Radiat Med (2008) 26:474–480 DOI 10.1007/s11604-008-0260-9
ORIGINAL ARTICLE
Is targeted reconstruction necessary for evaluating contrast-enhanced chest computed tomography using a liquid crystal display monitor? Yoshiyuki Ozawa · Masaki Hara · Hidekazu Oshima Masanori Kitase · Kazuya Ohashi · Yuta Shibamoto
Received: February 22, 2008 / Accepted: May 27, 2008 © Japan Radiological Society 2008
Abstract Purpose. The aim of this study was to examine whether 20-cm field-of-view (FOV) targeted reconstruction (TR) on contrast-enhanced (CE) chest computed tomography (CT) might improve the diagnostic value compared with simple zooming (SZ) from whole-thorax FOV images using a 2 million (2M)-pixel liquid crystal display (LCD) monitor. Materials and methods. We prospectively evaluated 44 patients. SZ images were magnified from a FOV of 26– 34 cm (mean 29.7 cm). Parameters were 512 × 512 matrix and 3 mm thickness and interval. Images were reconstructed using a soft-tissue kernel. Three radiologists evaluated contour, spiculation, notch, pleural tag, invasion, and internal characteristics of the lesions using 5scale scores. We also performed a phantom study to evaluate the spatial resolution of images. Results. The diagnostic value of the TR images was similar to that of the SZ images, with the findings identified in 88%–100% of the cases. Artifacts from highdensity structures deteriorated the image quality in six (14%), and the SZ images were judged to be preferable in five of them. In the phantom study, there was little difference in spatial resolution between the two images.
Y. Ozawa (*) · H. Oshima · M. Kitase · Y. Shibamoto Department of Radiology, Nagoya City University Graduate School of Medical Sciences, 1 Kawasumi, Mizuho-cho, Mizuho-ku, Nagoya 467-8601, Japan Tel. +81-52-853-8276; Fax +81-52-852-5244 e-mail:
[email protected] M. Hara · K. Ohashi Division of Central Radiology, Nagoya City University Hospital, Nagoya, Japan
Conclusion. The SZ images from whole-thorax FOV on CE chest CT were similar in quality to TR images using a 2M-pixel LCD monitor. Key words CT · Reconstruction · FOV · PACS · Monitor
Introduction With the introduction of a filmless picture archiving and communication system (PACS), image interpretations are usually performed by soft copy using high-resolution liquid crystal display (LCD) viewers. In our institution, LCD viewers with 2 to 5 million (M) pixels have been used since 2004. In this situation, we have evaluated how to optimize the ways and means for interpreting the soft copies of radiological information, particularly about computed tomography (CT) and magnetic resonance imaging (MRI), which produce numerous numbers of images. Although field-of-view (FOV) targeted reconstruction (TR) had been used in our institution to (1) improve the spatial resolution by optimizing the pixel size and (2) improve the conspicuousness of chest CT by magnifying the picture size in the film-based interpretation, we have had a question whether this technique would be essential for evaluating the chest CT images under the conditions utilizing a PACS with LCD viewer. The purpose of the present study was to examine whether 20-cm FOV TR on contrast-enhanced (CE) chest CT might improve the diagnostic value compared with simple zooming (SZ) from whole-thorax FOV images using a LCD monitor.
Radiat Med (2008) 26:474–480
Subjects and methods From December 2005 to February 2006, we prospectively evaluated 44 patients who underwent chest CE-CT before surgery. The patients were 21 females and 23 males, whose age ranged from 15 to 89 years (mean 65 years). Their lesions were primary lung cancers in 20 patients, pulmonary metastases in 5, chondromatous hamartoma in 1, mediastinal tumors in 5, and undiagnosed lesions in 13. The size of the lesions ranged from 6 to 95 mm (mean 29 mm) in the long axis. All CT scans were performed with a 16-row multidetector-row CT (MDCT) (IDT16; Philips Medical Systems, Cleveland, OH, USA). The parameters of CT scanning were 120 kVp, 200 mAs, 0.75 mm × 16 collimation, pitch 0.9, 0.5 s/rotation, and matrix of 512 × 512. The flow rate of administrating a contrast medium was 2 ml/s through the vein in the upper extremity, and the scan delay was 30 s. All CT scans were reconstructed at 3 mm thickness and interval with a soft tissue kernel. The data were sent to PACS (Centricity; General Electric Medical Systems, Milwaukee, WI, USA) and were displayed at window and level settings appropriate for the lung parenchyma (level −550 HU, width 1500 HU), mediastinum (level 50 HU, width 300 HU), and bone (level 500 HU, width 2500 HU). We evaluated and compared the following CT images.
