A Technic for Cross-Calibration of X-Ray Units Utilizing Half-Value-Layer Determinations 1 Leon Pape , M.S. , S. Baker , R.T. and Hyman L. Gildenhorn , M.D. 1500 E. Duarte Rd., Duarte, Calif. Excerpt In radiographic installations of more than one x-ray unit the need to reproduce a film technic with different units makes it necessary that they be cross-calibrated. In standard practice the establishment of a film technic requires that visual examination be made of representative films for various anatomical structures. Because of possible inconsistencies in the physical calibration factors of x-ray units, as reported in a previous paper (1), the establishment of a set of technical factors for one apparatus would not necessarily apply to a second. The process of trial and error must be repeated with each one, and this results, in addition to the time and effort required, in undesirable patient exposure. In principle, the quality of a film depends on beam energy and intensity, with all other things being constant. A knowledge of these two factors for any individual apparatus, therefore, should suffice to establish radiographic technics based on an established technic chart for another unit. A system for providing this type of cross-calibration was suggested in the authors' earlier paper. Incidentally, the use of half-value layer and output determinations for the establishment of film technic is not limited to interdepartmental standardization. The possibility exists that other departments would also be able to duplicate film qualities by this means. Simply stated, an x-ray unit must first be calibrated for technical factors in the usual manner of visual evaluation. This unit serves as the standard and it is then possible to calibrate any additional units without application of visual methods, purely by virtue of the comparison of half-value layer and output. The situations in which calibration of radiographic installations becomes necessary include the following: ( a ) change of tubes or other parts of a unit, ( b ) replacement of an installation, and ( c ) the addition of new units to an existing department or the creation of an entirely new department. Method Three x-ray units were studied, the conditions under which measurements were made being maintained constant. Half-value layer determinations were made with aluminum filters with a Landsverk 200-mr pocket chamber calibrated for energy dependence at the Landsverk factory. The values were plotted for each of the units on a single graph, so that the effective kilovoltage for equivalent half-value layer could be readily determined (see Fig. 1). After half-value layer determinations were made, the output of each x-ray unit was measured with the Landsverk chamber at an equivalent kilovoltage as determined from the plot of half-value layers. A plot of output per 100 milliampere-seconds at equivalent kilovoltage is shown in Figure 2. After establishment of half-value layer and output equivalences, the next step was to take one of the units as a standard and establish a set of technics which produced visually acceptable roentgenograms.
Radiology – Radiological Society of North America, Inc.
Published: Sep 1, 1961
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