Diagnostic accuracy of ultra-low-dose chest computed tomography in an emergency department

Abstract

Background

This study examined whether ultra-low-dose chest computed tomography (ULD-CT) could improve detection of acute chest conditions.

Purpose

To determine (i) whether diagnostic accuracy of ULD-CT is superior to supine chest X-ray (sCXR) for acute chest conditions and (ii) the feasibility of ULD-CT in an emergency department.

Material and Methods

From 1 February to 31 July 2019, 91 non-traumatic patients from the Emergency Department were prospectively enrolled in the study if they received an sCXR. An ULD-CT and a non-contrast chest CT (NCCT) scan were then performed. Three radiologists assessed the sCXR and ULD-CT examinations for cardiogenic pulmonary edema, pneumonia, pneumothorax, and pleural effusion. Resources and effort were compared for sCXR and ULD-CT to evaluate feasibility. Diagnostic accuracy was calculated for sCXR and ULD-CT using NCCT as the reference standard.

Results

The mean effective dose of ULD-CT was 0.05±0.01 mSv. For pleural effusion and cardiogenic pulmonary edema, no difference in diagnostic accuracy between ULD-CT and sCXR was observed. For pneumonia and pneumothorax, sensitivities were 100% (95% confidence interval [CI] 69–100) and 50% (95% CI 7–93) for ULD-CT and 60% (95% CI 26–88) and 0% (95% CI 0–0) for sCXR, respectively. Median examination time was 10 min for ULD-CT vs. 5 min for sCXR (P<0.001). For ULD-CT 1–2 more staff members were needed compared to sCXR (P<0.001). ULD-CT was rated more challenging to perform than sCXR (P<0.001).

Conclusion

ULD-CT seems equal or better in detecting acute chest conditions compared to sCXR. However, ULD-CT examinations demand more effort and resources.

Keywords

Diagnostic accuracy,ultra-low-dose chest CT,pneumonia,pneumothorax,pleural effusion,cardiogenic pulmonary edema

Introduction

From 2006 to 2018 the number of patients with acute respiratory diseases admitted to the emergency departments in Denmark (population of 5.8 million citizens) increased from 62,743 to 108,198 (1,2). Chest X-ray (CXR) is typically the first-line imaging modality for assessing acute chest conditions. Studies have shown that standard CXR has a moderate to low sensitivity for detecting conditions such as pneumonia, pleural effusion, pulmonary edema, and pneumothorax (3 –6). For elderly, frail, or severely ill patients a supine CXR (sCXR) is often performed. Unfortunately, this modality has an even lower diagnostic accuracy than posteroanterior CXR (3,6). This makes image interpretation more difficult, potentially leading to overlooked disease and delayed treatment. In a study by Ray et al. (7), inappropriate initial treatment in the emergency department was associated with increased mortality in elderly patients. Computed tomography (CT) of the chest is more accurate than CXR in detecting chest conditions but has drawbacks such as longer examination time and increased radiation exposure, which can increase the risk of developing cancer (8).

There is the need for an accurate first-line imaging modality with reduced radiation exposure for patients who cannot stand for a standard CXR examination. In theory, this could be achieved through an ultra-low-dose CT (ULD-CT) scan of the chest, which renders minimum radiation exposure while maintaining acceptable diagnostic accuracy for the most acute chest conditions. Some studies have evaluated different ULD-CT protocols, but most of them were done in a specifically selected group of patients and often in lung nodule evaluation (9 –11). Few studies have evaluated ULD-CT in an emergency setting with the aim of comparing ULD-CT with the current first-line image modalities (4,12). To our knowledge, this study is the first to evaluate a ULD-CT protocol in an emergency department setting for patients admitted with acute respiratory symptoms and referred for sCXR as the first-line imaging modality.

It has been suggested that studies on ULD-CT should explore the feasibility of ULD-CT and whether it could replace CXR in patients with acute chest conditions (13). The aim of the present prospective study was to assess whether ULD-CT could be implemented as an alternative to sCXR for patients with acute respiratory symptoms admitted to the emergency department. The diagnostic accuracy of ULD-CT and sCXR was compared with non-contrast chest CT (NCCT) as a reference standard. Furthermore, ULD-CT and sCXR were compared regarding their image quality and resources: number of involved staff, examination time, and perceived level of effort.

Material and Methods

The study was conducted according to the Standards for Reporting of Diagnostic Accuracy Studies criteria (14). (See checklist in Supplemental Files.)

