Optimising Beam Geometry in Orthopaedic X-Rays: A Phantom Study

Abstract

Introduction

Antoni Cieszynski, a 19th-century Polish dentist, pioneered a rule of isometry for accurate dental radiography. His bisecting angle technique improved the precision of imaging teeth by angling the x-ray tube perpendicular to a ‘bisecting line’ between the object and the image receptor, thereby minimising geometric distortion and achieving more anatomically accurate images. This study explored the potential use of this rule for x-ray imaging of long bones when routine positioning techniques are compromised.

Methods

Using an adult phantom forearm, an experiment was conducted to quantify the effectiveness of the bisecting angle technique on a long bone. A control image using a straight tube with the bone parallel to the digital image receptor (IR) was taken. This was followed by x-rays of the phantom forearm at 15° increments with a straight tube, matching tube angle and the bisecting angle technique. The resulting images were analysed by a single scribe and reviewed by one other peer for geometric distortion using the calliper measurement tool.

Results

The control image measured 21.4 cm. When the central ray matched the angle of the phantom forearm, the maximum elongation exceeded the length of the detector (> 43 cm). When a straight tube angle was applied, the maximum foreshortening measured 13.5 cm. Meanwhile, the maximum length of the phantom forearm, when the bisecting angle was applied, was 24.1 cm. While the use of the bisecting angle technique did not eliminate geometric distortion, it greatly reduced it. The experiment identified that common radiographic practice has the potential to be improved.

Conclusions

The bisecting angle technique offers a promising method to improve long bone imaging. While it does not fully eliminate geometric distortion, it effectively minimises elongation, suggesting a potential to enhance imaging accuracy for long bones in clinical settings.