X-ray Computed Tomography: From Medical Imaging to Dimensional Metrology
A recently published technical paper by the journal Precision Engineering Volume 60, Pages 544-569), titled – ‘X-ray computed tomography: from medical imaging to dimensional metrology’ is available for download. The paper authors are HerminsoVillarraga-Gómez, Ericka L.Herazo and Stuart T.Smith.
Below is an extract of the technical paper along with its and introduction.
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.
Tomography is a representation created from sections or sectioning, and ‘X-ray tomography’ means imaging by a superposition of sections or layers based on X-rays. Classical tomography or laminography, also termed ‘conventional tomography’, conceived of during the first half of the 20th century, aims to produce an X-ray image of a single plane in focus (imaged sharply) with other specimen planes blurred out by synchronously moving the radiographic detection system and the X-ray tube in opposite directions across the object or patient. Detailed historical studies of classical tomography development can be found elsewhere [27,[100], [101], [102], [103], [104]]. During the second half of the 20th century, with further developments in electronics and the advent of modern computers, ‘X-ray computed tomography’ came into development, which is today commonly known as simply CT (or CAT scan for either ‘computed axial tomography’ or ‘computer-aided tomography’).
At the beginning, in the early 1970s, CT was used solely in the medical field, but early 1980s adaptations from medical CT technology started to appear for industrial nondestructive evaluation, see Table 1. Soon after CT’s emergence into the field of nondestructive testing, in the early 1990s the concept of extracting dimensional characteristics from CT data emerged [[78], [79], [80], [81], [82], [83]] with the technology and measurement techniques continuingly being developed [[86], [87], [88], [89]]. The arrival of the dedicated-type dimensional CT machine, first exhibited at the Control international trade fair for quality assurance at Germany in 2005 [105,106], is a reasonable marker of the beginning of a commercial market. Since then, two review articles focusing on X-ray CT as a tool for dimensional metrology have appeared [91,107]. However, a published literature review on the topic of X-ray CT metrology that explores its historical roots and its progression to current status—to 2019—currently does not exist in a peer-reviewed journal publication (to the authors’ knowledge). To fill this gap, based on research conducted at the University of North Carolina at Charlotte [108], the present paper contains a novel collection of material that describes the progression of X-ray CT from medical imaging to current status in dimensional metrology.
The structure of the paper is organized as follows. Section 2 provides a brief review of the history of X-ray CT, which is followed in Section 3 by a general overview of the industrial X-ray CT technology with emphasis on dimensional metrology. A review of the market’s adoption of CT is presented in Section 4 from which an ‘early adoption’ stage is indicated. Section 5 reviews some of the major challenges of dimensional metrology with X-ray CT based on the current state of the art, and this continues in Section 6 with an overview of current standardization efforts that aim to design verification tests for evaluating the metrological performance of industrial X-ray CT systems. Lastly, Section 7 provides concluding remarks that summarize the main points presented through the present review of the expansion of X-ray CT toward dimensional metrology, including a table with relevant bibliographical references related to the current use of industrial X-ray CT that provide complementary resources for more detailed information. Overall, finding the spatial location of the ‘real surface’ of an object with a high level of accuracy as well as concepts, such as how to define and determine the structural metrological resolution still present major challenges in industrial X-ray CT metrology.
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