Analog and digital systems of imaging in roentgenodiagnostics

Abstract

In the recent years, we have been witnessing a very dynamic development of diagnostic methods of imaging. In contemporary radiology, the carrier of the diagnostic information is the image, obtained as a result of an X-ray beam transmitted through the patient's body, with modulation of intensity, and processing of data collected by the detector. Depending on the diagnostic method used, signals can be detected with analog (x-ray film) or digital systems (CR, DR and DDR). Each of these methods of image acquisition, due to its own technological solutions, determines a different quality of imaging (diagnostic data). The introduction of digital image receptors, instead of conventional SF systems, increased the patient dose, as a result of a gradually increasing exposure. This followed from the fact that in digital systems, the increased radiation dose reduces image noise and improves image quality, and that is owing to the data capacity of these systems (impossible in SF systems with a limited data capacity of the image detector). The availability of the multitude of imaging systems, each characterized by disparate qualitative and quantitative parameters, implies the problem of evaluation and enforcement of a proper efficiency from manufacturers of these systems.At the same time, there is a legal problem present in our country, i.e. the lack of laws and regulations regarding standards of the scope of quality control (parameters) and measurement methodology for the systems of digital image acquisition. In the European countries, the scope and standards of control are regulated by the manufacturers and European Guidelines, whereas in the United States, AAPM Reports have been introduced, that specifically describe methods of tests performance, their frequency, as well as target values and limits. This paper is a review of both, the scope of quality control parameters of image detectors in analog and digital systems of imaging, and the measurement methodology. The parameters determining the image quality are as follows: detection efficiency, dynamic range, spatial sampling, contrast resolution, spatial resolution, noise, and quantitative detection efficiency. Validation of the measurement methods, establishing standards of radiographic techniques for the performed examinations, and creating a uniform system of supervision, appears to be the only way to ensure an effective control of imaging systems and to eliminate an increasing exposure.

Keywords: CR Radiography; Digital Radiography; SF Radiography.

Figure 1.

Cross-sectional diagram of the X-ray film.

Figure 2.

Definite points on the characteristic curve.

Figure 3.

Principle of operation of the intensifying screen - impact on the sensitivity and resolution of the imaging system. (by Andrew P. Smith, Fundamental Digital Mammography, Physics, Technology and Practical Considerations.)

Figure 4.

Structure of an imaging plate.

Figure 5.

Scheme of image processing and recording on imaging plate.

Figure 6.

The process of image readout from an imaging plate – point-scan system.

Figure 7.

The process of reading the image from the imaging plate – line-scan system. Schematic diagram of a line-scan PSP system. (According to the AAPM Report No. 93).

Figure 8.

Thin-film transistor structure (according to MEDTECH).

Figure 9.

Construction of digital detectors – direct and indirect detection (according to MEDTECH and Performance Assessment of DR Systems, JA Seibert, Ph.D. UC Davis Medical Center Sacramento, CA, http://www.aapm.org/meetings/03AM/pdf/9787-48370.pdf).

Figure 10.

Direct digital system detector: (A) microphotography, (B) the physical structure of a single pixel of TFT matrix (C) a schematic diagram of the structure of two pixels (according to Mammographic detectors, G. PANAGIOTAKIS UNIV OF PATRAS).

Figure 11.

The sensor matrix construction in displays and the methods of readout control (according to Medtech).