Survey of the publics' preferences for communication of medical radiation risk.

To comply with the Ionising Radiations (Medical Exposures) Regulations 2017, patients need to be adequately informed of medical radiation risks prior to exposure. This study used a survey developed in partnership with patients and members of the public to explore patient preferences for radiation risk communication. It was distributed through social media between 28/4/2020 and 18/7/2020. All respondents (N= 376) wanted to be informed about radiation risk, though the threshold at which they wished to be informed varied. The current practice of displaying posters in waiting areas does not meet the expressed preference of the patients if used in isolation. Only 6% of respondents were satisfied with the commonly used statement that the 'risk is low' if used in isolation. The majority of respondents (73%) said they would not be concerned about an increase in the risk of cancer of less than 1 in 10 000. The level of risk at which patients express a concern and the methodology for risk communication has been evaluated and based on these findings, and pre-existing literature, a graded approach to radiation risk communication based on modality is proposed. Patients must be involved throughout the evolution of this practice.

Perioperative Cerebral Microbleeds After Adult Cardiac Surgery.

Background and Purpose- Cerebral microbleeds (CMBs) have been observed using magnetic resonance imaging in patients with cardiovascular risk factors, cognitive deterioration, small vessel disease, and dementia. They are a well-known consequence of cerebral amyloid angiopathy, chronic hypertension, and diffuse axonal injury, among other causes. However, the frequency and location of new CMBs postadult cardiac surgery, in association with cognition and perioperative risk factors, have yet to be studied. Methods- Pre- and postsurgery magnetic resonance susceptibility-weighted images and neuropsychological tests were analyzed from a total of 75 patients undergoing cardiac surgery (70 men; mean age, 63±10 years). CMBs were identified by a neuroradiologist blinded to clinical details who independently assessed the presence and location of CMBs using standardized criteria. Results- New CMBs were identified in 76% of patients after cardiac surgery. The majority of new CMBs were located in the frontal lobe (46%) followed by the parietal lobe (15%), cerebellum (13%), occipital lobe (12%), and temporal lobe (8%). Patients with new CMBs typically began with a higher prevalence of preexisting CMBs ( P=0.02). New CMBs were associated with longer cardiopulmonary bypass times ( P=0.003), and there was a borderline association with lower percentage hematocrit ( P=0.04). Logistic regression analysis suggested a ≈2% increase in the odds of acquiring new CMBs during cardiac surgery for every minute of bypass time (odds ratio, 1.02; 95% CI, 1.00-1.05; P=0.04). Postoperative neuropsychological decline was observed in 44% of patients and seemed to be unrelated to new CMBs. Conclusions- New CMBs identified using susceptibility-weighted images were found in 76% of patients who underwent cardiac surgery. CMBs were globally distributed with the highest numbers in the frontal and parietal lobes. Our regression analysis indicated that length of cardiopulmonary bypass time and lowered hematocrit may be significant predictors for new CMBs after cardiac surgery. Clinical Trial Registration- URL: http://www.isrctn.com . Unique identifier: 66022965.

Transcervical Carotid Stenting With Dynamic Flow Reversal Demonstrates Embolization Rates Comparable to Carotid Endarterectomy.

PURPOSE: To evaluate a series of patients treated electively with carotid endarterectomy (CEA), transfemoral carotid artery stenting with distal filter protection (CASdp), and transcervical carotid stenting with dynamic flow reversal (CASfr) monitored continuously with transcranial Doppler (TCD) during the procedure to detect intraoperative embolization rates. METHODS: Thirty-four patients (mean age 67.6 years; 24 men) with significant carotid stenosis underwent successful TCD monitoring during the revascularization procedure (10 CEA, 8 CASdp, and 16 CASfr). Ipsilateral microembolic signals were segregated into 3 phases: preprotection (until internal carotid artery cross-shunted or clamped if no shunt was used, filter deployed, or flow reversal established), protection (until clamp/shunt was removed, filter retrieved, or antegrade flow reestablished), and postprotection (after clamp/shunt or filter removal or restoration of normal flow). RESULTS: CASdp showed higher embolization rates than CEA or CASfr in the preprotection phase (p<0.001). In the protection phase, CASdp was again associated with more embolization compared with CEA and CASfr (p<0.001). In the postprotection phase, no differences between the revascularization therapies were observed. CASfr and CEA did not show significant differences in intraoperative embolization during any of the phases. CONCLUSION: TCD recordings demonstrated a significant reduction in embolization to the brain during transcervical carotid artery stent placement with the use of dynamic flow reversal compared to transfemoral CAS using distal filters. No significant differences in microembolization could be detected between CEA and CASfr. The observed lower embolization rates and lack of adverse events suggest that transcervical CAS with dynamic flow reversal is a promising technique and may be the preferred method when performing CAS.

