Prospective MR image alignment between breath‐holds: Application to renal BOLD MRI

Purpose To present an image registration method for renal blood oxygen level–dependent (BOLD) measurements that enables semiautomatic assessment of parenchymal and medullary R2* changes under a functional challenge. Methods In a series of breath‐hold acquisitions, three‐dimensional data were acquired initially for prospective image registration of subsequent BOLD measurements. An algorithm for kidney alignment for BOLD renal imaging (KALIBRI) was implemented to detect the positions of the left and right kidney so that the kidneys were acquired in the subsequent BOLD measurement at consistent anatomical locations. Residual in‐plane distortions were corrected retrospectively so that semiautomatic dynamic R2* measurements of the renal cortex and medulla become feasible. KALIBRI was tested in six healthy volunteers during a series of BOLD experiments, which included a 600‐ to 1000‐mL water challenge. Results Prospective image registration and BOLD imaging of each kidney was achieved within a total measurement time of about 17 s, enabling its execution within a single breath‐hold. KALIBRI improved the registration by up to 35% as found with mutual information measures. In four volunteers, a medullary R2* decrease of up to 40% was observed after water ingestion. Conclusion KALIBRI improves the quality of two‐dimensional time‐resolved renal BOLD MRI by aligning local renal anatomy, which allows for consistent R2* measurements over many breath‐holds. Magn Reson Med 77:1573–1582, 2017. © 2016 International Society for Magnetic Resonance in Medicine