Australian Dental Journal

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1 Australian Dental Journal The official journal of the Australian Dental Association Australian Dental Journal 2015; 60: doi: /adj Digital display monitor performance in general dental practice A Butt,* NW Savage *Metro North Oral Health Services, Queensland Health. School of Dentistry, The University of Queensland. Royal Brisbane and Women s Hospital, Queensland Health. ABSTRACT Background: The performance of computer displays represents an important factor influencing the quality of digital radiographs. The aim of this study was to evaluate the performance of computer displays used for the purposes of diagnostic radiology in a sample of dental practices in one Australian state. Methods: Twelve dental practices comprising 29 displays elected to participate in a detailed performance evaluation of their computer displays according to the AAPM TG18 and DICOM part 14 GSDF standards. Results: None of the 29 displays tested passed the primary or secondary acceptance criteria developed by the AAPM TG18. The greatest contributor to display failure, both prior to and following calibration, were specular and diffuse reflection. When the parameter of display reflection was ignored, the most frequent parameters contributing to display failure following calibration included the primary grade acceptance criteria of noise (n = 29, 100%), contrast ratio (n = 9, 31%) and maximum luminance (n = 12, 41%). However, display calibration resulted in a significant improvement in the parameter of contrast response. Conclusions: This study demonstrated significant problems concerning the performance of display monitors in the population surveyed. In recognition of the growing utilization of digital imaging in dentistry the importance of the computer display should be considered. Keywords: AAPM TG18, DICOM part 14 GSDF, digital radiography, display monitor performance. Abbreviations and acronyms: AAPM = American Association of Physicists in Medicine; AAPM TG18 = American Association of Physicists in Medicine Task Group 18; AMLCD = Active Matrix Liquid Crystal Display; DICOM = Digital Imaging and Communications in Medicine; DICOM part 14 GSDF = Digital Imaging and Communications in Medicine part 14 Standards for Grey Scale Display Function; DVI-D = Digital Visual Interface Dual Link; NEMA = National Electric Manufacturers Association; RDP = research digital population; VGA = Video Graphics Array. (Accepted for publication 28 July 2014.) INTRODUCTION Radiography makes an essential contribution to the processes of examination, diagnosis and treatment planning in dentistry. While the use of film-based imaging still predominates, digital imaging is gaining wider acceptance, and the use of this modality is anticipated to expand in the future. 1 4 In transitioning to digital radiography, resultant image quality should at least parallel that of plain film. The recognized stages of digital image acquisition include obtaining the raw data, data processing, image display and final visual perception by the observer. This has also been referred to as the digital imaging chain 1,5 7 or modulation transfer function of a system. 8 By necessity, the entire process of generating a digital image encompasses a greater number of steps when compared to plain film imaging. Each step in this chain is critical, as deficiencies at any point may compromise image quality. 1 Many authors have indicated that computer displays represent an important element in optimizing the quality of digital radiographs. 1,9 15 As highlighted by Badano in 2004, 11 display monitors always degrade information contained in an image and it is vital to ensure that certain measures are taken to limit this loss as much as possible. 10 Of additional concern is the recognition that the performance of computer displays lessens as a consequence of ageing. 16,17 However, due to the subtle nature of these changes, the daily user is often unable to perceive this decline Australian Dental Association

2 Digital display monitor performance in general dental practice In recognizing that display performance can affect image quality and diagnostic outcomes, 1,18 guidelines have been developed to assess and optimize the performance of these devices. 7,16,19 The American Association of Physicists in Medicine Task Group 18 (AAPM TG18) 10 and the National Electric Manufacturers Association (NEMA) 19 have developed recommendations regarding the appropriate operating characteristics of displays used for the purposes of diagnostic radiology. 10 In addition, the AAPM TG18 has also defined displays according to whether they are used for the purposes of establishing a diagnosis (primary grade displays) or to review images (secondary grade displays). Subsequently, many hospitals have introduced periodic quality assurance checks on the basis of these recommendations ,20 23 Research concerning the operating characteristics of display devices used in digital radiography in dentistry has been undertaken in other countries. 