Transcript bez titulu - Radiological Society of North America

Reduction of effective and organ dose
to the eye lens in cerebral MDCT scans
using iterative image reconstruction
Zizka J, Jandura J, Kvasnicka T, Klzo L, Grepl J
Dept. of Radiology
Charles University Teaching Hospital
Hradec Kralove
Czech Republic
ECR 2012
Introduction
• medical radiation exposure steadily rises
• CT accounts for > 2/3 cumulative exposure
• it reached the level of natural radiation background
in the USA
• ALARA
• dose reduction
– iterative reconstruction
Source:
www.oecd.org
Iterative reconstruction (IR)
• significantly lower image noise
• radiation dose reduction (25 – 60 %)
FBP
full dose
IR
50 % dose
Purpose
• to compare effective radiation dose and dose to the
eye lens in MDCT brain examinations utilizing
either standard filtered back projection (FBP) or
iterative reconstruction in image space (IRIS)
Material and Methods
• 400 routine adult brain CT exams:
– 200 with FBP
– 200 with IRIS
• single source MDCT scanner
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collimation: 64 x 2 x 0.6 mm
tube voltage: 120 kV (FBP & IRIS)
reference mAs: 300 (FBP) and 200 (IRIS)
rotational period 1 s
pitch 0.55
automatic dose modulation switched on
Material and Methods
• recorded were
– CT Dose Index (CTDIvol)
– Dose Length Product (DLP)
– effective dose calculated by ImPACT software
– organ dose to the lens was derived from the actual tube current-time product
value applied to the slices with lens included
– image noise (SD) in a 1.5 cm2 ROI in the centrum semiovale
• qualitative visual assessment: two experienced radiologists blinded
to the type of image reconstruction using a visual analogue scale
1 = very low noise, optimal diagnostic quality
2 = low noise, good diagnostic quality
3 = increased noise, diagnostic quality
4 = high level noise, limited diagnostic quality
5 = unacceptable noise, non-diagnostic scan
Results: radiation dose
FBP
(n = 200)
IRIS
(n = 200)
Reduced against
FBR by
CTDIvol [mGy]
33.29
22.41
- 32.7 %
DLP [mGy.cm]
589.7
396.2
-32.8 %
Effective dose [mSv]
1.47 ± 0.26
0.98 ± 0.15
- 33.3 %
Organ dose
to the eye lens [mGy]
40.0 ± 3.3
26.6 ± 2.0
- 33.5 %
European Commission Quality Criteria for MDCT 2004
CTDI = 60 mGy
- 36 %
- 58 %
Results: image quality
image noise
[median]
image quality
[mean]
FBP
IRIS
p-value
3.9
4.2
< 0.01
2.17
0.17
(kappa = 0.76)
2.11
Image quality
FBP
IRIS
dose reduced by 34 %
Discussion
ICRP ref 4825-3093-1464
https://rpop.iaea.org/RPOP/RPoP/Content/News/ICRP-statement-threshold-eye-lens.htm
Discussion
• eye lens radiosensitivity is higher than previously thought
• the lens is frequently exposed to the primary beam in
spiral MDCT scans
• the organ dose to the lens leading to cataract formation
(0.5 Gy) might be acquired in as low as 7 (!) nonoptimized CT head scans (CTDI ≈ 60 mGy)
• with optimized iterative reconstruction acquisition the
number of CT head scans leading to lens opacities
formation rises approx. 3-fold (20 MDCT head scans)
Conclusion
• even in the settings of optimized FBP based CT
protocols, iterative reconstruction is capable of
further substantial radiation dose savings which,
in our population, further reduced both the
effective and eye lens dose by more than 33 %
compared to optimized FBP protocols and by
58 % compared to reference standard