In Lee’s study of 351 patients with acute subdural hematoma SDH, the subdural hematoma appeared hyperdense in 98.6 % of cases, isodense in 1.1 %, and hypodense in 0.3 % of cases 1).

In a small proportion of cases, an acute SDH may appear isodense or even hypodense compared with the adjacent parenchyma. This situation is encountered in cases of anemia, disseminated intravascular coagulation, or if the hematoma is diluted with cerebrospinal fluid 2).

A hyperdense subdural component was present in all acute subdural hematomas in anemic patients. Therefore, anemia alone is not a sufficient explanation for a homogenous low-density acute subdural hematoma 3)

1)
Lee KS, Bae WK, Bae HG, Doh JW, Yun IG. The computed tomographic attenuation and the age of subdural hematomas. J Korean Med Sci. 1997 Aug;12(4):353-9. doi: 10.3346/jkms.1997.12.4.353. PMID: 9288636; PMCID: PMC3054217.
2)
Deb S, Bhaumik S, Pal H. Isodense acute subdural haematoma in anaemic patients. Neurol India. 2000 Sep;48(3):298-9. PMID: 11184455.
3)
Duy L, Badeeb A, Duy W, Alqahtani E, Champion W, Kim DH, Martin D, Vartanians V, Coffin P, Small JE. CT Attenuation of Acute Subdural Hematomas in Patients with Anemia. J Neuroimaging. 2019 Jul;29(4):536-539. doi: 10.1111/jon.12608. Epub 2019 Feb 16. PMID: 30771278.