X-ray field size and patient dosimetry.

Purpose: Limiting the size of the x-ray field during radiography is an important radiation safety practice and the sole responsibility of the radiologic technologist. Collimation reduces the volume of tissue irradiated and therefore reduces patient exposure and improves image quality. The purpose of the research experiment was to investigate the effect of decreasing the x-ray field size on patient dosimetry during lumbar spine imaging.

Methods: The x-ray field size was decreased from 14 × 17 in (35 × 43 cm) to 8 × 17 in (20 × 43 cm) with an increase in mAs to maintain exposure to the image receptor. Patient dosimetry was investigated by measuring the thermoluminescent dosimeter (TLD) exposure to the abdominal region of an anthropomorphic adult male phantom. Seven TLD chips were placed in the abdominal region, exposed, and replaced for each of 10 exposures, totaling 70 exposed TLDs. A 2-way factorial analysis of variance for independent samples was calculated to determine whether x-ray field size and the TLD locations altered the milliroentgen (mR) reading.

Results: The TLDs located closest to the lumbar spine yielded no significant change in patient dose for the 8 × 17 in (20 × 43 cm) x-ray field size following the increase in mAs. However, the TLDs located closest to the lateral edge of the 8 × 17 in (20 × 43 cm) collimated x-ray beam received a significantly (P < .001) lower mR exposure (> 60%).

Conclusion: Radiologic technologists should make considerable effort to limit the x-ray field size to the region of interest to reduce patient exposure.