【国外标准】 Standard Practice for Determining Contrast Sensitivity in Radiology

5.1 The contrast sensitivity gauge measures contrast sensitivity independent of the imaging system spatial resolution limitations. The thickness recess dimensions of the contrast sensitivity gauge are large with respect to the unsharpness limitations of most imaging systems. Four levels of contrast sensitivity are measured: 4 %, 3 %, 2 %, and 1 %.5.2 The contrast sensitivity gauge is intended for use in conjunction with a high-contrast resolution measuring gauge, such as Practice E2002, ISO 19232 – 5 Duplex Wire Image Quality Indicator7, or a line-pair gauge. Such gauges measure system unsharpness essentially independent of the imaging system's contrast sensitivity. Such measurements are appropriate for the qualification and performance monitoring of radiographic and radioscopic imaging systems with film, realtime devices, Computed Radiography (CR) and Digital Detector Arrays (DDA).5.3 Radioscopic/radiographic system performance may be specified by combining the measured contrast sensitivity expressed as a percentage with the unsharpness expressed in millimetres of unsharpness. For the duplex wire image quality indicator, the unsharpness is equal to twice the wire diameter. For the line pair gauge, the unsharpness is equal to the reciprocal of the line-pair/mm value. As an example, an imaging system that exhibits 2 % contrast sensitivity and images the 0.1 mm paired wires of the duplex wire IQI (equivalent to imaging 5 line-pairs/millimeter resolution on a line-pair gauge) performs at a 2 %–0.2 mm sensitivity level. A standard method of evaluating overall radioscopic system performance is given in Practice E1411 and in EN 13068–1. A conversion table from duplex wire read out to lp/mm can be found in Practice E2002. For CR system performance evaluation, this contrast sensitivity gauge is used in Practice E2445.1.1 This practice covers the design and material selection of a contrast sensitivity measuring gauge used to determine the minimum change in material thickness or density that may be imaged without regard to unsharpness limitations.1.2 This practice is applicable to transmitted-beam radiographic imaging systems (film, radioscopy, computed radiography, and digital detector array image detectors) utilizing X-ray and gamma ray radiation sources.1.3 The values stated in inch-pound units are to be regarded as standard. The SI units given in parentheses are for information only.1.4 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. For specific safety statements, see NIST/ANSI Handbook 114 Section 8, Code of Federal Regulations 21 CFR 1020.40 and 29 CFR 1910.96.1.5 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.