Room: Exhibit Hall | Forum 9
Purpose: Half-value layer (HVL) is a beam quality metric used in quality control programs on projection-type imaging equipment. Here, we present a reliable and practical methodology for measuring the HVL in computed tomography (CT).
Methods: A patent-pending apparatus(MuCTâ„¢), a cylindrical step wedge (CSW), presents a surface area of different thicknesses of aluminum perpendicular to an incident x-ray beam from a rotating CT x-ray tube while positioned at the center of rotation. A pencil ion chamber was used to evaluate HVL in a CT scanner (LightSpeedâ„¢ RT 16, GE, Milwaukee, WI) using an axial scan (one rotation/s) for a single 1.25, 5.0 and 10 mm slice thickness (ST). The procedure was repeated for multiple kVp settings and bow-tie (BT) filters. HVL-CSW measurements were fitted using linear regression models to HVL values quoted by the manufacturer (HVL-GE); the Pearson product-moment correlation and confidence intervals (CI) were also calculated.
Results: The average percent error between the HVL-GE and the HVL measured using the scout method was 4.79%. The maximum percent-difference between the average HVL-CSW of the three different ST for each kVp (small BT) and the HVL-CSW for ST=5 mm (small BT) was 0.90 %. The maximum percent-difference between the average HVL-CSW for three STâ€™s at each kVp (large BT) and the HVL-CSW for ST=5 mm (large BT) was 1.14%. For the linear regression models; the coefficients of determination were rÂ²â‰¥0.9985, (pâ‰¤0.0007), with percent errorsâ‰¤4.22%, and Pearson product-moment correlation coefficients were râ‰¥0.99 (pâ‰¤0.0007, 95% CI = 0.96-0.99).
Conclusion: An apparatus for measuring the HVL in CT and correlating broad-beam geometry to narrow-beam geometry was successfully designed and validated. In performing beam attenuation measurements, the patent-pending MuCTâ„¢ CSW can be used without the need to stop the x-ray tubeâ€™s rotation for HVL measurements with standard equipment.
Funding Support, Disclosures, and Conflict of Interest: The apparatus (the MuCT device) has a U.S. Patent Pending No. 62/654016 and commercialization is forthcoming.