[Intl_DxMedPhys] CTDIw - why 2:1 periphery/center weighting?

[Szczykutowicz, Timothy P]

If you assume a linear dose profile from periphery to center and calculate average value of this function over disc of ctdi phantom you get these 1/3 2/3 weights. Essentially they come from the linear dose assumption. See attached photo, I just did the math.

Amazing this isn't in a paper I could ever find, nor Dixon's book (I waited for a bit before posting so he could respond... I think the last time I brought this up with him he said the math was too easy to include in a book ;) )

[cid:e8eb1e03-0e8f-4650-b9ba-b593f339e50c]


-stick


Timothy P. Szczykutowicz, Ph.D., DABR
Professor
Departments of Radiology, Medical Physics and BME
University of Wisconsin Madison
Cell# 1-716-560-7751<tel:(716)%20560-7751>
Office# 1-608-263-5729
he/him/his



________________________________
From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces at lists.osu.edu> on behalf of Peter Watson, Mr via Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu>
Sent: Monday, April 13, 2026 10:18 AM
To: DxMedPhys List <intl_dxmedphys_wd_osu_list at lists.osu.edu>
Subject: [Intl_DxMedPhys] CTDIw - why 2:1 periphery/center weighting?


WARNING: This email appears to have originated outside of the UW Health email system.
DO NOT CLICK on links or attachments unless you recognize the sender and know the content is safe.

Hi List, I was recently contemplating the definition of CTDIw, and wondered why weighting factors of 2/3 periphery to 1/3 center CTDI100 were chosen. Anyone know the history here? If CTDIw is meant to estimate the average dose in the axial slice,
Hi List,

I was recently contemplating the definition of CTDIw, and wondered why weighting factors of 2/3 periphery to 1/3 center CTDI100 were chosen. Anyone know the history here?

If CTDIw is meant to estimate the average dose in the axial slice, then of course the periphery must have a higher weighting than the center due to geometric arguments (the area of the "periphery " region is larger than the "center" region) but it seems rather arbitrary and simplistic to pick a 2:1 ratio. In a CTDI phantom of radius R, if you consider the "center" region to have radius R/2, and thus the "periphery" to be an annulus of outer-inner radius R-R/2, then the areal ratio would be 3:1 (or ¾ to ¼ periphery to center weighting). Is it one of those things where a 2:1 ratio is "close enough" to experimental measurements and MC simulation data, and easy to remember?

Curious to hear what people know!
-P




Peter Watson, PhD, MCCPM

(masculine pronouns)

Medical Physicist, McGill University Health Centre

Assistant Professor, Medical Physics Unit and Department of Oncology, McGill University
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osu.edu/pipermail/intl_dxmedphys_wd_osu_list/attachments/20260413/71f0ed41/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image.png
Type: image/png
Size: 613485 bytes
Desc: image.png
URL: <http://lists.osu.edu/pipermail/intl_dxmedphys_wd_osu_list/attachments/20260413/71f0ed41/attachment.png>