Renal transplantation is a common and effective procedure for children with chronic kidney disease. In order to assess whether the procedure has been successful, MRI scanning is commonly performed using techniques such as diffusion-weighted imaging and R2* mapping [1]. A key parameter to assess functional viability of the transplanted kidney is renal perfusion, which can be measured using dynamic contrast enhanced (DCE) MRI [2]. However, safety concerns associated with the use of paramagnetic contrast agents limit the clinical applicability of DCE-MRI, particularly in a paediatric patient population with renal deficiencies. Arterial spin labelling offers a contrast-free alternative for the quantitative measurement of renal perfusion using MRI [3].To demonstrate the capability and value of ASL as a non-invasive fully quantitative technique for following the evolution of renal blood flow over one year in a longitudinal study of children who have undergone renal transplantation.Twenty children (age: median 13; range 9-17) were recruited for a one year longitudinal study of renal blood flow following renal replacement therapy. Each subject was scanned on 3 occasions after kidney transplantation (Table 1) at time points: (A) immediately post-transplant; (B) "1 month" post-transplant and (C) "1 year" post-transplant (variability of scanning timings due to patient availability, scanner scheduling, etc.). Three children had to be excluded as the first scan was undertaken more than one month following the transplant. One child was excluded as a block transplant of two kidneys was used. Four children received a cadaveric kidney while the remaining 12 received a kidney from a living donor. The donors were aged 27-51 years (median 42). At one year follow up all the children were alive and well with a normal serum creatinine. All children were scanned in a 1.5T Siemens Avanto scanner. Coronal-oblique ASL data volumes were obtained using a segmented FAIR 3D-GRASE acquisition scheme with background suppression and respiratory triggering [4]. One control/tag pair was sampled at fourteen equally spaced inflow times (TIs) with shortest TI of 100ms and increments of 200ms for a nominal scan time of 4m12s. The matrix size was 128x104x12 with voxel size 3.1x3.1x5.0mm, providing whole kidney coverage. Segmentation was performed along the first phase encoding direction (3 shots) and Partial Fourier (factor 3/4) was applied along the second phase direction (partition). Quantification was performed off-line using home-written MATLAB scripts. The ASL data were fitted to the general kinetic model [5] in a voxel wise manner to yield quantitative renal blood flow (RBF), bolus width, and arterial transit time maps. A $$$T_1$$$ value of 966ms was assumed for the renal cortex [6] and a single $$$M_0$$$ was chosen as the median $$$M_0$$$ value in whole-kidney regions of interest, obtained from a separate reference scan using the same readout as the ASL scan but without background suppression.To assess longitudinal changes in renal perfusion, cortical RBF (in ml/100g/min) of the first post-transplant scan were compared to the second scan and to the final scan. In 14 of the 16 children, the highest blood flow was found in the initial post-transplant scan compared to the second ("1 month") scan (Fig 1 a)). The differences in RBF between scan A and B were statistically significant (two-tailed paired t-test, p<0.01). When comparing the first scan to the "1 year" post-transplant scan, RBF was also significantly different (two-tailed paired t-test, p<0.01). Four children showed no change in RBF (<20ml/100g/min); 11 children had a higher RBF on the first scan and only one child had a higher RBF on the 1 year scan (table 2 and Fig 1 b)). The highest renal cortical blood flow was seen on the first scan in the majority of children. This could be due to the fact that all these children received an adult kidney so that the first scan was obtained when the "steal" phenomenon of blood by the donor kidney was present, while in later scans equilibrium between child and kidney had been reached (no significant in RBF between scans B and C, two-tailed paired t-test, p=0.92). In addition the anti-rejection drug regime may also have contributed to the reduction in blood flow after the first month post-transplantation. When comparing the anatomical images (coronal) of the first to the final scan, the anatomy of the kidney was noted to have changed despite the identical positioning of the two scans (Fig 2).In this study, we non-invasively evaluated renal perfusion changes in children within the first year post-transplant using ASL. This technique has provided unique data that has added valuable information in a complex clinical situation.