Noninvasive in vivo MRI detection of neuritic plaques associated with iron in APP[V717I] transgenic mice, a model for Alzheimer's disease.

Transgenic mice overexpressing the London mutant of human amyloid precursor protein (APP[V717I]) in neurons develop amyloid plaques in the brain, thus demonstrating the most prominent neuropathological hallmark of Alzheimer's disease. In vivo 3D T2*-weighted MRI on these mice (24 months of age) revealed hypointense brain inclusions that affected the thalamus almost exclusively. Upon correlating these MRI observations with a panel of different histologic staining techniques, it appeared that only plaques that were positive for both thioflavin-S and iron were visible on the MR images. Numerous thioflavin-S-positive plaques in the cortex that did not display iron staining remained invisible to MRI. The in vivo detection of amyloid plaques in this mouse model, using the intrinsic MRI contrast arising from the iron associated with the plaques, creates an unexpected opportunity for the noninvasive investigation of the longitudinal development of the plaques in the same animal. Thus, this work provides further research opportunities for analyzing younger APP[V717I] mouse models with the knowledge of the final outcome at 24 months of age.     

3He MRI in mouse models of asthma.

In the study of asthma, a vital role is played by mouse models, because knockout or transgenic methods can be used to alter disease pathways and identify therapeutic targets that affect lung function. Assessment of lung function in rodents by available methods is insensitive because these techniques lack regional specificity. A more sensitive method for evaluating lung function in human asthma patients uses hyperpolarized (HP) (3)He MRI before and after bronchoconstriction induced by methacholine (MCh). We now report the ability to perform such (3)He imaging of MCh response in mice, where voxels must be approximately 3000 times smaller than in humans and (3)He diffusion becomes an impediment to resolving the airways. We show three-dimensional (3D) images that reveal airway structure down to the fifth branching and visualize ventilation at a resolution of 125 x 125 x 1000 microm(3). Images of ovalbumin (OVA)-sensitized mice acquired after MCh show both airway closure and ventilation loss. To also observe the MCh response in naive mice, we developed a non-slice-selective 2D protocol with 187 x 187 microm(2) resolution that was fast enough to record the MCh response and recovery with 12-s temporal resolution. The extension of (3)He MRI to mouse models should make it a valuable translational tool in asthma research.     

High-resolution MRI with cardiac and respiratory gating allows for accurate in vivo atherosclerotic plaque visualization in the murine aortic arch.

Genetically engineered mouse models provide enormous potential for investigation of the underlying mechanisms of atherosclerotic disease, but noninvasive imaging methods for analysis of atherosclerosis in mice are currently limited. This study aimed to demonstrate the feasibility of MRI to noninvasively visualize atherosclerotic plaques in the thoracic aorta in mice deficient in apolipoprotein-E, who develop atherosclerotic lesions similar to those observed in humans. To freeze motion, MR data acquisition was both ECG- and respiratory-gated. T(1)-weighted MR images were acquired with TR/TE approximately 1000/10 ms. Spatial image resolution was 49 x 98 x 300 micro m(3). MRI revealed a detailed view of the lumen and the vessel wall of the entire thoracic aorta. Comparison of MRI with corresponding cross-sectional histopathology showed excellent agreement of aortic vessel wall area (r = 0.97). Hence, noninvasive MRI should allow new insights into the mechanisms involved in progression and regression of atherosclerotic disease.     

Automatic segmentation of amyloid plaques in MR images using unsupervised support vector machines

Deposition of the β-amyloid peptide (Aβ) is an important pathological hallmark of Alzheimer's disease (AD). However, reliable quantification of amyloid plaques in both human and animal brains remains a challenge. We present here a novel automatic plaque segmentation algorithm based on the intrinsic MR signal characteristics of plaques. This algorithm identifies plaque candidates in MR data by using watershed transform, which extracts regions with low intensities completely surrounded by higher intensity neighbors. These candidates are classified as plaque or nonplaque by an unsupervised learning method using features derived from the MR data intensity. The algorithm performance is validated by comparison with histology. We also demonstrate the algorithm's ability to detect age-related changes in plaque load ex vivo in amyloid precursor protein (APP) transgenic mice that coexpress five familial AD mutations (5xFAD mice). To our knowledge, this study represents the first quantitative method for characterizing amyloid plaques in MRI data. The proposed method can be used to describe the spatiotemporal progression of amyloid deposition, which is necessary for understanding the evolution of plaque pathology in mouse models of Alzheimer's disease and to evaluate the efficacy of emergent amyloid-targeting therapies in preclinical trials.     

