National Physical Laboratory

X-ray Computed Tomography

Driven by the need for quality control of complex three-dimensional engineering components, and enabled by progress in medical imaging and consumer graphics processing, X-ray computed tomography (XCT) has been increasingly used for industrial inspection in the last decade. The most important merit of the technique is that it allows the measurement of the geometry of external (surface) and internal features without the need to dissemble a component. Whilst many XCT instruments are used for non-destructive testing, they are increasingly being used in industry as dimensional metrology tools, akin to the conventional co-ordinate measuring machine.


Recent XCT event

Title: Dimensional X-ray Computed Tomography Conference 2016
Date: 10 May 2016
Venue: National Physical Laboratory

More info: www.npl.co.uk/dxct2016

XCT: 225kV system


XCT system at NPL

The NPL XCT system was installed in 2013. The system employs a cone-beam X-ray source with the maximum energy of 225 kV. Details of the system are as follows:

  • Cone-beam X-ray computed tomography
  • Maximum 225 kV
  • Temperature control of ±1°C
  • 40 cm × 40 cm scintillation detector
  • 200 µm × 200 µm pixel size on detector
  • Magnification up to ~120
  • Spatial frequency: up to 3 µm

Typical penetration depth of materials are given below (an energy of 200 kV is chosen in the estimation):

Material Atomic number Approximate depth
of penetration (cm)
Carbon 6 14.3
Aluminum 13 9.0
Titanium 22 5.0
Iron 26 2.6
Nickel 28 2.1
Copper 29 2.1


Development of XCT reference samples at NPL

NPL has designed a series of reference samples with the aim to:

  • Investigate systematic errors associated with the XCT technology
  • Verify the performance of XCT systems

Reference sample made by selective laser melting:

Reference sample made by selective laser melting - camera image    Reference sample made by selective laser melting - CT image
Camera image CT image


Calibrated two sphere reference sample:

Calibrated two sphere reference sample

Ball plate:

XCT: Ball plate

Hole plate:

Hole plate


Current research projects

  • Investigation of influence parameters and systematic errors
  • Development reference objects for XCT
  • Investigation of geometrical errors of the XCT systems and development of geometrical error model to calibrate the system
  • Metrology for Additive Manufacturing (AM)


Research team

  • Dr Michael McCarthy
  • Mr Stephen Brown
  • Dr Wenjuan Sun
  • Mr Sean Woodward
  • Mrs Nadia Flay (PhD with University of Southampton)
  • Mr Massimiliano Ferrucci (PhD with KU Leuven)
  • Ms Hannah Corcoran (PhD with UCL)


Recent publications

  • Characterisation of a scintillation flat panel detector employed by an X-ray computed tomography system: measurements of MTF, NPS and DQE
    Sun W, Konstantinidis A, Billas I, McCarthy M, Brown S, Corcoran H
    Proceedings of 16th euspen International Conference, Nottingham, UK, 2016
  • Characterisation of X-ray source for X-ray computed tomography for dimensional metrology
    Corcoran H, Sun W, Brown S, Robson S, Speller R, McCarthy M
    International Conference on Industrial Computed Tomography, Wels, Austria
  • Investigation of the focal spot drift in industrial cone-beam X-ray computed tomography
    Flay N, Sun W, Brown S B, Leach R K, Blumensath T
    The e-Journal of Nondestructive Testing, 20, 9 (2015)
  • A preliminary study of characteristics of X-ray computed tomography
    Sun W, Flay N, Brown S, McCarthy M, Corcoran H, Ferrucci M
    Proceedings of Digital Industrial Radiology and Computed Tomography, Ghent, Belgium (2015)
  • Towards geometrical calibration of X-ray computed tomography systems - a review
    Ferrucci M, Leach RK, Giusca C, Dewulf W, Carmignato S.
    Measurement Science and Technology, 26, 092003 (2015)
  • A new method of performance verification for x-ray computed tomography measurements
    Leonard F, Brown S, Withers P, Mummery P, McCarthy M
    Measurement Science and Technology, 25 (2014)
  • Application of the optical transfer function in x-ray computed tomography - a review
    Flay N, Leach R K
    NPL Report ENG 41 (2012)
  • An overview of industrial x-ray computed tomography
    Sun W, Brown S B, Leach R K
    NPL Report ENG 32 (2012)
Last Updated: 26 May 2016
Created: 3 Apr 2013

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