Jacob Geleijns PhD, Presenter: Nothing to Disclose

Marcal Salvado Artells PhD, Abstract Co-Author: Nothing to Disclose

Paul de Bruin PhD, Abstract Co-Author: Nothing to Disclose

Richard Mather PhD, Abstract Co-Author: Employee, Toshiba Corporation

Yoshihisa Muramatsu PhD, Abstract Co-Author: Nothing to Disclose

Michael F. McNitt-Gray PhD, Abstract Co-Author: Member, Physics Committee, American College of Radiology Imaging Network (ACRIN) Consultant, ACRIN ACR Image Metrix Consultant, Rapiscan Systems UCLA Radiological Sciences has a research agreement with Siemens Medical Solutions Research grants from National Cancer Institute (NCI) and National Institute of Biomedical Imaging and Bioengineering (NIBIB).

Computed tomography (CT) dosimetry should be adapted to the fast developments in CT in order to keep pace with new CT technology. Recently, a volume CT scanner that challenges the existing Computed Tomography Dose Index (CTDI) dosimetry paradigm was introduced (Aquilion ONE, Toshiba Medical Systems, Japan). The purpose of this study was to assess the appropriateness of existing CT dose metrics for the volume CT scanner and to suggest new approaches for CT dosimetry.

Dose measurements with a small Farmer type ionization chamber, and 100mm and 300mm long pencil ionization chambers were performed free-in-air. Measurements were also performed in 150mm and 350mm long CT head and CT body dose phantoms with 100mm and 300mm long ionization chambers. To explore effects that cannot be measured with ionization chambers, Carlo calculations of the dose distribution in 150mm, 350mm and 700mm long CT head and CT body phantoms were performed. To overcome inconsistencies in the definition of CTDI, doses were also expressed as CT_Dose100 (CT_Dose100=160/100 x CTDI100).

Measurements free-in-air revealed good-to-excellent correspondence between CTDI300air and CT_Dose100air; CTDI100air substantially underestimates CTDI300air. Measurements in phantoms and Monte Carlo calculations revealed good correspondence between CTDI300w, CT_Dose100w and CTDI600w; CTDI100w substantially underestimates CTDI300w. Measurements at different positions within CT dose phantoms with an ionization chamber that is smaller than the cone beam are fundamentally different compared to readings of pencil chambers that are longer than the cone beam.

CT_Dose100air and CT_Dose100w are pragmatic metrics for characterizing the dose of the volume CT scanner; these quantities can be measured with the widely available 100mm pencil ionization chambers and 150mm long CT dose phantoms. CTDI300air and CTDI300w in a 350mm long CT dose phantom can serve as appropriate standards of reference for characterizing the dose of the volume CT scanner. Geometrical considerations explain the fundamental differences of different CT dose metrics when measurements are performed at different position within CT dose phantoms.

CT_Dose100 is a pragmatic metric for characterizing the dose of the volume CT scanner, CTDI300 can serve as an appropriate and accurate standard of reference.

Geleijns, J, Salvado Artells, M, de Bruin, P, Mather, R, Muramatsu, Y, McNitt-Gray, M, Dose Descriptors and Dose Assessment for a 320-slice Volume CT Scanner.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6014506.html