1,3,17,24 26 However, to date no research applicable to Australian dental practices has been conducted. Therefore, the purpose of this study was to assess the operating characteristics of diagnostic display monitors from a sample of dental practices using the internationally recognized parameters established by the AAPM TG MATERIALS AND METHODS Questionnaire The purpose of the questionnaire was twofold. Firstly, it was necessary to determine how many dentists within a selected sample in one Australian state use digital radiography. Secondly, the questionnaire would give respondents an opportunity to participate in a detailed performance evaluation of their display monitors, used for the purposes of diagnostic radiology. Ethics approval for this study was granted following an institutional board review from The University of Queensland. Radiation Health, Queensland 27 mailed the study questionnaires to all private dental practitioners holding radiology equipment possession licenses in the state. Surveys were not distributed to public dental clinics, medical imaging facilities or veterinary practices. Respondents returned the questionnaire in a reply paid envelope. Responses received from specialists in the field of dentomaxillofacial radiology were excluded from the study. Evaluation of display monitor performance Evaluation of display performance encompassed visits to the practices of a subset of respondents who had elected to participate. Owing to study constraints, selection of a subset population was narrowed by geographic location (practices had to be located in the south-eastern corner of Queensland, type (Active Matrix Liquid Crystal Displays; AMLCD) and display age (less than 12 months old). AMLCD devices were selected as the evaluation and calibration software was only compatible with this display type. Display selection was narrowed on the basis of age to limit the negative effects of excessive luminance degradation, which can be associated with displays of greater age. 17 All displays were tested in their as found native environment. Prior to testing, all displays were cleaned, warmed up for a period of 30 minutes and the optimum display resolution set. All display evaluation and calibration functions were performed using a proprietary software package and calibrated photometer (EIZO RadiCS) containing the test patterns and reference standards developed by the AAPM TG18. Because the AAPM recommends that appropriately qualified personnel conduct the tests, the researcher completed training over a 12-month period with a senior medical physicist experienced in display performance assessment. The software was installed and operated from the research computer utilizing a dedicated graphics card capable of supporting 32-bit colour or 10-bit greyscale images, as recommended by the AAPM TG18. Prior to testing, the TG18-QC test pattern was used to review and correct the horizontal and vertical alignment of the active display field, and to ensure there were no gross defects inherent in the display. Both pre- and post-calibrated display performance was measured using the primary and secondary acceptance criteria of the AAPM TG18. Pre-calibrated display performance determined the behaviour of displays in as found condition without any adjustment for compliance to either AAPM TG18 and DICOM part 14 GSDF standards. 19 Displays were then calibrated to these standards and the performance of displays assessed a second time (postcalibration testing). Data were recorded using an Excel spreadsheet and individual monitor performances relative to the individual criteria were scored as either a pass or fail. In summary, a total of 30 tests were applied to each display, encompassing 16 tests for primary acceptance standards and 14 tests for secondary acceptance standards. A summary of the parameters and associated tolerances is outlined in Table 1. The test patterns employed may be freely accessed from the AAPM TG18 taskforce website Australian Dental Association 241

3 A Butt and NW Savage Table 1. Acceptance criteria for display monitor performance adapted from the AAPM TG18 Operating parameter Acceptance criteria primary grade display device Acceptance criteria secondary grade display device AAPM TG18 test patterns used Geometric distortions 2% deviation 5% deviation TG18-QC Reflection L min 1.5L amb L min 1.5L amb TG18-AD, black background (ideally 4Lamb) (ideally 4Lamb) Luminance response L max 170 cd/m 2 L max 100 cd/m 2 TG18-LN8-01 to TG18-LN8-18 LR 250 DLmax 10% j d 10% LR 100 DLmax 10% j d 20% Luminance dependencies/ Non-uniformity 30% Non-uniformity 30% TG18-UNL10 and TG18-UNL80 luminance uniformity Resolution 0 Cx 4 DL 30% RAR= AR Cx 6 DL 50% TG18-QC Noise All targets visible except Two largest sizes visible TG18-AFC the smallest Veiling glare 3 targets visible GR target visible GR 150 TG18-GV and TG18-GVN Chromaticity D (u, v ) 0.01 N/A TG-18UNL80 L min = minimum luminance;, L amb = ambient luminance; L max = maximum luminance in the presence of an ambient luminance component; L max = maximum luminance with no ambient luminance component; DL max = change in maximum luminance; k d = contrast response; LR = max/min luminance ratio; Cx = a measure for resolution used in the TG18-CX or TG18-QC test pattern; DL = change in luminance; RAR = resolution addressability ratio; AR = astigmatism ratio (ratio of the large versus short axis of a circle); GR = glare ratio; D (u, v ) = colour uniformity. RESULTS Questionnaire In total 784 questionnaires were mailed to the individuals or companies in private dental practices holding radiology equipment possession licences in the state of Queensland. Two hundred and eighty-eight questionnaires were returned which equates to a response rate of 36.7%. Of those who responded 92.4% (n = 266) were general dental practitioners and 7.6% (n = 22) were dental specialists. One hundred and thirteen respondents indicated they used digital imaging (39.2%). The remainder used plain film and the ratio of people using digital versus plain film was approximately 1:2. All respondents who used digital imaging were defined as the research digital population or RDP (n = 113). A total of 603 display monitors used for the purposes of