Optimisation of Chest Computed Tomography Using a Phantom: Impact of mAs and Reconstruction Techniques on Image Quality C.S. Reis1; T. Faqir2; V. Harsaker3; P. Hogg 2; L. Kristoffersen3; I.L. van Rein4; K. Stancombe2; N.C. Warmerdam4; C. Wergeland3

Outline Introduction Objectives Methods Results & Discussion Conclusion Further work

Introduction

Optimisation is needed Introduction Methods Results/Discussion Conclusion Further Work Why CT Optimisation is needed Highest growth(1) Used in screening programs(2) High effective dose(3) 1.Commission E. EUROSTAT - Your key to European statistics [Internet]. Eurostat. 2015 [cited 2015 Aug 5]. p. Needs for health care. Available from: http://ec.europa.eu/eurostat/web/health/health-care/data/database 2. Schmidt CW. CT scans: balancing health risks and medical benefits. Environ Health Perspect. 2012;120(3):118–21. 3. Team TNLSTR. Reduced Lung-Cancer Mortality with Low-Dose Computed Tomographic Screening. N Engl J Med [Internet]. 2011 Aug 4;365(5):395–409. Available from: http://www.nejm.org/doi/abs/10.1056/NEJMoa1102873

How to reduce dose Exposure Parameters Reconstruction techniques Introduction Methods Results/Discussion Conclusion Further Work How to reduce dose Exposure Parameters Reconstruction techniques Filtered back projection (FBP) Iterative Reconstructions (IR) 1-Klink T, Obmann V, Heverhagen J, Stork A, Adam G, Begemann P. Reducing CT radiation dose with iterative reconstruction algorithms: The influence of scan and reconstruction parameters on image quality and CTDIvol. Eur J Radiol [Internet]. Elsevier Ireland Ltd; 2013;83(9):1645–54. Available from: http://dx.doi.org/10.1016/j.ejrad.2014.05.033 2-Willemink MJ, De Jong P a., Leiner T, De Heer LM, Nievelstein R a J, Budde RPJ, et al. Iterative reconstruction techniques for computed tomography Part 1: Technical principles. Eur Radiol. 2013;23(6):1623–31.

Introduction Methods Results/Discussion Conclusion Further Work Objectives To verify if the mAs variation affects detection of anatomical details To verify if reconstruction techniques affects the observation of anatomical details

Methods

Phases of Research Image Acquisition Image Analysis Phantom – LUNGMAN Introduction Methods Results/Discussion Conclusion Further Work Phases of Research Image Acquisition Phantom – LUNGMAN Positioning: supine, head-first Siemens Somatom Definition AS 128 slice CT scanner Exposure parameters: 3 different fixed mAs values (10, 20, 30); 120 kVp, 1.2 pitch, 0.6 mm slice thickness and 512 x 512 matrix 4 reconstruction techniques: FBP or SAFIRE level 1, 3, and 5 Image Analysis

Phases of Research Image Acquisition VISUAL: Anatomical criteria(1) Introduction Methods Results/Discussion Conclusion Further Work Phases of Research The subjective anatomical visualisation for each structure was rated using a 4-point Likert scale as: 1 - It is not visible 2 - I can see it partially 3 - I can see it 4 - It is clearly defined The subjective noise with a 5-point Likert scale: 1 - very poor: excessive noise or poor vessel wall definition 2 - poor: poor vessel wall definition and prominent image noise 3 - adequate: some image noise, vessel walls definition is minimal 4 - good: minimal image noise definition of vessel walls are visible 5 - very good: excellent definition of vessel walls, limited perceptual image noise. The subjective overall image quality using a 5-point Likert scale: 1 - very poor: poor image quality due to artefacts, no definition between anatomical structures 2 - poor; prominent artefacts, minimal definition between anatomical structures 3 - adequate: minor artefacts present, definition between anatomical structures 4 - good: no perceptual artefacts present, clear definition between anatomical structures 5 - very good: no perceptual artefacts present, total definition between anatomical structures. Image Acquisition VISUAL: Anatomical criteria(1) Noise Global IQ Wilcoxon-test with Bonferroni correction - statistical differences between IQ for all images Image Analysis Commission E. European Guidelines on Quality Criteria for Computed Tomography European Guidelines on Quality Criteria [Internet]. Menzel H, Jessen K, Panzer W, Shripton P, Tosi G, editors. Europe. European Commission´s Radiation Protection Actions; 1999. 1-71 p. Available from: http://www.msct.info/CT_Quality_Criteria.htm

