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British Journal of Radiology (2005) 78, 922-927
© 2005 British Institute of Radiology
doi: 10.1259/bjr/75926824
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Full Paper

Comparison of a digital flat-panel versus screen–film, photofluorography and storage-phosphor systems by detection of simulated lung adenocarcinoma lesions using hard copy images

K Ono, PhD, RT1, T Yoshitake, RT2, K Akahane, PhD3, Y Yamada, MD1, T Maeda, MD1, M Kai, PhD4 and T Kusama, PhD4

1 Oita Prefectural Hospital, 476 Bunyo Oita City 870-8511, 2 Shin Beppu Hospital, 3898 Turumi Beppu City 874-0833, 3 National Institute of Radiological Sciences, 4-9-1 Anakawa, Inageku, Chiba City 263-8555 and 4 Oita University of Nursing and Health Sciences, 2944-9 Megusuno, Oita 870-1201, Japan


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 References
 
The purpose of this study was to compare hard copy images from a flat-panel detector digital radiography system with conventional radiography, photofluorographic radiography and storage phosphor radiography for the detection of simulated lung adenocarcinoma lesions and also for radiation dose. To test the diagnostic performance of these four systems, the authors used 15 types of lung adenocarcinoma phantom according to Noguchi's classification and an anthropomorphic chest phantom. The visual evaluation of tumour detectability by four radiologists and two general thoracic surgeons was examined with a five-level confidence scale. Lung doses were measured with glass dosemeters for the chest radiology systems under the conditions used by each hospital and centre. Our results indicated that flat-panel detector digital radiography and storage phosphor radiography are not necessarily superior to conventional radiography and photofluorographic radiography for detecting lung adenocarcinomas when only hard copy images are used, and this suggests a need to carefully optimize chest radiography.


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 References
 
Recent studies show that adenocarcinoma has become the most common type of lung carcinoma [1]. Recent technical advances, such as helical CT, have been developed and evaluated for lung cancer screening [2, 3]. However, helical CT is not often used because of cost and dose implications. Plain chest X-ray radiography is the main screening method. Pulmonary adenocarcinoma is characteristically demonstrated as an area of ground-glass opacity (GGO) [4–6]. It is difficult to detect GGO by chest X-ray examination [7]. Noguchi et al [8] reported a classification of small adenocarcinoma defined as Type A~F. Type C appears to be an advanced stage of Types A and B. The 5-year survival rate for Types A and B were 100%, and for Type C was 74.8%. It is important to evaluate chest X-ray image quality among various X-ray systems as it has been reported that the detection rate of simulated chest lesions varies according to the chest radiography systems used [9–11]. In previous phantom studies for simulated lung adenocarcinomas, however, no theoretical consideration was given to the pathological and biological features of lung adenocarcinoma. The shape and density of a simulated phantom strongly influence performance in the detection of lung adenocarcinoma. We have developed a phantom based on clinical information from CT to compare both image quality and doses among various chest radiography systems. The objective of this phantom study was to compare lesion detection on hard copy and lung dose between a flat-panel detector digital radiography system, conventional radiography, photofluorographic radiography and storage phosphor radiographs.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 References
 
Test phantom
Our study setup was similar to that of previous reports [9–11]. We used an anthropomorphic chest phantom (PBU-S-21; Kyotokagaku Co. Ltd, Japan) and 15 types of simulated lung adenocarcinomas according to Noguchi's classification (Table 1Go). The shape of lung adenocarcinoma was assumed to be spherical with different density in the core. The core corresponds to foci of structural collapse of alveoli and active fibroblastic foci. A cross-sectional drawing of the simulated small adenocarcinoma is shown in Figure 1Go. The shape was obtained by calculating the equivalent thickness from X-ray transmission which leads to the same shadow as the sphere. The simulated lesions were constructed from water-equivalent material (Kyotokagaku Co. Ltd., Japan). A selection of the test objects are shown in Figure 2Go and were attached to the anthoropomorphic chest phantom, as illustrated in Figure 3Go.


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  		Table 1. The characteristics of small adenocarcinoma phantoms

 


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  		Figure 1. A cross-sectional drawing of the spherical simulation of small adenocarcinoma. The core corresponds to foci of structural collapse of alveoli and active fibroblastic foci.

 


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  		Figure 2. Simulated small adenocarcinomas.

 


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  		Figure 3. A schematic diagram showing the location of the spherical simulation of a small adenocarcinoma on radiography.

