THE CONSISTENCY OF EXPOSURE INDICATOR VALUES IN DIGITAL RADIOGRAPHY SYSTEMS

THE CONSISTENCY OF EXPOSURE INDICATOR VALUES IN DIGITAL RADIOGRAPHY SYSTEMS Abstract After years of establishment of computed radiography (CR) and digital radiography (DR), manufacturers have introduced exposure indicator/index (EI) as a feedback mechanism for patient dose. However, EI consistency is uncertain for CR. Most manufacturers recommended EI values in a range of numbers for all examination, instead of giving the exact range for a specific body part, raising a concern of inappropriate exposure given to the patient in clinical practice. The aims of this study were to investigate the EI consistency in DR systems produced in constant exposure parameters and clinical condition, and to determine the interaction between the anatomical part and EI. A phantom study of skull, chest, abdomen and hand was carried out and four systems were used for comparison—Fuji CR, Carestream CR, Siemens DR and Carestream DR. For each projection, the phantom positioning and exposure parameters were set according to the standard clinical practice. All exposure parameters and clinical conditions were kept constant. Twenty (20) exposures were taken for each projection and the EI was recorded. Findings showed that EI is not consistent in DR systems despite constant exposure parameters and clinical condition except in Siemens DR, through skull examination. Statistical analysis showed a significant interaction between anatomical parts and EI values (P < 0.05). EI alone was proven to be less reliable to provide technologist a correct feedback on exposure level. The interaction between anatomical parts and EI values intensifies the need for an anatomical-specific EI values set by all manufacturers for accurate feedback on the exposure parameters used and the detector entrance dose. INTRODUCTION For decades, As Low as Reasonably Achievable (ALARA) is a standard clinical practice holding the purpose to obtain a plain radiograph using the optimum radiation exposure. Unfortunately, the amount of radiation exposure is more difficult to assess with digital imaging than with a screen-film system(1). Unlike screen-film imaging, image display in digital radiography (DR) is independent of image acquisition(2). Due to the wide exposure latitude and post-processing capabilities associated with DR, resultant images will have similar appearances in terms of contrast and density when compared to film-screen technologies; independent of the exposure, however, if images are underexposed, increased quantum mottle will be evident in the image(3). This condition encourages medical imaging technologists to rely on exposure indicator/index (EI) as a feedback mechanism. Nonetheless, there is lack of literature debating about the consistency of EI values in DR. In addition to uncertainty in EI consistency(4), most manufacturers recommended a range value for all anatomical parts as a guideline, raising a concern of unintentional overexposing by technologists in clinical practice. EIs were first developed for CR system technologies by Fujifilm with the introduction of the sensitivity ‘S’ number(3). Each manufacturer developed its own EI to reflect exposure and each index had its own name and value range(1). The naming of EIs varies between manufacturers. Carestream (formerly Kodak) and Philips named theirs the EI value, Fujifilm the S-number, Agfa the log mean (lgM) value and Siemens the EI value(4). The S-numbers range should be ~200. S-numbers under 75 should be considered overexposed, even though they may look normal in appearance and S-numbers >500 is considered underexposed(5). The S-numbers are inversely proportional thus, the Fuji S-number decreases if the exposure increases(6). While all other manufacturers suggested EI in range for all examinations, Fuji recommended ranges of S-numbers including head, chest, abdomen, spine, pelvis, upper and lower extremities as well as gastrointestinal (GI) studies(7). Carestream recommended that for all examinations, the EI should fall between 1700 and 1900(6). In another literature, Carestream’s recommendation is to achieve an EI range of 1500–1800(8)—this EI range was used for Carestream throughout this study since it was the latest recommendation. This literature also stated that after a review by radiologists and radiographers of CR image quality across the radiology department, a lower level EI of 1400 was deemed as providing acceptable diagnostic image quality. If the EI is increased by 300, the exposure to the IP (detector) is doubled(9). For Siemens, the EI value should fall in 150–400(10) and doubling of the absorbed dose in the image receptor results in a doubling of the EI(11). Generally, EI is influenced by the patient size, artefacts, source to image receptor distance, collimation, centering and IP plate size(4). The EI value simply indicates the detector exposure. If a thick body part is imaged on a detector, the exposure technique must be adjusted upward to compensate for increased X-ray attenuation. If a thin patient is imaged on the same detector, the X-ray technique must be reduced to achieve the same number of transmitted X-ray to the detector (same signal-noise-ratio), with the result being the same EI, at a lower dose to the patient(12). In addition, a correct radiographic projection must be selected, the correct anatomy must be projected onto the detector, vendor-specific collimation rules must be followed, and appropriate means for scatter control must be properly used because the detector cannot distinguish between primary and scatter radiation(13). EI is also influenced by the reproducibility of the exposure system(14). Factors contributing to the S-number are; scatter (more scatter—higher S-number), distance—source-image-distance and object-focus-distance (dose and scatter), collimation (good collimation reduces scatter), examination selected at the IIP (due to histogram analysis) and delay in processing from time of exposure(5). According to Siemens, EI depends on collimation, beam quality and examined organ and does not depend on organ program name, selected exposure method (manually or automatically) and