Imaging the rat brain on a 1.5 T clinical MR-scanner
Introduction
Rodent models are a widely used and accepted method for studying pathologies in the field of neuroscience. Most of the analyses, however, rely on the post-mortern examination of tissue. The application of imaging modalities has brought the advantage of in vivo monitoring of pathological changes for longitudinal studies, and leads therefore to a higher efficiency in animal experiments. Due to the small size of the rat brain, high-resolution imaging is required. 1H magnetic resonance imaging (MRI) has been found to meet these requirements providing a noninvasive, non-harmful technique. In most studies purpose-dedicated, small bore nuclear magnetic resonance systems have been used (Johnson et al., 1987, Peschanski et al., 1988, Wang et al., 1991, Detre et al., 1992, Sauer et al., 1992, Jackson et al., 1994, Mellin et al., 1994, Kamiryo et al., 1995, Mason et al., 1995, Silva et al., 1995, Ito et al., 1996, John et al., 1996, van Lookeren Campagne et al., 1996). This equipment is very expensive and not readily accessible to all research laboratories, moreover they have characteristics that differ strongly from clinical scanners. Even though clinical MR scanners are widely available, the spatial resolution routinely used to examine the human brain is not sufficient for the study of the rat brain. Therefore only few studies have been carried out on clinical MR scanners using specially developed radio frequency (RF)-coils (Martos and Petersen, 1993, Smith et al., 1993, Kennedy et al., 1995). We addressed this issue by studying the feasibility of imaging the rat brain using a clinical MR-scanner and clinically available RF-coils. Special considerations have to be directed towards the design of the sequences in term of resolution on the one hand, and signal to noise ratio (SNR) on the other hand. The spatial resolution in MRI is dependent of the sample volume that is acquired. This so called voxel size can be altered, either by reducing the field of view (x–y plane) or the slice thickness (z plane). Reducing the field of view while keeping the same matrix and using thin slice thickness is required to avoid too much volume-averaging. This is done at the expense of the signal-to-noise ratio, which increases linearly with the voxel size. Reduction of the voxel size results in diminished signal and therefore reduces the signal-to-noise ratio (Smith et al., 1993). Signal averaging of multiple excitations provides an increase in signal-to-noise ratio proportional to the square root of the number of excitations which, however, leads to increased measurement time.
The goal of the present study was to obtain MR images with good resolution in a consistent and repetitive way using a clinical whole body MR scanner and a commercially available RF-coil. In order to image the rat brain in the clinical scanner we designed an installation that allows to put the rat in the MR-scanner and to fix the rat in order to avoid movement artifacts. The present study shows that accurate imaging in the rat brain can be performed on clinically available material. Moreover we describe the potential of a clinical MR-scanner to detect changes in the brain after excitotoxic lesions and neural transplantation.
Section snippets
MR-imaging and coil
MR-scannings were performed on a Siemens Magnetom Vision at 1.5 T (Siemens Erlangen, Germany) with a field gradient strength of 25 mT/M using a flexible surface coil. Rats were anesthetized (Nembutal, 40 mg/kg i.p.) and placed into the polyvinyl chloride (PVC) rat holder. The rat head was fixed with two ear bars screwed on the sliding head-rest (Fig. 1A). The fixation of the head avoids image artifacts due to bulk movement. The tail was accessible for intravenous contrast agent injections
Anatomy
T2-weighted images typically showed high contrast between gray matter and highly myelinated white matter which appeared black and structures such as the corpus callosum or the arbor vitae of the cerebellum could be differentiated. The cerebrospinal fluid filled spaces are seen as high intensity areas (Fig. 2). It was possible to discern anatomical structures within the rat brain. The transverse plane at the level of the eyes depicted the neocortex, the caudate putamen and the cerebellar
Discussion
The present study aimed at investigating the rat brain by means of a clinical MR-scanner. MRI reveals to be a powerful approach in the study of brain pathologies in small laboratory animals. In order to demonstrate the resolution capacity of a clinical MR-scanner in the study of the rat brain, the excitotoxic and neurotransplantation rat models for Parkinson's and Huntington's disease were used.
MR imaging of small animal brains is technically challenging. The quality of the resulting image is
Conclusions
In summary, high resolution rat brain imaging has been performed on a clinical magnetic resonance scanner and a clinical transmitter–receiver coil demonstrating that clinical whole body MR scanners are suitable for in vivo study of small animals. This technique was found applicable in the study of neural excitotoxicity and neural grafting and may well be expanded to other experimental models in the field of neuroscience research.
Acknowledgements
Tanja Bosnjak, Beatrice Bühler, Sandra Krebs and Benoı̂t Schaller are gratefully acknowledged. The study was supported by the Swiss National Science Foundation (Grant No. 31-52947.97).
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