Limits of uncertainty in measured values of embolus-to-blood ratio due to Doppler sample volume shape and location

Measurements of the relative Doppler power scattered by an embolus and the surrounding blood are widely used to infer the composition of the embolus. For a given embolus, measured embolus-to-blood ratio (MEBR) is affected by the Doppler sample volume shape, the geometrical relationship between the sample volume and the vessel, and the embolus trajectory through the blood vessel. The likely magnitudes of such effects were quantified using a model that allowed calculation of theoretical values of MEBR as a function of the geometrical relationship between a blood vessel and a defined sample volume. Overall, the effects of embolus trajectory, likely insonation angles, and plausible vessel misalignments introduced uncertainties in MEBR values of approximately 10 to 12 dB for a given vessel size. In practice, the only operator-controlled factors are the position and orientation of the transcranial Doppler probe on the patients' heads. Probe positioning can significantly affect MEBR and suboptimal positioning may result in the reduced detection of emboli.     

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.     

New technique for emboli detection and discrimination based on nonlinear characteristics of gas bubbles

Detection and characterization of emboli in the blood stream is of high clinical importance for making decisions after surgery. In this study, a new technique based on the nonlinear oscillations of gas bubbles was applied to gaseous emboli detection, characterization and sizing. To simulate gaseous emboli, an experimental system was developed to produce air bubbles of uniform diameters ranging from 19 microm up to 200 microm. The ultrasonic setup consisted of low-frequency transducers operating at 130 kHz and 250 kHz and using low acoustic pressures (30 kPa and 55 kPa). The experimental and theoretical results show that, depending on the transmitted frequency and the bubble sizes, higher harmonic components were produced in the frequency spectrum of the backscattered echo. Nonresonating bubbles scatter either linearly when their sizes are far away from the resonance size or nonlinearly at the second or third harmonic frequency when their sizes are getting close to the resonance size. Only resonant bubbles or bubbles very close to the resonance size are able to scatter at higher harmonic frequencies (fourth and fifth). This property is used to discriminate resonating bubbles from other bubble sizes. The appearance of harmonic component in the frequency spectrum seems to be an unambiguous tool to differentiate gaseous emboli from solid emboli that scatter linearly.     

Barker code in TCD ultrasound systems to improve the sensitivity of emboli detection

In present research, Barker-coded excitation is applied to improve the sensitivity of emboli detection in transcranial Doppler (TCD) ultrasound systems. A 13-chip Barker code with a base pulse sequence that comprises two cycles at 2 MHz is used to compare with a 20-cycle pulse in both simulation studies and in vitro experiments. The results show that the system using coded excitation gains a 2.78-dB improvement in measured embolus-to-blood ratio (MEBR) while keeping a similar signal-to-noise ratio (SNR). The simulation results also indicate that the MEBR of the emboli are related to their velocities-a fast embolus has lower MEBR compared with a slow one with the same backscatter amplitudes, which makes fast emboli more difficult to detect. TCD system using coded excitation is more sensitive for faster and smaller emboli, which is significant for early diagnosis of stroke.     

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.     

Pulmonary emboli from therapeutic sodium hyaluronate

A patient presented with shortness of breath and pleuritic pain shortly after bilateral knee synovial injections with sodium hyaluronate (HA). He was discharged after a brief hospitalization without a diagnosis when no Doppler or radiologic evidence of deep vein thrombosis or pulmonary emboli was found. Radiologic studies found patchy ground glass opacities that were predominantly peripheral in disposition, with prominent septal lines in the lungs; a subsequent pulmonary function test showed a reduced diffusing capacity of the lung for carbon monoxide (D(LCO)). These results prompted a lung biopsy that revealed multiple emboli composed of HA and fibrin in medium size pulmonary arteries, enlarged lymphatic vessels, and a bone marrow embolus. This is the first report of HA emboli following therapeutic HA injections and demonstrates that pulmonary function tests can be used to infer the reduction in pulmonary vascular area consequent to pulmonary emboli, and so can contribute to the detection of pulmonary emboli in unusual presentations.     

