What are the MRI signal characteristics of extracellular methemoglobin in a subacute intracerebral hemorrhage, including the timeframe, red blood cell status, and T1 and T2 signal intensities?

Extracellular methemoglobin is typically seen 10-21 days after hemorrhage, when red blood cells have lysed. On MRI, it appears bright on both T1 and T2-weighted images. Key diagnostic features include the hyperintense signal on T1 due to the paramagnetic effects of methemoglobin, which can help differentiate it from other stages of hemorrhage. A common pitfall is confusing it with other causes of T1 hyperintensity, such as fat or proteinaceous fluid, but the clinical context and evolution of signal characteristics over time can aid in differentiation.