The utilisation of the air gap technique in general radiography is limited due to the need for equipment facilitation to create the air gap when it is not inherent in the standard technique.
Horizontal-beam lateral hip
There are many different methods of performing the horizontal beam lateral hip projection depending on the patient’s ability to move and the equipment available. Many methods have an air gap present due to the position of the patient and image detector. Due to the thickness of the proximal anatomy, an anti-scatter grid may also be necessary for optimal scatter reduction.
Recent research found that the optimum dose and image quality scenario for the projection phantom study involves using both an anti-scatter grid and a 45 cm air gap 1.
For pelvic radiography, an air gap of 10 cm proved adequate for diagnosis in both the CR and DR systems. The use of the 10 cm air gap technique is found to be sufficient to replace a 10:1 ratio anti-scatter grid, and replacing a grid with an air gap in a DR system could result in a dose reduction of 69.6 to 79.4% 2.
Lateral cervical spine
The standard positioning of the imaged patient, image receptor, and x-ray tube for a lateral cervical spine image includes an inherent air gap created by the distance between the patient's neck and the image receptor which is against their shoulder.
Using an anti-scatter grid despite the inherent air gap can result in a dose increase factor of an average 2.21 when compared to only using the air gap as an anti-scatter method 3.
The use of an anti-scatter grid and filter combination for the lateral cervical spine projection can improve the visualisation of C7-T1 vertebral junction. It was found that within the scenarios examined, 49.8% of the images produced were adequate to visualise C7-T1, while 61.1% of the images produced using a grid and filter combination were adequate 4. The increased adequacy with grid and filter use may justify the use of an anti-scatter grid due to the decreased need for further imaging to define the C7-T1 level.
The decision to use either an air gap alone or a grid and air gap combination for scatter reduction should be made on an equipment and patient basis.
- 1. Charnley C, England A, Martin A, Taylor S, Benson N, Jones L. An option for optimising the radiographic technique for horizontal beam lateral (HBL) hip radiography when using digital X-ray equipment. Radiography. 2016;22(2):e137-e42.
- 2. Dose optimization in pelvic radiography by air gap method on CR and DR systems – A phantom study. Radiography. 3 (21): 214. doi:10.1016/j.radi.2014.11.005
- 3. Bell N, Erskine M, Warren-Forward H. Lateral cervical spine examinations: an evaluation of dose for grid and non-grid techniques. Radiography. 9 (1): 43-52. doi:doi:10.1016/S1078-8174(02)00078-0
- 4. Goyal N, Rachapalli V, Burns H, Lloyd DCF. Cervical spine imaging in trauma: Does the use of grid and filter combination improve visualisation of the cervicothoracic junction? Radiography. 2011;17(1):39-42.
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Physics and imaging technology: x-ray
- x-ray production
- x-ray tubes
- tube rating
- interaction with matter
- beam collimators
- air gap technique
- intensifying screen
- x-ray film
- image intensifier
- digital radiography
- digital image
- x-ray artifacts
- radiation units
- radiation safety
- as low as reasonably achievable (ALARA)
- radiation protection
- background radiation
- background radiation equivalent time
- deterministic effect
- dose limits
- inverse square law
- lead apron
- radiation damage (biomolecular)
- radiation damage (skin injury)
- stochastic effect
- radiation detectors