Accuracy in pharmacokinetic modelling of DCE-MRI data can be impaired due to a delay between the contrast agent arrival in the tissue of interest and an artery further upstream. To correct the delay, bolus arrival times (BATs) are estimated from the concentration curves. However, the state-of-the-art method for estimating BATs may give unsatisfactory results if the curves do not exhibit a fast up-slope. We propose a spline-based method for BAT estimation for concentration curves without fast up-slopes which are often observed in small animal data. The proposed method gives accurate results on simulated and in vivo acquired rat data.