Journal Club: A new look at CT dose measurement: Beyond CTDI

Hope whoever was reading was amused by the last journal club article.

Figuring out how to measure CT does is getting more and more difficult, with the increasing popularity of multi-slice helical CT scanners that can now acquire upwards of 64 slices per gantry rotation and with beam widths approaching 20 or 30 mm. Current methods of measuring CT dose (CTDI) date back to when single slice scanners were the only thing available. So perhaps it’s time for something better and more accurate. As long as it’s easy to do and doesn’t require much more in the way of extra equipment.

Dixon RL, “A new look at CT dose measurement: Beyond CTDI”, Med Phys (30), 1272-1280 (2003)

Abstract:

Equations are derived for generating accumulated dose distributions and the dose line integral in a cylindrical dosimetry phantom for a helical CT scan series from the single slice dose profiles using convolution methods. This exposition will better clarify the nature of the dose distribution in helical CT, as well as providing the medical physicist with a better understanding of the physics involved in dose delivery and the measurement process. Also addressed is the concern that as radiation beam widths for multi-slice scanners get wider, the current methodology based on the measurement of the integral of the single slice profile using a 10 cm long ion chamber (CTDI₁₀₀) may no longer be adequate. It is shown that this measurement would underestimate the equilibrium dose and dose line integral by about 20% in the center of the body phantom, and by about 10% in the center of the head phantom for a 20 mm nominal beam width in a multi-slice scanner. Rather than making the ion chamber even longer to collect the broad scatter tails of the single slice profile, an alternative to the CTDI method is suggested which involves using a small volume ion chamber, and scanning a length of phantom long enough to establish dose equilibrium at the location of the chamber. With a modern CT scanner, such a scan length can be covered in 15 s or less with a helical or axial series, so this method is not significantly more time-consuming than the long chamber method. The method is demonstrated experimentally herein. ©2003 American Association of Physicists in Medicine.