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Home医源资料库在线期刊中风学杂志2001年第1卷第2期

Performance of a Flat-Panel Detector in Detecting Artificial Bone Lesions: Comparison with Conventional Screen-Film and Storage-Phosphor Radiography

来源:中风学杂志
摘要:1FromtheDepartmentofClinicalRadiology,UniversityofMuenster,Albert-Schweitzer-Strasse33,D-48129Muenster,Germany(K。L。,H。L。,T。M。L。,S。D。,D。W。,W。H。)。andPhilipsMedicalSystems,Hamburg,Germany(K。F。K。)。Fromthe2000RSNAscientificassembly。ReceivedJanuary......

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1 From the Department of Clinical Radiology, University of Muenster, Albert-Schweitzer-Strasse 33, D-48129 Muenster, Germany (K.L., H.L., T.M.L., S.D., D.W., W.H.); and Philips Medical Systems, Hamburg, Germany (K.F.K.). From the 2000 RSNA scientific assembly. Received January 11, 2001; revision requested February 26; final revision received August 6; accepted August 15. 


     ABSTRACT

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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
 
PURPOSE: To compare a large-area direct-readout flat-panel detector system with a conventional screen-film system and a storage-phosphor system in detecting small artificial osseous lesions simulating osteolytic disease and to assess diagnostic performance with decreasing exposure dose.

MATERIALS AND METHODS: Artificial lesions (0.5–3.0 mm) were created in 100 of 200 predefined regions in 20 porcine femoral specimens. Specimens were enclosed in containers filled with water to create absorption and scatter radiation conditions comparable with those in a human extremity. Imaging was performed with a flat-panel detector system, a conventional screen-film system, and a storage-phosphor system. Levels of exposure equivalent to speed classes 400, 800, 1600, and 3200 were used. In all images, the presence or absence of a lesion was assessed by three radiologists using a five-point confidence scale. Receiver operating characteristic (ROC) analysis was performed for 4,800 observations (600 for each imaging modality and exposure level) and diagnostic performance estimated with the area under the ROC curve (Az). The significance of differences in diagnostic performance was tested with analysis of variance.

RESULTS: ROC analysis showed Az values of 0.820 (speed class 400), 0.780 (class 800), 0.758 (class 1600), and 0.676 (class 3200) for the flat-panel detector; 0.761 (class 400), 0.725 (class 800), and 0.662 (class 1600) for the storage-phosphor system; and 0.788 (class 400) for the conventional screen-film system. The Az value for the flat-panel detector at speed class 400 was significantly higher than that for all other systems (P < .05). Az values for the speed class 400 screen-film system and flat-panel detector system at speed class 800 were not significantly different.

CONCLUSION: The flat-panel detector has diagnostic performance superior to that of conventional screen-film and storage-phosphor radiography for detecting small artificial osseous lesions at clinical exposure settings. With the flat-panel detector, exposure dose can be reduced by 50% to obtain diagnostic performance comparable with that of a conventional speed class 400 screen-film system.

 

Index terms: Bones, radiography, 444.121 • Radiography, digital, 444.121 • Radiography, flat panel, 444.121


     INTRODUCTION

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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
 
During the past years, digital radiography detector systems based on various physical principles have been developed. The interest in these detectors results from the increasing importance of imaging systems directly connectable to digital picture archiving and storage systems and by efforts to improve image quality and reduce radiation dose. To our knowledge, the most widely used technology at the time this article was written is storage-phosphor radiography, followed by selenium drum detector technology, which has been used in chest imaging, with good results. Large-area direct-readout flat-panel detector systems that use either a semiconductor or a scintillator-photodiode combination for x-ray conversion have been developed (14).

Initial study (511) results suggested superior image quality and considerable potential for reducing radiation dose with these flat-panel detectors, compared with standard systems. Spatial resolution with flat-panel detectors, however, is lower than that with high-speed conventional screen-film systems (12). Therefore, with the introduction of this new-generation technology, special attention must be focused on its performance in the detection of osseous lesions, which requires not only contrast resolution but also spatial resolution.

The purpose of our study was to compare a large-area direct-readout flat-panel system with state-of-the-art conventional screen-film and storage-phosphor systems for detecting artificial osseous lesions simulating osteolytic disease and to assess its potential in reducing exposure dose.


