What monitor to use for viewing medical images

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By Ken Compton and Herman Oosterwijk

The question of what monitor to use for diagnostic purposes with digital medical images has been a recurring theme ever since these images were first generated. Variations on this theme are: Can I use a commercial monitor for diagnosing medical images? Is a 3-megapixel (MP) display sufficient to look at chest radiographs? Related questions that we are often asked are: How should I calibrate these monitors? What is the impact of environmental light on the diagnostic capability of displays in the ER or the ICU?

This white paper provides practical guidelines for the selection of the right monitor for the right diagnostic application. We also will describe how to keep your monitor operating at diagnostic quality. In addition, we provide an introduction to the characteristics of displays and the key parameters that describe monitor performance.

TERMINOLOGY NEEDED TO UNDERSTAND MONITOR PERFORMANCE

Defining image quality involves a number of metrics. These range from terminology created before the invention of the incandescent light to the modern definitions found in DICOM Part 14, Grayscale Standard Display Function (GSDF). Some of the terms still in use were developed for analog TV. Their use in application to Liquid Crystal Displays (LCD’s) can lead to some errors in understanding. As the Cathode Ray Tube (CRT) has—for the most part—departed as a medical imaging display platform, the following explanations are focused on LCD technology.

Luminance is the correct term for photon energy that reaches the eye; it is not Brightness, as many assume. The unit of measure for luminous intensity is the candela (cd). One candela, which means candle in Latin, is roughly the luminous intensity of one common candle. The  preferred system of measure is expressed as candelas per square meter (cd/m2).

As with most standards of measures, there is another luminance unit. The foot-Lambert (fL) is the US/English unit of luminance. A fL can be converted (rounded) from candelas with the following formula: 1 fL = 3.7 cd/m2. The fL used to be used by the motion picture industry to express the luminance of images on a projection screen; it is still in use in the flight-simulation industry as a measurement of its visual display systems.

The maximum luminance capacity of an LCD display is determined by the backlight system. This can be either a Cold-Cathode Fluorescent (CCF) or Light Emitting Diode (LED) system.

A key difference between medical and commercial displays is that medical displays have a closed-loop control circuit to maintain a stable peak luminance from a cold start to thermal stability (when fully warmed up).

CCF technology could take as much as 2 hours to stabilize from a cold start. This correction requires a precision photometer mounted near the input source on the back of the display to monitor the luminance output. The actual output is compared to the desired peak value and a corrective signal is generated for the control circuit. This is all done automatically in the background, many times per second.

Commercial displays have a control circuit with a variable input selection, unfortunately called a brightness control by some vendors. It may have five to seven steps or provide a smooth variability over a predetermined range. There is no reference to an absolute luminance value associated with the setting.

Note: CCF lamps cannot be dimmed down like an incandescent lamp; they have a minimum threshold to function. The absence of closed-loop control and a photometer is the cause of drift over time (cold to hot on start up) and the absence of any correction as the CCF lamps (or LED) age from use.

The lack of quality control capabilities with most commercial-grade monitors will require frequent calibration checks to maintain a close approximation to the desired peak luminance.

Contrast on a display is the range of luminance available from an “off” pixel to a “100% on” pixel; the off pixel does not mean a total absence of luminance. Luminance Ratio, LR, (the old terminology is Contrast Ratio, CR) is calculated by dividing the minimum luminance into the maximum as measured with a full screen driven at the respective values. For example: An Lmin of 1 cd/m2 and Lmax of 450 cd/m2 would yield an LR of 450:1.

However, this does not include two factors that influence the resultant image quality. Adjacent pixels at min and max luminance values do not produce a 100% contrast difference (modulation). In addition, luminance from the max pixel influences the min pixel as perceived by the eye, and this simple calculation does not include ambient light contributions.

A medical-grade 3-MP LCD monochrome monitor should be able to produce a contrast modulation of approximately 93% between an “off” vs. “on” pixel.

As a reference, the venerable CRT just barely produced 60% contrast modulation at screen center. This measurement also fell precipitously at the corners of the display. Color CRT’s had even worse contrast modulation, although color LCD’s can perform at the same level as monochrome LCD’s.

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