Synthetic Haematocrit Formula at 3T Field Strength for the Assessment of Extracellular Volume on Cardiac MRI

Abstract

The calculation of extracellular volume (ECV) on cardiac MRI relies on T1 relaxation times of the myocardium and cardiac blood pool both pre- and post-contrast administration, as well as measured blood haematocrit levels (ECV=1 – haematocrit x (1/T1myo post – 1/T1myo pre)/(1/T1blood post/1/T1blood pre). The formula to estimate a synthetic haematocrit (synHCT) value is (synHCT=866 x 1/T1blood pre - 0.1232). This formula was validated at 1.5 Tesla (1.5T) field strength, and while it is considered applicable to 3T there is limited data on its accuracy. To evaluate the performance of the synthetic haematocrit (synHCT) formula at 3T field strength in comparison to measured haematocrit (mHCT) when calculating ECV. 40 patients underwent pre- and post-contrast T1 mapping on a Siemens 3T MAGNETOM Skyra machine, with a blood sample drawn immediately prior. Blood pool native T1 values were obtained using an automated blood pool mask (Siemens). There was a linear relationship between mHCT and synHCT with modest correlation (r=0.672). However, there was a significant difference between mean mHCT and synHCT (0.417 vs 0.367 L/L, p<0.00001). Synthetic ECV (synECV) had reasonable correlation with measured ECV (mECV) (r=0.895), although there was still a significant difference between mean mECV and synECV (23.0 vs 25.1%, p<0.00001). The synthetic haematocrit 1.5T formula predicts actual HCT with limited accuracy at 3T. Consequently, the derived synthetic ECV measurements were significantly higher than measured ECV. Further research is needed to derive a more accurate synthetic haematocrit formula for 3 Tesla field strength.