Lung nodule volumetry is used to diagnose the likelihood of malignancy in nodules detected during thoracic CT scans. These measurements are unreliable when the patient is subject to respiratory motion. We seek to understand the relationship between reconstructed images and the actual size of nodules subject to motion induced by quiet breathing. CT images of solid spheres of varying size and composition were acquired while travelling through a known path to approximate the motion of a pulmonary nodule during respiration. The measured size of the sphere’s image was found to increase non-linearly with speed. However, these relationships were dependent on the CT number of the sphere and the reconstruction filter used to generate the image. From these results we expect that for a specific CT number we can estimate the size of an object from a CT image if the speed of the object at the time of the scan is known.

Lung nodule volumetry is used to diagnose the likelihood of malignancy in nodules detected during thoracic CT scans. These measurements are unreliable when the patient is subject to respiratory motion. We seek to understand the relationship between reconstructed images and the actual size of nodules subject to motion induced by quiet breathing. CT images of solid spheres of varying size and composition were acquired while travelling through a known path to approximate the motion of a pulmonary nodule during respiration. The measured size of the sphere’s image was found to increase non-linearly with speed. However, these relationships were dependent on the CT number of the sphere and the reconstruction filter used to generate the image. From these results we expect that for a specific CT number we can estimate the size of an object from a CT image if the speed of the object at the time of the scan is known.