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1 All authors: Department of Diagnostic Radiology, University Hospital Regensburg, Franz-Josef-Strauss-Allee 11, 93042 Regensburg, Germany.
Received June 7, 2001;
accepted after revision August 6, 2001.
Address correspondence to M. Strotzer.
Abstract
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MATERIALS AND METHODS. The chest radiographs of 50 patients (age range, 16-79 years; mean age, 57 years) were obtained at two different detector dose levels. Digital images were taken from the same patients in posteroanterior and lateral views with detector doses of 2.5 µGy and 1.8 µGy, respectively, at 125 kVp tube voltage. The cesium iodide—amorphous silicon active-matrix imager had a panel size of 43 x 43 cm, a matrix of 3000 x 3000, and a pixel pitch of 143 µm. Images were presented in a random order to three independent radiologists who were unaware of the dose level at which the images had been obtained. They subjectively rated image quality on a 4-point scale, according to six criteria (presentation of obscured lung, unobscured lung, airways, mediastinum and hilum, bony thorax, and overall impression). Statistical significance of differences was evaluated with Student's t test for paired samples (confidence level, 95%).
RESULTS. Digital radiographs obtained at 2.5 and 1.8 µGy were equivalent on all quality criteria. No statistically significant differences and no tendency toward a preference for images obtained at one or the other dose level were observed. According to the registered mAs values, the average difference in patient dose was 33%.
CONCLUSION. Use of flat-panel digital imagers based on the cesium iodide—amorphous silicon technique allows a considerable dose reduction during routine chest radiography without loss of image quality.
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We evaluated the image quality achieved using a large-area, solid-state X-ray detector based on the CsI/a—Si technique by studying routine chest images of the same patients obtained at different dose levels.
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All images were acquired at 125 kVp using a phototimer for exposure control. The system was calibrated to stop exposure at 2.5 µGy (for the first study period) and 1.8 µGy (for the second study period). Exposures have been measured with a calibrated diagnostic dosemeter (DIADOS; PTW, Freiburg, Germany) as incident air kerma. The amount of registered backscattered radiation depends on collimation and spectral sensitivity of the dosemeter. Therefore, to improve reproducibility of the dose measurement, the contribution of backscattered radiation had to be decreased via the measurement procedure during calibration. We achieved this reduction by collimating the dose measurement chamber (10 x 10 cm), which was placed in front of an antiscatter grid. Because the absorption rate of the grid at specific kilovoltage and filter settings is known, we were able to correct these dose measurement results.
A dose of 2.5 µGy corresponds to the dose that would be used with a 400-speed screen-film system. Thus, a detector dose of 1.8 µGy results in a calculated speed of 560. Patient dose was not registered. The constant voltage setting allowed us to calculate relative radiation exposure based on the milliampere-second values assuming comparable average field sizes in both dose groups.
Patient Selection
At our institution, the flat-panel detector has been used for routine chest
imaging since August 2000. Initially, a detector dose of 2.5 µGy was
chosen. In January 2001, we switched to a detector dose of 1.8 µGy,
resulting in an expected decrease in radiation exposure of 30%. Using our
radiology information system software (version 7.2; Medos, Langenselbold,
Germany), we randomly selected 50 patients (24 women, 26 men; age range, 16-79
years; mean age, 57 years) who had chest radiographs obtained in
posteroanterior and lateral projections during both the period in which the
detector dose was 2.5 µGy and the period in which the dose was 1.8 µGy.
In cases of multiple available studies, the studies with the shortest time
elapsed between them were used for further evaluation (interval range, 13-73
days; mean interval, 41 days).
Clinical diagnosis or expectation of certain pulmonary abnormalities did not influence patient selection. The studies included both radiographs of patients with normal findings (n = 6 obtained at 2.5 µGy, n = 7 obtained at 1.8 µGy) and patients with diseases such as pleural effusion, pneumothorax, pneumonia, lung nodules, and atelectases as well as those with foreign bodies like chest tubes, central venous catheters, or pacemakers (n = 44 obtained at 2.5 µGy, n = 43 obtained at 1.8 µGy).
