2018 ACC/HRS/NASCI/SCAI/SCCT - Heart Rhythm Society
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JOURNAL OF THE AMERICAN COLLEGE OF CARDIOLOGY VOL. -, NO. -, 2018 ª 2018 BY THE AMERICAN COLLEGE OF CARDIOLOGY FOUNDATION PUBLISHED BY ELSEVIER EXPERT CONSENSUS DOCUMENT 2018 ACC/HRS/NASCI/SCAI/SCCT Expert Consensus Document on Optimal Use of Ionizing Radiation in Cardiovascular Imaging— Best Practices for Safety and Effectiveness, Part 2: Radiological Equipment Operation, Dose-Sparing Methodologies, Patient and Medical Personnel Protection A Report of the American College of Cardiology Task Force on Expert Consensus Decision Pathways Developed in Collaboration With Mended Hearts Writing John W. Hirshfeld, JR, MD, FACC, FSCAI, Chair Gilbert L. Raff, MD, FACCk Committee Victor A. Ferrari, MD, FACC, Co-Chair Geoffrey D. Rubin, MD, MBA, FNASCI{ Members Donnette Smith# Frank M. Bengel, MD* Arthur E. Stillman, MD, PHD, FNASCI Lisa Bergersen, MD, MPH, FACC Suma A. Thomas, MD, MBA, FACC Charles E. Chambers, MD, FACC, MSCAIy Thomas T. Tsai, MD, MSC, FACC Andrew J. Einstein, MD, PHD, FACC Louis K. Wagner, PHD Mark J. Eisenberg, MD, MPH, FACC L. Samuel Wann, MD, MACC Mark A. Fogel, MD, FACC Thomas C. Gerber, MD, FACC *Society of Nuclear Medicine and Molecular Imaging Representative. David E. Haines, MD, FACCz ySociety for Cardiovascular Angiography and Interventions Warren K. Laskey, MD, MPH, FACC, FSCAI Representative. zHeart Rhythm Society Representative. xAmerican Society Marian C. Limacher, MD, FACC of Nuclear Cardiology Representative. kSociety for Cardiovascular Kenneth J. Nichols, PHDx Computed Tomography Representative. {North American Society for Cardiovascular Imaging Representative. #Mended Hearts Representative. Daniel A. Pryma, MD This document was approved by the American College of Cardiology Clinical Policy Approval Committee in November 2017 and the approval bodies of the Heart Rhythm Society, North American Society for Cardiovascular Imaging, Society for Cardiovascular Angiography and Interventions, and Society of Cardiovascular Computed Tomography in January 2018. The American College of Cardiology requests that this document be cited as follows: Hirshfeld JW Jr, Ferrari VA, Bengel FM, Bergersen L, Chambers CE, Einstein AJ, Eisenberg MJ, Fogel MA, Gerber TC, Haines DE, Laskey WK, Limacher MC, Nichols KJ, Pryma DA, Raff GL, Rubin GD, Smith D, Stillman AE, Thomas SA, Tsai TT, Wagner LK, Wann LS. 2018 ACC/HRS/NASCI/SCAI/SCCT expert consensus document on optimal use of ionizing radiation in cardiovascular imaging—best practices for safety and effectiveness, part 2: radiologic equipment operation, dose-sparing methodologies, patient and medical personnel protection: a report of the American College of Cardiology Task Force on Clinical Expert Consensus Documents. J Am Coll Cardiol 2018;71:XXXX–XXXX. Copies: This document is available on the World Wide Web site of the American College of Cardiology (www.acc.org). For copies of this document, please contact Elsevier Reprint Department via fax (212) 633-3820 or e-mail (reprints@elsevier.com). Permissions: Multiple copies, modification, alteration, enhancement, and/or distribution of this document are not permitted without the express permission of the American College of Cardiology. Requests may be completed online via the Elsevier site (http://www.elsevier.com/about/our- business/policies/copyright/permissions). ISSN 0735-1097/$36.00 https://doi.org/10.1016/j.jacc.2018.02.018
2 Hirshfeld Jr. et al. JACC VOL. -, NO. -, 2018 Radiation Safety ECD, Part 2 -, 2018:-–- ACC Task Force James L. Januzzi, JR, MD, FACC Pamela Bowe Morris, MD, FACC on Expert Luis C. Afonso, MBBS, FACC Robert N. Piana, MD, FACC Consensus Brendan Everett, MD, FACC Karol E. Watson, MD, FACC Decision Adrian F. Hernandez, MD, MHS, FACC Barbara S. Wiggins, PHARMD, AACC Pathways** William Hucker, MD, PHD Hani Jneid, MD, FACC **Formerly named ACC Task Force on Clinical Expert Dharam Kumbhani, MD, SM, FACC Consensus Documents. Joseph Edward Marine, MD, FACC TABLE OF CONTENTS ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 4. MODALITY-SPECIFIC DOSE REDUCTION STRATEGIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - PREAMBLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 4.1. General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . - 4.1.1. Case Selection . . . . . . . . . . . . . . . . . . . . . . . . . - 1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 4.1.2. Dose-Determining Variables . . . . . . . . . . . . . - 2. PURPOSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 4.1.3. Image Quality Issues . . . . . . . . . . . . . . . . . . . - 4.2. X-Ray Fluoroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . - 3. MODALITY-SPECIFIC RADIATION 4.2.1. General Principles . . . . . . . . . . . . . . . . . . . . . - EXPOSURE DELIVERY . . . . . . . . . . . . . . . . . . . . . . . . . . - 4.2.2. Digital X-Ray System Operating Modes . . . . - 3.1. General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . - 4.2.3. X-Ray System Calibration, Operation, 3.1.1. Characteristics of Medical and Dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - Diagnostic Radiation . . . . . . . . . . . . . . . . . . . . - 4.2.4. Determinants of Total Dose for an 3.1.2. Tools Used to Estimate Absorbed Dose . . . . . - Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 4.2.5. Procedures and Practices to Minimize 3.2. X-Ray Fluoroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . - Patient and Personnel Exposure . . . . . . . . . . - 3.2.1. X-Ray Fluoroscopy Subject and 4.2.6. Pregnant Occupationally Exposed Operator Exposure Issues . . . . . . . . . . . . . . . - Workers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 3.2.2. Basics of Operation of an X-Ray 4.2.7. Alternative Imaging Techniques . . . . . . . . . . - Cinefluorographic Unit . . . . . . . . . . . . . . . . . . - 4.2.8. Summary Checklist for Dose-Sparing in 3.2.3. Measures and Determinants of X-Ray Fluoroscopy . . . . . . . . . . . . . . . . . . . . - Subject and Operator Exposure . . . . . . . . . . - 4.3. X-Ray Computed Tomography . . . . . . . . . . . . . . . . - 3.2.4. Measures and Determinants of Physician 4.3.1. X-Ray CT General Principles . . . . . . . . . . . . . - Operator and Healthcare Worker 4.3.2. Equipment Quality and Calibration . . . . . . . - Occupational Exposure . . . . . . . . . . . . . . . . . - 4.3.3. Variables That Affect Patient Dose 3.3. X-Ray CT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - for X-Ray CT . . . . . . . . . . . . . . . . . . . . . . . . . - 3.3.1. X-Ray CT Subject and Operator 4.3.4. Summary Checklist of Dose-Sparing Dose Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . - Practices for X-Ray CT . . . . . . . . . . . . . . . . . . - 3.3.2. Basics of Operation of an 4.4. Nuclear Cardiology Techniques . . . . . . . . . . . . . . . . - X-Ray CT Unit . . . . . . . . . . . . . . . . . . . . . . . . - 4.4.1. Nuclear Cardiology General Principles . . . . . - 3.3.3. X-Ray CT Measures of 4.4.2. Nuclear Cardiology Equipment Quality, Subject Exposure . . . . . . . . . . . . . . . . . . . . . . - Calibration, and Maintenance . . . . . . . . . . . . - 3.3.4. X-Ray CT Measures of Effective Dose . . . . . - 4.4.3. Nuclear Cardiology Spatial Resolution and Image Detector Dose . . . . . . . . . . . . . . . . . . . - 3.4. Patient and Medical Personnel Exposure in 4.4.4. Procedures and Practices to Minimize Nuclear Cardiology . . . . . . . . . . . . . . . . . . . . . . . . . . - Patient Exposure . . . . . . . . . . . . . . . . . . . . . . - 3.4.1. Patient Exposure in Nuclear Cardiology . . . . - 4.4.5. Procedures and Practices to Protect 3.4.2. Personnel Exposure in Nuclear Occupationally Exposed Healthcare Cardiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . - Workers in Nuclear Cardiology Facilities . . . -