selected measuring field(11). With reference to the significance, the aims of this study were to investigate the EI consistency in DR systems produced in constant exposure parameters and clinical condition, and to determine the interaction between the anatomical part and EI. MATERIALS AND METHODS Study design The study was an experimental phantom study between four (4) DR systems, two (2) computed radiography (CR) and two (2) direct DR—Fuji CR, Carestream CR, Siemens DR and Carestream DR. Skull, chest, abdomen and hand projections were carried out on the four (4) systems for comparison. For each projection, the phantom positioning and exposure parameters were set according to the standard clinical practice. All exposure parameters and clinical conditions were kept constant. Twenty (20) exposures were taken for each projection and the EI was recorded. Study instruments Two CR systems and two DR systems were chosen to assist in comparing the consistency which were Fuji FCR PRIMA CR (FUJIFILM Medical Systems, Inc., USA), Carestream DirectView Vita LE CR (Carestream Health Inc., Rochester, NY), Siemens Ysio DR (Siemens Healthineers Limited, Erlangen, DE) and Carestream DRX-Evolution Plus DR (Carestream Health Inc.), Rochester, NY. Phosphor imaging plates and readers used in CR correspond with the systems. CT Whole Body Phantom PBU-60 (KYOTO KAGAKU, Kyoto, Japan) were used to develop clinically relevant images and simulate typical examination conditions of the skull (PH-2B-1), thorax (PH-2B-2), abdomen and hand projection (PH-2B-4). Ethics approval The study began with the approval from the Research and Ethics Committee, School of Health Sciences KPJ Healthcare University College, Board of Ethics KPJ Damansara Specialist Hospital and KPJ Ampang Puteri Specialist Hospital. Data was then collected in the locations aforementioned. Statistical analysis The recorded data was used to describe the consistency of EI values in both systems using descriptive analysis and tested using Kruskal–Wallis test to determine the interaction between EI value produced in different types of DR systems and anatomical parts. Data collection Figure 1 shows the flowchart of data collection method in the research. Figure 1. View largeDownload slide Flowchart of data collection method in the research. Figure 1. View largeDownload slide Flowchart of data collection method in the research. RESULTS The consistency of EI values Tables 1–4 show the minimum and maximum EI values, and the ranges of fluctuation in CR and DR systems. Table 1. Fuji CR S-numbers in 20 exposures. Anatomical parts S-numbers Recommended S-number Output level Skull 340–365 250–600 Optimum Chest 875–960 300–800 Underexpose Abdomen 258–290 300–700 Overexpose Hand 145–159 120–400 Optimum Anatomical parts S-numbers Recommended S-number Output level Skull 340–365 250–600 Optimum Chest 875–960 300–800 Underexpose Abdomen 258–290 300–700 Overexpose Hand 145–159 120–400 Optimum Table 1. Fuji CR S-numbers in 20 exposures. Anatomical parts S-numbers Recommended S-number Output level Skull 340–365 250–600 Optimum Chest 875–960 300–800 Underexpose Abdomen 258–290 300–700 Overexpose Hand 145–159 120–400 Optimum Anatomical parts S-numbers Recommended S-number Output level Skull 340–365 250–600 Optimum Chest 875–960 300–800 Underexpose Abdomen 258–290 300–700 Overexpose Hand 145–159 120–400 Optimum Table 2. Carestream CR EI values in 20 exposures. Anatomical parts EI values Recommended EI value Output level Skull 1223.05–1326.10 1500–1800 Underexpose Chest 464.20–532.01 Underexpose Abdomen 1695.38–1871.54 Optimum Hand 1459.31–1620.36 Optimum Anatomical parts EI values Recommended EI value Output level Skull 1223.05–1326.10 1500–1800 Underexpose Chest 464.20–532.01 Underexpose Abdomen 1695.38–1871.54 Optimum Hand 1459.31–1620.36 Optimum Table 2. Carestream CR EI values in 20 exposures. Anatomical parts EI values Recommended EI value Output level Skull 1223.05–1326.10 1500–1800 Underexpose Chest 464.20–532.01 Underexpose Abdomen 1695.38–1871.54 Optimum Hand 1459.31–1620.36 Optimum Anatomical parts EI values Recommended EI value Output level Skull 1223.05–1326.10 1500–1800 Underexpose Chest 464.20–532.01 Underexpose Abdomen 1695.38–1871.54 Optimum Hand 1459.31–1620.36 Optimum Table 3. Siemens DR EI values in 20 exposures. Anatomical parts EI values Recommended EI value Output level Skull 403 150–400 Overexpose Chest 531–533 Overexpose Abdomen 987–988 Overexpose Hand 2165–2172 Overexpose Anatomical parts EI values Recommended EI value Output level Skull 403 150–400 Overexpose Chest 531–533 Overexpose Abdomen 987–988 Overexpose Hand 2165–2172 Overexpose Table 3. Siemens DR EI values in 20 exposures. Anatomical parts EI values Recommended EI value Output level Skull 403 150–400 Overexpose Chest 531–533 Overexpose Abdomen 987–988 Overexpose Hand 2165–2172 Overexpose Anatomical parts EI values Recommended EI value Output level Skull 403 150–400 Overexpose Chest 531–533 Overexpose Abdomen 987–988 Overexpose Hand 2165–2172 Overexpose Table 4. Carestream DR EI values in 20 exposures. Anatomical parts EI values Recommended EI value Output level Skull 1651–1658 1500–1800 Optimum Chest 1778–1782 Optimum Abdomen 1811–1816 Overexpose Hand 1429–1439 Underexpose Anatomical parts EI values Recommended EI value Output level Skull 1651–1658 1500–1800 Optimum Chest 1778–1782 Optimum Abdomen 1811–1816 Overexpose Hand 1429–1439 Underexpose Table 4. Carestream DR EI values in 20 exposures. Anatomical parts EI values Recommended EI value Output level Skull 1651–1658 1500–1800 Optimum Chest 1778–1782 Optimum Abdomen 1811–1816 Overexpose Hand 1429–1439 Underexpose Anatomical parts EI values Recommended EI value Output level Skull 1651–1658 1500–1800 Optimum Chest 1778–1782 Optimum Abdomen 1811–1816 Overexpose Hand 1429–1439 Underexpose Figures 2–5 show the box-and-whisker plot of the EI values for all anatomical parts in four different CR and DR systems. Figure 2. View largeDownload slide Distribution of skull EI values in different systems. Figure 2. View largeDownload slide Distribution of skull EI values in different systems. Figure 3. View largeDownload slide Distribution of chest EI values in