Computed tomography dose index free-in-air and single scan dose profile analysis of multi-slice scanners

Abstract

The increasing use of computed tomography (CT) in clinical practice marks the needs to understand the dose descriptor and dose profile. The purpose of the current study is to determine the computed tomography dose index free-in-air (CTDIair) and to evaluate the single scan dose profile (SSDP) in two types of multi-slice CT (MSCT) scanners. The MSCT scanners involved were 128-slice CT scanner Siemens SOMATOM Definition AS+ at Hospital Sultanah Aminah (HSA), Johor Bahru and 16-slice CT scanner Siemens SOMATOM Emotion 16 at Hospital Permai, Johor Bahru. Two types of dosimeters were used for the measurements which were thermoluminescence dosimeter (TLD-100) and optically stimulated luminescence dosimeter (nanoDot OSLD). For each CT scanner, all protocols were set based on the routine CT abdominal examinations for adult male. The scan time, slice thickness and nominal beam width were held on constant while the tube current and tube potential were modified. In total, there were seven protocols for 128-slice CT scanner and six protocols for 16-slice scanner. For both CT scanners, when the tube current or the tube potential was increased, the CTDIair was increased as well. For 128-slice CT scanner using TLD-100, when the tube current was constant at 100 mAs, the CTDIair ranged from 3.560 mGy (80 kV) to 13.585 mGy (140 kV). When the tube potential was constant at 120 kV, the CTDIair ranged from 8.825 mGy (100 mAs) to 21.528 mGy (250 mAs). From OSL dosimeter, when the tube current was constant at 100 mAs, the CTDIair ranged from 2.391 mGy (80 kV) to 7.751 mGy (140 kV). When the tube potential was constant at 120 kV, the CTDIair ranged from 6.084 mGy (100 mAs) to 14.604 mGy (250 mAs). For 16-slice CT scanner using TLD-100, when the tube current was constant at 100 mAs, the CTDIair ranged from 1.924 mGy (80 kV) to 4.624 mGy (130 kV). When the tube potential was constant at 110 kV, the CTDIair ranged from 2.456 mGy (50 mAs) to 6.493 mGy (200 mAs). From OSL dosimeter, when the tube current was constant at 100 mAs, the CTDIair ranged from 0.747 mGy (80 kV) to 1.649 mGy (130 kV). When the tube potential was constant at 110 kV, the CTDIair ranged from 1.058 mGy (50 mAs) to 3.833 mGy (200 mAs). For both MSCT scanners, the SSDP plotted using OriginPro 9.0 software showed that peaks in which the tube current was varied were higher and sharper than the SSDP peaks when the tube potential was changed. The full width at half maximum (FWHM) obtained from the SSDP presented the nominal beam width set for the MSCT scanner. For 128-slice CT scanner, the FWHM of OSLD was more reliable as it coincided with the nominal beam width which was 38.4 mm while for 16-slice CT scanner, the FWHM of TLD-100 was more reliable as it coincided with the nominal beam width which was 10.0 mm. The results showed good agreement between FWHM and nominal beam width. In conclusion, the X-ray tube output generated from the MSCT scanners was in line with the CT protocol setup.