BEAM QUALITY MEASUREMENTS IN DIAGNOSTIC ROENTGENOLOGY

Our study based on roentgenograms made on skull and pelvis phantoms incorporating human skeletal material imbedded in tissue-equivalent material (3M) indicates that the twentieth-value layer is a meaningful number from which roentgenographic exposures can be predicted. We know that the indicated kilovoltage was not correct on some of the units used in this study. These differences were reflected in the twentieth-value layer and so were compensated for in the predicted exposure values.

It will be noted from Table 1 that there were great variations in the first half-value layers, resulting partly from variations in kilovoltage but largely due to variations in the total filtration in the beam. These variations, while having a pronounced effect on incident exposure rate, have a decreasing effect on exposure rate at increasing depths in tissue. For this reason it is our conclusion that the half-value layer is of little value in predicting the technique in roentgenographic procedures. We would recommend that the half-value layer continue to be used only to determine compliance with minimum filtration recommendations. The twentieth-value layer accounts for those variables which determine effective beam energy incident to the grid, and with some modification by the grid, to the film-screen combination. It is the measure of this, not that of the incident beam, that is important in determining the exposure (mas.) required to produce the roentgenogram.

In the coming year this study will be expanded to cover three-phase equipment, other phantoms, and to make use of the information so derived in an installation where patient roentgenograms are made.

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