By Kenneth D. Compton:
Computed Radiography (CR) provides rapid image acquisition with excellent dynamic range to render subtle differences in luminance of adjacent pixels as defined by the phosphor screen detector. When defined in terms of line-pairs/mm (lp/mm), CR at 5 lp/mm holds approximately half the line-pairs/mm of film at 8 to 10 lp/mm. CR detectors generally fall on either side of 100 micron resolution (0.100 mm) as the native image capture.
The critical criteria when selecting a display is how it will be used and the ambient lighting. The American Association of Physicist in Medicine (AAPM, Task Group 18) produced a public document listed as OR_3 (1) that suggests minimum performance criteria for primary and secondary reading conditions. The focus of this article will be primary AAPM (2). considerations for radiologists working in controlled reading rooms and attending physicians using CR in the emergency room environment.
Primary (diagnostic) reading specifications inherently include ACR-NEMA DICOM Part 3.14 that defines the Grayscale Standard Display Function (GSDF) for both displays and printers (3). The GSDF is a defined response in absolute luminance that all medical grade displays must comply with to be FDA listed with a 510K for general radiography.
The ideal display format for CR is a three megapixel monochrome at 1536 x 2048 in portrait orientation in either a reading room or ER. An alternative for the reading room is a color three megapixel display to provide greater flexibility to present other modalities. A color display can deliver diagnostic image quality in low ambient reading room conditions, but will generally struggle to meet minimums in brightly lit ER’s and lack the long term capacity to hold minimum values.
The reading room is assumed to be an optimal environment for diagnostic utilization wherein fine details are searched for diagnostic reporting purposes. The ER physician tends to take a macro approach to diagnostic detail aiming instead for immediate decision-making information directed at life-saving. It is within the latter criteria that a two (2) megapixel monochrome or color format can be effectively utilized. Unlike older display technology, the Liquid Crystal Display (LCD) retains excellent contrast modulation regardless of the pixel format (assuming quality LC cores as part of a medical grade display), so that the ability to resolve pixels (details) on a three or two megapixel display should be functionally equal. The difference is in the efficacy of a first look.
A three megapixel display provides more detail closer to the native image as captured than a two megapixel display. However, the use of digital tools, pan-zoom-magnify, provides the ability to achieve a 1:1 relationship with the captured image and yield reading results with equal confidence and accuracy. The three megapixel format is ideally suited for a reading distance of 0.66M (ergonomic workstation) with a pixel pitch of 0.206mm. Close proximity in ER is not always possible and the larger pixel of a two megapixel format of 0.250mm provides additional viewing distance capability (based on viewing distance vs. pixel pitch to distinguish two distinct pixels separated by one).
- OR_3 website for download: http://deckard.mc.duke.edu/~samei/tg18
- Refer to OR_3, Page 121, Table 7 for the minimum performance criteria, see below.
(This applies to both monochrome and color under either the primary or secondary criteria) - American College of Radiology – National Electrical Manufacturers Association, jointly produced the Digital Imaging & Communications in Medicine (DICOM) GSDF calibration criteria for displays intended for medical image interpretation, Part 3.14.
RFI/RFQ/IFB Considerations for vendor compliance:
Luminance Minimum (defined by Digital Driving Level [DDL] zero) should be greater than or equal to 1.5 times and ideally 4 times the measured ambient surface reflectance. Example: if the ambient contribution is 1 cd/m2, then DDL zero should be set to produce no less than 1.5 cd/m2 or ideally 4 cd/m2.
Luminance Maximum (defined by DDL 255) is the peak luminance set point that the backlight stabilization is intended to maintain. Although AAPM indicates 170 cd/m2 is acceptable as long as a Luminance Ratio (LR’) of 250:1 is achieved, this requires a very low fractional black level. ACR-NEMA and noted physicists feel this is better served at 250 cd/m2 as a minimum peak. A low peak luminance also compresses the space available to proportion the Just Noticeable Differences (JND’s) within; this will mask low contrast information in a reading room and more so in a bright ER.
A proper reading room can be set to L’max of 400-450 cd/m2 for monochrome and 350-400 cd/m2 for color as the peak and provide an excellent Luminance Ratio. Not noted within AAPM is another limit that needs to be considered. If the LR’ (LR prime includes ambient) goes above 350:1, the low-level grayscale data is lost to the eye’s detection. This is equivalent to increasing the optical density of film so that a hot light is required to see detail within dark areas of interest.
a.) Monochrome LCD should be capable of an L’max setting between 350 and 550 cd/m2.
b.) Color LCD should be capable of an L’max setting between 300 and 400 cd/m2.
c.) Both monochrome and color LCD’s should be able to maintain the L’max setting within ± 10% for a guaranteed period of three years under continuous operation with or without energy conservation including standby and/or sleep modes that the display can recover from within 20 seconds to a stable luminance output.
d.) All displays must be DICOM Part 14 (3.14) compliant to the GSDF calibration as defined in said document. Accuracy of the GSDF cannot exceed ± 10% of target values per AAPM for Primary reading; medical grade displays with a factory calibration should achieve ± 5% tolerance given current technology.
e.) All displays must store operating history of said display to the GSDF and pertinent parameters including but not limited to: Lmax, Lmin, calibrated accuracy to the GSDF (Max Std. Deviation), run time clock, and date/time stamp of data capture. Said data is to be available and reported to a central archive area at specified intervals by the end user. Data format, albeit linked to the QC software of the vendor, must also be exportable to an industry standard spreadsheet.
f.) Pixel format for optimal reading performance is 2048 x 1536 pixels (3MP) in portrait orientation.
g.) If color and monochrome displays are to be paired (both medical grade of the same pixel format such as 3MP) together, they should both be set to the same L’max, Lmin, and LR’. Other than color deference, there should be no difference in readability.
h.) Vendor to supply appropriate light meter system (handheld photometer) for QC software; there shall be no limit on the number of workstations under any licensing agreement. Preference is for a vendor neutral light meter, but this is not mandatory.
Table inserted for reference to noted areas of “Reflection” and “Luminance Response” ……
AAPM OR_3 Public Document, Page 121:
By: Kenneth D. Compton, contributing editor Health Imaging Hub
