Emphysema quantification and lung volumetry in chest X-ray equivalent ultralow dose CT – Intra-individual comparison with standard dose CT

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Highlights

  • Lung volumetry and emphysema quantification was compared in 84 patients in standard dose and ultralow dose CT of the chest.

  • Fully automated software-based lung volumetry and emphysema quantification in chest CT is feasible at chest X-ray equivalent dose.

  • Compared to FBP, advanced modeled IR (ADMIRE) leads to an underestimation of low-attenuation areas as an indicator of emphysema volume.

  • Single-energy ultralow dose CT may be used for assessing the distribution and extent of emphysema (e.g. prior to surgery).

Abstract

Objectives

To determine whether ultralow dose chest CT with tin filtration can be used for emphysema quantification and lung volumetry and to assess differences in emphysema measurements and lung volume between standard dose and ultralow dose CT scans using advanced modeled iterative reconstruction (ADMIRE).

Methods

84 consecutive patients from a prospective, IRB-approved single-center study were included and underwent clinically indicated standard dose chest CT (1.7 ± 0.6 mSv) and additional single-energy ultralow dose CT (0.14 ± 0.01 mSv) at 100 kV and fixed tube current at 70 mAs with tin filtration in the same session. Forty of the 84 patients (48%) had no emphysema, 44 (52%) had emphysema. One radiologist performed fully automated software-based pulmonary emphysema quantification and lung volumetry of standard and ultralow dose CT with different levels of ADMIRE. Friedman test and Wilcoxon rank sum test were used for multiple comparison of emphysema and lung volume. Lung volumes were compared using the concordance correlation coefficient.

Results

The median low-attenuation areas (LAA) using filtered back projection (FBP) in standard dose was 4.4% and decreased to 2.6%, 2.1% and 1.8% using ADMIRE 3, 4, and 5, respectively. The median values of LAA in ultralow dose CT were 5.7%, 4.1% and 2.4% for ADMIRE 3, 4, and 5, respectively. There was no statistically significant difference between LAA in standard dose CT using FBP and ultralow dose using ADMIRE 4 (p = 0.358) as well as in standard dose CT using ADMIRE 3 and ultralow dose using ADMIRE 5 (p = 0.966). In comparison with standard dose FBP the concordance correlation coefficients of lung volumetry were 1.000, 0.999, and 0.999 for ADMIRE 3, 4, and 5 in standard dose, and 0.972 for ADMIRE 3, 4 and 5 in ultralow dose CT.

Conclusions

Ultralow dose CT at chest X-ray equivalent dose levels allows for lung volumetry as well as detection and quantification of emphysema. However, longitudinal emphysema analyses should be performed with the same scan protocol and reconstruction algorithms for reproducibility.

Introduction

Computed tomography (CT) of the lung has evolved to be the most widely used diagnostic tool for detailed imaging of the lung parenchyma [1], [2], [3], [4]. Chest CT allows for a reliable detection of pulmonary emphysema and further enables to assess the pattern, severity and distribution of disease [5]. In addition the severity of emphysema can be quantified with software-based CT densitometry [6], [7], [8] by computing the low-attenuation areas (LAA) of the lung parenchyma, which represent the histological correlate of the disease (i.e. permanent airspace destruction) [9]. Various studies have shown a high correlation of LAA values by CT with pathological findings and pulmonary functional test results [10], [11].

Meanwhile, the ongoing debate about potential harmful effects of CT-related radiation exposure has urged the radiological community to optimize examination protocols. The strategies for reducing radiation dose include lowering of the tube current and tube voltage, automatic exposure control and iterative reconstruction (IR) [12].

A recently introduced CT device includes a tin-filter for single-energy imaging, which allows for spectral shaping of the X-ray beam. In combination with advances in IR – i.e. advanced modeled iterative reconstruction (ADMIRE) – both measures contribute to a drastic dose reduction [13], [14]. A recent study investigated the value of chest CT with radiation dose levels as low as 0.13 mSv for detecting solid pulmonary nodules with the help of computer-aided detection software [15].

The value of these new techniques for emphysema quantification and lung volumetry was not investigated so far. Further, only a few studies have investigated the influence of IR techniques on the CT evaluation of pulmonary emphysema and lung volumetry [16], [17], but none have evaluated latest generation IR in a clinical setting.

The purpose of this study was therefore to determine whether ultralow dose chest CT with tin filtration can be used for emphysema quantification and lung volumetry and to assess differences in emphysema measurements and lung volume between standard dose and ultralow dose CT scans using ADMIRE.

Section snippets

Subjects

This study was performed in subjects enrolled in an ongoing prospective single-center trial (clinicaltrials.gov identifier NCT02468609). The inclusion criteria of the trial have previously been described [18]. Briefly, patients who were referred for a standard of care chest CT to our department (hereafter called standard dose CT), (a) aged ≥ 18 years, (b) excluded pregnancy, (c) and provided informed consent were enrolled and underwent an ultralow radiation dose CT that was conducted in

Results

The CT scans of 84 patients were quantitatively assessed, including four different reconstructions of standard and three of ultralow dose scans yielding a total of 588 datasets to be evaluated (see Fig. 1). Forty of the 84 patients (48%) had no emphysema and 44/84 patients (52%) had emphysema.

Discussion

This study sought to determine whether ultralow dose chest CT with tin filtration can be used for emphysema quantification and lung volumetry and to assess differences in emphysema measurements and lung volume between standard dose and ultralow dose CT scans using model-based IR.

The major findings of our study are as follows: First, the use of software-based lung volumetry and emphysema quantification in chest CT is feasible at effective radiation dose levels as low as 0.14 mSv. Second,

Acknowledgements

We thank Elisabeth Wismer and her team for their excellent technical support.

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