The fundamental parameters of radiographic image quality on film are contrast, unsharpness or definition and graininess. In RTR, graininess is replaced by the noise. The density difference between the image of the object and the background on the radiograph is called the image contrast or response (R) or modulation. The RTR system can be studied completely by knowing the Modulation Transfer function (MTF) of the whole system. The MTF curve is a special form of contrast/detail-size diagram in which the image contrast is plotted against the spatial frequency of a test object measured in line-pairs per millimetre (lp/mm). MTF curves are widely used to measure the characteristics of optical equipment, particularly for assessing the contribution of individual items in a complex image transfer system. The MTF characterises the performance of the image intensifier system. However, generation of such curves is a tedious task. This is practically impossible in the field conditions and it requires elaborate theoretical knowledge that a radiographic technician would not normally have. Codes of practice indicate that the image intensifier systems should be checked periodically to assess its performance through the use of MTF curves and step wedges for contrast ratio. Authors, instead, suggest the use of performance curves which are simple to obtain and can be easily interpreted by radiographers.

This paper describes the procedure for obtaining such curves and also the results obtained for the authors' system. The performance curves have been generated by the authors for their real time systems comprising of a trifield 9" image intensifier tube with a central resolution better than 44 lp/cm, a plumbic on and high resolution TV monitor. A step wedge with a maximum thickness of 58 mm and minimum thickness of 8 mm with steps in increments of 4 mm was used in narrow and broad beam geometry conditions. Both wire and plaque type penetrameters were used to assess the achievable image sensitivities as a function of focal spot and energy. A plot of the best sensitivity achieved based on wire contrast ratio as a function of thickness indicated that the shape of the curves depends on geometric factors such as source size and source to object distance, energy of the radiation and the thickness of the object. In general, it has been experimentally observed that with real time image intensifier systems, the best sensitivity of the order of 2% and less occurs in the thickness range of 20 - 60 mm steel equivalent.