CT Systems and their standards

Transcription

1 CT Systems and their standards Stephen Brown Engineering Measurement 11 th April 2012 Industrial X-ray computed tomography: The future of co-ordinate metrology? Burleigh Court, Loughborough University

2 An overview of this presentation Functions of a standard Do we need standards How do we design a standard Some standards bodies What standards are available A sample of associated standards Calibration or Verification The standards Present and Future

3 Function of a standard? Desirable Interoperability/Compatibility Quality Variety reduction/optimisation Information/Measurement Undesirable Good practice guide Instrument specific PhD Thesis Theory of how it works One persons opinion/bias Patented information

4 Do we need a standard? Simple question, simple answer: Yes We need confidence in the measurements we take We need confidence in the measurements someone else takes There needs to be agreement There needs to be traceability

5 How do we Design a standard? We need to understand the system What are the controllable variables? For XCT, e.g. its spot size & voltage What are the uncontrollable variables? e.g. vibration & operator interpretation Is it an operator manual only or do we need a calibrated material standard? Where do we get the traceability from? is it robust? does it meet requirements?

6 Some standard bodies ASTM BS ASTM International, formerly known as the American Society for Testing and Materials (ASTM), is a globally recognized leader in the development and delivery of international voluntary consensus standards. Are used around the world to improve product quality, enhance safety, facilitate market access and trade, and build consumer confidence. Put at its simplest, a standard is an agreed, repeatable way of doing something CEN A standard (French: Norme, German: Norm) is a technical document designed to be used as a rule, guideline or definition. It is a consensus-built, repeatable way of doing something. ISO VDI/VDE ISO: 'A document established by consensus and approved by a recognized body that provides for common and repeated use, rules, guidelines or characteristics for activities or their results, aimed at the achievement of the optimum degree of order in a given context'. Association of German engineers/electrical

7 Some different standard

8 Calibration or Verification? What is the difference between them? Verification is a check on performance Calibration provides assurance on specification (and will always lead to a measurement uncertainty) When do we need Calibration/Verification? Stability of instrument/control in manufacturing Legal requirement Recognition of compliance

9 ISO Draft XCT proposal ISO supports CMMs. Currently six parts have been developed. To achieve comparability with the characteristics of coordinate measuring machines with tactile and with optical sensors. Orientated towards tactile sensors, adds the additional aspects when using CT sensors. Based around procedure used for tactile measurements. But, is this the appropriate framework?

10 ISO Two Parts, Principles and Examination practices Principles: A tutorial addressing the principles of X-ray CT as they apply to industrial imaging. A guideline for a consistent set of performance parameter definitions. Examination practice: A guideline for the procedure for preforming CT examination

11 ISO System capability: to produce a 3D image Spatial resolution, PSF Statistical noise, due to number of photons, instrument (sensitivity of array) Artefacts, beam hardening, cupping Apparatus: object, mechanics, computer Source, tube, linear accelerator, isotope Detectors, gas ionization, scintillation

12 ISO A bit of theory: X-ray interaction, Photoelectric effect, Compton scattering, pair production. Transforms (Radon) Lambert s law of absorption, same thickness, same fraction. Matrix inversion, to big for modern computers?. Finite series expansion, iteratively alter the reconstruction matrix - slow Transform method: convolution-back-projection, and direct Fourier transform each image to get 3D

13 ISO Interpretation of Results: Contrast,a function of incident energy, thickness and absorption. %= µ f -µ b /µ b Contrast detail dose, the ability to resolve, Rayleigh criteria. Resolution, finite beam width causes blurring PSF, modulus of ID Fourier transform gives MTF, a measure of system performance. Noise, electronic, scatter. Precision, repeatability, operator

14 ISO Monoch romatic Back Projection Attenuation Transform Radon After glow Filter Maths/ Beam Edge response function PSF KeV,kV,kVp Cupping MTF After glow Radiogram Compton Scattering Voxel

15 ISO Scope CT examination interpretation System configuration Documentation System set-up and optimisation Records, reports and identification of accepted materials Safety conditions Performance measurement Precision and bias

16 ISO Configuration: Source; generation Synchrotron, tube, isotope. Detector system; single sensor, linear, 2D array. Computer; capacity, graphics. Software; Speed, algorithms, GUI Operator; repeatability, interpretation

17 ISO Documentation: Qualification; list of features, is it up to the job. Scan plan; objects, scan parameters and performance measurements Acquisition parameters; source energy, intensity, current. Reconstruction parameters Display parameters Image analysis

18 ISO System set-up and optimization System set-up; Consideration to; specimen size an composition, resolution. Source set-up; Health and safety, scatter, beamhardening, spot size. Image quality; Contrast sensitivity, intensity, integration time typical 14% transmission Artefacts; where the signal can go wrong,

19 ISO Performance Interpretation Reports Safety Regular assessment Operator Record Leakage Resolution, Contrast, etc Software

20 BS EN Part 1 - Terminology Part 2 Principle, equipment & samples Part 3 Operation & interpretation Part 4 - Qualification

21 ASTM E Although written by ASTM, this guide is the same as ISO part 1

22 ASTM E Examination This practice provides the approach for preforming CT examination System configuration, source, focal spot, detector, manipulation, reconstruction Documentation, scan plan, reconstruction Performance measurement, regular testing of phantom A general overall picture of CT systems

23 ASTM system performance Test method for spatial resolution and contrast sensitivity Quality of CT image degraded by, geometric unsharpness and random noise Object, disk phantom, a right cylinder of homogeneous material same as object Results, ERF, differentiate LRF or PSF, Fourier transform and normalise MTF Contrast discrimination function, CDF taken from tiled pattern at the centre of the disk

24 Conclusion Current standards give a robust insight to CT systems Quality of systems covered by MTF There is a void of traceability for metrology Material standards being developed Specification standards under construction

25 The End Thanks for listening