Ultrasound assessment of brain supplying arteries (transcranial)

Ultrasonography of intracranial arteries is a non-invasive and highly efficient method for the diagnosis and follow-up of patients with cerebrovascular diseases, also in the bedside setting of the critically ill. For reliable assessment and interpretation of sonographic findings, the technique requires - apart from dedicated anatomic and pathophysiological knowledge of cerebral arteries and their hemodynamics - the comprehension of alternative imaging modalities such as CT or MR angiography. This article reviews the transcranial color-coded duplex sonographic (TCCS) examination technique including the transcranial Doppler sonography (TCD) for a standardized ultrasound assessment of the intracranial arteries and typical pathological cases. As a complementary tool, transorbital ultrasound for the assessment of the optic nerve sheath diameter and adjacent structures is also described in this article.

Accuracy of contrast-enhanced ultrasound in diagnosing extracranial carotid occlusion: A meta-analysis

AIM

This study aimed to assess the accuracy of contrast-enhanced ultrasound (CEUS) in detecting extracranial carotid artery occlusion.

MATERIALS AND METHODS

A systematic literature search was conducted in the Cochrane, PubMed, and EMBASE databases. Prospective or retrospective studies that reported sensitivity and specificity of CEUS for the diagnosis of carotid artery occlusion were selected. Eight studies (354 arteries) were included in the meta-analysis. A bivariate random-effect model was used to estimate overall sensitivity and specificity. The results were also summarized by developing a summary receiver operating characteristic (SROC) curve.

RESULTS

The overall sensitivity, specificity, positive, and negative likelihood ratios were 0.99 (95% CI: 0.83-1.00), 0.97 (95% CI: 0.90-0.99), 30.0 (95% CI: 9.8-91.4), and 0.01 (95% CI: 0.00-0.21), respectively; the odds ratio for diagnosis was 4,796 (95% CI: 119-192,584).

CONCLUSION

The diagnostic test accuracy suggests that CEUS is a reliable tool for diagnosis of extracranial carotid artery occlusion.

Comparison Between Transcranial Color-Coded Duplex Doppler and Contrast Enhanced Transcranial Color-Coded Duplex Doppler After Subarachnoid Aneurysmal Hemorrhage

BACKGROUND

Transcranial color-coded duplex Doppler (TCCD) is commonly used to detect and monitor vasospasm in subarachnoid aneurysmal hemorrhage (aSAH). However, contrast enhanced TCCD (CE-TCCD) may be more effective. The objective of this study was to compare the accuracy of TCCD and CE-TCCD in the detection of vasospasm.

METHODS

This study was a prospective comparison of TCCD and CE-TCCD for the detection of vasospasm, using computed tomography angiography (CT Angio) as a reference examination. The setting was the Department of Anesthesiology and Intensive Care at the Bicêtre University Hospital in Le Kremlin Bicêtre, France. TCCD and CE-TCCD were performed in 47 patients admitted to the intensive care unit (ICU) following aSAH over a 7-month period. TCCD and CE-TCCD were performed at ICU admission and between days 7 and 10. We aimed to visualize the seven intracranial arteries of the circle of Willis. Vasospasm diagnosis was assessed by CT Angio  and graded as moderate when the percentage change in arterial diameter since admission was between 25 and 50% or as severe when the percentage change was greater than 50%.

RESULTS

On ICU admission, TCCD allowed visualization of all intracranial arteries in 16 (34%) of 47 patients, whereas CE-TCCD allowed visualization of all vessels in 37 (79%) of 47 patients (p < 0.001). These results were consistent between days 7 and 10. The proportions of middle cerebral arteries (MCAs), anterior cerebral arteries (ACAs) and posterior cerebral arteries (PCAs) visualized were greater with CE-TCCD. There was no difference in the visualization of basilar arteries (BAs). We performed vasospasm analysis on 67 of 94 MCAs in 47 patients. Area under the curve (AUC) of mean flow velocity to detect MCA vasospasm (moderate and severe) was 0.86 (0.58-1.00) for TCCD and 0.90 (0.77-1.00) for CE-TCCD. AUC of mean velocity to detect severe MCA vasospasm was 0.86 (0.58-1.00) for TCCD and 0.90 (0.77-1.00) for CE-TCCD, without any significant difference between the two techniques. For other arteries, the accuracy of TCCD and CE-TCCD to diagnose vasospasm was poor.

