Cholecystography: Optimum Voltage

Abstract

GRAHAM'S (1) original success in visualization of the gall bladder was with tetrabromphenolphthalein. For a year this was used by several workers, given by vein and by mouth, but was soon replaced everywhere by tetraiodophenolphthalein. (Some of the early tetraiodo. preparations had been toxic.) Actual measurements of the densities of shadows produced were apparently not made. Newell (2), in attempting to standardize gall-bladder shadows, made no study of the relation of contrast to tube voltage. He took it to be obvious that one ought to use the lowest practical voltage consistent with exposures short enough to avoid movement, accepting 60 kv. as the best compromise. With the advent of the rotating anode tube, lower voltages have become practical. We, therefore, set about making a set of gall-bladder standards for 50 and 40 kv. To our surprise, the shadows looked the same as at 60 kv. (Fig. 2). Reference to Figure 1 makes this paradox understandable. At 50 kv. constant potential (60 to 70 kv. pulsating) the peak of the x-ray spectrum comes just on the short wave side of the K absorption edge of iodine. At 30 kv. constant potential (say, 40 kv. pulsating) the peak falls right in the trough of selective transparency on the long wave length side of the absorption edge. Having been led thus to the practical conclusion that 70 kv. is as good as 40 for tetraiodo. examinations, we wondered if we might not make the higher voltage show an even better shadow by using a filter selectively absorptive for just those wave lengths in which iodine is selectively transparent. We made some preliminary trials with filters of barium sulphate and of cerium sulphate, but shadows did not appear any better. Then at moderate voltages we tried filters of silver and of molybdenum, hoping to cut off the short end of the spectrum, yet avoid the low efficiency of extremely low tube voltages. This scheme also yielded nothing worth while. Another possibility was to go back to tetrabromphenolphthalein. There is a long range of wave lengths (from 0.92 Å. to 0.37 Å.) for which bromine is more than twice as opaque as iodine (gram for gram). If we use this range of wave lengths, tetrabromphenolphthalein ought to cast better shadows than tetraiodophenolphthalein. Figure 3 confirms this . We could show tetrabrom. in half the concentration needed for tetraiodo. (but only at lowest practical kv. and smallest depth of immersion in water). With care, a visible shadow can be obtained of one-eighth per cent NaBr, while one-eighth per cent NaI remains completely invisible (in a 34 mm. rubber bag in 4 cm. of water). For practical visualization of the gall bladder, tetrabromphenolphthalein will not be better than tetraiodophenolphthalein, unless it proves, on further trial, to accumulate in higher percentage concentration, or to lead to fewer unpleasant reactions. Tetraiodo. by mouth produces unpleasant symptoms of some degree in 60 per cent of patients (3).