Abstract
The computed tomography dose index (CTDI) measured with a 10 cm long pencil ionization chamber placed in a 14 cm long PMMA phantom is typically used to evaluate the doses delivered during CT procedure. For the new generation of CT scanners, the efficiency of this methodology is low because it excludes the contribution of radiation scattered beyond the 100 mm range of integration along z. The AAPM TG111 Report proposes a new measurement modality using a small volume ionization chamber positioned in a phantom long enough to establish dose equilibrium at the location of the chamber. In this work, the AAPM report was implemented. The minimum scanning length needed to obtain cumulative dose equilibrium was evaluated. The equilibrium dose was determined and compared to CTDI values informed by the CT scanner, and the dose values were confirmed with TLD measurements. The difference between doses measured with TLD and with the ionization chamber (IC) was below 1% and the repeatability of the measurements' setup was 0.4%. The measurements showed that the scanning lengths needed to reach the cumulated dose equilibrium were 450 mm and 380 mm for the central and peripheral axes, respectively, which justifies the phantom length. For the studied clinical protocols, the doses measured were about 30% higher than those informed by the CT scanner. For the new generation of CT systems with wider longitudinal detector size or cone‐beam technology, the current CTDI measurements may no longer be adequate, and the informed CTDI tends to undervalue the dose delivered. It is therefore important to evaluate CT radiation doses following the AAPM TG111 methodology.PACS number: 87.57.qp, 87.53.Bn
Highlights
Computed tomography (CT) is one of the most frequently used diagnostic imaging methods
The aim of the present work was to follow the AAPM TG111 guidelines in order to evaluate the doses delivered during commonly used CT exam protocols, and to compare them with the doses informed by the CT scanner at the end of each scanning
As predicted by the AAPM TG111 Report, this work showed the limitation of the computed tomography dose index (CTDI) and, more of the index CTDI100
Summary
Computed tomography (CT) is one of the most frequently used diagnostic imaging methods. Recent overdose accidents[1] led to new interest in evaluating the dose delivered during CT exams.[2] The current paradigm used to evaluate these doses is the computed tomography dose index (CTDI), which represents the absorbed dose along the longitudinal axis (z-axis) of the CT scanner measured during a single rotation of the X-ray source. It is commonly measured with a 100 mm long pencil ionization chamber (CTDI100) placed in a 14 cm long and 16 cm or 32 cm diameter cylindrical PMMA phantom representing adult head and body, respectively.[3]. The methods and equations described in this report can be used for axial or helical scanning modes, for cone- or fan-beam geometries, with or without table translation
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