JACC VOL. -, NO. -, 2018 Hirshfeld Jr. et al. 3 -, 2018:-–- Radiation Safety ECD, Part 2 4.4.6. Summary Checklist of Dose-Sparing ABSTRACT Practices for Nuclear Cardiology . . . . . . . . . - 4.5. Summary of Dose Minimization Strategies in The stimulus to create this document was the recogni- X-Ray Fluoroscopy, X-Ray CT, and tion that ionizing radiation-guided cardiovascular pro- Cardiovascular Nuclear Scintigraphy . . . . . . . . . . . - cedures are being performed with increasing frequency, leading to greater patient radiation exposure and, 5. SUMMARY, CONCLUSIONS, AND potentially, to greater exposure to clinical personnel. RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . - While the clinical benefit of these procedures is sub- stantial, there is concern about the implications of 5.1. The Issue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - medical radiation exposure. ACC leadership concluded 5.1.1. Patient Participation in Clinical that it is important to provide practitioners with an Imaging Decisions . . . . . . . . . . . . . . . . . . . . . . - educational resource that assembles and interprets the current radiation knowledge base relevant to cardiovas- 5.2. Clinical Value of Radiation-Based Imaging Studies and Radiation-Guided Therapeutic Procedures . . . - cular procedures. By applying this knowledge base, car- diovascular practitioners will be able to select 5.3. Individual Patient Risk and Population Impact procedures optimally, and minimize radiation exposure (Including Occupationally Exposed Workers) . . . . . - to patients and to clinical personnel. 5.4. The ALARA Principle . . . . . . . . . . . . . . . . . . . . . . . . - “Optimal Use of Ionizing Radiation in Cardiovascular Imaging - Best Practices for Safety and Effectiveness” is a 5.5. The Potential to Minimize Exposure to comprehensive overview of ionizing radiation use in Patients and Personnel . . . . . . . . . . . . . . . . . . . . . . . - cardiovascular procedures and is published online. To 5.5.1. Imaging Modality Choice . . . . . . . . . . . . . . . . - provide the most value to our members, we divided the 5.5.2. Procedure Conduct Choice . . . . . . . . . . . . . . - print version of this document into 2 focused parts. 5.5.3. Protecting Occupationally Exposed “Part I: Radiation Physics and Radiation Biology” ad- Workers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - dresses radiation physics, dosimetry and detrimental 5.6. Physician Responsibilities to Minimize biologic effects. “Part II: Radiologic Equipment Operation, Patient Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . - Dose-Sparing Methodologies, Patient and Medical 5.6.1. Case Selection . . . . . . . . . . . . . . . . . . . . . . . . . - Personnel Protection” covers the basics of operation and 5.6.2. Procedure Conduct . . . . . . . . . . . . . . . . . . . . . - radiation delivery for the 3 cardiovascular imaging mo- 5.6.3. Facility Management . . . . . . . . . . . . . . . . . . . - dalities (x-ray fluoroscopy, x-ray computed tomography, and nuclear scintigraphy). For each modality, it includes 5.7. Patient Radiation Dose Tracking . . . . . . . . . . . . . . . - the determinants of radiation exposure and techniques to 5.8. Need for Quality Assurance and Training . . . . . . . . - minimize exposure to both patients and to medical personnel. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - APPENDIX A PREAMBLE Author Relationships With Industry and Other Entities (Relevant) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - This document has been developed as an Expert Consensus Document by the American College of Cardi- APPENDIX B ology (ACC) in collaboration with the American Society of Peer Reviewer Relationships With Industry and Nuclear Cardiology, Heart Rhythm Society, Mended Other Entities (Relevant) . . . . . . . . . . . . . . . . . . . . . . . . . - Hearts, North American Society for Cardiovascular Imaging, Society for Cardiovascular Angiography and APPENDIX C Interventions, Society for Cardiovascular Computed To- Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - mography, and Society of Nuclear Medicine and Molecu- lar Imaging. Expert Consensus Documents are intended to APPENDIX D inform practitioners, payers, and other interested parties Operator Education, Quality Assurance, of the opinion of ACC and document cosponsors con- Radiation Dose Monitoring, and Tracking . . . . . . . . . . . - cerning evolving areas of clinical practice and/or
4 Hirshfeld Jr. et al. JACC VOL. -, NO. -, 2018 Radiation Safety ECD, Part 2 -, 2018:-–- technologies that are widely available or new to the 1. INTRODUCTION practice community. Topics chosen for coverage by expert consensus documents are so designed because the evi- 1.1. Document Development Process and Methodology dence base, the experience with technology, and/or clin- 1.1.1. Writing Committee Organization ical practice are not considered sufficiently well The writing committee consisted of a broad range of developed to be evaluated by the formal ACC/American members representing 9 societies and the following areas Heart Association practice guidelines process. Often the of expertise: interventional cardiology, general cardiol- topic is the subject of considerable ongoing investigation. ogy, pediatric cardiology, nuclear cardiology, nuclear Thus, the reader should view the Expert Consensus medicine, electrophysiology, cardiac computed tomogra- Document as the best attempt of the ACC and document phy (CT), cardiovascular imaging, and the consumer pa- cosponsors to inform and guide clinical practice in tient perspective. Both a radiation safety biologist and areas where rigorous evidence may not yet be available physicist were included on the writing committee. or evidence to date is not widely applied to clinical This writing committee met the College’s disclosure practice. requirements for RWI as described in the Preamble. To avoid actual, potential, or perceived conflicts of interest that may arise as a result of industry relation- 1.1.2. Document Development and