     MATERIALS AND METHODS

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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
 
Animal Model
The experimental model we used was similar to those used by Link et al (13) and Ludwig et al (14). Twenty porcine femoral bones were used as specimens because their radiographic appearance resembles that of human femoral bones. All specimens were obtained from a slaughterhouse. As depicted in , 10 regions of approximately equal size were defined in each femur by using a grid. Cylindrical lesions with different diameters were created in 100 of the 200 predefined regions by means of a standard drilling device. To avoid any bias due to patterns of lesion distribution, for each region a random mechanism was used to determine whether it included a lesion. Lesions were located centrally within the predefined regions. Creation of artificial lesions and imaging of all specimens were performed within 24 hours after the animals’ death.


fig.ommitted  Figure 1. Schematic drawing of experimental setting: Porcine femoral bone is fixed with surgical k-wires in cylindrical polypropylene container filled with water. Horizontal defects (arrowheads) are created in five of the predefined areas marked by the grid.

 

 
Optimum lesion sizes were determined in a preliminary study (14). To obtain good discrimination between imaging systems, lesions of 0.5–3.0-mm diameter (0.5-mm increments) were chosen. Whenever a lesion was created, the random mechanism was used to choose one of the lesion sizes defined earlier.

To simulate the surrounding soft tissue of a human extremity, each femoral bone was enclosed in a cylindrical polypropylene container (12-cm diameter) filled with water. To assure correct repositioning and collimation, each femoral bone was fixed within the container by using surgical k-wires (Synthes, Bochurn, Germany). The surgical k-wires were located in the proximal femur only, so that they were not visible on images.

Imaging Technique
Images were obtained with three imaging systems:

1. A large-area direct-readout flat-panel detector (Philips Medical Systems, Hamburg, Germany) that uses a 500-µm layer of cesium-iodide alloyed with a small amount of thallium for conversion of x rays to light and an amorphous silicone matrix for conversion of light to electric charge; the detector provides a pixel size of 143 x 143 µm (Nyquist limit, 3.6 line pairs per millimeter [lp/mm]) in a 3,000 x 3,000-pixel matrix, resulting in a 43 x 43-cm field of view;

2. A storage-phosphor system (ADC compact; Agfa, Leverkusen, Germany) that provides a pixel size of 118 x 118 µm (Nyquist limit, 4.2 lp/mm) when used with an 18 x 24-cm film size; and

3. A conventional speed class 400 screen-film system (Insight Skeletal Medium; Kodak, Rochester, NJ) that provides a spatial resolution of 5.5 lp/mm.

Special care was taken to match the exposure conditions for the three imaging systems as precisely as possible. All imaging was performed with a standard x-ray tube and generator (Rotalix/Optimus; Philips Medical Systems). The focal spot–object plane distance was 135 cm, and the object plane–detector distance was 8 cm for all images. To avoid any bias caused by differences in scatter radiation, a constant collimation of 11 x 16 cm was used. All imaging was performed with a moving antiscatter grid (grid ratio, 12:1; 40 lines per centimeter).

Corresponding to imaging of the human femur, a voltage of 70 kVp with a total filtration of 3 mm Al was used. In a series of images of one of the specimens obtained with different exposure doses, the optimal clinical exposure dose (detector entrance dose of 3.9 µGy with a 25-mm-Al phantom) was determined for the conventional speed class 400 screen-film system. This exposure dose was used for all three imaging systems, hereafter referred to as "equivalent to speed class 400." Automatic exposure control was arbitrarily avoided as a potential cause of unsteady exposure dose with the different systems. With the digital flat-panel detector system and storage-phosphor system, additional imaging was performed at lower exposure doses, and . For the flat-panel system, an additional of this current-time product also was used (with the storage-phosphor system in our study, however, use of this low-exposure dose was technically not possible). The resulting images are hereafter referred to as "equivalent to speed classes 800, 1600, and 3200." All imaging was performed by one of the authors who was not involved in image evaluation (K.L.).

The flat-panel and storage-phosphor images were printed on film (Imation; Kodak) by using a laser printer (Imation DryView 8700; Kodak). To avoid any bias caused by differences in brightness or contrast between imaging modalities, look-up tables for both digital imaging modalities were chosen, such that the optic densities, measured with densitometry (Unilight D; Wellhoefer, Schwarzenbruck, Germany) for three predefined locations (ie, the diaphyseal cortex, diaphyseal spongiosa, and directly lateral to the diaphyseal cortex), were identical to those in the conventional images.

