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(Radiology. 2000;215:512-515.)
© RSNA, 2000

Experimental Studies

Simulated Bone Erosions in a Hand Phantom: Detection with Conventional Screen-Film Technology versus Cesium Iodide-Amorphous Silicon Flat-Panel Detector1

Michael Strotzer, MD, Markus Völk, MD, Thomas Wild, MD, Philipp von Landenberg, MD and Stefan Feuerbach, MD

1 From the Department of Diagnostic Radiology, University Hospital of Regensburg, Franz-Josef-Strauss-Allee 11, 93053 Regensburg, Germany. Received February 4, 1999; revision requested April 1; revision received August 23; accepted September 14. Address correspondence to M.S.


   Abstract
TOP
 Abstract
Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
PURPOSE: To assess the diagnostic performance of an active-matrix flat-panel x-ray detector for reduced-dose imaging of simulated arthritic lesions.

MATERIALS AND METHODS: A digital x-ray detector based on cesium iodide and amorphous silicon technology with a panel size of 43 x 43 cm, matrix of 3,000 x 3,000 pixels, pixel size of 143 µm, and digital output of 14 bits was used. State-of-the-art screen-film radiographs were compared with digital images obtained at doses equivalent to those obtained with system speeds of 400, 560, and 800. The phantom was composed of a human hand skeleton on an acrylic plate with drilled holes simulating bone erosions of different diameters and depths. Results of four independent observers were evaluated with receiver operating characteristic curve analysis.

RESULTS: The cesium iodide and amorphous silicon detector resulted in better diagnostic performance than did the screen-film combination, with the dose being the same for both modalities (P < .05). For digital images obtained at reduced doses, no significant differences were found.

CONCLUSION: The improved diagnostic performance with digital radiographs obtained with the cesium iodide and amorphous silicon detector suggests that this detector technology holds promise in terms of dose reduction for specific diagnostic tasks, without loss of diagnostic accuracy.

Index terms: Bones, radiography, 40.1215 • Flat panel detector • Radiography, comparative studies, 40.11, 40.1215 • Radiography, digital, 40.1215 • Radiography, technology • Screens and films


   Introduction
TOP
 Abstract
 Introduction
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Active matrix flat-panel x-ray detectors based on cesium iodide (CsI) and amorphous silicon (aSi) provide digital radiographs with high spatial and contrast resolutions and have the potential to reduce radiation dose (1–4). Clinical and phantom studies (1–4) have been performed with a prototype detector with a limited panel size. In this article, we describe a phantom study in which a large-area flat-panel detector was used. The phantom was designed to simulate specific arthritic lesions in the bones of a human hand. Given the high detective quantum efficacy and the good spatial resolution of CsI-aSi detectors, we sought to determine whether radiation exposure could be reduced without loss of diagnostic performance, with state-of-the-art screen-film radiography as the standard of reference.


   MATERIALS AND METHODS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Imaging Systems
The experimental x-ray system (Siemens Medical Systems, Forchheim, Germany) consisted of an x-ray tube (focal spot size, 0.6 mm), a carbon fiber horizontal floating table, and a CsI-aSi flat-panel detector (Trixell, Moirans, France). The full-size active-matrix detector had a pixel size of 143 µm and a matrix of approximately 3,000 x 3,000 pixels, which provided a total field size of 43 x 43 cm. This large area was made possible by tiling four individual aSi plates together. The plates were mounted on a common glass substrate. The theoretic limit of spatial resolution was 3.5 line pairs per millimeter (lp/mm). At 2.5 lp/mm, the modulation transfer function was determined to be 20%. A detective quantum efficacy of approximately 60% was measured at 0 lp/mm and 70 kVp with 21-mm-thick aluminium filtering. The dynamic range exceeded 1:6,000. A 500-µm-thick layer of CsI alloyed with a small amount of thallium was implemented for the conversion of x rays to light. The light-channeling properties of the needlelike crystalline structure of the CsI, together with the small pixel size, resulted in high spatial resolution. The impinging light was converted to electric charge in the photodiodes of the aSi matrix. The electric charge was measured and converted to a digital signal (14 bits, 16,384 gray levels). The digital image was finally transferred to a workstation (Magic View; Siemens Medical Systems), where individual window and level settings were performed for global adjustment of opacity and contrast. No spatial frequency processing algorithms (eg, unsharp-mask filtering) were applied. A logarithm-like gradation curve was simulated by means of a look-up table to help ensure comparable image contrast levels. Transparent hard copies were printed on film (Ektascan DHG; Eastman Kodak, Rochester NY) by using a laser printer (Ektascan XLP; Eastman Kodak).

