Systematic analysis of a radiological diagnostic system : A method for application in the effective use of x-rays in intraoral radiology

Sammanfattning: The effective use of an imaging system in diagnostic radiology implies an optimisation process. This demands knowledge and relevant descriptions of the various components, the X -ray units, the objects and receptors. Thus, in this work photon energy spectra have been measured, phantomsresembling human tissue constructed and receptor characteristics investigated. Essential image quality parameters (contrast, signal-to-noise ratio) and a radiation risk indicator for the patient (energyimparted) have been defined and their variation with photon energy spectrum determined for different objects and details. Results are presented as basic infonnation to be used by radiologists in collaboration with physicists in optimising examinations according to the patient and the particular diagnostic task. For intraoral radiology with Ultra-speed fihn, it is shown that to achieve equalised radiographic contrasts of an ivory wedge, kV-settings have to be decreased by 5-8 kV when single pulse X-ray generators are replaced by high-frequency constant potential ones. When imaging the ivory wedge in a PMMA phantom with Ektaspeed and Ultra-speed films at equal contrast, 7-9 kV lower kV-setting must be used with the Ektaspeed film. Ektaspeed film then gives a 35-40% decrease in the energy imparted tothe patient which can be compared to 45-55% decrease observed if the loss of contrast is not compensated for by lowering the kV-setting. Comparison of the contrasts of both films shows that Ultra-speed has higher contrast than Ektaspeed, but the latter has a wider dynamic range and higher values of base and fog optical densities, which contribute to its lower contrast at low optical densities. The radiographic contrast of details in the object is the product of object and film contrast. Objectcontrast depends on photon energy and is the same for both films. The energy imparted to the patient is calculated using conversion factors derived in this work. To simulate the large variety of anatomical structures encountered in intraoral radiology, a multimaterial compound hard tissue phantom was constructed. The constituent elements and their fractions by weight were carefully determined so as to allow computational methods to be used tocomplement experimental data. Different types of imaging system imply different optimal photon energy spectra. Strategies of optimising photon energy spectra with respect to image quality parameters and patient dose are described for both fihn and a digital system (Digora) using an imaging plate. In the digital system, the characteristics of the receptor affect image acquisition similarly to film but digitalized image formation and display may limit image quality (contrast resolution).