SIST EN 16016-2:2012
(Main)Non destructive testing - Radiation method - Computed tomography - Part 2: Principle, equipment and samples
Non destructive testing - Radiation method - Computed tomography - Part 2: Principle, equipment and samples
This European Standard specifies the general principles of computed tomography (CT), the equipment used and basic considerations of sample, materials and geometry.
Zerstörungsfreie Prüfung - Durchstrahlungsverfahren - Computertomographie - Teil 2: Grundlagen, Geräte und Proben
In diesem Dokument sind die allgemeinen Grundlagen der Computertomographie (CT) aufgeführt. Es werden die angewendeten Geräte und die grundsätzlichen Überlegungen zu Proben, Materialien und Geometrie beschrieben.
Essais non destructifs - Méthodes par rayonnements - Tomodensitométrie - Partie 2 : Principes, équipements et échantillons
La présente partie donne les principes généraux de la tomographie informatisée (TI). Elle décrit l'équipement utilisé et les considérations de base relatives à l'échantillon, aux matériaux et à la géométrie.
Neporušitveno preskušanje - Sevalne metode - Računalniška tomografija - 2. del: Osnove, oprema in vzorci
Ta evropski standard določa splošna načela računalniške tomografije (CT), uporabljeno opremo in temeljne zamisli glede vzorcev, materialov in geometrije.
General Information
Relations
Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Zerstörungsfreie Prüfung - Durchstrahlungsverfahren - Computertomographie - Teil 2: Grundlagen, Geräte und ProbenEssais non destructifs - Méthodes par rayonnements - Tomodensitométrie - Partie 2 : Principes, équipements et échantillonsNon destructive testing - Radiation method - Computed tomography - Part 2: Principle, equipment and samples19.100Neporušitveno preskušanjeNon-destructive testingICS:Ta slovenski standard je istoveten z:EN 16016-2:2011SIST EN 16016-2:2012en,fr,de01-marec-2012SIST EN 16016-2:2012SLOVENSKI
STANDARD
SIST EN 16016-2:2012
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 16016-2
August 2011 ICS 19.100 English Version
Non destructive testing - Radiation methods - Computed tomography - Part 2: Principle, equipment and samples
Essais non destructifs - Méthodes par rayonnements - Tomographie numérisée - Partie 2 : Principes, équipementset échantillons
Zerstörungsfreie Prüfung - Durchstrahlungsverfahren - Computertomographie - Teil 2: Grundlagen, Geräte und Proben This European Standard was approved by CEN on 29 July 2011.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre:
Avenue Marnix 17,
B-1000 Brussels © 2011 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members. Ref. No. EN 16016-2:2011: ESIST EN 16016-2:2012
EN 16016-2:2011 (E) 2 Contents Page Foreword .3Introduction .41Scope .52Normative references .53Terms and definitions .54General principles.54.1Basic principles .54.2Advantages of CT .54.3Limitations of CT.64.4Main CT process steps .64.4.1Acquisition .64.4.2Reconstruction .74.4.3Visualisation and analysis .74.5Artefacts in CT images .85Equipment and apparatus .85.1General .85.2Radiation sources .95.3Detectors . 105.4Manipulation . 105.5Acquisition, reconstruction, visualisation and storage system . 106CT system stability . 116.1General . 116.2X-Ray Stability . 116.3Manipulator stability . 117Geometric alignment . 128Sample considerations . 128.1Size and shape of sample . 128.2Materials (including table voltage / thickness of penetration) . 12Annex A (informative)
CT system components . 15A.1Radiation sources . 15A.1.1Open Tube X-ray sets . 15A.1.2Sealed Tube X-ray Sets . 16A.1.3Linear Accelerators . 16A.1.4X-ray target assemblies . 17A.2Detectors . 18A.2.1Ionisation detectors . 18A.2.2Scintillation detectors . 18A.2.3Semiconductor detectors . 19A.3Manipulation . 19A.4Acquisition, reconstruction, visualisation and storage system . 19A.4.1Acquisition system . 19A.4.2Reconstruction system . 20A.4.3Visualisation system . 20A.4.4Storage system . 20Bibliography . 21 SIST EN 16016-2:2012
