Efficiency and reporting confidence analysis of sequential dual-energy subtraction for thoracic x-ray examinations.

Rationale and objectives: We aimed to report and compare accuracy, reproducibility, and reporting confidence between thoracic dual-energy subtraction (DES) and routine posterior-anterior chest radiography (PA-CR) techniques. Materials (patients) and methods: We obtained DES (D1-D4) images from 96 patients using DES and a high-resolution dynamic flat-panel detector in combination. We compared the DES images of these patients with their PA-CR images. The maximum time interval between performing DES and PA-CR was nine weeks. Two radiologists evaluated abnormal findings on DES and PA-CR images using a three-point scale, and reporting confidence was scored using a four-point scale. The intra- and interobserver agreement values of the scores were analyzed. Further, the radiation exposure doses during PA-CR and DES acquisitions were calculated.
Results: The intra- and interobserver agreement values of PA-CR and DES images were good. The reporting confidence scores for DES were generally higher than those for PA-CR. Between bone-subtracted (D3) and soft-tissue-subtracted (D4) images, the former was more successful and useful in the evaluation of bone structures, whereas the latter was better in the evaluation of consolidation and/or solitary nodules. Conclusions: DES has the potential to improve the accuracy, reproducibility, and reporting confidence of thoracic radiography. It also has the potential to provide a better diagnosis of chest pathologies using relatively low dose radiation. © 2019 Gezer, Algin, Durmaz, Arslan, licensee HBKU Press.

Introduction

Routine posterior–anterior chest radiography (PA-CR) remains the mainstay for the diagnosis of many thoracic abnormalities.[1] Accurate and efficient evaluation of radiographs is crucial for avoiding unnecessary chest computed tomography (CT) examinations.[2-4] Generally, CT is preferred as a secondary imaging technique for accurate characterization of lesions previously detected using CR. However, the use of CT increases the radiation exposure and financial burden on the healthcare system.[3] The radiation exposure increases due to the widespread use of multidetector CT devices for CT.[3-5]

The purpose of dual-energy subtraction (DES) acquisitions is to highlight the tissue of interest with radiological images obtained using different energy levels and image-processing techniques.[2] Using DES to highlight the tissue of interest, radiologists can achieve tissue-specific examination by the detection of energy levels at which the K-edge peak sensitivity is specific to that tissue.[1,4] Unlike PA-CR, DES may be used to obtain soft-tissue-subtracted images wherein bone structures remain in the background or bone-subtracted images wherein bone structures do not have soft tissue superimposed on them.[5] This property of DES is useful for evaluating obscured lesions in lung parenchyma, particularly for nodule detection.[5] Similarly, by eliminating superimposed soft tissues from images, DES may allow for better evaluation of bone lesions. However, DES is not widely used in routine practice.[4] Therefore, we aimed to determine the efficiency and reliability of raw and subtracted images obtained with high-resolution DES for the detection of thoracic lesions and to determine the reporting confidence for this technique.

Material and Methods

We obtained approval from the local ethics committee for the present retrospective study. We identified all patients (n = 1945) who underwent CR within a one-year period using the picture and archiving communicating (PACS) system of our hospital. Among these, 144 patients underwent CR with PA-CR and DES during this period. We obtained the clinical and laboratory data of these patients from PACS. We excluded the following patients: those with poor-quality images and those who underwent chemotherapy, antibiotherapy, thoracic intervention, or medication for cardiovascular diseases. Consequently, we included 96 patients (50 male and 45 female; mean age: 52.5 ± 14.5 years) in the study.

DES and PA-CR were performed in the same radiology center and with the patient in the same position. DES and PA-CR exams were randomly performed. DES was performed using a 4343R flat-panel detector (FPD) (Toshiba, Tokyo, Japan), which had a MTF value of 40% at a frequency of 2 lp/mm; A/D converter with 14-bit (pixel pitch: 143 × 143 μm, pixel matrix: 9.2 million, DQE: 70%); and DDR-line device (Istanbul, Turkey) with an energy of 50 kW, which could produce X-ray at a maximum peak voltage of 150 kVp and used a high-frequency generator that could irradiate with the precision of 1 ms. PA-CR images were obtained using a generator that had a maximum output power of 32–100 kVp (CPI, Ontario, Canada) and 14-bit A/D converter and was 42.7 × 42.7 cm in diameter (pixel pitch: 139 × 139 μm, pixel matrix: 9.2 million, DQE: 70%) large FPD (4343R, Paxscan, Varian Medical Systems, Palo Alto, California, USA).

We obtained DES images using two consequent exposures at peak voltages of 120 (mAs: 0.2–1) and 65 (mAs: 1–4) kVP at 100-ms intervals. As a result of these exposures, we obtained two radiographs at two different energy levels, including high (D1) and low (D2) peak voltage. We subtracted these images from each other using the methods described previously[6-9] to obtain bone- and soft-tissue-subtracted (D3 and D4, respectively) images. Therefore, we obtained four images (D1–D4) in total using DES.

