A study of the short- to long-phantom dose ratios for CT scanning without table translation

For CT scanning in the stationary-table modes, AAPM Task Group 111 proposed to measure the midpoint dose on the central and peripheral axes of sufficiently long phantoms. Currently, a long cylindrical phantom is usually not available in many clinical facilities. The use of a long phantom is also challenging because of the heavy weight. In order to shed light on assessing the midpoint dose in CT scanning without table movement, the authors present a study of the short- to long-phantom dose ratios, and perform a cross-comparison of CT dose ratios on different scanner models.

The authors performed Geant4-based Monte Carlo simulations with a clinical CT scanner (Somatom Definition dual source CT, Siemens Healthcare), and modeled dosimetry measurements using a 0.6 cm3 Farmer type chamber and a 10-cm long pencil ion chamber. The short (15 cm) to long (90 cm) phantom dose ratios were computed for two PMMA diameters (16 and 32 cm), two phantom axes (the center and the periphery), and a range of beam apertures (3-25 cm). The results were compared with the published data of previous studies with other multiple detector CT (MDCT) scanners and cone beam CT (CBCT) scanners.

The short- to long-phantom dose ratios changed with beam apertures but were insensitive to beam qualities (80-140 kV, the head and body bowtie filters) and MDCT and CBCT scanner models.

The short- to long-phantom dose ratios enable medical physicists to make dosimetry measurements using the standard CT dosimetry phantoms and a Farmer chamber or a 10 cm long pencil chamber, and to assess the midpoint dose in long phantoms. This method provides an effective approach for the dosimetry of CBCT scanning in the stationary-table modes, and is useful for perfusion and interventional CT.