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RAMIN KHORASANI, MD, MPHBITS AND BYTES

© 2009

isplay Monitors for Digital Medical Imaging

atherine P. Andriole, PhD

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he choice of diagnostic displayonitors was relatively straightfor-ard for early adopters of picture

rchiving and communication sys-ems. Hardware was of a singleype, cathode ray tube (CRT) tech-ology, and was usually oriented inortrait mode, emulating the shapef film. Monitors had high bright-ess, typically 60 to 100 foot-lam-erts (FL), relative to other com-uter and television monitors andigh refresh rates of greater than 72z to reduce flicker visible to the

uman eye. The devices themselvesere physically large, heavy, and

xpensive. They generated notice-ble quantities of heat while con-uming relatively high amounts ofower, and their display quality de-raded quickly, requiring frequentonitor replacement.Early medical-grade monitors

ere available in two spatial reso-utions (high and low) reflectingheir pixel matrix sizes (2K, or048 columns by 2500 rows; andK, or 1024 columns by 1280 col-mns, respectively). Medium-resolu-ion 1.5K monitors of 1500 columnsy 1500 rows were later added to theix. Because of the exponentially

igher cost of 2K monitors com-ared with 1K monitors, radiologyepartments typically had a combi-ation of a few strategically placedigh-resolution displays and many

ow-resolution or medium-resolu-ion displays. The ACR recom-ended that 2K monitors be used

or primary diagnosis of digital pro-ection radiographs because a singlemage could be displayed per mon-tor in its full inherent acquired spa-ial resolution. The cross-sectionalodalities with slice matrix sizes of

12 � 512 for computed tomogra- p

005 American College of Radiology1-2182/05/$30.00 ● DOI 10.1016/j.jacr.2005.02.019

hy and 256 � 256 for magneticesonance imaging were considereddequately displayed on 1K moni-ors. As display application soft-are and graphical user interfaces

GUIs) improved, many radiolo-ists became comfortable readingrom 1K monitors even for projec-ion radiography, as long as the im-ges were acquired at their full spa-ial and contrast resolutions and theUI allowed for the easy manipu-

ation, magnification, and compar-son of images.

Today, in addition to the matu-ation of software, a richer array ofardware technologies exist for theurposes of displaying digital med-cal images. Unfortunately, therere currently no formally definedtandards or specification guide-ines to clarify choices of monitorsor users. This article summarizeshe different display devices avail-ble today and the specifications toonsider when purchasing moni-ors for use in radiologic imaging.n explanation of monitor types,

ncluding CRTs, active-matrix liq-id crystal displays (LCDs), andlasma technologies, is given, alongith a discussion of spatial resolu-

ion capabilities and requirements,ontrast resolution and monitor lu-inance, the orientation or shape

nd number of displays necessary,nd a comparison of color andonochrome or grayscale moni-

ors. Device calibration and qualityssurance practices are also ad-ressed.Two technology types of hard-

are displays are currently used inedical imaging: the half-century-

ld, mature CRTs and what theopular literature refers to as flat-

anel technology, of which there m

re several types [1]. Of the tworoad categories of flat-panel dis-lays, one filters reflected light or

ight from a source behind the fil-er, whereas the second creates lighty exciting a phosphor. Note thathe term flat panel is not meant toefer to the face of the monitor, be-ause some CRTs have flat faces2]. Rather, it refers to the thin-filmransistor array panel that addressesach pixel.

Cathode ray tubes produceight by exciting a phosphor-lu-

inescent coating with a focusedlectron beam. Light is generatedn an emissive structure, in whicht diffuses in a controlled manner,orming the displayed image. Theighest spatial resolution CRTonitors available have a display

rea of 2048 � 2560 pixels, oroughly 5 megapixels. They comen low-brightness and high-rightness versions of 50 to 60nd 100 FL or greater, respec-ively. High-resolution and low-esolution (2K and 1K) monitorsypically come in portrait mode,ith a 9:16 or 3:4 aspect ratio,

mulating the shape of film. Mostedium-resolution CRTs (1.5K)

re square or in landscape mode,ith an aspect ratio of 16:9 or 4:3.he choice of a portrait or land-

