SCATTER REJECTION BY AIR GAPS IN DIAGNOSTIC-RADIOLOGY - CALCULATIONS USING A MONTE-CARLO COLLISION DENSITY METHOD AND CONSIDERATION OF MOLECULAR INTERFERENCE IN COHERENT SCATTERING

The air gap technique is an old method for scatter rejection. It is st ill used in lung examinations and may be reconsidered for use in digit al radiography. Using magnification techniques, for example in mammogr aphy, the air gap thereby introduced simultaneously yields scatter rej ection. A Monte Carlo collision density method is exploited to investi gate the physical parameters relevant to this technique. Radiation qua ntities of scattered photons at points behind a water slab both on and laterally displaced from the central axis are calculated and their de pendence on field area, slab thickness, air gap length and detector ty pe is derived. Values of 'scatter-to-primary' ratios of the plane ener gy fluence (the energy imparted to a totally absorbing detector) are g iven for perpendicularly incident 30, 70 and 130 kV energy spectra, sl ab thicknesses of 0.05 and 0.2 m (30 kV: 0.05 m), air gaps of length 0 .002-1.0 m and field areas from 8 x 10(-5) to 0.3 m(2). Contrast degra dation factors are derived for both totally absorbing and thin detecto rs. The influence on the scatter-to-primary ratios of using divergent instead of parallel beams and of neglecting molecular interference in coherent scattering is analysed.