Air gap technique

An air gap between the patient and the film-screen will improve the contrast of the film by reducing the divergent scattered rays. Also, because of the inverse square law, air gap causes a mild reduction in the intensity of primary radiation that comes from the target from a larger distance and significant reduction in intensity of scattered radiation produced from points within the patient which are much nearer. This will result in a reduction of the scattered radiation thus improving contrast. However, patient exposure is increased due to the the inverse-square effect (the patient will be closer to the primary beam). Also, the use of an air gap generates magnification. Therefore, a larger receptor size is required to obtain the same patient area coverage. If the air gap is obtained by increasing the tube-to-receptor distance, the x-ray equipment must be operated at a higher output (higher dose to the patient) to obtain adequate receptor exposure. Air gap technique obviates the need offor the use of grids, resulting in a reduction in dose and an increase in contrast of the film.

A common use of the air gap is in magnification mammography. Since an air gap is produced by separating the breast from the receptor to produce magnification, it can be used for scatter reduction. Spatial resolution required to study the microcalcifications in magnification mammography is obtained with the reduction of the focal spot. The exposure time gets longer because the microfocus has  lower mAlower mAs values and lowerless power than the standard fine focus; in addition, the exposure time must be increased to compensate the greater distance between the breast and the image receptor. Because the breast is nearest to the x-ray source, the amount of given radiation would be very higher. As the radiation dose is much higher, it is justified only in symptomatic patients. The usual procedure is to remove the grid and rely on the air gap.