Purpose: Accurate longitudinal relaxation time (T₁) of arterial blood is important in evaluating blood flow in tissue by arterial spin labeling magnetic resonance (MR) imaging. Few studies have reported the T₁ of human arterial blood in vivo, especially using 3-tesla MR imaging. T₁ values of human venous blood in vivo have been reported, but they differ from those measured in vitro. We aimed to evaluate the accurate T₁ of human arterial blood in vivo.
Methods: We measured T₁ values of blood in 10 healthy volunteers in vivo using an inversion-recovery fast gradient-echo sequence and 3-tesla MR imaging unit. We also measured hematocrit (Hct) values of venous blood samples. After nonselective application of the inversion pulse using a body coil, we obtained MR imaging signals of arterial blood in the abdominal aorta. Similarly, we measured the signals of venous blood in the internal jugular vein. Inversion times varied between 200 and 5000 ms for imaging of the abdominal aorta and 200 and 2500 ms for imaging of the jugular vein. We also acquired signals without the inversion pulse. We estimated T₁ values from the data by nonlinear least squares fitting of a 3-parameter model.
Results: The T₁ value (mean±standard deviation) of arterial blood was 1779±80 ms and of venous blood, 1694±77 ms. The average Hct value was 0.47. The R₁ (=1/T₁) of arterial blood was related to the Hct value as: R₁=(0.59±0.16)Hct+(0.29±0.07) (mean±standard error) s⁻¹. For the venous blood, R₁=(0.70±0.11)Hct+(0.27±0.05) s⁻¹.
Conclusion: We observed a T₁ of human arterial blood in vivo of 1779±80 ms at a mean hematocrit value of 0.47 as determined by 3T MR imaging; an even longer T₁ value is expected with a hematocrit value less than 0.47.