Optimal speed for dual-energy subtraction neck imaging with flat-panel detector radiography

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1 Optimal speed for dual-energy subtraction neck imaging with flat-panel detector radiography Poster No.: C-2938 Congress: ECR 2010 Type: Scientific Exhibit Topic: Physics in Radiology Authors: H. Machida 1, M. Asanuma 1, T. Yuhara 1, E. Ueno 1, J. Sabol 2 ; 1 2 Tokyo/JP, Waukesha, WI/US Keywords: DOI: dual energy subtraction, flat panel detector neck radiography, optimal speed /ecr2010/C-2938 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 10

2 Purpose Currently, conventional radiography examinations using full-field digital X-ray detector radiography systems based on a cesium iodine scintillator and an amorphous silicon flat panel TFT array are being increasingly performed in clinical settings, because these systems enable rapid access to the image for diagnosis, improved image quality relative to that of film-screen and storage phosphor systems, and possibilities for reduced radiation exposure. Due to the high detective quantum efficiency of these detectors, chest radiography with a flat panel detector (FPD) has been shown to enable a significant dose reduction without degrading image quality compared to film-screen radiography or computed radiography systems [1-3]. Specifically, the image quality and visibility of anatomic structures were comparable between those modalities even if the radiation dose was approximately 50% with the FPD system. Development of a fast, high-efficiency FPD enables the dual energy subtraction (DES) technique to be integrated into the traditional clinical workflow. The DES technique has been applied to chest radiography, and reported to improve detection of pulmonary nodules by reducing overlying bony opacities [4-6]. This technique exposes the patient with two different energy X-ray beams, and can produce a soft-tissue only image by exploiting the difference in the energy dependence of attenuation between bone and soft tissue to eliminate bony opacity. We have previously described the clinical feasibility of the DES technique for easy, swift, and noninvasive diagnosis of vocal cord paralysis, even in institutions without an otolaryngologist [7]. By eliminating overlapping cervical spine opacity, the DES technique provides better delineation of the vocal cord, subglottis, and pyriform sinus, and more accurate diagnosis of vocal cord paralysis than the conventional technique on the anterior-posterior (AP) view in FPD laryngeal radiography. In addition, DES radiography can obtain images similar to pharyngolaryngography for evaluating mucosal changes of the hypopharynx and larynx and screening for extrinsic mass lesions in these regions without the use of contrast media. This technique can also delineate a wider field of view that includes the subglottic space, which is difficult to assess with laryngoscopy. However, increased caution regarding radiation exposure is required with this examination because the DES technique uses a dual-exposure method with slightly increased dose to the patient. Furthermore, this examination requires exposure during both phonation and inspiration, which is critical for assessing vocal cord motion. Although we have previously optimized the high- and low-energy tube voltages, the optimization of the speed has not been investigated in FPD neck radiography using the DES technique. Speed should be optimized in order to adequately reduce radiation exposure and decrease motion artifact, because we cannot alter tube voltage and tube current in this examination at our discretion. The purpose of the present study was to perform a phantom experiment to Page 2 of 10

3 investigate the optimal speed for balancing image quality and radiation dose in DES neck radiography using a FPD. Methods and Materials FPD neck radiography We obtained DES neck radiographs of an anthropomorphic phantom (RANDO TM Phantom, RAN100, the Phantom Laboratory, Salem, NY) in the upright position using a FPD system (Definium 8000, GE-Yokogawa Medical Systems, Tokyo, Japan). This system includes a cesium iodide scintillator and an amorphous silicon photodiodetransistor array. The detector has an image size of 41 cm 41 cm and a pixel dimension of 0.2 mm 0.2 mm. The DES radiographic technique consisted of the standard digital AP radiograph, the bone only image, and the soft-tissue only image. DES images were acquired in a dual-exposure method with less than 150 msec between the high- and lowenergy exposures (tube voltage, 130 and 60 kvp; 160 and 250 ma, respectively) with varying speeds ranging from the default value at our institution of 100 to 500 (100, 125, 160, 200, 250, 320, 400, and 500) [8]. The source-to-image distance was 100 cm. Finally, we printed the soft-tissue only image of the AP view of the neck radiograph for each speed onto a film using a laser imager (Dry Pix 7000, Fuji Film Medical, Tokyo, Japan). Data analysis We measured standard deviation (SD) of pixel values in 3 regions-of-interest in the softtissue region near the hypopharynx and larynx of the phantom image as a measure of image noise for each speed, and those values were averaged. We performed linear regression analysis to assess the relationship between the averaged SD and speed. Seven readers visually graded the delineation of hypopharynx and larynx shadow of the phantom image of each speed in the printed film on a score from 1 (poor) to 5 (excellent), and determined the acceptable speed using defined by having an average delineation score # 4. We determined the dose reduction rate by the optimal speed compared to a speed of 100, which was the default value and the standard parameter for this examination, using the following equation; the dose reduction rate (%) = (the radiation dose at a speed of that dose at the optimal speed) / that dose at a speed of 100 x 100. All data were expressed as mean ± SD. A p value of less than 0.05 was regarded as statistically significant. Page 3 of 10

