MRI of animal models of brain disease

Magnetic resonance assessment of the cerebral alterations associated with obesity development

Obesity is a current threat to health care systems, affecting approximately 13% of the world's adult population, and over 18% children and adolescents. The rise of obesity is fuelled by inadequate life style habits, as consumption of diets rich in fats and sugars which promote, additionally, the development of associated comorbidities. Obesity results from a neuroendocrine imbalance in the cerebral mechanisms controlling food intake and energy expenditure, including the hypothalamus and the reward and motivational centres. Specifically, high-fat diets are known to trigger an early inflammatory response in the hypothalamus that precedes weight gain, is time-dependent, and eventually extends to the remaining appetite regulating regions in the brain. Multiple magnetic resonance imaging (MRI) and spectroscopy (MRS) methods are currently available to characterize different features of cerebral obesity, including diffusion weighted, T2 and volumetric imaging and 1H and 13C spectroscopic evaluations. In particular, consistent evidences have revealed increased water diffusivity and T2 values, decreased grey matter volumes, and altered metabolic profiles and fluxes, in the brain of animal models and in obese humans. This review provides an integrative interpretation of the physio-pathological processes associated with obesity development in the brain, and the MRI and MRS methods implemented to characterize them.

Post-processing correction of magnetization transfer effects in FENSI perfusion MRI data

Magnetization transfer effects induced by repetitive saturation pulses employed in flow enhancement of signal intensity imaging sequences currently prevent quantitative, in vivo, cerebral perfusion studies. This study investigates the magnitude of these effects and introduces a post-processing correction protocol. The study shows that the magnetization transfer effect is consistent across individuals, which enables the derivation of a correction factor to be applied in post-acquisition. Our results, obtained for cerebral flux in white and gray matter in rodent brains, are in agreement with cerebral blood flow measurements previously reported in the literature.

Regional differences in cerebral edema after traumatic brain injury identified by impedance analysis

OBJECTIVE

Cerebral edema is a common and potentially devastating sequel of traumatic brain injury. We developed and validated a system capable of tissue impedance analysis, which was found to correlate with cerebral edema.

METHODS

Constant sinusoidal current (50 microA), at frequencies from 500 to 5000 Hz, was applied across a bipolar electrode unit superficially placed in a rat brain after traumatic brain injury. Rats were randomized to three groups: severe controlled cortical injury (CCI), mild CCI, or sham injury. At 60 h post-CCI, cerebral voltage and phase angle were measured at each frequency at the site of injury, at the penumbral region, at the ipsilateral frontal region, and in the contralateral hemisphere. Impedance measurements were also obtained in vivo. The electrical properties of varied injuries and specified locations were compared using a repeated measures analysis of variance (RMANOVA), were correlated with regional tissue water percentage using regression analyses, and were combined to generate polar coordinates.

RESULTS

The measured voltage was significantly different at the site of injury (P<0.0001), in the penumbra (P=0.002), and in the contralateral hemisphere (P=0.005) when severe, mild, and sham CCI rats were compared. Severely injured rats had statistically different voltage measurements when the various sites were compared (P=0.002). The ex vivo measurements correlated with in vivo measurements. Further, the impedance measurements correlated with measured tissue water percentage at the site of injury (R2=0.69; P<0.0001). The creation of a polar coordinate graph, incorporating voltage and phase angle measurements, enabled the identification of impedance areas unique to normal, mild edema, and severe edema measurements in the rat brain.

CONCLUSIONS

Electrical measurements and tissue water percentages quantified regional and severity differences in rat brain edema after CCI. Impedance was inversely proportional to the tissue water percentage. Thus, impedance measurement can be used to quantify severity of cerebral edema in real time at specific sites.

T1 relaxation in in vivo mouse brain at ultra-high field

Accurate knowledge of relaxation times is imperative for adjustment of MRI parameters to obtain optimal signal-to-noise ratio (SNR) and contrast. As small animal MRI studies are extended to increasingly higher magnetic fields, these parameters must be assessed anew. The goal of this study was to obtain accurate spin-lattice (T(1)) relaxation times for the normal mouse brain at field strengths of 9.4 and 17.6 T. T(1) relaxation times were determined for cortex, corpus callosum, caudate putamen, hippocampus, periaqueductal gray, lateral ventricle, and cerebellum and varied from 1651 +/- 28 to 2449 +/- 150 ms at 9.4 T and 1824 +/- 101 to 2772 +/- 235 ms at 17.6 T. A field strength-dependent increase of T(1) relaxation times is shown. The SNR increase at 17.6 T is in good agreement with the expected SNR increase for a sample-dominated noise regime.

Catheter confocal fluorescence imaging and functional magnetic resonance imaging of local and systems level recovery in the regenerating rodent sciatic nerve

The goal of the present work was to develop minimally invasive imaging techniques to monitor local regeneration of peripheral nerves and to determine the extent of return to function of brain cortical regions associated with that nerve. The sciatic nerve crush model was applied to Sprague-Dawley rats and conventional histological staining for myelin, axons and cell architecture was carried out, as well as traditional behavioral testing, to verify that nerve regeneration was occurring. The rate of sciatic nerve regeneration was measured by determining the distance a lipophilic, fluorescence probe (DiO) would move along the nerve's membrane following a direct injection into the sciatic nerve. This movement was monitored using a catheter based, confocal fluorescence microscope. Two to five days after the crush, the dye moved 1.4 + 0.6 mm/day, as compared to a distance of 5.3 + 0.5 mm/day in the normal nerve. Between 9 and 13 days following the crush, the distance the dye moved increases to 5.5 + 0.5 mm/day, similar to the control, and by 15 days following the crush, the distance increased to 6.5 + 0.9 mm/day. Functional Magnetic Resonance Imaging (fMRI) measurements were performed on alpha-chloralose anesthetized rats to monitor the return of somatosensory cortical functions, which were activated by the stimulation of the lesioned peripheral nerve. fMRI results showed the return of cortical activation around 15 days following the crush procedure. However, the somatosensory cortical region activated by stimulating the crushed hindpaw was significantly smaller in extent than the intact hindpaw stimulation. These findings demonstrate that fluorescence imaging and fMRI can integrate local and system level correlates of nerve regeneration in a non-destructive manner, thus enabling serial imaging of individual animals.

Neuroprotective effects of HSP70 overexpression after cerebral ischaemia--an MRI study

Heat shock proteins (HSPs) have been reported to increase cell survival in response to a wide range of cellular challenges. However, the role of HSP70 overexpression is still a matter of debate, with some reports showing protection and others not. In order to resolve these discrepancies and further investigate the action of these proteins in vivo, transgenic mice overexpressing HSP70 have been compared to wild-type mice in a middle cerebral artery occlusion model of permanent cerebral ischaemia. Previously, the effect of HSP70 was assessed histologically postmortem. In this report, magnetic resonance imaging (MRI) was used to assess the mice in vivo after the onset of stroke. The lesion volume, as measured at 24 h using T(2)-weighted MRI, was significantly smaller in HSP70 transgenic mice compared with wild-type mice. The smaller lesion size in HSP70 transgenic mice could not be attributed to differences in vascular anatomy or in cerebral blood flow during occlusion. Additionally, the apparent diffusion coefficient showed different spatial and temporal patterns between the groups, suggesting that the damage within the lesion may be less severe for HSP70 transgenic mice. Thus, we conclude that overexpression of HSP70 reduces the overall lesion size and may also limit the tissue damage within the lesion.