We investigated the reliability of different T1 and T2 relaxometry methods for arterial and venous blood. While TRIR enables measurements of both venous blood T1 and T2, T2 estimates from TRIR showed poorer repeatability compared to TRUST. Moreover, significantly higher venous blood T2 values were observed using TRIR. Lastly, arterial blood T1 measurements showed a better repeatability compared to venous T1 measurements using TRIR. These findings advocate for the use of the arterial T1 measurements instead of venous T1 and the use of TRUST to measure venous blood T2.