Methods Used to Attain High Speed in Roentgenography

Abstract

AN important problem in roentgenography is to obtain sharp shadowgraphs of moving objects, in particular of the heart and lungs in living persons. The solution of this problem involves taking roentgenograms with extremely short exposure times. By thus reducing to a minimum the blurring due to the motion of the object during exposure, the quality and detail of the x-ray film is greatly enhanced. The conventional way of decreasing exposure time is to increase the x-ray tube current (milliamperage) but there is a rather definite limit to which this can be carried. The limiting factors depend upon the rating of the x-ray tube focal spot and the load-carrying capacity of the supply line. Larger currents through the x-ray tube, even though applied only momentarily, require high voltage transformers with heavy windings to reduce ohmic resistance, and a supply line of large current-carrying capacity. Such heavy loads are generally limited to three phase systems. The heavy loads and violent surges on the supply line and x-ray transformer can be eliminated by using condensers to store the energy required for making an x-ray shadowgraph. The condensers are charged slowly through rectifiers over an interval of several seconds, and then discharged rapidly through the x-ray tube in a small fraction of a second. The capacitance of the condenser must be such that sufficient energy is released in a single discharge to produce an x-ray film of satisfactory density. Since electrical energy is drawn from the supply line at a low rate, even small x-ray transformers suffice to charge a set of condensers for high speed roentgenography. Hence a small fluoroscopic x-ray unit may be converted into the most powerful roentgenographic machine. Types of Controls Used to Discharge Condensers.—Perhaps the most simple method of discharging the condenser is through a sphere gap. With the x-ray tube filament set to emit a current of approximately 1,000 milliamperes, the condensers are charged to their predetermined voltage and then discharged through the gap. An appreciable amount of noise always accompanies the discharge, but due to the effective impedance of the x-ray tube being in the neighborhood of 80,000 ohms, it is not excessive. Even this noise is undesirable and unnecessary and would certainly frighten a patient unless the noise were muffled. An improvement is accomplished by immersing the sphere gap in an oil tank and closing the gap under oil. Silent operation can be obtained by using a vacuum switch to excite the x-ray tube. In this device (1) the separation of the electrodes is controlled by a magnetizing coil outside the vacuum chamber. These gaps withstand the total voltage of the transformer during the charging interval. Voltage is applied to the x-ray tube only during the discharge time.