The assessment and characterization of the built‐in internal photometer of primary diagnostic monitors

Abstract The purpose of this work was to perform the initial evaluation of primary diagnostic monitor (PDM) characteristics following the implementation of New York City quality assurance (NYC QA) regulations on January 1, 2016, and compare the results of the QA measurements performed by an external photometer and the PDM manufacturer's built‐in photometer. TG‐18 and Society of Motion Picture and Television Engineers test patterns were used to evaluate monitor performance. Overall, 79 PDMs were included in the analysis. The verification of grayscale standard display function (GSDF) calibration, using a built‐in photometer, showed that only 2 out of 79 PDMs failed calibration. However, the same measurements performed by the external luminance meter showed that 15 out of 79 monitors had failed GSDF calibration. Measurements of the PDMs maximum luminance (Lmax), using an external photometer showed that 10 out of 53 PDMs calibrated for Lmax = 400 cd/m2 and 17 out of 26 PDMs calibrated for Lmax = 500 cd/m2 do not meet the manufacturer's recommended 10% tolerance limit for the target Lmax calibration. Two PDMs did not pass the Lmax ≥ 350 cd/m2 NYC QA regulations with Lmax = 331 cd/m2 and Lmax = 340 cd/m2. All tested PDMs exceeded the minimum luminance ratio (LR) of 250:1 as required by NYC QA regulations. Measurements taken of Lmax and LR performed by a built‐in photometer showed that none of the PDMs had failed the NYC QA regulations. All PDMs passed the luminance uniformity test with a maximum nonuniformity of 17% (according to NYC regulations it must be less than 30%). The luminance uniformity test could only be performed using an external photometer. The evaluation of 79 PDMs of various ages and models demonstrated up to 18% disagreement between luminance measurements performed by the manufacturer's built‐in photometer when compared with those performed by an externally calibrated luminance meter. These disagreements were larger for older PDMs.


| INTRODUCTION
The conformance of Primary Diagnostic Monitors (PDMs) used to make a final interpretation from images generated by radiological devices to the Digital Imaging and Communication in Medicine Grayscale Standard Display Function is increasingly required by several state and city regulators. For example, effective from 1 January 2016, all radiological equipment registrants in New York City (NYC) are required to extend their Quality Assurance (QA) program to their PDMs. This work describes our experience in implementing the PDM performance and conformance with the NYC QA program 1 and characterization of the luminance measurement performance of the internal, built-in photometer of Barco monitors against an externally calibrated luminance meter.

| ME TH ODS
During the on-site annual testing of PDMs, several luminance characteristics were evaluated, including GSDF conformance, maximum luminance (L max ), luminance ratio (LR), and luminance uniformity.
Measurements were performed on 79 PDMs: color display system models MDCC-6130 (41), MDCC-6230 (9), MDCC-6330 (1) and Grayscale display system models MDCG 3120-CB (2), MDCG-10130 (2), MFGD 3420 (6) and MFGD 5421 (18) (Barco; Duluth, Georgia) as part of annual QA testing following the NYC regulations. 1 Monitors were located at various hospital sites, radiology physicians' offices, and radiology reading rooms. All measured PDMs were liquid crystal displays equipped with the manufacturer's built-in photometers (Barco, I-Guard) and connected to Barco MediCal QAWeb service for manual and automatic quality control measurements. PDM combinations included 1, 2, and 4 monitors, depending on the location. TG-18 and Society of Motion Picture and Television Engineers test patterns were used to evaluate monitor performance. An externally calibrated photometer (RaySafe Solo Light; Billdal, Sweden) was used to measure the luminance values.

2.A | Measurement and verification of Grayscale standard display function
For each PDM, the luminance response is expressed as the change in luminance between gray levels (dL/L) for the just noticeable difference plotted against the digital driving level (DDL). 2

2.B | Maximum luminance and luminance ratio
The perceived contrast characteristics of an image depend on the ratio of maximum luminance L max (all white) to minimum luminance L min (all black). 3 Luminance values were measured with an external photometer using TG18-LN01 and TG18-LN18 test patterns. Following NYC regulations 1 and the ACR technical standard, 5 the maximum luminance L max of nonmammography PDMs should be at least 350 cd/m 2 . The LR was calculated using the following equation: where LR is the luminance ratio, L max is the maximum luminance, and L min is the minimum luminance.

2.C | Evaluation of luminance uniformity
Using an external photometer, luminance values were measured at five locations on the PDM: the center and at each quadrant using the TG18-UNL80 test pattern. The luminance uniformity (LU) of a display was calculated using the following equation: where L 2 is the maximum and L 1 is the minimum measured luminance of the quadrants and the center.

2.D | Ambient light measurements
A RaySafe Solo Light photometer used in the study was equipped with the clip-on cover that eliminates the effect of ambient light on luminance measurements. All measurements of the luminance test patterns are performed with direct contact of the photometer with the measured PDM. The ambient luminance values were measured as a reflected ambient light from the screen of a turned off monitor (one foot away from the center of the monitor). These measurements were used to adjust the monitor GSDF calibration using the formula: where L meas is a measured luminance and L amb is a measured ambient luminance.
In addition to luminance measurements, ambient light was measured to confirm that the monitors did not receive outside light flow and they provide reading conditions mentioned in the ACR manual. 5 Illuminance was measured using the same RaySafe light pho-     Fig. 2) and L max = 500 cd/m 2 (Fig. 3). Measurement results showed that 10 out of 53 PDMs with a target calibration of L max = 400 cd/  3.C | Evaluation of luminance uniformity Figure 5 represents the results of luminance uniformity measurements performed by an external photometer.

3.D | Ambient light and illuminance measurements
The  Table 1

| CONCLUSION
We have investigated internal and external photometer performance of PDMs and have found that there are many more monitors that fail external photometer luminance measurements compared with those measured with an internal photometer when tested according to NYC PDM guidelines.
The discrepancy between the luminance measurement by external and built-in photometers should be followed up with the hospital IT department and manufacturer shortly after PDM testing.