3.1 Normal MRI anatomy of the Brain

The human adult brain is one of the largest organs of the body and can be divided into four main principal parts: brain stem, diencephalon, cerebrum and cerebellum.  The brain stem consists of the medulla oblongata, pons and midbrain and is continuous with the spinal cord.  Above the brainstem is the diencephalon, consisting primarily of the thalamus and hypothalamus. 

Diagram 1 Sagittal section of the brain and spinal cord (adapted from Snell, 1995).

 

The cerebrum occupies most of the cranium and spreads over the diencephalon.  The cerebrum is made of two halves the right and left cerebral hemispheres, which are further, divided into lobes: frontal, parietal, temporal and occipital.  The cover of each hemisphere is called the cortex and is composed of gray matter, while the underlying myelinated axons are composed of white matter.  Inferior to the to the cerebrum and posterior to the to the brain stem is the cerebellum (Snell, 1995; Tortora and Grabowski, 1993).  Grey and white matter contrast, and contrast within the different white matter regions is far greater on MRI than any other imaging modality.  A major contribution to this contrast is myelin density, which is by far greater in white matter.  Also, the water content of gray matter is greater than that in white matter.

Due to the lower water content of white matter, it will have a very short T1 and T2 relaxation times than that of gray matter.  On T1-weighted images gray matter will have an intermediate density (gray) and darker than white matter.  On T2-weighted images the signal intensities tend to be reversed i.e. the white matter appears darker than the gray matter (Woodward, 2001).

The cranial bones and the cranial meninges protect the brain where the outer layer is the dura mater, the middle arachnoid and the inner is the pia mater.  Bone, such as the cranial bones, will appear dark on T1 and T2-weighted images of the brain.  However, beam-hardening artefacts of CT when imaging through the petrous region are not present in MRI.  Calcification within the brain, as a calcified choroid plexus or basal ganglia will also appear dark on T1 and T2-weighted images (Woodward, 2001). 

The two cerebral hemispheres are separated by an extension of the dura mater referred to as the falx cerebri.  The falx cerebelli and the tentorium cerebelli are also an extension of the dura mater where the former separates the cerebellar hemisphere and the latter separates the cerebrum from the cerebellum.  The brain is nourished and protected against chemical or physical injury by cerebrospinal fluid (CSF), which circulates through the subarachnoid space and the four ventricles within the brain. 

 Figure 1 Mid-Sagittal T1-Weighted MR image

1=rostrum of corpus Callosum, 2=genu of Corpus Callosum, 3=body, 4=splenium, 5=cingulategyrus, 6=gyrus rectus, 7=precentral gyrus, 8=postcentral gyrus, 9=frontal lobe, 10=occipital lobe, 11=lateral ventricle, 12=Fornix, 13=thalamus, 14=third ventricle, 15=mammillary body, 16=cerebellar peduncle, 17=pons, 18=medulla, 19=vermis of cerebellum,  20=fourth ventricle, 21=parietal lobe, 22=tentorium cerebelli

Figure 2 Proton Density MR image showing the basal ganglia.

1=gray matter, 2=white matter, C=caudate nucleus, P=putamen, G=globus pallidus, I=internal capsule, T=thalamus.