 
Theory of the simulated lung adenocarcinoma lesions
We assumed that the shape of the lung adenocarcinoma is simulated by the spherical model with radius R that has CT values which are constant in the centres (density {rho}in) and that vary linearly with radius at the periphery from {rho}in to {rho}out where {rho}out is the density at the periphery of the spherical model. The radius of the inner circle is expressed as a fraction, f, of R. The shape of the simulated lung adenocarcinoma lesion to be placed on the surface of a chest phantom is that which gives the same X-ray transmission as the spherical model. The density as a function of radius r, {rho}(r), is expressed as: Go


{922equ1}

(See Figure 1Go).

Generally, for low atomic number materials, it has been assumed that the linear attenuation coefficient is proportional to the density of the material traversed. If the incident photon traverses a material in which the density varies with transmission distance, the logarithm of transmission will be proportional to the integral of the density along the track. This will be called the density integral.

The density integral of the core in the spherical model in the direction of the y-axis, Pcore(x) (0≤x≤fR), is: Go


{922equ2}

where {922equ5} and {922equ6}

The density integral of the outer part in the spherical model, Pouter(x), is also given by: Go


{922equ3}

The density integral for a normal lung without lung adenocarcinoma is: Go


{922equ4}

where {rho}lung is the density of normal lung.

In the mathematical deduction for the spherical simulation used in our study, the density integral given by Equation (4)Go was subtracted from the density integrals obtained by Equation (2)Go and Equation (3)Go.

Systems studied
Posteroanterior chest radiographs were obtained by using photofluorographic, conventional screen–film and digital detector systems. All imaging was performed using the individual hospital's protocols. Exposure parameters are summarized in Table 2Go. The eight imaging systems are listed in Table 3Go. The development conditions and films used for hard copy for each system are given in Table 4Go.


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  		Table 2. Exposure conditions for the radiographic systems tested

 

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  		Table 3. Exposure conditions for the radiographic systems tested

 

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  		Table 4. Development conditions of the film processors for tested radiographic systems

 
Image evaluation
All images were assessed independently by four radiologists and two general thoracic surgeons, who recorded the presence or absence of a lesion. Each image was ranked according to the following five-point confidence scale: 1=definitely no lesion; 2=probably no lesion; 3=indeterminate; 4=lesion probably present; and 5=lesion definitely present. There was no limit imposed on reading time. This resulted in a total of 720 observations (15 images x 6 readers x 8 imaging systems). Readers were blinded to exposure setting and imaging systems. Photofluorographic radiography was viewed on a special light box. All other images were viewed on the same light box with adjustable shutters under subdued ambient light.

Dosimetry
Digital and conventional chest radiographic doses were measured using a thorax phantom (THRA1, Kyotokagaku Co. Ltd, Japan) in which 20 glass dosemeters were inserted. The RPL glass dosemeters used in this study are GD-352M of the RPL system (Dose Ace of Chiyoda Technol Co. Ltd, Japan). The diameter of the glass dosemeter is 1.5 mm. Each glass dosemeter was put in a yellow plastic case which has a Sn filter for adjusting photon energy dependence. After irradiation, the dosemeters were read in the FGD-1000 reader. Calibration of the glass dosemeters was carried out using a Capintec PH-05 chamber, whose own calibration was traceable to national dosimetric standards. The calibration of the reader was automatically performed using an internal calibration glass. The doses were measured at 20 points in lung, each point contributing three readings.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 References
 
The result of image evaluation is shown in Table 5Go. For 96 observations (2 images x 6 readers x 8 imaging systems) of normal images with no lesion, 95 indicated "definitely no lesion" and one was "probably no lesion". The detection rates of Nos. 5, 10, 11, 14 and 15 simulated lesions (Type A) were 2.1–14.6% and Nos. 1, 2, 3, 8, 9, 12 and 13 simulated lesions (Type B) were 25.0–100%. No. 4 simulated lesions (Type C) averaged 93.8% among all the radiography systems. The results were characteristic of three types of adenocarcinoma in Noguchi's classification. The average performance in the detection of simulated lesions in storage phosphor radiography (System D) was higher than that in the other systems. Our study indicated that digital radiography (the flat-panel detector digital radiography system and the storage phosphor radiography system) showed no statistically significant difference from analogue radiography systems (the photofluorographic system and the conventional screen–film system) for hard copy reporting. The storage phosphor radiography system (System D) was superior to System C for the detection of simulated lesions (p<0.05). Among the flat-panel detector digital radiography systems with different processing and film size, there were no statistically significant differences.


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  		Table 5. Percentage detection rate of lesions simulated by each phantom, obtained as a result of visual evaluation by observers

 
Table 6Go shows the lung dose for each radiography system. The smallest dose was 73 µGy on the conventional screen–film system. The highest dose was 198 µGy on the photofluorographic system. The dose from the flat-panel detector digital radiography systems ranged from 82 µGy to 126 µGy. The relation between the dose and the detection rate of the simulated lesion is shown in Figure 4Go. The increase in the detection rate of the simulated lesion with dose was not observed at all. No evidence was obtained to demonstrate that the digital system was superior to the analogue system with regard to dose effectiveness.


