The effect of automatic exposure control with inappropriate scout image on radiation dose: a chest phantom experiment with three different CT machines

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1 The effect of automatic exposure control with inappropriate scout image on radiation dose: a chest phantom experiment with three different CT machines Poster No.: C-0891 Congress: ECR 2015 Type: Authors: Keywords: DOI: Scientific Exhibit J. H. Kim, K. B. Lee; Seoul/KR Radioprotection / Radiation dose, Lung, CT, CT-Quantitative, Experimental investigations, Comparative studies, Technology assessment /ecr2015/C-0891 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 9

2 Aims and objectives Due to development of MDCT, Computed tomography is taking major role in diagnostic area. Good resolution and short examination time is major advantage of CT in medical environment where speed is virtue. However, radiation dose of CT is inevitable because CT is performed by X-ray. In this reason, scanner manufacturers develop and commercialize various functions to reduce radiation dose and to make optimized images. Automatic exposure control that adapts radiation dose to size and shape of patient is one of those functions. This method depending on manufacturers is different, but it is common using information of patient like shape, size, density and data obtained during scan. Each name of this function is called by different way but the same object is to decrease of radiation dose. A recent research shows that automatic exposure control can reduce 20~40% of radiation dose. However, in 2007, Jianhai Li reveals that misusage of this function results in an increase of tube current and image noise. Recently, there are almost no CT examinations that don't use automatic exposure control. However, there are some cases of CT examinations that underwent without fully scanned scout image because of an overestimation of auto exposure control or shortage of time, or laziness. In this study, we performed phantom studies to investigate changes of tube current by setting a poor topogram(scout image) that doesn't include entire scan range. Methods and materials 128 MDCT scanners that used in this study are Somatom Definition AS +(Siemens, Erlangen, Germany) and Optima 660(GE healthcare, US), Ingenuity(Philips, Netherlands). A Lung/Chest Phantom of model RS-330(Fluke Biomedical, USA) was used. Scan parameters of the chest examination protocol on each scanner are recommended. All scan data are obtained with each automatic exposure control mode(caredose4d in Somatom Definition AS+, Auto ma in Optima 660, idose in Ingenuity). Scan parameters of Siemens scanner are 120kV, 200mAs of reference image quality and that of GE scanner is 120kV, mA and Noise index. That of Philips scanner is 120kV, 142mAs, 21 dose right index. The position of phantom was fixed on isocenter of scanner using positioning laser beam. First, we got normal topogram at first scan. And then we scanned a chest range in two ways(cranio-caudal and caudo-cranial direction). Slice thicknesses of each scan are 5mm and no gap. In same way, we got data from poor topogram. Types of poor topogram are two. Second scan was done by setting the shoulder level cut. As a results, this type can't include the information while scanning was progressing from high density area(shoulder) to low density area(neck). At third scan, topogram was got by setting the Liver level cut instead of that of neck level. this type of topogram had been cut while scanning was progressing from low density area(lung field) to high density area(liver). After CT scan using three types of topogram, we measured changes of mas between cranio-caudal scan data and caudo-cranial scan Page 2 of 9

3 data of each CT scan. Then, we compared changes of mas in a chest scan using a normal topogram with those using an abnormal topogram. Scan range of each CT scan is 30cm. Figure 2,3 are example for Siemens scanner. Images for this section: Fig. 1: A image of the equipment and phantom for our study. Fig. 2: Three kinds of study designs. normal topogram(a), topogram by setting shoulder level cut(b), topogram by setting the liver level cut(c) Page 3 of 9

4 Fig. 3: The white arrows are described in mas. Page 4 of 9

5 Results 1. Pattern of change in mas according to types of topogram Scan data from topogram that has no information above shoulder level(shoulder cut) shows high mas than that from normal topogram in 3 types of scanner. In contrast to shoulder cut, the usage of a topogram that has no information below lower lobes of lung(liver cut) decreases mas compared with scan data using normal topogram in 3 types of scanner. In two cases of scan data from abnormal topogram, there are no significant change in mas due to scan direction(figure 4,5,6). 2. The comparison of mas on neck when using a topogram of shoulder level cut When we use a topogram of shoulder level cut, each average mas is 2.38 times to that of using normal topogram in GE scanner(112 mas to 267 mas), 5.75 times in Phillips scanner(65 mas to 374 mas), 2.27 times in Siemens scanner(138 mas 314 mas) when scan direction is cranio-caudal. In caudo-cranial scan direction, average mas is 2.47 times to that of using normal topogram in GE scanner(104 mas to 257 mas), 6.82 times in Phillips scanner(56 to 382), 6.34 times in Siemens scanner(44 mas to 279 mas)(table 1) 3. The comparison of mas on liver when using a topogram of liver level cut Using liver level cut topogram, average mas is 0.61 times to that of using normal topogram in GE scanner(190 mas to 116 mas), 0.73 times in Phillips scanner(191 mas to 140), 0.70 times in Siemens scanner(119 mas 84 mas) when scan direction is craniocaudal. In caudo-cranial scan direction, average mas is 0.53 times to that of using normal topogram in GE scanner(191 mas to 101 mas), 0.68 times in Phillips scanner(204 mas to 139 mas), 0.54 times in Siemens scanner(151 mas to 81 mas)(table 2). Images for this section: Fig. 4: Graph of mas change using Optima 660(GE) in 3 types of topogram(normal topogram, shoulder cut, liver cut). Page 5 of 9

