Kidney oxygenation can be measured with time-resolved renal BOLD MRI where changes in renal oxygenation are induced by a functional challenge (1–5) (e.g., by drinking water) which is detected as an R2* change in the renal tissue. Renal R2* measurements are usually performed within individual, subsequent breath holds before, during and after the challenge. Because of inconsistencies in the kidney locations from one measurement to another, motion compensation is required. Ideally, displacements would be corrected prospectively to prescribe the correct slice positions for each measurement. Here, a new method, Kidney ALIgnment for BOLD Renal Imaging (KALIBRI), is presented for prospective 3D motion correction of the kidneys.For prospective motion correction, KALIBRI acquires a 3D VIBE data set before the acquisition of a 2D multi-echo gradient echo (mGRE) sequence for BOLD imaging. Both data sets (3D VIBE, 2D BOLD) are measured in a single breath hold. Data are acquired repeatedly, and the 3D VIBE data from the very first breath hold serves as a reference data set. VIBE image data are transferred automatically to an external PC where an ITK-based (6) rigid image registration (RIGR) of the current to the reference VIBE data set is done. To save computation time, the registration is performed only within pre-defined regions for each kidney. Position updates for both kidneys are then sent to the MR system to realign the imaging slices of the subsequent 2D mGRE acquisition (Fig. 1). The sequence employs a vendor-specific feedback mechanism and is executed once the slice update is received. The whole procedure is designed to be completed within a single breath hold of about 20 seconds. To remove residual in-plane displacements and deformations, the dynamic 2D BOLD images are retrospectively registered to the reference BOLD images using translational and non-rigid image registration using MIRT (7). The KALIBRI method was implemented on a 3T whole body system (Siemens PRISMA, Erlangen, Germany) using the following imaging parameters: 3D VIBE: TR = 3.34 ms, TE = 1.19 ms, a = 10°, matrix: 256x174, voxel size: 1.6x1.6x4.0 mm³, partitions: 22, GRAPPA with 24 reference lines, TA = 3.4 s; 2D BOLD: 2 coronal slices, TR = 35 ms, TE₁ = 2.42 ms, ΔTE = 2.66 ms, 12 echoes, matrix: 192x192, voxel size: 2.2x2.2x5.0 mm³, a = 25°, GRAPPA with 24 reference lines, bandwidth = 810 Hz/Px, TA = 7.6 s. Time-resolved renal BOLD MRI was performed in 6 healthy volunteers who had no food or water at least 5 h before the exam. After 4-7 baseline R2* measurements, the volunteers drank 1000 ml tap water in the magnet, and another 25-40 BOLD data sets were collected over about 50 min. The KALIBRI method was evaluated by calculation of the Mutual Information (MI) (8,9), the spatial overlap between two segmentations as Dice Coefficient (DC) (10) as well as the Standard Deviation (SD) of each kidney before and after registration.The total acquisition time including motion correction was 17 s. Prospectively corrected images showed better alignment of internal renal structures than data without correction: improvements are illustrated in Fig. 2 as difference images between the reference data and the images acquired without registration, with prospective and retrospective registration, respectively. On average, MI values improved after the prospective registration by up to 25%; and an additional 10% after retrospective correction. Prospective registration alone did not change DC and SD, but the combined registrations led to an improvement of up to 4% for DC and 10% for SD. Results of the R2* analysis of time-resolved BOLD MRI using small ROIs in medullar and cortical regions are summarized in Fig. 3. The average R2* baseline value was about 30.6 ± 4.3 s^-1 in medullar regions and 17.7 ± 1.5 s^-1 in the cortex. In 4 out of 6 volunteers, the medullar R2* values showed a decrease during water challenge of up to 40%, and a recovery afterwards. For all volunteers, the cortical values did not change.The proposed method KALIBRI for motion correction showed to improve the quality of time-resolved renal BOLD MRI, since local regions in the medulla and the cortex are better aligned and thus can be compared consistently over many breath holds. We demonstrated its functionality with in vivo experiments during a water challenge performing also a semi-automatic full time-resolved analysis. The KALIBRI method could also be applied to other serial measurements in the kidney, e.g. for MR-guided radiotherapy, for longitudinal studies, and, after adaptation, for other abdominal organs.