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SU‐E‐I‐06: Further Understanding of DLP and DLI as Radiation Dosimetry of Cone‐Beam CT
Author(s) -
Qi Z,
Zhang J,
Alsip C,
Lemen L
Publication year - 2013
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 - 10.1118/1.4814106
Subject(s) - ionization chamber , dosimetry , nuclear medicine , percentage depth dose curve , beam (structure) , dose profile , pencil (optics) , imaging phantom , optics , physics , materials science , ionization , ion , medicine , quantum mechanics
Purpose: Traditional 100mm pencil‐shaped ion‐chamber cannot be directly applied to dose estimation of cone‐beam CT. IEC recommended adjusting traditional CTDI 100 by nominal beam width and then calculating Dose‐Length‐Product (DLP) by multiplying scan length. An alternative approach is to measure dose in the central scan plane using a small volume ionization chamber. If the scan length is sufficiently long to achieve scattered equilibrium, the dose at the central location asymptotically approaches the equilibrium dose . Patient dose can then be estimated using Dose Line Integral (DLI= x scan length). This study aims to further understand these concepts, their limitations and the relationship. Methods: A phantom study was performed on a Toshiba Aquilion One CT scanner (nominal beam width up to 160mm). Three CT head phantoms were joined to capture all possible scattered radiation. First a 100mm ion‐chamber was used to measure CTDI with variable nominal beam width, from 16mm to 160mm. Adjusted CTDI was calculated according to IEC Standards (IEC 60601‐2‐44.ed3). Then an elongated 300mm ion‐chamber was used for the same measurement. For DLI, a 0.6cc Farmer ion chamber was used to measure the equilibrium dose. Results: With the increase of nominal beam width (from 16mm to 160mm), traditional CTDI 100 significantly underestimated CT radiation dose (12.6mGy vs. 7.5mGy). Adjusted CTDI 100 could account for this underestimation (12.6mGy vs. 11.9mGy) in some extent, though the estimation was still low compared to CTDI 300 measurements (approximately 30%). For measurements with Farmer chamber, the method using multi‐scan with a narrow beam width to reach equilibrium scan length overestimated CT dose by approximately 40%. For the same scan length, DLI overestimated CT dose, compared to DLP calculated by CTDI 300 and CTDI 100 . Conclusion: Both DLP and DLI may be used for dose estimation of cone‐beam CT but cannot be exchanged. A conversion factor is needed for cross comparison.