NEMA XR 28-2013 Supplemental Requirements for User Information and System Function Related to Dose in CT.pdf

1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA XR 28-2013Supplemental Requirements for User Information and SystemFunction Related to Dosein CTNEMA XR 28-2013 Supplemental Requirements for User Information and System Function Related to Dose in CT Published by: National

2、Electrical Manufacturers Association 1300 North 17th Street, Suite 900 Rosslyn, Virginia 22209 www.nema.org Copyright 2013 by the National Electrical Manufacturers Association. All rights including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convent

3、ion for the Protection of Literary and Artistic Works, and the International and Pan American Copyright Conventions. XR 28-2013 Page ii 2013 National Electrical Manufacturers Association NOTICE AND DISCLAIMER The information in this publication was considered technically sound by the consensus of pe

4、rsons engaged in the development and approval of the document at the time it was developed. Consensus does not necessarily mean that there is unanimous agreement among every person participating in the development of this document. The National Electrical Manufacturers Association (NEMA) standards a

5、nd guideline publications, of which the document contained herein is one, are developed through a voluntary consensus standards development process. This process brings together volunteers and/or seeks out the views of persons who have an interest in the topic covered by this publication. While NEMA

6、 administers the process and establishes rules to promote fairness in the development of consensus, it does not write the document and it does not independently test, evaluate, or verify the accuracy or completeness of any information or the soundness of any judgments contained in its standards and

7、guideline publications. NEMA disclaims liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on this document. NEMA dis

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10、perform any duty owed by any person or entity to someone else. Anyone using this document should rely on his or her own independent judgment or, as appropriate, seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. Information and othe

11、r standards on the topic covered by this publication may be available from other sources, which the user may wish to consult for additional views or information not covered by this publication. NEMA has no power, nor does it undertake to police or enforce compliance with the contents of this documen

12、t. NEMA does not certify, test, or inspect products, designs, or installations for safety or health purposes. Any certification or other statement of compliance with any health or safetyrelated information in this document shall not be attributable to NEMA and is solely the responsibility of the cer

13、tifier or maker of the statement. XR 28-2013 Page iii 2013 National Electrical Manufacturers Association Table of Contents NOTICE AND DISCLAIMER . ii FOREWORD . iv Section 1 - OVERVIEW .1 1.1 SCOPE .1 1.2 RATIONALE .1 1.3 REFERENCES 1 1.3.1 Normative References1 Section 2 - Additional User Informati

14、on 3 2.1 General .3 2.2 Perfusion Scanning3 2.3 Automatic Exposure Control vs. Manual mA-Control Considerations 3 2.4 Listing of Reference Protocols 3 2.5 Estimated Phantom Peripheral Dose .4 2.6 Organization of Dose Information in User Manuals .4 Section 3 - System Function .5 3.1 General .5 3.2 Pr

15、e-population of the Dose Check Alert Value 5 3.3 Functional Options When Switching Between Automatic Exposure Control and Manual mA Control .5 Appendix A Computed Tomography Perfusion.6 Appendix B - Automatic Exposure Control .12 Appendix C - Template for Listing of Reference Clinical Protocols 15 A

16、ppendix D - Text of FDA Letter to MITA, November 8, 2010 17 Appendix E - Estimated Phantom Peripheral Dose .20 XR 28-2013 Page iv 2013 National Electrical Manufacturers Association FOREWORD This first edition of this standard is intended to be used by medical imaging device manufacturers in the desi

17、gn and manufacture of CT scanner equipment. This standard was developed by the CT Group of the X-Ray Imaging Section of the Medical Imaging a list of parameters to check; and/or fill-in boxes for parameter values that need to be selected. 2. Default to blank values in parameter fields as a way to ge

18、t operator confirmation of ownership of the mode switch: blanks would force entry of values in order to proceed with scanning. 3. Default to reasonable values in parameter fields that would correspond to operating conditions representative of the mode before switching. 4. No scan-time switching allo