Image Evaluation
All images were assessed independently by three radiologists (T.M.L., S.D., D.W.), who recorded the presence or absence of a bone lesion. This resulted in a total of 4,800 observations (600 observations per imaging modality and exposure dose level). To prevent learning bias, all images were shown in random order. None of the readers was involved in preparing the phantoms. No time constraints were used. All images were viewed under subdued ambient light by using a light box with adjustable shutters. Exposure settings were not indicated on the images. The different appearance of digital and conventional images made it impossible, however, to blind readers to imaging modality. In all images, cortical bone was covered by an individual mask so that the readers could see only the trabecular bone. Thus, only the spongeous extent of the lesions was assessed. Readers were asked to rate the presence of a lesion according to a five-point confidence scale (a rating of 1 indicated that findings were definitely positive; 2, probably positive; 3, uncertain; 4, probably negative; or 5, definitely negative).

Data Analysis
Data were analyzed with ROC analysis (15). ROC curves were created with a maximum-likelihood curve-fitting algorithm. Lesion detectability was estimated by using values of the area under the ROC curve (Az). Because a range of lesion sizes instead of a single lesion size was used to guarantee variations in lesion visibility, lesion size was not taken into account.

By following methods described by Metz (15), Az values were calculated for the eight combinations of imaging modality and exposure dose (screen-film system 400; flat-panel system 400, 800, 1600, and 3200; and storage-phosphor system 400, 800, and 1600) for each of the three readers and as averaged Az values for all three readers. As described by DeLong et al (16), analysis of variance was performed to test the statistical significance of differences between mean Az values. ROCFIT (Metz CE, Department of Radiology, University of Chicago, Ill, 1989) and SAS (SAS, Cary, NC) software packages were used for statistical analysis.


     RESULTS

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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
 
  show Az values for individual observers and imaging modalities at the various exposure levels, averaged for all three observers.  shows the results of analysis of variance for comparison of averaged Az values.


fig.ommitted  TABLE 1. Az Values for the Different Imaging Systems and Exposure Doses

 

 

fig.ommitted  Figure 2. Graphic depiction of Az values for the different imaging modalities and exposure doses shows dependance of system performance on exposure. System performance of the flat-panel system (speed class 800) is close to that of the conventional screen-film system (speed class 400).

 

 

fig.ommitted  TABLE 2. Results of Analysis of Variance for Comparison of Az Values for the Eight Imaging System and Exposure Dose Combinations

 

 
 shows ROC curves for the three imaging systems, with an exposure dose equivalent to that of a speed class 400 system: With this exposure dose, diagnostic performance with the flat-panel system (Az = 0.820) was significantly higher than that with the conventional screen-film system (Az = 0.788) and the storage-phosphor system (Az = 0.761).  show examples of images obtained with this exposure dose.


fig.ommitted  Figure 3. ROC curves (combination of three readers’ results) for flat-panel (solid line), conventional screen-film (dashed line), and storage-phosphor system (dotted line), with exposure doses equivalent to that of a speed class 400 system. Diagnostic performance of the flat-panel system is superior to that of the conventional screen-film and storage-phosphor system.

 

 

fig.ommitted  Figure 4a. Images obtained at exposure doses equivalent to those of speed class 400 system with (a) flat-panel, (b) conventional screen-film, and (c) storage-phosphor system show differences in lesion depiction. With identical exposure dose, the flat-panel system offers the best lesion (arrowheads) depiction, followed by the conventional screen-film system.

 

 

fig.ommitted 
 
Figure 4b. Images obtained at exposure doses equivalent to those of speed class 400 system with (a) flat-panel, (b) conventional screen-film, and (c) storage-phosphor system show differences in lesion depiction. With identical exposure dose, the flat-panel system offers the best lesion (arrowheads) depiction, followed by the conventional screen-film system.

 

 

fig.ommitted 
 
Figure 4c. Images obtained at exposure doses equivalent to those of speed class 400 system with (a) flat-panel, (b) conventional screen-film, and (c) storage-phosphor system show differences in lesion depiction. With identical exposure dose, the flat-panel system offers the best lesion (arrowheads) depiction, followed by the conventional screen-film system.

 

 

fig.ommitted  Figure 5a. Optic magnification of predefined area depicts lesions (arrowheads) in images obtained with flat-panel system at exposure doses equivalent to those of speed classes (a) 400, (b) 800, (c) 1600, and (d) 3200 and with (e) conventional screen-film system shows that lesion visibility decreases with exposure dose. Lesion visibility with the flat-panel system at speed class 800 is comparable with that of the conventional screen-film system at speed class 400.