Microemboli detection on extracorporeal bypass circuits

Numerous authors have associated gaseous microembolization with adverse cerebral outcomes during cardiopulmonary bypass (CPB). The introduction to this review provides background on the connection between microemboli and adverse cerebral outcomes. This connection is often difficult to quantify, as outcomes depend on a number of factors, including the size of the bubble, where it passes through the patient, patient comorbidities and other factors. Nonetheless, numerous studies have shown statistically significant differences in the mean number of cerebral emboli detected in patients that stroked and those that did not, as well as for patients with major cardiac complications and patients with a longer length of hospital stay. Our introduction is followed by case reports and laboratory studies showing how monitoring for gaseous microemboli (GME) can be used to reduce the embolic load delivered to the patient through the bypass circuit. These methods include improved qualification of bypass circuit design prior to surgery, modification of priming procedures to reduce air in the circuit at the start of surgery, new methods for injecting drugs into the circuit during surgery, and better detection of removal of sources of air during surgery. The review concludes with background on the ultrasonic detection of GME, comparing through-transmission gross air detectors and Doppler ultrasound technology with fixed-beam ultrasonic imaging of emboli, a new ultrasonic technique that images moving emboli in the blood using a single ultrasound transducer element in a fixed position. This overview is meant to shed light on why different ultrasonic detection technologies report widely varying counts and emboli loads, and why fixed-beam ultrasonic imaging represents an improvement in the ability to monitor, measure and quantitate embolic load during CPB.     

Acoustical bubble trapper applied to hemodialysis

Gaseous microemboli can arise in extracorporeal lines and devices such as dialysis machines. They are associated with severe pulmonary side effects in patients undergoing chronic hemodialysis sessions. The goal of this study was to develop a gaseous emboli trapper using ultrasound waves to remove any air bubble from the tubing system before they reach the patient. A homemade bubble trapper, developed in the laboratory, consists of a Perspex block containing a main channel connected to the tubing of a hemodialysis machine and a second subchannel positioned perpendicularly to the main one, used to trap the air microemboli. The microemboli flowing in the main channel were insonified through an acoustic window with an ultrasound wave, at a frequency of 500 kHz and with a maximal acoustic pressure of 500 kPa, generated by a single-element transducer positioned 3 cm away from the main flow. The radiation force induced by the ultrasound beam acts directly on the flowing air emboli, by pushing them into the subchannel. Two Doppler probes operating both at 2 MHz, connected to a DWL Doppler machine were placed before and after the bubble trapper to count sequentially the number of embolic events. The flow of the machine was varied between 200 mL/min and 500 mL/min. Depending on the flow velocity, the number of microembolic signals (MES) detected by the Doppler probes before and after the trapping system was identical and ranged from 5 to 150 MES/min in absence of the ultrasound irradiation. When the air bubble trapper was activated, a reduction of the number of MES, up to 70%, was achieved. Doppler recordings suggest that the circulating bubbles were either fragmented into smaller bubble fragments or directly got pushed into the second subchannel where they were collected. This simple approach using an ultrasound-based trapping system was shown to operate adequately with the current settings and can be used to filter air microemboli.     

The matching pursuit: a new method of characterizing microembolic signals?

Detection of clinically silent circulating microemboli within cerebral arteries by transcranial Doppler ultrasound (US) is now being widely investigated in the hope of identifying patients at increased risk for stroke. However, the widespread application of embolus detection is still limited in clinical practice because current transcranial Doppler systems have not the required sensitivity and specificity to analyze microembolic signals, particularly to distinguish between gaseous, or solid brain emboli and artefacts. In this work, we proposed to investigate the potential of a new approach for the analysis of microembolic signals via the so-called matching pursuit, which is closely related to wavelet transform and is not subject to the same limitations as the fast Fourier transform. Our preliminary results clearly indicate that matching pursuit is well suited to this task.     

体外模拟实验研究心腔内压力对微气泡后散射强度的影响

目的 通过体外模拟实验，研究不同压力对声振白蛋白后散射强度的影响，并观察显微镜下其直径的变化，以进一步证实微气泡散射强度与其直径变化之间的关系，并评价运用这一方法无创性估测心腔内压力的可行性。方法 设计一体外模拟心脏模型，注入5％声振白蛋白后，密封容器，使微气泡处于悬浮状态并模拟心脏收缩和舒张时的压力变化，观察升压和解压后微气泡的后散射强度，显微镜同步观察单个微气泡的外形。结果 微气泡后散射时间强度曲线随压力的升高和降低出现周期性的变化，压力升高后散射强度达到最低，压力解除后散射强度恢复。显微镜下观察到压力升高微气泡体积变小，压力解除微气泡体积回复。结论 压力增加显微镜下观察到微气泡直径变小，声学密度分析软件则显示后散射强度减弱，从而推导后散射强度的减弱主要与微气泡的暂时被压缩有关。然而该方法在检测压力变化的敏感性和准确性上有待进一步提高，此外用于检测压力变化的绝对值时还需建立一个准确的校正公式。     

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. (E-mail: dhe@le.ac.uk)     

Impending Paradoxical Embolism: A Case Report

Background

A thrombus straddling a patent foramen ovale (i.e., impending paradoxical embolism) is a very rare event. Most cases have been reported at autopsy only after finding a patent foramen ovale and arterial emboli. Patent foramen ovale in the population is common.

