Evaluation of methemoglobin as an autologous intravascular MRI contrast agent

Methemoglobin (MetHb) was evaluated as an intravascular paramagnetic contrast agent. Methemoglobin formation was induced by 4‐dimethylaminophenol (4‐DMAP), causing a reduction in blood T 2 * in vitro. The 4‐DMAP generated metHb with a time constant of 62 s. A 4‐DMAP bolus did not decrease measurably the signal intensity in the in vivo rabbit kidney in the first pass. At steady state, a MetHb concentration of 24.8 ± 2.3% resulted in a signal decrease of 9.2 ± 2.6% in the kidney. Methemoglobin is an effective vascular T 2 * relaxation agent, but the formation of MetHb by 4‐DMAP is too slow for first‐pass imaging. A more effective conversion agent resulting in a bolus of at least 25% MetHb within 5 s would result in a detectable first‐pass signal and a viable contrast technique.