ASTM_E_1935_-_97_2013.pdf

Designation:E193597(Reapproved 2013)Standard Test Method forCalibrating and Measuring CT Density1This standard is issued under the fixed designation E1935;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.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-ray computed tomography(CT)systems and for 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 image,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 unknown 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 attenuationcoefficient,or one representative of the feature.1.2 CT provides an excellent method of nondestructivelymeasuring density variations,which would 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 SI 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 Documents2.1 ASTM Standards:2E1316 Terminology for Nondestructive ExaminationsE1441 Guide for Computed Tomography(CT)ImagingE1570 Practice for Computed Tomographic(CT)Examina-tion3.Terminology3.1 Definitions:3.1.1 The definitions of terms relating to CT,that appear inTerminology E1316 and Guide E1441,shall apply to the termsused 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 energyfor a polyenergetic CT system.Thus,the actual,polyenergeticCT system yields the same measured attenuation coefficient foran examination object as a theoretical,monoenergetic CTsystem at the effective energy.3.2.3 phantoma part or item being used to calibrate CTdensity.3.2.4 examination objecta part 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 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 proceduremay 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 the X-ray attenu-ation process.1This test method is under the jurisdiction of ASTM Committee E07 onNondestructive Testing and is the direct responsibility of Subcommittee E07.01 onRadiology(X and Gamma)Method.Current edition approved June 1,2013.Published June 2013.Originallyapproved in 1997.Last previous edition approved in 2008 as E1935-97(2008).DOI:10.1520/E1935-97R13.2For referenced ASTM standards,visit the ASTM website,www.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.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 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 imageartifacts.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