Approval ships or personal interests among the writing committee, The Writing Committee convened by conference call and e- all members of the writing committee, as well as peer mail to finalize the document outline, develop the initial reviewers of the document, are asked to disclose all cur- draft, revise the draft per committee feedback, and ulti- rent healthcare-related relationships, including those mately approve the document for external peer review. existing 12 months before initiation of the writing effort. All participating organizations participated in peer review, The ACC Task Force on Expert Consensus Decision resulting in 21 individual reviewers submitting 299 Pathways (formerly the ACC Task Force on Clinical Expert comments. Comments were reviewed and addressed by Consensus Documents) reviews these disclosures to the writing committee. A member of the ACC Task Force on determine which companies make products (on market or Expert Consensus Decision Pathways served as lead in development) that pertain to the document under reviewer to ensure that all comments were addressed development. Based on this information, a writing com- adequately. Both the writing committee and the task force mittee is formed to include a majority of members with approved the final document to be sent to the ACC Clinical no relevant relationships with industry (RWI), led by a Policy Approval Committee. This committee reviewed chair with no relevant RWI. Authors with relevant RWI are the document, including all peer review comments and not permitted to draft or vote on text or recommenda- writing committee responses, and approved the document tions pertaining to their RWI. RWI is reviewed on all in November 2017. The Heart Rhythm Society, North conference calls and updated as changes occur. Author American Society for Cardiovascular Imaging, Society for and peer reviewer RWI pertinent to this document are Cardiovascular Angiography and Interventions, and Soci- disclosed in Appendixes A and B, respectively. Addi- ety of Cardiovascular Computed Tomography endorsed the tionally, to ensure complete transparency, authors’ document in January 2018. This document is considered comprehensive disclosure information— including RWI current until the Task Force on Expert Consensus Decision not pertinent to this document—is available online Pathways revises or withdraws it from publication. (see Online Appendix). Disclosure information for the ACC Task Force on Clinical Expert Consensus Documents 2. PURPOSE is also available online, as is the ACC disclosure policy for document development. This print-published document is part 2 of an abbreviated The work of the writing committee was supported version of a larger, more comprehensive document that is exclusively by the ACC without commercial support. published concurrently online. The online version con- Writing committee members volunteered their time to tains additional technical detail for readers who wish to this effort. Conference calls of the writing committee understand a topic in greater depth. The online published were confidential and attended only by committee document, in addition to covering the topics in the 2 members and ACC staff. print-published documents in greater depth, also covers additional topics not covered in the print-published doc- James L. Januzzi, MD, FACC uments including 1) dose reduction strategies; 2) operator Chair, ACC Task Force on Clinical Expert education and certification; 3) quality assurance; and Consensus Documents 4) patient radiation tracking.
JACC VOL. -, NO. -, 2018 Hirshfeld Jr. et al. 5 -, 2018:-–- Radiation Safety ECD, Part 2 This document covers equipment operation for the 3 1% to 5% of the incident x-ray penetrates the subject cardiovascular procedure classes that employ ionizing reaching the image detector to form the image. radiation: x-ray fluoroscopy, x-ray CT, and radionuclide scintigraphy. For the 3 modalities, it includes discussions 3.1.2. Tools Used to Estimate Absorbed Dose of radiation delivery and strategies to minimize dose both Estimates of absorbed dose for x-ray fluoroscopy and to patients and to occupationally exposed medical x-ray CT are based on models developed by exposing personnel. In addition, it covers issues of quality assur- instrumented phantoms to incident x-ray beams that ance, radiation monitoring, and tracking. replicate the beams used in diagnostic imaging and The document’s purpose is to provide a comprehensive measuring absorbed dose at different points within the information source about ionizing radiation use in car- phantom. Estimating absorbed dose from radionuclides is diovascular procedures. The writing group has assembled an entirely different discipline that is discussed in Section this information to assist cardiovascular practitioners to 4.4 of “Part I: Radiation Physics and Radiation Biology”. provide optimal cardiovascular care when employing ionizing radiation-based procedures. The goal is to 3.2. X-Ray Fluoroscopy enhance cardiovascular practitioners’ ability to select the optimal imaging technique for a given clinical circum- 3.2.1. X-Ray Fluoroscopy Subject and Operator Exposure Issues stance, balancing a technique’s risk and benefits, and to X-ray fluoroscopy differs from other ionizing radiation apply that technique optimally to generate high-quality imaging techniques in that the beam entrance port is diagnostic images of greatest clinical value and minimal relatively small. Consequently, the skin at the beam radiation exposure. entrance port is the most intensely exposed tissue. Subject skin doses can reach levels that cause skin tissue reactions. X-ray photons are also scattered within the 3. MODALITY-SPECIFIC RADIATION subject. These deliver dose to subject tissues outside of EXPOSURE DELIVERY the imaging field (Figure 1). Scattered photons that exit the subject can expose nearby medical personnel 3.1. General Principles (Figure 2). Consequently, assessment of the implications 3.1.1. Characteristics of Medical Diagnostic Radiation of subject exposure from x-ray fluoroscopy must For all 3 imaging modalities (x-ray fluoroscopy, x-ray CT, consider entrance port skin dose, which is the dose and nuclear scintigraphy), 95% to 99% of radiation energy received by internal structures within the imaging field that enters or is released within the subject is either and by other internal structures outside of the imaging absorbed or scattered within the subject. The remaining field. F I G U R E 1 Diagrammatic Representation of an X-Ray Fluoroscopy System to Illustrate X-Ray Exposure Modality The primary beam, collimated to a rectangular cross section, enters the patient, typically through the patient’s back. It is attenuated and scattered within the imaging field. The primary beam exposes the subject within the imaging field. The scattered primary beam radiation can expose structures within the subject that are remote from the imaging field.
