SAFETY OF DIAGNOSTIC ULTRASOUND
Category: Diagnostic Radiology
Abstract : SAFETY OF DIAGNOSTIC ULTRASOUND : Within the field of clinical diagnostic ultrasound, it is currently accepted that there is insufficient evidence for any deleterious effects at diagnostic levels and that the benefits to patients outweigh the risks. As new techniques and technological developments come on to the market, new biophysical conditions may be introduced which require evaluation with reg
SAFETY OF DIAGNOSTIC ULTRASOUND : Within the field of clinical diagnostic ultrasound, it is currently accepted that there is insufficient evidence for any deleterious effects at diagnostic levels and that the benefits to patients outweigh the risks.
As new techniques and technological developments come on to the market, new biophysical conditions may be introduced which require evaluation with regard to safety and we cannot afford to become complacent about the possible effects. The situation remains under constant review.
Several international bodies continue to consider the safety of ultrasound in clinical use. The European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) has confirmed the safety of diagnostic ultrasound and endorsed its informed use. Whilst the use of pulsed Doppler is considered inadvisable for the developing embryo during the first trimester, no such exceptions are highlighted for abdominal ultrasound. The European Committee for Ultrasound Radiation Safety (ECURS) confirms that no deleterious effects have yet been proven in clinical medicine. It recommends, however, that equipment is used only when designed to national or international safety standards and that it is used only by competent and trained personnel. The World Federation for Ultrasound in Medicine and Biology (WFUMB) confirms that the use of B-mode imaging is not contraindicated, concluding that exposure levels and duration should be reduced to the minimum necessary to obtain the required diagnostic information. Ultrasound intensities used in diagnostic ultrasound vary according to the mode of operation.
Pulsed Doppler usually has a higher level than B-mode scanning, which operates at lower intensities, although there may be overlap with colour or power Doppler. The American Institute for Ultrasound in Medicine (AIUM) has suggested that ultrasound is safe below 100 W/cm. This figure refers to the spatial peak temporal average intensity (ISPTA). The use of intensity, however, as an indicator of safety is limited, particularly where Doppler is concerned, as Doppler intensities can be considerably greater than those in B-mode imaging. The Food and Drug Administration (FDA) sets maximum intensity levels allowed for machine output, which differ according to the application.
Biological effects of ultrasound : Harmful effects from ultrasound have been documented in laboratory conditions. These include thermal effects and mechanical effects.
Thermal effects are demonstrated as a slight rise in temperature, particularly in close proximity to the transducer face, during ultrasound scanning. This local effect is usually of no significance but the operator must be aware of the phenomenon. The most significant thermal effects occur at bone/tissue interfaces and are greater with pulsed Doppler. Increases in temperature of up to 5°C have been produced. Areas at particular risk are fetal bones and the interfaces in transcranial Doppler ultrasound
scans. Pulsed Doppler has a greater potential for heating than B-mode imaging as it involves greater temporal average intensities due to high pulse repetition frequency (PRF) and because the beam is frequently held stationary over an area while obtaining the waveform. Colour and power Doppler usually involve a greater degree of scanning and transducer movement, which involves a potentially lower heating potential than with pulsed Doppler. Care must be taken to limit the use of pulsed Doppler and not to hold the transducer stationary over one area for too long.
Mechanical effects, which include cavitation and radiation pressure, are caused by stresses in the tissues and depend on the amplitude of the ultrasound pulse. These effects are greatest around gasfilled organs, such as lungs or bowel and have, in laboratory conditions, caused small surface blood vessels in the lungs to rupture. Potentially, these effects could be a hazard when using contrast agents which contain microbubbles.
Safety indices (thermal and mechanical indices) : In order to inform users about the machine conditions which may potentially be harmful, mechanical and thermal indices are now displayed as an output display standard (ODS) on all equipment manufactured after 1998. This makes operators aware of the ultrasound conditions which may exceed the limits of safety and enables them to take avoiding action, such as reducing the power or restricting the scanning time in that area. In simple terms the mechanical index (MI) is related to amplitude and indicates how big an ultrasound pulse is, giving an indication of the chances of mechanical effects occurring. It is therefore particularly relevant in the abdomen when scanning gas-filled bowel or when using microbubble contrast agents. Gas bodies introduced by contrast agents increase the probablility of cavitation.
The thermal index (TI) gives an indication of the temperature rise which might occur within the ultrasound beam, aiming to give an estimate of the reasonable worst-case temperature rise. The TI calculation alters, depending upon the application, giving rise to three indices: the soft-tissue thermal index (TIS), the bone-at-focus index (TIB) and the bone-at-surface, or cranial index (TIC). The first of these is obviously most relevant for abdominal applications. In well-perfused tissue, such as the liver and spleen, thermal effects are less likely due to the cooling effect of the blood flow. The display of safety indices is only a general indication of the possibility of biological hazards and cannot be translated directly into real heating or cavitation potential. These safety indices are limited in several ways. They require the user to be educated with respect to the implications of the values shown and they do not take account of the duration of exposure, which is particularly important in assessing the risk of thermal damage. In addition, the TI does not take account of the patients temperature, and it is logical to assume that increased caution is therefore required in scanning the febrile patient. MI and TI are also unlikely to portray the optimum safety information during the use of contrast agents, in which, theoretically, heating effects and cavitation may be enhanced.