EN 16016-2:2011 (E) 3 Foreword This document (EN 16016-2:2011) has been prepared by Technical Committee CEN/TC 138 “Non-destructive testing”, the secretariat of which is held by AFNOR. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by February 2012, and conflicting national standards shall be withdrawn at the latest by February 2012. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights. EN 16016 consists of the following parts: Non destructive testing Radiation methods Computed tomography Part 1: Terminology; Non destructive testing Radiation methods Computed tomography Part 2: Principle, equipment and samples; Non destructive testing Radiation methods Computed tomography — Part 3: Operation and interpretation; Non destructive testing Radiation methods Computed tomography — Part 4: Qualification. According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and the United Kingdom. SIST EN 16016-2:2012
EN 16016-2:2011 (E) 4 Introduction This document gives guidelines for the general principles of X-ray computed tomography (CT) applicable to industrial imaging (in the context of this standard, industrial means non-medical applications); it also gives a consistent set of CT performance parameter definitions, including how these performance parameters relate to CT system specifications. This document deals with computed axial tomography and excludes other types of tomography such as translational tomography and tomosynthesis. SIST EN 16016-2:2012
EN 16016-2:2011 (E) 5 1 Scope This European Standard specifies the general principles of computed tomography (CT), the equipment used and basic considerations of sample, materials and geometry. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 473, Non-destructive testing Qualification and certification of NDT personnel General principles EN 16016-1:2011, Non destructive testing Radiation method Computed tomography Part 1: Terminology EN 16016-3:2011, Non destructive testing Radiation methods Part 3: Operation and interpretation EN 16016-4:2011, Non destructive testing Radiation methods Part 4: Qualification 3 Terms and definitions For the purposes of this document, the terms and definitions given in EN 16016-1:2011 apply. 4 General principles 4.1 Basic principles Computed tomography is a radiographic inspection method which delivers three-dimensional information of an object from a number of radiographic projections either over cross-sectional planes (CT slices) or over the complete volume. Radiographic imaging is possible because different materials have different X-ray attenuation coefficients. In CT images, the X-ray linear attenuation coefficients are represented as different CT grey values (or in false colour). For conventional radiography the three-dimensional object is X-rayed from one direction and an X-ray projection is produced with the corresponding information aggregated over the ray path. In contrast, multiple X-ray-projections of an object are acquired at different projection angles during a CT scan. From these projection images the actual slices or volume are reconstructed. The fundamental advantage compared to radiography is the preservation of full volumetric information. The resulting CT image (2D CT slice or 3D CT volume), is a quantitative representation of the X-ray linear attenuation coefficient averaged over the finite volume of the corresponding volume element (voxel) at each position in the sample. The linear attenuation coefficient characterizes the local instantaneous rate at which X-rays are attenuated as they propagate through the object during the scan. The attenuation of the X-rays as they interact with matter is the result of several different interaction mechanisms: Compton scattering and photoelectric absorption being the predominant ones for X-ray CT. The linear attenuation coefficient depends on the atomic numbers of the corresponding materials and is proportional to the material density. It also depends on the energy of the X-ray beam.
