1、Designation: E 1935 97 (Reapproved 2008)Standard Test Method forCalibrating and Measuring CT Density1This standard is issued under the fixed designation E 1935; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revis
2、ion. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers instruction for determining thedensity calibration of X- and g-ray computed tomography (CT)systems and f
3、or using this information to measure materialdensities from CT images. The calibration is based on anexamination of the CT image of a disk of material withembedded specimens of known composition and density. Themeasured mean CT values of the known standards are deter-mined from an analysis of the im
4、age, and their linear attenu-ation coefficients are determined by multiplying their measuredphysical density by their published mass attenuation coeffi-cient. The density calibration is performed by applying a linearregression to the data. Once calibrated, the linear attenuationcoefficient of an unk
5、nown feature in an image can be measuredfrom a determination of its mean CT value. Its density can thenbe extracted from a knowledge of its mass attenuation coeffi-cient, or one representative of the feature.1.2 CT provides an excellent method of nondestructivelymeasuring density variations, which w
6、ould be very difficult toquantify otherwise. Density is inherently a volumetric propertyof matter. As the measurement volume shrinks, local materialinhomogeneities become more important; and measured valueswill begin to vary about the bulk density value of the material.1.3 All values are stated in S
7、I units.1.4 This standard does not purport to address the safetyconcerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety andhealth practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Docu
8、ments2.1 ASTM Standards:2E 1316 Terminology for Nondestructive ExaminationsE 1441 Guide for Computed Tomography (CT) ImagingE 1570 Practice for Computed Tomographic (CT) Exami-nation3. Terminology3.1 Definitions:3.1.1 The definitions of terms relating to CT, that appear inTerminology E 1316 and Guid
9、e E 1441, shall apply to theterms used in this test method.3.2 Definitions of Terms Specific to This Standard:3.2.1 density calibrationcalibration of a CT system foraccurate representation of material densities in examinationobjects.3.2.2 effective energythe equivalent monoenergetic en-ergy for a po
10、lyenergetic CT system. Thus, the actual, polyen-ergetic CT system yields the same measured attenuationcoefficient for an examination object as a theoretical, monoen-ergetic CT system at the effective energy.3.2.3 phantoma part or item being used to calibrate CTdensity.3.2.4 examination objecta part
11、or specimen being sub-jected to CT examination.4. Basis of Application4.1 The procedure is generic and requires mutual agreementbetween purchaser and supplier on many points.5. Significance and Use5.1 This test method allows specification of the densitycalibration procedures to be used to calibrate
12、and performmaterial density measurements using CT image data. Suchmeasurements can be used to evaluate parts, characterize aparticular system, or compare different systems, provided thatobserved variations are dominated by true changes in objectdensity rather than by image artifacts. The specified p
13、roceduremay also be used to determine the effective X-ray energy of aCT system.5.2 The recommended test method is more accurate and lesssusceptible to errors than alternative CT-based approaches,because it takes into account the effective energy of the CTsystem and the energy-dependent effects of th
14、e X-ray attenu-ation process.5.3 This (or any) test method for measuring density is validonly to the extent that observed CT-number variations arereflective of true changes in object density rather than image1This test method is under the jurisdiction of ASTM Committee E07 onNondestructive Testing a
15、nd is the direct responsibility of Subcommittee E07.01 onRadiology (X and Gamma) Method.Current edition approved Dec. 15, 2008. Published December 2008. Originallyapproved in 1997. Last previous edition approved in 2003 as E 1935 - 97(2003).2For referenced ASTM standards, visit the ASTM website, www
16、.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.artif
17、acts. Artifacts are always present at some level and canmasquerade as density variations. Beam hardening artifacts areparticularly detrimental. It is the responsibility of the user todetermine or establish, or both, the validity of the densitymeasurements; that is, they are performed in regions of t
18、heimage which are not overly influenced by artifacts.5.4 Linear attenuation and mass attenuation may be mea-sured in various ways. For a discussion of attenuation andattenuation measurement, see Guide E 1441 and PracticeE 1570.6. Apparatus6.1 Unless otherwise agreed upon between the purchaserand sup
