Evaluation and comparison of water-equivalent CTDI dosimetry phantoms

The determination of radiation dose characteristics of CT scanners in clinical use is becoming increasingly important for patient protection and optimization of dose reduction measures. This paper focuses on the evaluation and the use of recently developed water-equivalent cylindrical head and body CTDI phantoms. CT doses measured in the new phantoms are compared with measurements in standard polymethylmethacrylate (PMMA) phantoms. The CTDI phantoms used for this study were manufactured by QRRI, Germany, and are based on a polyurethane resin technology with additives to fine-tune Hounsfield Units (HU), effective atomic number Z_eff and physical density p to those of water. A pencil ionization chamber with an active length of 10 cm was used for all measurements at our clinical CT scanner Siemens Somatom PLUS 4. All CTDI values assessed in PMMA CTDI phantoms are significantly higher (measured centrally up to 15.4%) than in the water-equivalent CTDI phantoms at all tube voltages (80 kVp, 120 kVp and 140 kVp) investigated. This is surprising, as the attenuation (μ/ρ)*ρ of PMMA is higher than the attenuation of water or the water-equivalent polyurethane resin. This is due to a higher ratio of scattered radiation in the PMMA phantom as the authors verified by Monte Carlo simulations. In practice, using PMMA phantoms for CT dosimetry is leading to an overestimation of patient dose. This may be even more relevant in a time when diagnostic reference dose levels are implemented in clinical routine