6 Hirshfeld Jr. et al. JACC VOL. -, NO. -, 2018 Radiation Safety ECD, Part 2 -, 2018:-–- F I G U R E 2 Diagrammatic Representation of the Pattern of X-Ray Scatter From a Subject Undergoing X-Ray Fluoroscopy Note that scattered x-ray emanates from the subject in all directions. 3.2.2. Basics of Operation of an X-Ray Cinefluorographic Unit to image formation. Layers of aluminum and copper in An x-ray cinefluorographic unit generates controlled x- the x-ray tube exit port filter out these “undesirable” rays in an x-ray tube that are collimated to regulate the photons. size and shape of the beam. The beam passes through the subject forming images that are detected by a flat panel 3.2.3. Measures and Determinants of Subject and detector (Figure 1). The x-ray tube output (and accord- Operator Exposure ingly the exposure to the subject) is modulated by feed- There are 2 different x-ray fluoroscopic system parame- back circuitry from the unit’s imaging chain to achieve an ters (described in detail in Section 4.4.1 of Part 1) that optimally exposed image. characterize x-ray exposure and dose: X-Ray Cinefluorographic Unit Operating Parameters There are multiple imaging parameters that influence the 1) Cumulative air kerma at the interventional reference x-ray exposure associated with an x-ray cinefluorographic point. Kerma is an acronym for “kinetic energy examination. These are: released in material.” 2) Cumulative kerma-area product (KAP). 1. X-ray image detector dose per pulse. The dose for each x-ray pulse (typically measured in nanogray [nGy]) that reaches the x-ray system detector. This parameter Cumulative Air Kerma at the is set by the x-ray unit calibration. It determines image Interventional Reference Point clarity and detail. A procedure’s cumulative air kerma at the interventional 2. X-ray unit framing (pulsing) rate. The number of pul- reference point is a more meaningful measure of subject ses that the x-ray system generates per unit time. This exposure than the total fluoroscopic time, which does is an operator-selectable parameter that generally not account for selected detector dose, subject density, ranges between 4 and 30 pulses/s. It determines image cine acquisition time, or changes in frame rate and temporal resolution. angulation. 3. Imaging field size. The area of the x-ray beam that X-ray exposure to the subject is not uniform. As an impinges on the subject. x-ray beam passes through a subject, tissue absorption 4. X-ray beam filtration. An x-ray tube produces a attenuates it. Tissue closer to the beam entrance port spectrum of x-ray photon energies. Photon receives a larger dose than deeper-lying tissue (Figure 3). energies
JACC VOL. -, NO. -, 2018 Hirshfeld Jr. et al. 7 -, 2018:-–- Radiation Safety ECD, Part 2 3.2.4. Measures and Determinants of Physician Operator and F I G U R E 3 Diagram Showing the Estimated Decreasing Intensity of X-Ray Exposure With Depth Within the Subject Healthcare Worker Occupational Exposure Application of KAP in Cardiovascular X-Ray Fluoroscopy to Estimates of Effective Dose to Medical Personnel Medical personnel who conduct x-ray fluoroscopic pro- cedures are exposed by scattered radiation (Figure 2). The cumulative quantity of scattered radiation is directly related to the procedure’s cumulative KAP. The quantity of scattered radiation that reaches and delivers dose to medical personnel is determined by: 1. The distance of the exposed medical personnel from the x-ray source—scattered x-ray intensity decreases proportionately to the square of the distance from the source. 2. The effectiveness of shielding employed by the exposed medical personnel. Physician and Medical Personnel Exposure Monitoring In this example, (right anterior oblique projection), the beam enters Estimates of radiation dose to exposed medical the left side of the subject’s back. Beam intensity decreases with depth personnel are based on measurements made by personal within the subject due to a combination of beam divergence with radiation monitors (formerly known as “film badges”). distance (inverse square law) and absorption within the subject. The overall effect of these processes is to attenuate the beam intensity The outside badge mounted at collar level outside that exits the subject to 5% or less of the incident intensity. protective garments) measures the dose that reaches unshielded structures of the head. A badge worn underneath protective garments measures the dose that tissue within the imaging area, but is not negligible penetrates the protective apron reaching the subject. (Figure 1). These measure total exposure in mGy. The personal Kerma-Area Product radiation monitor readings are converted using an KAP, the product of air kerma output and image field size, algorithm to estimate effective dose to the subject sometimes referred to as dose-area product, is commonly in mSv (2–5). The details of these measurements and used as a metric to estimate a subject’s total absorbed calculations are included in the full version of this dose. It incorporates both dose intensity and exposed document published online. tissue volume into a single measurement. KAP is also directly related to the quantity of scattered radiation that Exposure Levels for Operating Physicians leaves the subject’s body and, accordingly, to the Most studies of operating physician dosimetry find a magnitude of exposure to nearby medical personnel range of 0.02 to 0.12 m Sv/Gy$cm 2 KAP for the procedure (Figure 2). with typical values clustering about 0.1 mSv/Gy$cm 2 (6,7). 2 KAP is expressed in units of Gy$cm . It is calculated by (Note that the estimated patient exposure is 200 m Sv/ multiplying the beam air kerma by its cross-sectional Gy$cm 2, indicating that operator exposure is roughly area. Some x-ray system manufacturers report KAP in 1/2,000 of patient exposure.) Applying these values, a units of m Gy$m 2 (1 Gy$cm 2 ¼ 100 m Gy$m 2). It should also be “typical” combined coronary arteriogram and straight- noted that air kerma and KAP represent cumulative doses forward coronary interventional procedure utilizing a from an exposure, not exposure rates. cumulative KAP of 50 Gy$cm2 would deliver a 5-m Sv Application of KAP in Cardiovascular X-Ray effective dose to the physician operator standing roughly Fluoroscopy to Estimates of Effective Dose to Patients 1 m from the center of the primary beam while delivering The most commonly used estimate of the relationship 10 mSv to the patient. between KAP exposure to the thorax in Gy$cm 2 and Special considerations for occupationally exposed effective dose in Sieverts (Sv) is 0.20 mSv/Gy$cm2 (1). By workers who are pregnant or may become pregnant are this estimate, a combination coronary arteriography and discussed in Section 4.2 of this document and in greater percutaneous coronary intervention that delivers a KAP detail in section 5.4.4 of “Part I: Radiation Physics and exposure of 50 Gy$cm 2 would impart an effective dose to Radiation Biology” and in the longer, online-published the subject of 10 mSv. version of this document.
