistent opening of BBB as a consequence of hydrocephalus in young rats. However, increase in focal BBB permeability suggests that periventricular blood vessels may be disrupted.

Age-dependence of intracranial viscoelastic properties in living rats.

To explore the effect of maturation on intracranial mechanical properties, viscoelastic parameters were determined in 44 live rats at ages 1-2, 10-12, 21, 56-70, and 180 days using instrumented indentation. With the dura mater intact, the apparent modulus of elasticity, the indentation modulus, and viscous behavior were measured in vivo, as well as 1 h after death. In a separate group of 25 rats, brain water, and protein content were determined. A significant increase of the elastic and indentation moduli beginning at 10-12 days after birth and continuing to 180 days was observed. The creep behavior decreased in the postnatal period and stabilized at 21 days. Changes in intracranial biomechanical properties corresponded to a gradual decrease of brain water, and an increase in total protein content, including glial fibrillary acidic protein, myelin basic protein, and neurofilament light chain. Elastic properties were not significantly different comparing the live and dead states. However, there were significant postmortem changes in viscous behavior. Viscoelastic properties of living rat intracranial contents are shown to be age dependent, reflecting the physical and biochemical changes during postnatal development. This may be important for understanding why young and mature brains respond differently in situations of brain trauma and hydrocephalus.

Histochemical visualization and diffusion MRI at 7 Tesla in the TgCRND8 transgenic model of Alzheimer's disease.

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that has been characterized by gross cortical atrophy, cellular neurodegeneration, reactive gliosis, and the presence of microscopic extracellular amyloid plaques and intracellular neurofibrillary tangles. Earlier diagnoses of AD would be in the best interest of managing the patient and would allow for earlier therapeutic intervention. By measuring the apparent diffusion coefficient (ADC) using diffusion-weighted imaging (DWI), a type of magnetic resonance imaging (MRI), one can quantify alterations in water diffusivity resulting from microscopic structural changes in the cell at early stages that are associated with pathophysiological processes of brain injury and/or disease progression. Whether or not this methodology is useful for AD is a question under examination. For example, DWI in suspected AD patients has shown increases in mean ADC values in the hippocampus and diminished diffusion anisotropy in the posterior white matter. However, in some cases, hippocampal ADC values appear not to change in AD patients. Moreover, to our knowledge, all DWI studies in suspected AD patients to date are technically incomplete in experimental design, because corresponding histological sections demonstrating actual plaque deposition are lacking and so it is not clear that ADC changes actually correspond to plaque deposition. In our study, we used DWI in the TgCRND8 transgenic model of Alzheimer's disease in conjunction with histological techniques and found robust plaque deposition in the transgenic strain in older animals (12-16 months old). However, we did not find statistically significant changes (p > 0.05) in ADC values (although ADC values in TgCRND8 mice did decrease in all regions examined) in mice 12-16 months old. Collectively, recent results from human studies and in rodent AD transgenic models support our findings and suggest that amyloid beta plaque load is not likely the major or primary component contributing to diffusional changes, if they occur.

A comparison of rapid-scanning X-ray fluorescence mapping and magnetic resonance imaging to localize brain iron distribution.

The clinical diagnosis of many neurodegenerative disorders relies primarily or exclusively on observed behaviors rather than measurable physical tests. One of the hallmarks of Alzheimer disease (AD) is the presence of amyloid-containing plaques associated with deposits of iron, copper and/or zinc. Work in other laboratories has shown that iron-rich plaques can be seen in the mouse brain in vivo with magnetic resonance imaging (MRI) using a high-field strength magnet but this iron cannot be visualized in humans using clinical magnets. To improve the interpretation of MRI, we correlated iron accumulation visualized by X-ray fluorescence spectroscopy, an element-specific technique with T1, T2, and susceptibility weighted MR (SWI) in a mouse model of AD. We show that SWI best shows areas of increased iron accumulation when compared to standard sequences.

Intracerebral hemorrhage models in rat: comparing collagenase to blood infusion.

Many therapies have shown promise in preclinical stroke studies, but few benefit patients. A greater understanding of stroke pathophysiology is needed to successfully develop therapies, and this depends on appropriate animal models. The collagenase and blood infusion models of intracerebral hemorrhage (ICH) are widely used; yet, investigators often prefer using one model for a variety of reasons. Thus, we directly compared these to highlight advantages and limitations of each as well as the assessment approach. An ICH was created by infusing blood or bacterial collagenase into the rats' striatum. We matched initial hematoma volume in each model (Experiment 1) and assessed the time course of bleeding (Experiment 2). Functional deficits and the progression of injury were tracked over 6 weeks using behavior, magnetic resonance imaging, and histology (Experiment 3). Despite similar initial hematoma volumes, collagenase-induced ICH resulted in a greater blood-brain barrier breakdown and more damage to the striatum, substantia nigra, white matter, and cortex. Magnetic resonance imaging revealed faster hematoma resolution in the blood model, and little increase in the volume of tissue lost from 1 to 6 weeks. In contrast, tissue loss continued over 4 weeks in the collagenase model. Finally, functional deficits recovered more quickly and completely in the blood model. This study highlights key differences between these models and that neither closely replicates the human condition. Thus, both should be used whenever possible taking into account the significant differences between these models and their limitations. Furthermore, this work illustrates significant weaknesses with several outcome measures.