Arterial spin labeling (ASL) provides quantitative tissue perfusion information for basic and clinical research ^{1, 2}. Models linking ASL signal changes to quantitative perfusion include several other parameters that must be measured or assumed. One such parameter is blood T1, used to correct for signal decay between labeling and imaging. In ASL quantification blood T1 is typically assumed using either a fixed value³ or a modeled value based on age and sex⁴. An alternative strategy is to calculate blood T1 based on measured hematocrit³ since blood T1 depends on hematocrit⁵. This may be critically important, for studies in patients with anemia. Here we evaluated these approaches to blood T1 used to model ASL CBF measurements in a cohort of children with chronic kidney disease (CKD) and controls. 61 patients with CKD (defined as estimated glomerular filtration rate, eGFR <90 ml/min/1.73m² using modified Schwartz formula, on dialysis, and post-transplant) and 47 age-matched control subjects were included in this analysis (Table 1). A pCASL labeling scheme was implemented with 2D GE EPI sequence on a Siemens 3T scanner (Verio) using a 32-channel head coil. The labeling and control RF duration was 1.5 sec with post-labeling delay of 1.2 sec. Multi-slice perfusion maps were acquired with the following parameters: TR/TE = 4000/17 ms, flip angle=900, bandwidth = 1532 Hz/pixel, slice thickness = 4mm with 25% distance factor, matrix size = 64×64, FOV = 240×240 mm², slice number = 20, GRAPPA factor = 2 in Ky, and 40 label/control pairs. Quantitative CBF values were calculated by using a standard one-compartment perfusion model⁶. Blood T1 values were implemented using three different approaches: (1) a fixed value of 1664 msec for all subjects, (2) Hct-based estimation using the equation derived by Lu et al.: T1=1/(0.52*Hct+0.38)³ based on a Hct measurement performed at the time of the MRI scan, and (3) age+sex based estimation according to the method derived by Wu et al. (T1=2115.6-21.5*age-73.3*sex, where sex=1 for males and 0 for females)⁴.Table 2 shows the results of calculated T1 and CBF values for each subgroup using different T1 methods. By using paired t-test, blood T1 based on both Hct and age+sex showed significant larger T1 values than the assumed value (P<0.05), except in control males where the fixed value of T1 was very close to the result using Hct correction (P=0.756). In controls, age+sex based estimation overestimated T1 for both males and females as compared with Hct method (Table 3, P=0.0002 and P <0.00005 for males and females, respectively). In the CKD group, age+sex based T1 estimation was closer to HCT-corrected T1, but T1 was still overestimated (Table 3, P=0.86 and P=0.04 for males and females, respectively). Use of a fixed T1 value without considering the effect of Hct produced strong group differences in CBF between CKD and controls (P=0.007, 0.048 and 0.0009 for GM, WM and global CBF, respectively, Figure 1), and Hct based corrected T1 still showed significant group differences in GM and global CBF (P=0.010 and 0.012, respectively, Figure 1) while age+sex estimated T1 did not yield any significant group differences (P=0.130, 0.669, 0.168 for GM, WM and global CBF, respectively, Figure 1). In controls, females showed higher CBF values as compared with males using all of the T1 methods, though significant differences using fixed T1 (P=0.02) were reduced in HCT based corrected T1 method (P=0.11). Sex differences in CBF are not present in CKD using any of the models (P>0.05).We observed significant changes in blood T1 depending on the approach used, leading to different results for both sex and group differences in CBF. Sex differences of CBF in controls using fixed T1 were reduced with Hct based T1 correction, suggesting that at least some of these effects are mediated by Hct. Age+sex based T1 estimation eliminated group differences in CBF expected due to known rheological effects of anemia⁷. The absence of sex difference in CBF in patients with CKD using any of the models is consistent with delayed sexual differentiation that is known to occur in children with CKD⁸. These results highlight the importance of blood T1 in ASL CBF quantification and suggest that Hct-based T1 may be the optimal approach if Hct can be measured at the time of the scan.