8 Hirshfeld Jr. et al. JACC VOL. -, NO. -, 2018 Radiation Safety ECD, Part 2 -, 2018:-–- F I G U R E 4 Comparison of Retrospective ECG Gating With Prospective ECG Gating With retrospective gating, the intensity-modulated x-ray beam is on for the entirety of the R-R intervals during imaging. With prospective gating, the x-ray beam is on for about 26% of every other R-R interval. Reproduced with permission from Shuman et al. (8). 3.3. X-Ray CT Electrocardiographic gating, which can have a major 3.3.1. X-Ray CT Subject and Operator Dose Issues impact on dose, is important in cardiovascular imaging to minimize motion artifact. Although x-ray CT, like x-ray fluoroscopy, is an external There are 2 types of gating (Figure 4): beam exposure technique, unlike x-ray fluoroscopy the incident beam is distributed circumferentially around the n Retrospective gating involves x-ray exposure contin- subject. Consequently, x-ray CT subject skin doses should ually over the cardiac cycle. Because exposure occurs never approach levels that could cause skin injury, and continuously, retrospective gating delivers greater subject-harm issues should be confined to stochastic risk. exposure than prospective triggering. The dose delivered by an x-ray CT examination is not n Prospective triggering involves synchronizing expo- uniform, delivering greater dose to more superficial lo- sure to a selected portion of the cardiac cycle. The goal cations compared with deeper locations closer to the of prospective triggering is for exposure to occur only exposed volume center. when cardiac motion is minimal. 3.3.2. Basics of Operation of an X-Ray CT Unit 3.3.3. X-Ray CT Measures of Subject Exposure The dose delivered by an x-ray CT examination can vary The dose delivered by an x-ray CT examination should be substantially depending on patient characteristics and considered from 2 perspectives: the settings of multiple scanner operating parameters. n Dose Intensity: Dose per unit mass of tissue. This is a Configurable CT technique parameters that can affect measure of the intensity of the dose used to generate dose include x-ray tube potential (measured in kV), x-ray the images. tube current (measured in milliamperes [mA]), scan pro- n Volume of Tissue Exposed. The total dose delivered to tocol (e.g., axial or helical), pitch, gating protocol, scan a subject is the product of the dose intensity and the rotation time, beam width, scan length, and beam volume of tissue exposed. filtration. Image quality is affected by imaging parameter selec- Although CT dose metrics are derived from the mea- tion. This selection involves a conscious balancing of surement of x-ray tube air kerma, in the CT lexicon, the image quality and dose. Other parameter selections, such term “dose” is widely used. as gating protocol, do not necessarily affect image quality CT Dose Index—A Measure of Dose Intensity but do affect the amount of radiation used to acquire an Computed tomography dose index (CTDI) was first image set. defined in 21 CFR 1020.33(c) as the average dose detected
JACC VOL. -, NO. -, 2018 Hirshfeld Jr. et al. 9 -, 2018:-–- Radiation Safety ECD, Part 2 over a 100-mm scan length from an imaging acquisition of phantoms for the determination of DLP. For that reason, 14 slices. It is a measure of dose intensity, that is, the dose when reporting CTDI or DLP in children, the phantom size imparted by a unit scan length. used should always be specified. In addition, in children, CTDI100 sensitivity to a stochastic event varies substantially with CTDI 100 is a refinement of CTDI that standardizes all dose subject age. Consequently, in children, European guide- index measurements to a scan length of 100 mm. lines for chest CT conversion factors (19), based on the 32- Weighted CTDI 100 cm phantom, range from 0.013 mSv $ mGy 1 $ cm 1 (age 10 The CTDIw, or weighted CTDI 100, is an index developed to years) to 0.039 mSv $ mGy 1 $ cm 1 (age 0 years). Only 2 approximate the average radiation dose delivered to a studies using contemporary cardiac scanners have deter- cross section of a subject’s body, allowing for dose vari- mined cardiac CT-specific conversion factors for children. ation with depth. Normalized to the 32-cm phantom, conversion factors (20) Volume CTDI range from 0.092 to 0.099 mSv $ mGy 1 $ cm 1 for age 1 CTDI vol is the weighted absorbed dose to air of a 1 cm year, 0.049 to 0.082 mSv $ mGy 1 $ cm 1 for age 5 years, axial length of the examined subject located in the (19) and 0.049 mSv $ mGy 1 $ cm 1 for age 10 years. middle section of a 100-mm length scan of an acrylic cylinder for a specific CT technique. It accounts for both 3.4. Patient and Medical Personnel Exposure in the exposure directly delivered to the 1-cm thick slice Nuclear Cardiology and the exposure to that slice by scatter from adjacent 3.4.1. Patient Exposure in Nuclear Cardiology imaged tissue. Unlike x-ray imaging, which principally exposes the CTDI vol Special Considerations for Exposure in Children imaged structures, an injected radioactive tracer exposes It is noteworthy that for identical techniques, smaller the entire body. Organs receiving the highest radiation subjects receive a higher dose than larger subjects. Esti- dose may not be the imaged structures. The patient’s mates of CTDI vol for body imaging made utilizing a 32-cm behavior after study completion can alter the rate of thick phantom underestimate the dose received by radiopharmaceutical excretion, affecting the overall ra- smaller individuals by a factor of 2. diation dose. Size-Specific Dose Estimate Estimating the effective dose from a radiopharmaceu- Size-specific dose estimate is a normalization of CTDIvol tical exposure incorporates: that takes into account subject size. Its incorporation into practice is still to be determined. 1. Quantity of radioactivity administered. Dose-Length Product—A Measure of the Total Dose 2. Radiopharmaceutical distribution within the subject. Absorbed by the Subject 3. Kinetics of distribution to and elimination from each Dose-length product (DLP) is the product of CTDI vol and organ. the axial scan length. It is a measure of total dose to the 4. Radiosensitivity of each exposed organ. subject and is analogous to KAP for x-ray fluoroscopy. 5. Physical half-life of the radionuclide and its emitted Accordingly, for x-ray CT, DLP is the best predictor of photon or particle energy. stochastic risk. Medical internal radiation dose is a commonly used framework for estimating the radiation dose from radio- 3.3.4. X-Ray CT Measures of Effective Dose pharmaceuticals. The medical internal radiation dose For CT imaging, European Commission–sponsored method uses the radiopharmaceutical’s “effective” half- guidelines from 2000 (9) and 2004 (10) have suggested a life—the combination of radionuclide organ residence simple approximation of the effective dose that can be times and physical decay rates—to estimate the total dose obtained by multiplying the DLP by a conversion factor k 1 1 (in mGy) received by each organ. These values are (unit: mSv $ mGy $ cm ) that varies dependent on the multiplied by the individual organ radiation sensitivities radiation sensitivity of different body regions and patient to yield the individual organ equivalent doses, which are ages. There are specified conversion factors for CT of the then summed to calculate the whole-body effective dose head, neck, chest, abdomen, pelvis, and legs (11). The for the subject in mSv. most common conversion factor for adult chest CT is Additional dose issues: 0.014 mSv $ mGy 1 $ cm1 (12), with values for children being greater. For CT examinations confined to the car- 1. A renally excreted radiopharmaceutical will deliver a diac region, estimated conversion factors are greater, with radiation dose to the bladder wall. If the subject voids an average value of 0.026 mSv $ mGy 1 $ cm 1 (13–18). infrequently, the dose to the bladder will be higher. X-Ray CT Measures of Effective Dose in Children 2. Radiopharmaceutical imaging studies, both positron Pediatric CT dosimetry is complicated by the fact that imaging (positron emission tomography [PET]) and scanners and studies have variably used 32- or 16-cm single-photon emission computed tomography