different systems. Figure 3. View largeDownload slide Distribution of chest EI values in different systems. Figure 4. View largeDownload slide Distribution of abdomen EI values in different systems. Figure 4. View largeDownload slide Distribution of abdomen EI values in different systems. Figure 5. View largeDownload slide Distribution of hand EI values in different systems. Figure 5. View largeDownload slide Distribution of hand EI values in different systems. Interaction between EI values produced in different DR systems and anatomical parts The results were tested using Kruskal–Wallis to test the interaction between EI value produced in different DR systems and anatomical parts. The Kruskal–Wallis test indicated that there are statistically significant differences between the EI values in all DR systems to the anatomical parts (P < 0.05). In other words, there is a significant interaction between all anatomical parts and the EI values for all DR systems (P < 0.05). DISCUSSION S-numbers and EI values obtained were inconsistent throughout 20 exposures in all examinations in Fuji CR, Carestream CR and Carestream DR. EI values in Siemens DR showed a consistent value in skull examination only, with EI value of 403.00, while other anatomical parts were inconsistent throughout 20 exposures. Results of the previous study showed that the EI value was not consistent in CR, and thoroughly consistent in DR(4). A similar result has been noted in Carestream CR, but EI in Siemens DR was inconsistent. One possible cause of varying results was the type of DR used—direct or indirect DR. Butler failed to include the exact model of Siemens DR used in her study, which causes the current study to wonder the effect of DR types on EI consistency. The highest fluctuation was seen within hand projection in Carestream CR which yields an increased value of 110.80. Note that an increase in the EI by 300 values represents a doubling of the screen exposure(15). Even though the increase of EI value in hand from the first exposure was considered in the range of optimum exposure, the exposure was increased one-third from the original EI. The fluctuation value was apparent in Carestream CR hand compared to other anatomical parts in other systems possibly due to centering point. In addition, constant repositioning of hand before every exposure probably contribute the fluctuation and there might be a slight room for error in reproducing the exact same positioning of the hand on the IP plate. This study did not consider the calibration and correction factor in the four systems investigated. EI requires careful calibration of the image detector if they are to be used as a surrogate for proper exposure of detector and even more so if they are to be used for patient entrance dose or effective dose estimation(16). The readout time in both CR were <2 min and this could be the reason of inconsistency of S-number and EI value in Fuji CR and Carestream CR. However, latent image will lose ~25% of the stored signal between 10 min and 8 h after an exposure(11). Not only most of the results show inconsistency, note that most values do not fall in the manufacturer’s recommended value. The most possible cause of the outcome is the use of same exposure factors for all systems. Since the systems’ detection quantum efficiency are different from one another, the values obtained might viewed as confusing, take into account that not only the values are inconsistent, but most of them suggests overexposure especially in Siemens DR. This study disregards the effect of using overexposed value and underexposed value, as this probably affect the consistency of values obtained. Thus, the use of the same exposure factor for all manufacturers should be avoided since there are no standardized exposure parameters for all systems. If a future study were to be conducted, it is advised that the exposure factors is first determined to achieve an optimum exposure for every anatomical parts and every manufacturer being investigated to observe if variations or inconsistency might be influenced by under- or overexposures. Based on the results obtained by comparing means of the EI values from different DR systems, the exposure factor taken from the standard exposure factor guidelines were proven to be an optimum exposure factor when using Carestream DR whereas in Siemens DR, the image produced with the same exposure factor contributed to an overexposure. Factor that could not be controlled in this study includes the effect of heat and the time for the next exposure on the EI values obtained, thus this study has neglected those effects although EI might be affected by the delay between exposures(17). Nonetheless, the data were taken continuously without any interruption or if so, the data were collected all over again, regardless the data that has been obtained beforehand. EI is also influenced by the reproducibility of the exposure system(14), which was also one of the factors that could not be controlled in this study. The box-and-whisker plots in Figures 2–5 show both DR systems data were close to the median except in hand projection in Carestream DR. While in CR systems, there are outliers noted in Carestream CR abdomen and Fuji CR hand. Again, hand projection probably has the greatest outliers in both CR and DR systems due to the direct-exposure technique, where the part needs to be repositioned every time the exposure needs to be made. Statistical analysis done showed that there was an interaction between anatomical parts and EI values. This corresponds to the assertion of International Electrotechnical Commission which stated that EI can be defined as a measure of a digital detector’s response to radiation in the relevant image region (RIR) of an image acquired using DR(18). The generated EI value also depends critically on the RIR thus different EI values are possible when a different image region is segmented and used for estimating the incident exposure(12). This intensifies the need for an anatomical-specific EI values set by all manufacturers for accurate feedback on the exposure parameters used and the detector entrance dose. The limitation of this study included the