1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA XR 16-1991 (R2001)Test Standard for the Determination of the System Contrast Ratio and the System Veiling Glare Index of an X-Ray Image Intensifier SystemNEMA Standards Publication No. XR 16-1991 (R1996, R2001) Test Standard
2、 for the Determination of the System Contrast Ratio and the System Veiling Glare Index of an X-Ray Image Intensifier System Published by: National Electrical Manufacturers Association 1300 North 17th Street, Suite 1847 Rosslyn, VA 22209 O Copyright 2001 by the National Electrical Manufacturers Assoc
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13、of the certifier or maker of the statement. NEMA XR*Lb 71 61i70247 0500709 5 Foreword This NEMA Standards hblication is one of a series of test standards developed by the X-ray Imaging Products Section and the medical diagnostic imaging industry for the measurement of performance characteristics gov
14、erning the image quality of X-ray image intensifier (XRiI) systems. These test standards are intended for the use of equipment manufacturers, prospective purchasers, and users alike. Manufacturers may find these standards useful in establishing standard test conditions for the determination of syste
15、m performance specifications. Prospective equipment purchasers may benefit from these Standards :the employment of standard test conditions and methods enables meaningful comparisons to be made of NEMA-referenced, competitive, equipment performance specifications. Users may also benefit hm these sta
16、ndards through a better appreciation of the performance of XRII systems. Characterizing the parameters that govern image quality is a non-trivial task. The assessment of image quality lies in eye of the beholder and so is a somewhat subjective measure influenced by the spatial, temporal, contrast, a
17、nd noise characteristics of the image. Historically, many techniques have been developed employing varying degrees of sophistication to establish subjective and objective measures of these parameters, and the foilowing non-exhaustive list of parameters can be considered as relevant to image quality:
18、 Contrast Ratio Detective Quantum Efficiency (DQE) Fixed Pattern Noise Gain Image Signal Uniformity hg Limiting Spatial Resolution Modulation Transfer Function (MTF) Noise Power Spectrum Radial Image Distortion Signal to Noise Ratio Visible Entrance Field Size These parameters are not all independen
19、t and some of them may partially overlap in assessing the image quality. Moreover, it must be recognized that not all of these parameters lend themselves to measurement at the instailation site. Some parameters require instrumentation better suited to factory measurements while others require the fa
20、cilities of an instrumentation laboratory to assure the stable test conditions necessary for reliable measurements. The NEMA X-ray Image Intensifier Systems Subcommittee intends to make a selection from the above-mentioned list of parameters where practical and technically feasible measurements can
21、be made at the system level and to prepare a set of NEMA standards. This publication has been prepared by the X-ray Imaging Products Section of NEMA which periodically reviews it for any revisions necessary to keep it up to date with advancing technology. Comments or proposed revisions are welcomed
22、and should be submitted to: Vice-President, Engineering Department National Electrical Manufacturers Association 2101 L Street, N.W., Suite 300 Washington, D.C. 20037 11 NENA XR*1b 91 m 6470247 0500710 1 m Introduction The system contrast ratio (SCR) and the system veiling glare index (SVGI) are mea
23、sures of the spatial frequency response of an imaging system at low frequencies and, as such, complement the limiting spatial resolution, which is a measure of the systems high frequency response. The contrast represented by the difference in X-ray fluence between an object and its surround can be r
24、educed by most of the elements of the imaging chain. Generally, this loss of contrast is due to scattering of the image-carrying quanta away from ideal, geometric paths. The luminance of the displayed image due to imaging the scaUered quanta is cailed “veiling glare.“ Processes that contribute to ve
25、iling glare include, for example, the scattering of X-rays at the XRTI input and the scattering of light at the XRII output, in the lenses, and in the base of the film or in the faceplate of the video monitor. For an image formed by an XRII system, the SCR is the ratio of the signal at the center of
26、 the image with no test object in the X-ray beam to the signal at the center of the image with a specified contrast ratio test object in the beam. The SVGI is the reciprocal of the SCR. All signals used in calculating the SCR and SVGI are in terms of X-ray fluence or fluence rate or quantities linea
27、rly related to the fluence or fluence rate. While the SCR is unbounded, ranging from 1 to infinity, the SVGI is bounded between O and 1. An increase of an already large value of SCR yields only a small change in the SVGI. Although the SCR and the SVGI could, in principle, be calculated from the modu
28、lation transfer function, it is somewhat simpler to measure them directly. Additionally, the contrast ratio of the XRII itself has found general acceptance as an image quality parameter. Consequently, the determination of SCR and SVGI for XRII systems is deemed worthy of coverage in a NEMA Standards
29、 Publication. Scope The procedures in this standard determine the system contrast ratio (SCR) and the system veiling glare index (SVGI) at the center of the image produced by an XRII system under a given set of test conditions. The determination is made on a system set up according to the specificat
30、ions of the system designer or manufacturer. The measurement procedures described pertain to images formed by the photofluorographic film system, the cine film system, the video system, and the directviewing system. iii NENA XR*Lb 9L 6470247 05007LL 3 W XR 16-1991 Page 1 Section 1 REFERENCE STANDARD
