Microbubble signal properties from PFO tests using transcranial Doppler ultrasound

Background

A limitation of transcranial Doppler (TCD) ultrasound is the inability to distinguish tiny benign bubbles from potentially hazardous particulate emboli based on analysis of the intensity of backscattered ultrasound. This study examines the Doppler characteristics of small microbubbles detected during screening of patients for a patent foramen ovale (PFO). The aim of this study was to identify unique microbubble properties that could differentiate between solid and gaseous emboli.

Methods

Bilateral TCD monitoring of the middle cerebral arteries (MCA) was performed for 34 patients during PFO screening using agitated saline. Patients were injected up to three times and asked to perform a valsalva manoeuvre. The raw audio data was recorded onto an external laptop for subsequent analysis.

Results

Eleven patients tested positive for a PFO, yielding 331 embolic signals with intensities <35 dB. The median peak measured-embolus-blood-ratio (MEBR) was 25.7 dB and the median duration was 33.0 ms. The majority of signals lasted between 12 and 92 ms, which are much longer than previously reported for particulate thrombus where the majority of signal durations are between 6 and 41 ms. Pearson correlation tests revealed a weak positive correlation between estimated microbubble velocity and signal duration (0.26, p < 0.0001).

Conclusions

Doppler signal properties were analysed for over 300 microbubbles recorded in vivo. Microbubble signal duration was found to be higher than measured for solid emboli. Further work to develop a clinically useful model based on microembolus properties to differentiate solid and gaseous is ongoing.