Ethics

The study was approved by the Danish Data Protection Agency (“Datatilsynet via Region Syddanmark”, Journal no. 2012-58-0018) and the regional ethics board (“De Videnskabsetiske Komitéer for Region Syddanmark”, Journal no. S-20160041). Informed written consent was obtained from all included patients. Patients could only be enrolled twice to avoid accumulation of radiation.

Study design and patients

The present study was designed as a prospective single-center study conducted in the emergency department at Odense University Hospital in Denmark. Patients were non-traumatic and consecutively included between 1 February and 31 July 2019 if they were admitted to the emergency department and referred for sCXR.

Exclusion criteria were as follows: informed written consent was not obtained; patients aged < 40 years; participation would delay life-saving treatment; interval between sCXR and CT > 4 h; and patients already enrolled in the study twice.

SCXR and ULD-CT

SCXR was performed using Siemens Ysio (Siemens Healthcare GmbH, Erlangen, Germany) according to department guidelines.

Patients were examined with the GE Revolution CT (GE Healthcare, Waukesha, IL, USA) with a scan range of 350 mm and detector configuration of 128 × 0.625 mm. All patients were examined in the supine position with feet first and separate breath-holds for each scan. Scan parameters are shown in Table 1. The ULD-CT exams were performed without scout view using fixed tube current. A NCCT was performed consecutively and was chosen as a reference standard in this study to avoid exposing patients to unnecessary contrast media.

Table 1.

Scan acquisition parameters for NCCT and ULD-CT protocols.

Scan mode	NCCT	ULD-CT
Tube voltage (kVp)	100	80
Tube current (mA)	20–579; modulated	10
Bowtie filter (cm)	Body 32	Body 32
Image quality metric	Noise Index 27	Not applicable
Pitch	0.992	0.992
Rotation time (s)	0.35	0.5
Field of view (mm)	350	350
Image reconstruction	30% ASiR-V*	100% ASiR-V*
Reconstruction algorithm	Lung	Lung
Contrast-enhanced	No	No

ASiR-V, adaptive statistical iterative reconstruction; CT, computed tomography; NCCT, non-contrast chest computed tomography; ULD-CT, ultra-low-dose chest computed tomography.

Data collection

Study data were managed using REDCap hosted at Open Patient data Explorative Network (OPEN) (Odense University Hospital, Denmark) (15,16).

Age, sex, and body mass index (BMI) were systematically recorded for all patients.

The ULD-CT and sCXR examinations were assessed by OG, PG, and MF, who had eight, four, and two years of experience in acute radiology, respectively. A chest radiologist with 10 years of experience (MA) assessed the NCCT examinations. None of the readers had previous experience with ULD-CT.

The readers were to look for the following predefined chest conditions: cardiogenic pulmonary edema; pneumonia; pneumothorax; and pleural effusion.

Image quality was rated independently by the readers using the following scale: 1 = non-diagnostic; 2 = poor; 3 = suboptimal; 4 = optimal; and 5 = near perfect.

For exact wording of the scale, see Supplemental Material A.

Patient data and clinical status were anonymized. The readers evaluated the examinations independently and at different times in between their clinical work. No time interval was set between the evaluation of ULD-CT and sCXR examinations. All ULD-CT examinations were evaluated before sCXR to prevent consecutive examinations of the same patient.

Evaluation was performed using GE Centricity RA1000 picture archiving and communication system (PACS) (GE Healthcare, Waukesha, IL, USA) workstations. ULD-CT images were reconstructed in sagittal, axial, and coronal planes using a slice thickness of 5 mm to minimize image noise and were displayed using lung window setting (window width = 1465 HU, window level = –498 HU) (Table 1).

The effective dose was calculated from dose length product (DLP) from each CT examination by: effective dose = dose length product × E_DLP, with E_DLP being the specific chest coefficient (E_DLP = 0.014 mSv/mGy cm) (17). A mean Volumetric CT Dose Index and effective dose with standard deviation (SD) and range were calculated for ULD-CT and standard NCCT, respectively.

The staff resource data were collected from both ULD-CT and sCXR regarding examination and staff time spent, number of staff members needed, and staff-rated effort regarding performance of the examination. Staff time was defined as both examination and preparation time. The effort was rated on a scale of 0–10, with 10 being the highest level of effort.

Statistical analysis

For patient baseline data means, SD, and ranges were calculated.

The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of ULD-CT and sCXR were calculated with 95% confidence intervals (95% CI) for the predefined chest conditions.

Differences between sCXR and ULD-CT were evaluated using Wilcoxon signed-rank test for examination and staff time, number of staff, and staff-rated effort. A significance level of P < 0.05 was chosen. For examination and staff time, medians, ranges, and interquartile range (IQR) in minutes were calculated.