31、S AND TERMINOLOGY 1.1 REFERENCED STANDARDS None. 1.2 DEGREES OF REQUIREMENTS A statement incorporating the auxiliary verb “shall“ indicates that compliance with a requirement is mandatory for compliance with the standard. Statements incorporating other auxiliary verbs such as “should,“ “may,“ etc.,
32、refer to authorized engineering in- formation and not to procedures required for compliance with the standard. 1.3 DEFINITIONS 1.3.1 Actual Focal Spot cepts the beam of electrons. 1.3.2 Automatic Brightness Control Area on the surface of the X-ray tube target that inter- NEMA Standard 8-28-1991. A d
33、evice that automatically controls the brightness at a preselected location in the output of the XRII by control of one or more of the technique factors influencing the electrical energy supplied to the X-ray tube, e.g., X-ray tube current and voltage. NEMA Standard 8-28-1991. 1.3.3 Automatic Exposur
34、e Control In an X-ray generator, the mode of operation in which one or more loading factors are controlled automatically in order to obtain at a preselected location a desired quan- tity of radiation. Loading factors are equivalent to tech- nique factors. NEMA Standard 8-28-1991. 1.3.4 Automatic Gai
35、n Control A device that automatically controls one or more of the factors influencing the magnitude of the output of a video camera, e.g., degree of optical filtration, aperture of the video lens iris, or electronic video signal amplification, in order to obtain at a preselected location in the outp
36、ut video signal a desired signal magnitude. NEMA Standard 8-28-1991. 1.3.5 Center of the Image That point in the entrance plane of the XRII system which corresponds to the center of the smallest chle circumscribing the image. NEMA Standard 8-28-1991. 1.3.6 Central Axis of the X-ray Field The line pe
37、rpendicular to the entrance plane of the XRII system passing through the center of the actual focal spot. 1.3.7 Central Magnification With a smali test object of known length placed symmet- ricaliy about the center of the image, the ratio of the test object length in the image to the test object len
38、gth itself. NEMA Standard 8-28-1 991. 1.3.8 Entrance Plane of the XRII System The plane perpendicular to the axis of the XRTI and grazing that part of the XRII, including its housing, that protrudes most in the direction of the radiation source. The following components shali not be considered eleme
39、nts of the XRII housing: NEMA Standard 8-28-1991. antiscauer grid collision sensors ionization chambers protective cover removable magnetic shields and coils spot-film device NEM4 Standard 8-28-1991. 1.3.9 Focal Spot of the actual focal spot. 1.3.10 Half Value Layer The thickness of a specified mate
40、rial required to de- crease the exposure rate of a beam of X-rays to one half its initial value. For the purpose of this definition, the solid angle of the radiation beam shall be as small as possible so that the quantity of scatteredradiation included in the beam is minimized. 1.3.1 1 Kilovolts Pea
41、k (kVp) The maximum potential difference, in kilovolts, applied, during an exposure, between the anode and cathode of an X-ray tube. NEMA Standard 8-28-1991. 1.3.1 2 Nominal Image Size The linear dimension of a smali-test object in the en- trance plane of the XRIL system multiplied by the central ma
42、gnification, NEMA Standard 8-28-1991. The projection along the central axis of the X-ray field NEMA Standard 8-28-1991. NEMA Standard 8-28-1991. NENA XR*Lb 91 = 6470247 0500712 5 E XR 16-1 991 Page 2 1.3.13 Output Image The image produced by the XRII system that is visually perceivable by a human ob
43、server. Output channels of the system include: processed photofluorographic film processed cine film television monitor direct viewing system. NEMA Standard 8-28-1 991. 1.3.14 Radiation Quality The characteristic of ionizing radiation determined by the spectral distribution of amdiation quantity wit
44、h respect to radiation energy. NEMA Standard 8-28-1 991. 1.3.15 Source to Entrance Plane Distance The distance hm the actual focal spot to the entrance plane of the XRII system measured along the centrai axis of the X-ray field. (SED) NEMA Standard 8-28-1991. 1.3.16 Useful Entrance Field Diameter Fo
45、r an electro-optical XRII system, the diameter of the field in the entrance plane of the XRII system that can be used for the transmission of an X-ray pattern when the SED equals 1.00 meter. Limitations of this diameter other than by the XRII itself shall not be considered. For an XRII with more tha
46、n one magnification mode, the useful entrance field diameter for each of the magnification modes shall correspond to the same diameter of the XRII output image occurring with the largest useful entrance field diameter. NEMA Standard 8-28-1991. 1.3.17 X-ray Image Intensifier System (XRII System) That
47、 portion of an X-ray imaging system consisting of the XRII with its housing and the subsequent components of the imaging chain responsible for transmitting and displaying the detected X-ray pattern. NEMA Standard 8-28-1991. NEMA XR*3b 93 6470247 0500733 7 XR 16-1 991 Page 3 Section 2 TEST EQUIPMENT
48、2.1 CONTRAST RATIO TEST OBJECTS The contrast ratio test objects shall be lead disks having a thickness of at least 3 mm. One disk shallhave a diameter of 10 f 0.5 mm. Other disks shall have diameters such that the area of a disk is 10 f 0.5 percent of the area determined by the useful entrance field
49、 diameter of the mode of the XRII used for the measurement. All disk diameters shall refer to the entrance plane of the XRII system. If the measurement cannot be performed with a disk in that plane, then the disk diameter shall be scaled such that its projection in that plane is as specified above. NEMA Standard 8-28-1991. 2.2 IMAGE POSITION CALIBRATION TEMPLATE An image position calibration template shali be used to locate the center of the image. The linear dimensions of the template shall equal or exceed the useful entrance field diameter of the XRII system being evaluated. A suggested