diagnostic radiology were possessed by the RDP; 46.9% (n = 53) of practices possessed between two and six displays. Of the total number of displays, 90.2% (n = 544) were of the AMLCD type. Seventyeight per cent (n = 471) of displays owned by the RDP were less than three years of age. Evaluation of display monitor performance From the RDP, 10.6% (n = 12) respondents volunteered to participate in the study. Twenty-nine AMLCD display monitors of less than or equal to 12 months of age were tested. All displays tested were used for radiographic interpretation, but no displays were used exclusively for this purpose. Other functions included general office work, patient record keeping and Internet access as required. Twenty-four displays were located chairside, four were situated in private offices and one was located in a sterilization room. None of the displays tested were of medical grade. Eighty-three per cent (n = 24) of monitor displays exhibited a matt finish, and prior to testing 86% (n = 25)ofdisplaystestedwereeither visibly unclean or marked. Display monitor performance according to AAPM TG18 standards Geometric distortions Geometric distortion was quantitatively assessed using the TG18-QC test pattern. All 29 display monitors passed this criterion prior to, and following, calibration. Display reflection Both the specular and diffuse reflection characteristics of displays were evaluated qualitatively using the black screen and TG18-AD test patterns respectively. All displays failed to meet the criteria for specular and diffuse reflection under normal clinical lighting conditions. Following a maximum possible reduction in room lighting, only one display passed. Luminance response All three luminance response parameters of maximum luminance, contrast ratio and contrast response were evaluated quantitatively utilizing the photometer in Australian Dental Association

4 Digital display monitor performance in general dental practice conjunction with the TG18-LN8-01 to TG18-LN8-18 test patterns. Maximum luminance When assessing displays to primary maximum luminance standards, 37.9% (n = 11) failed pre-calibration and 41.4% (n = 12) failed post-calibration. A similar trend was observed when testing displays to secondary standards, where one display (3.5%) failed pre-calibration and three (10.3%) failed post-calibration. Following calibration, a mean reduction in maximum luminance and increase in minimum luminance values was observed. Calibration did not improve either the minimum or maximum luminance response of the displays tested. Contrast ratio Prior to calibration, two (6.9%) displays failed the primary acceptance criteria for this parameter. However, following calibration nine (42.9%) did not meet the recommended standards. Therefore, calibration was seen to significantly worsen the contrast ratio according to primary acceptance standards. However, all displays passed the contrast ratio standards applicable to secondary displays in both pre- and post-calibrated states. Contrast response All displays failed the recommended acceptance criteria for contrast response prior to calibration. Following calibration nine displays (42.8%) passed the primary acceptance criteria and 25 (96.1%) passed the secondary acceptance criteria. Luminance dependencies (luminance uniformity) Luminance dependencies were evaluated quantitatively using the photometer in conjunction with the TG18- UNL10 and TG18-UNL80 test patterns. The tolerances are such that variation in light emission across the display face should not exceed 30%. Following the application of low luminance values, the research sample expressed mean luminance uniformity percentages between 20% to 22% (range %). When high luminance values were applied, luminance dependencies between 18% to 20% (range %) were observed. All displays passed this parameter both preand post-calibration. Display resolution Display resolution was assessed qualitatively using the TG18-QC test pattern. All displays passed the acceptance criteria for display resolution in both pre- and post-calibrated states. Display noise Display noise was evaluated qualitatively using the TG18-AFC test pattern. All displays failed the primary acceptance criteria for this parameter before and after calibration. However, when the lower tolerances of the secondary acceptance criteria were employed, all displays passed. Veiling glare Veiling glare was qualitatively assessed by determining the total number of targets visible using the TG18-GV and TG18-GVN test patterns. As suggested by the AAPM TG18, a masking device manufactured from black cardboard was employed to exclude peripheral light from the display and environment during testing. Prior to calibration, 20 displays (69.0%) failed the primary acceptance criteria and one display (3.5%) failed the secondary acceptance criteria for veiling glare. Following calibration, all displays passed this parameter. Chromaticity Chromaticity was assessed using the photometer and the TG18-UNL80 test pattern. One display failed this parameter prior to calibration. Following calibration, all displays met the acceptance criteria. DISCUSSION Suboptimal display performance represents an important factor that may contribute to diagnostic errors and adverse outcomes in the field of diagnostic medical radiology. 18 In recognition of this, the AAPM TG18 16 and NEMA 19 have established recommendations to standardize and optimize the performance of medical grade displays. Most of current published research pertaining to medical radiology has evaluated the performance of medical grade displays. From a thorough search of the literature only six studies evaluated commercial grade displays according to the aforementioned standards. 1,20,24,26,28,29 However, to the best of the author s knowledge, this is the only study that has assessed the performance of a number of commercial grade displays according to DICOM part 14 GSDF and all primary and secondary acceptance standards established by the AAPM TG18. In the present study, none of the 29 displays tested passed all of the primary or secondary acceptance criteria developed by the AAPM TG18. The greatest contributor to display failure was specular and diffuse reflection (n = 28, 96.6%). Specular reflection occurs when the display face behaves like a mirror, reflecting 2015 Australian Dental Association 243