Images obtained with a high-amplification screen-film combination with a system speed of 400 (Lanex Regular screen and T-MAT Plus DG film; Eastman Kodak) were used as the standard of reference. The spatial resolution achievable with this technique is 2.8 lp/mm at 20% of the modulation transfer function and 6.2 lp/mm at 4%. Calculation of system speed was determined on the basis of the x-ray dose required to produce an optical density of 1.0 above base plus fog (5), such that speed equalled 1 Gy divided by the x-ray dose in micrograys.

In addition, images were obtained with mammographic techniques that included use of MIN-R2 screens and MIN-R DH film (Eastman Kodak). The low-speed screen-film combination had a high spatial resolution (approximately 13 lp/mm at 10% of the modulation transfer function) combined with a large dynamic range.

Phantom Design and Image Acquisition
The phantom design was derived from that of Grote et al (6). The hand phantom consisted of a human hand skeleton embedded in transparent plastic that had been molded in the shape of the soft tissues of the hand (Fig 1). Sixty potential locations of bone lesions were defined (Fig 2). These artificial lesions were simulated by means of precisely located and drilled holes in an underlying 6-mm-thick polymethyl acrylate panel. A total of six polymethyl acrylate templates, each containing 30 vertical burr holes, were produced. The holes had a diameter of 1.5, 2.0, 2.5, or 3.0 mm and a depth ranging from 1 to 6 mm. The holes were round or cylindric in shape. One-half of the lesions had a diameter of 1.0 or 1.5 mm. To simulate lesions with low conspicuity, the smaller and shallower holes were placed over the fingers, and the larger and deeper holes were located over the wrist.



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  		Figure 1. Digital radiograph of the hand phantom obtained with a simulated system speed of 400.

 


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  		Figure 2. Schematic shows the locations of the 60 simulated bone erosions, which are indicated by the circled areas.

 
Images were obtained at a focus-to-detector distance of 145 cm, to avoid parallax effects resulting from the divergence of x rays and the axis of the drilled holes. To achieve homogeneous opacity, especially for conventional images with their reduced dynamic range, a wedge-shaped aluminium filter was mounted beneath the x-ray tube for all (conventional and digital) exposures. At a voltage of 45 kVp, a setting of 5.6 mAs proved to provide optimum opacity for conventional images. Digital images were obtained with 45 kVp and 5.6, 4.0, or 2.8 mAs to simulate a system speed of 400, 560, or 800, respectively. Images on mammographic film were obtained at 45 kVp and 25 mAs to achieve adequate optical density.

Data Analysis
The 30 images on transparencies were presented in random order to four independent observers (M.S., M.V., P.v.L., T.W.), who were unaware of the applied radiation dose. Soft-copy interpretation was not performed. For each of the 60 potential lesion sites per image, a diagnosis was graded with a five-point scale: score of 1, definitely negative; score of 2, probably negative; score of 3, uncertain; score of 4, probably positive; and score of 5, definitely positive.

The resultant 7,200 observations were analyzed by using a receiver operating characteristic (ROC) technique (7). Areas under the ROC curves (Az values) were calculated with a maximum likelihood algorithm and represent an estimate of observer performance (ie, detectability of lesions). The statistical significance of the differences between the areas under ROC curves was assessed with the two-tailed Student t test for paired samples for individual ROC data at the 95% confidence level.


   RESULTS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
The Table lists the Az values for each observer. When dose was the same (system speed of 400), use of the CsI-aSi detector resulted in significantly improved diagnostic performance in comparison with that of the high-speed screen-film method (mean Az value, 0.6819 vs 0.6495; P = .045). The corresponding ROC curves are shown in Figure 3. There were no significant differences between conventional and reduced-dose digital images: The mean Az values were 0.6495 for screen-film radiography versus 0.6518 and 0.6446 for CsI-aSi radiography at system speeds of 560 and 800, respectively. Thus, the mean Az values indicated a correlation between detector performance and radiation dose.


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  		Az Values for Four Observers and Radiographic Modalities
 


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  		Figure 3. ROC curves show pooled results from four observers for screen-film radiography (SFR) and the CsI-aSi detector method. At a simulated system speed of 400, use of the CsI-aSi technique resulted in better diagnostic performance than did use of screen-film radiography. Reduced radiation dose resulted in Az values that were nearly identical to those achieved with screen-film radiography.

 
Mean Az values achieved with mammographic film were slightly higher than those achieved with the high-speed screen-film system (0.6626 vs 0.6495) and lower than those achieved with the digital radiographs obtained with a simulated system speed of 400 (0.6626 vs 0.6819).


   DISCUSSION
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
For artificial bone erosions in a hand phantom, our results revealed the superiority of the CsI-aSi radiographic technique over the conventional screen-film technique at the same radiation dose. Equivalent observer performance was found when the dose was reduced by 30% and 50%. These results confirm those of previous studies (1–4) in which a smaller detector prototype was used. The lowest possible dose for the given diagnostic task, however, could not be determined.