cape monitor shape is a functionf personal user preference, witho technical issues bearing on the

ssue.The flat-panel display type pre-

ominantly used in medical imag-ng is the active- matrix LCD. Liq-id crystal displays use a transistor-riven matrix of organic liquidrystals that filter reflected light.iquid crystal displays use a light-

odulating as opposed to a light-

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mitting mechanism for creatingisplay images. Polarization con-rols the light intensity such thathe maximum intensity is perpen-icular to the LCD panel. Conse-uently, this technology suffersrom marked variations in lumi-ance and contrast depending oniewing angle [1]. This is the off-xis-viewing or angle-of-regardroblem, in which images can ap-ear quite different if viewed fromifferent angles or heights abovend below the center axes of thecreen. Newer LCD designs haveore uniform luminance and con-

rast profiles within larger viewingngle cones (some as high as 170°).sers should inquire about the hor-

zontal and vertical viewing angleapabilities and, better yet, ask ven-ors for demonstration monitorsor clinician testing. Liquid crystalisplays typically have the capabil-

ty to display in both portrait andandscape modes.

Plasma display panels are cur-ently being developed largely forigh-definition television viewingith 37-inch or larger screens. A

urrent passed through ionized gasNe-Xe) contained between a pairf glass layers causes the emission ofltraviolet light, which excites visi-le light–emitting phosphors toroduce the display image. Plasmaisplay panels are very expensivend have roughly the same numberf addressable pixels as 17-inchCDs, can be hung on a wall, havewide viewing angle with no loss ofuality, and have high brightnessut relatively slow response time2]. They are not used currently inedical imaging because of their

igh cost, slow refresh rates, ghost-ng artifacts, and contrast limita-ions. Other types of displays, suchs field-emissive displays and or-anic light-emitting diodes, are un-

ergoing heavy developmental ef-

orts but are not yet viable for medicalmaging display purposes [2].

Although the spatial resolutionerminology used for both CRTsnd LCDs is based on the deviceixel matrix dimensions—1K toK for CRTs and 3, 5, and 9egapixels for LCDs—not allonitors are created equal. For ex-

mple, 1K and 2K CRT monitorsend to have standard diagonals sohat a larger pixel matrix size con-otes a smaller pixel size and henceetter spatial resolution capabili-ies, and all 1K monitors have hadquivalent spatial resolution, asid all 2K monitors. This is not thease for LCDs. For example,-megapixel monitors come withifferent sized diagonals, that is,ifferent physical sizes such that ahysically bigger monitor actuallyas a larger pixel size and henceoorer spatial resolution. Userseed to understand what the pixelr spot size is, because this directlyeflects spatial resolution and theerception of fine detail, and notecessarily choose the largestcreen. Pixel size can be determinedrom the physical screen size, typi-ally given as a display area diagonaln inches, and total pixel count ororizontal and vertical matrix reso-

ution. Often, vendors give the de-ice pixel density or pixel pitchpacing, and to confuse the issue,his is often given in millimeters. Asor comparison between CRT andCD monitors, the 1K CRTs at024 � 1280 pixels correspond to-megapixel monitors, the 1.5KRTs at 1500 � 1500 pixels corre-

pond to 2-megapixel monitors,nd CRTs at 1760 � 1760 corre-pond to 3-megapixel displays. TheK CRTs at 2048 � 2500 pixels cor-espond to 5-megapixel LCDs. Theecently introduced 9-megapixelCDs have 200 pixels/inch on a 22-

nch diagonal screen.

The brightness of a monitor, or a

ts luminance, affects perceivedontrast or the number of discern-ble gray levels. Studies have shownhat diagnostic accuracy increasess monitor luminance increases. Toain a perspective on luminancealues, a typical lightbox or alterna-or used to display film is on therder of 400 to 600 FL, whereas atandard PC color monitor isoughly 20 to 40 FL. An LCD coloronitor has monitor luminance of

5 to 75 FL, whereas the low-rightness, medical-grade CRTonitors have monitor luminance

f 50 to 60 FL, and the high-bright-ess CRTs have monitor lumi-ance of 100 FL or greater. Amonghe device specifications reflectingonitor brightness and affecting

ontrast resolution are the monitornd display card bit depth (typicallybits for 256 potential gray values)

nd the monitor dynamic range orontrast ratio, reflecting the maxi-um discernable luminance over

he minimum, with typical valuesf 600:1 or greater.