4.2 Advantages of CT Computed tomography (CT) is a radiographic method that can be an excellent examination technique whenever the primary goal is to locate and quantify volumetric details in three dimensions. In addition, since SIST EN 16016-2:2012
EN 16016-2:2011 (E) 6 the method is X-ray based it can be used on metallic and non-metallic samples, solid and fibrous materials and smooth and irregularly surfaced objects. In contrast to conventional radiography, where the internal features of a sample are projected onto a single image plane and thus are superposed on each other, in CT images the individual features of the sample appear separate from each other, preserving the full spatial information. With proper calibration, dimensional inspections and material density determinations can also be made. Complete three-dimensional representations of examined objects can be obtained either by reconstructing and assembling successive CT slices (2D-CT) or by direct 3D CT image (3D-CT) reconstruction. Computed tomography is thus valuable in the industrial application areas of non-destructive testing, 2D and 3D metrology and reverse engineering. CT has several advantages over conventional metrology methods: acquisition without contact; access to internal and external dimensional information; a direct input to 3D modelling especially of internal structures. In some cases, dual energy (DE) CT acquisitions can help to obtain information on the material density and the average atomic number of certain materials. In the case of known materials the additional information can be traded for improved discrimination or improved characterization. 4.3 Limitations of CT CT is an indirect test procedure and measurements (e.g. of the size of material faults; of wall thicknesses must be compared with another absolute measurement procedure, see EN 16016-3). Another potential drawback of CT imaging is the possible occurrence of artefacts (see 4.5) in the data. Artefacts limit the ability to quantitatively extract information from an image. Therefore, as with any examination technique, the user must be able to recognize and discount common artefacts subjectively.
Like any imaging system, a CT system can never reproduce an exact image of the scanned object. The accuracy of the CT image is dictated largely by the competing influences of the imaging system, namely spatial resolution, statistical noise and artefacts. Each of these aspects is discussed briefly in 4.4.1. A more complete description will be found in EN 16016-3. CT grey values cannot be used to identify unknown materials unambiguously unless a priori information is available, since a given experimental value measured at a given position may correspond to a broad range of materials. Another important consideration is to have sufficient X-ray transmission through the sample at all projection angles (see 8.2) without saturating any part of the detector. 4.4 Main CT process steps 4.4.1 Acquisition During a CT scan, multiple projections are taken in a systematic way: the images are acquired from a number of different viewing angles. Feature recognition depends, among other factors, on the number of angles from which the individual projections are taken. The CT image quality can be improved if the number of projections of a scan is increased. As all image capture systems contain inherent artefacts, CT scans usually begin with the capture of offset and gain reference images to allow flat field correction; using black (X-rays off) and white (X-rays on with the sample out of the field of view) images to correct for detector anomalies. The capture of reference images for SIST EN 16016-2:2012
EN 16016-2:2011 (E) 7 distortion correction (pin cushion distortion in the case of camera-based detector systems with optical distortion), and centre of rotation correction can also take place at this stage. Each subsequent captured image for the CT data set has these corrections applied to it. Some systems can be configured to either the X-ray settings or enhance the image to ensure that the background intensity level of the captured images remains constant throughout the duration of the CT scan. The quality of a CT image depends on a number of system-level performance factors, with one of the most important being spatial resolution. Spatial resolution is generally quantified in terms of the smallest separation at which two features can be distinguished as separate entities. The limits of spatial resolution are determined by the design and construction of the system and by the resolution of and number of CT projections. The resolution of the CT projection is limited by the maximum magnification that can be used while still imaging all parts of the sample at all rotation angles. It is important to notice that the smallest feature that can be detected in a CT image is not the same as the smallest that can be resolved spatially. A feature considerably smaller than a single voxel can affect the voxel to which it corresponds to such an extent that it appears with a visible contrast so that it can be easily detected with respect to adjacent voxels. This phenomenon is due to the “partial-volume effect”. Although region-of-interest CT (local tomography) can improve