Physical and psychophysical properties of digital intraoral radiography

Abstract

Objectives: This thesis aimed to the evaluation of different digital intraoral radiographic systems, partly by defining objective measures of their physical properties, namely their resolution, signal and noise characteristics, and partly by employing methods to measure their psychophysical properties including viewer performance, such as the Perceptibility Curve test. In a next step, a mathematical model was developed to predict Perceptibility Curves without employing viewers. An additional aim was to compare data on objective calculations of theoretically perceptible limit frequencies with viewer performance.

Methods: Specially designed computer programs were used in order to determine physical and psychophysical properties of several digital intraoral radiographic systems, namely the Sens-A-Ray® the Dixel®, the CDR®, the Digora®, and the Dixi®. The physical properties that define resolution, modulation transfer, noise, signal-to-noise ratio and efficiency of the system to detect x-ray photons were calculated. The psychophysical properties of some of the above named systems were determined by means of the Rose model and the Perceptibility Curve test. Threshold SNRs were determined according to the Rose model, and perceptible frequency limits were theoretically calculated and compared with experimentally obtained data employing viewers. Perceptibility curves were constructed for the Sens-A-Ray system using the conventional technique. A simplified method to construct Perceptibility Curves was tested. Additionally, a method to predict Perceptibility Curves without directly involving viewers was developed and tested and then compared with the conventional technique.

Results: The measurements of the physical properties of all digital systems returned results that indicated that they are suitable for intraoral radiography, especially when considering their sensitivity and the efficiency of the systems to detect x-ray photons. Experimentally obtained data on the performance of the systems with respect to their limit resolution were in agreement with the theoretically calculated perceptible frequency limits. A very good agreement was observed between theoretically constructed Perceptibility Curves and curves obtained using the conventional techniques.

Conclusions: The physical properties of digital intraoral radiographic systems tested in this study show that these systems have such features that they are considered suitable for clinical intraoral radiography. The agreement between experimentally obtained perceptible frequency limits employing viewers and theoretical calculations shows that the former may be applied as a practical test of the performance of digital radiographic systems with respect to their limit resolution where both signal contrast and frequency are taken into account. The agreement between the predicted Perceptibility Curves and the ones obtained with the conventional technique shows that it is possible to predict psychophysical properties of digital intraoral radiographic systems as defined by the Perceptibility Curve employing a limited number of system parameters together with predetermined data on the performance of an average observer.