From the Guidelines
Yes, apparent low lung volumes on chest CT can definitely be due to a patient exhaling during image acquisition. When a patient exhales rather than holds a full inspiration during the scan, the lungs appear smaller and denser than they actually are. This breathing artifact is a common technical limitation in chest CT interpretation, as noted in studies such as 1, which highlights the importance of inspiration level in lung density measurements. Radiologists typically instruct patients to take a deep breath and hold it during the scan to maximize lung inflation and improve visualization of pulmonary structures. When images are obtained during exhalation, the decreased air volume causes increased lung density, potentially obscuring subtle pathologies or mimicking conditions like atelectasis or infiltrates. This phenomenon is particularly important to recognize because it can lead to misinterpretation of findings. Radiologists often look for signs of expiratory imaging such as dome-shaped diaphragms positioned higher in the thorax, narrowed intercostal spaces, and overall decreased lung volumes symmetrically affecting both lungs. In cases where expiratory artifact is suspected, repeat imaging with proper breath-holding technique may be necessary for accurate assessment, as suggested by the principles outlined in 1 and 1, which discuss the role of CT scanning in assessing lung function and structure. Given the potential for misinterpretation and the importance of accurate diagnosis, it is crucial to ensure proper breathing technique during chest CT scans, especially in the context of diagnosing and managing conditions like COPD, as discussed in 1 and 1.
From the Research
Apparent Low Lung Volumes on Chest CT
- Apparent low lung volumes on chest Computed Tomography (CT) can be due to various factors, including the patient's respiratory phase at the time of imaging.
- A study published in the AJR. American journal of roentgenology in 1998 2 found that lung volumes measured using helical CT at expiration were equivalent to intrathoracic gas volume, suggesting that expiration can affect lung volume measurements on CT.
- Another study published in Pediatric radiology in 2010 3 described a volume-monitored (VM) method for performing reproducible, motion-free full inspiratory and end expiratory chest CT examinations in children, highlighting the importance of controlling respiratory phase during CT imaging.
Effect of Respiratory Phase on Lung Volume Measurements
- The study by 2 found that using expiratory helical CT, they overestimated residual volume by 850 ml, indicating that expiration can lead to apparent low lung volumes on CT.
- The VM-CT technique described by 3 allows for reproducible high-quality full inspiratory and end expiratory lung CT images, suggesting that controlling respiratory phase can help minimize errors in lung volume measurements.
- A study published in Rheumatology (Oxford, England) in 2016 4 used quantitative chest CT analysis to evaluate lung abnormalities in patients with systemic sclerosis, and found that total lung volume increased after autologous stem cell transplantation, highlighting the importance of accurate lung volume measurements.
Clinical Implications
- Apparent low lung volumes on chest CT due to patient exhaling at the time of image can have significant clinical implications, as it may lead to incorrect diagnoses or misinterpretation of disease severity.
- The study by 5 found that chest CT scan scores were associated with future lung disease progression in children with cystic fibrosis, highlighting the importance of accurate lung volume measurements for predicting disease outcomes.
- Controlling respiratory phase during CT imaging, as described by 3, can help minimize errors in lung volume measurements and ensure accurate diagnoses and disease monitoring.