spatial resolution in specified regions of larger objects, it introduces artefacts (due to incomplete data) which can sometimes be reduced with special processing. Radiographic imaging as used for CT examination is always affected by noise. In radiography this noise arises from two sources: (1) intrinsic variation corresponding to photon statistics related to the emission and detection of photons and (2) variations specific to instruments and processing used. Noise in CT projections is often amplified by the reconstruction algorithm. In the CT images statistical noise appears as a random variation superimposed on the CT grey value of each voxel and limits density resolution. Although statistical noise is unavoidable, the signal to noise ratio can be improved by increasing the number of projections and/or time of exposure for each of them, the intensity of the X-ray source or the voxel size. However, some of these measures will decrease spatial resolution. This trade-off between spatial resolution and statistical noise is inherent in computed tomography. 4.4.2 Reconstruction A CT scan initially produces a number of projections of an object. The subsequent reconstruction of the CT image from these individual projections is the main step in computed tomography, which distinguishes this examination technique from other radiographic methods. The reconstruction software may apply additional corrections to the CT projections during reconstruction, e.g. reduction of noise, correction of beam hardening and/or scattered radiation. Depending on the CT system, either individual CT slices or 3D CT images are reconstructed. 4.4.3 Visualisation and analysis This step includes all operations and data manipulations, for extracting the desired information from the reconstructed CT image. Visualisation can either be performed in 2D (slice views) or in 3D (volume). 2D visualisation allows the user to examine the data slice-wise along a defined axis (generally it can be an arbitrary path). For 3D imaging, the CT volume or selected surfaces derived from it, are used for generating the desired image according to the optical model underlying the algorithm. The main advantage of this type of visualisation is that the visual perception of the image corresponds well with the natural appearance of the object for the human eye, although features may appear superimposed in the 2D-representation on a screen.
SIST EN 16016-2:2012
EN 16016-2:2011 (E) 8 During visualisation, additional artefacts of different origin can occur, especially in the 3D imaging of the CT volume. Such artefacts due to sampling, filtering, classification and blending within the visualisation software are dependent on the hardware and software used, as well as the visualisation task at hand. Therefore such artefacts are not included in the definition of artefacts as found in 4.5. Nevertheless, the user should be aware that misinterpretation of the data might also occur in this process step. To highlight features of interest during visualisation different digital filter operations can be performed. A characteristic of all these operations is that although they enhance one or more properties of the data, they simultaneously deteriorate other properties (for example: highlighting the edges deteriorates recognition of inner structures of an object). Therefore digital filters should always be used cautiously for specific tasks, being aware which benefits and which detriments they are associated with. A computer used for 3D visualisation should be able to process the complete volume of interest in the main memory. The corresponding monitor should have a resolution, a dynamic range and settings sufficient for the given visualisation task. Adequate vision of the personnel is to be ensured, see EN 473. 4.5 Artefacts in CT images An artefact is an artificial feature which appears on the CT image but does not correspond to a physical feature of the sample. Artefacts result from different origins; they can be classified into artefacts arising from the measurement itself and the equipment (artefacts due to a finite beam width, scattered radiation, instabilities and detector peculiarities) and artefacts inherent to the method (e.g. beam hardening). Artefacts can also be divided into acquisition artefacts (e.g. scattered radiation, ring artefacts) and reconstruction artefacts (e.g. cone beam artefacts). Some artefacts can be eliminated by using an appropriate measurement technique with suitable parameters, while others can only be reduced in their extent. Artefacts may be detrimental for specific measurement or analysis tasks, but may have no impact on certain other analyses. With this fact in mind, the type and extent of artefacts in a data set has to be evaluated in the context of the corresponding analysis task. Noise and the partial volume effect are not considered as artefacts in this standard. More details are given in EN 16016-3:2011, 5.5. 