Objectives

The objective of this case report is to remind physicians that common presentations can have uncommon causes. Some of these uncommon causes are easy to find and may significantly change outcomes if treated early.

Case Report

We present the case of a dyspneic patient with concomitant pulmonary embolism, deep vein thrombosis, and impending paradoxical embolism. Emergency Physicians should be aware that dyspnea may be the only initial symptom. Although dyspnea may be linked to a pulmonary embolus, it may not represent the entire clinical picture. A thrombus formed within a patent foramen ovale portends the possibility of a larger pulmonary embolus and an arterial embolus.

Conclusion

Early detection of an impending paradoxical embolism may result in an improved outcome. Treatment choices consist of anticoagulation, thrombectomy, or thrombolysis. Choice of treatment is difficult but should be made quickly to reduce the possibility of adverse patient outcomes.     

Acoustically stimulated transient power scattering explains enhanced detection of the very low velocities in myocardial capillaries by power Doppler imaging: an in vitro study.

BACKGROUND: Although enhanced detection of myocardial perfusion signals by power Doppler imaging during contrast echocardiography has been noted, flow velocities in the coronary microvasculature should generally be below the threshold for Doppler motion detection. It has been suggested that in this situation nonlinear scattering related to acoustically stimulated microsphere oscillation or destruction may be responsible for the detected Doppler shift. METHODS AND RESULTS: This study examined the behavior of MRX 115 (ImaRx Pharmaceuticals) microbubbles during harmonic and nonharmonic power Doppler imaging at varying power outputs (mechanical indexes 0. 3, 0.5, 0.7, and 0.9) in a perfusion tube model under zero-flow conditions. Boluses of MRX 115 0.5-mL suspension were introduced into the model, and flow was halted during each imaging period. Once power Doppler imaging was implemented, a signal was detected as unique sparkling color pixels corresponding to individual bubble destruction events, even in the absence of contrast movement. This phenomenon continued until all contrast bubbles disappeared from the region subjected to power Doppler imaging, usually within 35 to 40 seconds. Off-line videointensity measurements showed that initial power Doppler signal intensity and maximum signal decay rates increased parallel to increasing power output and were substantially greater for nonharmonic than for harmonic imaging modes. CONCLUSION: This relationship between signal intensity and decay rate and acoustic power output suggests that transient scattering related to bubble destruction is responsible for generation of the power Doppler signal in the absence of flow. This would explain the enhanced detection of the very low velocity flows in the myocardial capillaries by power Doppler contrast imaging.     

Improved methods for measurement of gaseous microbubbles during extracorporeal circulation

The detection and quantification of gaseous microbubbles in the arterial line of the extracorporeal circuit (ECC) are very important aims for quality assurance of perfusion. A system that allows a continuous measurement of microbubble distribution in the range of 10 and 120 microm was tested. The two-channel ultrasonic bubble counter (UBC) was based on a 2-MHz ultrasound Doppler system with propriety ultrasound probes. The bubble size was determined using the backscattered Doppler signal and was corrected by means of a reference signal based on measurement conditions. Our studies have shown that the quality of this signal can be negatively affected in the clinical environment. Different influences are involved, such as electrocoagulation or electromagnetic disturbances. Various algorithms were tested and new ones were developed in order to minimize the effect of such interferences on the accuracy of the bubble detection. The on-line data were recorded during the entire surgical time to allow an off-line evaluation with different algorithms. This allowed us to obtain more exact results. Two clinical studies with 91 patients were performed with microbubbles measured in the arterial line during coronary artery bypass grafting (CABG) and valve replacement. The results confirmed the expected occurrence of microbubbles during various phases of surgery. The measurement itself proved to be resistant to different external disturbances.     

