This poster is published under an
open license. Please read the
disclaimer for further details.
Keywords:
Radioprotection / Radiation dose, Lung, CT, CT-Quantitative, Experimental investigations, Comparative studies, Technology assessment
Authors:
J. H. Kim, K. B. Lee; Seoul/KR
DOI:
10.1594/ecr2015/C-0891
Conclusion
Radiation exposure due to CT examinations takes 11% in total radiation dose of medical area and take 67% of total radiation dose in population[3].
In these reasons,
radiation dose of CT is long-standing issue.
Manufacturers of CT have been made effort to reduce radiation exposure by developing various technologies,
AEC is also developing in same way.
Usefulness of AEC system was proved by advanced studies.
Lehmann KJ showed rotational AEC can reduce radiation dose almost 20% with information about geometrical structure of patient[11].
In phantom study of on-line angular modulation,
Gies M showed on-line angular modulation technique can reduce radiation dose nearly 50% compared to conventional angular modulation[12].
Dose modulation with AEC is varied for each manufacturers in ways of combination of parameters like patient size,
thickness change due to tube angle,
z-axis perception[4,8,9].
Topogram is basic information in CT scan.
Through this image,
CT user can determine scan range according to prescription.
In aspect of scanner,
this image take major role in setting radiation dose.
With the advances in AEC techniques,
an online modulation technique has been developed that does not require the information provided by topograms.
This technique calculates the modulation function data (an objective image quality parameter) from the online patient attenuation.
These data are sent to the generator control for dose modulation with a delay of 180° from the x-ray generation angle.
Thus,
the system makes use of attenuation data from the previous rotation and modulates tube current to accommodate patient attenuation “on the fly” [5,6].
In angular dose modulation,
more dose modulations occur in asymmetric regions and the variation in image noise throughout the examination can be minimized.
This rotational AEC is also helpful in reducing photon starvation artifacts,
especially in the shoulder[5].
Advanced study in 2011,
Kwan-Joong Park showed that If the part of scan range is not contained in topogram,
even if scan range is same,
radiation dose increase nearly 15.6% compared to case with normal topogram[7].
In this study,
radiation dose is increased when using shoulder cut topogram,
too.
However,
when we use liver cut topogram,
radiation dose is decreased.
It is considered that the scanner keep applying former mAs if information from topogram is insufficient.
In this case,
it seems that on-line modulation(real time feedback) doesn't operate[10].
We think that this is happened because large z-axis coverage ends scan before mAs is changed by on-line modulation.
AEC system reduce radiation dose generally,
but there is possibility to increase dose by applying AEC[10].
CT user should understand AEC of his scanner precisely before apply it to patient.
To conclude,
if CT user use incomplete topogram to scan,
scanners can't react to change of object.
Then radiation dose for patient get abnormally larger or smaller.
Therefore,
don't over-estimate AEC system and take appropriate topogram to exposure patient with proper radiation dose according to examinations.
If topogram is not appropriate,
we recommend to retake topogram again.
Limitation First,
In our study,
We only compare three different scanner except for Tosiba CT.
In fact,
we examed a Tosiba scanner for our study.
But,
The AEC function of this scanner is poor due to very wide detector row.
We think there are few chance to fluctuate the mAs during the scan.
So,
that scanner is exclude for our study and we can’t compare the AEC function in all types of CT.
Second,
we can’t unify our chest protocol.
It’s very difficult to fit same parameter because of different machine and different AEC mode.