Physical properties of a photostimulable phosphor system for intra-oral radiography

To determine physical properties of the Digora digital intra-oral radiographic system (Soredex Orion Corporation, Helsinki, Finland) for different calibration settings and beam energies.

The line spread function (LSF) and the modulation transfer function (MTF) were determined from radiographs of a slit. Noise power spectra (NPS) were determined from radiographs exposed to homogeneous radiation fields at 10, 50 and 100% of the calibration exposure for three tube potentials. All calculations were performed using relative values of exposure comprised of gray level, the signal at the photomultiplier tube and the amplified signal in order to confirm agreement between these different approaches. Noise equivalent quanta (NEQ) were calculated from the one-dimensional NPSs and the MTF. Detective quantum efficiencies (DQE) were determined from the NEQs and representative values of the photon fluence. Signal-to-noise ratios (SNR) were calculated for different signal contrasts applying the NEQs.

The MTF of the system exhibited typical characteristics and falls to a value close to zero at the Nyquist frequency of about 7 cycles/mm. Noise as expressed by the NPS was found to be relatively low, i.e. about 10(-5) to 10(-6) mm2 depending on exposure and frequency. There was no significant difference between data obtained at different beam energies. The NEQ and hence the DQE were relatively high. DQE decreased with increased exposure. For exposures in the clinical range of the DQE reached a peak value of about 25%. SNRs are favorable.

The physical properties of the Digora intra-oral system indicate that it is suitable for digital intra-oral radiography.