The purpose of this work was to compare standard desktop display systems with dedicated medical display systems. The set of image tests proposed by the American Association of Physicists in Medicine (AAPM TG18) was used to assess a Philips 107S desktop display system and a Siemens medical display. Three observers performed the subjective assessment, in a non-concerted manner. The objective assessment was performed using a CCD camera according to the AAPM TG18 procedure. The results clearly demonstrate the inadequacy of standard desktop display systems in the framework of diagnostic radiology. Moreover, a good correlation between the subjective and objective assessment methods was obtaine

Gutierrez, D.

Monnin, P.

Valley, J.-F

Verdun, F. R.

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A STRATEGY TO QUALIFY THE PERFORMANCE OFRADIOGRAPHIC MONITORSD. Gutierrez, P. Monnin, J.-F. Valley and F. R. VerdunUniversity Institute for Applied Radiophysics, Grand-Pre´ 1, CH-1007 Lausanne, SwitzerlandThe purpose of this work was to compare standard desktop display systems with dedicated medical display systems. Theset of image tests proposed by the American Association of Physicists in Medicine (AAPM TG18) was used to assess aPhilips 107S desktop display system and a Siemens medical display. Three observers performed the subjective assessment,in a non-concerted manner. The objective assessment was performed using a CCD camera according to the AAPMTG18 procedure. The results clearly demonstrate the inadequacy of standard desktop display systems in the frameworkof diagnostic radiology. Moreover, a good correlation between the subjective and objective assessment methods wasobtained.INTRODUCTIONThe weakest link in conventional radiology is thefilm processing. To assure the stability of imagequality produced over time, a quality assurance pro-gramme requiring frequent control of film pro-cessing is mandatory. In mammography screeningprogrammes, for example, the stability of film pro-cessing is required at least once a day(1,2). The trans-ition from conventional to digital poses another typeof problem for us, since the film printers remove thisweakness, because these processors have self-controldevices, which ensure the production of films ofconstant quality.However, one should not ignore that a weaknessstill exists in the radiological chain. The new weakestlink is the monitor on which radiologists read theirimages. In this context, acceptance test and stabilitytests should be applied to diagnostic monitors to assurethat no diagnostic information is lost over time.To help radiologists, medical physicists and radio-graphers control the adequacy of their monitors theAmerican Association of Physicists in Medicine(AAPM) has proposed a set of test images allowingboth subjective and objective assessments of monit-ors(3). The goal of the work is to apply several ofthese test images in order to evaluate their sensitivityand to establish a policy concerning the quality con-trol of the monitors in our institution by followingwell-known image quality parameters such as resolu-tion and noise(4).MATERIALS AND METHODSDisplay systemsThe medical display system used in this study was aSiemens Model SMM-21140P (Siemens, Erlangen,Germany), having a maximum resolution of1280 1600 pixels (i.e. 2 megapixels). According tothe manufacturer, its highest luminance was 350 and800 cd m2 when using a full screen (i.e. 30 40 cm2)and a 10 10 cm2 display area, respectively. Its pixelsize was equal to 234 250 mm2. The raster fre-quency used was systematically set equal to 75 Hz.This display system was connected to a DOMEmodel Md2/PCI graphic board (DOME imagingsystems Inc., Waltham, MA), using a 10 to 8 bitDAC with a video memory of 4 MB. This adaptorhas an attached photometer of the same mark thatserved to set up the LUT response of the screen(5).The desktop display system used was a PhilipsModel 107 S (Royal Philips Electronics, TheNetherlands) using a resolution of 1024 768 pixelsover the 32.5 24.5 cm2 screen surface leading to apixel size of 317 319 mm2. There was no technicaldata available concerning its luminance; however, itshighest measured luminance in the 10 10 cm2 geo-metry was equal to 140 cd m2. The raster frequencyused was systematically set equal to 70 Hz. Thisscreen was connected to a standard Trident videoaccelerator 3D graphic board with a video memoryof 8 MB. This system was a 32 bit colour display.PhotometerThe photometer used to perform all luminancemeasurements