Effects of Focus Film Distance (FFD) variation on entrance testicular dose in lumbar-pelvic radiography.

Introduction: With the steady increase in public and professional concern regarding the biological effects of ionising radiation, there is a need for both the Chiropractic and Radiography professions to improve imaging techniques for the lowering of patient radiation doses. Lumbar radiographs are essential in chiropractic general practice for biomechanical diagnosis and postural analysis. Detailed anatomical measurements are taken from spinal radiographs for the determination of various biomechanical alterations for clinical purposes. The quality of spinal radiograph is dependent on a number of factors, including Focus Film Distance (FFD), magnification ratios, penumbra, contrast and density. Variation in FFD will vary magnification factor (MF) and Penumbra.

Objectives: The study aims to investigate the relationship between FFD and received radiation dose to patients, where the radiation dose to the testes may be significantly lowered whilst still maintaining acceptable image quality.

Methods: Radiographic images and dosimetry were obtained with a Seimans wall-mounted X-ray unit. All anterior/posterior (AP) and lateral lumbar-pelvic radiographs were taken of an anthromorphological phantom that resembles human tissues, at both 100cm and 200cm FFD. Five central beam air doses were measured for all parameters to demonstrate patient entrance doses.

Results: For AP lumbar-pelvic radiography, increasing FFD by a factor of two resulted in an approximately 30% decrease in entrance dose to the testes. For lateral lumbar-pelvic radiography a two fold increase in FFD resulted in a 70% reduction in entrance dose.

Conclusions: The study suggests for the first time that an FFD of 200 cm, which is largely utilised by the chiropractic profession, is an efficient method of minimising radiation dose to patient, during lumbar radiography.