Noninvasive Brain Imaging in Small Animal Stroke Models: MRI and PET
Acute brain damage after stroke produces remarkable changes in the brain that can be visualized with a variety of neuroimaging techniques. Some of these techniques are used in patients for diagnostic purposes and are now available to image the rodent brai
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1. Magnetic Resonance Imaging 1.1. Introduction
Magnetic Resonance Imaging (MRI) is a widely used technique to image stroke patients for diagnostic and therapeutic purposes (1). With the accessibility of high magnetic field horizontal magnets, animal-dedicated MRI has emerged as a powerful tool to investigate the brain of living animals. MRI has great potential for studying animal models of brain ischemia as it can provide information on the progression of the brain lesion in vivo and on the magnitude of the alterations (2). Animals can be followed up in longitudinal studies and multiparametric studies can be carried out as the various MRI techniques give information on different aspects of the brain lesion (3–6). The aim of this text is to provide an overview of the MRI imaging techniques useful for studying the brain in animal models of stroke. Neuroimaging techniques have a complex theoretical and physical base, and for this reason, the readers
Ulrich Dirnagl (ed.), Rodent Models of Stroke, Neuromethods, vol. 47, DOI 10.1007/978-1-60761-750-1_11, © Springer Science+Business Media, LLC 2010
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will be directed to specialized literature for detailed technological information on physical principles (7–9) and quantification (10). 1.2. Materials 1.2.1. Instrumentation
1.2.2. Anesthesia
MRI is based on the effect that a magnetic field has on protons (mainly hydrogen nuclei of organic water molecules) as they align according to the direction of the field. Image acquisition requires that electromagnetic radiofrequency pulses are applied using gradient coils. Protons then absorb energy that is later released at a radiofrequency that can be detected in the receiver coil (9). The main requisites for MRI are instrumentation and a multidisciplinary team of specialized and well-trained personnel. Imaging rats at 1 Tesla (T), 1.5 T, 2 T, or 3 T (11–13) is possible and findings in the field of stroke research have been reported using clinical equipment. However, animal-dedicated MRI systems are preferred because they provide better spatial resolution. Also, current legislation in some countries does not allow the use of clinical equipment for animal experimentation. Most animal studies are now performed with horizontal magnets with a magnetic field of 4.7 T, 7 T, or 9.4 T. However, horizontal magnets with very high magnetic fields (11.7 T or more) are also available for animal research and results in the field of ischemia have been published, for instance, using a model of neonatal ischemia in rodents (14). Many images of interest in brain ischemia such as diffusion-weighted imaging (DWI) and T2-weigthed (T2W) imaging (see below) offer sufficiently good quality at 4.7 T and 7 T in adult rats and mice. However, a higher magnetic field is useful to increase image quality by improving the signal-to-noise ratio or to reduce acquisition times. These factors may be critical in certain MRI techniques such as diffusion tensor imaging (DTI) or functional MRI (fMRI), in which increasing the strength of the magnetic f
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