usage of same exposure factor for all manufacturers (resulting in variations of exposure output level), lack of number of the same manufacturer for comparison, lack of manufacturers which uses the same means of EI values, lack of anatomical parts being investigated and the negligence of EI value association with image quality. Future study should determine the optimum exposure parameters for that particular manufacturer before the consistency test is conducted increase the number of the same manufacturer in the study, adding more anatomical parts being investigated and associate the EI values obtained with image quality. CONCLUSION In conclusion, the consistency of EI remained uncertain in CR and DR based on the findings of this study. The inconsistency might due to the calibration factor, inconsistent delay of exposures and readout time and the inconsistent placement of IP due to the readout of the direct-exposure (without grid) approach in hand examinations conducted using CR. Although the findings were contradictory to the previous study by Butler et al. in relations to DR, this might be due to the difference in the type of DR used. The study could also conclude that there must be an additional EI range set by the manufacturer that is anatomical-specific in order to set a limit to the exposures given in clinical practice according to the anatomical part being exposed since the EI depends on the anatomical part being examined. Using the same range of exposure for all anatomical parts and examination would potentially cause overexposure incidents left overlooked over time. ACKNOWLEDGEMENTS The author would like to thank all who have contributed to this study direct and indirectly towards its completion, as well as KPJ Damansara Specialist Hospital and KPJ Ampang Puteri Specialist Hospital for their support and active involvement throughout the period of this study being conducted. REFERENCES 1 Cohen , M. D. , Cooper , M. L. , Piersall , K. and Apgar , B. K. Quality assurance: using the exposure index and the deviation index to monitor radiation exposure for portable chest radiographs in neonates . Pediatr. Radiol. 41 ( 5 ), 592 – 601 ( 2011 ). Google Scholar CrossRef Search ADS PubMed 2 Shepard , S. J. , Wang , J. , Flynn , M. , Gingold , E. , Goldman , L. et al. . An exposure indicator for digital radiography: AAPM Task Group 116 (Executive Summary) . Med. Phys. 36 ( 7 ), 2898 – 2914 ( 2009 ). Google Scholar CrossRef Search ADS PubMed 3 Mothiram , U. , Brennan , P. C. , Lewis , S. J. , Moran , B. and Robinson , J. Digital radiography exposure indices: a review . J. Med. Radiat. Sci. 61 ( 2 ), 112 – 118 ( 2014 ). Google Scholar CrossRef Search ADS PubMed 4 Butler , M. L. , Rainford , L. , Last , J. and Brennan , P. C. Are exposure index values consistent in clinical practice? A multi-manufacturer investigation . Radiat. Prot. Dosimetry 139 ( 1–3 ), 371 – 374 ( 2010 ). Google Scholar CrossRef Search ADS PubMed 5 Fujifilm Medical Systems . FUJIFILM Medical Systems CR Users Guide. 1–40 ( 2004 ). 6 Warren-Forward , H. M. , Arthur , L. , Hobson , L. , Skinner , R. , Watts , A. et al. . An assessment of exposure indices in computed radiography for the posterior–anterior chest and the lateral lumbar spine . Br. J. Radiol. 80 ( 949 ), 26 – 31 ( 2007 ). Google Scholar CrossRef Search ADS PubMed 7 Seeram , E. Optimization of the Exposure Indicator of a Computed Radiography Imaging System as a Radiation Dose Management Strategy, Ph.D. Thesis, Charles Sturt University. ( 2012 ). 8 Gibson , D. J. and Davidson , R. A. Exposure creep in computed radiography: a longitudinal study . Acad. Radiol. 19 ( 4 ), 458 – 462 ( 2012 ). Google Scholar CrossRef Search ADS PubMed 9 Seeram , E. , Davidson , R. , Bushong , S. and Swan , H. Radiation dose optimization research: exposure technique approaches in CR imaging—a literature review . Radiography 19 , 331 – 338 ( 2013 ). Google Scholar CrossRef Search ADS 10 Mothiram , U. , Brennan , P. C. , Robinson , J. , Lewis , S. J. and Moran , B. Retrospective evaluation of exposure index (EI) values from plain radiographs reveals important considerations for quality improvement . J. Med. Radiat. Sci. 60 , 115 – 122 ( 2013 ). Google Scholar CrossRef Search ADS PubMed 11 An Exposure Indicator for Digital Radiography . Report of AAPM Task Group 116. American Association of Physicists in Medicine ( 2009 ). 12 Seibert , J. A. and Morin , R. L. The standardized exposure index for digital radiography: an opportunity for optimization of radiation dose to the pediatric population . Pediatr. Radiol. 41 ( 5 ), 573 – 581 ( 2011 ). Google Scholar CrossRef Search ADS PubMed 13 Willis , C. E. Strategies for dose reduction in ordinary radiographic examinations using CR and DR . Pediatr. Radiol. 34 , 196 – 200 ( 2004 ). Google Scholar CrossRef Search ADS 14 Schaefer-Prokop , C. and Neitzel , U. Computed radiography/digital radiography: radiologist perspective on controlling dose and study quality . In (Ed): RSNA categorical course in diagnostic radiology physics: From invisible to visible – The science and practice of x-ray imaging and radiation dose optimization. pp. 85 – 98 ( 2006 ). 15 Peters , S. E. and Brennan , P. C. Digital radiography: are the manufacturers’ settings too high? Optimisation of the Kodak digital radiography system with aid of the computed radiography dose index . Eur. Radiol. 12 ( 9 ), 2381 – 2387 ( 2002 ). Google Scholar CrossRef Search ADS PubMed 16 IAEA . Avoidance of Unnecessary Dose to Patients While Transitioning From Analogue to Digital Radiology. International Atomic Energy Agency ( 2011 ). ISBN 978 92 0 121010 4. 17 Acceptance Testing and Quality Control . of Photostimulable Storage Phosphor Imaging Systems Report of AAPM Task Group 10 October 2006. American Association of Physicists in Medicine ( 2006 ). 18 International Electrotechnical Commission . Medical electrical equipment—Exposure index of digital X-ray imaging systems—Part 1: Definitions and requirements for general radiography. IEC, Geneva, Switzerland, International standard 62494 – 1 ( 2008 ). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

THE CONSISTENCY OF EXPOSURE INDICATOR VALUES IN DIGITAL RADIOGRAPHY SYSTEMS