Data Analysis
Three general radiologists experienced in digital radiography were invited
to evaluated a total of 200 images. The radiographs were presented to the
independent radiologists in a random order on a computer workstation (Magic
View). Luminance of the screen (Simomed HM, Siemens) was 260
candela/m2, the matrix was 1000 x 1000, and the diameter was
52 cm. Changes of window and density were allowed. Presentation of the full
detector matrix was provided by a zoom function. The radiologists were unaware
of any information related to the applied radiation dosage or patient history.
The following anatomic regions and imaging features were evaluated
[5,6,7]:
unobscured lung (not obscured by heart shadow or diaphragm); obscured lung
(partially obscured by heart shadow or diaphragm); airways (trachea and main
bronchi); mediastinum and pulmonary hilum; chest wall and bony thorax; and
overall impression.
Each radiologist subjectively analyzed image quality independently by visual assessment and rated the quality on a 4-point scale: 1, excellent (no limitations); 2, good (minor limitations, full diagnostic information); 3, moderate (major limitations, limited diagnostic information); and 4, poor visualization (nondiagnostic). Intermediate scores at 0.5 intervals were allowed.
Mean values were calculated for each criterion, each radiologist, and each imaging modality. The resulting values for the different digital images were compared by using two-tailed Student's t test for paired samples (confidence level, 95%). A total of 3,600 observations were analyzed.
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As for the average milliampere-second values, a reduction of 24% (0.85 vs 1.12 mAs) for posteroanterior and 35% (3.054 vs 4.697 mAs) for lateral views was achieved, resulting in a total reduction of 33% (3.904 vs 5.817 mAs). The milliampere-second values of the lateral views were much higher than those of the posteroanterior projections. This finding reflects the fact that radiation has to penetrate both lungs and the mediastinum in the lateral chest radiographs as opposed to having to penetrate only one lung or the mediastinum in the frontal radiographs.
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A detector dose of 2.5 µGy results in a similar amount of radiation exposure for the patient as that found during conventional radiography using a 400-speed screen-film system. The CsI/a—Si detector exposes the patient to 33% less radiation than a state-of-the-art screen-film radiography system does.
A potential reduction of up to 75% (depending on the clinical query) has been reported for skeletal radiography [8], but we did not test a dose reduction beyond 33% for several reasons. First, extremely short exposure times are necessary to achieve detector doses of less than 1.8 µGy in the posteroanterior images. Such exposure times were not possible with the CsI/a—Si detector in this study without additional filtering. Second, a visually perceivable increase of image noise at detector doses of less than 1.8 µGy might disturb the image impression, specifically in the area of the mediastinum. Third, the dose reduction was applied independently from the clinical query. Thus, potential impairment of diagnostic performance because of image noise was not acceptable.
The overall good performance of the self-scanning, solid-state, flat-panel CsI/a—Si detector is a result of the relatively high spatial resolution (pixel size, 143 µm) and the high detective quantum efficiency (60% at 70 kVp; filtering, 21-mm aluminum).
Diagnostic performance was subject of a previously reported phantom study that indicated advantages of CsI/a—Si technique over conventional screen-film radiography regarding linear and micronodular opacities at equal dose levels [3]. In studying the detectability of simulated lung lesions, the researchers found that the digital system at a dose reduction of 50% had no statistically significant inferiority to conventional screen-film radiography.
There are disadvantages to the digital system, one being that its pixel size is limited compared with that of conventional radiography. Therefore, various studies have dealt with spatial resolution requirements in digital lung imaging. A pixel size of 200 µm seems acceptable [9,10,11]. The promising results of recent phantom and clinical studies [3, 4] confirm our impression that a pixel size of 143 µm, as was used in the our detector unit, is sufficient. A disadvantage of the flat-panel detector images is the large digital storage capacity required (18 megabytes per full-size image), more than that required for storage phosphor images. This fact has to be taken into account, especially if a picture archiving and communication system is in use or planned for future use. In addition, the heaviness of flat-panel detectors makes the production of a mobile system impractical. Therefore, storage phosphor radiography or conventional radiography will still be required for bedside chest imaging.
In conclusion, this clinical study confirms that CsI/a—Si detector technology fully satisfies the requirements for chest radiography at a wall stand, even at a reduced detector dose.
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