Magnetic resonance imaging of amyloid plaques in transgenic mouse models of Alzheimer's disease

A major objective in the treatment of Alzheimer's disease is amyloid plaque reduction. Transgenic mouse models of Alzheimer's disease provide a controlled and consistent environment for studying amyloid plaque deposition in Alzheimer's disease. Magnetic resonance imaging is an attractive tool for longitudinal studies because it offers non-invasive monitoring of amyloid plaques. Recent studies have demonstrated the ability of magnetic resonance imaging to detect individual plaques in living mice. This review discusses the mouse models, MR pulse sequences, and parameters that have been used to image plaques and how they can be optimized for future studies.     

High-resolution magnetic resonance imaging at 2 Tesla: potential for atherosclerotic lesions exploration in the apolipoprotein E knockout mouse

INTRODUCTION: The aim of the present study was to evaluate the potential of high-resolution MRI at 2 Tesla (T) for direct noninvasive imaging of the aortic wall in a mouse model of atherosclerosis. MATERIAL AND METHODS: A specific mouse antenna was developed and sequence parameters were adjusted. T(1)- and T2-weighted images of abdominal aorta were obtained at 2 T with a spatial resolution of 86 x 86 x 800 microm3 in vivo. With a dedicated small coil, ex vivo MRI of the aorta was performed with a spatial resolution of 54 x 54 x 520 microm3. RESULTS: In vivo, the aortic wall was clearly defined on T(2)-weighted images in 15 of 16 mice: along the aorta the lumen circumference ranged from 1.07 to 3.61 mm and mean wall thickness from 0.11 to 0.67 mm. In vivo measurements of plaque distribution were confirmed by ex vivo MR imaging and by histology, with a good correlation with histology regarding lumen circumference (r = 0.94) and wall thickness (r = 0.97). CONCLUSION: Magnetic resonance imaging at 2 T to analyze in vivo atherosclerotic lesions in mice is possible with a spatial resolution of 86 x 86 x 800 microm3 and thus can be used for noninvasive follow-up in evaluation of new drugs.     

Gradient-echo and CRAZED imaging for minute detection of Alzheimer plaques in an APPV717I x ADAM10-dn mouse model.

Different strategies to visualize amyloid plaques with MRI at 17.6 Tesla were investigated in a novel mouse model of Alzheimer's disease (AD). Large iron-containing plaques were observed in the thalamus, but cortical plaques did not show iron deposits. Plaques in the thalamus were visualized in vivo with the use of low-resolution, 3D gradient-echo (GRE) imaging in 82 s, and with 94-microm resolution in 34 min. The feasibility of obtaining bright contrast from plaques using the COSY revamped with asymmetric z-GRE detection (CRAZED) technique was investigated in experiments on fixed brains. The original CRAZED approach provided reduced signal near the plaques (similarly to GRE imaging) and additionally emphasized small structures in the brain. In CRAZED images acquired with mismatched gradients, elevated signal near the plaques was obtained, while background signal was suppressed almost to the noise level. Bright-contrast images were acquired in 2.6 min with the use of a 2D GRE sequence with slightly mismatched slice refocusing gradients. For future detection of plaques in patients, such bright-contrast visualization protocols may be of particular value when contrast agents that allow labeling of early plaques with iron oxide nanoparticles become available.     

Passive staining: a novel ex vivo MRI protocol to detect amyloid deposits in mouse models of Alzheimer's disease.

Amyloid plaques are one of the hallmarks of Alzheimer's disease (AD). This study evaluated a novel microMRI strategy based on "passive staining" of brain samples by gadoteric acid. The protocol was tested at 4.7 T on control animals and APP/PS1 mice modeling AD lesions. T(1) was strongly decreased in passively stained brains. On high-resolution 3D gradient echo images, the contrast between the cortex and subcortical structures was highly improved due to a T2* effect. The brains of APP/PS1 mice revealed plaques as hypo-intense spots. They appeared larger in long compared to short TE images. This suggests that, after passive staining, plaques caused a susceptibility effect. This easily performed protocol is a complementary method to classic histology to detect the 3D location of plaques. It may also be used for the validation of in vivo MRI protocols for plaque detection by facilitating registration with histology via post mortem MRI.     