19、wed. 5. List of alternate protocols, e.g., AEC protocols that could represent an original manual-mode protocol. XR 28-2013 Page 6 2013 National Electrical Manufacturers Association Appendix A Computed Tomography Perfusion Normative Purpose of CT Perfusion Studies Computed tomography (CT) perfusion s

20、tudies are used to assess the delivery and perfusion of blood to an organ and/or its tissues. Such studies may be valuable for evaluating blood supply to neoplastic and non-neoplastic tissue (including normal and ischemic tissue). In particular, CT perfusion imaging allows the evaluation of cerebral

21、 ischemia or of the extent of angiogenesis associated with a tumor. CT perfusion should be performed only for a valid medical reason and with the minimum radiation dose necessary to achieve an adequate exam. Use of perfusion scans in children should be particularly reviewed for clinical impact and j

22、ustified. Pediatric patients are more radiosensitive than adults and have a longer post-exam life expectancy, so particular attention should be paid to displayed CTDIVOL when modifying protocols. CT perfusion imaging relies on the linear relationship between CT attenuation, expressed in Hounsfield U

23、nits (HU) and represented in a particular pixel of an image, versus the amount of iodinated contrast material perfusing the corresponding region of tissue attenuating the x-rays. Dynamic CT scanning enables the calculation of perfusion parameter maps, e.g., anatomic images where the pixel value repr

24、esents mean transit time, blood flow, blood volume, or permeability, depending upon the post-processing algorithm used. Scan technique parameters (e.g., kV, mAs) for CT perfusion studies should be set at values lower than those used for routine diagnostic scanning of the same anatomical area. Perfus

25、ion imaging involves visualization of temporal changes in iodine enhancement, rather than resolution of small or subtle anatomical detail. The post-scan software processing of the data is relatively insensitive to increased noise levels; hence perfusion scans do not require use of the same radiation

26、 levels. In general, lower kV improves visualization of iodine contrast and consequently allows use of lower radiation doses. Lower kV settings are therefore recommended to be used as long as sufficient image quality for perfusion post-processing can be obtained. Body perfusion imaging of obese pati

27、ents, for example, may be an application that requires use of higher kV values. Users should carefully review the manufacturers reference perfusion protocols, which reflect the recommended kV, mA, and scan time for a typical perfusion acquisition. Additional guidance may be obtained from professiona

28、l societies, regulatory agencies, educational textbooks, or peer-reviewed literature. The American Association of Physicists in Medicine provides a set of reasonable scan protocols for CT brain perfusion imaging that is freely available to users via its public webpage. (See Recommended Reading.) Bec

29、ause CT perfusion requires specialized post-processing software, a CT perfusion acquisition should not be performed unless this software is readily available to the institution. All users should be trained in both CT perfusion acquisitions and post-processing and should follow professional society p

30、erfusion practice guidelines. Before any changes are made to the manufacturers reference protocols, both a radiologist and medical physicist familiar with CT perfusion should be consulted. Changes in protocol and the reason for the changes should be communicated to the radiologic technologist. Any c

31、hanges to the protocols should be evaluated with respect to the image quality (less than diagnostic level), temporal sampling and radiation dose of the manufacturers original reference perfusion protocols. It is essential that all users understand that CT perfusion images will be much noisier than i

32、mages of the same body region acquired for most other diagnostic purposes, and that this level of image quality is sufficient for the calculation of perfusion parameters. XR 28-2013 Page 7 2013 National Electrical Manufacturers Association Components of a CT Brain Perfusion Study Assessment of tissu

33、e perfusion for stroke includes a diagnostic quality non-contrast brain exam, an optional CT angiogram of the circle of Willis that may include the carotid arteries, and a CT perfusion exam. It may also include a post-contrast CT scan of the brain for assessment of residual lesion enhancement. In th

34、e assessment of tumors, a non-contrast scan for localization of the area of interest is often done prior to the CT perfusion exam. In all cases, the CT perfusion exam should have technique factors that are lower than those used for the other components of the study (e.g. the non-contrast, post-contr