 

 

fig.ommitted  Figure 5b. Optic magnification of predefined area depicts lesions (arrowheads) in images obtained with flat-panel system at exposure doses equivalent to those of speed classes (a) 400, (b) 800, (c) 1600, and (d) 3200 and with (e) conventional screen-film system shows that lesion visibility decreases with exposure dose. Lesion visibility with the flat-panel system at speed class 800 is comparable with that of the conventional screen-film system at speed class 400.

 

 

fig.ommitted 
 
Figure 5c. Optic magnification of predefined area depicts lesions (arrowheads) in images obtained with flat-panel system at exposure doses equivalent to those of speed classes (a) 400, (b) 800, (c) 1600, and (d) 3200 and with (e) conventional screen-film system shows that lesion visibility decreases with exposure dose. Lesion visibility with the flat-panel system at speed class 800 is comparable with that of the conventional screen-film system at speed class 400.

 

 

fig.ommitted  Figure 5d. Optic magnification of predefined area depicts lesions (arrowheads) in images obtained with flat-panel system at exposure doses equivalent to those of speed classes (a) 400, (b) 800, (c) 1600, and (d) 3200 and with (e) conventional screen-film system shows that lesion visibility decreases with exposure dose. Lesion visibility with the flat-panel system at speed class 800 is comparable with that of the conventional screen-film system at speed class 400.

 

 

fig.ommitted  Figure 5e. Optic magnification of predefined area depicts lesions (arrowheads) in images obtained with flat-panel system at exposure doses equivalent to those of speed classes (a) 400, (b) 800, (c) 1600, and (d) 3200 and with (e) conventional screen-film system shows that lesion visibility decreases with exposure dose. Lesion visibility with the flat-panel system at speed class 800 is comparable with that of the conventional screen-film system at speed class 400.

 

 
For both digital systems, reduction of exposure dose resulted in lower diagnostic performance, as shown in  and . With 50% reduction of exposure dose, however, the diagnostic performance of the flat-panel system (Az = 0.780) was comparable with that of the conventional screen-film system (Az = 0.788). The performance of the storage-phosphor system with 50% reduction of exposure dose, however, was significantly lower (Az = 0.725) than that of the conventional screen-film system.  shows images obtained with the digital imaging systems at different exposure levels.


fig.ommitted  Figure 6a. Optic magnification of predefined area depicts lesions (arrowheads) in images obtained with storage-phosphor system at exposure doses equivalent to those at speed classes (a) 400, (b) 800, and (c) 1600 and with (d) conventional screen-film system. With storage-phosphor system, lesion visibility decreases with exposure dose. Lesion depiction is best with the conventional screen-film system.

 

 

fig.ommitted 
 
Figure 6b. Optic magnification of predefined area depicts lesions (arrowheads) in images obtained with storage-phosphor system at exposure doses equivalent to those at speed classes (a) 400, (b) 800, and (c) 1600 and with (d) conventional screen-film system. With storage-phosphor system, lesion visibility decreases with exposure dose. Lesion depiction is best with the conventional screen-film system.

 

 

fig.ommitted 
 
Figure 6c. Optic magnification of predefined area depicts lesions (arrowheads) in images obtained with storage-phosphor system at exposure doses equivalent to those at speed classes (a) 400, (b) 800, and (c) 1600 and with (d) conventional screen-film system. With storage-phosphor system, lesion visibility decreases with exposure dose. Lesion depiction is best with the conventional screen-film system.

 

 

fig.ommitted Figure 6d. Optic magnification of predefined area depicts lesions (arrowheads) in images obtained with storage-phosphor system at exposure doses equivalent to those at speed classes (a) 400, (b) 800, and (c) 1600 and with (d) conventional screen-film system. With storage-phosphor system, lesion visibility decreases with exposure dose. Lesion depiction is best with the conventional screen-film system.

 

 

     DISCUSSION

Top
ABSTRACT
INTRODUCTION
ATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
 
Large-area direct-readout flat-panel detector systems are the most recent development in digital radiography. The detector quantum efficiency of these systems is considerably higher than that of storage-phosphor or screen-film systems (14). Measured for low spatial frequencies with a detector exposure of 2.5 µGy, they offer a detector quantum efficiency of 0.6, whereas state-of-the-art storage-phosphor or screen-film systems offer detector quantum efficiency of only 0.3 (12). The spatial resolution of flat-panel detectors, limited by pixel size, is comparable with that of storage-phosphor systems and is lower than that of conventional screen-film systems (12). With a pixel size of 143 µm, the flat-panel system has a Nyquist frequency of 3.6 lp/mm; the storage-phosphor system used in our study, with a pixel size of 118 µm, has a Nyquist frequency of 4.2 lp/mm (cassette size, 18 x 24 cm). Conventional screen-film systems of the 400-speed class, however, offer spatial resolution of 5.5 lp/mm.