19、plier, the density calibration phantom shall be con-structed as follows (see Fig. 1):6.1.1 A selection of density standards bracketing the rangeof densities of interest shall be chosen. For best results, thematerials should have known composition and should bephysically homogeneous on a scale compar
20、able to the spatialresolution of the CT system. It is a good idea to radiographi-cally verify homogeneity and to independently verify chemicalcomposition. All materials should be manufactured to repro-ducible standards. Solids should be readily machinable and notsusceptible to surface damage.6.1.2 O
21、ne or more cylinders of each density standard shallbe machined or prepared, or both. Selecting cylinders overrectangles reduces the uncertainties and streaks that sharpcorners have on volumetric determination and verificationmethods. The cylinders should be large enough that the meanCT number corres
22、ponding to each standard can be computedover a hundred or more uncorrupted (see 8.1.3) pixels but smallenough relative to the dimensions of the host disk that radialeffects are minimal.6.1.3 The physical density of each density standard shall bedetermined empirically by weighing and measuring the sp
23、eci-mens as accurately as possible. It is a good idea to indepen-dently verify the measured densities using volumetric displace-ment methods.6.1.4 The mass attenuation coefficient, /r, at the effectiveenergy of the system (see 8.3) shall be determined from areference table. For compounds, /r can be
24、obtained by takingthe weighted sum of its constituents, in accordance with thefollowing equation:m5 /r5(iwi/r!i(1)where:wi= the weight fraction of the ith elemental component.6.1.5 For each density standard, the measured density, r,shall be multiplied by its corresponding mass attenuationcoefficient
25、, /r, as determined in 6.1.4. The linear attenuationcoefficient, , thus obtained shall be permanently recorded foreach density calibration standard.6.1.6 A host disk to hold the density standards shall befabricated. The opacity of the disk should approximate theattenuation range of the examination o
26、bjects. If possible, thehost disk should be of the same material as the examinationobjects, but other requirements take precedence and maydictate the selection of another material.6.2 In general, it is very difficult to find acceptable materialsfor density standards. Published density data are gener
27、ally notreliable enough for calibration purposes. Homogeneity oftenvaries on a local scale and negatively influences the calibrationprocedure. Machine damage can increase the density at thesurface of a sample, making it difficult to determine the densityof the interior material crucial to the calibr
28、ation process.Lot-to-lot variations in composition or alloy fraction can makeit difficult to compute mass attenuation coefficients. For theseand other reasons, development of a good density calibrationphantom takes effort, resources and a willingness to iterate theselection and production of standar
29、ds until acceptable resultsare obtained.6.2.1 Liquids make the best standards, because they can beprecisely controlled and measured. However, liquids requirespecial handling considerations, are sensitive to temperatureFIG. 1 Density Calibration PhantomE 1935 97 (2008)2variations, and often tend to p
30、recipitate, especially high-concentration aqueous solutions. It is hard to find organicliquids with densities above 1.5 g/cm3or inorganic liquidsabove 4.0 g/cm3; but for many purposes, they offer a suitablechoice.6.2.2 Plastics are popular but in general make the worststandards. Most plastics have a
31、t best an approximately knownpolymerization and often contain unknown or proprietaryadditives, making them poor choices for calibration standards.They also tend to vary more than other materials from batch tobatch. Notable exceptions to these generalizations are brand-name acrylics and brand-name fl
32、uorocarbons.6.2.3 Metals are also popular, but they are generally avail-able only in a limited number of discrete densities. They canexhibit important lot-to-lot variations in alloy fractions; butwith careful selection or characterization, they can make gooddensity calibration standards. Pure elemen
33、ts or very wellknown specimens offer an excellent option when they can beobtained in the density range of interest.6.2.4 Each material must be treated on a case-by-case basis.Reactor-grade graphite provides a good case study. Reactor-grade graphite is available in a variety of shapes, in very purefo
34、rm, and in a number of densities. At first glance, it appears tooffer an attractive choice in a density range without manyviable alternatives. However, upon closer examination, thematerial is found to be susceptible to surface damage duringmachining and to exhibit important inhomogeneities in densit
