Mechanical Index Limits are being Exceeded

Jasmine Lister, Sheffield Teaching Hospitals NHS Foundation Trust – winner of the award for Best Scientific Poster at BMUS Ultrasound 2023

Aim

This audit aimed to assess local compliance to the stricter MI limits imposed on lung and eye ultrasound examinations by professional bodies (MI < 0.7 and MI < 0.23, respectively), and the associated guidance.

Method

Study descriptions for lung (US thorax and pleural cavity) and eye (US eye left/right/both) ultrasound examinations were queried on PACS across the radiology and A&E department. The maximum MI across each study was noted alongside the pre-set, transducer, frequency, acoustic power and imaging mode. For studies with more than one transducer and/or pre-set, the maximum MI and associated parameters were noted per each variation. The station number was recorded for future investigation of pre-set values should any MI or imaging parameters have exceeded the specified limits. The maximum, minimum, range and modal values were calculated across both departments for each examination type, along with a tally of pre-set choice.

Results

In A&E (n=33), 45% of examinations had an MI > 0.7, nine of which had used a dedicated lung pre-set. In contrast, 90% of examinations in radiology (n=55) had an MI > 0.7, of which there were no examinations using a dedicated lung pre-set. In A&E, only one instance (5%, n =19) of ultrasound on the eye was performed with a suitable MI, which used a dedicated orbit pre-set. Of the remaining 18 examinations, 21% studies were performed using a dedicated orbital pre-set and 61% utilised colour Doppler. No eye examinations were performed in radiology, due to the trust having a dedicated ophthalmological ultrasound service.

Conclusions

Both lung and eye ultrasound examinations have exceeded the limits imposed by professional bodies. The pre-sets in both radiology and A&E require revision, along with potential retraining of staff to understand the safety considerations that arise when scanning these anatomical sites. A wider, multisite audit is advisable due to discovery of ineffectual pre-sets.

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