X-ray computed tomography: from medical imaging to dimensional metrology

X-ray computed tomography (also referred to as X-ray CT, CAT scan, or simply ‘CT’) is a technological advancement with expanding applications, from medical imaging and nondestructive evaluation to, more recently, dimensional metrology. The CT technique is now used to measure a specimen's geometrical dimensions (of both internal and external features). As a result, CT presently contributes to dimensional inspection and geometric analysis for technology companies spanning a variety of industries such as aerospace, automotive, electronics, medical devices, plastic components, metalworking, and additive manufacturing (one of the main drivers presently pushing the use of CT for dimensional measurement). For medical diagnoses or other qualitative analyses that depend mainly on feature recognition dimensional accuracy is not necessary. In contrast, for precision engineering applications accurate dimensional measurement is the essence of X-ray CT metrology. This article describes the development of X-ray CT metrology beginning with a historical overview that spans the discovery of X-rays to the invention of CAT scan and focuses with greater detail on its expansion toward industrial dimensional measurements. Following this overview is a brief review of the current state of the art of the technology—specifically focused on issues of metrology—and of the present standardization efforts in the design of acceptance tests for evaluating the metrological performance of X-ray CT. As of writing, the CT metrology technique is still evolving with several technical issues yet to be resolved, in particular, to find better ways of expressing uncertainties associated with CT dimensional measurements. Supported by data indicating a growing commercial/industrial market, this technology appears to be in an ‘early adoption’ phase.