5 Equipment and apparatus 5.1 General In relation to performance, a CT system can be considered as comprising four main components: the X-ray source, detector, sample manipulation stages (the latter including any mechanical structure that influences image stability) and reconstruction/visualisation system. Generally the source and detector will be fixed whilst the sample rotates in the beam to acquire the necessary set of projections. In scanners for example designed for in vivo animal studies or for imaging large structures, the source and detector may orbit around the sample, as in medical scanners. In the majority of micro-/nano- or sub-micro-tomography systems, the resolution is determined primarily by the X-ray focal spot size. Geometric magnification allows the detector element spacing to be much larger than the computed voxel size and a thicker and therefore more efficient scintillator to be used. A disadvantage of this approach is that to obtain high magnification ratios, the sample should be located very close to the source. This is a particular problem if the sample is to be mounted in some form of environmental chamber or, for example, an in situ loading stage. This imposes a lower limit on the source to sample distance, thus reducing X-ray fluence (resulting in a lower signal to noise ratio and/or increased acquisition time) and requiring the detector to be mounted proportionately further away in order to achieve the same magnification factor. Alternatively, if the sample to detector distance is low compared with the source to sample distance, the detector resolution becomes the limiting factor, rather than the spot size. In this case, the increased source to detector distance again means reduced X-ray fluence and high resolution detectors tend to require thinner and hence less efficient scintillators. SIST EN 16016-2:2012
EN 16016-2:2011 (E) 9 CT systems may be optimised for resolution, energy, speed of acquisition or simply cost. Although a particular system may operate over a wide range of conditions, it will operate optimally over a much smaller range and the user should consider the prime application when selecting one model over another and not simply over-specify. For example, a high resolution CT system (small X-ray focal spot size) may have a considerably lower flux output at more modest resolution settings than one designed to operate at such resolution. Furthermore, a high performance rotation stage for a high resolution scanner will have a much smaller load limit. Similarly, a system designed for high energy imaging will require a thicker phosphor screen, giving poorer resolution compared with a thinner screen, which is adequate at lower energies. Some CT systems may provide interchangeable X-ray target heads (transmission or reflection, see Annex A) and/or interchangeable detectors, but these will come at a higher price. When comparing resolution and scan times on different CT systems, it is important to consider the signal to noise ratio (SNR), see EN 16016-3:2011, 5.1.3. This is dependent on the X-ray exposure and thus the faster the scan, the worse the SNR. It is also dependent on the sample type and geometry. A sample with a high void volume fraction (or with a high proportion of relatively low absorbing regions), such as a foam or cancellous bone sample, will exhibit a better SNR than a more homogeneous sample. For a given exposure, the best SNR is obtained with the X-ray accelerating voltage set to give approximately 10 % – 20 % transmission through the sample. If the transmission is too low, the low number of photons detected will give rise to excessive noise. Conversely, if it is too high, the contrast (signal in SNR) will be too low. The SNR does not vary sharply with voltage however, and simulations of X-ray attenuation in aluminium indicate that the SNR only drops by 20 % of the peak value if the voltage is set for 35 % or 40 % transmission. For a given sample size, the required X-ray exposure to maintain a fixed SNR is proportional to the fourth power of resolution (for a given detector). Thus, for example, doubling the resolution will require a 16-fold increase in exposure whilst a 10-fold increase in resolution will demand a 10,000 fold increase in exposure. There is therefore a critical need to use the same or similar samples when comparing the image quality from one system with that of another. 5.2 Radiation sources
Most industrial Computed Tomography systems will use an electrically generated X-ray source, and these can be sub-divided into three main types:
Open Tube (or Vacuum Demountable) X-ray sets ; Sealed Tube Constant Potential X-ray sets ; Linear Accelerators. Each source type has a speciality, sometimes systems are supplied with more than one source so they can be used over a broader range of samples. Besides cost considerations, selection of a suitable X-ray source is dictated by the range of samples (size, composition and material density) that will be inspected and the resolution they are to be inspected at. X-ray set manufacturers will often quote a single focal spot size, this is a ‘nominal’ measurement at a specific energy setting, the size of the focal spot will vary depending upon the voltage (kV) and current (µA/mA) settings used, the higher the power the larger the focal spot will become. Focal spot size and the feature recognition (which is sometimes referred to by system manufacturers) are not the same as the spatial resolution of the CT syst
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.