Background: A higher Detector Air Kerma (DAK) lowers the noise level in a radiograph and delivers a crisp and sharp appearance, which is favoured by the radiologist. Lowering the DAK and consequently also the signal, delivers more visible noise that can mask a detail. A higher DAK should deliver a higher image quality. Therefore, a linear relation between DAK and image quality is expected. However, previous analyses of a set of clinically approved radiographs demonstrated a lack of such a relation. The aim of the current study is to investigate the contribution of post-processing on the Contrast-Noise Ratio (CNR) as an attempt to clarify the absence of relationship between image qual-ity and DAK in a set of clinically accepted images.
Methods and Materials: A TOR18FG phantom, enveloped with 5cm of PMMA, is exposed to a large range of dose levels. To investigate the impact of post-processing on the low and high contrast regions, the radiographs were analysed with and without post-processing. The effect of post-processing on the contrast is evaluated by calculating the CNR for a low and a high contrast element.
Results: The post-processing nullifies the relation between the DAK and the CNR for the low contrast element. It has a smaller influence on the high contrast element: It weakens the DAK-CNR relationship above 4µGy.
Conclusion: The hypothesis of CNR saturation, a stagnation of CNR in function of DAK, may explain the absence of a relation between DAK and image quality found in the previous study. These findings indicate that post-processing should be handled with care and raises the question how and if guidelines for the daily practice concerning DAK and image quality can be defined.