Optimization of retrospective respiratory-gated high speed micro-CT for free-breathing mice

Technological advances in micro-CT scanners have introduced dynamic, flat-panel scanners, which allow the acquisition of volume images in a few seconds. However, motion artefacts associated with normal respiratory motion arise when imaging the thorax or abdomen. To reduce these artefacts, and the accompanying loss of spatial resolution, and to enable the study of rodent respiratory function, we developed a retrospective respiratory gating technique for volume micro-CT imaging of free-breathing rodents. Anaesthetized male C57BL6 mice were placed in the prone position on a custom-made bed containing an embedded pressure chamber that was connected to a pressure transducer. Inhalation motion caused an increase in the chamber pressure, which was monitored as a surrogate for the respiratory waveform, and measured throughout the scan. Projection images of the mouse thorax were acquired using a GE Locus Ultra micro-CT scanner, at 80 kVp, 50 mA (entrance exposure of approximately 2.7 cGy per rotation), over ten rotations in less than 1 minute. Respiratory gating was performed retrospectively by selecting projections that were obtained during the same portion of the respiratory cycle prior to reconstruction; CT images reconstructed from three to ten rotations were evaluated. The nominal voxel spacing was 0.15 mm. isotropic. Images were assessed for image noise, artefacts and measurement accuracy of physiologically relevant structures. These measurements showed no significant differences for images reconstructed from projection images from five to ten rotations. The optimum number of rotations for imaging mouse lungs was found to be six, corresponding to a 3 0 second (16.2 cGy) scan.