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  		Table 6. Lung dose from a chest radiograph in each radiography system. The numbers in parentheses show the range over 20 measured points covering the whole volume of the lungs

 


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  		Figure 4. Relation between dose and the detection rate of simulated lesions of lung adenocarcinomas for Type B, phantom No. 1 (•: analogue system, {blacktriangleup}: digital system). Each error bar shows the standard deviation of the detection rate of simulated lesions among observers (four radiologists and two general thoracic surgeons).

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
References
 
To test the diagnostic performance of hard copy images from analogue and digital systems, the authors used 15 types of simulated lung adenocarcinoma on the surface of an anthropomorphic chest phantom. The detection of the small adenocarcinoma on the radiographs by visual evaluation is influenced by tumour size and the extent of GGO. Tsubamoto et al [6] reported that the detection of small peripheral carcinoma on chest radiographs is influenced by tumour size and the extent of GGO as seen at thin-section CT. Yang et al [7] reported that the detection rates of adenocarcinomas with lepidic growth (0% for type A, 29% for type B and 68% for type C) were less than those with hilic growth (100% for types D–F). The results here are similar to those reported by Yang et al. According to Leppert et al [12], no significant differences were found between three imaging systems (a conventional screen–film combination, an asymmetric screen–film system and storage phosphor radiography) used for the detection of nodules. However, in the patient study, an asymmetric screen–film system was significantly superior to both the storage phosphor radiography and the conventional system in the detection of pulmonary nodules.

Schaefer-Prokop et al [13] compared lesion detection with storage phosphor (250 speed), selenium radiography (250 speed) with an antiscatter grid, selenium radiography (450 speed) without an antiscatter grid, an asymmetric screen–film system (400 speed), and a conventional screen–film system (250 speed). It appeared that the selenium detector improves detection of simulated fine line and low-contrast micronodular details and is superior to other detector systems for chest radiography. Yang et al [14] studied the detection of small peripheral lung cancer by storage phosphor chest radiography and concluded that the sensitivity increased with tumour size. Sato et al [15] evaluated the imaging performance of a flat-panel detector digital radiography system and a storage phosphor radiography system. Receiver operating characteristic (ROC) analysis indicated that the flat-panel detector digital radiography system was superior in overall performance. Awai et al [16] showed that selenium-based digital radiography was superior to high-resolution storage phosphor radiography for the detection of pulmonary nodules. Goo et al [17] reported that in the evaluation of soft copy images, the flat-panel detector digital radiography system appears to be superior to the storage phosphor radiography system for the detection of pulmonary nodules. Our study, using hard copy reading only, indicated that digital radiography (the flat-panel detector digital radiography system and the storage phosphor radiography system) gave no statistically significant difference from analogue radiography systems (photofluorographic system and the conventional screen–film system). There was no statistically significant difference between the hard copy images derived from the flat-panel detector digital radiography system and the storage phosphor radiography system for the detection of simulated pulmonary adenocarcinomas. Hard copy images from digital modalities are normally used for reporting in Japan and very few studies are reported from soft copies. The wide dynamic range which characterizes digital images is thus virtually redundant. For visualization of pulmonary adenocarcinomas, the storage phosphor system was best. However, we observed a significant difference (p<0.05) between two hospitals using this modality. The reading technology of the storage phosphor system differed between Konica Co. Ltd, Tokyo, Japan and Fuji Co. Ltd, Tokyo, Japan. One used dual side reading technology, and the other used point scan reading technology.

The dose from the flat-panel detector digital radiography systems ranged from 82 µGy to 126 µGy. However, we observed no significant difference (p>0.05) between two hospitals for detection of pulmonary adenocarcinomas. The increased dose did not necessarily bring a better image quality. It was evident that current practical use of each modality in our study indicated a lack of optimization of exposure or image quality. The automatic setup function for exposure conditions in digital systems caused more marked difference among hospitals, compared with analogue systems.


   Acknowledgments
 
We thank Yoshinobu Kondo of the Nankai Hospital and Tuneyuki Fusatune of the Oita Area Health Support Center. The authors gratefully acknowledge and appreciate the participation of the following radiologists and general thoracic surgeons in the panel of observers: Naoya Yamasaki, PhD, Syuichi Tanoue, MD, Yuriko Okino, MD and Katsuro Furukawa, MD.

Received for publication November 20, 2003. Revision received January 7, 2005. Accepted for publication May 11, 2005.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 

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