Practical Considerations with the Clinical Implementation of TG-18 Guidelines

Quality control of soft-copy displays is critical to ensure the proper contrast rendition of medical images. The American Association of Physicists in Medicine's (AAPM) Task Group 18 (TG-18) has developed a set of testing parameters for the acceptance testing and quality control of medical grade displays. This paper addresses practical challenges associated with the broad implementation of TG-18 in a clinical setting. First, a computer model was developed to determine the effects of ambient light variations on the contrast response of a DICOM GSDF calibrated display. The model was based on an LCD displays with diffuse reflection coefficients of 0.0017 sr-1 , 0.0060 sr-1, 0.0080 sr-1, and 0.0200 sr-1. Second, the influence on display assessment due to inter-device variability and measurement techniques was established. Finally, the utility of a commercially available quality control program for remote monitoring of soft-copy displays was examined by confirming the accuracy and precision of the program. In terms of ambient light effects, the results suggest that the maximum allowable increase in ambient lighting can be determined for primary and secondary class displays by the following equations.

E_max^Primary ≤(-521.62R_d^2+18.822R_d+0.2511) E_cal+(0.2169R_d^(-1.002) )+E_cal

E_max^Primary ≤(-423.03R_d^2+22.306R_d+0.5126) E_cal+(2.1328R_d^(-0.753) )+E_cal

Restricting ambient light increases to less than the ΔE_max value will ensure that GSDF calibration is maintained. Assessment of the displays can be performed with either telescopic or contact luminance meters provided the device behaves linearly and the diffuse reflected luminance (L_{amb) is added to the contact measurements to generate L', the luminance perceived by the human eye. Finally, some tests recommended by TG-18 can be implemented by the use of an automated QC system to perform many of the routine measurements. A soft-copy display quality control program can be implemented effectively and efficiently. When performing the TG-18 recommended tests, any calibrated luminance meter can be used provided it captures L'. A commercial program can be used to facilitate these measurements. However, the contact luminance meters used by such systems should be characterized and calibrated against a stand-alone calibrated luminance meter with the required compensation for ambient lighting and reflections.}