Fig. 1. Example of targeted reconstruction (TR) and simple zooming (SZ) images. a In this case, the SZ and TR images, in the upper and lower panels, respectively, are of the same size. b The radiologists were not informed of the position of each image, and the monitor surface was covered with a hand-made frame to conceal the characteristics of each image
475
• SZ images from large FOVs (mean 29.7, range 26– 34 cm) covering the entire thoracic cage to the same size corresponding to that of FOV 20 cm • TR images from raw data with an FOV of 20 cm that demonstrated unilateral lung parenchyma We used a 2M LCD monitor (1600 × 1200 × 10 bit pixels) (ME203L, Totoku, Tokyo, Japan). These two CT images were displayed at upper and lower panels randomly with the same size on the stack mode. The radiologists were not informed of the location of each image (Fig. 1) and evaluated the images using the following 5-grade score (+2, the upper image was preferable; +1, between +2 and 0; 0, both images showed similar quality (Figs. 2, 3); (−1, between 0 and −2; −2, the lower image was preferable). The evaluated CT findings were contour, spiculation, notch, pleural tag, invasion to adjacent structure, and internal characteristics (attenuation, cavity, calcification) of the lesions. Three experienced radiologists (M.H., M.K., H.O.) with 23, 12, and 10 years of experience in diagnostic radiology, respectively, subjectively evaluated these findings, with the decisions being made by consensus. Finally, we evaluated whether the quality of the TR images was superior or inferior to that of the SZ images. Significance of difference between the two images was evaluated with the Wilcoxon signed rank test.
476
Radiat Med (2008) 26:474–480
Fig. 2. A 76-year-old man with adenocarcinoma in the left lower lobe. a TR image. b SZ image. The window and level settings are matched for lung parenchyma. Computed tomography (CT) findings of these two images are similar and cannot be distinguished from each other
Fig. 3. Same patient as in Fig. 2. a TR image. b SZ image. The window and level settings are matched for the mediastinum. CT findings of these two images are similar and cannot be distinguished from one another
We also performed an experimental study using a phantom with air holes of 0.40–1.75 mm in diameter to evaluate the spatial resolution of CT images under various conditions (Fig. 4). The phantom was scanned with a CT scanner IDT 16 (Philips Medical Systems) using the parameters of CT scanning equal to those of our clinical examination. The difference of spatial resolution between the reconstruction kernels (FOV 50 mm), and between the FOV 20 and 30 cm images was examined using the 2M LCD monitor.
Results Regarding the lesion contour, the quality of SZ and TR images was found to be equal in all but one case (2%), in which the TR image was slightly superior. The finding
of spiculation was evaluated in 20 of 44 cases (45%), and there was no difference in 18 of 20 (90%) cases. In one case (5%) the SZ image was slightly superior, and in the other (5%) the TR image was slightly superior. The finding of a notch was evaluated in 18 of 44 cases (41%), and there was no difference in all cases (100%). The finding of a pleural tag was evaluated in 26 of 44 cases (59%), and there was no difference in 23 of 26 cases (88%). In 3 cases (12%) the TR image was evaluated as slightly superior. The finding of invasion to adjacent structures was evaluated in 19 of 44 cases (43%), and there was no difference in all cases (100%). Internal characteristics of the lesion were evaluated in all 44 cases, and there was no difference in 41 (93%) cases. Among the other 3 cases, the TR image was evaluated as slightly superior in 2 cases (5%), and the SZ image was oppositely evaluated as superior in 1 (2%) case (Table 1).