We performed measurements of the radiation exposure dose for PA-CR and DES in a sample group containing 25 individuals. We obtained these measurements using the Accu-pro 9096 dose-measuring device (Radcal, California, USA) during the examinations. Further, we obtained PA-CR dose results via a single shot during exposure. DES results were the cumulative summation of two exposures depending on the dual energy used. We added the measured doses under automatic exposure control for the DES and PA-CR systems in the DICOM files of the patients. Subsequently, we recorded the results of these measurements.

The images of the patients obtained using both techniques were evaluated using the PACS system using the following parameters:

Investigated parameters on chest radiographs:

Each finding was recorded in the Excel data and scored according to the following system:

1. Morphology of thoracic bones, trachea, and fissures

2. Integrity of the diaphragm and heart borders

3. Presence of fibrotic lung shrinkage, nodule, atelectasis, pleural effusion, cardiomegaly, Kerley lines, consolidation, and/or air bronchogram

4. Hilar, aortic, or bronchovascular abnormality

Scoring schema of chest radiograph findings:

We scored the reporting confidence of the radiologists regarding the assessment of the mentioned findings as follows.

Score 0: There are no pathological findings or anatomical integrity is normal

Score 1: There is a suspicious pathological finding or the anatomy of the tissue of interest cannot be clearly evaluated

Score 2: There is a definite pathological finding or the anatomical integrity is impaired

Reporting confidence scoring scale:

We compared the four images obtained using DES [image obtained using a high kVp (D1), that obtained using a low kVp (D2), subtracted images obtained by subtracting D1 from D2 (D3), and subtracted images obtained by subtracting D2 from D1 (D4)] and the results of conventional radiography in terms of the mentioned variables.

Score 0: Ineffective/doubtful radiological assessment; the radiologist has low reporting confidence

Score 1: Moderate reporting confidence

Score 2: Good reporting confidence

Score 3: Absolute radiological assessment; the radiologist has high reporting confidence

Scoring was performed by two radiologists with four and three years of experience in determining interobserver reliability. In addition, the first radiologist performed scoring again three weeks later to evaluate intraobserver reliability. The duration of assessment of radiography was not limited, and all images were evaluated within the same PACS system using the same computer and screen.

Clinical and laboratory tests as well as thoracic CT and/or surgical–pathological (if results were available) examination were used as gold standard methods for determining the accuracy of PA-CR and DES.

Statistical analysis

SPSS version 15.0 (SPSS Inc., Chicago, IL, USA) and R program were used for all statistical analyses. A module called ‘nparLD’ was used for analyzing the LD_F1 design on R program. Shapiro–Wilk test was used to determine whether the age of the study patients were normally distributed. Cohen's kappa (κ) coefficient was used for evaluating inter- and intra-observer reliability. Differences between DES and PA-CR or between the four images obtained with DES (subgroup DES images: high/low kV and subtracted images) were assessed by performing ANOVA using the LD_F1 design. This was performed to assess pathological findings, abnormalities on chest radiographs, and reporting confidence scores. The relative treatment effect was evaluated for the variables in which a significant difference was obtained for pairwise comparison. Radiation dose measurements performed for DES and PA-CR were assessed using the MATLAB platform and ANOVA methods. A p value of < 0.05 was considered statistically significant.

Results

We found no significant differences in terms of age and sex among the patients included in the present study (p>0.05). The intra-observer agreement values for chest radiograph findings and reporting confidence scores were generally excellent for DES, PA-CR, and subgroup DES (D1–D4: raw and subtracted versions) images (Tables 1–4). On the other hand, the inter-observer agreement values for chest radiograph findings and reporting confidence scores were generally good for these images (Tables 5–8).

Table 1

Intra-observer agreement results between DSR and PA-CR findings for chest roentgenogram findings (parameters) scores.

