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Dose equations for shift‐variant CT acquisition modes using variable pitch, tube current, and aperture, and the meaning of their associated CTDI vol
Author(s) -
Dixon Robert L.,
Boone John M.,
Kraft Robert A.
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4897246
Subject(s) - convolution (computer science) , aperture (computer memory) , scanner , physics , current (fluid) , modulation (music) , imaging phantom , nuclear medicine , optics , mathematics , computer science , medicine , acoustics , artificial neural network , thermodynamics , machine learning
Purpose: With the increasing clinical use of shift‐variant CT protocols involving tube current modulation (TCM), variable pitch or pitch modulation (PM), and variable aperture a ( t ), the interpretation of the scanner‐reported CTDI vol is called into question. This was addressed for TCM in their previous paper published by Dixon and Boone [Med. Phys. 40 , 111920 (14pp.) (2013)] and is extended to PM and concurrent TCM/PM as well as variable aperture in this work. Methods: Rigorous convolution equations are derived to describe the accumulated dose distributions for TCM, PM, and concurrent TCM/PM. A comparison with scanner‐reported CTDI vol formulae clearly identifies the source of their differences with the traditional CTDI vol . Dose distribution simulations using the convolution are provided for a variety of TCM and PM scenarios including a helical shuttle used for perfusion studies (as well as constant mA)—all having the same scanner‐reported CTDI vol . These new convolution simulations for TCM are validated by comparison with their previous discrete summations. Results: These equations show that PM is equivalent to TCM if the pitch variation p ( z ) is proportional to 1/ i ( z ), where i ( z ) is the local tube current. The simulations show that the local dose at z depends only weakly on the local tube current i ( z ) or local pitch p ( z ) due to scatter from all other locations along z , and that the “local CTDI vol ( z )” or “CTDI vol per slice” do not represent a local dose but rather only a relative i ( z ) or p ( z ). The CTDI‐paradigm does not apply to shift‐variant techniques and the scanner‐reported CTDI vol for the same lacks physical significance and relevance. Conclusions: While the traditional CTDI vol at constant tube current and pitch conveys useful information (the peak dose at the center of the scan length), CTDI vol for shift‐variant techniques (TCM or PM) conveys no useful information about the associated dose distribution it purportedly represents. On the other hand, the total energy absorbed E (“integral dose”) as well as its surrogate DLP remain robust (invariant) with respect to shift‐variance , depending only on the total mAs = 〈 i 〉 t 0 accumulated during the total beam‐on time t 0 and aperture a , where 〈 i 〉 is the average current.