Data were analyzed using a commercially available statistical software package (Stata Statistical Software: Release 16; StataCorp LLC, College Station, TX, USA).

Results

Informed written consent was obtained from 105 patients, and due to various issues 12 patients were unable to receive CT examinations and were excluded from analysis (Fig. 1). A total of 91 patients (48 women, 43 men) received sCXR, ULD-CT, and standard NCCT (93 examinations for each modality) and were included in the analysis (Fig. 1).

The patients had a mean age of 78 ± 10.6 years and a mean BMI of 25.9 ± 6.6 kg/m² (Table 2).

Table 2.

Baseline demographics of study patients (n = 91) included in this study.

	Sex	Mean
Patients (n)	48 F, 43 M
Age (years)		78 ± 10.6 (42–97)
	Female	78 ± 11.2 (42–97)
	Male	77 ± 9.9 (57–93)
Height (cm)		166 ± 10.8 (138–193)
	Female	160 ± 9.3 (138–184)
	Male	172 ± 8.3 (19–5)
Weight (kg)		72 ± 20.6 (39–139)
	Female	67 ± 20.8 (39–139)
	Male	77 ± 19.1 (51–130)
BMI (kg/m²)		25.9 ± 6.6 (16–47)
	Female	25.9 ± 7.6 (16–47)
	Male	25.8 ± 5.4 (17–42)

Values are given as mean ± SD (range).

BMI, body mass index; F, female; M, male; SD, standard deviation.

Time interval between examinations

Missing values were seen in four examinations due to errors in time registration. The sCXR and ULD-CT examinations were performed within 4 hours of each other. Median time interval between the sCXR and ULD-CT for 89 examinations was 14 min (IQR = 5–81 min, range = 0–209 min). No adverse events occurred during or between the performance of sCXR or CT examinations.

CT parameters

The mean DLP for NCCT was 238 ± 110.4 mGy*cm (range = 74.7–529.7 mGy*cm) and for ULD-CT was 3.8 ± 0.3 mGy*cm (range = 2.7–4.4 mGy*cm). Mean Volumetric CT Dose Index for NCCT was 6.6 ± 3.1 mGy (range = 2–15 mGy), and for ULD-CT mean Volumetric CT Dose Index was 0.11 mGy. The mean effective dose for NCCT examinations was 3.2 ± 1.6 mSv (range = 1–7.4 mSv) and mean effective dose for ULD-CT examinations was 0.05 ± 0.01 mSv (range = 0.04–0.06 mSv). The dose for sCXR was approximately 0.1 mSv according to the Radiology Department’s standard practice.

Diagnostic accuracy

Specificity for all acute chest conditions was similar between the sCXR and ULD-CT examinations. Specificity for pneumothorax and pleural effusion was in the range of 94%–100% and for pneumonia and cardiogenic pulmonary edema in the range of 71%–100% (Table 3).

Table 3.

Sensitivity and specificity in the detection of chest conditions with ULD-CT and sCXR.

	sCXR		ULD-CT
Prevalence of acute chest conditions	Sensitivity (%)	Specificity (%)	Sensitivity (%)	Specificity (%)
Cardiogenic pulmonary edema (n = 9/93)
Reader 1	78 (40–97)	83 (74–91)	56 (21–86)	98 (92–100)
Reader 2	0 (0–34)	100 (96–100)	0 (0–34)	100 (96–100)
Reader 3	11 (0–48)	96 (90–99)	11 (0–48)	100 (96–100)
Pneumonia (n = 10/93)
Reader 1	50 (19–81)	83 (73–91)	90 (56–100)	86 (76–92)
Reader 2	60 (26–88)	89 (80–95)	100 (69–100)	87 (78–93)
Reader 3	80 (44–98)	71 (60–81)	100 (69–100)	77 (67–86)
Pneumothorax (n = 4/93)
Reader 1	0 (0–0)	100 (96–100)	25 (1–81)	100 (96–100)
Reader 2	0 (0–60)	99 (94–100)	50 (7–93)	100 (96–100)
Reader 3	0 (0–0)	100 (96–100)	50 (7–93)	100 (96–100)
Pleural effusion (n = 29/93)
Reader 1	59 (39–77)	98 (92–100)	48 (29–68)	100 (94–100)
Reader 2	66 (46–82)	94 (85–98)	72 (53–87)	97 (89–100)
Reader 3	66 (46–82)	98 (92–100)	69 (49–85)	98 (92–100)

Values are reported separately for the three readers. Values in parentheses are 95% CI.

n, number of cases with the diagnosis according to the reference test (non-contrast chest CT).

