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Technical Innovation |
1
Department of Radiology, Hospital de Sant Pau, Universidad
Autónoma de Barcelona, Avda Sant Antoni
Ma Claret 167, Barcelona 08025, Spain.
2
Department of Radiology, Harborview Medical Center, University of Washington,
325 Ninth Ave., ZA-65 Seattle, WA 98104.
3
Department of Internal Medicine, Hospital de Sant Pau, Universidad
Autónoma de Barcelona, Barcelona 08025,
Spain.
Received December 28, 1998;
accepted after revision July 27, 1999.
Address correspondence to T. Franquet.
Introduction
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With conventional radiography, lateral decubitus positioning is advocated as a useful technique for the detection of air-trapping in infants [4]. We hypothesized that the same concept of lateral decubitus positioning could be used to detect air-trapping in patients with suboptimal standard dynamic CT.
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Thirty-two of the 49 patients had satisfactory paired inspiratory—expiratory CT scans showing a pattern of heterogeneous lung attenuation either in the inspiratory—expiratory series (n = 24) or in the expiratory series alone (n = 8). In the remaining 18 patients, the patient's inability to suspend respiration during CT and the presence of respiratory motion artifacts resulted in uninterpretable CT scans. These 18 patients, in whom we performed bilateral decubitus CT, form the basis of our study.
CT was performed using one of two scanners, a Toshiba 900 CT scanner (Toshiba Medical Systems, Tokyo, Japan) or a Somaton Plus 4 scanner (Siemens, Erlangen, Germany). Before CT, all patients were trained in the breathing technique by a radiology technologist. Other than verbal coaching, we made no attempt to control the patient's respiratory status during scanning. No IV contrast medium was injected. Lung attenuation was compared in matched sections for the presence of air-trapping. Satisfactory CT examinations and the presence and degree of air-trapping were based on a qualitative visual assessment of lung parenchyma and a decrease in the cross-sectional area between maximum inspiration and maximum expiration on CT. CT scans were analyzed by two chest radiologists by consensus.
In patients with suboptimal, inconclusive, or uninterpretable paired inspiration—expiration CT scans, additional CT sequences were performed at suspended full expiration with the patients in both the right and left lateral decubitus positions. In all patients, additional CT scans were obtained using the same methods previously described. The lateral decubitus CT scans were evaluated by the same radiologists using the same criteria to determine satisfactory CT, uninterpretable CT, and degree of air-trapping.
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In 11 of the 18 patients, areas of air-trapping were detected on the right or left lateral decubitus CT scans (Fig. 1A, 1B). These areas were lobular in three patients and segmental or multilobular in eight (Fig. 2A, 2B). Air-trapping had a bilateral distribution in eight of the 11 patients. Qualitative assessment showed the degree of air-trapping varied from mild (four patients) to severe (seven patients). Seven of the 18 patients had suboptimal or uninterpretable lateral decubitus expiratory CT scans.
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Normal CT lung attenuation varies in relation to the lung region studied, the degree of lung inflation, and the dependency of the region studied [6]. Changes in body posture increase lung attenuation in dependent lung areas because of normal gravitational density [7]. Furthermore, gravitational lung attenuation may vary depending on local abnormalities such as lung compliance, distribution of mechanical stresses, and differences in ventilation or perfusion.
The most common problem in obtaining useful dynamic CT scans is that many patients are unable to suspend respiration during the paired inspiratory—expiratory scanning sequence. Assessment of air-trapping can be extremely difficult in obese patients or in those who are unable to suspend respiration, especially at end-expiration. Therefore, before performing dynamic CT, it is important to instruct patients on breathing techniques. Breathing during CT causes motion artifacts in the lung parenchyma that are detrimental to the diagnosis of small airways disease. Lateral decubitus positioning causes the dependent hemithorax to be relatively splinted, thereby restricting movement of the thoracic cage on that side. The dependent lung is more opaque than the upper lung because of gravitational differences in perfusion and inflation. The lateral decubitus position also allows radiologists to take advantage of gravitational gradients, thereby accentuating the differences in lung attenuation. We found that airtrapping was visible on lateral decubitus CT scans in 11 (61%) of 18 patients with otherwise uninterpretable dynamic supine CT scans.
We did not perform lateral decubitus CT at full suspended inspiration. It is possible that lateral decubitus positioning improves the detection of air-trapping in patients with small airways disease. However, in another study [8], dynamic CT was performed in conjunction with spirometric control. Other than verbal coaching, we made no attempts to control respiratory status during scanning.
Despite the subjectivity of the visual assessment of mild air-trapping, we found that lateral decubitus CT is a helpful alternative in the detection and visualization of air-trapping in a small number of patients with suboptimal, inconclusive, or uninterpretable supine expiratory CT scans. The detection of air-trapping in some of our patients suggests that lateral decubitus CT may be more sensitive for the detection of air-trapping than scans obtained in the supine position. Lateral decubitus CT may also help to avoid the pitfalls associated with air-trapping diagnosis, such as those related to pulmonary embolic disease. In conclusion, we believe that lateral decubitus CT is a useful adjunct to conventional dynamic CT in the diagnosis of small airways disease.
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