Observations of MI Values During Neonatal Cardiac Ultrasound Scanning

Abstract

Aim: The on-screen Mechanical Index (MI), was recorded from eight diagnostic ultrasound machines used for cardiac scanning of neonates. The objective of the study was to compare values of MI used in neonatal cardiac scanning with those recommended by the British Medical Ultrasound Society safety guidelines. Methods: The eight scanners were based at Sheffield Teaching Hospitals NHS Trust (STH), Sheffield Children's Hospitals NHS Trust (SCH) and Leeds Teaching Hospitals NHS Trust (LTH). Two methods of recording the MI value were used. At the Sheffield sites the on-screen safety index was obtained from archived patient studies saved on the Trust PACS. At the Leeds site the on-screen safety index was recorded from the scanner when it was switched on to its default neonatal or paediatric pre-sets with the appropriate transducer connected. Results: Data from clinical examinations at the Sheffield sites show that on average the MI values, for cardiac scanning, in B-mode and colour/pulsed wave Doppler mode were 1·1±0·3 and 1·3±0·3 respectively. The maximum values of MI recorded during clinical cardiac examinations were 1·6 for B-mode and 1·7 for colour/pulsed wave Doppler mode. Data from the Leeds site recorded default MI values, using appropriate transducers with neonatal and paediatric presets. One scanner from the Leeds site defaulted to an MI value of 1·9 in colour/pulsed Doppler mode with a neonatal preset. All the MI values are well above BMUS guideline recommendations for neonates when lung tissue is present within the ultrasound beam. Conclusions: The implication of these results is that neonates undergoing cardiac ultrasound examinations, were regularly exposed to MI values at least four to six times that recommended by BMUS guidelines (MI<0·3).