Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Standard viewboxMammography viewboxGrayscale monitor (CRT or flat panel)Color monitor (CRT or flat panel)Standard personal computer color monitor1,500-3,5003,000-7,500 (ACR criterion: :2:3,000)120-800 (ACR standard for monitors used forinterpretation: :2:160)80-30065-130Note: ACR, American College of Radiology; CRT,cathode ray tube."Information on viewbox luminance from Mammography Quality Control Manual. Reston, VA: AmericanCollege of Radiology, 1999:286-294. Note: 1 cd/m 2 = 0.292 foot-Iamberts.displays, provide much higher maximal luminance than do color monitors. CRTmonitors of large matrix formats provide lower maximal luminance than do CRTmonitors of smaller matrix formats, but the maximal luminance of a flat panel monitoris not significantly affected by the matrix format. The luminance of a CRTmonitor is highest at the center and diminishes toward the periphery. The minimalluminance of a monitor is called the black level, and CRT monitors have lower measuredblack levels than flat panel monitors do. In practice, the minimal luminanceis limited by veiling glare, the scattering of light inside the monitor. The amount ofveiling glare is determined by the size and brightness of other areas of the displayedimage and the distances between bright and dark areas. Flat panel monitors suffermuch less from veiling glare than do CRT monitors. The dynamic range of a monitoris much less than that of radiographic film on a viewbox, and windowing andleveling are needed to compensate.Manufacturers often describe spatial resolution in terms of a monitor'saddressable pixel format (e.g., 1,280 X 1,024 pixels), but this is only a crude indicationof spatial resolution. In fact, two monitors with the same pixel format mayhave quite different spatial resolutions. The spatial resolution of a CRT videomonitor, in the direction perpendicular to the scan lines, is determined mainly bythe number of scan lines and the width, in the perpendicular direction, of theelectron beam striking the phosphor. The profile of the electron beam and thusthe point-spread function (PSF) of the light emitted by the phosphor are approximatelyGaussian in shape. The full width at half-maximum of the PSF must becomparable to the width of a scan line to reduce the visibility of the scan lines.This causes a significant fraction of the energy in the beam to be deposited in thetwo adjacent scan lines, degrading the spatial resolution. Simply put, a CRTmonitor cannot produce a completely black scan line immediately adjacent to abright one.The spatial resolution in the direction parallel to the scan lines is determinedmainly by the bandwidth of the video circuitry and the width of the electron beamin that direction. The bandwidth determines how quickly the electron beam intensitycan be changed in relation to the time required to scan each line. Spatial resolutionoften becomes worse toward the periphery of the CRT. A flat panel monitorusing active-matrix LCD technology provides spatial resolution superior to that ofa CRT monitor with the same addressable pixel format, because a bright pixel hasless effect on its neighbors.