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Abstract

Abstract After years of establishment of computed radiography (CR) and digital radiography (DR), manufacturers have introduced exposure indicator/index (EI) as a feedback mechanism for patient dose. However, EI consistency is uncertain for CR. Most manufacturers recommended EI values in a range of numbers for all examination, instead of giving the exact range for a specific body part, raising a concern of inappropriate exposure given to the patient in clinical practice. The aims of this study were to investigate the EI consistency in DR systems produced in constant exposure parameters and clinical condition, and to determine the interaction between the anatomical part and EI. A phantom study of skull, chest, abdomen and hand was carried out and four systems were used for comparison—Fuji CR, Carestream CR, Siemens DR and Carestream DR. For each projection, the phantom positioning and exposure parameters were set according to the standard clinical practice. All exposure parameters and clinical conditions were kept constant. Twenty (20) exposures were taken for each projection and the EI was recorded. Findings showed that EI is not consistent in DR systems despite constant exposure parameters and clinical condition except in Siemens DR, through skull examination. Statistical analysis showed a significant interaction between anatomical parts and EI values (P < 0.05). EI alone was proven to be less reliable to provide technologist a correct feedback on exposure level. The interaction between anatomical parts and EI values intensifies the need for an anatomical-specific EI values set by all manufacturers for accurate feedback on the exposure parameters used and the detector entrance dose. INTRODUCTION For decades, As Low as Reasonably Achievable (ALARA) is a standard clinical practice holding the purpose to obtain a plain radiograph using the optimum radiation exposure. Unfortunately, the amount of radiation exposure is more difficult to assess with digital imaging than with a screen-film system(1). Unlike screen-film imaging, image display in digital radiography (DR) is independent of image acquisition(2). Due to the wide exposure latitude and post-processing capabilities associated with DR, resultant images will have similar appearances in terms of contrast and density when compared to film-screen technologies; independent of the exposure, however, if images are underexposed, increased quantum mottle will be evident in the image(3). This condition encourages medical imaging technologists to rely on exposure indicator/index (EI) as a feedback mechanism. Nonetheless, there is lack of literature debating about the consistency of EI values in DR. In addition to uncertainty in EI consistency(4), most manufacturers recommended a range value for all anatomical parts as a guideline, raising a concern of unintentional overexposing by technologists in clinical practice. EIs were first developed for CR system technologies by Fujifilm with the introduction of the sensitivity ‘S’ number(3). Each manufacturer developed its own EI to reflect exposure and each index had its own name and value range(1). The naming of EIs varies between manufacturers. Carestream (formerly Kodak) and Philips named theirs the EI value, Fujifilm the S-number, Agfa the log mean (lgM) value and Siemens the EI value(4). The S-numbers range should be ~200. S-numbers under 75 should be considered overexposed, even though they may look normal in appearance and S-numbers >500 is considered underexposed(5). The S-numbers are inversely proportional thus, the Fuji S-number decreases if the exposure increases(6). While all other manufacturers suggested EI in range for all examinations, Fuji recommended ranges of S-numbers including head, chest, abdomen, spine, pelvis, upper and lower extremities as well as gastrointestinal (GI) studies(7). Carestream recommended that for all examinations, the EI should fall between 1700 and 1900(6). In another literature, Carestream’s recommendation is to achieve an EI range of 1500–1800(8)—this EI range was used for Carestream throughout this study since it was the latest recommendation. This literature also stated that after a review by radiologists and radiographers of CR image quality across the radiology department, a lower level EI of 1400 was deemed as providing acceptable diagnostic image quality. If the EI is increased by 300, the exposure to the IP (detector) is doubled(9). For Siemens, the EI value should fall in 150–400(10) and doubling of the absorbed dose in the image receptor results in a doubling of the EI(11). Generally, EI is influenced by the patient size, artefacts, source to image receptor distance, collimation, centering and IP plate size(4). The EI value simply indicates the detector exposure. If a thick body part is imaged on a detector, the exposure technique must be adjusted upward to compensate for increased X-ray attenuation. If a thin patient is imaged on the same detector, the X-ray technique must be reduced to achieve the same number of transmitted X-ray to the detector (same signal-noise-ratio), with the result being the same EI, at a lower dose to the patient(12). In addition, a correct radiographic projection must be selected, the correct anatomy must be projected onto the detector, vendor-specific collimation rules must be followed, and appropriate means for scatter control must be properly used because the detector cannot distinguish between primary and scatter radiation(13). EI is also influenced by the reproducibility of the exposure system(14). Factors contributing to the S-number are; scatter (more scatter—higher S-number), distance—source-image-distance and object-focus-distance (dose and scatter), collimation (good collimation reduces scatter), examination selected at the IIP (due to histogram analysis) and delay in processing from time of exposure(5). According to Siemens, EI depends on collimation, beam quality and examined organ and does not depend on organ program name, selected exposure method (manually or automatically) and selected measuring field(11). With reference to the significance, the