Effect of CT Bore size on Radiation Dose during Head CT Acquisition.

Abstract

Abstract: With the development of technology, the use of CT extended beyond the diagnostic purposes and made a room for complex radiotherapy treatment planning. As the traditional diagnostic CT scanners have small bore-size (typically 65–70 cm) and curved patient couch which is not suitable for virtual simulation with immobilised devices. Therefore wide-bore CT scanners with increased bore size (80- 100 cm) were introduced for radiotherapy treatment planning. These virtual simulation CT scanners are further equipped with an external patient positioning laser system, flat patient couch and specialized visualization software. Due to the unavailability of traditional CT scanner, theses wide (or large)-bore CT simulators are used for routine diagnostic procedures. The main purpose of this study was to compare the patient doses delivered by a wide (LB) and a small-bore (AQ1) CT scanners to ensure the safe use of wide-bore simulator for diagnostic purpose. A standard head CT phantom (16 cm diameter and 15 cm length) made out of Polymethyl methacrylate (PMMA) was positioned at the iso-centre and 100 cm pencil ionizing chamber was positioned at the centre and periphery of the phantom. The phantom was scanned under different combinations of tube voltages (80, 100, 120, 135 kVp) and tube currents (100,150, 200, 300, 400 mA) and the Dose Length Product (DLP) in cGy were measured for each location of the ion chamber using an electrometer. The pitch and scan lengths (0.813, 15 cm) were kept constant for each measurement. Our study confirmed that the wide-bore is delivering considerably more dose than observed in AQ1 for head CT with the same exposure parameters. This increasement is more pronounced in exposures above 200 mA and 120 kVp. Therefore, the use of wide-bore simulator for routine diagnostic CT examinations is safer for lower kVp and mA but not encouraged for all the routine diagnostic purpose without further study.