How many individual monitorsoes a user need per display work-tation: one, two, four, eight? Manyhink that for primary diagnosis,ual-headed configurations areost efficient for the comparison of

urrent and prior relevant studies,articularly for projection radio-raphs. Note that a good GUI de-ign can reduce the need for multi-le monitors. The ability to pagehrough and move images aroundhe screen, the ability to instanta-eously switch between tile andtack or cine modes of display, andhe ability to view multiple studiesn one monitor as well as performhe side-by-side comparison oftudies are critical to reducing themount of hardware and physicalisplay space required. In mostases, a 2-monitor setup is suffi-ient for primary diagnosis and im-

ge-intensive use, with perhaps a

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hird (color) monitor for work listreation and access to other rele-ant medical data. The most com-on configuration for enterprise

sers is the single-headed or-monitor display.In comparing CRT and LCD

echnologies, the advantages ofCD over CRT monitors includeetter stability for longer device

ifetime. The change in brightnessf standard LCD monitors haseen measured at less than 0.5%er month [1]. Liquid crystal dis-lays are not prone to the geometricistortion typical of CRTs, theyend to consume less power, andhey have reduced sensitivity andeflection artifacts from ambientoom lighting. The disadvantagesf LCD monitors compared withRTs include the aforementionedff-axis-viewing or angle-of-regardistortion of LCDs, backlight in-tabilities, liquid crystal fluctua-ions with temperature, and manu-acturing defects creating dead oronresponsive pixel areas.Receiver operating characteristic

tudies are currently the best meth-dology available to compare mon-tor quality and associate it witheader performance, that is, diag-ostic accuracy, sensitivity, andpecificity. Numerous clinical stud-es have been done, most showingo significant difference betweeniagnostic performance on CRTsnd LCDs. Recent studies repre-entative of CRT versus LCD com-arisons for radiologic diagnosis in-lude one that examined brainomputed tomography scans fordentifying early infarction [3] andnother that looked at computedadiographs of the chest for thevaluation of interstitial lung dis-ase [4]. The computed tomogra-hy study showed no statisticallyignificant differences in diagnosticerformance between a 21-inch,

onochrome CRT monitor with a b

ixel matrix of 1280 � 1600 and arightness of 175 FL and an 18-nch, color LCD monitor with aixel matrix of 1024 � 1280 and a

uminance of 55 FL, when 10 radi-logists were asked to rate the pres-nce or absence of disease on a-point scale. Similarly, a receiverperating characteristic study com-aring the efficacy of a 5-megapixelRT display and a 3-megapixelCD for the evaluation of intersti-ial lung disease in digital chest ra-iography showed no statisticallyignificant change in observer per-ormance sensitivity between thewo types of monitors.

Several studies have investi-ated the comparison of color ver-us monochrome (technicallychromatic) or grayscale moni-ors, and there does not seem toe a clear consensus. This is anmportant issue because color

onitors tend to have decreaseduminance, contrast, and spatialesolution capabilities comparedith monochrome monitors, and

he human visual system has de-reased spatial resolution percep-ion in the color channels, butreater dynamic range (500 just-oticeable differences vs. 60 to 90

ust-noticeable differences inrayscale). On the other hand,igh-performance monochromeonitors are expensive and have a

elatively short lifetime of approxi-ately 3 years, and color is becom-

ng increasingly useful in diagnosticmaging with the emergence of 3Display renderings. Although atudy comparing monochromaticnd color CRT monitors found notatistically significant differencesn the display of computed radiog-aphy chest images for the detec-ion of subtle pulmonary disease, itid find higher sensitivity rates forpecialty chest radiologists on theonochromatic monitor, perhaps

ecause of the lower maximum lu- m

inance levels of the color displays5]. Another study comparing pul-onary nodule detection on P45

