Reduction of effective and organ dose Reduction of effective and organ dose

to the eye lens in cerebral MDCT scans to the eye lens in cerebral MDCT scans

using iterative image reconstructionusing iterative image reconstruction

Zizka J, Jandura J, Kvasnicka T, Klzo L, Grepl JZizka J, Jandura J, Kvasnicka T, Klzo L, Grepl J

Dept. of RadiologyDept. of RadiologyCharles University Teaching HospitalCharles University Teaching Hospital

Hradec KraloveHradec KraloveCzech RepublicCzech Republic

ECR 2012ECR 2012

IntroductionIntroduction

• medical radiation exposure steadily rises medical radiation exposure steadily rises

• CT accounts for CT accounts for > 2/3 > 2/3 cumulative cumulative exexposureposure

• it reached the level of natural radiation background it reached the level of natural radiation background

in the USAin the USA

• ALARAALARA

• dose reductiondose reduction

– iterative reconstructioniterative reconstruction

Source:www.oecd.org

Iterative reconstruction (IR)Iterative reconstruction (IR)

• significantly lower image noisesignificantly lower image noise

• radiation dose reduction (25 – 60 %)radiation dose reduction (25 – 60 %)

FBPFBP IR IRfull dosefull dose 50 % dose 50 % dose

PurposePurpose

• 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 MethodsMaterial and Methods

• 400 routine adult brain CT exams:400 routine adult brain CT exams:– 200 with FBP200 with FBP

– 200 with IRIS200 with IRIS

• single source MDCT scannersingle source MDCT scanner– collimation: 64 x 2 x 0.6 mmcollimation: 64 x 2 x 0.6 mm

– tube voltage: 120 kV (FBP tube voltage: 120 kV (FBP && IRIS) IRIS)

– reference mAs: reference mAs: 300300 (FBP) and (FBP) and 200200 (IRIS) (IRIS)

– rotational period 1 srotational period 1 s

– pitch 0.55pitch 0.55

– automatic dose modulation switched onautomatic dose modulation switched on

Material and MethodsMaterial and Methods• recorded wererecorded were

– CT Dose Index (CTDIvol) CT Dose Index (CTDIvol)

– Dose Length Product (DLP)Dose Length Product (DLP)

– effective dose calculated by ImPACT softwareeffective dose calculated by ImPACT software

– organ dose to the lens was derived from the actual tube current-time product organ dose to the lens was derived from the actual tube current-time product

value applied to the slices with lens includedvalue applied to the slices with lens included

– image noise (SD) in a 1.5 cmimage noise (SD) in a 1.5 cm22 ROI in the centrum semiovale ROI in the centrum semiovale

• qualitative visual assessment: two experienced radiologists blinded qualitative visual assessment: two experienced radiologists blinded to the type of image reconstruction using a visual analogue scaleto 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 doseResults: 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 %

CT

DI

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Gy

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200

4

- 36 %

- 58 %

Results: image qualityResults: image quality

FBP IRIS p-value

image noise [median]

3.9 4.2 < 0.01

image quality [mean]

2.11 2.170.17

(kappa = 0.76)

FBPFBP IRIS IRIS dose reduced by 34 %dose reduced by 34 %

Image qualityImage quality

DiscussionDiscussion

ICRP ref 4825-3093-1464

https://rpop.iaea.org/RPOP/RPoP/Content/News/ICRP-statement-threshold-eye-lens.htmhttps://rpop.iaea.org/RPOP/RPoP/Content/News/ICRP-statement-threshold-eye-lens.htm

• eye lens radiosensitivity is higher than previously thought eye lens radiosensitivity is higher than previously thought • the lens is frequently exposed to the primary beam in the lens is frequently exposed to the primary beam in

spiral MDCT scansspiral MDCT scans• the organ dose to the lens leading to cataract formation the organ dose to the lens leading to cataract formation

(0.5 Gy) might be acquired in as low as 7 (!) non-(0.5 Gy) might be acquired in as low as 7 (!) non-optimized CT head scans (CTDI ≈ 60 mGy)optimized CT head scans (CTDI ≈ 60 mGy)

• with optimized iterative reconstruction acquisition the with optimized iterative reconstruction acquisition the number of CT head scans leading to lens opacities number of CT head scans leading to lens opacities formation rises approx. 3-fold (20 MDCT head scans)formation rises approx. 3-fold (20 MDCT head scans)

DiscussionDiscussion

• even in the settings of optimized FBP based CT even in the settings of optimized FBP based CT

protocols, iterative reconstruction is capable of protocols, iterative reconstruction is capable of

further substantial radiation dose savings which, further substantial radiation dose savings which,

in our population, further reduced both the in our population, further reduced both the

effective and eye lens dose by more than 33 % effective and eye lens dose by more than 33 %

compared to optimized FBP protocols and by 58 compared to optimized FBP protocols and by 58

% compared to reference standard% compared to reference standard

ConclusionConclusion