Phases of Research Image Acquisition OBJECTIVE Measurements on ImageJ Introduction Methods Results/Discussion Conclusion Further Work Phases of Research Manning DJ, Ethell SC, Donovan T. Detection or decision errors? Missed lung cancer from the posteroanterior chest radiograph. Br J Radiol [Internet]. 2004 Mar;77(915):231–5. Available from: http://www.birpublications.org/doi/abs/10.1259/bjr/28883951 Image Acquisition OBJECTIVE Measurements on ImageJ Edges of anatomical structures Pixel values in a plot profile(6). Trend line was added to the linear points of the plot profile Edge gradient was calculated Image Analysis

Results & Discussion

Visualisation of Anatomical Structures Introduction Methods Results/Discussion Conclusion Further Work Visualisation of Anatomical Structures RT and mAs: variation in visibility within the acceptance level > variation for 10 mAs and FBP in axial images Not significant (Wilcoxon Test p = 0.491; p = 0.124; p = 0.384) To analyse the anatomical structures globally we added the scores to achieve a global analysis of the anatomical structures and as you can see (line on the graph) the acceptable score to have total visualization of the structures is 12. Almost all images have this score except at 10 mAs w/ FBP in axial images. The best results were observed at 20 mAs w/ SAFIRE5. However, these variations and these differences are not statistically significant.

Introduction Methods Results/Discussion Conclusion Further Work Noise and Overall IQ No significant difference Wilcoxon Signed Rank Test (FBP with SAFIRE 1, 3 and 5 respectively: p = 0.405; p = 0.251; p = 0.083; SAFIRE 1,3 and 5: p = 0.046; p = 0.926). The same results were obtain for the other two parameters that we ask to the observers to analyse: Noise and overall IQ.

Edge Gradient - Sharpness Introduction Methods Results/Discussion Conclusion Further Work Edge Gradient - Sharpness These results were also confirmed by the analysis of edge gradient. This gradient allows to see how sharp is the edge. So the measure should be around 90º to guarantee that the edge is well visible. With our results It is possible verify that the mAs and RT promote differences in images and the best results were obtained for 20 mAs. BUT (move to the table) when we look to the this table that shows how much different are these differences in edge we can see that the differences are very small. Higher sharpness for 20 mAs – SAFIRE 5 Lower noise, easier detection of edges

Clinical Relevance mAs Introduction Methods Results/Discussion Conclusion Further Work Clinical Relevance mAs Important to study mAs further and observe impact on image quality Clinical indication Real anatomical structures and lesions can be affected in different ways depending on clinical indication This study showed that visualisation of anatomical structures was possible even at a low mAs value of 20, partial visibility was made at 10 mAs, therefore future research needs to consider mAs and other clicical indications because the diferent characteristics of lesions and anatomy can affect the visualisation

Dose (DLP) Comparison with the European Guidelines: Introduction Methods Results/Discussion Conclusion Further Work Dose (DLP) Comparison with the European Guidelines: The dose that was used to produce the Dose reduction of 91.1% can be achieved whilst maintaining visualisation of anatomical structures

Introduction Methods Results/Discussion Conclusion Further Work Limitations Phantom Artefacts cannot be simulated Lesion variety Anatomical variations Patient size Observers with different clinical experience – impacts analysis (1) Kakinuma R, Ashizawa K, Kobayashi T, Fukushima A, Hayashi H, Kondo T, et al. Comparison of sensitivity of lung nodule detection between radiologists and technologists on low-dose CT lung cancer screening images. Br J Radiol [Internet]. 2012 Sep;85(1017):e603–8. Available from: http://www.birpublications.org/doi/abs/10.1259/bjr/75768386

Conclusion

Introduction Methods Results/Discussion Conclusion Further Work How does the mAs and reconstruction techniques affect the detection of anatomical structures in lung CT Detection of anatomical structures was largely unaffected by reconstruction techniques and mAs variations

Further work

Introduction Methods Results/Discussion Conclusion Further Work Next step To verify if there is a relationship between visual and physical measures of image quality Gain a better understanding of visual and clinical value of reconstruction techniques at lower doses Bigger variety in clinical indications, patient size and exposure parameters.

Acknowledgments Ruurd Visser for support - statistical analysis 5 observers Staff of Hanze University and UMCG that helped us acquire the images for this research

Thank you for your attention! christinewe@live.no