CONCLUSIONS

CE-TCCD allows better visualization of intracranial arteries in patients with aSAH. The accuracy of CE-TCCD to screen severe MCA vasospasm is similar to that of TCCD. CE-TCCD is an alternative tool for monitoring patients with aSAH without a temporal bone window for an ultrasound.

Ultrasound methods of imaging atherosclerotic plaque in carotid arteries: examinations using contrast agents

The primary technique for detecting the presence and monitoring the development of carotid atherosclerotic plaque is ultrasound. The development of ultrasound techniques has made it possible to precisely visualise not only blood flow, but also vessel walls, including atherosclerotic plaque. Contrast-enhanced ultrasound examination enables one to make an objective observation of atherosclerotic plaque neovascularisation, clearly indicating active inflammation, which is an inherent feature of vulnerable (unstable) plaque. Depending on the examination method used, it is possible to precisely visualise different components of the plaque and its behaviour during blood flow through the vessel lumen or through the neovessels of the plaque, and, consequently, determine the possible presence of inflammation, which is a defining feature of plaque stability. The full utilisation of physical phenomena that underlie contrast-enhanced ultrasound will bring further enormous progress of diagnostic and probably also therapeutic methods for carotid atherosclerosis. The selection of the right examination method significantly accelerates diagnosis and adequate classification of plaque, and makes it possible to monitor the progression of atherosclerosis. However, one needs to bear in mind that ultrasound remains a very subjective method. The success of contrast-enhanced ultrasound also depends on the skills and experience of the examiner. Current attempts at increasing the objectivity of contrast-enhanced ultrasound examination using artificial intelligence will make it possible in the future to make a definitive evaluation of atherosclerotic plaque stability. This will allow one to assess the risk of ischaemic stroke adequately.

A cross-sectional feasibility study of neurovascular ultrasound in Malawian adults with acute stroke-like syndrome

Background

In sub-Saharan Africa, there is a dearth of epidemiologic data on the burden of cerebral atherosclerosis. This is explained by the limited availability and the high cost of standard vascular imaging techniques. Neurovascular ultrasound is portable, cheaper and non-invasive and could, therefore, represent a reasonable alternative to fill this knowledge gap. We explored the feasibility of neurovascular ultrasound in Malawian adults with acute stroke-like syndrome to inform the design of future large stroke studies comparing its diagnostic performance to that of gold standard vascular imaging techniques in sub-Saharan Africa.

Methods

We enrolled consecutive patients diagnosed with acute stroke-like syndrome based on the World Health Organization definition. Clinical and demographic data were recorded, and a comprehensive neurovascular ultrasound was performed. Fisher’s exact and Kruskal-Wallis tests were used to study the relationship between atherosclerosis and potential risk factors.

Results

Sixty-six patients were enrolled (mean age: 58.7 years). The frequency of extracranial atherosclerosis was 39.4% (n = 26, 95% CI: 28.6–52.2). There were 12 patients with abnormal carotid intima media thickness (18.2%, 95% CI: 9.8–29.6) and 14 patients with a carotid plaque (21.2%, 95% CI: 12.1–33.0). The frequency of intracranial atherosclerosis was 19.2% (95%CI: 6.6–39.4) in 26 patients with successful transcranial insonation. Hypertension (80.8 versus 52.5%, p = 0.03) and hypercholesterolemia (11.5 versus 0.0%, p = 0.05) were more prevalent in patients with extracranial atherosclerosis.