5 A Butt and NW Savage incident light directly back to the observer. Diffuse reflection, sometimes referred to as haze, describes the internal scattering of incident light that is reflected diffusely from the display surface. 16 Problems with reflection in our study were predominantly due to the intensity and orientation of ambient room lighting relative to the display face. Hellen-Halme et al. 1 made similar observations in their study, and observed that environmental lighting was largely responsible for the excessive reflection of light from the display monitor. The results from numerous investigators, 2,25,30 34 have demonstrated that optimal perception of image contrast is influenced by ambient room lighting and display reflection. Bright room lighting is detrimental to radiographic interpretation due to the combined effects of inducing pupillary contraction and interfering with the primary emission of light from the display. Both factors are critical to the perception of contrast in images. 34,35 Ideally, the appropriate levels of environmental lighting should be established based on both the reflection and luminance response characteristics of displays. 16,22 When the parameter of display reflection was ignored, the highest parameter contributing to display failure was the primary grade criterion of display noise (n = 29, 100%). However, all displays passed the secondary acceptance criteria, which was largely due to the lower tolerances as defined by the AAPM TG18. Display noise refers to unwanted signals, which are not part of the image and can compromise true image detail. 16 Many factors induce noise in AMLCD displays but the predominant causes include input voltage/signal fluctuations, the physical pixel architecture of the device and the type and length of cable used to connect the display to the hard drive. 16 Medical and graphics displays demonstrate fewer problems because the architecture and electronics of these devices are of a higher quality. Gutierrez et al. 28 quantitatively assessed the noise characteristics of a medical and standard office grade display, and observed that the noise level of the standard display was 10 times greater when compared to the medical device. The displays tested in our sample had not been manufactured to medical grade standards. Consequently, this would make it difficult to achieve the display noise criterion expected of primary medical grade displays. Another important factor is that the displays tested were only capable of supporting VGA and not the recommended DVI-D connecting cables, and this may have unduly impacted on the performance of our sample. After display reflection and noise, the two final parameters contributing most to display failure comprised the primary grade luminance response characteristics of maximum luminance and contrast ratio. The luminance response characteristics of displays are comprised of the three individual but closely interrelated criteria of maximum luminance (the maximum amount of light emitted from a display), contrast ratio (the ratio of the maximum to minimum luminance) and contrast response (the consistency of light emission in response to a specified input electronic signal). 7,16 These three parameters represent the foundation of the DICOM part 14 GSDF standards. This standard ensures the consistent presentation of greyscale in digital images by the display, such that it matches the contrast perception of the human eye. In our study it was observed that calibration adversely affected display performance, by consistently decreasing the maximum luminance values and raising the minimum luminance levels expressed by displays. Subsequently, this resulted in greater failures concerning the parameter of contrast ratio. The reasons why a greater number of displays failed the acceptance criteria for maximum luminance and contrast ratio are a consequence of how calibration affects display function. Critical to display calibration is the establishment of compliance with the DICOM standards for greyscale display function (DICOM part 14 GSDF). This is largely achieved through the application of a table of references (also called a Look Up Table or LUT) to the graphics card that drives the display. This table assigns a distinct input electronic signal for each possible shade of grey between the 0 and 256 pixel values. Application of this table results in a complete reassignment of the relationship between the electronic input signal from the graphics card and the pixel value or light emitted by the display device. This process of reassignment in essence fills in the missing or inadequately presented luminance levels presented by the display between the values of 0 to 256, and ensures the consistent presentation of greyscale by both the display and graphics card. The reassignment of pixel value to signal input consistently causes a reduction in maximum luminance, increases minimum luminance