Micro-CT, a technique for imaging small objects at high resolution using micro focused x-rays, is becoming widely available for small animal imaging. With the growing number of mouse models of pulmonary pathology, there is great interest in following disease progression and evaluating the alteration in longitudinal studies. Along with the high resolution associated with micro CT comes increased scanning times, and hence minimization of motion artifacts is required. We propose a new technique for imaging mouse lungs in vivo by inducing an intermittent iso-pressure breath hold (IIBH) with a fixed level of positive airway pressure during image acquisition, to decrease motion artifacts and increase image resolution and quality.
Mechanical ventilation of the respiratory system for such a setup consists of three phases, 1) tidal breathing (hyperventilated), 2) a breath hold during a fixed level of applied positive airway pressure, 3) periodic deep sighs. Image acquisition is triggered over the stable segment of the IIBH period.
Comparison of images acquired from the same mouse lung using three imaging techniques (normal breathing / no gating, normal breathing with gating at End Inspiration (EI) and finally the IIBH technique) demonstrated substantial improvements in resolution and quality when using the IIBH gating. Using IIBH triggering the total image acquisition time increased from 15 minutes to 35 minutes, although total x-ray exposure time and hence animal dosage remains the same. This technique is an important step in providing high quality lung imaging of the mouse in vivo, and will provide a good foundation for future longitudinal studies.
Regional lung ventilation can be measured via Xenon-enhanced computed tomography (Xe-CT) by determining washin (WI) and washout (WO) rates of stable Xe. It has been assumed that WI = WO, ignoring Xe solubility in blood and tissue and then other geometric isssues. We test this by measuring WO-WI in lung by Xe-CT. Also, we investigate the effect of tidal volume (TV) and end inspiratory (EI) vs end expiratory (EE) scan gating on WO and WI measurements. 3 anesthetized, supine sheep were scanned using multidetector-row computed tomography (MDCT). Imaging was gated to both EE and EI during a WI (33 breaths) and WO (20 breaths) maneuver using 55% Xe for WI and room air for WO. Time constants (TCs) of Xe WI and WO were obtained by exponential fitting. WO and WI TCs were compared: 1) apex and base 2) dependent, middle, and nondependent 3) EE and EI 4) three TVs. The vertical gradient of WO-WI showed WO > WI in dependent vs non-dependent regions. WO-WI in both dependent and nondependent region at the lung base and apex was larger when measured at EE compared to EI. As TV increases, the global WO-WI difference decreased. TV showed greater influence on WO than WI. Xe WO was longer than WI possibly reflecting Xe solubility in blood and tissue. Higher TVs and gating to EE provided greater effects on WO than WI TCs which may relate to the number of partial volumed conducting airways contributing to the regional voxel-based measures. We conclude that WO mode is more susceptible to errors caused by either xenon solubility or tidal volume than WI mode and EE scanning may more accurately reflect alveolar ventilation.