Lingke Fan

Interpretation of Embolic Phenomena During Carotid Endarterectomy

BACKGROUND AND PURPOSE Air and particulate emboli are a major source of morbidity during carotid endarterectomy (CEA); however, amplitude overload and poor time resolution have restricted the ability of transcranial Doppler ultrasound to differentiate between the two. METHODS We have now overcome these two limitations by (1) rerouting embolic signals away from the audio frequency amplifier to avoid amplitude overload and (2) substituting the Wigner distribution function for the fast Fourier transform to improve time and frequency resolution. Thus, we can now accurately determine embolic duration and embolic velocity, the product of which is the sample volume length (SVL). This measurement represents the physical distance over which an embolic signal can be detected. The underlying hypothesis was that air reflected more ultrasound and would therefore be detected over a greater SVL. RESULTS The median SVL (interquartile range) for 75 in vitro air emboli was 1.97 cm (range, 1.70 to 2.35) compared with 0.27 cm (range, 0.16 to 0.43) for 185 particulate emboli detected during the dissection phase of CEA. Off-line analysis on an additional 560 embolic signals detected during different phases of CEA suggested that 46 of 143 (32%) of emboli immediately after shunt insertion were particulate, as were 19 of 33 (58%) occurring during shunting, 28 of 78 (36%) after restoration of flow in the external carotid artery, 23 of 251 (9%) after restoration of flow in the internal carotid artery, and 55 of 55 (100%) of those emboli detected during the early recovery phase. CONCLUSIONS This development provides objective physical criteria upon which embolus characterization (particulate/air) can be based. This could have major implications for future patient monitoring with respect to modification of surgical technique and pharmacological intervention.

Differentiation between emboli and artefacts using dual-gated transcranial Doppler ultrasound

It is well documented that transcranial Doppler ultrasound has the ability to detect cerebral emboli. During intraoperative patient monitoring studies, many signals due to artefact (probe motion, patient movement or surgical manipulation) are also detected and can be difficult to distinguish from genuine embolic events. We have constructed a Doppler system that can simultaneously range-gate at two separate depths, in order to test the hypothesis that it should be possible to distinguish between emboli and artefact by comparing the signal from the two separate regions within the vessel. The classification algorithm is based on the principle that emboli propagate with blood motion (whereas artefacts do not) and thus will be detected sequentially at different depths along the insonated cerebral artery. One hundred thirty-eight (presumed) embolic and 170 artefact signals were analysed. The median (interquartile range) gate separation was 10.01 mm (7.41-10.78 mm). The time delay between detection of embolic signals in the two channels was 11.04 ms (6.24-16.41 ms, but was only 0.08 ms (-0.48(-)+0.64 ms) for artefact (p < 0.0001). Dual-gated Doppler ultrasound is a conclusive and independent method that differentiates emboli from artefact. Incorporation of this system for long-term monitoring may eliminate the need for an experienced observer to be present.

Processing Doppler ultrasound signals from blood-borne emboli

Abstract Analysing Doppler ultrasound signals from blood-borne emboli has been hindered by two basic problems. Firstly, the ratio of the signal levels from blood and emboli is greater than the dynamic range of conventional Doppler instruments used to detect such emboli. This causes amplitude overload, which must be eliminated before accurate analysis of embolic data can be performed. Secondly, the temporal resolution of fast Fourier transforms (FFT) usually used to analyse Doppler signals, is insufficient to quantify accurately such short duration signals. This paper describes methods to overcome these two problems. The Doppler signal is recorded from close to the front-end of the velocimeter, and a Wigner distribution analyser is used to provide Doppler spectra with both high temporal and frequency resolution. The resulting sonographic display allows quantitative measurement of embolic events to be made.

Enhanced Detection of Thromboemboli With the Use of Targeted Microbubbles

Background and Purpose— Targeted ultrasound contrast agents have recently been developed to adhere selectively to specific pathogenic materials such as plaque or thrombus. Administration of such microbubbles has potential to aid transcranial Doppler ultrasound (TCD) detection of emboli and to act as markers for distinguishing one embolic material from another. The purpose of this study was to investigate whether TCD detection of circulating thrombus emboli would be enhanced by the addition of targeted microbubbles. Methods— Binding of microbubbles to the surface of the thrombus was confirmed by scanning electron microscopy. Targeted and control bubbles were then introduced to thrombus and tissue-mimicking material circulated under pulsatile-flow conditions in an in vitro flow rig. Embolic signal intensities before and after introduction of the bubbles were measured by TCD. Results— Targeted microbubbles enhanced TCD signal intensities from thrombus emboli by up to 13 dB. The bubbles were capable of binding to moving thrombus when injected into the flow circuit in low concentrations (≈36 bubbles per 100 mL) and were retained on the thrombus under pulsatile-flow conditions. Signal intensities from similarly sized pieces of tissue-mimicking material were not enhanced by injection of targeted bubbles. Conclusions— Injection of appropriately targeted microbubbles significantly enhances TCD detection of circulating thrombus emboli in vitro.

