The T1 value of blood is an important parameter in several vascular imaging sequences such as arterial spin labeling to cerebral blood flow (CBF)¹, vascular space occupancy (VASO) MRI to measure cerebral blood volume (CBV)², and black-blood angiography³. At present, the standard practice is assuming a constant blood T1 across all individuals. However, blood T1 is known to have dependence on hematocrit (Hct) (e.g. due to individual variation or in anemia or polycythemia) and oxygenation^4-5 (e.g. in ischemia), and may vary with abnormal blood composition (e.g. in sickle cell disease). Here, we used a fast method to measure blood T1 in the internal carotid artery (ICA) in vivo, and verify the measurements with the ex vivo experiments from the participants’ sampled blood. The arterial T1 was measured using the fast inversion recovery technique that has been previously used at internal jugular vein (IJV)⁷, in which multiple inversion time points can be acquired rapidly due to constant refreshing of blood (Fig 1a). The main difference between the T1 measurement protocols at ICA and IJV is the position of the subjects. To acquire proper inversion recovery curves from the fast flowing arterial blood, it is crucial to invert all blood within heart, lung, aorta and common carotid before it reaches the ICA. Thus for the measurement of T1 at ICA, subjects need to be centered at the clavicle, instead of the eyes as for the protocol of measuring T1 of IJV (Figs 1b,c,d). To reduce artifacts due to the turbulence of arterial blood, the imaging plane is placed at the ICA above the bifurcation of the common carotid (Fig 1c). Fig 1e shows the acquired inversion recovery curve from the ICA (red) with the whole chest covered by the inversion, compared to the one corrupted by the fresh in-flow effect (black) from the venous protocol with only the whole brain inverted. An inversion pulse with less sensitivity to B0 and B1 inhomogeneities (a 20ms HSn inversion pulse (n=4, b=4, Dw=1250Hz, B1max=13.5mT)⁹) was employed to ensure more reliable inversion recovery curves. To validate our approach, arterial and venous T1 of 9 healthy volunteers (27-51 years old, 6 females) were measured both in vivo and ex vivo for comparison at a 3T Philips Achieva scanner (body coil for transmission and a 32-channel head coil for reception). In vivo experiments were conducted with multiple-shot TFE acquisition (FOV = 169x169mm², acquisition matrix=212x210, TFE shot=7, TFE factor=15, flip angle=50^o, TR/TE=13/4.9ms, reconstruction resolution 0.8x0.8mm², slice thickness=5mm, ~1min). The venous T1 was conducted using the protocol reported before⁷. Following the in-vivo experiments, 4mL blood samples were drawn from the volunteers and oxygenated to 96%-99% before T1 measurements. Oxygenation was again confirmed after the experiment. Traditional inversion-recovery experiments (Fig 2) were performed on these blood samples on the same day of the in vivo scans with a 2s fixed delay between acquisition and the next inversion pulse and TI = [0.2, 0.5, 1, 2, 3, 5, 9, 15]s. To prevent RBC precipitation and temperature variation, the ex vivo experiments were completed within 4 min. Fig. 3 shows linear regression and Bland-Altman plots of individual’s arterial T1 values measured in vivo comparing to the ones obtained ex vivo. Excellent correlation and agreement across different subjects are demonstrated. As expected, the arterial T1 values measured in vivo correlated significantly with Hct: 1000/T1(ms) = 0.61Hct+0.29 (P=0.006) (Fig. 4). For this group of healthy adult volunteers (Hct=0.41+/-0.04, range: 0.36-0.47), the average arterial T1 was 1864+/-92ms (range: 1720-2037ms) which is 68+/-27ms longer than the venous T1 (1796+/-84ms, range: 1685-1966ms) (Fig. 4). The measured­­ ICA T1 values compare well with a previous arterial T1 study on abdominal aorta (1779+/-80ms with Hct=0.47+/-0.03) using the traditional inversion-recovery method¹⁰ and with recent venous values in humans^6-8. The averaged arterial T1 value (1864ms with Hct=0.41) is 200ms longer than the result from the ex vivo bovine blood (1664ms with Hct=0.42)⁴. A recent theoretical model has shown that this is most likely due to increased methemoglobin in the bovine samples¹¹. It is known that CBF values estimated using ASL are negatively correlated with the arterial blood T1 values¹². For the highest arterial blood T1 value obtained in this study (2037ms, Hct: 0.36), using an 18% undervalued blood T1 (1664ms) would cause a 34% overestimation of CBF. The proposed fast method can be performed for a subject-based arterial blood T1 determination, When not performing such a determination, we recommend use of 1864ms for T1 of arterial blood.