Tube Current Settings Research Articles

Optimization of Cd1–xZnxTe Detectors for Digital Radiography

In this study, measurements of the electrical and detection parameters of the Cd1-xZnxTe detectors, within the x-ray diagnostic energy range, have been performed with the aim of optimizing the image quality parameters of these solid-state-ionization detectors. Namely, the leakage current and system capacitance of the x-ray imaging system have been measured as they relate to signal parameters. Similarly, the detected signal and noise contributions were measured and related to the radiation exposure and tube current setting. Furthermore, the detector contrast has been experimentally determined. The experimental results indicate that Cd1-xZnxTe detectors have low leakage current, high resistivity, and high detector contrast resolution. Therefore, they appear to be very attractive for imaging applications with applications in x-ray digital radiography.

A limited survey of radiation exposure from medical fluoroscopes.

With the co-operation of the University of Pennsylvania and dentists and physicians of Philadelphia, the Department of Public Health of the City of Philadelphia instituted in 1956 a study of radiation exposure from medical x-ray units. A survey of dental x-ray units has previously been reported (1). The present paper is concerned with the radiation characteristics of fluoroscopes surveyed in the Philadelphia area and some of the problems involved in controlling unnecessary radiation exposure from their use. Method of Survey Measurements made on 81 fluoroscopes are included in the present study. Twenty-two units were owned by 8 hospitals4 and 4 city clinics, and the remaining 59 were in the offices of physicians engaged in private practice. The distribution of the units according to ownership is presented in Table I. They are listed according to type and manufacturer in Table II. Sixty-seven fluoroscopes were located within the city limits of Philadelphia. The other 14 were in eastern Pennsylvania and southern New Jersey. Twenty of the fluoroscopes, owned by hospitals and radiologists, were inspected at the request of the owners. The others, all in Philadelphia, were chosen at random from a list registered by physicians with the Commonwealth of Pennsylvania Department of Health. A survey and questionnaire form was used to record systematically four categories of information for each fluoroscope: (a) data from the physician concerning age, manner of use, and average work load; (b) physical inspection; (c) radiation characteristics; (d) radiation exposure to the physician. Measurement Technic The technics used to measure the radiation characteristics of the fluoroscopes were adapted with modifications for field use from methods developed and described by Hale et al. (2). Measurements were made with the field size of the primary x-ray beam ad just ed to 10 × 10 cm. at the panel surface or table top. Tube potential and current settings were those normally used by the physician. Exposure Dose Rate and Half-Value-Layer Measurements: The “quality” of the radiation was determined by measuring the half-value layer (h.v.l.) in aluminum by an inferential method (2, 3). The design of aluminum caps,5 varying in thickness, which were machined to fit over the sensitive volume of the Victoreen 25-r condenser ionization chamber,6 is shown in Figure 1. The ratio of the chamber reading with cap on to the reading with cap off was calibrated as a function of the h.v.l. (Fig. 2). The calibration curves were obtained by the method previously described (2). It can be seen in Figure 2 that the h.v.l. calibration curve for aluminum caps is independent of tube potential. Most of the h.v.l. determinations were made with caps 2 mm. and 3 mm. thick; the 3-mm. thickness was found to be most satisfactory. A second 25-r chamber was always used as a monitor to avoid uncertainty in measurements due to electrical instability.

High-Voltage Generators for X-ray Apparatus

Modern methods of generating the high voltage required for the excitation of X-ray tubes are discussed from the electrical aspects of efficiency, effect on the apparatus, and possibility of direct measurement. The direct current supply from transformers and valve rectifiers with condensers is shown to be the best from the above electrical aspects, and it is suggested that standardisation on this or some similar circuit could well be discussed, to the ultimate advantage of the manufacturer and the user. The Greinacher circuit using two valves and two condensers is dealt with in some detail, and test results on an experimental low-voltage circuit of this type are given and discussed. From these results are deduced some points in design and an outline specification for a continuous voltage plant for 100 milliamperes and up to 80–90 kilovolts. These results are also analysed and found to conform to calculations. Other desirable features in design, include voltage selection in only three or four steps, control by tappings on the primary of the transformers, X-ray tube current setting by filament rheostat, and exposure controlled by a wattsecond relay so as to minimise the effect of line voltage fluctuations.