A real-time and fine resolution analyser used to estimate the instantaneous energy distribution of Doppler signals

Doppler ultrasound signal analysers in current use require that the signal be stationary within the time interval of processing, and yield average results for that interval. A real-time instantaneous frequency analyser based on the Wigner distribution function (WDF) has been developed, which provides a means of analysing time-varying signals or signals with short stationary time periods, and also produces results with very high instantaneous temporal resolution without causing significant deterioration of frequency resolution. In addition to the real-time processing, the most recent 2.4 s of Doppler signal is stored in the analyser so that the operator can perform further fine analysis and obtain results with very high resolutions in both the time and frequency domains. The pseudo-instantaneous mean frequency (PIMF) and the Pseudo-Instantaneous Power Distribution (PIPD) are calculated and displayed every 4.0 ms in the real-time processing mode, and with a resolution of between 80 microseconds and 2 ms in the fine resolution analysis mode. The analyser utilises an algorithm developed so that the WDF can be calculated efficiently using the conventional Fast Fourier Transform (FFT) method, and the PIPDs are calculated from data that contribute equally.

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 intraand 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

Impact of Clinical Environment on Embolus Detection: A Comparison of Automated and Manual Detection of Doppler Embolic Signals

Background: Transcranial Doppler ultrasound detection of weak embolic signals is inhibited by intrinsic limitations within the human auditory system. Psychoacoustics effects are likely to be exacerbated in a clinical environment, where automated embolus detection has potential to surpass manual detection. In this study we quantify the impact of clinical environment on manual detection of Doppler embolic signals following carotid surgery. We also discuss the implications of psychoacoustics considerations for the evaluation of automated detection systems. Method: Concurrent monitoring by vascular technologists and an automated embolus detection system were performed for 50 consecutive patients during postoperative recovery. Both detection methods were evaluated against a majority decision human expert panel analyzing under ideal conditions. Results: Clinical environment reduced the overall sensitivity of manual monitoring by ∼23%, mainly due to a ∼2-dB increase in the lower threshold for detection. Clinical environment was also associated with a reduction in positive predictive value for manual detection of ∼9% compared to ideal conditions. Automated monitoring, which is not affected by environment, was marginally more sensitive for detection of weaker embolic signals. Conclusions: One in 4 weak embolic signals was missed during routine clinical monitoring compared to ideal conditions. Automated detection (in this study) performed slightly better than human observers but did not approach the performance of the majority decision panel.

Differences in the power structures of Fourier transform and autoregressive spectral estimates of narrow-band Doppler signals

There is considerable interest in the application of autoregressive (AR) spectral analysis to ultrasonic Doppler signals. Sonograms produced using this technique are, however, very different from those produced using classic Fourier transform methods. Simulations have shown that the heights of the peaks in the AR spectra of narrow-band signals are not necessarily proportional to signal power, and should be used with caution in the context of Doppler signal processing.<<ETX>>

Development of a dual-gate automatic embolus detection system

Transcranial Doppler ultrasound (TCD) has been widely used to identify micro-emboli in the cerebral circulation and can provide valuable clinical information, but currently micro-embolic signal (MES) detection and analysis are significantly limited because they rely mainly on costly off-line analysis by human experts. Two new high-resolution real-time automated systems based on expert system classification theory have been designed and developed for the detection and archival of MESs. In addition to the Doppler signal processing features of the two systems, the later version is equipped with a built-in programmable TCD unit. The systems have been evaluated in-vitro and in-vivo, and preliminary results are presented.

Original contributionDifferentiation between emboli and artefacts using dual-gated transcranial Doppler ultrasound

It is well documented that transcranial Doppler ultrasound has the ability to detect cerebral emboli. During intraoperative patient monitoring studies, many signals due to artefact (probe motion, patient movement or surgical manipulation) are also detected and can be difficult to distinguish from genuine embolic events. We have constructed a Doppler system that can simultaneously range-gate at two separate depths, in order to test the hypothesis that it should be possible to distinguish between emboli and artefact by comparing the signal from the two separate regions within the vessel. The classification algorithm is based on the principle that emboli propagate with blood motion (whereas artefacts do not) and thus will be detected sequentially at different depths along the insonated cerebral artery. One hundred thirty-eight (presumed) embolic and 170 artefact signals were analysed. The median (interquartile range) gate separation was 10.01 mm (7.41-10.78 mm). The time delay between detection of embolic signals in the two channels was 11.04 ms (6.24-16.41 ms, but was only 0.08 ms (-0.48(-)+0.64 ms) for artefact (p < 0.0001). Dual-gated Doppler ultrasound is a conclusive and independent method that differentiates emboli from artefact. Incorporation of this system for long-term monitoring may eliminate the need for an experienced observer to be present.