Beam quality assessment: Evaluating aluminum purity's effects

This study evaluates the effect of aluminum (Al) purity on Half Value Layer (HVL) measurements and beam quality across mobile and fixed radiographic units. Beam quality assessment is a fundamental aspect in ensuring the safe and effective delivery of ionizing radiation to patients. Standards for this assessment vary nationally and internationally, leading to ambiguity within measurements, which may cause regulatory compliance issues.

Beam filtration was measured using Al filters of 99.0% and 99.5% purity, for mobile and fixed radiographic units utilizing radiation detectors from three manufacturers. Methods and geometry followed IEC standards for HVL measurements with techniques representative of those used for standard patients. Measurements were made across a range of thicknesses of aluminum incrementally centered around the projected HVL. HVL was determined through exponential fitting of exposure versus Al thickness. Additionally, for each solid-state detector, a single-shot HVL measurement was performed without aluminum in the x-ray beam, as this method is commonly used for HVL determination in clinical settings.

When Al filtration purity was increased from 99.0% to 99.5%, HVL measurements rose by 1.35% to 4.82%, with a median increase of 3.26% (95% confidence interval 2.68%-3.85%). The deviation between single-shot HVL measurements and interpolated values ranged from 0.16% to 8.99% (median: 4.90%; 95% CI: 3.32%-6.49%) for 99.0% purity Al, and from 0.43% to 4.79% (median: 2.62%; 95% CI: 1.60%-3.64%) for 99.5% purity Al.

There is a discrepancy in the purity of reference aluminum used for determining HVL specified in the United States Food and Drug Administration (FDA) and International Electrotechnical Commission (IEC) standards. FDA requires Type-1100 aluminum with a minimum of 99.0% purity, while IEC requires a minimum 99.9% purity. Our results indicate that measured HVL values increase with aluminum purity in a magnitude sufficient to result in units falsely failing regulatory requirements for minimum HVL if different types of aluminum are used by manufacturers and physicists. Medical physicists should be aware of this issue when testing HVL for compliance with regulatory standards. This study did not provide a direct comparison of FDA and IEC standards; instead, it demonstrates the direction and magnitude of HVL sensitivity to aluminum purity within clinically realistic materials, highlighting how seemingly small variations in aluminum purity can materially influence regulatory interpretation of beam quality.