Paradoxical embolus straddling patent foramen ovale demonstrated by computed tomographic pulmonary angiography

An elderly gentleman presented to the emergency department with a recent history of dyspnoea, collapse and transient neurological symptoms. He was noted to be hypoxic with a significantly elevated D Dimer. A computer tomography pulmonary angiogram demonstrated a large embolus with a further filling defects within the left and the right atria, abutting the inter-atrial septum. Suspicion of a paradoxical pulmonary embolus was raised and the patient subsequently underwent echocardiography which confirmed a patent foramen ovale (PFO). He was commenced on warfarin therapy. In patients with elevated right heart pressure, a PFO can be unmasked and give rise to cerebral emboli. Clinical suspicion should be raised in patients with pulmonary emboli or deep venous thrombosis if there is a concomitant history of focal neurological symptoms.     

Bilateral cerebral emboli monitoring during extracorporeal circulation.

Microembolism generated during extracorporeal circulation is thought to be responsible for stroke and neuropsychological deficits. Before one can investigate the pathogenetic role in more detail, reproducible and reliable quantitative methods need to be developed. In several previous studies, microemboli detection was performed unilaterally. We questioned if this reflects the bihemispheric embolic load. In 42 patients undergoing coronary artery bypass grafting, bilateral embolus detection was performed during extracorporeal circulation. The side-to-side correlation of microembolus counts was strong (0.91), but there was a significant difference in number (p < 0.01) comparing left and right emboli. The side of higher embolus counts cannot be predicted in the individual because either side may show higher counts. Doubling the unilateral count may deviate from the bilateral count by as much as 51% in the individual patient. The total embolic load to the brain during extracorporeal circulation cannot be precisely predicted from unilateral transcranial insonation alone.     

二维与彩色多普勒超声对布-加综合征病因学的诊断价值

目的 探讨二维超声与彩色多普勒超声对布-加综合征病因学诊断价值，以指导临床进行治疗。方法 应用二维超声与彩色多普勒超声检查19例布-加综合征患者，男12例，女7例，年龄12-69岁，平均41岁，综合分析其二维图象与彩色血流显像特点，对布-加综合征的病因作出诊断，并通过临床及其他影像学检查互相验证。结果 14例均出现肝脏左右叶不同程序增大，呈瘀血肝样表现，其中1例有肝叶巨大占位病变，5例肝脏体积减小，肝实质回声不均匀，呈肝硬化声像图表现，肝静脉或下腔静肪局限性陕窄，远端管腔扩张，血流速度减慢，甚至出现逆流，8例可见云雾状影，彩色血流显示狭窄处血流速度增快，远端血流颜色暗淡或消失。经临床及其他影像学检查，19例中先天发育异常6例，血栓型2例，癌栓型4例，肿块外压型3例，原因不明型4例，二维及彩色多普勒超声诊断除1例先天性发育异常漏诊1例癌栓型误诊为栓型外，其余均获确诊，总的病因诊断准确率94.7%，敏感性93.3%，特异性100%。结论 二维超声及彩色多普勒超声对面-加综合征病因学的诊断有较高的价值，对临床治疗方式的选择起着重要作用。     

Quantification of Embolic Showers Using Radio-Frequency Based TCD Analysis

During cardiac surgery and cardiology interventions, microemboli may be generated and disperse in the systemic circulation. The amount of microemboli that ends up in cerebral blood vessels is associated with postoperative neurologic complications. During cardiac surgery a large amount of cerebral microemboli can occur at once and create so-called “cerebral embolic showers.” To correlate postoperative neurologic outcome to cerebral embolic load, a quantitative evaluation of these embolic showers is necessary. The standard monitoring technology to visualize cerebral microemboli is transcranial Doppler (TCD). Although the conventional TCD systems are equipped with software claiming to detect microembolic signals, none of the existing TCD systems is capable of an accurate estimation of the number of cerebral microemboli in embolic showers. In this study, an algorithm with a high temporal resolution, based on the radiofrequency (RF) signal of a TCD system, has been designed to quantify these showers. Evaluation by three independent observers of a training set demonstrates that the proposed method has a sensitivity of at least one order of magnitude better than the automatic detection algorithm on the existing Doppler device used. RF-based emboli detection can possibly become a standard addition to conventional Doppler methods, considering that accurate estimation of the embolic load supports quantification of neurologic risk during various surgical procedures. (E-mail: l.sauren@ctc.unimaas.nl)     

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.