Reduction of effective and organ dose to the

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 – collimation: 64 x 2 x 0. 6 mm – tube voltage: 120 k. V (FBP & IRIS) – reference m. As: 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 Im. PACT 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 cm 2 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 [m. Gy] 33. 29 22. 41 - 32. 7 % DLP [m. Gy. cm] 589. 7 396. 2 -32. 8 % Effective dose [m. Sv] 1. 47 ± 0. 26 0. 98 ± 0. 15 - 33. 3 % Organ dose to the eye lens [m. Gy] 40. 0 ± 3. 3 26. 6 ± 2. 0 - 33. 5 %

European Commission Quality Criteria for MDCT 2004 CTDI = 60 m. Gy - 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/RPo. P/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 m. Gy) • 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