Impact of perioperative infarcts after cardiac surgery.

BACKGROUND AND PURPOSE: Brain injury after cardiac surgery is a serious concern for patients and their families. The purpose of this study was to use 3-T fluid attenuated inversion recovery MRI to characterize new and preexisting cerebral ischemic lesions in patients undergoing cardiac surgery and to test whether the accumulation of new ischemic lesions adversely affects cognition. METHODS: Digital comparison of before and after fluid attenuated inversion recovery MRI images was performed for 77 cardiac surgery patients. The burden of preexisting versus new ischemic lesions was quantified and compared with the results of baseline and postoperative neuropsychological testing. RESULTS: After surgery, new lesions were identified in 31% of patients, averaging 0.5 lesions per patient (67 mm(3) [0.004%] of brain tissue). Patients with preexisting lesions were 10× more likely to receive new lesions after surgery than patients without preexisting lesions. Preexisting ischemic lesions were observed in 64% of patients, averaging 19.4 lesions (1542 mm(3) [0.1%] of brain tissue). New lesions in the left hemisphere were significantly smaller and more numerous (29 lesions; median volume, 44 mm(3); volume range, 5-404 mm(3)) than those on the right (10 lesions; median volume, 128 mm(3); volume range, 13-1383 mm(3)), which is consistent with a cardioembolic source of particulate emboli. Overall, the incidence of postoperative cognitive decline was 46% and was independent of whether new lesions were present. CONCLUSIONS: New lesions after cardiac surgery added a small (≈4%) contribution to the burden of preexisting cerebrovascular disease and did not seem to affect cognitive function. CLINICAL TRIAL REGISTRATION URL: http://public.ukcrn.org.uk. Unique identifier: UKCRN ID: 11702.

The Presence of New MRI Lesions and Cognitive Decline After Cardiac Surgery: A Systematic Review.

Patients are commonly reported to experience postoperative cognitive decline (POCD) and new ischemic lesions following surgery, which many researchers have hypothesised to result from emboli entering the cerebral circulation during surgery. Modern magnetic resonance imaging techniques have enabled clear and accurate identification of ischemic lesions. However, difficulties in assessing subtle changes in cognitive impairment clinically remain. The purpose of this systematic review is to discuss the literature that has investigated cognitive outcome in relation to new ischaemic brain lesions after cardiac surgery.

Three-dimensional simulations of embolic stroke and an equation for sizing emboli from imaging.

Stroke simulations are needed to run in-silico trials, develop hypotheses for clinical studies and to interpret ultrasound monitoring and radiological imaging. We describe proof-of-concept three-dimensional stroke simulations, carrying out in silico trials to relate lesion volume to embolus diameter and calculate probabilistic lesion overlap maps, building on our previous Monte Carlo method. Simulated emboli were released into an in silico vasculature to simulate 1000 s of strokes. Infarct volume distributions and probabilistic lesion overlap maps were determined. Computer-generated lesions were assessed by clinicians and compared with radiological images. The key result of this study is development of a three-dimensional simulation for embolic stroke and its application to an in silico clinical trial. Probabilistic lesion overlap maps showed that the lesions from small emboli are homogeneously distributed throughout the cerebral vasculature. Mid-sized emboli were preferentially found in posterior cerebral artery (PCA) and posterior region of the middle cerebral artery (MCA) territories. For large emboli, MCA, PCA and anterior cerebral artery (ACA) lesions were comparable to clinical observations, with MCA, PCA then ACA territories identified as the most to least probable regions for lesions to occur. A power law relationship between lesion volume and embolus diameter was found. In conclusion, this article showed proof-of-concept for large in silico trials of embolic stroke including 3D information, identifying that embolus diameter could be determined from infarct volume and that embolus size is critically important to the resting place of emboli. We anticipate this work will form the basis of clinical applications including intraoperative monitoring, determining stroke origins, and in silico trials for complex situations such as multiple embolisation.

Are portable ankle brachial pressure index measurement devices suitable for hypertension screening?

