The ability to better delineate the anatomy of the central nervous system (CNS) in everyday neurological practice using modern imaging techniques has increased with improvements in technology and its widespread adoption in hospitals throughout the world. The major methods for imaging the nervous system are discussed in Chapter 53, but in general magnetic resonance imaging (MRI) is the best way to look at anatomical structure and its capacity to do this is dependent on the strength of the magnetic field that can be generated with the scanner. Most hospitals use a 1.5 Tesla (T) machine, but increasingly 3T machines are being used and for research purposes 7T scanners have been developed for human use. While the introduction of more sophisticated MRI and computed tomography (CT) sequences has enabled us to better define the vasculature of the brain, the gold standard is still formal angiography and indeed is the only way to visualize the blood vessels in the spine if this is needed, which is rare.
MRI of the cerebral hemispheres
MRI of the cerebral hemispheres clearly reveals a large number of structures which are illustrated in Figures 65.1 to 65.5. In particular:
· The different lobes of the brain can be clearly seen although the central sulcus in the human brain lies more posteriorly than one would imagine.
· The basal ganglia structures can be seen in terms of the caudate, putamen and globus pallidum. The subthalamic nucleus and substantia nigra are harder to see, although the latter is becoming easier to recognize with newer MRI scanners.
· The thalamus and the integrity of the ventricular system.
· The major pathways running in the internal capsule and the corpus callosum.
· The visual pathways can also be clearly seen up to the optic tracts. The optic radiations cannot be seen using standard imaging paradigms. Getting clear pictures of the optic pathway can be difficult and sometimes special sequences are needed to look at it in detail if there is a high suspicion of pathology.
· Limbic system structures are much harder to see, given their location on the medial aspects of the temporal lobe. The hippocampi can usually be seen, although if volumetric loss in this structure is being sought (e.g. in cases of possible Alzheimer’s disease) then special imaging protocols should be used as it is easy to mistakenly see atrophy in this structure using standard scan sequences.
MRI of the posterior fossa
CT scans can be used to look at the gross structure of the brain, but it is unable to give much information on smaller lesions, especially within the posterior fossa. Thus MRI is the modality of choice for delineating the anatomy of the brainstem and cerebellum. MRI of the posterior fossa can reveal a number of structures (Figures 65.4–65.6):
• The major divisions of the brainstem and its connection to the cerebellum and the ventricular system as it passes through the aqueduct to the fourth ventricle.
• The different lobes and parts of the cerebellum especially the cerebellar tonsil and where it lies relative to the foramen magnum (e.g. Arnold–Chiari malformations) (see Chapter 5).
· Within the brainstem itself a number of structures can normally be seen. On occasions higher-resolution scans can be undertaken to look at specific parts of the brainstem, e.g. acoustic neuromas with high-resolution CT scans through the internal auditory meati.
MRI of the spinal cord (Figure 65.7)
This is typically used to look at the integrity of the spinal cord and the spine around it, to ensure that there is no compression of the spine by lesions extrinsic to it (e.g. disc herniations) or lesions within it, such as tumours.
Vasculature of the brain (Figure 65.8)
Dye can be injected into the circulation followed by the rapid capture of images as the dye moves through the different arterial vessels before draining through the venous system. This is the best way to pick up any vascular abnormalities such as small aneurysms or arteriovenous malformations.