Digital intraoral radiography : Determination of technical properties and application evaluations
Abstract: Digital registration of intraoral radiographs was introduced about 1990. One of the first systems on the market was the Sens-A-Ray (Regain Medical Systems AB, Sundsvall, Sweden). At the time of the introduction, it was deemed important to determine basic technical properties for this system. Methods to determine such properties for digital systems did not exist and were developed as the scientific investigations progressed. The technical properties first determined for the Sens-A-Ray were the Dose response function, the resolution defined as the Line Spread Function (LSF) and as the Modulation Transfer Function (MTF), the Dark current and the Signal-to-noise ratio (SNR). In a further study the noise characteristics were calculated as the Noise Power Spectrum (NPS) and as the Noise Equivalent Quanta (NEQ) From this last parameter the efficiency of the system to detect incident photons was calculated, the Detective Quantum Efficiency (DQE). Employing the methods developed for determining the LSF and MTF for the Sens-ARay system, these parameters were also determined for three other systems, the RVG (Throphy Radiologie, Paris, France), the VIXA/Visualix (Gendex, Des Plaines, Ill., USA) and the Flash Denta (Villa Sistemi Medicale srd, Buccinasco, Italy). At the introduction of digital intraoral systems, the display function was linear on computer monitors, which means that grey level values were directly converted to light intensity levels. Dentists used to viewing conventional film radiographs found the digital images not "film-like", which is due to the contrast properties of digital images. In order to investigate the subjective preference of the display of digital images a study was performed, which showed that a curved display function was preferred. An analysis of the function preferred by the viewers showed that this function de facto made the digital images more "film-like". In this connection, an important fact turned out to be that the curved display function approximately compensates for the exponential attenuation of x-rays, as radiographs are exposed. The consequence of this compensation is, that equal steps in object thickness are shown with approximately equal steps in light intensity on a computer monitor. When new image registration systems are introduced, it is important also to test applications in addition to technical parameters. In this dissertation, the Sens-A-Ray system was employed in a longitudinal study of bone inflammation in rats. The inflammation was introduced by intravenous injection of Staphylococcus aureus Periodical identical radiographs were exposed. The results show several advantages of digital radiographic techniques in animal studies. Implant therapy has become a standard method to replace lost teeth. A literature search could find no study comparing digital and conventional techniques in assessing implants, and thus such a study was deemed important. The study shows that the diagnostic yield using digital techniques was fully comparable to conventional film techniques, and showed a tendency to better results. The determination of technical parameters as well as the evaluations of applications show that digital techniques have such properties that conventional film techniques can suitably be replaced.
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