RadBook 2013

147RAD·BOOK 2013 DISPLAYS/PRINTERS Sponsored by NDS Surgical Imaging DoingitRight High-precision lookup tables (LUTs) are one of the defining elements of a diagnostic display. When Dome introduced the first diagnostic LCD monitors, they incor- porated special internal LUTs to allow the display system to be precisely cali- brated to DICOM without depending on operating system software. Over the years, most other vendors have done the same, yet we continue to see claims indicating a real misunderstanding of the underlying technology. High-precision LUTs are necessary to allow the system to specify particu- lar colors or gray shades with greater accuracy than can be achieved with commercial display systems. Most di- agnostic imaging software will display images with 256 or 1024 shades of gray. However, you want a larger num- ber of shades to select from so that each targeted gray shade is correctly positioned on the DICOM curve. The number of shades needed depends on www.ndssi.com bled this to account for any non-linear behavior and designed our grayscale displays with more than 3,000 shades of gray. This yields an average error of about 0.2 JNDs. Conclusion; well below what can be perceived by the human eye. So why are some vendors touting proprietary improvements moving them from 0.9 JND to 0.3 JND error, as if that was a great accomplishment? And others have systems with 14,000 shades of gray for 0.05 JND error, if the human eye can’t see the difference? Don’t let confusing marketing mes- sages shape your understanding of true diagnostic image quality value. Best of all, a human person is subjective to see- ing changes in gray shades, whereas the Dome computer based approach yields more accurate and consistent results. how precisely you need to specify the brightness of each gray shade. More precision improves image quality but at some point more precision doesn’t make a perceivable difference. In medical imaging, the DICOM Grayscale Display Function (GSDF) is used to define the target brightness for each gray shade. It is based on the mini- mal brightness difference a human can perceive, known as a Just Noticeable Difference (JND). At any given bright- ness, this is the smallest step a person can see under ideal circumstances. If we can set the gray shades with less than a half a JND error, than any errors in the brightness steps will be invisible to the human eye. The brightest displays used in med- ical imaging have less than 700 JND steps from black to white. If we de- signed a system with 1,400 available gray shades, we could select shades with less than half a JND error on average. For Dome displays, we dou-

RadBook 2013

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