Groups	Parameters	Kappa value	SD	P-value
DSR scores	Diaphragm integrity	1.000	0.000	< 0.001
	Pleural effusion	1.000	0.000	< 0.001
	Atelectasis	1.000	0.000	< 0.001
	Heart borders	0.869	0.064	< 0.001
	Hilar abnormality	1.000	0.000	< 0.001
	Morphology of costae	0.795	0.200	< 0.001
	Vertebrae	0.976	0.024	< 0.001
	Clavicle	1.000	0.000	< 0.001
	Trachea	1.000	0.000	< 0.001
	Bronchovascular abnormality	0.928	0.072	< 0.001
	Aortic morphology	1.000	0.000	< 0.001
	Fissures	0.826	0.075	< 0.001
	Kerley lines	1.000	0.000	< 0.001
	Consolidation	0.883	0.115	< 0.001
	Presence of air bronchogram	1.000	0.000	< 0.001
	Solitary nodule	0.846	0.106	< 0.001
	Lung regions behind the heart	1.000	0.000	< 0.001
	Fibrotic lung shrinkage	0.852	0.083	< 0.001
PA-CR scores	Diaphragm integrity	1.000	0.000	< 0.001
	Pleural effusion	1.000	0.000	< 0.001
	Atelectasis	1.000	0.000	< 0.001
	Heart borders	0.833	0.072	< 0.001
	Hilar abnormality	1.000	0.000	< 0.001
	Morphology of costae	1.000	0.000	< 0.001
	Vertebrae	0.973	0.027	< 0.001
	Clavicle	1.000	0.000	< 0.001
	Trachea	1.000	0.000	< 0.001
	Bronchovascular abnormality	0.928	0.072	< 0.001
	Aortic morphology	1.000	0.000	< 0.001
	Fissures	0.826	0.075	< 0.001
	Kerley lines	1.000	0.000	< 0.001
	Consolidation	0.883	0.115	< 0.001
	Presence of air bronchogram	1.000	0.000	< 0.001
	Solitary nodule	0.904	0.095	< 0.001
	Lung regions behind the heart	1.000	0.000	< 0.001
	Fibrotic lung shrinkage	0.897	0.071	< 0.001

Table 4

Intra-observer agreement results of reporting confidence scores in each DSR subgroup (D1-D4) images.

GROUPS	D1			D2			D3			D4
Parameters	K values	SD	P-values	K values	SD	P-values	K values	SD	P-values	K values	SD	P-values
Diaphragm integrity	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Pleural effusion	1.000	0.000	< 0.001	1.000	0.000	< 0.001	0.937	0.063	< 0.001	1.000	0.000	< 0.001
Atelectasis	0.918	0.082	< 0.001	0.928	0.071	< 0.001	0.967	0.033	< 0.001	1.000	0.000	< 0.001
Heart borders	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Hilar abnormality	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Morphology of costae	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Vertebrae	0.904	0.038	< 0.001	0.916	0.036	< 0.001	0.916	0.036	< 0.001	0.921	0.034	< 0.001
Clavicle	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Trachea	0.853	0.145	< 0.001	0.853	0.144	< 0.001	0.928	0.050	< 0.001	1.000	0.000	< 0.001
Bronchovascular abnormality	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Aortic morphology	0.492	0.306	< 0.001	0.492	0.306	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Fissures	0.880	0.066	< 0.001	0.808	0.089	< 0.001	0.835	0.064	< 0.001	0.884	0.115	< 0.001
Kerley lines	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Consolidation	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Presence of air bronchogram	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Solitary nodule	0.866	0.093	< 0.001	0.848	0.106	< 0.001	0.656	0.184	< 0.001	0.796	0.199	< 0.001
Lung regions behind the heart	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Fibrotic lung shrinkage	0.800	0.097	< 0.001	0.759	0.115	< 0.001	0.830	0.095	< 0.001	0.796	0.199	< 0.001

Table 5

Inter-observer agreement results between DSR and PA-CR findings for chest roentgenogram parameters scores.

Groups	Parameters	Kappa values	SD	P-values
DSR	Diaphragm integrity	− 0.016	0.012	0.857
	Pleural effusion	0.716	0.101	< 0.001
	Atelectasis	0.795	0.080	< 0.001
	Heart borders	0.182	0.118	0.051
	Hilar abnormality	0.206	0.124	0.001
	Morphology of costae	0.420	0.210	< 0.001
	Vertebrae	0.478	0.074	< 0.001
	Clavicle	1.000	0.000	< 0.001
	Trachea	− 0.011	0.007	0.918
	Bronchovascular abnormality	0.468	0.111	< 0.001
	Aortic morphology	0.826	0.063	< 0.001
	Fissures	0.432	0.119	< 0.001
	Kerley lines	1.000	0.000	< 0.001
	Consolidation	0.556	0.229	< 0.001
	Presence of air bronchogram	0.558	0.225	< 0.001
	Solitary nodule	0.471	0.181	< 0.001
	Lung regions behind the heart	1.000	0.000	< 0.001
	Fibrotic lung shrinkage	0.498	0.125	< 0.001
PA-CR	Diaphragm integrity	0.312	0.253	0.002
	Pleural effusion	0.583	0.118	< 0.001
	Atelectasis	*	*	*
	Heart borders	*	*	*
	Hilar abnormality	*	*	*
	Morphology of costae	0.484	0.216	< 0.001
	Vertebrae	0.512	0.079	< 0.001
	Clavicle	1.000	0.000	< 0.001
	Trachea	*	*	*
	Bronchovascular abnormality	0.429	0.107	< 0.001
	Aortic morphology	0.826	0.063	< 0.001
	Fissures	*	*	*
	Kerley lines	1.000	0.000	< 0.001
	Consolidation	0.368	0.202	< 0.001
	Presence of air bronchogram	0.558	0.225	< 0.001
	Solitary nodule	0.579	0.189	< 0.001
	Lung regions behind the heart	0.662	0.316	< 0.001
	Fibrotic lung shrinkage	0.401	0.136	< 0.001

*: Statistical analyses cannot be performed due to small case numbers.