nd P104 monochrome and color600 � 1200 pixel monitors foundignificantly greater false-positivend false-negative responses withhe color monitors, as well as longerearch times [6]. So for primary di-gnosis of projection radiographs inarticular, monochrome monitorsay still be preferred. It also seems

hat users prefer color LCDs toolor CRTs. This may be related tohe Gaussian spot pixel and emis-ive structure of CRTs and the usef black matrix (shadow mask orperture grille), which separates theed-green-blue phosphor dots thatorm an arrangement of color dotsr stripes for luminance and chro-atic contrast [1]. Grille misalign-ent can degrade color purity and

ontrast.Early adopters of picture ar-

hiving and communication sys-ems equipped their radiologyeading rooms with the highestuality display monitors, someK and others 1K but all highrightness. The software applica-ions were more complex thanhose targeted for nonradiologistnterprise users. It was commono provide an intermediate appli-ation for use by image-intensivepecialists, such as orthopedists,eurosurgeons, and radiation on-ologists, as well as in image-in-ensive areas such as intensive carenits and emergency depart-ents. And finally, the lesser

uality monitors with stripped-own software capabilities weresed by enterprise image users. It

s interesting to note that as dis-lay hardware and software con-inue to evolve, display applica-ion software continues to meldnto one flexible, easily configu-able GUI, and one monitor type

ay in time meet most needs.

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Monitor calibration and qualityssurance practices are important toaintaining high-performing medi-

al displays. The Digital Imaging andommunications in Medicine 14ray-Scale Standard Display Func-

ion and the American Association ofhysicists in Medicine Task Group8 recommend that monitors be cal-brated to a perceptually linearizedisplay function, because this isatched to the perceptual capabili-

ies of the human visual system.onitors forced to follow these stan-

ards produce more uniform images,ith optimum contrast. Cathode ray

ubes are less stable than LCD mon-tors, requiring luminance calibra-ion and matching to be doneonthly, physically measuring light

evels with a photometer. ManyCDs have embedded luminanceeters for automated quality assur-

nce measurements, although sometudies have also recommended do-

ng external luminance measures,

ut less frequently. Liquid crystalisplays must still be manually in-pected for nonresponsive pixels.outine manual viewing of test pat-

erns such as the Society of Motionicture and Television Engineersest pattern are usually sufficient forvaluating overall monitor perfor-ance, low contrast, and fine detail

etection.A collaborative guideline on dig-

tal image quality, specifically forigital mammography, is being de-eloped under the auspices of theCR with participants from themerican Association of Physicists

n Medicine, the Radiological Soci-ty of North America, and the So-iety for Computer Applications inadiology. It will include recom-endations regarding display de-

ices and is due for release in 2006.

EFERENCES

. Badano A. Principles of cathode-ray tube and

liquid crystal display devices. In: Samei E,

Flynn MJ, editors. Advances in digitalradiography: categorical course in diagnosticradiology physics 2003 syllabus. Oak Brook(IL): Radiological Society of North America;2003. p. 91-102.

. Leachtenauer JC. Electronic image display:equipment selection and operation. Belling-ham (WA): SPIE Press; 2004.

. Partan G, Mayrhofer R, Urban M, et al. Di-agnostic performance of liquid crystal andcathode-ray-tube monitors in brain com-puted tomography. Eur Radiol 2003;13:2397-401.

. Langer S, Bartholmai B, Andriole K, et al.SCAR R&D Symposium 2003: comparingthe efficacy of 5-MP CRT versus 3-MP LCDin the evaluation of interstitial lung disease. JDigit Imaging 2004;17:149-57.

. Iwano S, Ishigaki T, Shimamoto K, et al.Detection of subtle pulmonary disease on CRchest images: monochromatic CRT monitorvs color CRT monitor. Eur Radiol 2001;11:59-64.

. Krupinski E, Roehrig H. Pulmonary noduledetection and visual search: P45 and P104monochrome versus color monitor displays.Acad Radiol 2002;9:638-45.

atherine P. Andriole, PhD, Department of Radiology, Center for Evidence-Based Imaging, Brigham and Women’s Hospital,arvard Medical School, 1620 Tremont Street, BC-3-010B, Boston, MA 02120; e-mail: kandriole@partners.org.