M u l t i d e t e c t o r Co m p u t e d Tomographic I maging in C h ron i c Ob s t r u c t i v e P u l m o n a r y Disease Emphysema and Airways Assessment Diana E. Litmanovich, MDa,*, Kirsten Hartwicka, Mario Silva, MDa,b, Alexander A. Bankier, MD, PhDa KEYWORDS COPD Emphysema Airway imaging CT Phenotyping
KEY POINTS Computed tomography (CT) imaging is crucial for both subjective and objective assessment of severity of emphysema and airway disease in chronic obstructive pulmonary disease (COPD). Standardization of the CT acquisition and reconstruction parameters is crucial for both subjective and objective assessment of COPD. Substantial correlation between imaging findings and clinical severity of emphysema and airway disease has been established. Investigation of the role of CT in phenotyping COPD and its contribution to large-scale studies is under way.
Chronic obstructive pulmonary disease (COPD) is defined as incompletely reversible expiratory airflow obstruction, likely caused by exposure to noxious inhaled particles.1 The airflow limitation that underlies functional obstruction is usually progressive and associated with an abnormal inflammatory response of the lung.2 Clinically, the severity of COPD is graded based on the Global Initiative for Chronic Obstructive Lung Disease (GOLD) classification, which relies on the spirometric parameters forced expiratory volume in
1 second (FEV1) and the ratio of FEV1 to forced vital capacity (FEV1/FVC). Because the GOLD classification was designed as an epidemiologic instrument rather than as a tool for assessing severity in individual patients, it has weaknesses, notably in the evaluation of patients with early disease and in patients with complex or complicated disease.2 These weaknesses can be explained by the complex nature of COPD itself, which seems to have evaded any sustainable evaluation by 1 classification system alone. What is clinically called COPD reflects a complex syndrome encompassing potentially overlapping
Disclosures: D.E. Litmanovich discloses being employed by Beth Israel Deaconess Medical Center and Harvard Medical Faculty Physicians and receiving research grants from Society of Thoracic Radiology and Radiological Society of North America. A.A. Bankier discloses being employed by Beth Israel Deaconess Medical Center and Harvard Medical Faculty Physicians. Dr A.A. Bankier is a consultant to Spiration and receives royalties from Amisrsys and Elsevier. K. Hartwick and M. Silva, MD have nothing to disclose. a Department of Radiology, Beth Israel Deaconess Medical Center, 330 Brookline Avenue-Shapiro 4, Boston, MA 02215, USA; b Section of Diagnostic Imaging, Department of Surgical Sciences, University of Parma, Parma, Italy * Corresponding author. E-mail address:
[email protected] Radiol Clin N Am 52 (2014) 137–154 http://dx.doi.org/10.1016/j.rcl.2013.09.002 0033-8389/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved.
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INTRODUCTION
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Litmanovich et al diseases such as pulmonary emphysema, chronic bronchitis, and small airways disease. In addition, there is increasing evidence of nonpulmonary contributors to (or consequences of) COPD, such as cardiac disease, neurologic and cognitive dysfunction, and musculoskeletal disorders. COPD seems to be related by unknown ties to generalized inflammation and metabolic syndrome. Diagnostic imaging is well established in the individual assessment of these disorders. This situation and the wide availability of imaging, combined with the growing expertise that radiologists have acquired with these diseases, have revived interest in the imaging of COPD in general. Thus, the current expectation of imaging is to complement the assessment of COPD using the GOLD classification with a more sophisticated definition of COPD subtypes, according to the prevailing underlying disease or diseases. This process, often referred to as phenotyping, could identify clinically meaningful subcategories of disease for a more tailored approach to both diagnosis and treatment. In this article, the role of imaging in COPD is discussed as related to the most prevalent subtypes of the disease. Given the importance of computed tomography (CT) in the assessment of the individual diseases contributing to COPD, most of the text is limited to this modality. The role of CT in phenotyping COPD and its contribution to largescale studies under way that aim to establish potential links between the imaging, clinical, and genetic manifestations of COPD are also discussed. Areas in which imaging could play a vital role in the discussion of COPD are highlighted.
of the images. Thin sections (0.5-mm–1-mm reconstructions) are generally recommended for assessing patients with COPD.4 Because intravenous iodinated contrast material influences the attenuation values of the organs imaged, non– contrast-enhanced volumetric CT acquisition is a standard technique for COPD imaging. Although a high-resolution reconstruction algorithm is appropriate for visual assessment of the lungs, a high-resolution reconstruction algorithm has been shown to increase the percentage of emphysema measured by the density mask method.5 Thus, the standard reconstruction algorithm is required for computerized analysis, implying that 2 separate sets of reconstructions are essential for CT assessment.2 Acquisition at full inspiration is required for quantification of pulmonary emphysema, because submaximal inspiration can cause underestimation of emphysema severity.6 Hence, appropriate breathing instructions during the scanning are of paramount importance.7 The solution would be spirometric gating in CT acquisitions for quantitative emphysema assessment.6 The CT radiation dose level used for evaluation of COPD is driven by the balance between radiation dose and image noise, although radiation exposure may be of secondary importance in this specific group, given their age profile. Because excessive image noise simulates emphysema, particularly on quantitative evaluation,8,9 standard kVp and mAs parameters are recommended. A proposed technique is shown in Table 1. Dr Madani has also shown that radiation dose does not substantially influence the strength of correlations between histopathologic indexes
CT Imaging in COPD CT technique Because chest radiography has shown little sensitivity in detecting COPD-related changes, and high costs and cumbersome technical procedures can restrict access to magnetic resonance imaging, CT has become the primary imaging modality in patients with suspected COPD, in both clinical and research contexts. The establishment of CT in this role was determined by rigorous validation and confirmation studies, as well as by recent large epidemiologically oriented studies. However, important questions related to the overall impact of CT in the workup of COPD remain to be answered. This section summarizes established knowledge of CT in patients with COPD and concludes with newly emerging areas of research. Technical CT parameters recommended by the COPD Gene study3 reflect the necessity to obtain image acquisition with high signal-to-noise ratio to secure both subjective and objective assessment
Table 1 Proposed technique for emphysema and airway imaging Tube potential (kVp) Tube current (mAs) Inspiratory acquisition End-expiratory acquisition Pitch Detector configuration (mm)
Reconstruction algorithms Section thickness (mm)
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