| Popis: |
We have developed a four-dimensional CT (4D CT) using continuous rotation of cone-beam x-ray. The maximum nominal beam width of the 4D CT is 128 mm at the center of rotation in the longitudinal direction. In order to obtain appropriate estimations of exposure dose, detailed single-slice dose profi les perpendicular to the rotation axis including scattered radiation were measured in PMMA cylindrical phantoms, which were cylindrical lucite phantoms of 160 mm and 320 mm diameter and 900 mm length. Dose profi les were measured with a pin photodiode detector at the center and a peripheral point of 10 mm depth. A pin silicon photodiode sensor with 3 × 3 × 3 mm sensitive region was used as an x-ray detector, which was scanned along longitudinal direction in the phantom for beam widths of 20, 42, 74, 106 and 138 mm. The dose profi les had long tails caused by scattered radiation more than 200 mm out of the beam width edge. The exposure dose covered 95 % was distributed along about 360 mm length at the center and about 310 mm at the periphery, which was independent of the beam width. Before the advent of multi-detector CT, CTDI100 was used to approximate integral dose for clinical scan conditions. However, for 4D CT employing a variable beam width, the standard CTDI was not a good estimation. This work was carried out to establish a method of the dose measurements including scattered radiation for cone-beam CT such as 4D CT. In order to perform the dose assessment including scattered radiation, dose measured length should be recommended to measure integral dose over beam widths plus at least 230 mm, which covered 95 % total exposure dose. |
