The effect to dose and diagnostic quality of limiting source-to-image distance on anterior-posterior semi-erect adult chest X-rays

Introduction
Mobile chest X-rays (CXR) examinations are requested for clinically unstable inpatients who cannot be transported to the radiology department. These examinations often have limitations, such as reduced source-to-image distance (SID) in anterior-posterior (AP) position due to limited space in ward environments. This study aimed to investigate the effect of entrance skin dose (ESD) and the diagnostic image quality of reduced SID from the standard 180cm to 150cm and 100cm on semi-erect AP CXRs.

Method
A human tissue equivalent Alderson Rando phantom was positioned in a hospital bed at an AP semi-erect 45-degree angle. Three exposures were taken for each SID; the dose was recorded with an Ion Chamber attached to the anterior chest wall to record the ESD and the dose area product (DAP). The mean dose from the three SID positions was compared using a t-test. Image quality was assessed using Image Quality Score (IQS) and RadLex criteria for diagnostic quality using Analysis of Variance (ANOVA).

Results
A linear response of decreasing the SID resulted in a 40.1 % increase in ESD (53.7 μGy; p = < 0.001) from 180 cm to 150 cm SID and a 121.5 % increase (329.9 μGy; p = < 0.001) from 180 cm to 100 cm SID. There was also a decrease in image quality from 180 cm to 100 cm SID (IQS p = 0.03), with 100 cm scoring poor against lower lung field visual reproduction and limited diagnostic quality on RadLex.

Conclusion
The findings demonstrate that in controlled X-ray settings, the ESD is increased when the SID is reduced from 180cm to 150cm SID (40.1 %) and to 100cm SID (121.5 %), which furthermore decreases image quality (p = 0.03) and the diagnostic value of the image. It is recommended that radiographers maintain the standard 180 cm SID during adult inpatient semi-erect AP CXR examinations to maintain as low as reasonably practicable (ALARP) diagnostic X-ray examinations.

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