MRI of thermally denatured blood: Methemoglobin formation and relaxation effects

Focal regions of T-1-shortening have been observed in magnetic resonance im aging (MRI)-monitored thermal ablations of perfused tissues. The aims of th is study were two-fold: to find evidence for heat-induced conversion of hem oglobin (Hb) to methemoglobin (mHb), and to investigate the effects of heat treatment of in-vitro blood components upon their MR relaxation times. Spe ctrophotometric studies were performed to confirm the heat-induced formatio n of methemoglobin. Preparations of whole and fractionated blood, previousl y submitted to elevated temperatures of 40 degrees C to 80 degrees C, were imaged and the relaxation times were calculated. Optical absorption spectra of samples containing free Hb, heated to 60 degrees C, showed increased li ght absorption at 630 nm, evident of mHb presence, Short T-1 values in whol e blood (1.13 s) and packed red blood cell (0.65 s) compartments, heated at 60 degrees C, compared to their baseline values (1.62 s and 0.83 s, respec tively), were attributed to mHb formation. In relation to MRI-guided therma l interventions, these results suggest a possible explanation for observati on of hyperintense regions on T-1-weighted images. (C) 1999 Elsevier Scienc e Inc.