Detection of high intensity transient signals (HITS): how and why?

HITS (high intensity transient signals) in transcranial Doppler recordings reflect either microemboli, both gaseous and solid, or artefacts. Various frequencies in number of microembolic signals (MES) have been reported in the same clinical condition. A possible explanation for these discrepancies may be different device settings and algorithms for embolus detection. For reproducibility of data, we suggest that studies on MES report the following parameters: (1) Ultrasound device; (2) transducer type; (3) insonated artery; (4) insonation depth; (5) algorithms for signal intensity measurement; (6) scale settings; (7) detection threshold; (8) axial extension of sample volume; (9) fast Fourier transform (FFT) size (number of points used); (10) FFT length (time); (11) FFT overlap; (12) transmitted ultrasound frequency; (13) high pass filter settings; and (14) recording time. No current system of automatic embolus detection has the full sensitivity and specificity required for clinical use. Therefore, each of the signals detected by these devices needs to be checked and verified by an experienced investigator. MES will help to identify the site and activity of the embolizing lesion. Microembolus detection might reduce the observation time and the number of patients needed to perform interventional trials. First, however, MES needs to be validated as a meaningful prognostic parameter. Microemboli originating from prosthetic cardiac valves are mainly gaseous. Therefore, they cannot serve as an indicator of the valves thromboembolic activity or the patient's stroke risk.