10 Hirshfeld Jr. et al. JACC VOL. -, NO. -, 2018 Radiation Safety ECD, Part 2 -, 2018:-–- (SPECT), which employ attenuation correction, utilize a is important always to seek to minimize patient radiation hybrid radiation-based technique to estimate attenua- exposure (this is a particular consideration in younger tion. This delivers an additional exposure. patients who have long natural life expectancies), it is equally important to not withhold appropriate studies 3.4.2. Personnel Exposure in Nuclear Cardiology due to undue concern of the radiation-related risk. Nuclear cardiology personnel receive exposure both from 4.1.2. Dose-Determining Variables handling radiopharmaceutical doses and from their The radiation dose delivered to patients and medical proximity to radioactive patients. There are substantive personnel (regardless of modality) is affected by 3 vari- differences, compared with x-ray environments, in the ables that are under the operator’s control. These are: variables affecting personnel exposure: 1. Equipment quality and calibration 1. The photons emitted from the subject from radio- 2. Equipment operating protocols pharmaceuticals are generally of higher energy than 3. Operator conduct the x-rays emitted from fluoroscopy or CT devices. Therefore, personal shielding devices such as lead As each of these variables influences the dose delivered aprons or leaded glasses are less effective and, conse- to the patient (and also, potentially to operating medical quently, are rarely used. Nuclear cardiology personnel personnel), each provides an opportunity to reduce dose. rely on the principles of time and distance, minimizing the time they spend in close proximity to either the 4.1.3. Image Quality Issues dose syringe or the injected patient. Image quality is a major determinant of an examination’s 2. Unlike x-ray environments, the radiopharmaceutical is diagnostic accuracy. Inadequate image quality may cause a continuous source of activity that can be excreted via either incorrect diagnoses or a need to repeat an exami- body fluids or spread during administration. Thus, nation—requiring additional patient exposure. Conse- subject blood and excreted body fluids are radioactive. quently, it is imperative that radiological equipment meet An accident or error can cause a healthcare worker to current image quality standards, be maintained in prime receive an exposure from contamination. working order, and are operated properly to produce high-quality diagnostic images. 4. MODALITY-SPECIFIC DOSE REDUCTION Radiological image quality is strongly influenced by the STRATEGIES detector dose—the quantity of radiation that reaches the image detector. Overall image quality is determined by 4.1. General Principles spatial and temporal resolution, the signal-to-noise ratio, Table 1 indicates core principles to follow for the use of the contrast-to-noise ratio, and presence of imaging arti- medical ionizing radiation for diagnostic and therapeutic facts. Most tactics that increase either spatial resolution (by procedures. improving signal-to-noise ratio and contrast-to-noise ra- tio) or temporal resolution (by increasing framing rate) do Core Principles for the Use of Medical Ionizing so at the cost of increased dose. The challenge is to optimize TABLE 1 Radiation for Diagnostic and Therapeutic these properties by balancing the tradeoffs between dose Procedures and image quality. There are circumstances in which the 1. The examination should be conducted such that the dose received by the “best” image that the system can deliver is better than patient and attendant medical personnel is the smallest necessary to yield satisfactory diagnostic efficacy. needed for diagnosis. Consequently, operators can choose 2. Diagnostic and therapeutic efficacy should not be compromised in the interest to accept a lower image quality, which is still sufficiently of sparing radiation dose. diagnostic, to reduce patient (and operator) radiation dose. 3. If the study’s purpose can be achieved employing a modality that does not Spatial Resolution—Detector Input Dose, Pulse Width, employ ionizing radiation, serious consideration should be given to the and Nuclear Scan Acquisition alternative modality. Image signal-to-noise ratio is inversely proportional to the square root of the detector dose. Low signal-to-noise 4.1.1. Case Selection ratio images have a “grainy” appearance because the The most effective way to reduce patient radiation image is formed by a small number of x-ray photons. This exposure is to perform a radiation-based procedure only grainy quality, termed “quantum mottle,” becomes when it is the preferred choice among alternative mo- smoother as dose increases, improving the ability to dalities that do not involve radiation exposure (e.g., stress perceive image detail. echo or stress cardiac magnetic resonance). Appropriate Examples of the impact of detector dose on image noise use criteria should be applied to select patients to un- for x-ray fluoroscopic imaging are presented in Figure 5. dergo diagnostic and therapeutic procedures. Although it These are images of a line pair phantom acquired at different
JACC VOL. -, NO. -, 2018 Hirshfeld Jr. et al. 11 -, 2018:-–- Radiation Safety ECD, Part 2 F I G U R E 5 Images of a Line Pair Phantom Acquired in an X-Ray Fluoroscopic System at Different Detector Doses (as Labeled on the Individual Images) Note the progressive decrease in image noise and the ability to perceive image detail as the dose increases: 10 nGy/frame, an unacceptably low dose; 18 nGy/ frame, representative dose for low-dose fluoroscopy; 40 nGy/frame, representative dose for standard-dose fluoroscopy; 200 nGy/frame, representative dose for cine acquisition; 1,200 nGy/frame, representative dose for digital subtraction imaging. detector doses ranging from 10 to 1,200 nGy/frame. counts per unit time, and the image acquisition time, with As the number of photons reaching the detector increases, longer acquisition times acquiring a larger number of image noise decreases and the image becomes smoother. counts. Over a defined range, as image noise decreases, perceptible The cardiovascular system moves. This imposes image spatial resolution increases. For each imaging additional requirements on cardiovascular imaging sys- modality there is an upper limit of dose beyond which tems. Spatial resolution is also determined by x-ray further dose increase, although it may produce a smoother- pulse width. Images acquired with pulse durations >8 appearing image, does not yield greater image detail of ms will be degraded by motion unsharpness just as diagnostic importance. photographs of moving objects are blurred if acquired at Similarly, the image noise in x-ray CT images is deter- slower camera shutter speeds. Typical pulse durations mined in part by detector dose. Larger doses will yield are 2 to 8 ms. images with less noise and, within limits, greater spatial Temporal Resolution—Pulse Frequency resolution. For x-ray CT, the spatial resolution required to If an image series (such as an x-ray fluoroscopy cine assess myocardial contours, and, accordingly, the dose acquisition) is acquired at too slow of a frame rate, events needed to achieve it, is smaller than that required to im- that occur during time periods shorter than the framing age coronary arteries. rate will not be resolved and object motion will cause the For nuclear scan images, the number of gamma ray image to have a jerky quality. counts that are acquired to construct the image de- termines the image noise and, accordingly, its spatial 4.2. X-Ray Fluoroscopy resolution, which improves as the number of counts ac- Of the 3 imaging modalities, x-ray fluoroscopy has the quired increases. The number of counts acquired is greatest variability in dose per procedure and has the determined by the amount of radioactivity administered potential to deliver the largest dose to patients, operators, for the examination, which determines the number of and nearby medical personnel. Dose is substantially