In vivo visualization of Alzheimer's amyloid plaques by magnetic resonance imaging in transgenic mice without a contrast agent.

One of the cardinal pathologic features of Alzheimer's disease (AD) is the formation of senile, or amyloid, plaques. Transgenic mice have been developed that express one or more of the genes responsible for familial AD in humans. Doubly transgenic mice develop "human-like" plaques, providing a mechanism to study amyloid plaque biology in a controlled manner. Imaging of labeled plaques has been accomplished with other modalities, but only MRI has sufficient spatial and contrast resolution to visualize individual plaques noninvasively. Methods to optimize visualization of plaques in vivo in transgenic mice at 9.4 T using a spin echo sequence based on adiabatic pulses are described. Preliminary results indicate that a spin echo acquisition more accurately reflects plaque size, while a T2* weighted gradient echo sequence reflects plaque iron content, not plaque size. In vivo MRI-ex vivo MRI-in vitro histologic correlations are provided. Histologically verified plaques as small as 50 microm in diameter were visualized in living animals. To our knowledge this work represents the first demonstration of noninvasive in vivo visualization of individual AD plaques without the use of a contrast agent.     

Visualization of beta-amyloid plaques in a transgenic mouse model of Alzheimer's disease using MR microscopy without contrast reagents.

The visualization of beta-amyloid plaque deposition in brain, a key feature of Alzheimer's disease (AD), is important for the evaluation of disease progression and the efficacy of therapeutic interventions. In this study, beta-amyloid plaques in the PS/APP transgenic mouse brain, a model of human AD pathology, were detected using MR microscopy without contrast reagents. beta-Amyloid plaques were clearly visible in the cortex, thalamus, and hippocampus of fixed brains of PS/APP mice. The distribution of plaques identified by MRI was in excellent agreement with those found in the immunohistological analysis of the same brain sections. It was also demonstrated that image contrast for beta-amyloid plaques was present in freshly excised nonfixed brains. Furthermore, the detection of beta-amyloid plaques was achieved with a scan time as short as 2 hr, approaching the scan time considered reasonable for in vivo imaging.     

In vivo measurement of plaque burden in a mouse model of Alzheimer's disease

PURPOSE: To demonstrate an MRI method for directly visualizing amyloid-beta (Abeta) plaques in the APP/PS1 transgenic (tg) mouse brain in vivo, and show that T1rho relaxation rate increases progressively with Alzheimer's disease (AD)-related pathology in the tg mouse brain. MATERIALS AND METHODS: We obtained in vivo MR images of a mouse model of AD (APP/PS1) that overexpresses human amyloid precursor protein, and measured T1rho via quantitative relaxometric maps. RESULTS: A significant decrease in T1rho was observed in the cortex and hippocampus of 12- and 18-month-old animals compared to their age-matched controls. There was also a correlation between changes in T1rho and the age of the animals. CONCLUSION: T1rho relaxometry may be a sensitive method for noninvasively determining AD-related pathology in APP/PS1 mice.     

In vivo magnetic resonance imaging of large spontaneous aortic aneurysms in old apolipoprotein E-deficient mice

Old ApoE-deficient mice were studied in vivo by magnetic resonance imaging (MRI) to prospectively evaluate vascular remodeling associated with atherosclerotic lesions. MATERIAL AND METHODS: Old female ApoE-/- mice on a normal diet were followed by MRI at 2 Tesla for a 3-month period and killed for histopathology. Aortic dimensions were measured and compared. RESULTS: High-quality in vivo MR images were obtained at 2 Tesla with in plane spatial resolution of 86 X 86 microm2. On MRI, aortic lumen enlargement (>1.5-fold dilation) was seen in 10 of 13 mice, located predominantly in the suprarenal portion of the aorta. The mean maximal diameter of the aneurysms and of the aorta above and below the aneurysm were, respectively, 1.12 +/- 0.32 mm and 0.53 +/- 0.08 mm by MRI and 1.3+/- 0.41 mm and 0.55 +/- 0.15 mm by histology. Matched histologic cross-sections of the aortic wall showed medial degradation with rupture of the internal elastic lamina at multiple sites, associated with fibrolipidic plaque containing cholesterol crystals. CONCLUSIONS: Aortic lumen enlargement was diagnosed in old ApoE-/- mice at sites with advanced atherosclerotic plaques. MRI has potential both as an in vivo imaging technique for screening mouse models for vascular wall pathology and to follow arterial remodeling associated with the disease progression.     

