IV.A.100.     An air gap technique will generally improve image contrast because:

    a.     It is used with a small focal spot.    No

    b.     The air absorbs scattered radiation.    No

    c.     It is used with a small field of view.    No

    d.     The scatter is more diverging than the primary beam.    Yes

   Discussion:    The air does not selectively absorb the scattered radiation.   Contrast can be improved because the scattered radiation is more diverging (because of the geometry) and the intensity of the scatter at the receptor is reduced.   The introduction of an air gap produces magnification and a small focal spot is desirable.   However, this is not what improves the contrast except for the improved visibility of detail of small objects such as calcification in mammography.   When a magnification technique is used in mammography the grid is usually removed because the air gap reduces the scattered radiation.

   Reference:    Physical Principles of Medical Imaging, 2nd Edition. p. 185