ASME B89.4.23:2020 pdf download

ASME B89.4.23:2020 pdf download.X-Ray Computed Tomography (CT) Performance Evaluation
1 SCOPE
This Standard specifies the dimensional measurement accuracy of industrial X-ray computed tomography (CT) systems for length, size, and form measurands of sphere-based test objects made of homogeneous materials. Medical CT systems are outside the scope of this Standard. The material properties of the measured test objects arerestrictedtothreeclassesofmaterialselectedtoberepresentativeofindustrialmaterials: plasticpolymers,aluminum alloys, and steel alloys; other materials are outside the scope ofthis Standard. However, this Standard may be used as a guide fortestingthe performance ofa CT system forothermaterials. The evaluation ofworkpieces composed ofmultiple materials orofmaterials withdensitygradients, i.e., gradual densityvariations withinthematerial, is outsidethe scopeof this Standard. This Standard is applicable to dimensional measurements made at the surface ofthe workpiece, i.e., at the workpiece material–air interface, including those ofinternal cavities. The effect ofcomplex workpiece-material influence is simu- lated bytestobjects composed ofspheres (servingas the metrological geometric elements) and an obstructingbody; the obstructingbody’sdimensionsareuncalibratedanditsshapemaybearbitrary.Thespheresandobstructingbodyshallbe ofthe same class ofmaterial and theircombined length shall notexceed the maximum penetration length forthis class of material, as stated by the CT system manufacturer. This Standard applies to a variety ofCT systems thatmay vary by scanning mode and system components involved in the acquisition of images. In cases where a system provides multiple configurations of X-ray sources, detectors, and scanning modes, the dimensional measurement accuracy may be specified for each scanning mode. This Standard does not mandate testing (by either the CT manufacturer or the user) to verify CT performance. The amountoftesting, andwhichpartywillbearthe costoftesting, is abusiness decisionandmustbe negotiatedbetweenthe two parties.
2.2 Specifications
The accuracyspecifications are the MPEs associated with the measurementofthe specific dimensional and geometric characteristics of the test objects. These values are specified by the CT system manufacturer, and they represent the largestCT measurement errors permitted when a CT system is tested per the ASME B89.4.23 testprotocol. A CT system that generates measurement errors greater than the MPE values is not in conformance with this Standard. These MPEs maybe ofsignificantvalue when actual workpieces are measured. OftentheMPE functionisjustasinglevalue; i.e.,thefunctionis aconstantover all rated operating conditions. Typical rated operating conditions include such quantities as the size ofthe system work zoneunderspecification, datacollectionspeeds, ambienttemperatureconditions, thenumberofradiographs inascan,X- ray source voltages and currents, operator training and skill requirements, and other metrological factors that are required to be satisfied for the system to function properly; these are described in section 5.
2.3 Objectives of the Test Protocol
Testing is designed to verify the metrological specifications of the CT system. The test protocol described in this Standard has several objectives: to provide a thorough yet efficient test, to limit time and money spent on testing, and to assure users of the capabilities of their CT system. This Standard meets these objectives in several ways. (a) To facilitate commerce, both the CT system manufacturer and the user need an agreed-upon amountoftesting to consider the system acceptable. A large amountoftestingrequested bya user can drive up the manufacturer’s costs, but insufficient testing might miss errors that could demonstrate nonconformance with MPE specifications. This Standard establishes the recommended amountoftesting required to be reasonably thorough without being overly burdensome. (b) By recommending test-object designs that can realize the measurands specified in this Standard, the CT system manufacturer need only maintain this limited set of test objects, thus avoiding the expense of multiple different test objects thatmightbe otherwise requestedbythe user. Similarly, the useris protectedbecause the manufacturerhas used carefully designed test objects that reveal CT errors in an effective manner. (c) Thetestprotocolmayrequirespecifictestingconditions(e.g.,locationsandorientations) forthetestobjectthatare sensitiveto knownerrorsources in the CTsystemandhence will reveal them withaminimum numberofmeasurements. This well-crafted protocol protects the inexperienced user from performing a weak test but also saves on the cost of testing because it provides a targeted and acceptable testing procedure. (d) The ASME B89.4 standards typicallydefine testprotocols with some user-selectable testconditions. These select- able testconditions ensure thatthe MPE specificationis testable overanypossible conditions withinthe rated conditions specified by the manufacturer, and not just those specific testing conditions required in this Standard.