12 Hirshfeld Jr. et al. JACC VOL. -, NO. -, 2018 Radiation Safety ECD, Part 2 -, 2018:-–- affected by operator choices, behavior, equipment qual- catheter placement can be accomplished with fluoro- ity, and calibration. scopic frame rates as slow as 4 frames/s. More complex procedures such as coronary and structural interventions 4.2.1. General Principles require greater temporal resolution and employ frame For an x-ray fluoroscopic examination, the total skin dose rates between 10 and 15 frames/s. (in Gy) is determined by the sum of air kermas of all the Cine acquisition frame rates also vary with the purpose frames (fluoroscopy and cine acquisition) in the exami- of the examination. For coronary arteriography, a frame nation. The total effective dose is proportional to the sum rate of 10 to 15 frames/s is generally adequate. For adult of the KAPs of all of the examination’s frames. ventriculography, 30 frames/s is preferred to achieve more precise identification of end diastole and end sys- 4.2.2. Digital X-Ray System Operating Modes tole. In pediatric applications, framing rates as fast as 60 Digital x-ray imaging systems operate in 3 modes that frames/s are occasionally needed. employ different detector doses to achieve different im- age spatial resolution. 4.2.4. Determinants of Total Dose for an Exposure Dose per Frame and Framing Rate 1. Fluoroscopy—the lowest-dose imaging protocol that The optimal parameter settings for a fluoroscopic yields images with the lowest spatial resolution. examination or a cine acquisition run are determined by Typical fluoroscopic detector doses range between 20 the patient’s particular circumstance’s and requirements and 40 nGy/frame. for spatial and temporal resolution. For fluoroscopy 2. Cine acquisition—an intermediate-dose imaging proto- mode, current x-ray units typically provide tableside- col intended to provide diagnostic quality images for selectable fluoroscopy detector dose per frame levels archiving and diagnostic interpretation. Cine acquisi- that produce different degrees of image noise. They also tion images have less image noise than fluoroscopic provide tableside fluoroscopy and cine acquisition frame images but should still have visible noise. Typical cine rates ranging from 4 to 30 pulses/s. For cine acquisition acquisition detector dose rates are 200 nGy/frame. mode, the detector dose per pulse is set by the service 3. Digital subtraction—Digital subtraction algorithms are engineer but the operator is able to select the frame highly sensitive to image noise and require high doses rate. to function effectively. Consequently, digital subtrac- X-Ray Imaging Field Size and System Positioning tion algorithms per frame dose rates are the largest Whereas the dose per pulse and the number of pulses (typically 1,200 nGy/frame). determine the total dose intensity (in mGy) delivered to the patient, the product of the total dose and the imaging 4.2.3. X-Ray System Calibration, Operation, and Dose field size determines the total amount of radiation energy The goals and purposes of an examination determine the (expressed as the KAP in Gy$cm 2) that the patient re- optimal balance between radiation exposure and image ceives. In addition to the examination’s total number of spatial and temporal resolution. For example, for x-ray pulses and the detector dose per pulse, the KAP is fluoroscopy, the spatial and temporal resolution required affected by 2 additional parameters that are under the for general catheter placement and manipulation is less operator’s control: the imaging field size selected and than that required to perform cardiac interventional system positioning. procedures. Current x-ray fluoroscopy systems are X-Ray Imaging Field Size capable of imaging at multiple frame rates and can adjust Current x-ray systems link brightness stabilization detector gain to utilize variable detector doses (21,22). detection to a collimator position that samples only the These capabilities enable the operator to select an optimal detector area receiving the collimated x-ray beam. Conse- imaging protocol for a particular situation. quently, the dose per pulse to the detector is not affected Temporal Resolution Issues and Dose Tradeoffs by collimator position. However, the KAP is directly related Because the cardiovascular system moves, x-ray fluoro- to the size of the imaged area. The consequence of this graphic imaging requires short pulse durations to limit phenomenon is that, for a given detector zoom (magnifi- image motion unsharpness (typically between 3 and 8 ms cation or input phosphor size) mode, smaller image area for adults, as short as 2 ms for children). sizes deliver proportionately smaller KAPs. Thus, at a given Fluoroscopic temporal resolution requirements vary detector zoom mode, reducing exposed field size by colli- substantially depending on the examination’s purpose. In mation to the smallest size necessary minimizes the KAP less demanding circumstances, the operator can decrease that the patient receives. This is not true for changing dose by utilizing slower frame rates and lower doses detector zoom modes. Detector dose per pulse increases as per frame without compromising effectiveness. General the zoom magnification increases.
JACC VOL. -, NO. -, 2018 Hirshfeld Jr. et al. 13 -, 2018:-–- Radiation Safety ECD, Part 2 X-Ray System Positioning the radiation scattered within the patient that would There is an optimal distance between the patient’s skin otherwise reach medical personnel; accordingly, x-ray surface and the x-ray source (typically approximately 70 detector positioning contributes to medical personnel cm). If the patient is positioned too close to the x-ray protection (Figure 6). source, the x-ray output is concentrated on a smaller area of the patient’s skin, increasing the patient’s beam 4.2.5. Procedures and Practices to Minimize Patient and entrance port exposure rate. This can increase the pa- Personnel Exposure tient’s skin injury risk. If the patient is positioned too far X-Ray Equipment Quality, Calibration, and from the x-ray source, the image receptor necessarily Maintenance must also be positioned further away from the source and Invasive cardiovascular x-ray imaging facilities have a the inverse square law requires a greater x-ray output to responsibility to maintain and update x-ray equipment to achieve the requisite detector dose, requiring increased produce quality images at the minimum detector dose. kVp and decreasing image contrast. Equipment should be well maintained and its calibration X-ray detector positioning is also an important should be surveyed periodically to verify that it is oper- determinant of dose to the patient as well as the expo- ating within appropriate specifications. The x-ray system sure to medical personnel from scattering. If the detec- should provide beam spectral filtering that is consistent tor is positioned substantially above the thorax, the with current standards. image magnification caused by beam divergence will The x-ray system should provide reduced-dose oper- decrease the size of the beam entrance port, causing the ating protocols for low-dose and low frame rate fluoros- patient to receive a larger skin dose. In addition, copy imaging programs. Cine acquisition detector input the x-ray image detector, when positioned close to the doses range should be set at the smallest detector dose patient’s chest, intercepts a substantial portion of that provides satisfactory diagnostic quality images. F I G U R E 6 Diagrammatic Representation of the Effect of System Positioning on Patient and Operator Radiation Exposure During X-Ray Fluoroscopy Note that in the “table too low” circumstance, the entrance port dose delivered to the patient is increased compared with optimal positioning. In the “table too low, detector too high” circumstance, the entrance port dose to the patient is further increased. In addition, in the “table too low” circumstance, the scattered dose to the operator increases because less of the scattered dose is intercepted by the detector (23).
