CME ArticleRole of high-resolution computed tomography in the detection of early cystic fibrosis lung disease
Introduction
High-resolution computed tomography (HRCT) has become an indispensable tool for imaging the airways and lung parenchyma.1 The cross-sectional perspective and high spatial resolution result in high quality images approaching gross histological detail. HRCT has the ability to delineate clearly the structural components of cystic fibrosis (CF) lung disease.2, 3 It has been demonstrated to be sensitive at detecting early disease, as well as detecting the extent and severity of more advanced disease, often before it is appreciated clinically.3 Despite this, the role of CT in young children with CF has yet to be defined.2 CT may have its greatest utility in the component of the CF population in whom it is most difficult to perform, i.e. infants and young children.2, 3 Asymptomatic infants with CF have evidence of significant airway inflammation and infection,4 as well as diminished lung function.5 Emerging evidence suggests that structural changes occur earlier in life than previously appreciated.6, 7
Difficulties inherent in performing HRCT in young children include: their disinclination to lie still, a rapid respiratory rate, the inability to perform breath-holding manoeuvres and the structures of interest being small.8 These characteristics offer significant challenges to the standardization of scanning protocols, image acquisition and data analysis. The repeated exposure of young children to ionizing radiation raises the concern of long-term risks.9 Currently, there is no consensus regarding when HRCT scans are indicated in infants and preschool-aged children with CF, nor guidelines regarding scan acquisition. As such there is no standardized approach to important variables, including the lung volumes at which to scan, sedation guidelines, scanner settings or image analysis.
Section snippets
Early cystic fibrosis lung disease detected by HRCT
In established CF lung disease the most significant structural finding on HRCT is bronchiectasis. However, currently we have an incomplete understanding of which features of early structural lung disease predict the development of bronchiectasis, or how, and at what rate, this occurs. Indeed, some early radiological features may be reversible and amenable to treatment.
There are a limited number of studies describing the structural features of early CF lung disease in the infant and preschool
Scanner settings
HRCT is considered the gold standard technique to diagnose bronchiectasis.13 However, diagnostic ability may be affected by image quality. Image quality depends on several factors: thin beam collimation (slice thickness) typically 0.5–1.0 mm,14, 15 a high spatial frequency or edge enhancing reconstruction algorithm;16 the largest available matrix size, which is typically 512 × 512;1 and a small field of view.1, 17 The optimal settings to delineate the airways is a window level of −450 and a window
HRCT Scoring
The ability of HRCT to demonstrate the structural features of CF lung disease with much greater detail and clarity than conventional chest X-ray has resulted in the development of an array of CT scoring systems.32, 33, 34, 35 The first CT score developed specifically for CF was that by Bhalla et al. in 1991 and it has formed the foundation for most subsequent scores.32 In 2004, Brody et al. published a substantial re-working of the Bhalla score, which focused on younger patients (6–10 years
Quantitative measures of CF lung disease
Quantitative measures of HRCT findings may be particularly relevant to the detection of early lung disease, as the features most amenable to quantitative scores are gas trapping and bronchial dimension measurements. Regions of gas trapping are best appreciated as low attenuation areas on expiratory scans which can be measured directly by recording the Hounsfield unit of each voxel on the image.19, 25 Areas below a threshold level, considered to be indicative of gas trapping, can then be
Radiation issues
Radiation exposure is measured in terms of the quantity of radiation energy ‘absorbed’ by body tissue. It is measured in joules per kilogram (J/kg) and is expressed in Sievert (Sv).38 A radiation dose >1 Sv can result in massive cell death and organ failure.39 Such large radiation doses are rarely encountered in medical radiation hence the mSv is more commonly used. The effect on the body of low dose radiation (<50 mSv) principally relates to damage to the genome, with a resultant increased
Conclusion
The role of CT in the CF population continues to be debated. It is important to distinguish between its roles as a research tool and as a clinical investigation. As a research tool CT is expanding our understanding of the early morphological changes that occur in the CF lung. It is providing new insights into the evolution and pathogenesis of airway and parenchymal changes. It may prove an invaluable tool to assess the efficacy of much needed interventions in treating early CF lung disease, and
Practice points
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HRCT is a sensitive tool capable of detecting structural changes associated with early CF lung disease.
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The acquisition of HRCT images in infants and young children requires some form of sedation or anaesthetic, along with augmented ventilation.
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There is currently a lack of consensus regarding the role of HRCT in the routine clinical care of patients with CF.
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Lifetime risk of fatal cancer secondary to repeated exposures to relatively low radiation doses is largely unknown but current evidence
Research directions
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Development of scoring systems for early CF lung disease which incorporate quantitative measures of lung and airway morphology.
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Development of low radiation CT scanning protocols.
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Description of the evolution of structural changes from early (possibly reversible) CF lung disease to established disease.
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Incorporation of CT findings into clinical trials as a surrogate outcome.
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Correlation of structural changes detected by HRCT with measures of lung function, markers of airway inflammation and
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Acknowledgements
B. Linnane is the recipient of a Medical Postgraduate Scholarship from the National Health and Medical Research Council, Australia. The authors are supported by a grant from the United States Cystic Fibrosis Fund.
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