There are many causes of basal ganglia T1 hyperintensity, but the majority relate to deposition of T1-shortening elements within the basal ganglia such as:
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calcium
idiopathic calcification
calcium and phosphate abnormalities
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hepatic dysfunction or bypass (specifically affecting globi pallidi)
hereditary hemorrhagic telangiectasia, often with hepatic arteriovenous malformations 5
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toxins/ischemia
carbon monoxide (usually low T1 signal; unless associated with hemorrhage)
hyperalimentation or long term parenteral nutrition (manganese)
hyperglycemia-associated choreaballism 2: non-ketotic hyperglycemic hemichorea (NHH)
previous administration of linear gadolinium chelates 3,4
global hypoxia
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blood
methemoglobin in intracranial hemorrhage
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others