Radiat Med (2008) 26:474–480
477
Fig. 4. Phantom for evaluating spatial resolution
Table 1. Comparison of CT findings between the TR and the SZ CT finding
TR preferable
TR slightly preferable
TR vs. SZ: no difference
SZ slightly preferable
SZ preferable
Contour Spiculation Notch Pleural tag Invasion Internal characteristics
0 0 0 0 0 0
1 1 0 3 0 2
43 18 18 23 19 41
0 1 0 0 0 1
0 0 0 0 0 0
TR, targeted reconstruction image; SZ, simply zoomed image
Thus, regarding all six findings evaluated, there were no significant differences between the SZ and TR images. Using a subjective criterion whether a faint artifact of stair ladder could be recognized, the TR and SZ images correctly distinguished it in 41 (93%) of 44 cases. Beamhardening artifacts from high-density structures (e.g., bone, contrast medium) obviously affected the image quality in 6 of 44 (14%). Among them, the SZ images were judged superior in five of six (83%) cases (Fig. 5). The phantom examination demonstrated that the pixel size of the TR image with an FOV of 20 cm became smaller than that of the SZ image from an FOV of 30 cm (Fig. 6). The spatial resolution of CT does not mean the calculated pixel size, and Fig. 6a,b show the same spatial resolution of 0.75 mm. The difference in spatial resolution between the reconstruction kernels for chest and abdomen contrastenhanced CT (CECT) and high-resolution CT (HRCT) was also examined. The FOV of the phantom study was 50 mm, and window and level settings were matched for lung parenchyma. On the reconstructed images using the kernel for chest and abdominal CECT and HRCT, the spatial resolution of the images using the kernel for chest and abdominal CECT was 0.75 mm even in the best
condition of FOV 50 mm and was slightly inferior to that of images reconstructed by the kernel for HRCT (Fig. 7). The difference in spatial resolution between the 20-cm FOV (TR) and 30-cm FOV (SZ) images displayed at both the same lung parenchyma and soft tissue settings reconstructed with the kernel for chest and abdominal CECT is shown in Fig. 8. The contour of the holes filled with air on the SZ images seemed slightly indistinct regarding spatial resolution compared to those of the TR images on a lung window setting. However, the spatial resolution of these two images was almost equal and ranged from 0.75 to 1.00 mm. On the mediastinal window setting, it was difficult to point out the difference between the two images.
Discussion The introduction of PACS has changed the modality for interpreting diagnostic images from hard copy to soft copy. Replacing hard copy by soft copy avoids the specific problems associated with films and changed the working environment of diagnostic radiologists.1 Soft
478
Radiat Med (2008) 26:474–480
Fig. 5. A 58-year-old man with squamous cell carcinoma in the left upper lobe. a, b TR images. c, d SZ images. There are no significant differences between these images except for beam-hardening artifacts (arrows) originated from undiluted contrast medium in the left brachiocephalic vein that is sharp and prominent on TR images
Fig. 6. Magnified CT findings of pixels in each field of view (FOV). a TR image (FOV 20 cm). b SZ image (FOV 30 cm). The pixel sizes of the TR and SZ images are about 0.4 mm and 0.6 mm,
respectively. The pixel size of the TR image becomes smaller than that of the SZ image
copy interpretation using PACS requires less time than hard copy interpretation.2 It was also reported that PACS has the potential for improved accuracy in CT interpretation compared with traditional film-based interpretation.3 In such a situation, however, there is a need to change and optimize the methods of interpreting medical images, and a number of publications have reported on them.1,2,4–7 Bennet et al.4 reported the changes by introducing PACS, and 77% of those who use PACS for diagnostic reading have either decreased the number
of images per monitor or decreased the number of monitors used. Mathie et al.5 reported that the stack mode was considerably faster than the tile mode in the interpretation time, and no mistakes were made in the stack mode. Regarding window width and level settings, Lev et al.6 reported that reading by soft copy with variable window widths and level settings improved the detection of ischemic brain parenchyma in nonenhanced head CT. Promerantz et al.7 also reported that use of multiple window and level settings might increase diagnostic
Radiat Med (2008) 26:474–480
479
Fig. 7. Difference between the spatial resolutions of reconstruction kernels of chest and abdominal contrast-enhanced (CE) CT and high-resolution (HR) CT. The FOV of the phantom study is 50 mm. a, b Reconstructed image using the kernel for chest and abdominal CECT. c, d Reconstructed image using the kernel for HRCT. The window and level settings are matched for lung parenchyma in a and c and for the conditions under which the spatial resolution could be evaluated precisely in b and d. The spatial resolution of the images using the kernel for chest and abdominal CECT is 0.75 mm even under the best conditions and slightly inferior to that of HRCT (0.6 mm)
accuracy, although multiple settings are impractical in the film-based environment. Before the introduction of the PACS system at our institution, we interpreted the CT images of localized pulmonary and mediastinal lesions using the TR technique to improve the conspicuousness of the lesion on CT scans. A PACS system has been used in our institution since 2004, and we have interpreted the images using LCD monitors. In such a PACS environment, we questioned the necessity of TR because we could use an SZ technique without constraint for the soft copy unlike the film-base reading. Our subjective initial impression was that the SZ images from whole-thorax FOV images might show a quality similar to that of the TR images from raw data in regard to using LCD monitors. We compared the 20-cm-FOV TR images with SZ images from wholethorax FOV images. The present study indicated that the latter images on chest CECT had an showed image quality almost equal to that of the TR images using an LCD monitor with a spatial resolution of 2M pixels. Although identification of the TR and SZ images was almost possible on the 2M LCD monitor when interpreters noted the difference of stepladder artifacts produced according to pixel size of the images, we could find little difference in image quality during clinical use.
