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The assessment of a CT scanner's performance is a complicated subject. The evaluation of a CT device cannot be simply an extension of the methodologies used to assess film, electronic cameras, and x-ray radiographs. A reconstructed CT image contains quantitative information that requires precision and accuracy; precision describes the reproducibility of a measurement, and accuracy characterizes the closeness of the measurement to the truth. Recent CT technology developments have extended CT images to four dimensions: three dimensions in space and one dimension in time. Clearly, it is impossible to deal with all aspects of these parameters in a short chapter, so we will address in this chapter only the key performance parameters that are not covered in the other chapters of this book. Some parameters are discussed in detail in a separate chapter (such as image artifacts covered in Chapter 7) and will be omitted here to avoid duplication. One parameter that has received increased attention in recent years, energy resolution, will be covered under dual-energy CT in Chapter 12. Because of the importance of the x-ray dose, that topic is also covered in a separate chapter together with a discussion on the biological effects of x-ray radiation and several dose-reduction approaches (Chapter 11). One topic not covered in this book is quality assurance of CT. Although quality control and acceptance testing of CT equipment (which addresses the installed CT system's conformance to the vendor's specification) is an important aspect of CT performance, we will refer interested readers to other literature on the subject.
5.1 High-Contrast Spatial Resolution
The high-contrast spatial resolution of a CT scanner describes the scanner's ability to resolve closely placed objects. Spatial resolution is often measured in two orthogonal directions: in-plane (x-y) and cross-plane (z). Historically, the CT scanner was used primarily to generate 2D images, and there was a large discrepancy between the in-plane and cross-plane spatial resolutions. However, the difference between the two is quickly disappearing due to the recent introduction of helical and multislice scanners.
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