CI, confidence interval; sCXR, supine chest X-ray; ULD-CT, ultra-low-dose chest computed tomography.

Pleural effusion was the most prevalent acute chest condition and was detected in 29 of 93 (31%) of the examinations (Table 3). For detection of pleural effusion and cardiogenic pulmonary edema, sensitivity varied among the readers for both modalities (Table 3).

Pneumothorax was present in 4 of 93 (4%) examinations (Table 3). Pneumothorax was not detected on any sCXR examinations; however, the condition was detected with ULD-CT in 25% (1/4; 95% CI = 1–81) or 50% (2/4; 95% CI = 7–93) of the examinations. Fig. 2 presents a case of pneumothorax from this study.

Two readers were able to detect all cases of pneumonic opacities on ULD-CT examinations (100%, 10/10, 95% CI = 69–100%). In contrast, the same readers detected 60% (6/10; 95% CI = 26%–88%) or 80% (8/10; 95% CI = 44–98) of cases of pneumonia on sCXR examinations (Table 3). Fig. 3 presents a case of pneumonia from this study.

PPV and NPV with 95% CI for ULD-CT and sCXR in detection of acute chest conditions can be seen in Supplemental Material B.

Image quality

The image quality of ULD-CT and sCXR examinations was mostly rated suboptimal (“3”) by the readers. Distributions of the ratings by each reader for both imaging modalities are shown in Fig. 4a and b.

Feasibility: time, resources, and effort

The median, quartiles, and range for examination and staff time can be seen in Table 4. One examination from both ULD-CT and sCXR was excluded due to technical issues that prolonged the examination time. The median examination and staff time was longer for ULD-CT compared to sCXR (P < 0.001).

Table 4.

Examination and staff time for performing sCXR and ULD-CT.

	Median	25%	75%	Range	Excluded values
sCXR
Staff time (min)	5	4	6	2–13	1
Examination time (min)	5	5	6	2–13	1
ULD-CT
Staff time (min)	10	9	12	6–16	1
Examination time (min)	10	8	11	6–16	1

sCXR, supine chest X-ray; ULD-CT, ultra-low-dose chest computed tomography.

The number of staff needed to perform ULD-CT and sCXR is shown in Supplemental Material C. For sCXR, 68 of 93 examinations were performed by two staff members, whereas for ULD-CT, 52 of 93 examinations were performed by three staff members. Generally, more staff members were needed to perform ULD-CT examinations compared to sCXR (P < 0.001).

The staff-rated effort when performing the ULD-CT and sCXR examinations is shown in Fig. 5. There were two missing values in the dataset for ULD-CT examinations and three missing values for sCXR examinations due to logistical issues during data collection. ULD-CT examinations were estimated to be more difficult to perform compared to sCXR (P < 0.001).

Discussion

The present study examined the feasibility of ULD-CT as a first-line image modality in an emergency setting and whether its diagnostic accuracy was superior to that of the sCXR. For cardiogenic pulmonary edema and pleural effusion, the diagnostic accuracy was similar when comparing ULD-CT and sCXR. For pneumonia and pneumothorax, 10 of 10 and 2 of 4 cases were detected with ULD-CT, whereas 6 of 10 and 0 of 4 cases were detected with sCXR. Image quality of both modalities was mostly rated suboptimal. Median examination and staff time (10 min) were longer for ULD-CT examinations compared to sCXR (P < 0.001). For ULD-CT examinations, 1–2 additional staff members were needed to perform the examination (P < 0.001) and the staff-rated effort was higher for ULD-CT than for sCXR (P < 0.001).

Other studies evaluated ULD-CT (0.1–0.2 mSv) for detection of lung consolidations (pneumonic and nodular opacities). In those studies, the study populations comprised 36–85 patients and ULD-CT detected consolidations with a sensitivity of 87%–100% (9 –11,18,19). These results resemble the results in the current study and, thus, ULD-CT can accurately detect pneumonia despite its low occurrence in the study population.

In the present study, sensitivity for pneumothorax is lower compared to the 100% sensitivity reported by Macri et al. (12) and Debray et al. (18). They included trauma and lung transplant patients with a higher occurrence of pneumothorax, than in the patients included in this study. The readers in the studies by Macri et al. and Debray et al. were either trained in ULD-CT assessment or had 12–22 years of expertise in chest CT (12,18). The lower sensitivity in the present study might be explained by the readers’ unfamiliarity with reading ULD-CT and suboptimal image quality, which might have influenced the detection of a smaller pneumothorax. Despite this, ULD-CT proved to be a better diagnostic tool for detecting pneumothorax compared to sCXR in this study.