was a Gossen Model MAVO-MONITOR (GOSSEN Foto und LichtmesstechnikGmbH, Nu¨rnberg, Germany) using a measurementsurface of 6.5 6.5 mm2 and an aperture diameter of18.5 mm by means of optical lenses. This device pro-vides a measurement range of 0.01–19,999 cd m2,split into four sub-ranges having a relative uncer-tainty of 2.5%.CameraThe objective measurements were performed using aPMI-1401 C camera, manufactured by ‘Q Imaging’Corresponding author: francis.verdun@chur.chRadiation Protection Dosimetry (2005), Vol. 114, Nos 1-3, pp. 192–197doi:10.1093/rpd/nch565ª The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oupjournals.org(Q Imaging, Burnaby, BC, Canada). It provides adigital output on 14 bits (16,384 grey levels) andcontains a Kodak CCD (Charged Coupled DeviceKAF-1401) made of 1317 1035 pixels (pixel size6.8 6.8 mm2). The camera was read out with anIEEE 1394 (Firewire) interface.Test patternsThe AAPM TG18 (AAPM Task Group 18) hasdeveloped the image test patterns used. Image qual-ity assessments performed in the framework of thisstudy follow the procedure proposed by AAPMTG18(3).Luminance assessmentThe first test to be performed should verify if adisplay system conformed to the DICOM 3.14standard(6) (which provides the relationship betweenthe grey levels values of the images and the displayedluminance levels). The objective assessment of thisparameter is obtained by measuring the luminance inthe middle of the 18 TG18-LN test patterns. Withthese results, the contrast values normalised to theirrespective just noticeable difference (JND)(3) can becalculated. A deviation of 10% from DICOM stand-ard is allowed for first class screens, while a 20%deviation is authorised for second class screens.The test image TG18-CT should be used to assessthis parameter in a subjective manner. It is made ofsmall low-contrast targets placed in 16 squares ofvarious grey levels. The detectability of all of thesetargets is required for first class screens.Spatial resolution assessmentThe test images TG18-RH and TG18-RV, whichprovide horizontal and vertical impulse lines,respectively, at various luminance levels must betaken with the CCD camera to calculate the pre-sampled modulation transfer function (MTF) ofmonitors, following the AAPM procedure(3). Oncethe MTF is obtained, one should verify that its valueat the Nyquist frequency is >25% for a first classcategory and for second category monitors this valuemust be >35%.The test image TG18-CX should be used to assessthis parameter in a subjective manner. It is madeof many CX patterns spread all over the entireimage at four luminance levels: maximum luminancelevel, 75, 50 and 25%. A 12-grade reference scale(from 2 and 1 for the defocused spot and 0–9for perfect to twice the spot size) is provided to assessthe spatial resolution of the monitor. The CX ele-ments should be scored between 0 and 4 for a firstclass display and between 0 and 6 for second classdisplays.Image noiseThe noise power spectra (NPS) of monitors canbe assessed again with the CCD camera at threeluminance levels with the three test images TG18-NS that contain large homogeneous areas. There arestill no reference values for this parameter.The test image TG18-AFC can be used to assessthis parameter in a subjective manner. It is made ofmany small boxes containing low-contrast featuresin a random location; these boxes are placed in fourquadrants depending on the contrast and size of thefeature. One must detect all the small features of atleast three quadrants for a first class display and intwo quadrants for a second class display.RESULTS AND DISCUSSIONLuminance assessmentThe response luminance curves obtained with thetwo monitors used in this study are shown inFigure 1. The contrast values normalised to theirrespective JND are presented in Figure 2. It can beobserved that the radiological monitor is withinthe 10% tolerance of the ideal curve (i.e. first classcategory monitor), whereas the desktop monitordiverges from this ideal curve in the low luminancerange, and it can be noticed that the desktop monitoris out of the 10% tolerance range in the low lumin-ance, which is applied to second class radiologicalmonitors.In visual examination all low-contrast targetscould be observed with the diagnostic monitor,while many targets could not be detected in the lowand high luminance range with the desktop monitor.This result agrees quite nicely with the results of theobjective assessment.Spatial resolution assessmentAs shown in Figure 3, regardless of the