Spirometry-Assisted High Resolution Chest Computed Tomography in Children: Is it Worth the Effort?

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Background

Image quality of high resolution chest computed tomographies (HRCTs) depends on adequate breath holds at end inspiration and end expiration. We hypothesized that implementation of spirometry-assisted breath holds in children undergoing HRCTs would improve image quality over that obtained with voluntary breath holds by decreasing motion artifact and atelectasis.

Methods

This is a retrospective case-control study of HRCTs obtained at a tertiary care children’s hospital before and after implementation of a spirometry-assisted CT protocol, in which children ≥8 years of age are first trained in supine slow vital capacity maneuvers and then repeat the maneuvers in the CT scanner, coached by a respiratory therapist. Spirometry-assisted CT scans (cases) were matched by age, gender and diagnosis (cystic fibrosis vs other) to CT scans obtained with voluntary breath holds in the 6 years before implementation of the spirometry assistance protocol (controls), and evaluated by 2 blinded pediatric radiologists.

Results

Among both cases and controls (N = 50 each), 10 carried the diagnosis of cystic fibrosis and 40 had other diagnoses. Mean age was 12.9 years (range: 7.5-20.1) among cases and 13.0 (7.1-19.7) among controls. Mean (SD) inspiratory image density among cases was −852 (37) Hounsfield units (HU) and −828 (43) among controls (p = 0.006). Mean (SD) expiratory image density was −629 (95) HU among cases and −688 (83) HU among controls (p = 0.002). Mean (SD) change in image density between inspiratory and expiratory images was +222 (85) HU among cases and +140 (76) HU among controls (p < 0.001). Motion artifact was present on inspiratory images in 5 cases and 9 controls (p = 0.39 by Fisher’s exact test), and on expiratory images in 20 cases and 18 controls (p > 0.80). Atelectasis was present on inspiratory images in 8 cases and 9 controls and on expiratory images in 9 cases and 10 controls (p > 0.80).

Conclusions

Spirometry-assisted CTs had a significantly greater difference in lung density between inspiratory and expiratory scans than those performed with voluntary breath holds, likely improving the ability to detect air trapping. No appreciable difference in image quality was detected for the presence of motion artifact or atelectasis.

Introduction

Optimizing image protocols for various clinical indications allows radiologists and clinicians to ensure the highest possible sensitivity and specificity for disease processes. This is particularly important for computed tomography (CT), where radiation dose and cost makes repeat examinations for poor quality undesirable. High resolution chest CT (HRCT) is often used to monitor patients with cystic fibrosis (CF) and to evaluate patients for interstitial lung disease. Although there are many potential findings possible, in these settings we are often searching for bronchiectasis, air trapping, and patterns of parenchymal interstitial disease. These findings can be subtle, especially early in the disease process.1 In addition, respiratory motion hinders image interpretation.2 Bronchiectasis is best evaluated on images obtained at end inspiration (total lung capacity) and air trapping on images at the end of a full expiratory maneuver (residual volume).3 In most centers, voluntary breath holds are used during scanning. Patients are instructed to breath hold after a maximal inhalation maneuver for the inspiratory scan and then to maximally exhale and breath hold for the expiratory scan. As many patients, particularly children, have difficulty following these instructions, images are often not obtained at true end inspiration or end expiration. In a pediatric study of 20 patients with CF (mean age = 12 years), voluntary breath hold lung volumes were compared with lung volumes measured by plethysmography before scanning.4 Mean inspiratory volume was 77% of total lung capacity but ranged from 55%-106%. Mean expiratory volume was 140% of residual volume (range: 83%-293%). Thus, while inspiratory images were generally obtained near total lung capacity, end-expiratory scans were obtained at lung volumes closer to functional residual capacity than residual volume.

Lung volumes can be controlled and motion artifact minimized in infants and toddlers under anesthesia using the controlled ventilation technique.5 The effect of suboptimal lung volumes on detection of bronchiectasis and air trapping was documented in a study of 16 young children undergoing sedated HRCT either with controlled ventilation or during tidal breathing. Bronchiectasis was identified on 30% of images obtained at end inspiration compared to 6% of images obtained during tidal breathing. Air trapping was detected on 45% of images obtained at end expiration, compared to 19% of images obtained near functional residual capacity.5 In school-age children, spirometry assistance is an alternative to voluntary breath holds to ensure reproducible lung volumes. In this technique, patients are first trained in the pulmonary function lab to perform a supine slow vital capacity maneuver to obtain full inflation and complete exhalation. They then perform this same maneuver in the CT scanner during image acquisition, coached by a trained respiratory technician.6 This technique may be particularly suitable for patients with prior experience with spirometry, such as patients with CF. Although spirometry-assisted HRCT has been shown to improve image quality in adults,7 it has been minimally evaluated in children. Kongstad et al showed that spirometry-assisted HRCT in pediatric patients with CF improved detection of gas trapping on expiratory images. However, they did not evaluate overall image quality.8 The objective of the current study was to test the hypothesis that spirometry-assisted HRCT would yield improved image quality (decreased motion artifact and atelectasis) as compared to scans obtained with voluntary breath holds.

Section snippets

Study Design and Eligibility

We conducted a retrospective case-control study of chest HRCTs obtained at Seattle Children’s Hospital before and after implementation of a spirometry-assisted CT protocol in July 2013. Eligibility to participate included age ≥ 8 years, ability to cooperate with a slow vital capacity maneuver and being a patient of the pulmonology or rheumatology services (inpatient or outpatient). Spirometry-assisted CT scans (cases) obtained from July 2013 to August 2014 were matched 1 to 1 by age (within one

Participant Characteristics

Among both cases and controls (N = 50 each), 10 carried the diagnosis of CF and 40 had other diagnoses. Mean age was 12.9 years (range: 7.5-20.1) among cases and 13.0 (7.1-19.7) among controls (Table 1).

Lung Volumes and Image Density

Among the cases, the slow vital capacity maneuvers performed in the CT scanner achieved a median (interquartile range) inspiratory volume of 95.0% (91.0%-98.9%) and a median (interquartile range) expiratory volume of 95.0% (89.2%-97.8%) of the best respective volumes achieved during the practice

Discussion

We have shown that a spirometry-assisted CT protocol resulted in significantly greater inspiratory lung volumes and significantly lower expiratory volumes compared to voluntary breath holds, as measured by lung density and chest wall diameter. Patients were able to achieve a median of 95% of their best inspiratory slow vital capacity during CT scanning, and for their expiratory effort a median of 95% of their best expiratory effort. Nonetheless, we were not able to detect an effect of the

Conclusion

Spirometry assistance during CT scanning improves the lung volumes at which inspiratory and, particularly, expiratory images are obtained and allows quantification of lung volumes relative to the patient’s best slow vital capacities. However, the additional time, cost, and scheduling considerations necessary for spirometry-assisted scans may make this protocol difficult to justify for routine use in all HRCT examinations. This is particularly true given that image quality did not change

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