and simultaneously decreases the contrast ratio. The negative impact of calibration is more likely to be observed when applied to standard office displays as opposed to medical grade devices. Medical displays are manufactured to DICOM part 14 GSDF standards, and reliably express luminance values at the extreme ends of the luminance spectrum. Calibration of the luminance response characteristics of such displays may not unduly affect final compliance to the AAPM TG18 and DICOM part 14 GSDF standards, which was successfully demonstrated by Jung et al. 22 Commercial or standard office displays (which encompassed all the devices tested in this project) are not designed to present maximum or minimum luminance values with the consistency expected of medical grade devices, and neither are they manufactured to comply with the DICOM Australian Dental Association

6 Digital display monitor performance in general dental practice part 14 GSDF standards. Therefore, calibration may unduly affect the performance of standard display devices in comparison to medical grade displays. The most important consequence of compromised performance in the luminance response characteristics of a display is the increased potential for inconsistent presentation of greyscale and inability to express luminance values at the extreme ends of the luminance spectrum. The consistent presentation of greyscale is particularly important when reviewing images demonstrating very subtle differences in contrast. In 2007, Buls et al. 18 found a statistically significant reduction in observer performance with displays demonstrating lower contrast ratio and maximum luminance levels below the AAPM TG18 16 recommendations. McIllgorm et al. 26 found that upon calibration of three standard office grade displays to DICOM part 14 GSDF standards, a statistically significant improvement in the perception of subtle contrast in images was observed. Irrespective of whether displays failed to meet the primary or secondary acceptance standards as defined by the AAPM TG18, calibration of displays resulted in a significant improvement in the perception of contrast in several of the AAPM TG18 test patterns. Improved visibility of the central low contrast central objects in the TG18-GV/TG18-GVN test patterns, and the 5% and 95% low contrast and Quality control luminance squares in the TG18-QC test pattern was noted. How this would translate to visibility of subtle contrast in digital dental radiographs was not determined in this study. However, recent research by McIllgorm et al. 26 suggests optimizing display performance to DICOM part 14 GSDF standards, of which display calibration to the AAPM TG18 standards ensures, does improve the visibility of subtle contrast in dental radiographs. The benefit of using displays compliant with both AAPM TG18 and DICOM part 14 GSDF standards would be best investigated with further studies. CONCLUSIONS Research in medicine has demonstrated that poorly performing displays are capable of compromising image quality and observer performance. In the present study it was established that the performance of a select sample of display monitors was suboptimal when measured against known standards, for the practice of diagnostic medical radiology. The main parameter compromising display performance in our sample following calibration to AAPM TG18 and DICOM part 14 GSDF standards included the criterion of specular and diffuse display reflection. This predominantly related to inappropriate ambient room lighting. Difficulty in achieving the criterions of display noise, and the luminance response characteristics of maximum luminance and contrast ratio, was also noted. As highlighted in the medical literature to date, our research suggests that standard office grade displays, even when calibrated to DICOM part 14 GSDF standards, do not represent a suitable substitute for primary medical grade devices. Three important limiting factors in this study include: the computer systems of the practices were not evaluated; testing was conducted from an optimized hardware/software platform; and only relatively new display devices were evaluated. However, the fact that a significant variation in the performance of individual displays was still evident is of concern. Consultation with a medical physicist specializing in display performance indicated this was most likely consequent to differences in build quality, brand and historical use of the device. The purpose of calibration to DICOM part 14 GSDF standards is to ensure inherent electronic information contained within any digital image is uniformly presented, irrespective of the age or type of display. Variation in display performance has the potential to negatively impact on the perception of subtle low contrast image detail, 29 and the potential for compromised diagnostic outcomes has been clearly demonstrated in the medical literature. 18,36 Although display performance is not the only factor that may impact on the quality of digital radiographs, employing displays compliant with DICOM part 14 GSDF standards for the purposes of diagnostic radiology must be considered. 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Soft-copy reading of digital chest radiographs: effect of ambient light and automatic optimization of monitor luminance. Invest Radiol 2005;40: Roehrig AC, Krupinski E, Fan J, Gandhi K. Why should you calibrate your display? Proceedings of the SPIE. 2004;5199: Address for correspondence: Dr Alison Butt Brisbane Dental Hospital and Adult Specialist Services Corner Albert and Turbot St Brisbane QLD alison.butt@health.qld.gov.au Australian Dental Association