We performed a number of phantom investigations to add technical evidence to this study using the 2M LCD monitor. Using a matrix size of 512 × 512, the pixel size theoretically becomes about 0.6 mm in a 30cm FOV and about 0.4 mm in a 20-cm FOV. Our phantom study showed that the potential spatial resolution using a soft tissue kernel was limited to 0.75 mm even in the best condition; and even using a kernel for HRCT, the spatial resolution was limited to 0.6 mm. Based on these results it is not surprising that the TR images from an FOV of 20 cm could not produce any benefit compared to the SZ images from an FOV of 30 cm on visual assessment. We suggest that even at the kernel for chest unenhanced CT and HRCT, the SZ images might have almost equal diagnostic values up to a FOV of 30 cm because of the limitation of the spatial resolution of 0.6 mm. We suggest that the results on chest CECT could be accommodated to unenhanced CT and HRCT in an appropriate setting and that it might not be necessary to reconstruct the TR images even using those kernels. Because the matrix numbers of the 2M LCD monitor (1600 × 1200) are enough for demonstrating the CT images with those of 512 × 512, the results in this study should not change using the LCD monitor larger than 2M.
480
Radiat Med (2008) 26:474–480
Fig. 8. Difference in spatial resolution between the 20-cm FOV (TR) and 30-cm FOV (SZ) images. a, b TR images obtained from an FOV of 20 cm. c, d SZ images from an FOV of 30 cm. The window and level settings are matched for lung parenchyma in a and c and for the mediastinum in b and d. The sharpness of the SZ image is inferior to that of the TR image for depicting the contours of holes filled by air but has similar spatial resolution even on a lung window setting (a, c). With the mediastinal window setting (c, d), it is difficult to see any difference between the two images
There were some cases (14%) in which the quality of the TR images was inferior to that of SZ images because of beam-hardening artifacts from high-density structures such as bone and contrast medium. We believe that the larger pixel size of the SZ images might have diminished the contrast of microlinear structures of the artifacts. On TR images, the information outside the FOV may be lost and never be appreciable. We emphasize that in the LCD monitor environment the advantage of reconstructing TR images is limited when the original FOV is larger than 38.4 (0.75 × 512) cm, in which case the appropriate SZ images would be enough for diagnosing thoracic disease. Conclusion The SZ images from the whole-thorax FOV on chest CECT showed diagnostic usefulness similar to that of TR images of 20 cm FOV using a 2M LCD viewer. We believe that it is not necessary to reconstruct the TR images on CECT.
References 1. Mertelmeier T. Why and how is soft copy reading possible in clinical practice? J Digit Imaging 1999;12:3–11. 2. Reiner BI, Siegel EL, Hooper FJ, Pomerantz S, Dahlke A, Rallis D. Radiologists’ productivity in the interpretation of CT scans: a comparison of PACS with conventional film. AJR Am J Roentgenol 2001;176:861–4. 3. Reiner BI, Siegel EL, Hooper FJ. Accuracy of interpretation of CT scans: comparing PACS monitor displays and hard copy images. AJR Am J Roentgenol 2002;179:1407–10. 4. Bennet WF, Vaswani JA, Mendiola JA, Spigos DG. PACS monitors: an evolution of radiologists’ viewing techniques. J Digit Imaging 2002;15:171–4. 5. Mathie AG, Strickland NH. Interpretation of CT scans with PACS image display in stack mode. Radiology 1997;203: 207–9. 6. Lev MH, Farkes J, Gemmete JJ, Hossaln ST, Hunter GJ, Koroshetz WJ, et al. Acute stroke: improved nonenhanced CT detection—benefits of soft-copy interpretation by using variable window width and center level settings. Radiology 1999; 213:150–5. 7. Promerantz SM, White CS, Krebs TL, Daly B, Sukumar SA, Hooper F, et al. Liver and bone window settings for soft-copy interpretation of chest and abdominal CT. AJR Am J Roentgenol 2000;174:311–4.
