Effects of breathing and cardiac motion on spatial resolution in the microscopic imaging of rodents

Abstract

One can acquire high-resolution pulmonary and cardiac images in live rodents with MR microscopy by synchronizing the image acquisition to the breathing cycle across multiple breaths, and gating to the cardiac cycle. The precision with which one can synchronize image acquisition to the motion defines the ultimate resolution limit that can be attained in such studies. The present work was performed to evaluate how reliably the pulmonary and cardiac structures return to the same position from breath to breath and beat to beat across the prolonged period required for MR microscopy. Radiopaque beads were surgically glued to the abdominal surface of the diaphragm and on the cardiac ventricles of anesthetized, mechanically ventilated rats. We evaluated the range of motion for the beads (relative to a reference vertebral bead) using digital microradiography with two specific biological gating methods: 1) ventilation synchronous acquisition, and 2) both ventilation synchronous and cardiac-gated acquisitions. The standard deviation (SD) of the displacement was < or =100 microm, which is comparable to the resolution limit for in vivo MRI imposed by signal-to-noise ratio (SNR) constraints. With careful control of motion, its impact on resolution can be limited. This work provides the first quantitative measure of the motion-imposed resolution limits for in vivo imaging.