In vivo quantification of T1, T2, and apparent diffusion coefficient in the mouse retina at 11.74T.

MRI has recently been used for noninvasive examination of retinal structure and function in rats and cats. However, the advantages of quantitative high-resolution MRI of retina from mice have not yet been explored. In the present study, T(1) and T(2) relaxation time constants and the directional apparent diffusion coefficient (ADC) in the retina of C57/BL6 mice were measured. Three MR-detected retina layers and a MR-detected choroid layer were observed on both T(1)- and T(2)-weighted images at an image resolution of 47 x 47 x 400 microm(3). The significantly higher ADC parallel to than that perpendicular to the optic nerve in the MR-detected outer retina layer at the central retina reflects the known cellular organization of the photoreceptor cells. This study establishes, for the first time, normative metrics of T(1), T(2), and ADC of the mouse retina. These MR parameters are expected to be useful in future evaluation of developmental and pathological alterations of retinal cell layers in mice.     

MR microimaging of amyloid plaques in Alzheimer's disease transgenic mice

INTRODUCTION: Alzheimer's disease (AD) is the most prevalent neurological condition affecting industrialized nations and will rapidly become a healthcare crisis as the population ages. Currently, the post-mortem histological observation of amyloid plaques and neurofibrillary tangles is the only definitive diagnosis available for AD. A pre-mortem biological or physiological marker specific for AD used in conjunction with current neurological and memory testing could add a great deal of confidence to the diagnosis of AD and potentially allow therapeutic intervention much earlier in the disease process. DISCUSSION AND CONCLUSION: Our group has developed MRI techniques to detect individual amyloid plaques in AD transgenic mouse brain in vivo. We are also developing contrast-enhancing agents to increase the specificity of detection of amyloid plaques. Such in vivo imaging of amyloid plaques will also allow the evaluation of anti-amyloid therapies being developed by the pharmaceutical industry in pre-clinical trials of AD transgenic mice. This short review briefly discusses our progress in these areas.     

Imaging noradrenergic influence on amyloid pathology in mouse models of Alzheimer's disease

INTRODUCTION: Molecular imaging aims towards the non-invasive characterization of disease-specific molecular alterations in the living organism in vivo. In that, molecular imaging opens a new dimension in our understanding of disease pathogenesis, as it allows the non-invasive determination of the dynamics of changes on the molecular level. IMAGING OF AD CHARACTERISTIC CHANGES BY microPET: The imaging technology being employed includes magnetic resonance imaging (MRI) and nuclear imaging as well as optical-based imaging technologies. These imaging modalities are employed together or alone for disease phenotyping, development of imaging-guided therapeutic strategies and in basic and translational research. In this study, we review recent investigations employing positron emission tomography and MRI for phenotyping mouse models of Alzheimer's disease by imaging. We demonstrate that imaging has an important role in the characterization of mouse models of neurodegenerative diseases.     

Detection of Alzheimer's amyloid in transgenic mice using magnetic resonance microimaging.

The presence of amyloid-beta (Abeta) plaques in the brain is a hallmark pathological feature of Alzheimer's disease (AD). Transgenic mice overexpressing mutant amyloid precursor protein (APP), or both mutant APP and presenilin-1 (APP/PS1), develop Abeta plaques similar to those in AD patients, and have been proposed as animal models in which to test experimental therapeutic approaches for the clearance of Abeta. However, at present there is no in vivo whole-brain imaging method to detect Abeta plaques in mice or men. A novel method is presented to detect Abeta plaques in the brains of transgenic mice by magnetic resonance microimaging (muMRI). This method uses Abeta1-40 peptide, known for its high binding affinity to Abeta, magnetically labeled with either gadolinium (Gd) or monocrystalline iron oxide nanoparticles (MION). Intraarterial injection of magnetically labeled Abeta1-40, with mannitol to transiently open the blood-brain barrier (BBB), enabled the detection of many Abeta plaques. Furthermore, the numerical density of Abeta plaques detected by muMRI and by immunohistochemistry showed excellent correlation. This approach provides an in vivo method to detect Abeta in AD transgenic mice, and suggests that diagnostic MRI methods to detect Abeta in AD patients may ultimately be feasible.     

Mapping of retinal projections in the living rat using high-resolution 3D gradient-echo MRI with Mn2+-induced contrast.