Table 8

Inter-observer agreement results of reporting confidence scores in each DSR subgroup (D1-D4) images.

GROUPS	D1			D2			D3			D4
Parameters	K values	SD	P-values	K values	SD	P-values	K values	SD	P-values	K values	SD	P-values
Diaphragm integrity	*	*	*	*	*	*	0.137	0.088	0.074	*	*	*
Pleural effusion	1.000	0.000	< 0.001	1.000	0.000	< 0.001	*	*	*	1.000	0.000	< 0.001
Atelectasis	0.225	0.202	0.011	*	*	*	*	*	*	*	*	*
Heart borders	1.000	0.000	< 0.001	1.000	0.000	< 0.001	*	*	*	1.000	0.000	< 0.001
Hilar abnormality	− 0.018	0.014	0.831	− 0.018	0.014	0.831	*	*	*	*	*	*
Morphology of costae	*	*	*	*	*	*	*	*	*	*	*	*
Vertebrae	0.214	0.077	0.001	0.133	0.061	0.012	0.070	0.082	0.304	0.091	0.052	0.073
Clavicle	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	*	*	*
Trachea	*	*	*	*	*	*	*	*	*	*	*	*
Bronchovascular abnormality	*	*	*	*	*	*	*	*	*	*	*	*
Aortic morphology	− 0.015	0.010	0.882	− 0.015	0.010	0.882	*	*	*	*	*	*
Fissures	*	*	*	*	*	*	*	*	*	*	*	*
Kerley lines	*	*	*	*	*	*	*	*	*	*	*	*
Consolidation	*	*	*	*	*	*	*	*	*	− 0.011	0.008	0.917
Presence of air bronchogram	1.000	0.000	< 0.001	1.000	0.000	< 0.001	*	*	*	1.000	0.000	< 0.001
Solitary nodule	− 0.009	0.010	0.175	0.003	0.002	0.707	0.048	0.079	0.560	*	*	*
Lung regions behind the heart	*	*	*	*	*	*	*	*	*	− 0.030	0.052	0.693
Fibrotic lung shrinkage	*	*	*	*	*	*	*	*	*	− 0.020	0.010	0.805

*: Statistical analyses cannot be performed due to small case numbers.

When we compared DES and PA-CR in terms of reporting confidence, we found a significant difference in terms of certain parameters such as diaphragm integrity, heart border definition, and hilar/costal/vertebral abnormalities.

The reporting confidence scores for these parameters were higher in DES than in PA-CR (Tables 9–10, Diagram 1).

Table 9

Statistical differences between DSR and PA-CR results for each chest roentgenogram findings and reporting confidence scores (ANOVA test results).

	Parameters	F-values	P-values
Chest roentgenogram findings	Diaphragm integrity	NA	NA
	Pleural effusion	1.000	0.317
	Atelectasis	1.000	0.317
	Heart borders	NA	NA
	Hilar abnormality	1.000	0.317
	Morphology of costae	1.000	0.317
	Vertebrae	1.048	0.306
	Clavicle	NA	NA
	Trachea	NA	NA
	Bronchovascular abnormality	NA	NA
	Aortic morphology	NA	NA
	Fissures	NA	NA
	Kerley lines	NA	NA
	Consolidation	NA	NA
	Presence of air bronchogram	NA	NA
	Solitary nodule	2.021	0.155
	Lung regions behind the heart	1.000	0.317
	Fibrotic lung shrinkage	1.000	0.317
Reporting confidence	Diaphragm integrity	472.297	< 0.001
	Pleural effusion	NA	NA
	Atelectasis	NA	NA
	Heart borders	5.300	0.021
	Hilar abnormality	2062.790	< 0.001
	Morphology of costae	11.047	< 0.001
	Vertebrae	13.901	< 0.001
	Clavicle	NA	NA
	Trachea	3973.634	< 0.001
	Bronchovascular abnormality	817.000	< 0.001
	Aortic morphology	1.000	0.317
	Fissures	1.180	0.277
	Kerley lines	NA	NA
	Consolidation	NA	NA
	Presence of air bronchogram	NA	NA
	Solitary nodule	NA	NA
	Lung regions behind the heart	1.000	0.317
	Fibrotic lung shrinkage	1.000	0.317

NA: Statistical analysis is not possible.

Table 10

Relative effects of the variables that were found statistically significant in the accordance with groups of DSR and PA-CR technique in reporting confidence results.

