How to Interpret the Ultrasound Output Display Standard for Diagnostic Ultrasound Devices: Version 3.

Abstract

Journal of Ultrasound in MedicineVolume 38, Issue 12 p. 3101-3105 AIUM Technical BulletinFree Access How to Interpret the Ultrasound Output Display Standard for Diagnostic Ultrasound Devices: Version 3 First published: 17 November 2019 https://doi.org/10.1002/jum.15159Citations: 1 The American Institute of Ultrasound in Medicine (AIUM) is a multidisciplinary organization dedicated to promoting the safe and effective use of ultrasound in clinical medicine. The purpose of a technical bulletin is to keep the ultrasound community informed about practical scientific developments relating to the clinical practice of diagnostic ultrasound. Topics are generally selected for their broad application to health care professionals who use diagnostic ultrasound technology in their practice of medicine. AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Diagnostic ultrasound has an excellent safety record, but the hypothetical possibility of tissue damage on a microscopic scale cannot be completely ruled out under all ultrasound conditions. Most diagnostic ultrasound devices that follow the output display standard (ODS) have the capability of higher acoustic output than ultrasound devices marketed before the inception of the ODS in 1992. Many users are not aware of the higher outputs that these devices can generate. These machines require a higher level of awareness, expanded user responsibilities, and instructions on how to interpret biological exposure index displays (thermal index [TI] and mechanical index [MI]).1-3 These indices are displayed in real time on the monitors of imaging systems. Users should be familiar with the location of the ODS indices and be able to adjust the acoustic output, thereby implementing the as low as reasonably achievable (ALARA) principle for all examinations. Recognizing the importance of understanding ultrasound bioeffects and the output display indices, the AIUM has issued several Official Statements relevant to safety. For example, statements on “Mammalian Biological Effects in Tissues with Naturally Occurring Gas Bodies” and “Mammalian Biological Effects in Tissues with Gas Body Contrast Agents” describe the potential for nonthermal bioeffects when ultrasound interacts with gas bodies due to the physical phenomenon of acoustic cavitation. The “Statement on the Safe Use of Doppler Ultrasound During 11–14 week scans (or earlier in pregnancy)” emphasizes caution when using spectral Doppler ultrasound with high TI settings in the first trimester. Furthermore, although the Statement on “Conclusions Regarding Epidemiology for Obstetric Ultrasound” states that there is insufficient justification to warrant concluding a causal relationship between diagnostic ultrasound and recognized adverse effects in humans, it adds that the epidemiologic evidence is based primarily on exposure conditions before 1992. In that year, the maximum recommended acoustic output levels of ultrasound machines were substantially increased, requiring the onscreen ODS. Educational programs developed jointly by organizations including the AIUM, the National Electrical Manufacturers Association (NEMA), and the US Food and Drug Administration (FDA) explain how to use these devices safely. The purpose of this technical bulletin is to provide information about the ODS and how its inception has conveyed responsibilities to the users of these devices. The Basic Facts In 1992, NEMA and the AIUM agreed on a voluntary standard—commonly called the ODS—for on-screen labeling of diagnostic ultrasound devices. The ODS originally was developed as a United States standard published by the AIUM and NEMA, but in 2001 it was replaced by an international standard through the International Electrotechnical Commission (IEC). This standard, designated IEC 60601-2-37, is titled, “Particular Requirements for the Basic Safety of and Essential Performance of Ultrasonic Medical Diagnostic and Monitoring Equipment,” with the latest edition dated 2016. This standard has become a part of the FDA's guidance to manufacturers. Beginning with the 1993 FDA guidance, manufacturers were given the option to market systems that exceed previously allowed application-specific levels of ultrasound exposure as long as the levels of exposure are displayed on screen. The exposure levels are now expressed in terms of the MI and TI, which reflect the potential for biological effects based on mechanical and thermal mechanisms. This policy requires greater user responsibility for patient exposure levels with these devices, while providing enhanced diagnostic capabilities. To maintain an equivalent level of safety, the FDA requires manufacturers to educate users about the capabilities of these higher-output devices, as well as the meaning of the ODS and the attendant responsibilities. The Changes To illustrate the change in acoustic output levels, Table 1 shows the maximum allowed acoustic output levels of the pre-ODS machines, which complied with application-specific limits set by the FDA, and the current, higher-output machines. For comparison, the old application-specific limits for the derated (ie, values corrected by a tissue attenuation factor) spatial-peak temporal-average intensity [ISPTA] are listed (units are in milliwatts per square centimeter): Table 1. Changes in Acoustic Output Levels Application Application-Specific Limits ISPTA (Derated): Pre-ODS, mW/cm2 Output Display Standard:Post-ODS Level, mW/cm2 Fetal imaging, neonatal, pediatric, and other 94 720 Cardiac Peripheral vessel 430 720 Ophthalmic 720 720 17 50 The higher-output devices are allowed to produce a derated maximum output of 720 mW/cm2 for all