OBJECTIVE: In a large-scale population cardiovascular screening programme, peripheral artery disease (PAD) and hypertension would ideally be rapidly assessed using a single device. The ankle-brachial pressure index (ABPI) is calculated by comparing the ankle and brachial blood pressure (BP). However, it is currently unclear whether brachial BP measurements provided by automated PAD screening systems are sufficiently accurate for simultaneous hypertension screening. METHODS: Two portable PAD screening devices, the MESI ABPI MD and Huntleigh's Dopplex ABIlity, were evaluated following the European Society of Hypertension International Protocol (ESH-IP) Revision 2010 using a mercury-free sphygmomanometer as a reference device. RESULTS: On average, the MESI slightly underestimated brachial systolic blood pressure (BP) with a bias and standard deviation (SD) of -3.5 (SD: 3.3) mmHg and diastolic BP with a bias of -1.5 (SD: 2.3) mmHg. For systolic BP estimates, the Dopplex was more accurate than the MESI with a lower bias of -0.5 (SD: 4.2) mmHg but less precise. The MESI successfully fulfilled all the requirements of the ESH-IP for hypertension screening. The Dopplex device failed the ESH-IP due to the absence of DBP measurements. CONCLUSIONS: The MESI device appears to be suitable for simultaneous PAD and hypertension screening as part of a preventative care programme. Huntleigh's Dopplex ABIlity failed to pass the ESH-IP validation test. Further clinical trials are underway to assess the use of the MESI for simultaneous screening for hypertension and PAD in a population screening setting.

Effects of Blood Pressure on Brain Tissue Pulsation Amplitude in a Phantom Model.

OBJECTIVE: The precise mechanism and determinants of brain tissue pulsations (BTPs) are poorly understood, and the impact of blood pressure (BP) on BTPs is relatively unexplored. This study aimed to explore the relationship between BP parameters (mean arterial pressure [MAP] and pulse pressure [PP]) and BTP amplitude, using a transcranial tissue Doppler prototype. METHODS: A phantom brain model generating arterial-induced BTPs was developed to observe BP changes in the absence of confounding variables and cerebral autoregulation feedback processes. A regression model was developed to investigate the relationship between bulk BTP amplitude and BP. The separate effects of PP and MAP were evaluated and quantified. RESULTS: The regression model (R2 = 0.978) revealed that bulk BTP amplitude measured from 27 gates significantly increased with PP but not with MAP. Every 1 mm Hg increase in PP resulted in a bulk BTP amplitude increase of 0.29 µm. CONCLUSION: Increments in BP were significantly associated with increments in bulk BTP amplitude. Further work should aim to confirm the relationship between BP and BTPs in the presence of cerebral autoregulation and explore further physiological factors having an impact on BTP measurements, such as cerebral blood flow volume, tissue distensibility and intracranial pressure.

Development of a Flow Phantom for Transcranial Doppler Ultrasound Quality Assurance.

Anecdotal evidence was recently brought to our attention suggesting a potential difference in velocity estimates between transcranial Doppler (TCD) systems when measuring high velocities (∼200 cm/s) close to the threshold for sickle cell disease stroke prevention. As we were unable to identify a suitable commercial TCD phantom, a middle cerebral artery (MCA) flow phantom was developed to evaluate velocity estimates from different devices under controlled conditions. Time-averaged velocity estimates were obtained using two TCD devices: a Spencer Technologies ST3 Doppler system (ST3 PMD150, Spencer Technologies, Seattle, WA, USA) and a DWL Dopplerbox (DWL Compumedics, SN-300947, Singen, Germany). These were compared with velocity estimates obtained using a Zonare duplex scanner (Zonare Medical Systems, Mountain View, CA, USA), with timed collection of fluid as the gold standard. Bland-Altman analysis was performed to compare measurements between devices. Our tests confirmed that velocities measured with the DWL TCD system were +4.1 cm/s (+3.7%; limits of agreement [LoA]: 2%, 5%; p = 0.03) higher than the Spencer system when measuring a velocity 110 cm/s and +12 cm/s higher (+5.7 %; LoA: 4.8%, 6.6%; p = 0.03) when measuring velocities of 210 cm/s, close to the diagnostic threshold for stroke intervention. We found our MCA phantom to be a valuable tool for systematically quantifying differences in TCD velocity estimates between devices, confirming that the DWL system gave consistently higher readings than the Spencer ST3 system. Differences become more pronounced at high velocities, which explains why they were not identified earlier. Our findings have clinical implications for centers using TCD to monitor patients with sickle cell disease, as extra care may be needed to adjust for bias between manufacturers when making treatment decisions about children with sickle cell with velocities close to the diagnostic threshold.

