The air gap technique
The air gap technique is a radiographic technique that improves image contrast resolution through reducing the amount of scattered radiation that reaches the image detector. In select situations, this technique can be used instead of an anti-scatter grid as the primary scatter reduction method in order to reduce patient dose 1. The air gap technique is utilised in both plain radiography and mammography.
The reduction in scattered radiation reaching the detector is achieved through increasing the distance between the effective scatter point source and the image receptor 1. This increased distance allows for more chance that the scatter radiations path will result in it not reaching the receptor and therefore not reduce image contrast 2.
The air gap technique is not as effective in high kVp settings because the scattered x-rays are aimed in more forward direction. In projections where the kVp selected is below 90, the scattered x-rays are generally directed more to the side and therefore the air gap technique is more effective in removing them 2.
The decision to use an air gap for scatter radiation control should be considered when an air gap is inherent in the projection positioning and in situations where scatter radiation must be reduced and the equipment allows for an increased object-image distance.
For situations where the air gap technique is chosen for scatter reduction and the object-image distance is not predetermined by projection and equipment requirements, an air gap of 10 to 15 cm has shown to be effective 2.
Radiation dose requirements
To maintain a constant radiation exposure to the image receptor, the exposure factor must be increased in order to compensate for the x-rays that no longer reach the receptor due to the larger object to image distance 1.
The increase in exposure factors needed is comparable to that of an 8:1 ratio grid. The use of an air gap in place of a high ratio anti-scatter grid for scatter reduction therefore results in a lower radiation dose requirement 2.
If the object-image distance is increased without increasing the source image distance, the patient would be positioned much closer to the x-ray source, which would result in the radiation dose being increased in accordance with the inverse square law. Increasing the source-image distance will prevent this 2.
The increased object-to-image distance will also result in increased magnification factor which can be calculated using the following formula 1: magnification factor = source-image-distance/source-object distance
To image the same amount of anatomy as a non-air gap image, a larger image receptor may need to be utilised.
To maintain spatial resolution and image sharpness, the source-image distance may need to be increased, though this may not always be possible with equipment limitations 3.
- 1.Horst Aichinger, Joachim Dierker, Sigrid Joite-Barfub, Sabel M. Radiation Exposure and Image Quality in X-Ray Diagnostic Radiology: Springer; 2004.
- 2.Bushong SC. Radiologic Science for Technologists: Physics, Biology, and Protection. 10 ed: Mosby/Elsevier; 2008.
- 3.Cover F, James N. Johnston TLF. Essentials of Radiographic Physics and Imaging. 2 ed: Mosby; 2012.