Top Tips- Neonatal Ultrasound Safety
Five top tips in neonatal ultrasound safety
- Know the risks
Although diagnostic ultrasound is one of the safest diagnostic imaging modalities, it has been shown to have the potential to produce undesired thermal (tissue heating) and non-thermal bio-effects (cavitation and radiation force), so caution should be exercised, especially in vulnerable premature infants. Studies in humans are difficult to carry out due to ethical considerations but in-vitro and pre-clinical trials have produced measurable bio effects at diagnostic ultrasound levels.
Obtaining a diagnostic image is paramount in answering the clinical question and not obtaining the best image quality can result in an undiagnostic or inaccurate scan, however, note should be made that manufacturers may not set their presets based on the ALARA principle and a diagnostic image can often be achieved by using safer techniques and settings. It is the user’s responsibility to ensure the scan is being performed as accurately and safely as possible. The risks can be influenced by both the tissue type being examined and the properties of the ultrasound beam.
- Be aware of vulnerable tissues particularly sensitive to thermal effects
Cranial and spinal tissues are vulnerable to potentially harmful thermal bio-effects. In some cranial examinations cranial bone may be included in the scan field. Fully calcified bone absorbs the most energy and has the highest degree of heating. Not only can the sensitive brain tissue undergo a heating effect from the direct ultrasound beam but can also undergo a local heating effect from the adjacent cranial bone and the self-heating of the probe itself. The scan is performed primarily through the small anterior fontanelle and therefore there is very little movement of the ultrasound beam at the brain periphery near the fontanelle, also contributing to the increased thermal effect. Research studies have shown this area of the brain near the fontanelle during cranial scans has the highest temperature increase.
- Be aware of particularly vulnerable tissues particularly sensitive to non-thermal effects
Ultrasound has the potential to cause undesired non-thermal bio-effects such as cavitation and radiation force in gas containing organs such as lungs and intestines. Research in animal studies has shown diagnostic lung ultrasound can cause pulmonary capillary haemorrhage (PCH). Shear wave elastography which uses long pulses is not recommended in neonatal lung ultrasound because of increased risk to cause PCH.
M mode can be used to assess the movement of the diaphragm during chest ultrasound. Despite M mode beam not having an increased output power compared to B mode scanning and therefore being preferred to Doppler, it does involve the transducer being held in a stationary position at the diaphragm where there is almost complete reflection of the ultrasound beam at the air/tissue interface. This could lead to an increased potential of bio-effects.
- Take care with high output power applications
Different ultrasound applications can also influence the potential of undesired bio-effects. Colour and spectral pulsed wave Doppler are used in cardiac scanning and both have an increased output power. Spectral Doppler has the highest and the most potential of causing undesired bio-effects. Note should also be made that occasionally during cardiac scanning neonatal lung will also be in the ultrasound field therefore also increasing the risk of cavitation and radiation force effects causing PCH.
Shear wave is not recommended for lung ultrasound. If you are planning innovative research or new uses of applications and are unsure about the safety implications, you could contact your Medical Physicist or the Safety Group of BMUS to ask advice.
- Consider the ALARA (As low as reasonably achievable) principle
Start scanning with the lowest power output at which a diagnostic image can be obtained. Ask Systems Applications Specialists or your Medical Physicist to alter scanning machine presets if the output powers are set too high or if you need help with locating machine controls.
Be conscious of the exposure time, especially during training when scan times may become prolonged. Only hold an active probe on the patient when actually scanning. Keep the probe moving to avoid overheating a particular area.
Monitor the safety indices which are displayed on the ultrasound machine, MI correlates with the potential to cause undesired non-thermal bio-effects, TI correlates with the potential to cause thermal bio-effects. Please see table 1 on page 8 of the BMUS Safety Guidelines for recommended levels. Microsoft Word - BMUS Safety Guidelines _2009 revision_ Feb 2010.doc
Further reading
Good infection prevention practice: using ultrasound gel - GOV.UK (www.gov.uk) Consult your manufacturer regarding use of the correct cleaning wipes as some can damage the probes.
Guidance_document_on_neonatal_ultrasound_safety_issues.pdf (bmus.org)
EFSUMB Safety Statement for Diagnostic Ultrasound https://www.thieme-connect.de/products/ejournals/pdf/10.1055/a-1010-6018.pdf
EFSUMB Doppler best practice https://efsumb.org/wp-content/uploads/2021/12/2022-doppler_bestpractice.pdf
*These top tips were co-authored by Pat Duffin, Paediatric Sonographer and Prashant Verma, Clinical Scientist, with contributions from the BMUS Physics & Safety group (Piero Miloro, Gail Ter Haar, Christoph Lees, Ben Stenberg & Caroline Shaw).