35、ast and angiogram scans). Specific acquisition times for perfusion exam depend on the post-processing algorithm used, but in all cases the exam must be performed over a relatively long period of time (typically 40-50 seconds and potentially up to 3 minutes; consult model-specific user manual and rad

36、iologist) in order to measure the time-dependent physiologic process of blood flow through the brain. Since the scan location is fixed, the same anatomy is irradiated repeatedly during this scan time. Scan times are also affected by the concentration, volume, and rate of delivery of the contrast age

37、nt. The lenses of the eyes are more radiosensitive than the skin. Scanning through the orbits should be avoided, if possible, by the use of patient positioning and/or gantry tilting. Consult the medical physicist to ascertain appropriate deterministic thresholds across the body. Body perfusion consi

38、derations Perfusion scanning of the torso, typically referred to as body perfusion CT is not currently performed as frequently as head perfusion scans. It is essential to refer to manufacturers reference protocols (if provided) and to involve a radiologist and medical physicist familiar with the pri

39、nciples and techniques for body CT perfusion imaging, as well as communicate with the radiologic technologist. Because of the higher attenuation of the torso, body perfusion scans may require a higher kV than head perfusion scanning. Again, the image quality obtained should be noisier than most conv

40、entional body CT scans as the post-processing algorithm is able to extract the needed time attenuation information from the noisy data set. Respiratory motion is an important consideration in body perfusion CT and methods to limit diaphragmatic motion during the scan, or realign anatomic regions aft

41、er the scan using registration algorithms should be used to minimize errors introduced from the movement of the tissue of interest during the course of the perfusion scan. In the rare event that a body perfusion scan would be performed in a pediatric patient, sedation in small children may be requir

42、ed. Perfusion Acquisition Types (insert or modify manufacturer-specific language below) Manufacturers should provide specific language appropriate to their systems in the following section Some perfusion scans are performed in a continuous exposure mode, in which the table does not move and the x-ra

43、ys are turned on over the entire scan period. This provides the highest degree of temporal sampling; however, such temporal sampling may not be required for a particular application. This acquisition mode delivers the highest dose to the patient, since the x-ray beam is always on. Other techniques a

44、nd recommended protocols may include a mode where the table does not move but the x-rays are turned on intermittently during the scan (intermittent scanning). This method can be used to reduce the dose if the temporal sampling rate remains adequate for the post processing software to be used. Other

45、types of data acquisitions are axial or helical “shuttle modes” which are specially designed for perfusion scanning and can extend the coverage of tissue imaged, thus dispersing the dose over a wider area and decreasing peak skin dose. In both cases the temporal sampling rate is reduced for any spec

46、ific anatomic location compared with continuous and intermittent exposure XR 28-2013 Page 8 2013 National Electrical Manufacturers Association modes where the table remains stationary. The user must ensure that the sampling frequency remains adequate for the post-processing software. In axial shuttl

47、e mode, scanning is performed at two adjacent axial locations by moving the table between x-ray exposures, thereby increasing the amount of anatomy that is imaged. As with intermittent scanning, the x-rays are not turned on continuously and thus peak skin dose and overall delivered x-ray radiation d

48、ose are reduced (compared with continuous mode), while overall exam time remains the same. In helical shuttle mode, the scanner emits x-rays continuously while the table continuously moves back and forth across the prescribed scan range. As a result, the amount of anatomy that is imaged is increased

49、. The peak skin dose is reduced because the dose is similar to that of the continuous acquisition mode, but it is spread out over a larger region. The total x-ray on time typically remains the same. In helical shuttle mode, the total irradiation exposure is similar to that of a continuous acquisition exposure because the x-rays remain on during the entire acquisition; the benefit of this mode is that a larger anatomic region can be scanned. The temporal sampling rate varies based on the acquisition mode selected and can affect the total dose for the scan. The following