35、yon linear scales of about 1 mm. Surface damage makes itnearly impossible to determine the core density of the samplegravimetrically, because the total weight is biased by a denserouter shell. Inhomogeneities make it difficult to extract accu-rate mean CT numbers from an image of a sample that is no
36、tlarge in diameter compared to 1 mm.7. Procedure7.1 Unless otherwise agreed upon between the purchaserand supplier, the density calibration phantom shall be scannedas follows:7.1.1 The phantom shall be mounted on the CT system withthe orientation of its axis of revolution normal to the scanplane.7.1
37、.2 The phantom shall be placed at the same location usedfor examination object scans.7.1.3 The slice plane shall be adjusted to intercept thephantom approximately midway between the flat faces of thedisk.7.1.4 The phantom shall be scanned using the same dataacquisition parameters, and the data shall
38、 be processed usingthe same steps (for example, beam-hardening corrections)applied to examination objects.8. Interpretation of Results8.1 Unless otherwise agreed upon between the purchaserand supplier, the image of the density calibration phantom shallbe analyzed as follows:8.1.1 The phantom scan da
39、ta shall be reconstructed usingthe same reconstruction parameters and post-processing steps,if any, used for examination object data.8.1.2 The phantom image shall be displayed using the samedisplay parameters used for viewing examination object im-ages.8.1.3 The mean CT numbers of the density standa
40、rds in theCT image shall be measured. Special attention needs to be paidto this part of the measurement process. As much of the area ofeach specimen as practical should be used, but care must betaken to insure that only valid pixels are included. For example,a square region of interest in a round sa
41、mple could yield biasedresults if there are significant radial effects, such as from beamhardening or a higher density around the perimeter due tosurface damage caused by machining or compression. Ideally,a circular region of interest should be used that includes ahundred or more pixels but avoids t
42、he boundary region aroundeach density standard, especially if edge effects of any type areclearly visible.8.1.4 A table of linear attenuation coefficients versus meanCT numbers shall be prepared.8.1.5 A least-squares fit to the equation NCT= a + b shallbe performed on the data stored in the table, w
43、here is thelinear attenuation coefficient and NCTis the CT number.8.1.6 The resulting linear curve shall be used as the densitycalibration. Using the inferred linear relationship between CTnumber and linear attenuation coefficient, the measured CTvalue, NCT, of any material can be used to calculate
44、a bestestimate of its associated linear attenuation coefficient, .8.2 Unless otherwise agreed upon between the purchaserand supplier, the density of a region of interest in an exami-nation object shall be determined as follows:8.2.1 The mean CT number in the region of interest shall bemeasured.8.2.2
45、 From the known calibration parameters, the linearattenuation coefficient of the region of interest shall be ob-tained using the equation NCT= a + b.8.2.3 The density of the region of interest shall be calculatedby dividing the obtained linear attenuation by the appropriatetabulated value of /r at t
46、he effective energy of the system (see8.3). If /r is not known for the feature of interest, a nominalvalue for /r may be used. Variations in /r are minor, andbasically independent of material in the energy range of about200 keV to about 2 MeV. Outside this range, the selection of anominal value is m
47、ore sensitive. Adoption of an appropriatenominal value is a matter of agreement between purchaser andsupplier.8.3 Unless otherwise agreed upon between the purchaserand supplier, the effective energy of the CT system shall bedetermined as follows:8.3.1 A table of linear attenuation coefficients versu
48、s meanCT numbers shall be prepared for several X-ray energiesbracketing the effective energy of the CT system, as shown in8.4.1.8.3.2 For each X-ray energy, a least-squares fit to theequation NCT= a + b shall be performed and the correlationcoefficient recorded.8.3.3 The energy value in the table th
49、at yields the best fit(that is, the largest value of the correlation coefficient) shall beselected as the effective energy of the CT system.E 1935 97 (2008)38.3.4 If the effective energy has been determined previouslyunder the same or similar conditions, this step may be skippedwith the consent of the buyer.8.4 Illustrative Examples:8.4.1 Effective Energy DeterminationThe process of de-termining the effective X-ray energy of a CT system isillustrated with the following example taken from actualpractice. The data presented were acquired with a 9-MVindustrial
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