What are the MRI signal characteristics of intracellular methemoglobin in a subacute hemorrhage, and how does this correlate with the time frame and status of red blood cells?

Intracellular methemoglobin appears in subacute hemorrhage, typically 2-14 days after the bleed. During this phase, red blood cells remain intact, and MRI shows T1 hyperintensity (bright) and T2 hypointensity (dark). Key diagnostic features include the characteristic 'bright on T1, dark on T2' appearance, which helps differentiate it from other stages of hemorrhage. A common pitfall is confusing this with other causes of T1 hyperintensity, such as fat or proteinaceous material.