luminancelevel, the diagnostic monitor offers a pre-sampledMTF, which is systematically >50% at its Nyquistfrequency (measured equal to 2.14 mm1), thus thismonitor can be placed in the first class category.Figure 4 presents the results obtained with thedesktop monitor where a loss of spatial resolutionappears with the increase in luminance. Its pre-sampled MTF, at its Nyquist frequency (measuredequal to 1.67 mm1), is <25% in the medium andhigh luminance conditions. This makes it a non-acceptable monitor. The degradation of the resolu-tion with the increase of brightness is due to the typeof phosphor used, since the diffusion of the light onthe surface increases with brightness.The subjective assessment has given a score of 0 tothe radiological monitor, while the desktop monitorobtained a score of 4 in the high luminance range,PERFORMANCE OF DISPLAY MONITORS1933 in the medium luminance range and 2 in the lowluminance range, confirming the objective results.Image noiseFigures 5 and 6 present the NPS obtained atthree luminance levels for the radiological anddesktop monitors, respectively. The results showthat for both monitors normalised noise increaseswith the luminance level. Moreover, the noiselevel of the radiological monitor is 10 timeslower than that of the desktop one. While thereis still no threshold defined by the AAPM standard,it is recommended to have a noise level noFigure 2. Contrast response from 18 computed patterns for both systems associated with DICOM response.Figure 1. Luminance response corresponding to DICOM 3.14 superposed with our measures.D. GUTIERREZ ET AL.194higher than the one of a conventional filmsystem.In both cases peaks corresponding to the rasterfrequency and associated with the pixel size can beobserved (i.e. 4.7 mm1 corresponding to a pixel sizeof 210 mm and 3 mm1 corresponding to a pixelsize of 300 mm).The results of the visual test have ranked the radio-logical screen as a first class monitor whereas the desk-top monitor has been ranked as a second class monitor.Figure 4. Pre-sampled MTF of the PHILIPS display screen.Figure 3. Pre-sampled MTF for three luminance levels of Siemens SMM-21140P calculated by the constructor.PERFORMANCE OF DISPLAY MONITORS195CONCLUSIONSSeveral test images proposed by TG18 of theAAPM were tested and provided both subjectiveand objective results concerning the assessment oftwo display monitors. These tests are fairly easy toperform and should be integrated into acceptancetesting protocols of monitors in radiologyFigure 6. Philips Weiner spectra for three levels of brightness.Figure 5. Siemens Weiner spectra for three levels of brightness.D. GUTIERREZ ET AL.196departments. The expected result was confirmed;according to the DICOM 3.14 standard, the desktopdisplay is not suitable to read diagnostic images. Acorrelation was systematically obtained between theobjective and subjective assessments of the monitors.Thus, acceptance tests of monitors do not necessarilyrequire the use of a high-quality level CCD camera.Finally, the weak link, which remains after havingaccepted a monitor for diagnostic purposes, is itsplacement. AAPM test images can also be used todemonstrate the importance of ambient light on per-ceived image quality.ACKNOWLEDGEMENTThe authors gratefully acknowledge the financialsupport of the Swiss Federal Office for Educa-tion and Science (grant no 99.0739—FifthEuropean Research programme no. FIGMCT 200000036).REFERENCES1. American College of Radiology. Mammography QualityControl forMedical Physicists (Reston, Va: ACR) (1992).2. van Voudenberg, S., Thijssen, M. and Young, K.European guidelines for quality assurance in mammo-graphy screening. Report EUR 14821, 3rd edition(Luxembourg: CEC) (1999).3. American Association of Physicists in Medicine TaskGroup 18. Assessment of display performance for med-ical imaging systems. Pre-print Draft (version 10.0),August 26 (2004).4. RadiologicalSocietyofNorthAmerica.Advancesindigitalradiography categorical course in diagnostic radiologyphysics. RSNA Syllabus (Oak Brook, IL: RSNA) (2003).5. Blume, H. R., Ho, A. M. K., Stevens, F. and Steven, P. M.Practical aspects of grayscale calibration of displaysystems. In: PACS and Integrated Medical InformationSystems: Design and Evaluation. Proc. SPIE 4323,28–41 (2001).6. Digital Imaging and Communications in Medicine.Grayscale Standard Display Function DICOM 3 Part14. National Electrical Manufacturers Association.PERFORMANCE OF DISPLAY MONITORS197

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