Mobile X-ray equipment

5. Exposure factor selection

Typically, a mobile CXR in this scenario is performed in antero-posterior (AP), in an intensive and less critical scenario (17). Mobile equipment due to its less performer generator (compared to a stationary generator), requires longer exposure times (18).  In a study performed with mobile X-ray equipment, the mean values used to expose an average adult chest were: 96 kVp, 1.25 mAs, 30 msec, no filtration and no grid (18). Other guidelines referred that kVp can varied between 70 or greater than 100 kVp, depending on patient thickness and on the use of scatter radiation grid. The beam intensity needs to be as short as possible to reduce the probability of having movement artefacts(19). Increasing the kVp can reduce the dose and the signal-to-noise ratio (SNR). Lowering x-ray tube potential from 80 to 60 kVp improved the SNR by 30–40 %, with a corresponding increase in phantom dose of 40–50 %. If patients are obese the use of grid can improve image quality of thicker regions (20). If it involves peadiatric patients with less than 14 cm the use of grids must be minimized(19).

The source-to-image distance (SID) needs also to be considered since it will have impact on dose and image quality. Images can be underpenetrated, mainly in lower lung fields and in obese patients, or they can be overpenetrated if the beam energy is too high or if the SID is to short(21). Using a longest SID magnification and unsharpness can be reduced. According to the recommendations of American College of Radiology (19)for mobile chest X-rays, SID must be at least 40 inches (1 meter) with the optimal distance as close as possible to 72 inches (1,8m).

If the context involves uncooperative or pediatric patients, the images can be performed in supine or semi-erect with one meter or greater SID. Increasing the SID from 1 to 1.3 m the entrance skin dose (ESD) can be reduced by 32.2% without lost of image quality (22).

Table 5 – Comparison of two phantoms image quality and dose based the use/no use of a grid and for different mAs  for CR mobile  system f (adapted from(20))