Phantoms in Medical Physics (RT) U. Oelfke. Division of Radiotherapy & Imaging

Transcription

1 in partnership with Phantoms in Medical Physics (RT) U. Oelfke Division of Radiotherapy & Imaging Making the discoveries that defeat cancer

2 1. Introduction

3 What is a phantom? Wiki: Latin Phantasmas Phantasy A phantom is a device which mimics a limited number of aspects of reality.

4 What is a phantom used for? Step 1: Basic hypothesis validation A phantom is used to provide answers to a simplified problem, mostly to validate a hypothesis, often with high accuracy Outcome: Real success, wonderful!

5 What is a phantom used for? Step 1: Basic hypothesis validation A phantom is used to provide answers to a simplified problem, mostly to validate a hypothesis often with high accuracy Outcome: Our favourite but then

6 Step 2: Extrapolation of the validity of the hypothesis to a more complex situation not covered by the phantom Dangerous: Scientist become believers Classical example: MC dose calculations for inhomogeneous patient geometries One cannot disprove it s validity, but one also cannot prove it

7 2. What do we verify with phantoms? Quality - Geometrical accuracy (images, dose distributions, equipment alignment, ) - Dose algorithm accuracy calibration of algorithms - Image quality (contrast, spatial resolution, geometrical accuracy) - Machine QA (dose delivery) - Patient QA - Workflow, Irradiation techniques end to end testing - Calibrations of measurements to required physical quantities -.

8 3. Material Phantoms

9 a) Water and Solid Water

10 Water phantom 10 Automated water phantom Source: PTW web page

11 Water phantom 11 Source: PTW web page

12 Solid Water 12

13 Solid water phantoms 13 Source: PTW web page

14 Hounsfield number calibration phantom 14 Source: SunNuclear web page

15 Mimicking patient anatomy for dosimetry and imaging or end to end a)static a)dynamic

16 Image quality/igrt irradiation geometries

17 CBCT Image quality assessment 17 Contrast Spatial resolution Imaging dose (CT, ) Geometrical accuracy (MRI) Source: phantomlab.com web page cirsinc.com web page

18 EPID phantoms IGRT calibrations 18 Source: PTW.com web page standardimaging.com

19 W. Mao et al.

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21 Determination of 9 parameters Source pos. Detector trans. Detector rot.

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23 Non-ideal projection geometry Real world: projection geometry is non-ideal due to gravitational sag of the imaging hardware determine projection matrix experimentally: calibration phantom alignment at the isocentre sample projection

24 Contrast/resolution phantom Geometrical calibration calibrated Not calibrated

25 kv/mv - Image Quality Images: examples Images: artifacts Images: doses

26 kv-cbct: Contrast phantom 1cGy 2cGy 440 projections over 220 degrees Estimated dose at the isocenter

27 Image Artifacts Courtesy of Jeffrey Siewerdsen Rings & flex Streaks Shading de Frise Metal Lag Truncation Motion

28 In room 3D-imaging MV/kV kv CBCT (cone beam, electron energy: kev,fpi) In room kv-ct (Spiral CT (fan), KeV, ion-chamber) MV CBCT (Cone beam, 6 MeV,FPI) MV-CT (Fan beam, tomo, 3.5 MeV,FPI) IBL ( inline kview, conebeam, 3.5 MeV, C-target,FPI)

29 Siemens Cone beam phantom Contrast slices I,II,III, spatial resolution slice, noise & scaling slice, MTF slice

30 Example: Image quality and dose

31 Geometrical distortions in MRI 31 Source: cirsinc.com phantomlab.com modusqa.com

32 Phantoms in RT 32 Multi modality phantoms (e.g. US/kV/MRI) Source: cirsinc.com web page

33 4. Mimicking patient anatomy for dosimetry

34 Anatomical Phantoms - Patient-sections - Organs (OARS) Geometrical shape (Internal Structure) Patient/Organ materials - e-density, Z, A Detector-compatible Mobility/Elasticity - Deformations - Organ movement - Un-/correlated movement of organs Connectivity to physiological processes - airflow - blood circulation Tumour Models?

35 Alderson Phantom 35 Source: rsdphantoms.com web page

36 Static Lung Phantoms 36 Attempt 1: Source: cirsinc.com web page

37 Attempt 2: Features of the phantom 50 mm Density: 0.27 g cm -3 Randomly filled 0.5 mm 3 structures Built using 3D-printer 50 mm 25 mm x 2

38 Attempt 3: Lung tissue 38

39 Pos 1 Pos 2

40 Experimental Results 90% +117% 90% +52% 10% 10% Phantom leads to degradation of Bragg peak

41 Patient lung? Physiological structure (not just e density) Motion means also change in physiology

42 Dynamic phantoms 42 Source: cirsinc.com modusqa.com

43 Phantoms in RT 43 MR compatible lung phantoms Source: cirsinc.com modusqa.com

44 Realistic lung phantoms? 44 Source: artichest.com web page PSI

45 Patient QA phantoms?? 45 Source: scandidos.com sunnuclear.com ptw.com

46 End to End Phantoms?? 46 Source: cirsinc.com

47 Conclusions Phantoms are an integral and extremely essential component of QA and R&D in medical physics All phantoms represent only limited aspects of reality and the relevance of achieved results needs to be assessed with outmost care Phantom development is an important area of research to advance the development and clinical integration of new imaging and treatment technologies

48 Conclusions So far, (almost) all phantoms are dead no standardized biological or physiological processes are accounted for

49 Wyss Institute Video