Brain tissue motion in acute hemorrhagic stroke using amplified MRI (aMRI).

Brain tissue pulsates with each cardiac cycle, however the effect of disease on this natural motion is still unclear. Current literature mainly focuses on healthy brain tissue, with only limited studies looking at disease states such as Chiari malformation and acute ischemic stroke. This case report advances on recent literature by describing the case of a patient with an acute intracerebral hemorrhage and demonstrating an amplified MRI cine of the brain's motion. A clearer understanding of the effects of disease on brain motion may guide clinical application of pulsation measurement.

Ultrasound shear wave elastography imaging of common carotid arteries in patients with Spontaneous Coronary Artery Dissection (SCAD).

BACKGROUND: Shear wave elastography (SWE) is emerging as a valuable clinical tool for a variety of conditions. The aim of this pilot study was to assess the potential of SWE imaging of the common carotid arteries (CCA) in patients with spontaneous coronary artery dissection (SCAD), a rare but potentially life-threatening condition, hypothesized to be linked to changes in vessel wall elasticity. METHODS: Ultrasound shear wave elastography (SWE) estimates of artery wall elasticity were obtained from the left and right CCAs of 89 confirmed SCAD patients and 38 non-dissection controls. SWE images obtained over multiple cardiac cycles were analysed by a blinded observer to estimate elasticity in the form of a Young's Modulus (YM) value, across regions of interest (ROI) located within the anterior and posterior CCA walls. RESULTS: YM estimates ranged from 17 to 133 kPa in SCAD patients compared to 34 to 87 kPa in non-dissection controls. The mean YM of 55 [standard deviation (SD): 21] kPa in SCAD patients was not significantly different to the mean of 57 [SD: 12] kPa in controls, p = 0.32. The difference between groups was 2 kPa [95% Confidence Interval - 11, 4]. CONCLUSIONS: SWE imaging of CCAs in SCAD patients is feasible although the clinical benefit is limited by relatively high variability of YM values which may have contributed to our finding of no significant difference between SCAD patients and non-dissection controls.

Emerging Detection Techniques for Large Vessel Occlusion Stroke: A Scoping Review.

Background: Large vessel occlusion (LVO) is the obstruction of large, proximal cerebral arteries and can account for up to 46% of acute ischaemic stroke (AIS) when both the A2 and P2 segments are included (from the anterior and posterior cerebral arteries). It is of paramount importance that LVO is promptly recognised to provide timely and effective acute stroke management. This review aims to scope recent literature to identify new emerging detection techniques for LVO. As a good comparator throughout this review, the commonly used National Institutes of Health Stroke Scale (NIHSS), at a cut-off of ≥11, has been reported to have a sensitivity of 86% and a specificity of 60% for LVO. Methods: Four electronic databases (Medline via OVID, CINAHL, Scopus, and Web of Science), and grey literature using OpenGrey, were systematically searched for published literature investigating developments in detection methods for LVO, reported from 2015 to 2021. The protocol for the search was published with the Open Science Framework (10.17605/OSF.IO/A98KN). Two independent researchers screened the titles, abstracts, and full texts of the articles, assessing their eligibility for inclusion. Results: The search identified 5,082 articles, in which 2,265 articles were screened to assess their eligibility. Sixty-two studies remained following full-text screening. LVO detection techniques were categorised into 5 groups: stroke scales (n = 30), imaging and physiological methods (n = 15), algorithmic and machine learning approaches (n = 9), physical symptoms (n = 5), and biomarkers (n = 3). Conclusions: This scoping review has explored literature on novel and advancements in pre-existing detection methods for LVO. The results of this review highlight LVO detection techniques, such as stroke scales and biomarkers, with good sensitivity and specificity performance, whilst also showing advancements to support existing LVO confirmatory methods, such as neuroimaging.

Embolus trajectory through a physical replica of the major cerebral arteries.

