ASTM_E_1441_-_11.pdf

Designation:E144111Standard Guide forComputed Tomography(CT)Imaging1This standard is issued under the fixed designation E1441;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.This standard has been approved for use by agencies of the U.S.Department of Defense.1.Scope*1.1 Computed tomography(CT)is a radiographic methodthat provides an ideal examination technique whenever theprimary goal is to locate and size planar and volumetric detailin three dimensions.Because of the relatively good penetra-bility of X-rays,as well as the sensitivity of absorption crosssections to atomic chemistry,CT permits the nondestructivephysical and,to a limited extent,chemical characterization ofthe internal structure of materials.Also,since the method isX-ray based,it applies equally well to metallic and non-metallic specimens,solid and fibrous materials,and smoothand irregularly surfaced objects.When used in conjunctionwith other nondestructive evaluation(NDE)methods,such asultrasound,CT data can provide evaluations of material integ-rity that cannot currently be provided nondestructively by anyother means.1.2 This guide is intended to satisfy two general needs forusers of industrial CT equipment:(1)the need for a tutorialguide addressing the general principles of X-ray CT as theyapply to industrial imaging;and(2)the need for a consistent setof CT performance parameter definitions,including how theseperformance parameters relate to CT system specifications.Potential users and buyers,as well as experienced CTinspectors,will find this guide a useful source of informationfor determining the suitability of CT for particular examinationproblems,for predicting CT system performance in newsituations,and for developing and prescribing new scan pro-cedures.1.3 This guide does not specify test objects and test proce-dures for comparing the relative performance of different CTsystems;nor does it treat CT inspection techniques,such as thebest selection of scan parameters,the preferred implementationof scan procedures,the analysis of image data to extractdensitometric information,or the establishment of accept/rejectcriteria for a new object.1.4 Standard practices and methods are not within thepurview of this guide.The reader is advised,however,thatexamination practices are generally part and applicationspecific,and industrial CT usage is new enough that in manyinstances a consensus has not yet emerged.The situation iscomplicated further by the fact that CT system hardware andperformance capabilities are still undergoing significant evo-lution and improvement.Consequently,an attempt to addressgeneric examination procedures is eschewed in favor ofproviding a thorough treatment of the principles by whichexamination methods can be developed or existing onesrevised.1.5 The principal advantage of CT is that it nondestructivelyprovides quantitative densitometric(that is,density and geom-etry)images of thin cross sections through an object.Becauseof the absence of structural noise from detail outside the thinplane of inspection,images are much easier to interpret thanconventional radiographic data.The new user can learn quickly(often upon first exposure to the technology)to read CT databecause the images correspond more closely to the way thehuman mind visualizes three-dimensional structures than con-ventional projection radiography.Further,because CT imagesare digital,they may be enhanced,analyzed,compressed,archived,input as data into performance calculations,com-pared with digital data from other NDE modalities,or trans-mitted to other locations for remote viewing.Additionally,CTimages exhibit enhanced contrast discrimination over compactareas larger than 20 to 25 pixels.This capability has noclassical analog.Contrast discrimination of better than 0.1%atthree-sigma confidence levels over areas as small as one-fifthof one percent the size of the object of interest are common.1.6 With proper calibration,dimensional inspections andabsolute density determinations can also be made very accu-rately.Dimensionally,virtually all CT systems provide a pixelresolution of roughly 1 part in 1000,and metrologicalalgorithms can often measure dimensions to one-tenth of onepixel or so with three-sigma accuracies.For small objects(lessthan 100 mm(4 in.)in diameter),this translates into accuraciesof approximately 0.1 mm(0.003 to 0.005 in.)at three-sigma.For much larger objects,the corresponding figure will beproportionally greater.Attenuation values can also be relatedaccurately to material densities.If details in the image are1This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-tive Testing and is the direct responsibility of Subcommittee E07.01 on Radiology(X and Gamma)Method.Current edition approved Jul