aims of this study were to investigate the EI consistency in DR systems produced in constant exposure parameters and clinical condition, and to determine the interaction between the anatomical part and EI. MATERIALS AND METHODS Study design The study was an experimental phantom study between four (4) DR systems, two (2) computed radiography (CR) and two (2) direct DR—Fuji CR, Carestream CR, Siemens DR and Carestream DR. Skull, chest, abdomen and hand projections were carried out on the four (4) systems for comparison. For each projection, the phantom positioning and exposure parameters were set according to the standard clinical practice. All exposure parameters and clinical conditions were kept constant. Twenty (20) exposures were taken for each projection and the EI was recorded. Study instruments Two CR systems and two DR systems were chosen to assist in comparing the consistency which were Fuji FCR PRIMA CR (FUJIFILM Medical Systems, Inc., USA), Carestream DirectView Vita LE CR (Carestream Health Inc., Rochester, NY), Siemens Ysio DR (Siemens Healthineers Limited, Erlangen, DE) and Carestream DRX-Evolution Plus DR (Carestream Health Inc.), Rochester, NY. Phosphor imaging plates and readers used in CR correspond with the systems. CT Whole Body Phantom PBU-60 (KYOTO KAGAKU, Kyoto, Japan) were used to develop clinically relevant images and simulate typical examination conditions of the skull (PH-2B-1), thorax (PH-2B-2), abdomen and hand projection (PH-2B-4). Ethics approval The study began with the approval from the Research and Ethics Committee, School of Health Sciences KPJ Healthcare University College, Board of Ethics KPJ Damansara Specialist Hospital and KPJ Ampang Puteri Specialist Hospital. Data was then collected in the locations aforementioned. Statistical analysis The recorded data was used to describe the consistency of EI values in both systems using descriptive analysis and tested using Kruskal–Wallis test to determine the interaction between EI value produced in different types of DR systems and anatomical parts. Data collection Figure 1 shows the flowchart of data collection method in the research. Figure 1. View largeDownload slide Flowchart of data collection method in the research. Figure 1. View largeDownload slide Flowchart of data collection method in the research. RESULTS The consistency of EI values Tables 1–4 show the minimum and maximum EI values, and the ranges of fluctuation in CR and DR systems. Table 1. Fuji CR S-numbers in 20 exposures. Anatomical parts S-numbers Recommended S-number Output level Skull 340–365 250–600 Optimum Chest 875–960 300–800 Underexpose Abdomen 258–290 300–700 Overexpose Hand 145–159 120–400 Optimum Anatomical parts S-numbers Recommended S-number Output level Skull 340–365 250–600 Optimum Chest 875–960 300–800 Underexpose Abdomen 258–290 300–700 Overexpose Hand 145–159 120–400 Optimum Table 1. Fuji CR S-numbers in 20 exposures. Anatomical parts S-numbers Recommended S-number Output level Skull 340–365 250–600 Optimum Chest 875–960 300–800 Underexpose Abdomen 258–290 300–700 Overexpose Hand 145–159 120–400 Optimum Anatomical parts S-numbers Recommended S-number Output level Skull 340–365 250–600 Optimum Chest 875–960 300–800 Underexpose Abdomen 258–290 300–700 Overexpose Hand 145–159 120–400 Optimum Table 2. Carestream CR EI values in 20 exposures. Anatomical parts EI values Recommended EI value Output level Skull 1223.05–1326.10 1500–1800 Underexpose Chest 464.20–532.01 Underexpose Abdomen 1695.38–1871.54 Optimum Hand 1459.31–1620.36 Optimum Anatomical parts EI values Recommended EI value Output level Skull 1223.05–1326.10 1500–1800 Underexpose Chest 464.20–532.01 Underexpose Abdomen 1695.38–1871.54 Optimum Hand 1459.31–1620.36 Optimum Table 2. Carestream CR EI values in 20 exposures. Anatomical parts EI values Recommended EI value Output level Skull 1223.05–1326.10 1500–1800 Underexpose Chest 464.20–532.01 Underexpose Abdomen 1695.38–1871.54 Optimum Hand 1459.31–1620.36 Optimum Anatomical parts EI values Recommended EI value Output level Skull 1223.05–1326.10 1500–1800 Underexpose Chest 464.20–532.01 Underexpose Abdomen 1695.38–1871.54 Optimum Hand 1459.31–1620.36 Optimum Table 3. Siemens DR EI values in 20 exposures. Anatomical parts EI values Recommended EI value Output level Skull 403 150–400 Overexpose Chest 531–533 Overexpose Abdomen 987–988 Overexpose Hand 2165–2172 Overexpose Anatomical parts EI values Recommended EI value Output level Skull 403 150–400 Overexpose Chest 531–533 Overexpose Abdomen 987–988 Overexpose Hand 2165–2172 Overexpose Table 3. Siemens DR EI values in 20 exposures. Anatomical parts EI values Recommended EI value Output level Skull 403 150–400 Overexpose Chest 531–533 Overexpose Abdomen 987–988 Overexpose Hand 2165–2172 Overexpose Anatomical parts EI values Recommended EI value Output level Skull 403 150–400 Overexpose Chest 531–533 Overexpose Abdomen 987–988 Overexpose Hand 2165–2172 Overexpose Table 4. Carestream DR EI values in 20 exposures. Anatomical parts EI values Recommended EI value Output level Skull 1651–1658 1500–1800 Optimum Chest 1778–1782 Optimum Abdomen 1811–1816 Overexpose Hand 1429–1439 Underexpose Anatomical parts EI values Recommended EI value Output level Skull 1651–1658 1500–1800 Optimum Chest 1778–1782 Optimum Abdomen 1811–1816 Overexpose Hand 1429–1439 Underexpose Table 4. Carestream DR EI values in 20 exposures. Anatomical parts EI values Recommended EI value Output level Skull 1651–1658 1500–1800 Optimum Chest 1778–1782 Optimum Abdomen 1811–1816 Overexpose Hand 1429–1439 Underexpose Anatomical parts EI values Recommended EI value Output level Skull 1651–1658 1500–1800 Optimum Chest 1778–1782 Optimum Abdomen 1811–1816 Overexpose Hand 1429–1439 Underexpose Figures 2–5 show the box-and-whisker plot of the EI values for all anatomical parts in four different CR and DR systems. Figure 2. View largeDownload slide Distribution of skull EI values in different systems. Figure 2. View largeDownload slide Distribution of skull EI values in different systems. Figure 3. View largeDownload slide Distribution of chest EI values in different systems. Figure 3. View largeDownload slide Distribution of chest