Advantages of thre-phase spot-film radiography.

Abstract

Most radiologists give considerable thought to the selection of a spot-film device. Their choice is usually based on mechanical convenience and mobility. There is evidence, however, that many other factors involved in spot-film radiography receive only slight consideration. This paper will deal with the possibilities of improving spot-film quality by utilizing 3-phase generators and controls. The usual fluoroscopic-radiographic unit in use in the United States is powered by a single-phase generator. Design of the control and a comparative lack of efficiency in the generators used in the single-phase systems present several handicaps in producing spot-films of highest quality and sharpness of detail. No choice of focal-spot size is permitted for making a spot-film. Fluoroscopy is done on the small focal spot, and spot-filming is permitted on the large focal spot only. In most cases the large focal spot is 2 mm. in diameter. Focal Spot To demonstrate the effect of focal-spot size on sharpness of detail, three radiographs of wire mesh laminated in plastic were made, one with a 2-mm. focal spot (Fig. 1, A), one with a 1-mm. focal spot (Fig. 1, B), and a third with a O.6-mm. focal spot (Fig. 1, C). Enlargement technic was used to simulate the increased part to film distances which are usually encountered in making spot-films. All other factors influencing detail were constant. A comparison of the three films demonstrates the marked blurring that results with large focal spots even though the object is stationary while being filmed. Versatility of Controls The controls used with most single-phase apparatus do not permit exploiting the fluoroscopic x-ray tube even to its capacity for single-phase operation. The rigidity of the milliamperage, time, and peak kilovolt relationships imposed by control design frequently preclude loading the tube at or near its maximum. Most 3-phase apparatus, on the other hand, permits the selection of any milliamperage for spot-film radiography within the range of the x-ray tube capacity for a given time and kilovolt setting. In contrast to the single-phase units, this flexibility is available for use with either the large or the small focal spot with the 3-phase unit; in other words, one has a wide choice of milliamperage stations and a choice of two focal spots in spot-filming as compared to a single milliamperage station and one focal spot with the customary single-phase units. “Efficiency” of X-ray Production and Tube Load Another important advantage of 3-phase equipment is the higher load that may be applied on the x-ray tube. In the spot-film timing ranges, the manufacturers' tube rating charts show up to 50 per cent greater loadability with 3-phase equipment as compared to single-phase.