Real-time identification and archiving of micro-embolic Doppler signals using a knowledge-based DSP system

Automatic detection of emboli in the TCD RF signal using principal component analysis

The transcranial Doppler (TCD) radio-frequency (RF) signal can provide additional information on events recorded during ultrasonic monitoring. Embolic signals appear as uniform and predictable shapes within the RF signal, enabling pattern recognition and image processing techniques to be used for their automated detection. This paper uses principal component analysis (PCA) to characterise the typical variation in embolic signal shape, within the RF signal, using training sets of in vitro and in vivo data. PCA techniques are then utilised to discriminate between previously unseen embolic and artifact signals. Although the results of this study show that the algorithms described in this paper do not yet have the accuracy required for their use in a clinical setting, it does demonstrate that this novel technique has the potential to be developed further.

Multigate transcranial Doppler ultrasound system with real-time embolic signal identification and archival

An integrated system for acquisition and processing of intracranial and extracranial Doppler signals and automatic embolic signal detection has been developed. The hardware basis of the system is a purpose-built acquisition/processing board that includes a multigate Doppler unit controlled through a computer. The signal-processing engine of the system contains a fast Fourier transform (FFT)-based, spectral-analysis unit and an embolic signal-detection unit using expert system reasoning theory. The system is designed so that up to four receive gates from a single transducer can be used to provide useful reasoning information to the embolic signal-detection unit. Alternatively, two transducers can be used simultaneously, either for bilateral transcranial Doppler (TCD) investigations or for simultaneous intra- and extracranial investigation of different arteries. The structure of the software will allow the future implementation of embolus detection algorithms that use the information from all four channels when a single transducer is used, or of independent embolus detection in two sets of two channels when two transducers are used. The user-friendly system has been tested in-vitro, and it has demonstrated a 93.6% sensitivity for micro-embolic signal (MES) identification. Preliminary in-vivo results also are encouraging.

Characteristics of Doppler embolic signals observed following carotid endarterectomy

Postoperative Doppler embolic signals following carotid endarterectomy (CEA) are associated with an increased risk of stroke, but the characteristics of these signals are rarely reported. In this study, we survey signals from 1485 emboli, assumed to consist predominantly of thrombus. Data were obtained by monitoring the middle cerebral arteries of 100 consecutive CEA patients during postoperative recovery. The distribution of embolic signal frequencies, intensities and durations revealed that embolic signals do not occur randomly in the sonogram. In particular, we find that the signals possess a characteristic distribution of velocities reflecting the preferred path of the embolus through the artery (at approximately 75% of the distance between the centre of the artery and the artery wall). Embolic signals were more likely to be observed at cardiac cycle positions between 35% and 80% from the start of systole than elsewhere. After eliminating other considerations, we hypothesized that this peak in the distribution of signals in the sonogram arose due to the localization of emboli trajectories and a strong tendency for emboli to detach from the carotid bifurcation during systole.

Detection of Doppler embolic signals: psychoacoustic considerations

The purpose of this study was to improve reliability in the identification of Doppler embolic signals by determining the decibel threshold for reproducible detection of simulated "emboli" as a function of signal duration, frequency and cardiac-cycle position. The auditory sensitivity of 16 participants to 574 simulated "emboli" was examined using psychoacoustic techniques to assess how the probability of detection varies with embolic signal parameters. Detailed measurements of the threshold for detection of simulated embolic signals are presented. These provide evidence that the measured embolus-to-blood threshold ranges between 2 dB and 14 dB as a continuous function of signal duration and frequency. The level of the threshold is closely linked to both embolic signal parameters and the properties of the blood flow signal. We conclude that the current fixed choice of threshold does not provide a good approximation to the true threshold of detection across the full range of embolic signal parameters.

RF signals provide additional information on embolic events recorded during TCD monitoring

Transcranial Doppler ultrasound (TCD) is commonly used to detect embolic signals in the cerebral circulation. However, current techniques to discriminate between signals from emboli and artifacts are subjective and ambiguous. The radiofrequency (RF) signal provides an extra dimension to the information available from conventional TCD systems that may help to interpret complex events. Artifacts generated by healthy volunteers and embolic signals recorded from a flow phantom were used to characterize the appearance of the two types of event. Characteristics of events, recorded during and immediately after carotid endarterectomy surgery, were compared with those from known sources. Additional information was provided by the RF signal on events recorded during TCD monitoring thus aiding classification. The RF signal may have a role as a "gold standard" for embolus detection.