For pleural effusions, two other studies reported a sensitivity of 90%–100%. Disagreement on findings were solved by consensus, which improved sensitivity, as one reader might find something the other had missed (9,18). In this study, each reader performed assessments individually, which might explain the variation in sensitivity. A third study reported a sensitivity of 65%, in which a chest radiologist assessed the reference standard (20). The same method was adopted in this study and yielded similar sensitivities. Both above-mentioned studies had the same readers assess the index and reference standard (9,18), increasing the chance of finding the same conditions on both tests, which might have biased true disease status. Separating the assessment of index and reference standard and the use of an expert chest radiologist for the reference standard could be considered a strength of this study.

Kroft et al. (4) reported a mean examination time of <3 minutes for ULD-CT. This differs from the median examination time of this study (10 min). In this study, examination time was prolonged by scout-acquisitions for the NCCT, which will not be included in future ULD-CT examinations. Furthermore, if a patient admitted to emergency department receives a CT examination for other purposes, performing an ULD-CT instead of sCXR will demand less time. Wetzl et al. (21) reported a mean staff time (examination and post-processing time) of 14 min for chest CT examinations. This was longer than the median staff time reported in this study (10 min). However, Wetzl et al. used only 1–2 radiographers for their examinations.

To our knowledge, the mean effective dose of ULD-CT in this study (0.05 mSv) is the lowest value to ever be reported by diagnostic accuracy studies, which might explain the low sensitivity, specificity, and suboptimal image quality reported. Other studies reported higher sensitivities and almost perfect image quality with only slightly increased effective dose (11,12,19). In future studies, it might be relevant to increase the radiation dose and compare different ULD-CT protocols for better image quality and accuracy. Furthermore, we used a fixed tube current and therefore, the radiation dose was not adjusted to the individual patient. Recently, Chen et al. (22) successfully performed ULD chest CT using automatic tube current modulation, but they included only patients with normal range BMIs (22).

A limitation of the present study was the possibility of reader fatigue as the readers were not time-limited in their assessment of examinations (23). Furthermore, the readers might have made different conclusions if clinical information had been available, especially in situations where treatment of the patient depended on their diagnosis. Another limitation was the small sample size relative to the broad inclusion criteria. This could explain the broad 95% CI for the diagnostic accuracy measurements. To our knowledge, this study was the first to evaluate an ULD-CT protocol for a broad non-traumatic patient population with respiratory symptoms from the emergency department. This is a strength of this study, as the results were more generalizable to a real-life setting. The use of ULD-CT instead of sCXR in an emergency department for non-traumatic patients has the potential to improve diagnostic precision. Some patients are admitted to the emergency department multiple times due to chronic conditions (e.g. chronic obstructive pulmonary disease) and thus, the ULD-CT would be preferable to an NCCT due to the low radiation exposure.

This study did not analyze the financial aspects of utilizing ULD-CT instead of sCXR, which should be explored in future studies.

In conclusion, ULD-CT seemed equal or better in detection of cardiogenic pulmonary edema, pleural effusion, pneumothorax, and pneumonia compared to sCXR. ULD-CT could potentially be implemented as an alternative to sCXR for acute respiratory patients. This, however, comes at the cost of increased time and resources.

Supplemental Material

sj-pdf-1-acr-10.1177_0284185121995804 - Supplemental material for Diagnostic accuracy of ultra-low-dose chest computed tomography in an emergency department

Supplemental material, sj-pdf-1-acr-10.1177_0284185121995804 for Diagnostic accuracy of ultra-low-dose chest computed tomography in an emergency department by Maria Tækker, Björg Kristjánsdóttir, Michael B Andersen, Maja L Fransen, Pernille W Greisen, Christian B Laursen, Bo Mussmann, Stefan Posth and Ole Graumann in Acta Radiologica

Footnotes

Acknowledgements

The authors thank Kim Pelle Christensen, Benedikte Klærke, and Kim Storm Rasmussen at Odense University Hospital, Denmark for the technical assistance.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received the following financial support for the research, authorship, and/or publication of this article: This work was supported by the private fund “Grosserer L.F. Foghts Fond” (grant number 21.689) and by Odense University Hospital’s “OUHs prægraduatpulje,” (grant number A3480). Funding was obtained for the salary of Maria Tækker and Björg Kristjánsdóttir for one year. Funders had no role in study design, data collection or analysis, decision to publish, or preparation of the manuscript.

ORCID iDs

Maria Tækker

Björg Kristjánsdóttir

Michael B Andersen

Bo Mussmann

Ole Graumann

Supplemental Material

Supplemental material for this article is available online.

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Supplementary Material

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