	Parameters	DSR Relative effects	PA-CR Relative effects
Reporting confidences	Diaphragm integrity	0.709	0.290
	Heart borders	0.513	0.487
	Hilar abnormality	0.740	0.260
	Morphology of costae	0.526	0.473
	Vertebrae	0.467	0.533
	Trachea	0.742	0.258
	Bronchovascular abnormality	0.724	0.276

Diagram 1.

Graphics of the reporting confidence scores for DES and PA-CR.

Additional findings that could not be determined using PA-CR were detected using DES in 10 patients (Figure 1). These additional findings included the following: pleural effusion (n = 2, 2%), indefinite diaphragm/heart/aortic borders (n = 5, 5%), atelectasis (n = 2, 2%), sclerotic costal lesion (n = 1, 1%), solitary nodules (n = 2, 2%), and fibrotic lung shrinkage (n = 2, 2%).

Figure 1.

PA radiograph (left), soft-tissue-subtracted DES (middle), coronal reformatted CT (right) images of a 72-year-old male demonstrates multiple focal consolidations and ground-glass opacifications, particularly in the right lung. These images clearly show that soft-tissue-subtracted DES images are more sensitive than conventional X-ray images. Remarkably, ground-glass opacification in the right upper lobe is present on soft-tissue-subtracted DES (arrow, middle) and CT (right) images but not on conventional X-ray image (left).

When we compared DES subgroup (D1–D4) images in terms of reporting confidence scores and relative treatment effect values, we found significant differences in terms of the following parameters: diaphragm integrity; pleural effusion; atelectasis; hilar abnormality; costal, vertebral, tracheal, and fissures morphology; bronchovascular abnormality; consolidation; air bronchograms; solitary nodules; and fibrotic lung shrinkage (Table 11). For instance, the reporting confidence scores for D3 images in the evaluation of hilar abnormality and presence of air bronchograms were lower than those in the assessment of bone structure (Diagram 2, Figure 2). However, the reporting confidence scores for D4 images in the evaluation of consolidation and solitary nodules were higher than for the other images (Table 11, Diagram 2).

Table 11

Relative effects of the variables that found statistically significant in accordance with the groups of D1, D2, D3 and D4 in the results of reporting confidence of the first observer for each group.

	Parameters	D 1 Relative effects	D 2 Relative effects	D 3 Relative effects	D 4 Relative effects
Reporting confidences	Diaphragm integrity	0.516	0.521	0.477	0.487
Pleural effusion	0.512	0.512	0.465	0.512
	Atelectasis	0.512	0.512	0.465	0.512
	Hilar abnormality	0.625	0.625	0.130	0.620
	Morphology of costae	0.376	0.376	0.875	0.372
	Vertebrae	0.434	0.432	0.706	0.428
	Trachea	0.509	0.502	0.143	0.847
	Bronchovascular abnormality	0.617	0.619	0.133	0.631
	Fissures	0.472	0.483	0.213	0.831
	Consolidation	0.390	0.390	0.364	0.856
	Presence of air bronchogram	0.625	0.625	0.130	0.620
	Solitary nodule	0.385	0.398	0.379	0.838
	Fibrotic lung shrinkage	0.506	0.506	0.150	0.838

Diagram 2.

Graphics of the reporting confidence scores for DES subgroup images.

Figure 2.

PA X-ray (upper line) and coronal CT (lower line) images of a 61-year-old male showing emphysematous areas (predominantly located in the lower lobes) and left hilar mass. Soft-tissue-subtracted DES images (D4, upper right) is more useful than conventional PA radiograph (upper left) for the evaluation of these pathological changes. Emphysematous areas were not reported in the conventional PA radiograph report.

There was a significant difference in terms of the radiation exposure dose between DES and PA-CR (Table 12). The mean radiation exposure dose was 107 ± 39.12 uSv for DES and 219 ± 44.18 uSv for PA-CR (p < 0.05). According to the mean values of the two datasets, DES used 48% radiation dose according to the PA-CR system.

Table 12

For dose comparison between DES and conventional CR devices. We have used the ANOVA test (variance analysis). The boxplot on this table shows us mean values are different for different X-ray techniques. Y-axis is for DES dose usage, the x-axis is for CR dose usage, boxplots are for the X-ray studies dose outputs correlated between CR and DES. CR dose results have been taken via a single shot during the exposure. DES results are the cumulative summation of two exposures depending on the dual energy. Depending on the means of two datasets DES system uses %48 percent radiation dose compared to the conventional CR system (mean doses for DES and CR are 218.85 and 106.72, respectively).

Discussion

Chest roentgenography is the main imaging technique in thoracic radiology.[1,2] Accurate and efficient use of imaging techniques such as CR decreases the need for CT as a secondary imaging technique, thus preventing additional radiation exposure.[3] The primary difficulty in assessing chest radiographs is the superimposition of several structures due to the projection of three-dimensional volumetric structures as a two-dimensional image.[3] For instance, the superimposition of bone structures can obscure the lesions in the lung parenchyma.[4] We obtained two raw images using DES (D1 and D2) and two subtracted images (D3 and D4); this facilitates better evaluation by reducing superimposition.[5,7] The data obtained in this study support this hypothesis.