applications except ophthalmic, as long as the TI and the MI are displayed for every possible setting of transducer type, output setting, focus, frame rate, and pulse rate. (A detailed definition of the TI and the MI and the specific applications under which each is displayed are described in the section entitled “A Brief Explanation of the Indices.”) This means that, for fetal applications, there could be an 8-fold increase in the allowed maximum derated ISPTA if users choose to use the highest output possible on their device. For ophthalmic use, there is a theoretical 3-fold increase in the allowed maximum derated ISPTA. However, because the generic ODS tissue models are not applicable in the eye, the maximum intensity recommended by the FDA was set much lower than for other applications (50 vs 720 mW/cm2). Similarly, the maximum MI for ophthalmic use was set at 0.23 rather than 1.9 (see below). Machines that use the ODS indices have preset default levels that are switched on automatically when the machines are first turned on or when a new application is chosen. For most machines, these preset levels correspond to less than maximum output for all applications, although some machines allow users to change the preset default levels to any level desired. The AIUM recommends that users verify the “preset” imaging characteristics. For example, in obstetrics, many ultrasound examinations are performed between 10 and 14 weeks using a first-trimester preset that defaults to a TI formula based on soft tissue (TIS). In this gestational age window, using a TI formula incorporating bone (TIB) would be more appropriate, as bone ossification has occurred. All personnel who use these devices should thoroughly read the information provided by the manufacturer concerning the ODS and the prudent use of the device to understand and be able to adjust the default levels for their particular machine. During all ultrasound examinations, ALARA should be followed by keeping the TI and MI indices as low as possible while obtaining the required diagnostic information. Education Effort An informational program designed to address the safety of higher–acoustic output devices and use of the output display began in 1990. The AIUM first published a booklet in 1994 entitled Medical Ultrasound Safety1 that includes sections on biological effects and prudent use (keeping the exposure low), and explains the ODS indices. Edition 3 of Medical Ultrasound Safety,1 dated 2014, was developed jointly by the AIUM, the FDA, NEMA, and others. FDA guidance to industry stipulates that manufacturers should include copies of this booklet, or similar text, as part of the user manual with every high-output device sold. If the booklet or similar information is not included with the device, users should request a copy from the manufacturer. AIUM members may download the PDF of this publication for free from the AIUM website's Resource Library. The ALARA Principle Higher–acoustic output ultrasound systems using the ODS have been commercially available since 1993. For users of these devices, there are mechanisms designed to help them keep the total patient exposure at ALARA levels while maintaining high image quality. This is achieved by using the information displayed on the screen in the form of biologically relevant exposure indices: the TI and the MI. The display of biological index information is only a tool. Hypothetical risk of these devices may be minimized through the users’ knowledge and the deliberate application of the ALARA principle. Users should read the information provided in the user manual on the possible biological effects of high-output intensities, prudent use of the equipment and controls, and correct interpretation of the exposure indices displayed on the screen. Users should make a conscious effort to remain continuously aware of the dwell time (exposure at a specific location in the body) and total exposure time (exposure throughout the entire scan). Body parts that naturally contain gaseous bodies (eg, lungs and the digestive tract) are at a higher potential for ultrasound-induced bioeffects from mechanical mechanisms than other parts of the body that lack gaseous inclusions. Additionally, the clinical infusion of echogenic microbubbles (eg, ultrasound contrast agents) into the circulatory system, while having the potential to yield increasing amounts of diagnostic information from an ultrasound examination, converts a seemingly gas nuclei–free “liquid tissue” (blood) to one that is richly gas nucleated and rapidly distributed throughout the body. Numerous scientific publications demonstrate that the interaction of ultrasound and gas nuclei can induce biological effects. Therefore, ALARA should be implemented with particular diligence under conditions of exposure to internal gas bodies. A Brief Explanation of the Indices To interpret the ODS, one must understand the TI and the MI. These indices are necessary as intensity levels alone do not accurately estimate the biological exposure. Other factors, such as temperature elevation and the potential for cavitation, determine the biological significance of tissue exposure. That is, under certain conditions, a rising temperature or mechanical vibration of tissue can alter or damage tissue. The temperature rise and the possibility of cavitation are, in turn, dependent on such factors as the total energy output, the mode, the shape of the ultrasound beam, the position of the focus, the center frequency, the shape of the waveform, the frame rate, and the duration of tissue exposure. The TI and MI account for all of these factors, with the exception of the dwell time, and give users instant information about the potential for thermal or mechanical