Increasing FFD: An effective dose-reducing tool for lateral lumbar spine investigations

Abstract

The lumbar spine radiographic examination is a relatively high dose examination and a high contributor to collective dose. This study investigated the effects on patient dose and image quality of increasing the focus-to-film distance (FFD) for lateral lumbar spine projections. The first part of the investigation employed an anthropomorphic phantom and thermoluminescent dosimeters (TLDs) to measure the entrance surface dose and dose to positions corresponding to the ovaries at 100 cm, 130 cm and 150 cm FFD at varying X-ray beam energies. Following statistical analysis, dose reductions of 47%, 57% and 62% were achieved for the entrance surface, right ovary and left ovary, respectively, when the FFD was increased from 100 cm to 130 cm at a beam energy of 109 kVp (P<0.0001). Increasing the distance from 130 cm to 150 cm offered no further advantage. The second part of the investigation involved patients (female) and showed a 44% reduction in both the entrance surface and effective dose when the FFD was increased from 100 cm to 130 cm at 109 kVp. No loss in total weighted image quality was noted at 130 cm FFD for either the phantom or patient study. The authors therefore recommend implementing 130 cm for lateral lumbar spine projections and encourage more studies establishing the effectiveness of increasing FFD for other projections. The lumbar spine radiographic examination is a relatively high dose examination and a high contributor to collective dose. This study investigated the effects on patient dose and image quality of increasing the focus-to-film distance (FFD) for lateral lumbar spine projections. The first part of the investigation employed an anthropomorphic phantom and thermoluminescent dosimeters (TLDs) to measure the entrance surface dose and dose to positions corresponding to the ovaries at 100 cm, 130 cm and 150 cm FFD at varying X-ray beam energies. Following statistical analysis, dose reductions of 47%, 57% and 62% were achieved for the entrance surface, right ovary and left ovary, respectively, when the FFD was increased from 100 cm to 130 cm at a beam energy of 109 kVp (P<0.0001). Increasing the distance from 130 cm to 150 cm offered no further advantage. The second part of the investigation involved patients (female) and showed a 44% reduction in both the entrance surface and effective dose when the FFD was increased from 100 cm to 130 cm at 109 kVp. No loss in total weighted image quality was noted at 130 cm FFD for either the phantom or patient study. The authors therefore recommend implementing 130 cm for lateral lumbar spine projections and encourage more studies establishing the effectiveness of increasing FFD for other projections.