DES used in the present study is a relatively new technique. It uses a full-field digital FPD with high quantum efficiency and frame rate, functions based on double shot (DS), and causes lower radiation exposure than single-shot systems.[7-12] Low-energy images are obtained with a low peak voltage (60–90 kVp) and high-energy images with a high peak voltage (120–150 kVp) in DS-DES.[2,5] Another advantage of DS-DES is a high DQE value.[8-13] A higher DQE value allows better identification of an object against a noisy background. The devices with higher DQE values provide better visualization of inadequate-quality images with less radiation exposure.[8-13] The exposure radiation dose for DS-DES using a new-generation FPD system is 35%–50% less than that for PA-CR, consistent with our results.[4,7-14]

Data composed of four images can be obtained after each DS-DES acquisition.[5] These images are as follows: raw image (D1) obtained with a high peak voltage, raw image (D2) obtained with a low peak voltage, bone-subtracted image (D3), and soft-tissue-subtracted image (D4).[5-10,14] To obtain subtracted images, the data from the raw image with the desired contrast are mathematically multiplied by two or three, and the other raw image is obtained from that enhanced image.[1,5]

Swensen et al.,[15] compared DES and PA-CR in a study on 50 patients with lung cancer. They found that mediastinal structures or pathological findings could be better demonstrated with DES. Tagashiara et al.,[16] also reported that DES could detect more nodules than PA-CR. Observer error, superimpositions of the surrounding tissues/bones, lesion characteristics (size, conspicuity, and location), and technical defects are the main causes of undetected lung cancer.[17] DS-DES may help reduce diagnostic errors and interpretation/reporting time.[17] To the best of our knowledge, there are few published reports focusing on the DS-DES used in the present study.

We found high-degree similarity between inter- and intra-observer agreement values for DES and PA-CR images. Our results show that DES is a reliable method for thoracic evaluation.

In general, reporting confidence scores for DES were higher than those for PA-CR. These results show that DES facilitates the evaluation of chest radiographs. DES can also prevent misdiagnosis caused by less-experienced radiologists.

In addition, the joint assessment of all DES subgroup images provides a more efficient analysis of several abnormalities compared with the assessment of PA-CR images. Our results are supported by those of Tagashira et al.,[16] For instance, in our study, atelectasis or solitary nodules could be seen on the DES images of two patients but not on the PA-CR images of the same patients.

To the best of our knowledge, our study is the first to compare DES subgroup images. D3 images emphasizing bone structures were usually more efficient in assessing bone pathologies than other images. For instance, costal disorder could be detected only using DES images (particularly the D3 images) in one patient. Similarly, D4 images were more effective in the detection of solitary nodules, fibrotic lung shrinkage, and consolidation. Previous reports show that DES is superior to other techniques in the evaluation of solitary nodules.[5,16] Austin et al.,[18] demonstrated that 81% of undetectable lung cancers occurred in the region that obscures solitary nodules (particularly in the upper lobes of the lungs). Uemura et al.,[19] and Ricke et al.,[20] indicated that the detectability of solitary nodules is higher in DES images than in PA-CR images in patients with a history of solitary nodules. This is most likely due to the elimination of bones in soft-tissue-subtracted (D4) images. This study also showed that DES could reduce the need to use CT for follow-up analysis of solitary nodules. Because there are no studies that demonstrate this claim in the literature, further studies are needed to test the validity of our suggestion.

Each subgroup image should be evaluated separately for more accurate assessment of DES images. Bone structures should be re-evaluated using bone-subtracted (D3) images after these assessments. The re-evaluation of soft tissues and lungs using soft-tissue-subtracted (D4) images could also increase reporting impact and confidence. However, DES is not the ideal technique. For instance, DES may not be useful in the detection of some small nodules obscured by soft tissues. Because DES relies on projection; it has some limitations related to superimposition similar to PA-CR.[21,22] In addition, subtracted images are highly susceptible to motion effects.[5,23] Thus, motion artifacts could be seen, particularly at the contours of the diaphragm and heart, on these images. Low-dose CT may be useful for overcoming this limitation.[22]

The major limitation of our study is that the number of patients who had abnormal findings was small. Therefore, the assessment of some parameters and more detailed statistical analyses could not be performed sufficiently for all chest radiograph findings. The second limitation is that the findings obtained on DES and PA-CR could not be compared with gold standard methods (such as CT) in all patients. In addition, the low-level experience of the radiologists in performing DES is a limitation. Furthermore, the scoring methods used are subjective and may be influenced by interpersonal differences in interpretation. As observed by Manji et al.,[23] the preliminary nature of our study as well as limited time and resources limited the numbers of readers and patients. Lastly, the interval between DES and PA-CR is another limitation. Further comprehensive studies are needed to overcome these limitations.