bioeffects, which is more significant than simple intensity levels. The TI gives a relative indication of the potential for a temperature increase at a specific point along the ultrasound beam. The reason for the term “relative” is that the actual conditions for heating in tissue are complex, and this index alone cannot give the actual increase in temperature for all possible conditions. Thus, a TI of “2” represents a higher temperature rise than a TI of “1” but does not necessarily represent a rise of 2°C. This temperature rise is a theoretical estimate based on experimental conditions that may not apply to clinical conditions. The important point to remember about the TI is that it is designed to make users aware of the possible temperature rise at a particular point in tissue. There are three specific TIs: the soft tissue TI (TIS), the bone TI (TIB), and the cranial bone TI (TIC). The TIS is used to provide information concerning the estimated temperature rise within homogeneous soft tissue, whereas the TIB gives information on the estimated temperature rise in bone at or near the focus of the beam, and the TIC gives the estimated temperature increase of bone at or near the surface, such as during a cranial examination. Although there are three subcategories of TI (ie, TIS, TIB, and TIC), only one of these is required to be displayed on the ultrasound screen. The machine must allow users to retrieve the other two if appropriate for specific imaging conditions. For example, in obstetric scanning, the TIB should be used at 10 weeks’ gestation or greater when bone ossification is evident. Users should contact their system's application specialists for details on how to display other TIs. The MI gives a relative indication of the potential for mechanical effects, such as inertial cavitation (the violent collapse of a bubble in tissue), which in some scanning modes may be more significant than thermal effects. (“Scanning modes” refers to internal equipment functions that steer the ultrasound beam in a sequential pattern, such as in real-time imaging, rather than the act of scanning a patient.) The potential for mechanical effects increases as the peak pressure increases, and it decreases as the frequency increases. The MI can range up to 1.9 for all uses except ophthalmic, which has a maximum MI limit of 0.23. The index levels do not indicate that a biological effect is actually happening but inform users concerning the relative probability of such a biological effect. This is the reason for implementing the ALARA principle, using the lowest possible TI and MI values, while obtaining diagnostic-quality images. All imaging machines capable of producing TI or MI values higher than 1 must display these index values when they exceed 0.4 to help users implement the ALARA principle. For every patient having an ultrasound examination, a prudent starting point is to set the machine at a low output setting and then modify (up or down) from this level until diagnostic-quality information is obtained. An important contributor to ALARA, not incorporated in the output display indices, is the element of time. Total exposure is directly proportional to the amount of time the ultrasound beam remains in one area. Thus, in addition to keeping the TI and MI as low as possible, the total exposure time and dwell time should be kept as short as possible. Users should refer to the AIUM statement on “Recommended Maximum Scanning Times for Displayed Thermal Index (TI) Values” for guidance on the duration of ultrasound exposure at a given TI. It is highly recommended that all users read the most current AIUM publication, Medical Ultrasound Safety, eg, Edition 3,1 which contains additional detail on implementing ALARA. Users should be familiar with the current AIUM publication, Recommended Ultrasound Terminology, 2 which contains definitions of technical terms used in sonography. AIUM members may download the PDF of the current version of Medical Ultrasound Safety1 and Recommended Ultrasound Terminology 4 for free from the AIUM website's Resource Library. By understanding information concerning biological risk factors and prudent use, one can implement the principles of ALARA by adjusting machine settings to use the lowest output that will yield the required diagnostic information while minimizing the exposure time and dwell time. With a clear understanding of these responsibilities, users will have more capabilities and potentially more diagnostic information available from these higher-output devices without compromising the record of safe use of diagnostic ultrasound devices. Technical Standards Committee American Institute of Ultrasound in Medicine Clinical Standards Committee American Institute of Ultrasound in Medicine Bioeffects Committee American Institute of Ultrasound in Medicine © Copyright 2019; Original approval 1998; Revised: 11/2003; 8/2019. American Institute of Ultrasound in Medicine, 14750 Sweitzer Ln, Suite 100, Laurel, MD 20707-5906 USA. Phone: 301-498-4100. www.aium.org References 1 American Institute of Ultrasound in Medicine. Medical Ultrasound Safety. 3rd ed. Laurel, MD. American Institute of Ultrasound in Medicine; 2014. 2Bigelow TA, Church CC, Sandstrom K, et al. The thermal index its strengths, weaknesses, and proposed improvements. J Ultrasound Med 2011; 30: 714– 734. 3 American Institute of Ultrasound in Medicine. Consensus report on potential bioeffects of diagnostic ultrasound: executive summary. J Ultrasound Med 2008; 27: 503– 515. 4 American Institute of Ultrasound in Medicine. Recommended Ultrasound Terminology. 3rd ed. Laurel, MD. American Institute of Ultrasound in Medicine; 2014. Citing Literature Volume38, Issue12December 2019Pages 3101-3105 ReferencesRelatedInformation