Journal of Medical Physics
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ABSTRACT
Year : 2017  |  Volume : 42  |  Issue : 5  |  Page : 49
 

Trade Talks



Date of Web Publication24-Oct-2017

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How to cite this article:
. Trade Talks. J Med Phys 2017;42, Suppl S1:49

How to cite this URL:
. Trade Talks. J Med Phys [serial online] 2017 [cited 2020 Nov 24];42, Suppl S1:49. Available from: https://www.jmp.org.in/text.asp?2017/42/5/49/217115



   TR-1: Precision and Uncertainties in CTDI Measurements on Axial computed tomography Scans Top


Erik Wikstrom, Soren Sören Sturesson

RTI Group AB, Mölndal, Sweden.

E-mail: [email protected]

Introduction: The Computed Tomography Dose Index (CTDI) formalism has been used for quality controls on computed tomography (CT) systems for many years. Concerns has been raised weather the CTDI formalism is a relevant parameter for today's CT technology such as wider beams, helical scans etc, when the CTDI formalism is based on measurements on Axial scans and relatively narrow beam widths.

A 100 mm pencil ion chamber have been used to evaluate uncertainties in traditional CTDI measurements, and it's use for comparison to CTDI values for helical scans. E.g. can the CTDIvol value that is displayed on the CT console for a helical scan be validated by measure on an Axial scan?

Methods: A 100 mm long pencil ionization chamber were used to measure the CTDI100,c and CTDI100,p in a PMMA CD dose phantom. The variation in measured results were studied to evaluate different influence parameters that contributes to the CTDI calculations.

Result: Major variations in CTDI100,p were found. The reason for the variation is that the detector, when paced in a peripheral position of the phantom, may randomly become irradiated twice during one axial scan. This is due to that the radiation may be present for more than 360° of rotation. The CT machine that was used in this study had the radiation present for 380° at a one second axial scan.

Table attenuation when placing the phantom on the patient table is a major influence parameter when measured CTDI values is compared to the the console values.

CTDIvol values that were shown on the console for helical scans could not be directly compared to CTDI values measured in Axial scan, due to that the same technical parameters were not selectable, such as collimation. Also the presence of “over-rotation” in axial scans makes the comparison difficult.

Discussion: There are many factors that effects the measurement uncertainty when measure CTDI on axial scans using a pencil ion chamber. Measurement errors and uncertainties can be reduced by making a lot of exposures followed by careful evaluation of measured data. Then compensation for various generator characteristics can be applied. The measured CTDI value on an Axial scan has a limited relevance when used to evaluate the CTDIvol for a helical scan.

By once carefully establishing the relation between the CTDI100,c and CTDI100,p, the CTDIw can be estimated with sufficient precision saving a lot of time, by significantly reducing number of exposures.

Conclusions: New methods for evaluating CT doses are of importance. Not only for wide beams and helical scans, but also for Axial scans.




 

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