Other hazards : Whilst most attention in the literature is focused on the possible biological effects of ultrasound, there are several other safety issues which are within the control of the operator. Electrical safety All ultrasound machines should be subject to regular quality control and should be regularly checked for any signs of electrical hazards. Loose or damaged wiring, for example, is a common problem if machines are routinely used for mobile work. Visible damage to a transducer, such as a crack in the casing, should prompt its immediate withdrawal from service until a repair or replacement is effected.
Microbiological safety It is the responsibility of the sonographer to minimize the risks of crossinfection. Most manufacturers make recommendations regarding appropriate cleaning agents for transducers, which should be carefully followed. Sterile probe covers should be used in cases where there is an increased risk of infection. Operator safety By far the most serious hazard of all is that of the untrained or badly trained operator. Misdiagnosis is a serious risk for those not aware of the pitfalls. Apart from the implications for the patient of subsequent incorrect management, the operator risks litigation which is difficult or impossible to defend if they have had inadequate training in ultrasound. Work-related musculoskeletal disorders There is increasing concern about WRMSD related to ultrasound scanning, as workloads increase and it has been estimated that a significant proportion of sonographers who practise full-time ultrasound scanning may be affected. One contributing factor is the ergonomic design of the ultrasound machines, together with the position adopted by the operator during scanning. While more attention is now being paid by ultrasound manufacturers to designs which limit WRMSD, there are various other contributing factors which should be taken into account when providing ultrasound services. Well-designed, adjustable seating for operators, adjustable patient couches, proper staff training for manoeuvring patients and a varied work load all contribute to minimizing the potential problems to staff. Hand-held, portable ultrasound machines are now available. Provided they are of sufficient functionality to provide the service required, they may also potentially limit the problems encountered when manoeuvring larger scanners around hospital wards and departments.
The safe practice of ultrasound : It is fair to say that the safety of ultrasound is less of an issue in abdominal scanning than in obstetric or reproductive organ scanning. Nevertheless it is still incumbent upon the operator to minimize the ultrasound dose to the patient in any practicable way.
The use of X-rays is governed by the ALARA principle that of keeping the radiation dose As Low As Reasonably Achievable. Although the risks associated with radiation are not present in the use of ultrasound, the general principle of keeping the acoustic exposure as low as possible is still good practice and many people still refer to ALARA in the context of diagnostic ultrasound.
Steps for minimizing the ultrasound dose :
● Ensure operators are properly trained, preferably on recognized training programmes.
● Minimize the output (or power) level. Use amplification of the received echoes to manipulate the image in preference to increasing the transmitted power.
● Minimize the time taken to perform the exam.
● Dont rest the transducer on the skin surface when not scanning.
● Make sure the clinical indications for the scan are satisfactory and that a proper request has been received. Dont do unnecessary ultrasound examinations.
● Be aware of the safety indices displayed on the ultrasound machine. Limit the use of pulsed Doppler to that necessary to contribute to the diagnosis.
● Make the best use of your equipment maximize the diagnostic information by manipulating the controls effectively.
MEDICOLEGAL ISSUES : Litigation in medical practice is increasing and the field of ultrasound is no exception to this. Although currently the majority of cases involve firstly obstetric and secondly gynaecological ultrasound, it is prudent for the operator to be aware of the need to minimize the risks of successful litigation in all types of scanning procedures. Patients have higher expectations of medical care than ever before and ultrasound practitioners should be aware of the ways in which they can protect themselves should a case go to court. The onus is upon the defendant to prove that he or she acted responsibly and there are several helpful guidelines which should routinely be followed. The medicolegal issues surrounding ultrasound may be different according to whether the operator is medically or non-medically qualified. Depending on their profession, operators are constrained by codes of conduct of their respective colleges and/or Councils. Either way, the operator is legally accountable for his or her professional actions. If non-medically qualified personnel are to perform and report on scans (as happens in the UK, USA and Australia), this task must be properly delegated by a medically qualified practitioner, for example a radiologist in the case of abdominal scanning. As the role of sonographers continues to expand, it is noteworthy that the same standard of care is expected from medically and non-medically qualified staff alike. To avoid liability, practitioners must comply with the Bolam test, in which they should be seen to be acting in accordance with practice accepted as proper by a responsible body of relevant medical people.
Guidelines for defensive scanning :
● Ensure you are properly trained. Operators who have undergone approved training are less likely to make mistakes.
● Act with professionalism and courtesy. Good communication skills go a long way to avoiding litigation.
● Use written guidelines or schemes of work.
● Ensure a proper request for the examination has been received.
● A written report should be issued by the operator.
● Record images to support your findings.
● Clearly state any limitations of the scan which may affect the ability to make a diagnosis.
● Make sure that the equipment you use is adequate for the job.
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