A.
ANATOMY / FUNCTION
Basal ganglia refer to the paired deep grey matter nuclei located,
consisting of:
- caudate nucleus
- putamen
- globus pallidus
- nucleus accumbens
- olfactory tubercle
From a functional standpoint,
many authors consider the substantia nigra and subthalamic nucleus as part of the basal ganglia.
The caudate nucleus,
putamen and globus pallidus compose the corpus striatum,
while the putamen along with the globus pallidus form the lentiform nucleus.
The basal ganglia are widely connected with the cerebral cortex,
thalami and brainstem and constitute the arena of numerous pathways.
Hence,
they are responsible for a vast range of vital functions,
especially movement,
but also memory and state of consciousness.
Subsequently,
when basal ganglia are injured or dysfunction,
the clinical features vary from movement disorders (e.g.
chorea,
tremor,
bradykinesia,
dystonia) and/or neurobehavioral disorders (e.g.
apathy,
apraxia,
hyperactivity,
psychiatric syndromes) to coma.
Close attention needs to be paid to the vascular supply of the deep grey matter nuclei,
as ischemia is a typical cause of basal ganglia and thalami abnormalities.
Arterial supply of basal ganglia is achieved chiefly by perforating branches of the anterior cerebral artery and middle cerebral artery (medial lenticulostriate arteries and lateral lenticulostriate arteries respectively) while the thalamus is mainly supplied by small perforating arteries arising from the posterior cerebral artery and posterior communicating artery.
Both the basal ganglia and the thalamus drain into the deep venous system (internal cerebral veins).
B.
NORMAL IMAGING FINDINGS
Computer Tomography (CT)
The caudate nucleus and putamen can be easily distinguished in a normal CT scan image.
Loss of this distinction should raise suspicion of pathology (e.g.
infarction).
Distinction between the globus pallidus and putamen is more subtle,
with the former being slightly hypodense.
Magnetic Resonance Imaging (MRI)
The anatomy of the deep grey matter structures is best visualised on PD or T2-weighted images.
They follow the signal intensity of the cerebral cortex,
while compared to the white matter,
they appear slightly hyperintense on T2-weighted images.
The globus pallidus is relatively T1 hyperintense compared to the putamen due to its larger myelin component,
while slightly more hypointense than the adjacent putamen on T2WI,
GRE,
and SWI images,
a normal feature attributable to higher iron deposition.
Substantia nigra (pars reticulata),
as well as red nuclei,
also demonstrate low signal intensity on T2 weighted sequences due to higher iron concentration.
Age-related iron deposition in the putamen results in its gradual decrease of signal intensity on T2-weighted images.
Dilated Virchow-Robin (perivascular) spaces often appear in the lentiform nuclei,
located typically around the lenticulostriate arteries.
This finding is characterised as normal,
more pronounced when aging and observed unilaterally.
The signal intensity of these foci follows that of the cerebrospinal fluid on all MR sequences.
Calcium deposition related to aging,
commonly in the globus pallidus,
initially results in increased T1 signal intensity followed by signal loss on all sequences at advanced stages.
Basal ganglia calcification is relatively common in older patients,
considered as an incidental and idiopathic finding,
although in patients younger than 40,
further workup is required to exclude other causes (Table 1).
C.
ABNORMALITIES OF THE BASAL GANGLIA AND THALAMI
Being highly metabolically active,
the deep grey matter nuclei tend to be affected by different metabolic,
inflammatory,
degenerative and vascular diseases (Table 2).
Diagnostic workup includes serum and CSF examination,
as well as electroencephalography,
while clinical history provides crucial input for further investigation.
Neuroimaging with MRI as the key modality is useful for evaluating brain damage and restricting the differential diagnosis process.
A helpful tool can be the classification of the imaging findings based on the extent to which basal ganglia are affected or possible involvement of the cerebral cortex,
white matter or brainstem.