Pseudohelical scan for the dose profile measurements of 160-mm-wide cone-beam MDCT

Our objective was to determine what the reasonable total phantom length should be for the measurements and determination of length equilibrium, specifically for the Aquilion ONE cone-beam MDCT system.

Radiation dose measurements of a 160-mm-wide cone-beam-MDCT scanner and its radiation dose profile require a different approach than the traditional or conventional method using thermoluminescent dosimeters or small ionization chambers, which have been suggested by some investigators. In order to obtain the radiation dose profile of a cone-beam MDCT, two key elements must be addressed: proper instrumentation for the detection of radiation beam and inclusion of the tails of the radiation dose profile. In this study, a small (2 x 2 x 0.3 mm) solid-state detector was used to measure the dose profile, which required the introduction of a stepping motor to pull the detector through the phantom. Inclusion of the tails of the radiation dose profile meant more than one standard CT dose index phantom would be required to encompass the dose profile tails as much as practically possible. In fact, at minimum, a total of five standard CT dose index (CTDI) phantoms would be required to ensure the entire dose profile is included and detected.

In the case of Toshiba Aquilion ONE MDCT with the maximum beam width of 160 mm, the phantom length that is required for the radiation dose profile measurement should be at least 750 mm, or 5 standard CTDI body phantoms. Current CTDI measurements utilizing 150 mm or 350 mm phantom lengths significantly underestimate the total dose of wide cone-beam MDCT.

The measurement method outlined in this study amounts to an introduction of a new CT dose profile measurement using a pseudohelical scan.