Dosimetry methods for multi-detector computed tomography.

The aim of this study is to compare four dosimetry methods for wide-beam multi-detector computed tomography (MDCT) in terms of computed tomography dose index free in air (CTDI free-in-air) and CTDI measured in phantom (CTDI phantom). The study was performed with Aquilion One 320-detector row CT (Toshiba), Ingenuity 64-detector row CT (Philips) and Aquilion 64 64-detector row CT (Toshiba). In addition to the standard dosimetry, three other dosimetry methods were also applied. The first method, suggested by International Electrotechnical Commission (IEC) for MDCT, includes free-in-air measurements with a standard 100-mm CT pencil ion chamber, stepped through the X-ray beam, along the z-axis, at intervals equal to its sensitive length. Two cases were studied-with an integration length of 200 mm and with a standard polimetil metakrilat (PMMA) dosimetry phantom. The second approach comprised measurements with a twice-longer phantom and two 100-mm chambers positioned and fixed against each other, forming a detection length of 200 mm. As a third method, phantom measurements were performed to study the real-dose profile along z-axis using thermoluminescent detectors. Fabricated PMMA tube of a total length of 300 mm in cylindrical shape containing LiF detectors was used. CTDI free-in-air measured with an integration length of 300 mm for 160 mm wide beam was by 194 % higher than the same quantity measured using the standard method. For an integration length of 200 mm, the difference was 18 % for 40 mm wide beam and 14 % for 32 mm wide beam in comparison with the standard CTDI measurement. For phantom measurements, the IEC method resulted in difference of 41 % for the beam width 160 mm, 19 % for the beam width 40 mm and 18 % for the beam width 32 mm compared with the method for CTDI vol. CTDI values from direct measurement in the phantom central hole with two chambers differ by 20 % from the calculated values by the IEC method. Dose profile for beam widths of 40, 32 and 16 mm, and analysis and conclusions are presented.