Comparing the standard knee X-ray exposure factor, 10 kV rule, and modified 10 kV rule techniques in digital radiography to reduce patient radiation dose without loss of image quality

Introduction
The 10 Kilovoltage (kV) rule was a historic exposure adaption technique designed for film screen X-ray imaging to reduce ionising radiation dose without loss of image quality. This study evaluates knee X-ray radiation dose and image quality between standard patient exposure factors, the historic 10 kV rule (−50 % Milliampere-second (mAs), and a modified 10 kV rule (−75 % mAs) using a digital radiography (DR) system.

Method
Applying the exposure factors of 63 kV and 8 mAs (standard pre-set exposure), 73 kV and 4 mAs (historic 10 kV rule) and 73 kV and 2 mAs (modified 10 kV) to a phantom knee and recording entrance skin dose (ESD) using thermoluminescence dosemeters (TLDs). The ESD was analysed with a t-test. The image quality was assessed using a Likert 5-point Visual Grading Analysis (VGA) by (n = 3) independent observers. The ESD data was analysed with Analysis of Variance (ANOVA) for differences between the techniques.

Results
The ESD reduction for the historic 10 kV rule was 32.1–33.7 % (20.9 μGy; p = 0.00), and the modified 10 kV rule 81.5–81.8 % (42.1–43.7 μGy; p = 0.00) compared to the standard pre-set exposure technique. The historic and modified 10 kV exposure parameters image quality for the AP views knee X-rays scored higher (p = 0.00) than the standard preset exposure images. The VGA for the lateral knee view using the historic (−0.1 VGA; p = 0.02) and the modified 10 kV (−0.3 VGA; p = 0.00) were slightly lower than the standard preset image quality, related to the trabeculae pattern and cortical outlines.

Conclusion
The findings suggest dose reductions could be made by modifying the exposure factors without reducing the quality of diagnostic images in the AP Knee position. The findings for the lateral knee X-rays indicate the image quality scored lower but was still within diagnostic range. Further research is required in laboratory conditions of exposure adaptations over a larger sample of anatomy thickness and applying a wider exposure (kV) range.

Implications for practice
One of a radiographer's many roles are to optimise techniques to improve image quality of anatomy and reduce the radiation dose to the patient. The findings have shown there is potential for further research using the modified 10 kV rule.

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