BACKGROUND AND PURPOSE: The observed distribution of cerebral infarcts varies markedly from expectations based on blood-flow volume or Doppler embolus detection. In this study, we used an in vitro model of the cerebral arteries to test whether embolus microspheres encountering the circle of Willis are carried proportionally to volume flow or express a preferred trajectory related to arterial morphology or embolus size. METHODS: Our model consisted of a patient-specific silicone replica of the cerebral macrocirculation featuring physiologically realistic pulsatile flow of a blood-mimicking fluid at approximately 1000 mL/min and an input pressure of approximately 150/70 mm Hg. Particles of 200, 500, and 1000 microm diameter with equivalent density to thrombus were introduced to the carotid arteries and counted on exiting the model outlets. RESULTS: The middle cerebral arteries (MCAs) of the replica attracted a disproportionate number of emboli compared with the anterior cerebral arteries; 98%+/-3% of 1000 microm and 93%+/-2% of 500 microm emboli entered the MCA compared with 82%+/-5% of the flow. The observed distribution of large emboli was consistent with the ratio of MCA:anterior cerebral artery infarcts, approximately 95% of which occur in territories supplied by the MCA. With decreasing embolus size, the distribution of emboli approaches that of the flow (approximately 89% of 200 microm emboli took the MCA). CONCLUSIONS: Embolus trajectory through the cerebral arteries is dependent on embolus size and strongly favors the MCA for large emboli. The 70:30 ratio of MCA:anterior cerebral artery emboli observed by Doppler ultrasound is consistent with the trajectories of small emboli that tend to be asymptomatic.

Acute ischemic stroke diagnosis using brain tissue pulsations.

Healthy brain tissue pulsates with the cardiac cycle, but whether brain tissue pulsations (BTPs) are impaired by tissue ischemia due to ischemic stroke is currently unclear. This study is the first to explore the clinical potential of measuring BTPs using ultrasound in acute ischemic stroke patients. BTPs were measured in 24 healthy volunteers (aged 52-82 years) and 14 acute ischemic stroke patients (aged 51-86 years) using a novel Transcranial Tissue Doppler (TCTD) method. Measurements were quick to perform and were well tolerated by all subjects. A mixed-methods approach was used for blinded analysis of recordings. This identified qualitative disruption of BTPs in acute stroke patients, which were used to create an analysis checklist. Blinded BTP analysis by novices using the checklist resulted in high sensitivity but low specificity for stroke detection. Quantitative analysis also identified differences between stroke and healthy participants, including weaker BTPs in stroke patients. This first study reporting BTP characteristics in acute ischemic stroke revealed weaker brain tissue pulsations and waveform disruption in acute stroke patients. However, further clinical evaluation using a larger sample size is required to confirm these findings and to explore whether TCTD monitoring might be beneficial for clinical neuromonitoring.

Brain Tissue Pulsation in Healthy Volunteers.

It is well known that the brain pulses with each cardiac cycle, but interest in measuring cardiac-induced brain tissue pulsations (BTPs) is relatively recent. This study was aimed at generating BTP reference data from healthy patients for future clinical comparisons and modelling. BTPs were measured through the forehead and temporal positions as a function of age, sex, heart rate, mean arterial pressure and pulse pressure. A multivariate regression model was developed based on transcranial tissue Doppler BTP measurements from 107 healthy adults (56 male) aged from 20-81 y. A subset of 5 participants (aged 20-49 y) underwent a brain magnetic resonance imaging scan to relate the position of the ultrasound beam to anatomy. BTP amplitudes were found to vary widely between patients (from ∼4 to ∼150 µm) and were strongly associated with pulse pressure. Comparison with magnetic resonance images confirmed regional variations in BTP with depth and probe position.

The Effects of Hypocapnia on Brain Tissue Pulsations.

Hypocapnia is known to affect patients with acute stroke and plays a key role in governing cerebral autoregulation. However, the impact of hypocapnia on brain tissue pulsations (BTPs) is relatively unexplored. As BTPs are hypothesised to result from cerebrovascular resistance to the inflow of pulsatile arterial blood, it has also been hypothesised that cerebral autoregulation changes mediated by hypocapnia will alter BTP amplitude. This healthy volunteer study reports measurements of BTPs obtained using transcranial tissue Doppler (TCTD). Thirty participants underwent hyperventilation to induce mild hypocapnia. BTP amplitude, EtCO2, blood pressure, and heart rate were then analysed to explore the impact of hypocapnia on BTP amplitude. Significant changes in BTP amplitude were noted during recovery from hypocapnia, but not during the hyperventilation manoeuvre itself. However, a significant increase in heart rate and pulse pressure and decrease in mean arterial pressure were also observed to accompany hypocapnia, which may have confounded our findings. Whilst further investigation is required, the results of this study provide a starting point for better understanding of the effects of carbon dioxide levels on BTPs. Further research in this area is needed to identify the major physiological drivers of BTPs and quantify their interactions with other aspects of cerebral haemodynamics.

Neurological impact of emboli during adult cardiac surgery.

