Guidelines for the Safe Use of Diagnostic Ultrasound - Recommendations

2. Recommendations

2.1 General

1. The use of diagnostic ultrasoundto obtain informationabout function or structure in human beings should be restricted to situations in which the medical benefit that may accrue from the diagnostic data outweighs any foreseeable risk. Most such situations are limited to clinical examinations of the ill or potentially ill patient, or pregnantwomen. Where available, Canadian clinical practice and operator training guidelines should be used to help maximize the benefit of an examination.
2. Situations of training, demonstration or research may also provide a medical benefit from diagnostic data that outweighs any foreseeable risk. Here, information is obtained for people who are not necessarilyin the categories of Recommendation (1), above. In all situations of training, demonstration or research, if either of the Thermal Index or Mechanical Index will be greater than 1, then a subjectshould be informedof the anticipated exposure condition and how it compares in safety with conditions for normal diagnostic practice.
3. Ultrasound should not be used for any of the following:
   
   
   1. to have a picture of the fetus, solely for non-medical reasons;
   2. to learn the sex of the fetus solely for non-medical reasons; and
   3. for commercial purposes, such as trade shows, or producing pictures or videos of the fetus.

2.2 Thermal Effects

1. M-mode, pulsed Doppler and Colour Flow Imaging are valuable clinical tools and, despite potential risks, are not contraindicated. However operators should be careful to limit exposure to critical structures and utilize the exposure information provided by the manufacturer.
2. In particular, users should employ exposures which are As Low As Reasonably Achievable(ALARA)¹because of the potential for ultrasonic heating of tissue during M-mode imaging and, normally to a significantly greater extent, Doppler ultrasound blood flow examinations. For devices which comply with the AIUM/NEMA Standard for Real-time Display of Thermal and Mechanical Acoustic Output Indices on Diagnostic Ultrasound Equipment, Revision 1, 1998 (Output Display Standard), implementation of the ALARA principle can be achieved by using the real-time display of the relevant Thermal Index to assess the potential for ultrasonic heating. Guidanceon potentially hazardous exposures is found in Section 3.2.
3. Exposure can be reduced by either reducing the Thermal Index usingoutput controlsor by reducing the dwell time, the amount of time that the transducer remains in one place (AIUM 1994).

2.3 Mechanical Effects

1. Users should employ exposures, in any relevant mode, which are
   As Low As Reasonably Achievable (ALARA) because of the potential for:
   
   
   1. ultrasonically induced capillary hemorrhaging in lung if it is exposed during pediatric diagnostic ultrasound examinations, particularly for infants and neonates, especially if they are pre-term;
   2. ultrasonically induced capillary hemorrhaging of the intestine where intestinalperistalsis is inhibitedor conditions promote intraluminal or submucosal gas collections;
   3. ultrasonically induced capillary hemorrhaging in other soft tissues when Gas Contrast Agents are used.
      
      
2. Use of Gas Contrast Agents in a diagnostic ultrasound examination is not recommended within 24 hours before extracorporeal shock wave lithotripsy.
3. Implementation of the ALARA principle can be achieved by usingthe real-time display of the MechanicalIndex,to assess the potential for capillary hemorrhaging. If the Mechanical Index (MI) can exceed 1, then, for devices to comply with the Output Display Standard, the MI must be given in B-mode.
4. Exposure can be reduced by lowering the Mechanical Index usingoutput controls. Reducing the dwell time is of use if threshold pressures are exceeded.
   Guidance on the likelihood and clinical significance of injury due to mechanical effects is given in Section 3.3.

2.4 Device Performance

1. It is recommendedthat diagnostic ultrasounddevices comply with the Output Display Standard (AIUM/NEMA 1998a).
2. It is recommendedthat the maximum attainable values for the
   Mechanical Index and the derated spatial peak time average in-tensity, I _spta.3, not exceed 1.9 and 720 mW/cm², respectively.
3. For ophthalmic devices or for ophthalmic applications of general purpose devices, the maximum attainable value for the Thermal Index should be less than or equal to 1, the maximum attainable value of the Mechanical Index should be less than or equal to 0.23 and the maximum attainable value of the derated spatial peak time average intensity,  I_spta.3 should be less than or equal to 50 mW/cm². With these limits, the Output Display Standard does not require a real-time output display.
4. For Fetal Heart Rate Monitors, the maximum attainable value of spatial average, temporal average intensity at the transducer face should be less than 20 mW/cm² for continuous wave devices and the maximum attainable value of the spatial average, pulse average intensity at the transducer face should be less than 20 mW/cm² for pulsed devices (FDA 1997). These recommended limits were chosen to be consistent with the output level limits in the U.S. FDA 510(k) guidance document (FDA 1997). With these limits for Fetal Heart Rate Monitors, it is unlikely that a Thermal Index display would be required under the Output Display Standard.

2.5 Quality Assurance

It is recommended that equipment operators implement quality assurance measures to maintain the capability of obtaining reliable diagnostic information at acoustic exposures which are As Low As Reasonably Achievable. Guidance on quality assurance methods can be found in several documents, including Guidelines of the Canadian Society of Diagnostic Medical (CSDMS 1998), as well as publications of the American Institute for Ultrasound in Medicine (AIUM 1991, AIUM 1995a, AIUM 1995b).

As the quality of diagnostic information depends, in part, on operator training, it is also recommended that sonographers (ultrasound technologists) be appropriately qualified and registered with either the Canadian Association of Registered Diagnostic Ultrasound Professionals (CARDUP) or the American Registry of Diagnostic Medical Sonographers (ARDMS).

1. Section 6 (Glossary) provides more informa tion about the extension of the ALARA principle to diagnostic ultrasound.