14 Hirshfeld Jr. et al. JACC VOL. -, NO. -, 2018 Radiation Safety ECD, Part 2 -, 2018:-–- Physician Operator Conduct date, theoretical, based upon anecdotal reports of Dose Awareness and Monitoring increased left-sided brain tumors in interventional car- Appropriate physician operator conduct begins with a diologists (29). commitment to minimize radiation exposure to patients The protection afforded by lead garments should be and to healthcare personnel. Operators should be cogni- augmented by portable shielding. Typical in-room zant of the variables that determine image quality and shielding includes a ceiling-mounted lead-impregnated dose to achieve the best balance of image quality and poly (methyl methacrylate) shield that can be placed radiation exposure (24,25). between the patient’s thorax and the operator’s upper Current x-ray units display real-time values for air body. The importance of ceiling-mounted shields kerma dose rates, and cumulative air kerma and KAP. The cannot be overstated. Proper use of these shields re- physician operator should be aware of these values and duces operator eye exposure by a factor of 19 (30). their interpretation throughout a procedure and consider Under-table mounted 0.5-mm lead-equivalent shielding total accumulated dose in making procedure conduct intercepts backscatter off of the patient and the x-ray decisions. table that would otherwise strike the operator’s lower X-Ray System Operational Issues body. Imaging modality, imaging time, and image field size The inverse square law is one of the best sources of are 3 important dose-affecting parameters that are under protection. X-ray intensity decreases as the square of the the operator’s direct control. Operators should select the distance from the source. This relationship has implica- lowest-dose imaging modality that is appropriate for a tions for physician operators, because the operator’s po- particular application. This includes using an image field sition in relation to the x-ray source can make a large size that confines exposure to the structures of interest, difference in exposure magnitude. using the lowest-dose fluoroscopy program, and using the Circulating personnel should be positioned remotely slowest fluoroscopy pulse rates that yield appropriate from the x-ray source and, as a result, should receive quality images (26). negligible exposure. When circulating personnel need to Operators should use the x-ray system collimator to approach close to the patient, the physician operator minimize the exposed field size. Operators should opti- has a responsibility to not operate the x-ray system mize system positioning with the procedure table at the (22,31). optimal distance from the x-ray tube and the image de- tector as close to the patient as possible. In addition, 4.2.6. Pregnant Occupationally Exposed Workers operators should employ radiation-sparing tactics Uterine Exposure Considerations for Pregnant or including “last image hold,” virtual collimator position Potentially Pregnant Occupationally Exposed Workers adjustment, and virtual patient positioning aides. As discussed in Section 5.4.4 of Part 1, no measurable Physician and Medical Personnel Shielding and increase in adverse fetal outcomes has been detected at Protection fetal or embryonic exposures below 50 mGy. For occu- Protective shielding of operators and personnel provides pationally exposed workers in an x-ray fluoroscopy substantial protection. Standard shielding for diagnostic environment, proper shielding and practices should keep x-ray ranges between 0.25 and 0.5 mm of lead or equiv- accumulated uterine exposures well below this level. alent. A 0.5-mm lead-equivalent apron absorbs 95% of 70 Because the uterus is a deep structure and is inside of kVp x-ray and 85% of 100 kVp (27,28). protective garments, the dose to the uterus delivered by Medical personnel working in an x-ray procedure scattered x-ray is greatly attenuated. Measurements made room should wear 0.25- or 0.5-mm equivalent lead in phantoms indicate that the uterine dose in a subject aprons augmented with neck thyroid shields and hu- wearing a 0.25-mm lead apron is
JACC VOL. -, NO. -, 2018 Hirshfeld Jr. et al. 15 -, 2018:-–- Radiation Safety ECD, Part 2 4.2.7. Alternative Imaging Techniques nondiagnostic either because of poor image quality or Alternative imaging techniques, such as intracardiac ul- because the images will not answer the clinical ques- trasound and electromagnetic mapping, can provide tions posed. Case selection should incorporate the structural and guidance information that can supple- appropriate use criteria formulated collaboratively by ment or replace x-ray fluoroscopic imaging. These the American College of Cardiology and other organi- should be employed in place of fluoroscopy when zations (37–39). appropriate. Procedure Planning and Patient Preparation In planning the examination, it is important to select the 4.2.8. Summary Checklist for Dose-Sparing in X-Ray Fluoroscopy acquisition protocol that provides a degree of spatial and temporal resolution that is consistent with the examina- Checklist of Dose-Sparing Practices for X-Ray Fluoroscopy tion’s purpose. Imaging should be confined to the region Case selection , Consider patient age, comorbidities, natural life of interest. expectancy , Consider appropriateness and utility of 4.3.2. Equipment Quality and Calibration nonradiation-based imaging techniques Equipment calibration and preventive maintenance as Equipment calibration , Fluoroscopic and cine doses as low as compatible with diagnostic image quality part of quality assurance and control programs play an Procedure conduct , Minimize beam-on time important role in reducing radiation dose by facilitating , Use lowest-dose fluoroscopy setting suitable for dose optimization. This is discussed in greater detail in a particular task the full online document. , Collimate imaging field size to the area of interest 4.3.3. Variables That Affect Patient Dose for X-Ray CT , Use the slowest framing rates suitable for a particular task The radiation dose to a patient is determined by a com- , Minimize cine acquisition run durations bination of the patient’s physical characteristics and , Minimize patient-detector distance scanner protocol selection. Larger patients require larger exposures. , Maximize employment of operator shielding Operator-selectable imaging protocols that influence patient dose include: 4.3. X-Ray CT 1. Scan length. Scan length should be kept to a minimum 4.3.1. X-Ray CT General Principles to encompass only the anatomy of interest. Achieving optimal images at minimal dose requires an 2. X-ray beam intensity. Dynamically modulated tube expert team to coordinate patient management and pro- current should be used for cardiovascular acquisitions tocol selection including image acquisition, reconstruc- Tube potential: The single most important factor in con- tion, and interpretation. The team needs to select the trolling radiation dose is adjustment of x-ray tube voltage imaging protocol most likely to acquire diagnostic-quality (in kV) (40–42). Increasing tube voltage increases the x-ray images that achieve the examination’s goals while beam’s mean photon energy level, and increases radiation exposing the patient to the smallest necessary radiation dose roughly proportionally to the square of the voltage. dose (34–36). Increasing x-ray tube voltage decreases image noise. The keys to minimizing radiation exposure in cardiac Tube current: The x-ray tube current (in milliamperes CT are: [mA]) is proportional to the number of x-ray photons produced per unit time and is linearly proportional to 1. Appropriate case selection. radiation dose. Image noise is inversely proportional to 2. Scanner capability and protocol selection. the square root of the tube current. Thus, decreasing 3. Proper patient preparation. tube current at a given tube potential decreases 4. Appropriate examination conduct. the radiation dose at the expense of increased image Greater detail of how to implement these procedures is noise. discussed in depth in the complete document published 3. Rotation time. The time required for the gantry to online. perform 1 rotation is a selectable parameter. Exposure Case Selection Appropriateness increases linearly with rotation time. The first principle to reduce patient radiation 4. X-ray beam filtration. Greater filtering decreases pa- exposure due to CT examinations is to avoid tient dose. The choice of filter depends on the size of performing examinations that will prove to be the patient and the acquisition field of view (36).
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