Performance of primary diagnostic monitors (PDMs) over time

Abstract In this work, we evaluated the change of primary monitor characteristics in two consecutive years. Sixty‐six primary monitors were included in the analysis. The monitors were located at radiology physicians' offices and radiology reading rooms. All primary monitors were equipped with the manufacturer's built‐in photometers and connected to the BarcoMediCalQA web service for manual and automatic quality control measurements. External photometer/illuminance meter (RaySafe Solo Light) was used to measure the luminance values. Measured luminance values of the TG18LN1‐18 and TG18UNL80 test patterns were used to evaluate the primary monitors performance. In a comparison of the quality assurance (QA) measurement results for the same monitors that were performed within 2 years, the luminance of 25 displays remained statistically the same (P > 0.01). The luminance of 17 displays decreased (P < 0.01) in 2017 when compared with 2016, the luminance of 24 displays increased (P < 0.01) in 2017 when compared with 2016. For the annual measurements of the MLD in 2016 and 2017, 25 out of 66 displays showed a decrease of MLD values in 2017 compared with the same measurements in 2016 and 41 displays showed an increase of MLD in 2017. All tested primary displays had the MLD value less than 17.2%. The mean value of illuminance measured in 2016 was 5.8 lux ± 3.1 lux. In 2017, the mean value of illuminance measured was 8.7 lux ± 5.3 lux. Although it is expected that monitors luminance values will decrease over time, we found displays with increased luminance. This is possibly due to the multiple monitor calibrations that were performed between two annual monitor QA tests. Based on the findings of this work, more efficient display QA programs with a shorter time interval than 1 year are needed.

lux. Although it is expected that monitors luminance values will decrease over time, we found displays with increased luminance. This is possibly due to the multiple monitor calibrations that were performed between two annual monitor QA tests.
Based on the findings of this work, more efficient display QA programs with a shorter time interval than 1 year are needed. Quality assurance programs for displays used for the primary interpretation of medical images are required by some state and city regulators. 5 Quality control (QC) measurements typically include an evaluation of display luminance characteristics such as the Grayscale Standard Display Function (GSDF), the luminance uniformity, and ambient light conditions of the display environment.
The GSDF curve is a luminance response curve of the display, which indicates that the display provides perceptual linearization for the human eyes. 4 The characterization of the GSDF curve was performed by luminance measurements in steps of 18 gray levels both at acceptance and at annual testing of the display. 5,6 The luminance uniformity is assessed to ensure that the luminance in the corners and at the center of the display agree within a designated percentage. The luminance measurement is usually performed with an external photometer, which measures the intensity of visible light emitted from a unit area of the display's surface. The results of the luminance measurements are part of the QC of the medical display. The illuminance on a display is the total luminous flux from all other light sources in the room and is evident on the viewing surface of the display. Illuminance is measured by an external photometer that that has been converted to an illuminance meter.
When assessing a display's performance parameters, it is important to note that its luminance performance can change over time.
This change may decrease a display's overall performance, depending on its age and hours of usage. [6][7][8][9] The primary interest for this work is to determine how much the luminance level changes after 1 year of display usage. Billdal, Sweden) was used to perform manual luminance measurements using the TG-18 test patterns. All displays were warmed up for at least 30 min before the luminance measurements were taken.

| ME TH ODS
The display output was left to stabilize after each switch of patterns, and then a stable reading on the luminance meter was obtained. To verify stability, several repeated recordings were made for each display to ensure that there was no drift in the photometer readings. The display performance data of the 66 primary displays was collected and analyzed for 2 years (2016 and 2017). The effect of changes in temperature and ambient light conditions on display luminance was assumed to be negligible because the locations of the primary displays in the diagnostic reading rooms remained the same over the year and ambient light conditions were controlled in those rooms.

2.A | Luminance measurements
Characterization of the GSDF curve was performed in steps of 18 gray levels, resulting in a total of 18 luminance measurements per year for each display. TG18LN1 through TG18LN18 test patterns were used to perform luminance measurements of the display. 2

2.B | Display Uniformity
The Maximum Luminance Deviation (MLD) is described as the quantitative measure of display uniformity in the TG-18 report. 2 Luminance values were measured at five locations on the diagnostic display: the center and at each of the four corners using the TG18UNL80 test pattern. The MLD of a display was calculated using the following equation: where L 2 is the highest recorded luminance, and L 1 is the lowest recorded luminance, regardless of the location on the display. 2

2.C | Illuminance measurements
In addition to diagnostic display luminance measurements, ambient illuminance was measured on each workstation display to confirm that the display's ambient illuminance did not exceed 50 Lux, as is recommended in the ACR-AAPM-SIIM standard. 3 The illuminance was measured using the same RaySafe Solo 3 | RESULTS

3.A | Luminance measurements
All diagnostic displays analyzed in this study were sorted by the inservice date and are presented in Table 1. All the displays were less than 8 years old at the time of the study.
Luminance measurements for each of the display paired t-tests of 2016 and 2017 are presented in Table 2

3.C | Illuminance measurements
The mean value of illuminance measured in 2016 was 5.

| DISCUSSION
In recent years, there has been an increase in the number of primary displays deployed in our radiology clinics. The quality and accuracy of a radiologist's reading depends on the performance of the display that he/she uses to review images and to perform diagnosis. Problems in display luminance, display uniformity, or ambient light could negatively affect image interpretation. To ensure that primary displays are performing according to their specifications, display performance parameters must be checked regularly as has been described in various publications. [7][8][9][10][11] One of the objectives of a well-established QC program for the primary displays is to follow and quantify any change of performance parameters over time, thereby detecting the problems that arise with the displays before they lead to clinical issues.
In this study, the changes in primary display performance parameters, such as luminance, were inconsistent across displays: they either stayed the same, decreased, or increased after 1 year of usage. Measurements of illuminance or luminance and their corre- In addition, all measurements were performed after at least a 30-min warm-up of the displays based on the results of our previous work, which was performed to estimate the stability of luminance measurements for all tested primary display models. 9 The results from that study confirmed that it is important to perform any luminance measurements after the display has been turned on for at least 30 min. 9 Another source of inaccuracy in QC measurements of primary displays is that all displays in the program undergo automated calibration biannually. Using an internal photometer reading, display software calibrates the luminance level according to company speci-