EI values in different systems. Figure 4. View largeDownload slide Distribution of abdomen EI values in different systems. Figure 4. View largeDownload slide Distribution of abdomen EI values in different systems. Figure 5. View largeDownload slide Distribution of hand EI values in different systems. Figure 5. View largeDownload slide Distribution of hand EI values in different systems. Interaction between EI values produced in different DR systems and anatomical parts The results were tested using Kruskal–Wallis to test the interaction between EI value produced in different DR systems and anatomical parts. The Kruskal–Wallis test indicated that there are statistically significant differences between the EI values in all DR systems to the anatomical parts (P < 0.05). In other words, there is a significant interaction between all anatomical parts and the EI values for all DR systems (P < 0.05). DISCUSSION S-numbers and EI values obtained were inconsistent throughout 20 exposures in all examinations in Fuji CR, Carestream CR and Carestream DR. EI values in Siemens DR showed a consistent value in skull examination only, with EI value of 403.00, while other anatomical parts were inconsistent throughout 20 exposures. Results of the previous study showed that the EI value was not consistent in CR, and thoroughly consistent in DR(4). A similar result has been noted in Carestream CR, but EI in Siemens DR was inconsistent. One possible cause of varying results was the type of DR used—direct or indirect DR. Butler failed to include the exact model of Siemens DR used in her study, which causes the current study to wonder the effect of DR types on EI consistency. The highest fluctuation was seen within hand projection in Carestream CR which yields an increased value of 110.80. Note that an increase in the EI by 300 values represents a doubling of the screen exposure(15). Even though the increase of EI value in hand from the first exposure was considered in the range of optimum exposure, the exposure was increased one-third from the original EI. The fluctuation value was apparent in Carestream CR hand compared to other anatomical parts in other systems possibly due to centering point. In addition, constant repositioning of hand before every exposure probably contribute the fluctuation and there might be a slight room for error in reproducing the exact same positioning of the hand on the IP plate. This study did not consider the calibration and correction factor in the four systems investigated. EI requires careful calibration of the image detector if they are to be used as a surrogate for proper exposure of detector and even more so if they are to be used for patient entrance dose or effective dose estimation(16). The readout time in both CR were <2 min and this could be the reason of inconsistency of S-number and EI value in Fuji CR and Carestream CR. However, latent image will lose ~25% of the stored signal between 10 min and 8 h after an exposure(11). Not only most of the results show inconsistency, note that most values do not fall in the manufacturer’s recommended value. The most possible cause of the outcome is the use of same exposure factors for all systems. Since the systems’ detection quantum efficiency are different from one another, the values obtained might viewed as confusing, take into account that not only the values are inconsistent, but most of them suggests overexposure especially in Siemens DR. This study disregards the effect of using overexposed value and underexposed value, as this probably affect the consistency of values obtained. Thus, the use of the same exposure factor for all manufacturers should be avoided since there are no standardized exposure parameters for all systems. If a future study were to be conducted, it is advised that the exposure factors is first determined to achieve an optimum exposure for every anatomical parts and every manufacturer being investigated to observe if variations or inconsistency might be influenced by under- or overexposures. Based on the results obtained by comparing means of the EI values from different DR systems, the exposure factor taken from the standard exposure factor guidelines were proven to be an optimum exposure factor when using Carestream DR whereas in Siemens DR, the image produced with the same exposure factor contributed to an overexposure. Factor that could not be controlled in this study includes the effect of heat and the time for the next exposure on the EI values obtained, thus this study has neglected those effects although EI might be affected by the delay between exposures(17). Nonetheless, the data were taken continuously without any interruption or if so, the data were collected all over again, regardless the data that has been obtained beforehand. EI is also influenced by the reproducibility of the exposure system(14), which was also one of the factors that could not be controlled in this study. The box-and-whisker plots in Figures 2–5 show both DR systems data were close to the median except in hand projection in Carestream DR. While in CR systems, there are outliers noted in Carestream CR abdomen and Fuji CR hand. Again, hand projection probably has the greatest outliers in both CR and DR systems due to the direct-exposure technique, where the part needs to be repositioned every time the exposure needs to be made. Statistical analysis done showed that there was an interaction between anatomical parts and EI values. This corresponds to the assertion of International Electrotechnical Commission which stated that EI can be defined as a measure of a digital detector’s response to radiation in the relevant image region (RIR) of an image acquired using DR(18). The generated EI value also depends critically on the RIR thus different EI values are possible when a different image region is segmented and used for estimating the incident exposure(12). This intensifies the need for an anatomical-specific EI values set by all manufacturers for accurate feedback on the exposure parameters used and the detector entrance dose. The limitation of this study included the usage of same exposure factor for all manufacturers (resulting in