Although technical characteristics provide a valuable basis for comparing detector systems, it is difficult to transfer them into terms of diagnostic performance: For detecting small low-contrast lesions, both spatial resolution and contrast resolution are important factors.

In this study, we examined the diagnostic performance of a large-area flat-panel detector in an experimental model of artificial lesions that mimic myeloma-like osteolytic disease. An experimental design was chosen for several reasons: Because lesions were created artificially, a well-defined reference standard was available, which is frequently difficult to obtain in clinical studies. The design allowed a large number of observations, and radiation exposure of patients was avoided. The range of lesion sizes and thus lesion detectability could be chosen to acquire good discrimination of imaging methods: Lesions detected too easily would have resulted in an Az value too close to 1.0, which would not have been useful in comparing imaging systems; lesions too small to detect would have resulted in an Az value close to 0.5, also useless for system comparisons. The fact that mean Az values in our study ranged between 0.662 and 0.820 shows that lesion sizes were chosen in an appropriate range. Because lesion sizes ranged to a minimum of 0.5 mm, which is 3–4 times the pixel length with the flat-panel system, spatial resolution can be expected to be an important contributing factor in lesion depiction. Because of lesion localization in the medullary cavity, however, the lesions in our study can be considered low-contrast lesions. Contrast resolution, therefore, is an important factor in their depiction, as well.

Exposure settings in our study corresponded exactly to those used in imaging of a human femur. Porcine femurs have diameters similar to those of human femurs. The water surrounding the femur accounts for absorption resembling that of soft tissue surrounding a human femur. Long bones, furthermore, have relatively homogenous absorption compared with that of the spine or pelvis. Therefore, the wider dynamic range of the digital imaging modalities could not compensate for their lower spatial resolution.

Our data show the diagnostic performance of the flat-panel system to be superior to that of screen-film and storage-phosphor systems in detecting the simulated lesions. The diagnostic performance is almost equal to that of conventional screen-film systems when using only 50% of exposure dose (speed class 800). Further reduction of radiation dose leads to lower diagnostic performance. A 25% radiation dose (speed class 1600) with the flat-panel detector, however, still offers the diagnostic performance of a storage-phosphor system used with a 400–800 speed class dose equivalent.

The results of our study agree with those of previous studies. Using a medical physics contrast-detail phantom, Neitzel et al (12) demonstrated the superior performance of a flat-panel system in comparison with two conventional and two digital imaging modalities in detecting low-contrast objects. Strotzer et al (8) compared a flat-panel with a conventional screen-film system in detecting artificial rheumatoid lesions. They demonstrated superior image quality with the flat-panel system, with equal exposure dose and an equal image quality, with an exposure dose reduction of 30%–50%. With a minimum lesion size of 1.5 mm, however, their study design did not address the effect of spatial resolution. Vandevenne et al (10) evaluated a selenium-based flat-panel system in an experimental study. They showed that, even with an exposure dose reduced by 56%, image quality was comparable with that obtained with a conventional screen-film system. The authors, however, used only a qualitative descriptive approach, and their study was based on analysis of a single specimen.

Limitations of our study must be taken into account: An in vitro model was used instead of patient images. However, in patient images, a true reference standard usually is not available. The shape of artificial lesions was cylindrical rather than circular in most true osteolytic lesions. Complete elimination of observer bias was not possible because of the specific appearance of digital and conventional images. Our study, however, was based on a large number of observations and a study design characterized by precisely matched exposure conditions and maximum effort to avoid systematic errors.

Practical application: A digital flat-panel detector offers diagnostic performance superior to that of conventional screen-film radiography and storage-phosphor radiography in detecting experimental osteolytic lesions of various sizes at exposure settings equivalent to those of a speed class 400 system, as well as diagnostic performance comparable with that of a speed class 400 conventional screen-film system, with a dose reduction of approximately 50%.

 

     ACKNOWLEDGMENTS
 
We are grateful to Cristina Sauerland, PhD (Institute of Medical Informatics and Biomathematics, University of Muenster, Germany), for her help with statistical analysis.



     REFERENCES

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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
 

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作者: Karl Ludwig MD Horst Lenzen MS Karl-Friedrich Ka 2007-5-14
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