Quantitative and Qualitative Assessment of Thorax Cone Beam CT Image Quality across Multiple Imaging Systems

Transcription

1 Quantitative and Qualitative Assessment of Thorax Cone Beam CT Image Quality across Multiple Imaging Systems Matthew Williams Pre-registration Clinical Scientist Velindre NHS Trust, Cardiff

2 Computed Tomography (CT) Using x-rays to obtain 3D anatomical information Often viewed in cross-sectional slices

3 Cone Beam Computed Tomography (CBCT) Conventional (Helical) CT: multiple rotations of a fan beam while the patient moves through the scanner CBCT: Single rotation of a cone beam around a static patient (Rotation time 1 minute) CT CBCT

4 Cone Beam Computed Tomography (CBCT) Treatment beam source CBCT detector CBCT source Isocentre

5 Cone Beam Computed Tomography (CBCT) 3D images on set allows soft tissue matching with the planning CT Measurement and correction of patient setup error

6 Background to Investigation CBCT systems at Velindre: Varian TrueBeam STx with OBI (v2.0) Varian Trilogy with OBI (v2.1) 4x Elekta Agility with XVi (v4.5) Following TrueBeam commissioning Thorax CBCT scans thought to be of poorer quality than Elekta XVi Predominantly due to streaking artefacts Concern over use for SABR Lung treatments (high dose per fraction to small volumes)

7 Clinical Example Slice from TrueBeam OBI Thorax CBCT

8 Summary of Investigations Trial of new protocols on the TrueBeam, including varying scan speed Recalibration of TrueBeam OBI Small two patient qualitative study (TrueBeam OBI vs Trilogy OBI) Quantitative assessment with static Catphan phantom comparing all three systems Dynamic phantom investigation More comprehensive qualitative study of clinical images across the three systems

9 Anthropomorphic Phantom Streaking artefacts not present in TrueBeam OBI images of static Thorax phantom (Rando)

10 Motion Artefacts Quasar phantom scanned on all three systems Low density lung insert with higher density rod half way through, driven by motor Consistent slice location and window and level settings displayed

11 Motion Artefacts: Quasar Phantom TrueBeam OBI: Static phantom, 1 min scan

12 Motion Artefacts: Quasar Phantom TrueBeam OBI: 10 BPM, 1 min scan

13 Motion Artefacts: Quasar Phantom TrueBeam OBI: 10 BPM, 2 min scan

14 Motion Artefacts: Quasar Phantom TrueBeam OBI: 10 BPM, 4 min scan

15 Motion Artefacts: Quasar Phantom Trilogy OBI: 10 BPM, 1 min scan

16 Motion Artefacts: Quasar Phantom XVi: 10 BPM, 1.5 min scan

17 Motion Artefacts: Quasar Phantom XVi: 10 BPM, 2 min scan

18 Motion Artefacts: Quasar Phantom TrueBeam XVi

19 Artefact Analysis/Quantification Matlab script to measure a profile around a semicircle

20 Artefact Analysis/Quantification Background non-uniformity apparent Filtered low and high frequencies in the profiles with code written in Python

21 Artefact Analysis/Quantification

22 Artefact Analysis/Quantification Peak detection

23 Artefact Analysis/Quantification Relative CT Nu umber Average Peak - Average Trough TrueBeam OBI Trilogy OBI XVi Scan Time / mins

24 Artefact Analysis/Quantification Relative CT Number Max Peak - Min Trough TrueBeam OBI Trilogy OBI XVi Scan Time / mins

25 Qualitative Study Eight patients scanned with Thorax CBCT protocol from each of the TrueBeam OBI, Trilogy OBI and XVi Images from third fraction Anonymised and randomised Assessed by Imaging Specialist Radiographers Displayed alongside planning CT including PTV (Planning Target Volume) outline Used two preset, consistent window and level settings for Lung and Soft Tissue evaluation (equivalent settings for OBI and XVi)

26 Qualitative Study

27 Qualitative Study Image Quality for matching Five point scale 1 unacceptable 2 substandard 3 acceptable 4 above average 5 superior Assessment of specific anatomy Assessment of overall image quality, and comments

28 Definition of areas for scoring Base of Lung: Periphery Aorta Base of Lung: Midline

29 Definition of areas for scoring Mid Lung: Periphery Oesophagus Mid Lung: Midline

30 Average Results for Specific Anatomy 5 XVi Trilogy OBI TrueBeam OBI 4 Me ean Score Acceptable in 5-point scale 0 Mid: Midline Mid: Peripheral Base: Midline Lung Base: Peripheral Apex Aorta Oeso - phagus Tumour

31 Average Results for Overall Image Quality Metrics 5 4 XVi Trilogy OBI TrueBeam OBI Mean Score Acceptable in 5-point scale 0 Sharpness Contrast Overall impact Artefacts Worst area Overall IQ for matching

32 Qualitative Study: Results Compared systems using Mann-Whitney test on the scores for each metric for each observer No significant (p<0.05) difference between any systems for any metric for either user

33 Qualitative Analysis Some examples of comments made about especially good or poor images (compared with the equivalent CT slice) Patient related factors (e.g. size) often reason for low/high score

34 Qualitative Analysis Tumour not well defined. Lots of streaking artefacts around the bronchi resulting in poor midline scores.

35 Qualitative Analysis Very good for SABR as tumour shape is identical to viewed on CT. Great detail around the bronchi.

36 Qualitative Analysis The artefacts at the base, shadowing the liver, are the reason for the low scores, as well as the circular artefact.

37 Conclusions & Further Work Image Quality must allow matching of CBCT to CT with confidence No stages of the investigation to date indicate a significant difference in image quality between any of the systems Follow up studies of this nature may be necessary

38 Conclusions & Further Work Work towards better harmonisation of protocols, including reconstruction filters, preset window and level settings, etc. Development of patient size dependent protocols Consideration of motion at commissioning in Consideration of motion at commissioning in future

39 Acknowledgements Phil Wheeler Clinical Scientist Sam Hall and Lisa Davies Imaging Specialist Radiographers Leo Garcia Pre-Registration Clinical Scientist