ASTM E1695-20

4.1 The major factors affecting the quality of a CT image are total image unsharpness (U_T^image), contrast (Δµ), and random noise (σ). Geometrical and detector unsharpness limit the spatial resolution of a CT system, that is, its ability to image fine structural detail in an object. Random noise and contrast response limit the contrast sensitivity of a CT system, that is, its ability to detect the presence or absence of features in an object. Spatial resolution and contrast sensitivity may be measured in various ways. In this test method, spatial resolution is quantified in terms of the modulation transfer function (MTF), and contrast sensitivity is quantified in terms of the contrast discrimination function (CDF). The relationship between contrast sensitivity and spatial resolution describing the resolving and detecting capabilities is given by the contrast-detail-diagram (CDD metric, see also Guide E1441 and Practice E1570). This test method allows the purchaser or the provider of CT systems or services, or both, to measure and specify spatial resolution and contrast sensitivity and is a measure for system stability over time and performance acceptability.

1.1 This test method provides instruction for determining the spatial resolution and contrast sensitivity in X-ray and γ-ray computed tomography (CT) volumes. The determination is based on examination of the CT volume of a uniform cylinder of material. The spatial resolution measurement (Modulation Transfer Function) is derived from an image analysis of the sharpness at the edges of the reconstructed cylinder slices. The contrast sensitivity measurement (Contrast Discrimination Function) is derived from an image analysis of the contrast and the statistical noise at the center of the cylinder slices.

1.2 This test method is more quantitative and less susceptible to interpretation than alternative approaches because the required cylinder is easy to fabricate and the analysis easy to perform.

1.3 This test method is not to predict the detectability of specific object features or flaws in a specific application. This is subject of IQI and RQI standards and standard practices.

1.4 This method tests and describes overall CT system performance. Performance tests of systems components such as X-ray tubes, gamma sources, and detectors are covered by separate documents, namely Guide E1000, Practice E2737, and Practice E2002; c.f. 2.1, which should be consulted for further system analysis.

1.5 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.