Conclusion

DES is a technique that allows more accurate and comfortable evaluation of soft tissues and bone structures. DES can increase the visibility of parenchymal and bone abnormalities obscured in the background by eliminating structural noise. It can decrease the need for chest CT examination, which prevents further radiation exposure. The inter- and intra-observer reliability values of DES are generally good. Further, DES may decrease the radiation exposure dose.

Table 2

Intra-observer agreement results between DSR and PA-CR findings for reporting confidence scores.

Groups	Parameters	Kappa values	SD	P-values
DSR	Diaphragm integrity	0.930	0.000	< 0.001
	Pleural effusion	1.000	0.000	< 0.001
	Atelectasis	1.000	0.000	< 0.001
	Heart borders	1.000	0.000	< 0.001
	Hilar abnormality	1.000	0.000	< 0.001
	Morphology of costae	1.000	0.000	< 0.001
	Vertebrae	0.977	0.023	< 0.001
	Clavicle	1.000	0.000	< 0.001
	Trachea	1.000	0.000	< 0.001
	Bronchovascular abnormality	1.000	0.000	< 0.001
	Aortic morphology	1.000	0.000	< 0.001
	Fissures	0.795	0.200	< 0.001
	Kerley lines	1.000	0.000	< 0.001
	Consolidation	1.000	0.000	< 0.001
	Presence of air bronchogram	1.000	0.000	< 0.001
	Solitary nodule	1.000	0.000	< 0.001
	Lung regions behind the heart	1.000	0.000	< 0.001
	Fibrotic lung shrinkage	1.000	0.000	< 0.001
PA-CR	Diaphragm integrity	0.758	0.134	< 0.001
	Pleural effusion	1.000	0.000	< 0.001
	Atelectasis	1.000	0.000	< 0.001
	Heart borders	1.000	0.000	< 0.001
	Hilar abnormality	0.742	0.175	< 0.001
	Morphology of costae	1.000	0.000	< 0.001
	Vertebrae	0.963	0.036	< 0.001
	Clavicle	1.000	0.000	< 0.001
	Trachea	1.000	0.000	< 0.001
	Bronchovascular abnormality	0.642	0.146	< 0.001
	Aortic morphology	1.000	0.000	< 0.001
	Fissures	0.905	0.094	< 0.001
	Kerley lines	1.000	0.000	< 0.001
	Consolidation	1.000	0.000	< 0.001
	Presence of air bronchogram	1.000	0.000	< 0.001
	Solitary nodule	1.000	0.000	< 0.001
	Lung regions behind the heart	1.000	0.000	< 0.001
	Fibrotic lung shrinkage	1.000	0.000	< 0.001

Table 3

Intra-observer agreement results of chest roentgenogram findings scores in each DSR subgroup (D1-D4) images.

GROUPS	D1			D2			D3			D4
Parameters	K values	SD	P-values	K values	SD	P-values	K values	SD	P-values	K values	SD	P-values
Diaphragm integrity	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Pleural effusion	0.871	0.073	< 0.001	0.871	0.073	< 0.001	0.905	0.066	< 0.001	0.871	0.073	< 0.001
Atelectasis	0.969	0.031	< 0.001	0.969	0.031	< 0.001	0.969	0.030	< 0.001	0.968	0.032	< 0.001
Heart borders	0.855	0.063	< 0.001	0.855	0.063	< 0.001	0.882	0.058	< 0.001	0.855	0.063	< 0.001
Hilar abnormality	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Morphology of costae	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Vertebrae	0.878	0.048	< 0.001	0.882	0.046	< 0.001	0.850	0.059	< 0.001	0.875	0.049	< 0.001
Clavicle	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Trachea	1.000	0.000	< 0.001	1.000	0.000	< 0.001	0.954	0.046	< 0.001	1.000	0.000	< 0.001
Bronchovascular abnormality	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Aortic morphology	0.808	0.064	< 0.001	0.833	0.060	< 0.001	0.855	0.057	< 0.001	0.833	0.060	< 0.001
Fissures	0.817	0.079	< 0.001	0.786	0.084	< 0.001	0.850	0.073	< 0.001	0.850	0.073	< 0.001
Kerley lines	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Consolidation	0.884	0.115	< 0.001	0.884	0.115	< 0.001	0.883	0.115	< 0.001	0.884	0.115	< 0.001
Presence of air bronchogram	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Solitary nodule	0.846	0.106	< 0.001	0.846	0.106	< 0.001	1.000	0.000	< 0.001	0.784	0.120	< 0.001
Lung regions behind the heart	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Fibrotic lung shrinkage	0.852	0.083	< 0.001	0.852	0.083	< 0.001	0.855	0.082	< 0.001	0.852	0.083	< 0.001

Table 6

Inter-observer agreement results between DSR and PA-CR findings for reporting confidence scores.