OBJECTIVES: This study draws on advances in Doppler ultrasound bubble sizing to investigate whether high volumes of macro-bubbles entering the brain during cardiac surgery increase the risk of new cerebral microbleeds (CMBs), ischemic MR lesions, or post-operative cognitive decline (POCD). METHODS: Transcranial Doppler (TCD) ultrasound recordings were analysed to estimate numbers of emboli and macrobubbles (>100 µm) entering the brain during cardiac surgery. Logistic regression was used to explore the hypothesis that emboli characteristics affect the incidence of new brain injuries identified through pre- and post-operative MRI and neuropsychological testing. RESULTS: TCD, MRI, and neuropsychological test data were compared between 28 valve and 18 CABG patients. Although valve patients received over twice as many emboli per procedure [median: 1995 vs. 859, p = .004], and seven times as many macro-bubbles [median: 218 vs. 28, p = .001], high volumes of macrobubbles were not found to be significantly associated with new CMBs, new ischaemic lesions, or POCD. The odds of acquiring new CMBs increased by approximately 5% [95% CI: 1 to 10%] for every embolus detected in the first minute after the release of the aortic cross-clamp (AxC). Logistic regression models also confirmed previous findings that cardiopulmonary bypass time and valve surgery were significant predictors for new CMBs (both p = .03). Logistic regression analysis estimated an increase in the odds of acquiring new CMBs of 6% [95% CI: 1 to 12%] for every minute of bypass time over 91 mins. CONCLUSIONS: This small study provides new information about the properties and numbers of bubbles entering the brain during surgery, but found no evidence to substantiate a direct link between large numbers of macrobubbles and adverse cognitive or MR outcome. Clinical Trial Registration URL - http://www.isrctn.com. Unique identifier: 66022965.

Doppler ultrasound detection of side-vessel occlusion: an in vitro study.

BACKGROUND AND PURPOSE: Small-vessel knock is a recently reported Doppler ultrasound finding that has been identified in patients with cerebral ischemia. It has been hypothesized that knock-type signals are linked to the presence of either small-vessel occlusion or wall motion. The aim of this study was to investigate the origins of "knock-type" signals by reproducing occlusion of a peripheral artery model in vitro. METHODS: Synthetic bifurcations were fabricated from glass and latex and placed in a flow-rig mimicking physiological blood-flow conditions. The glass model permitted study of fluid flow in the absence of wall motion, whereas the latex model also produced wall motion effects. Vessels were artificially obstructed to examine Doppler signal characteristics associated with blood flow and wall motion. RESULTS: Complete obstruction of the peripheral branch of the glass model revealed discrete (<100 ms) knock-type signals caused by local fluid flow in the occluded branch. Imaging of the obstructed vessel using color Doppler revealed forward and reflected flow. The walls produced periodic bidirectional knock-type signals, which occurred during systole and were not related to the presence of an obstruction. CONCLUSIONS: In our laboratory model, transcranial Doppler ultrasound was found to be capable of detecting knock signals produced by circulating fluid within an occluded branch. However, because similar signals are also generated by nonpathological wall motion, these results cannot be directly translated to a clinical setting. Clinicians should be careful to avoid casual overinterpretation of transcranial Doppler ultrasound data.

Thrombus size and Doppler embolic signal intensity.

BACKGROUND: Migration of thrombus through the cerebral arteries is a common cause of stroke. Thrombus emboli can be detected non-invasively using Doppler ultrasound, but even where the embolus composition is known, there is currently no method for estimating the size of an embolus based on the returned ultrasound signal. Here we report the results of in vitro experiments investigating the relationship between size and embolic signal intensity for fresh thrombus emboli with a view to estimating the sizes of thrombi detected following carotid surgery. METHOD: Thrombi were formed from whole blood using the 'Chandler loop' method under flow conditions similar to those associated with arterial thrombus formation in vivo. A total of 390 Doppler embolic signals were then measured from 37 pieces of thrombus circulated in a pulsatile closed-flow circuit. The dimensions of each of the thrombi were measured before and after circulation using an optical microscope. Relationships between thrombus size and embolic signal properties were then investigated using standard statistical methods with a view to size estimation of thrombi during clinical monitoring. RESULTS: Although embolic signals generally became more intense with increasing thrombus size, strong oscillations due to resonance effects were observed. Pearson tests revealed strong positive correlations between embolus diameter, signal intensity and duration (r > 0.8, p < or = 0.01). CONCLUSIONS: This study provides experimental evidence supporting theoretical predictions relating Doppler embolic signal intensity to thrombus size. In our discussion, we tentatively suggest how this information might be used to size emboli in clinical practice.