CONTROL OF SCATTERED RADIATION BY AIR GAP TECHNIQUES: APPLICATIONS TO CHEST RADIOGRAPHY

The ratio of the light emitted by an intensifying screen in the cassette due to scattered radiation, L_s, to the amount of light emitted due to primary radiation, L_p, has been determined as a function of the parameters that must be considered in establishing air gap techniques in chest radiography. Data were obtained varying the tube potential from 60-140 kVp. Measurements were made using a 14x17 inch field at 6 and 10 foot TFD with absorbers simulating water phantoms varying in thickness from 7.4 to 21.7. The air gap was varied between and 15 inches.

It was found that there was no systematic variation of L_s/L_p as a function of tube potential. This experimental result was explained by calculations based on the theory of the Compton differential cross-section. It was found that when the incident photon energy varies from 10 keV to 140 keV the most probable angle of scattering changes only from about 53° to 45° and the crosssection changes only by about 15 per cent.

To compare the air gap technique with conventional grids, contrast improvement factors were calculated. It was found that a 10 inch air gap is comparable to a medium weight grid and that for thin absorbers, a 5 inch air gap is comparable to a light grid. These conclusions were confirmed in the instance of chest radiography by calculations of the resulting visual contrast.

In air gap techniques the TFD must be increased to compensate for the magnification introduced by the air gap. Calculation showed that a 10 foot TFD with a 5 inch air gap gave magnification essentially the same as that with conventional techniques. The magnification for a 10 inch air gap was about 5 per cent greater in both posteroanterior and the lateral projections. Calculations of the modulation transfer functions showed that the MTF with a 5 inch air gap at 10 foot TFD was essentially the same as conventional technique. When the air gap was increased to 10 inches, the high frequency response of the system was slightly poorer.

Patient exposure is about the same for air gap techniques as conventional techniques in the posteroanterior projection. Air gaps of 5 or 10 inches at 10 foot TFD reduce patient exposure by factors of 5.5 and 3.8, respectively, compared to a conventional technique using a 12:1 grid.

Finally, although the interest of this investigation was in chest radiography, a set of measurements were made of L_s/L_p as a function of field size. It was found that this ratio is strongly dependent on the area irradiated.

In summary, for a posteroanterior projection in chest radiography, air gap techniques at 10 foot TFD produce somewhat better visual contrast with no change in resolution or patient exposure. For the lateral projection the visual contrast is poorer than that obtained with a conventional technique using a grid, the resolution is about the same, but patient exposure is reduced by a factor of 4 to 6.

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