In this study a new digital projector was tested as an alternative display station. In a ROC study observer performance of detection of lung nodules and bone ...
Detection of artificial bone and lung lesions using a high resolution projector M. Fiebich, T.M. Link, W. Strebel, W. Mueller, R.R. VohsbeckPetermann, P.E. Peters Department of Clinical Radiology, University of Muenster, Germany Graphical-Interactive Systems Group, Darmstadt Technical University, Germany
Summary One problem employing PACS in radiology departments is the viewing system. Monitors employed in a digital radiology department have a reduced observer performance compared to regular lightboxes. In this study a new digital projector was tested as an alternative display station. In a ROC study observer performance of detection of lung nodules and bone erosions was tested. Detection of lung and bone lesions was reduced significantly ( p< 0.05) by using digital projection. Nevertheless in the available set up the projector is well suited for demonstrations and conferences.
Introduction The increasing use of digital modalities in radiology, technical developments and lower costs of computer systems make PACS now feasible. For economical and organizational reasons the implementation of a PAC system demands a filmless radiology department. However, one problem is still the viewing system. Monitors employed in a digital radiology department have a reduced observer performance compared to regular lightboxes.This is caused by low brightness and contrast, different image sizes and flickering of monitors. In this study a new digital projector, orginally build for High Definition Television (HDTV) was tested as an alternative display station. Observer performance was examined using ROC analysis in comparison to digital radiographs (hard copies) on viewing screens. Material and Methods The new LCD-projector (XH-L 100, Sharp, Japan) worked with the HDTV standard (Muse, NHK), and had a high resolution (1920 x 1035 pixel, landscape mode) and
brightness (450 cd/m²). It was using three 14 cm TFT-Liquid Cristal Displays (LCD), each with 1.2 million thinfilm transistors, for three different colors. In this study it was only used in a black and white mode. The used projection size was about 140 cm x 80 cm and for this size the brighness distribution was measured. The system allows two different types of presentation, a projection on a screen or a backprojection. For this study the backprojection mode was used, due to a higher brightness and to obtain a standardized viewing position to the screen. Nevertheless radiologists are using a simular viewing mode with films hanging in front of lightboxes. To compare the HDTV projector and the viewing boxes we used two different image sets, both with a matrix of about 2k to 2k. The first one consisted of 25 chest x-rays obtained for screening purposes. None of them displayed pathological findings as verified independently by two radiologists. This images were acquired with 125 kVp, a SID of 1.70 m, a grid with 40 lp/mm and a ratio of 8, and a computed radiography (CR) system (KEICCS, Eastman Kodak, USA). For the obliviation of the goldstandard we used simulated lesions. Each of the lung images was divided in quadrants and 50 lung lesions (nodules) were digitally superimposed with a maximum of one lesion per quadrant. The second set of images consisted of 100 x-rays displaying bone specimens. An image of each bone specimen was obtained before and after creation of 50 bone lesions using dental burs. In each of the specimens one lesion was created simulating rheumatoid joint erosions. This image set was aquired with 45 kVp, 1.00 m SID, no grid, and a CR system (FCR 7000, Fuji, Japan). The images were transferred to the HDTV projector using a scan converter (Chromatek 9135, Chromatek, Japan) adapted to a Sun workstation. The evaluation of observer performance was done using ROC methology (5 level scale receiver operating characteristic [ROC]). One half of the readers viewed the images first on the lightbox and six weeks later on HDTV projection, the other half vice versa to reduce the influence of learning effects. Statistical analysis was done using the program FEASIBLE (Bazis, Netherlands). The images were viewed by five radiologists, respectively.
Results
The brightness distribution of the screen shows a inhomogeneous pattern (fig. 1). The brightness was significantly decreased close to the corners. Therefore the images were presented in a centered position to reduce the disadvantage of an inhomogeneous brightness.
245 40
175
150
40
405 440
450 160
330
445
415
340
175
420 350 55
155
170
55
210
Figure 1: Brighness distribution of the HDTV-LCD projection. The circles are indicating the area of measurement, values in candela per m 2.
Detection of lung and bone lesions was reduced significantly by using digital projection. With pooled data, the area under curve (AUC) for lung lesions was 0.68 compared to 0.79 (table 1 and figure 2). Two of the readers had a better diagnostic accuracy using HDTV projection than two other readers using the conventional lightbox. However, all readers performed better using the lightbox. The area under curve (pooled data) for rheumatoid joint erosions was 0.67 compared to 0.72 for artificial bone erosions (table 2 and figure 3). Every reader performed better on the conventional lightbox than all of the readers using HDTV projection.
lightbox
HDTV-projector
Reader 1
0.864
0.779
Reader 2
0.790
0.683
Reader 3
0.871
0.823
Reader 4
0.707
0.652
Reader 5
0.791
0.655
mean value
0.8046
0.7184
pooled data
0.786
0.681
Table 1: Results obtained with the chest images containing lung nodules (Areas under Curve)
lightbox
HDTV-projector
Reader 1
0.731
0.697
Reader 2
0.809
0.673
Reader 3
0.743
0.628
Reader 4
0.755
0.635
Reader 5
0.718
0.619
mean value
0.7512
0.6504
pooled data
0.724
0.675
Table 2: Results obtained with the bone image with rheumatoid joint erosions (Areas under Curve) The time for reporting was slightly increased using the HDTV projector (72 sec vs. 57 sec) due to loading time of the digital images compared to prehung images on the lightbox.
TPF
1 Lung lesions lightbox AUC=0.786
0.8 0.6
Lung lesions HDTV projector AUC=0.681
0.4 0.2 0 0
0.2
0.4
0.6
0.8
1
0.8
1
FPF
Figure 2: ROC curves for the detection of lung nodules
1
TPF Bone lesions lightbox AUC=0.724
0.8 0.6
Bone lesions HDTV projector AUC=0.675
0.4 0.2 0 0
0.2
0.4
0.6 FPF
Figure 3: ROC curves for detection of bone lesions
Conclusion ROC analysis showed a significantly (p < 0.05) reduced detection of small artificial lung and bone lesions in digital projection compared to viewing hard copies on a regular lightbox. Therefore it is not suitable for reporting in a clinical setting. Nevertheless, performance can be improved by the use of a monochrome display to avoid chromatic aberations and the use of a portrait mode to improve resolution, because most x-rays are obtained in portrait mode. Especially an integration of a framebuffer instead of a scanconverter may improve image quality significantly.
However, in the available setting the HDTV projector is well suited for demonstrations. Meanwhile a newer projector is available with higher brightness, better contrast and a more homogenous brightness.
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