This study describes the neuroaxonal tracing of the visual pathway in the living rat using high-resolution T1-weighted 3D gradient-echo MRI (195 x 195 x 125 microm3) at 8, 24, 48, and 72 h after intraocular Mn2+ injection (0.1 microl of 1 M aqueous MnCl2). Best results were obtained at 24 h postinjection, revealing a continuous pattern of anterograde labeling from the retina, optic nerve, and chiasm to the contralateral optic tract, the dorsal and ventral lateral geniculate nucleus, the superior colliculus and its brachium, the olivary pretectal nucleus, the nucleus of the optic tract, and the suprachiasmatic nucleus. These results underline the feasibility of repeated MRI tract tracing in living animals after a single injection of Mn2+. The approach is expected to advance studies of neuroaxonal function in behaving animals with special emphasis on applications in developmental neurobiology.     

Longitudinal assessment of Alzheimer's beta-amyloid plaque development in transgenic mice monitored by in vivo magnetic resonance microimaging

PURPOSE: To assess the development of beta-amyloid (Abeta) plaques in the brain with age in the transgenic mouse model of Alzheimer's disease (AD) pathology by in vivo magnetic resonance microimaging (microMRI). MATERIALS AND METHODS: Live transgenic mice (Tg2576) and nontransgenic littermates (control) were studied at regular intervals between the ages of 12 and 18 months. Plaques were visualized using a T(2)-weighted rapid acquisition with relaxation enhancement (RARE) sequence. Changes in T(2) relaxation times were followed using a multislice multiecho (MSME) sequence. Plaque load and numerical density in MR images were calculated using SCIL image software. RESULTS: Abeta plaques were clearly detected with the T(2)-weighted RARE sequence in the hippocampal and cortical regions of the brain of Tg2576 mice but not in control mice. Following the plaque development in the same animals with age showed that plaque area, number, and size increased markedly, while T(2) relaxation time showed a decreasing trend with age. CONCLUSION: These results demonstrate that microMRI is a viable method for following the development of Abeta plaques in vivo, and suggest that this method may be feasible for assessing the effect of therapeutic interventions over time in the same animals.     

Cardiac MRI of the normal and hypertrophied mouse heart

With the development of recent transgenic techniques, studies involving mice offer opportunities to increase understanding of cardiac disease. This provides motivation for the current study to perform noninvasive evaluation of the normal and hypertrophied mouse heart with MRI. By acquiring EGG and respiratory signals, the MR image acquisition was gated to both the cardiac and respiratory cycles. Combining a spin-warp imaging sequence with an RF surface coil resulted in short-axis images that allowed quantification of in vivo cardiac mass. Excellent agreement between MRI-determined (y) and postmortem heart weight (x) was obtained: y = 0.991x + 1.43 (r = 0.996). Isoproterenol, at 282 μMmol/kg body weight (BW) and 573 μMmol/kg BW, induced a dose-dependent increase in the ratio of heart weight to BW of 16.8 ± 1.09% and 24.1 ± 1.71 %, respectively, which was accurately measured by MRI. These results demonstrate the ability of MRI to non-invasively monitor cardiac anatomy in the mouse.     

Quantitative assessment of coronary artery plaque vulnerability by high-resolution magnetic resonance imaging and computational biomechanics: a pilot study ex vivo.

The risk of atherosclerotic plaque disruption is thought to be closely related to plaque composition and rupture triggers such as external mechanical forces. The purpose of this study was to integrate MR imaging and computational techniques for the quantification of plaque vulnerability with morphologic data and biomechanical stress/strain distributions that were all based on high-resolution MR images of coronary artery plaque specimens ex vivo. Twenty-two coronary artery plaque specimens were selectively collected from 10 cadavers. Multislice T(2)-weighted spin echo images were acquired with a resolution of 100 x 100 microm(2). Histopathological images were used as the gold standard for the identification of plaque components and vulnerability. Plaque components were classified on MR images, and the stress/strain components were calculated with a two-dimensional computational model with fluid-structure interactions. As expected, vulnerable plaques appeared to result from a large lipid pool, a thin fibrous cap, and some critical stress/strain conditions. An empiric vulnerability marker was derived and was closely related to the vulnerability score that was determined through pathologic examination. Noninvasive quantification of the MR contrast and mechanical properties of plaque may provide a comprehensive biomarker for the assessment of vulnerability of atherosclerotic plaques.