4 Results There was a significant linear correlation between the SD of pixel values as image noise and speed on DES neck radiography using FPD (SD = 0.039x , r = 0.99, p < ) (Figure 1 on page 4). The average visual score of hypopharynx and larynx delineation was improved as speed decreased, and was acceptable with a speed of 250 or less (Figure 2a on page 5and Figure 2b on page 6). The dose reduction rate was approximately 58% with a speed of 250 compared to that of 100. In Figure 3 on page 7, we present an illustrative case to show the difference in image quality under identical conditions other than speed on DES neck radiography. We measured the SD of pixel values in 3 regions-of-interest in the soft-tissue region near the hypopharynx and larynx, and obtained the averaged SD values of 43 ± 2 for a speed of 100 (Figure 3a on page 7) and 53 ± 2 for a speed of 250 (Figure 3b on page 7). The image noise using a speed of 250 was thought not to significantly degrade the diagnostic accuracy of vocal cord paralysis and mass lesions in this area in clinical settings. Images for this section: Page 4 of 10

5 Fig. 1: Figure 1 Linear regression analysis assessing the correlation between speed (x axis) and image noise (y axis) on DES neck radiography using FPD. The image noise is defined as the average standard deviation of pixel values in 3 regions-of-interest in the soft-tissue region near the hypopharynx and larynx of an anthropomorphic phantom. The regression lines, equation, and correlation coefficients are shown. Page 5 of 10

6 Fig. 2: Figure 2a a. Line graph showing the relationship between speed (x axis) and averaged visual grading score of 7 readers (y axis) regarding the delineation of hypopharynx (the pink line) and larynx shadow (the yellow line) of an anthropomorphic phantom on DES neck radiography using FPD. b. The hypopharynx (1) and larynx shadow (2) of the anthropomorphic phantom are shown. This grading score ranges from 1 (poor) to 5 (excellent), and that of 4 or more is regarded as acceptable. The average visual score of hypopharynx and larynx delineation is improved as speed decreased, and remains acceptable with a speed of 250 or less. Page 6 of 10

7 Fig. 3: Figure 2b a. Line graph showing the relationship between speed (x axis) and averaged visual grading score of 7 readers (y axis) regarding the delineation of hypopharynx (the pink line) and larynx shadow (the yellow line) of an anthropomorphic phantom on DES neck radiography using FPD. b. The hypopharynx (1) and larynx shadow (2) of the anthropomorphic phantom are shown. This grading score ranges from 1 (poor) to 5 (excellent), and that of 4 or more is regarded as acceptable. The average visual score of hypopharynx and larynx delineation is improved as speed decreased, and remains acceptable with a speed of 250 or less. Page 7 of 10

8 Fig. 4: Figure 3 DES neck radiographs of a 40-year-old male healthy volunteer at speeds of 100 (a) and 250 (b). The measured standard deviation of pixel values in 3 regionsof-interest (circle) in the soft-tissue region near the hypopharynx and larynx was 43 ± 2 for speed of 100 and 53 ± 2 for speed of 250. The image noise using speed of 250 is thought not to significantly degrade the diagnostic accuracy of vocal cord paralysis and mass lesions in this area in clinical settings. Page 8 of 10

9 Conclusion This anthropomorphic phantom study has shown that DES imaging with FPD neck radiography can reduce radiation dose by 58% using a speed of 250 without significantly degrading the image quality. This increased speed is beneficial for reducing radiation dose and also decreasing motion artifact to some degree, although further study is warranted to establish the clinical feasibility of this low dose technique. References 1. Fink C, Hallscheidt PJ, Noeldge G, Kampschhulte A, Radeleff B, Hosch WP, et al. Clinical comparative study with a large-area amorphous silicon flat-panel detector: image quality and visibility of anatomic structures on chest radiography. Am J Roentgenol 2002;178: Strotzer M, Gmeinwieser JK, Volk M, Frund R, Seitz J, Feuerbach S. Detection of simulated chest lesions with normal and reduced radiation dose: comparison of conventional screen-film radiography and a flat-panel X-ray detector based on amorphous silicon. Invest Radiol 1998;33: Baucher K, Smeets P, Bonnarens K, Hauwere AD, Verstraete K, Thierens H. Dose reduction in patients undergoing chest imaging: digital amorphous silicon flat-panel detector radiography versus conventional film-screen radiography and phosphor -based computed radiography. Am J Roentgenol 2003;181: Ricke J, Fishbach F, Freund T, Teichgraber U Hanninen EL, Rottgen R, et al. Clinical results of CsI-detector-based dual-exposure dual energy in chest radiography. Eur Radiol 2003;13: McAdams HP, Samei E, Dobbins J 3rd, Tourassi GD, Ravin CE. Recent advances in chest radiography. Radiology 2006;241: Tagashira H, Arakawa K, Yoshimoto M, Mochizuki T, Murase K. Detectability of lung nodules using flat panel detector with dual energy subtraction by two shot method: evaluation by ROC method. Eur J Radiol 2007;64: Machida H, Yoda K, Arai Y, Nishida S, Masukawa A, Asanuma M, Yuhara T, et al. Dualenergy subtraction imaging for diagnosing vocal cord paralysis with flat panel detector radiography. Korean J Radiol 2010 (in press). Page 9 of 10

10 8. Sabol JM, Avinash GB, Nicolas F, Claus B, Zhao J, Dobbins JT. The development and characterization of a dual-energy subtraction imaging system for chest radiography based on CsI:Tl amorphous silicon flat-panel technology. Proc. SPIE Medical Imaging 2001;4320: Personal Information Haruhiko Machida, MD Department of Radiology, Tokyo Women's Medical University Medical Center East machira@dnh.twmu.ac.jp Page 10 of 10