Groups	Parameters	Kappa values	SD	P-values
DSR	Diaphragm integrity	*	*	*
	Pleural effusion	1.000	0.000	< 0.001
	Atelectasis	− 0.011	0.007	0.918
	Heart borders	*	*	*
	Hilar abnormality	1.000	0.000	< 0.001
	Morphology of costae	1.000	0.000	< 0.001
	Vertebrae	0.150	0.046	< 0.001
	Clavicle	1.000	0.000	< 0.001
	Trachea	0.489	0.311	< 0.001
	Bronchovascular abnormality	1.000	0.000	< 0.001
	Aortic morphology	*	*	*
	Fissures	1.000	0.000	< 0.001
	Kerley lines	1.000	0.000	< 0.001
	Consolidation	1.000	0.000	< 0.001
	Presence of air bronchogram	− 0.011	0.007	0.918
	Solitary nodule	1.000	0.000	< 0.001
	Lung regions behind the heart	− 0.014	0.010	0.883
	Fibrotic lung shrinkage	1.000	0.000	< 0.001
PA-CR	Diaphragm integrity	*	*	*
	Pleural effusion	1.000	0.000	< 0.001
	Atelectasis	*	*	*
	Heart borders	0.167	0.132	0.042
	Hilar abnormality	*	*	*
	Morphology of costae	*	*	*
	Vertebrae	0.236	0.060	< 0.001
	Clavicle	1.000	0.000	< 0.001
	Trachea	0.029	0.022	0.066
	Bronchovascular abnormality	− 0.021	0.021	0.003
	Aortic morphology	1.000	0.000	< 0.001
	Fissures	0.426	0.208	< 0.001
	Kerley lines	1.000	0.000	< 0.001
	Consolidation	1.000	0.000	< 0.001
	Presence of air bronchogram	1.000	0.000	< 0.001
	Solitary nodule	1.000	0.000	< 0.001
	Lung regions behind the heart	*	*	*
	Fibrotic lung shrinkage	1.000	0.000	< 0.001

*: Statistical analyses cannot be performed due to small case numbers.

Table 7

Inter-observer agreement results of chest roentgenogram findings scores in each DSR subgroup (D1-D4) images.

GROUPS	D1			D2			D3			D4
Parameters	K values	SD	P-values	K values	SD	P-values	K values	SD	P-values	K values	SD	P-values
Diaphragm integrity	0.314	0.251	0.001	0.314	0.251	0.001	0.314	0.251	0.001	0.314	0.251	0.001
Pleural effusion	0.524	0.126	< 0.001	0.524	0.126	< 0.001	*	*	*	0.215	0.133	0.038
Atelectasis	0.754	0.088	< 0.001	0.754	0.088	< 0.001	*	*	*	0.354	0.121	0.001
Heart borders	0.186	0.115	0.040	0.186	0.020	0.040	0.186	0.115	0.040	0.008	0.089	0.925
Hilar abnormality	0.205	0.124	0.001	0.205	0.124	0.001	*	*	*	*	*	*
Morphology of costae	0.657	0.225	< 0.001	0.657	0.225	< 0.001	0.657	0.225	< 0.001	*	*	*
Vertebrae	0.391	0.073	< 0.001	0.380	0.075	< 0.001	0.404	0.078	< 0.001	0.112	0.074	0.137
Clavicle	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Trachea	− 0.011	0.008	0.917	− 0.011	0.008	0.917	*	*	*	*	*	*
Bronchovascular abnormality	0.471	0.127	< 0.001	0.471	0.127	< 0.001	0.429	0.121	< 0.001	0.298	0.132	0.001
Aortic morphology	0.627	0.090	< 0.001	0.657	0.087	< 0.001	0.657	0.087	< 0.001	0.163	0.109	0.117
Fissures	0.411	0.129	< 0.001	0.411	0.129	< 0.001	0.435	0.132	< 0.001	0.247	0.130	0.010
Kerley lines	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Consolidation	0.657	0.225	< 0.001	0.657	0.225	< 0.001	0.657	0.225	< 0.001	− 0.030	0.015	0.762
Presence of air bronchogram	0.657	0.225	< 0.001	0.657	0.225	< 0.001	0.657	0.225	< 0.001	− 0.030	0.015	0.762
Solitary nodule	0.470	0.181	< 0.001	0.470	0.181	< 0.001	0.578	0.190	< 0.001	0.264	0.169	0.005
Lung regions behind the heart	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001	1.000	0.000	< 0.001
Fibrotic lung shrinkage	0.524	0.125	< 0.001	0.524	0.125	< 0.001	*	*	*	0.216	0.132	0.035