Skip to search form Skip to main content. Medicine Published in Magnetic resonance imaging…. The exquisite detail provided by brain magnetic resonance imaging scans can make interpretation simultaneously straightforward and complicated, particularly to the novice. For this reason, it is essential to become familiar with normal structures before describing the pathologic condition. This article serves as a practical reference point to further enhance knowledge of the intracranial anatomy.
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- Cross-sectional anatomy of the brain.
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Submitted comments are subject to editing and editor review prior to posting. Web page addresses and e-mail addresses turn into links automatically. Lines and paragraphs break automatically. Interestingly, a comparison of atrophy patterns and histology showed that atrophied animals have severe intracellular amyloid depositions as well as astrogliosis Kraska et al.
The in vivo detection and follow-up of the atrophy process using MRI offers the possibility to search for the biological origin of ongoing neurodegeneration. Typical profiles of cerebral atrophy in lemurs. In comparison, profile of an animal without any atrophic process. In animals such as dogs Bagley, ; Borras et al. However, the biochemical composition of cerebral vasculature changes with age Sobin et al.
MRI is particularly sensitive to the detection of hemorrhages and such lesions lead to hypointense spots on MR images. As an example, the figure 7A shows the detection of an artificially induced hemorrhage in a rat brain. The figure 7B displays patterns of microhemorrhages in aged mouse lemurs.
These data suggest that vascular alterations are part of the alterations associated to cerebral aging in lemurs. A Example showing detection by MRI of a hypointense lesions arrow corresponding to a hemorrhage in a rat brain. B displays patterns of microhemorrhages arrows, hypointense spots in aged mouse lemurs. The regions that are particularly involved in such accumulations are the globus pallidus and the substantia nigra. Other regions such as the red nucleus, the putamen, the caudate nucleus, the dentate nucleus as well as the subthalamic body also show large iron load Hallgren et Sourander, Abnormal accumulation of iron has been reported in particular during the course of neurodegenerative diseases such as Parkinson's or Alzheimer's diseases Zecca et al.
MRI analyses in humans have shown a rapid T2 signal decrease in young individuals, less rapid in middle-aged and slower in aged individuals Pujol et al. Stronger signals in key regions involved in AD or PD have also been observed, thus confirming histological results concerning the modulation of iron by aging and neurodegenerative diseases Drayer et al. More specifically, iron accumulates in the globus pallidus, substantia nigra, neocortical and cerebellar white matter, thalamus, and in anterior forebrain structures, including the nucleus basalis of Meynert Dhenain et al.
In the basal forebrain, the areas of iron accumulation largely overlap the distribution of choline acetyltransferase ChAT -immunoreactive neurons Gilissen et al. However, unlike in humans, the signal decrease is continuous over the whole life span of the mouse lemur Dhenain et al.
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Also very high correlations can be observed between the T2-w magnetic resonance imaging signal decrease and the natural logarithm of mouse lemur age in the pallidum, the substantia nigra, and in the thalamus Dhenain et al. The T2 signal decreases rapidly until the age of 4 years. After this age, in middle-aged and older animals the signal decrease becomes less important. T2-w images from a young A and an old mouse lemurs B showing a reduced signal at the level of the globus pallidus arrow in the old animal. This signal decrease in the old animal is caused by iron accumulation in this brain region.
In rats or mice, such injections rely on dedicated atlases that are used to calculate injection points relative to reference points such as the bregma. This region located on the top of the skull is the junction of the sagittal and coronal sutures. In the mouse lemur, a stereotaxic atlas has been published Bons et al. However the bregma position is changing as compared to the position of the brain Fig. Thus, the bregma can not be used as a reference as it is the case in rodents. In our laboratory, we implemented a method based on MRI to inject drugs in the brain of mouse lemurs.
The method consists in sticking two capillaries filled with distilled water on the skull of mouse lemurs. One capillary is stuck on the bregma and another one is stuck parallel to the sagittal suture Fig. A third visual cue capillary is placed to the right of the animal to know the position of the left and the right on the MR images. The animal is then placed into the scanner and MR images allow to visualize the location of the tubes. MRI showing capillary tubes arrows stuck at the level of the bregma on the skull of six mouse lemurs.
One can see that the anatomical level varied greatly from one animal to the other. In A-C, the bregma was at the level of the hippocampus Hipp and posterior part of the caudate nucleus pCd. In D, the bregma was at the level of the posterior part of the hippocampus pHipp. In E-F, the bregma was at the level of the anterior part of the caudate nucleus aCd. Mouse lemur in a stereotaxic frame. Capillary tubes are positioned at the level of the bregma, sagittal suture, and on the right side of the animal.
Positron emission tomography is one of these modalities. This method is based on the injection of radioligands such as the 2--fluorodeoxy-D-glucose FDG. It allows to detect local metabolism in the brain of mouse lemurs. The resolution of PET images 0. These conditions do not allow to detect clearly anatomical regions Fig.
This allows for a fine discrimination of the regions of interest in which the radioligand uptake is measured. MR and PET images are recorded for each animal and rigidly co-registered. The co-registration can be performed automatically or manually. The co-registration accuracy is checked thanks to benchmarks like eyes, olfactory bulbs, cerebellum, spinal cord and temporal lobes Fig.
Brain Anatomy and Magnetic Resonance Imaging
PET images are the colored images, while the MR images are the images with anatomical details. In A, the shapes of the eye e and of the cerebellum c are outlined on the MR black doted lines and PET images red doted lines. C-D show registered images. In D, the caudate nucleus Cd was outlined. The region of interest outlined in MR images can be used to evaluate the metabolism from the registered PET images. Other annotations: OB: olfactory bulb. C-D montrent les images mises en registre.
Autres annotations : OB : bulbe olfactif.
https://ananergreen.tk For example, they are used to evaluate the effect of immunotherapies against Alzheimer's disease Trouche et al. Two difficulties are associated to the evaluation of therapies in lemurs. The first one is related to the heterogeneity of aging phenomena in lemurs. MRI can be used to select the animals with a given atrophy level in order to incorporate them in a therapeutic evaluation.
The second one is related to the follow-up of animals during therapeutic evaluation.
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For example, Figure 11 shows the longitudinal follow-up of two mouse lemurs during immunotherapy treatment. In this study, animals were scanned before vaccination, and two, six and nine months after the beginning of the treatment. The in vivo MRI follow-up allows for the detection of not only a basal heterogeneity, but also visible cerebral changes in some cases Fig.
Follow-up of two mouse lemurs by MRI before and during immunotherapy. In the first case A the mouse lemur presented visible modulation of CSF pattern arrow. In the second case B there was no visible modification of CSF level in the brain. The same sequence was used for the two animals and for all the sessions for parameters see Fig. Because of their small size, they can be studied with widely available preclinical MR spectrometers.
Many MRI studies can thus be performed in lemursto evaluate the brain in normal and pathological conditions.