6 Fig. 5: Graph of mas change using Ingenuity(Phillips) in 3 types of topogram(normal topogram, shoulder cut, liver cut). Fig. 6: Graph of mas change using Definition AS+(Siemens) in 3 types of topogram(normal topogram, shoulder cut, liver cut). Table 1: Comparison of mas of scan data using normal topogram to that using shoulder level cut topogram on neck portion Page 6 of 9

7 Table 2: Comparison of mas of scan data using normal topogram to that using liver level cut topogram on liver portion Page 7 of 9

8 Conclusion Radiation exposure due to CT examinations takes 11% in total radiation dose of medical area and take 67% of total radiation dose in population[3]. In these reasons, radiation dose of CT is long-standing issue. Manufacturers of CT have been made effort to reduce radiation exposure by developing various technologies, AEC is also developing in same way. Usefulness of AEC system was proved by advanced studies. Lehmann KJ showed rotational AEC can reduce radiation dose almost 20% with information about geometrical structure of patient[11]. In phantom study of on-line angular modulation, Gies M showed on-line angular modulation technique can reduce radiation dose nearly 50% compared to conventional angular modulation[12]. Dose modulation with AEC is varied for each manufacturers in ways of combination of parameters like patient size, thickness change due to tube angle, z-axis perception[4,8,9]. Topogram is basic information in CT scan. Through this image, CT user can determine scan range according to prescription. In aspect of scanner, this image take major role in setting radiation dose. With the advances in AEC techniques, an online modulation technique has been developed that does not require the information provided by topograms. This technique calculates the modulation function data (an objective image quality parameter) from the online patient attenuation. These data are sent to the generator control for dose modulation with a delay of 180 from the x-ray generation angle. Thus, the system makes use of attenuation data from the previous rotation and modulates tube current to accommodate patient attenuation "on the fly" [5,6]. In angular dose modulation, more dose modulations occur in asymmetric regions and the variation in image noise throughout the examination can be minimized. This rotational AEC is also helpful in reducing photon starvation artifacts, especially in the shoulder[5]. Advanced study in 2011, Kwan-Joong Park showed that If the part of scan range is not contained in topogram, even if scan range is same, radiation dose increase nearly 15.6% compared to case with normal topogram[7]. In this study, radiation dose is increased when using shoulder cut topogram, too. However, when we use liver cut topogram, radiation dose is decreased. It is considered that the scanner keep applying former mas if information from topogram is insufficient. In this case, it seems that online modulation(real time feedback) doesn't operate[10]. We think that this is happened because large z-axis coverage ends scan before mas is changed by on-line modulation. AEC system reduce radiation dose generally, but there is possibility to increase dose by applying AEC[10]. CT user should understand AEC of his scanner precisely before apply it to patient. To conclude, if CT user use incomplete topogram to scan, scanners can't react to change of object. Then radiation dose for patient get abnormally larger or smaller. Therefore, don't over-estimate AEC system and take appropriate topogram to exposure patient with proper radiation dose according to examinations. If topogram is not appropriate, we recommend to retake topogram again. Limitation First, In our study, We only compare three different scanner except for Tosiba CT. In fact, we examed a Tosiba scanner for our study. But, The AEC function of this scanner is poor due to very wide detector row. We think there are few chance to fluctuate the mas during the scan. So, that scanner is exclude for our study and we can't compare the AEC function in all types Page 8 of 9

9 of CT. Second, we can't unify our chest protocol. It's very difficult to fit same parameter because of different machine and different AEC mode. Personal information Jung Hoon Kim, Radiological Technologist Department of Radiology, Seoul Asan Medical Center, Seoul, Korea Ki Baek Lee, Radiological Technologist Department of Radiology, Seoul Asan Medical Center, Seoul, Korea References 1. McCollough CH. Automatic exposure control in CT: are we done yet? Radiology 2005; 237: Jianhai Li, Unni K. Udayasankar, Thomas L. Toth, et al. Automatic Patient Centering for MDCT: Effect on Radiation Dose, AJR 2007;188: International Commission on Radiological Protection, Managing Patient Dose in Computed tomography,in Ann ICRP 2000, ICRP Publication 87,Vol.30, Issue Gynthia H.McCollough,PhD. Michael R. Bruesewitz,RT(R). James M.Kofler, PhD CT Dose Reduction and Dose Manaement Tools: Overview of Available Options¹ Radiographics 2006:26: Kalender WA, Wolf H, Suess C. Dose reduction in CT by anatomically adapted tube current modulation: phantom measurements. Med Phys 1999;26: Gies M, Kalender WA, Wolf H, Suess C. Dose reduction in CT by anatomically adapted tube current modulation: simulation studies. Med Phys 1999;26: Guan-Jang Park, Sung-Ho Kim, yoon-chul Nam et al. Automatic Exposure Control Systems with Inappropriate Topogram : A Phantom study in Chest CT on Radiation dose and Image quality between different manufactures. Journal of Korean Society of Computed Tomographic Technology 2011;13: Brenner D, Elliston C, Hall E, Berdon W. Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR Am J Roentgenol 2001;176: Lewis M, Keat N, Edyvean S. Report 06013: 32 to 64 slice CT scanner comparison report version 14. London, England: ImPACT, Available at: impactscan.org/reports/report Accessed June 16, Keat N. Report CT scanner automatic exposure control systems. London, England: Im- PACT, Available at: org/reports/ Report Accessed June 16, Lehmann KJ, Wild J, Georgi M. Clinical use of software-controlled x-ray tube modulation with ""Smart-Scan"" in spiral CT. Aktuelle Radiol 1997; 7: Gies M, Kalender WA, Wolf H, Suess C. Dose reduction in CT by anatomically adapted tube current modulation. I. Simulation studies. Med Phys 1999;26: Page 9 of 9