High-speed screen-film radiography is not the reference standard for diagnosis of early changes in the hand due to rheumatoid arthritis. Imaging with mammographic screen-film combinations or magnification radiography are thought to be superior for this diagnostic task (8). The dose at the detector, however, is much higher in magnification radiography, especially in mammographic screen-film systems. Link et al (8) measured a dose at the detector of 80 µGy with mammographic film used for radiography of the hand. Therefore, the use of mammographic screen-film combinations cannot be recommended for routine purposes. Magnification radiography in combination with a CsI-aSi flat-panel detector might be an alternative approach.

Our initial study design included high-speed screen-film radiography and imaging with a mammographic screen-film combination. The main features of this mammographic screen-film combination are extremely high spatial resolution and a wide dynamic range. The diagnostic task in our phantom study, however, mainly required high contrast resolution and was limited by the signal-to-noise ratio rather than the modulation transfer function. Owing to the inverse relationship between contrast and latitude, the contrast resolution of the mammographic screen-film system was limited. Thus, the diagnostic performance with images recorded on mammographic film was only slightly superior to that with images recorded with a high-speed screen-film system. Owing to the high radiation dose required, we did not consider the mammographic technique to be a useful reference method.

A potential limitation of this study arises from the nature of the simulated bone lesions. One feature of early rheumatoid arthritis is destruction of the subchondral bone margins of the finger joints. In our study, we did not destroy any osseous structures but simulated relatively homogeneous areas of hyperlucency, which resulted in an increase in film density. Although the true nature of the simulated lesions was different from that of bone erosions, the image appearance was similar. Other radiographic features of arthritis were not the subject of this study.

The images were presented to the readers in a random order without any technical information, to avoid bias resulting from direct comparison of the different imaging modalities with the same object. However, digital and conventional images always have a different appearance and can be easily discriminated. Thus, complete exclusion of bias was not possible.

In conclusion, our results confirm that CsI-aSi flat-panel detectors can yield a potential reduction in dose without loss of diagnostic accuracy.

Practical application: Flat-panel detectors based on CsI and aSi can be used to demonstrate small low-contrast lesions in the bones of the hand, with a radiation dose that is reduced relative to that of high-speed screen-film radiography.


   Acknowledgments
 
We thank the engineers and physicists at Siemens Medical Engineering (Forchheim, Germany) for their technical support.


   Footnotes
 
Abbreviations: Az = area under the ROC curve aSi = amorphous silicon CsI = cesium iodide ROC = receiver operating characteristic

Author contributions: Guarantors of integrity of entire study, M.S., S.F.; study concepts, M.S., S.F.; study design, M.S., M.V.; definition of intellectual content, M.S., S.F.; literature research, M.S., M.V.; experimental studies, M.S., M.V.; data acquisition, M.S., M.V.; data analysis, M.S., M.V., T.W., P.v.L.; statistical analysis, M.S.; manuscript preparation and editing, M.S.; manuscript review, all authors.


   References
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

  1. Völk M, Strotzer M, Gmeinwieser J, et al. Flat-panel x-ray detector using amorphous silicon technology: reduced radiation dose for the detection of foreign bodies. Invest Radiol 1997; 32:373-377.[Medline]
  2. Strotzer M, Gmeinwieser J, Spahn M, et al. Amorphous silicon (a-Si), flat-panel, x-ray detector versus screen-film radiography: effect of dose reduction on the detectability of cortical bone defects and fractures. Invest Radiol 1998; 33:33-38.[Medline]
  3. Strotzer M, Gmeinwieser J, Völk M, Fründ R, Seitz J, Feuerbach S. Detection of simulated chest lesions with reduced radiation dose: comparison of conventional screen-film radiography and a flat-panel x-ray detector based on amorphous silicon (a-Si). Invest Radiol 1998; 33:98-103.[Medline]
  4. Strotzer M, Gmeinwieser J, Völk M, et al. Clinical application of a flat-panel x-ray detector based on amorphous silicon technology: image quality and potential for dose reduction in skeletal radiography. AJR Am J Roentgenol 1998; 171:23-27.[Abstract/Free Full Text]
  5. Haus AG. The AAPM/RSNA physics tutorial for residents: measurement of screen-film performance. RadioGraphics 1996; 16:1165-1181.[Abstract]
  6. Grote R, Nast J, Rosenthal H, Calleja R, von Falkenhausen U, Rieder P. Optimization of filtering and exposure parameters in pictures of the hand using digital luminescence radiography. Rofo Fortschr Geb Röntgenstr Neuen Bildgeb Verfahr 1992; 156:264-269[German].[Medline]
  7. Metz CE. ROC methodology in radiologic imaging. Invest Radiol 1986; 21:720-733.[Medline]
  8. Link TM, Rummeny EJ, Lenzen H, Reuter I, Roos N, Peters PE. Artificial bone erosions: detection with magnification radiography versus conventional high-resolution radiography. Radiology 1994; 192:861-864.[Abstract/Free Full Text]



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