variations of exposure output level), lack of number of the same manufacturer for comparison, lack of manufacturers which uses the same means of EI values, lack of anatomical parts being investigated and the negligence of EI value association with image quality. Future study should determine the optimum exposure parameters for that particular manufacturer before the consistency test is conducted increase the number of the same manufacturer in the study, adding more anatomical parts being investigated and associate the EI values obtained with image quality. CONCLUSION In conclusion, the consistency of EI remained uncertain in CR and DR based on the findings of this study. The inconsistency might due to the calibration factor, inconsistent delay of exposures and readout time and the inconsistent placement of IP due to the readout of the direct-exposure (without grid) approach in hand examinations conducted using CR. Although the findings were contradictory to the previous study by Butler et al. in relations to DR, this might be due to the difference in the type of DR used. The study could also conclude that there must be an additional EI range set by the manufacturer that is anatomical-specific in order to set a limit to the exposures given in clinical practice according to the anatomical part being exposed since the EI depends on the anatomical part being examined. Using the same range of exposure for all anatomical parts and examination would potentially cause overexposure incidents left overlooked over time. ACKNOWLEDGEMENTS The author would like to thank all who have contributed to this study direct and indirectly towards its completion, as well as KPJ Damansara Specialist Hospital and KPJ Ampang Puteri Specialist Hospital for their support and active involvement throughout the period of this study being conducted. REFERENCES 1 Cohen , M. D. , Cooper , M. L. , Piersall , K. and Apgar , B. K. Quality assurance: using the exposure index and the deviation index to monitor radiation exposure for portable chest radiographs in neonates . Pediatr. Radiol. 41 ( 5 ), 592 – 601 ( 2011 ). Google Scholar CrossRef Search ADS PubMed 2 Shepard , S. J. , Wang , J. , Flynn , M. , Gingold , E. , Goldman , L. et al. . An exposure indicator for digital radiography: AAPM Task Group 116 (Executive Summary) . Med. Phys. 36 ( 7 ), 2898 – 2914 ( 2009 ). Google Scholar CrossRef Search ADS PubMed 3 Mothiram , U. , Brennan , P. C. , Lewis , S. J. , Moran , B. and Robinson , J. Digital radiography exposure indices: a review . J. Med. Radiat. Sci. 61 ( 2 ), 112 – 118 ( 2014 ). Google Scholar CrossRef Search ADS PubMed 4 Butler , M. L. , Rainford , L. , Last , J. and Brennan , P. C. Are exposure index values consistent in clinical practice? A multi-manufacturer investigation . Radiat. Prot. Dosimetry 139 ( 1–3 ), 371 – 374 ( 2010 ). Google Scholar CrossRef Search ADS PubMed 5 Fujifilm Medical Systems . FUJIFILM Medical Systems CR Users Guide. 1–40 ( 2004 ). 6 Warren-Forward , H. M. , Arthur , L. , Hobson , L. , Skinner , R. , Watts , A. et al. . An assessment of exposure indices in computed radiography for the posterior–anterior chest and the lateral lumbar spine . Br. J. Radiol. 80 ( 949 ), 26 – 31 ( 2007 ). Google Scholar CrossRef Search ADS PubMed 7 Seeram , E. Optimization of the Exposure Indicator of a Computed Radiography Imaging System as a Radiation Dose Management Strategy, Ph.D. Thesis, Charles Sturt University. ( 2012 ). 8 Gibson , D. J. and Davidson , R. A. Exposure creep in computed radiography: a longitudinal study . Acad. Radiol. 19 ( 4 ), 458 – 462 ( 2012 ). Google Scholar CrossRef Search ADS PubMed 9 Seeram , E. , Davidson , R. , Bushong , S. and Swan , H. Radiation dose optimization research: exposure technique approaches in CR imaging—a literature review . Radiography 19 , 331 – 338 ( 2013 ). Google Scholar CrossRef Search ADS 10 Mothiram , U. , Brennan , P. C. , Robinson , J. , Lewis , S. J. and Moran , B. Retrospective evaluation of exposure index (EI) values from plain radiographs reveals important considerations for quality improvement . J. Med. Radiat. Sci. 60 , 115 – 122 ( 2013 ). Google Scholar CrossRef Search ADS PubMed 11 An Exposure Indicator for Digital Radiography . Report of AAPM Task Group 116. American Association of Physicists in Medicine ( 2009 ). 12 Seibert , J. A. and Morin , R. L. The standardized exposure index for digital radiography: an opportunity for optimization of radiation dose to the pediatric population . Pediatr. Radiol. 41 ( 5 ), 573 – 581 ( 2011 ). Google Scholar CrossRef Search ADS PubMed 13 Willis , C. E. Strategies for dose reduction in ordinary radiographic examinations using CR and DR . Pediatr. Radiol. 34 , 196 – 200 ( 2004 ). Google Scholar CrossRef Search ADS 14 Schaefer-Prokop , C. and Neitzel , U. Computed radiography/digital radiography: radiologist perspective on controlling dose and study quality . In (Ed): RSNA categorical course in diagnostic radiology physics: From invisible to visible – The science and practice of x-ray imaging and radiation dose optimization. pp. 85 – 98 ( 2006 ). 15 Peters , S. E. and Brennan , P. C. Digital radiography: are the manufacturers’ settings too high? Optimisation of the Kodak digital radiography system with aid of the computed radiography dose index . Eur. Radiol. 12 ( 9 ), 2381 – 2387 ( 2002 ). Google Scholar CrossRef Search ADS PubMed 16 IAEA . Avoidance of Unnecessary Dose to Patients While Transitioning From Analogue to Digital Radiology. International Atomic Energy Agency ( 2011 ). ISBN 978 92 0 121010 4. 17 Acceptance Testing and Quality Control . of Photostimulable Storage Phosphor Imaging Systems Report of AAPM Task Group 10 October 2006. American Association of Physicists in Medicine ( 2006 ). 18 International Electrotechnical Commission . Medical electrical equipment—Exposure index of digital X-ray imaging systems—Part 1: Definitions and requirements for general radiography. IEC, Geneva, Switzerland, International standard 62494 – 1 ( 2008 ). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Radiation Protection DosimetryOxford University Press

Published: May 15, 2018

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