Conclusions

This study demonstrates the feasibility of neurovascular ultrasound to assess cervical arteries in adults with stroke-like syndrome in sub-Saharan Africa. There is a high rate of transcranial insonation failure in this setting, highlighting the need for echocontrast agents.

[Contrast-enhanced ultrasonography as the most perspective diagnostic method for unstable atherosclerotic plaque of carotid artery]

Problem of internal carotid artery disease diagnosis appears to be crucial today. Complications of this pathology are strokes and transient ischemic attacks. There is no technology for their prediction or at least stratifying risks. Some recent researches are devoted to a new diagnostic method. This new technology is called Contrast Enhanced Ultrasonography (CEUS) and followed by outstanding results in studying the morphological peculiarities of internal carotid artery plaques and predicting the probability of complications. CEUS is a new way for atherosclerotic process analysis because it is able to detect intraplaque neovascularization and vascular wall inflammation.

Molecular imaging of carotid plaque vulnerability

BACKGROUND

Carotid endarterectomy (CEA) has been shown to be beneficial in patients with high-grade symptomatic carotid artery stenosis. Patients with high-grade asymptomatic stenosis may only exceptionally benefit from CEA during periods of increased plaque vulnerability. Imaging modalities to characterize unstable, vulnerable plaques are strongly needed for better risk stratification in these patients.

SUMMARY

Contrast-enhanced ultrasound (CEUS) is a novel and noninvasive technique capable to identify several surrogate markers of vulnerable carotid plaques. The use of specific ultrasound microbubbles allows a reliable detection of microulcerations due to an optimized visualization of the plaque-lumen border. As microbubbles are strictly intravascular tracers, the detection of individual microbubbles within the plaque corresponds to intraplaque neovessels. The accuracy of CEUS in the visualization of newly formed microvessels has been confirmed in histological studies on carotid endarterectomy specimens. Together with the formation of adventitial vasa vasorum, intraplaque neovascularization is a strong predictor for symptomatic disease. The phenomenon of late phase contrast enhancement is based on the adherence of microbubble-containing monocytes on inflamed endothelium. Recent studies suggest that late phase contrast enhancement may reflect endothelial inflammation or activation within carotid plaques. The development of conjugated microbubbles that bind to specific ligands such as thrombotic material or neovessels has led to the term 'molecular imaging'. CEUS with microbubbles targeted to P-selectin and VCAM-1, key molecules in leukocyte trafficking, was used to detect an inflammatory plaque phenotype, whereas microbubbles coupled to the VEGF-receptor may allow for a detection of neovascularization. Even though imaging with targeted microbubbles is yet in an experimental stage, this technique can visualize active plaque reorganization with increased vulnerability leading to generation of arterio-arterial embolism. Key Messages: The use of contrast-enhanced ultrasound can be recommended to assess atherosclerotic carotid lesions at risk for rupture. Prospective clinical studies are needed to validate the use of CEUS in patients with high risks of recurrent large artery strokes. In particular, this applies to the detection of intraplaque neovascularization, a well-established marker in preclinical and observational studies, while the clinical significance of late phase contrast enhancement still needs to be determined..

Image-guided transcranial Doppler sonography for monitoring of defined segments of intracranial arteries

BACKGROUND

Transcranial Doppler sonography (TCD) is widely used in neurointensive care. Image guidance (IG) could simplify secure vessel identification and reduce interinvestigator and intrainvestigator variability. The present study was purposed to investigate the precision and reproducibility of image-guided TCD.

METHODS

The Kolibri IG system (Brainlab AG, Feldkirchen, Germany) was used to track a hand-held Doppler probe of a DWL Multi-Dop® T digital device (Compumedics Germany GmbH, Singen, Germany). The patient's head was registered noninvasively to the IG system. Distance between predefined vascular target and optimal Doppler signal was evaluated to assess spatial accuracy of image-guided TCD. To investigate reproducibility, spatial accuracy of trajectories acquired during an initial examination using the IG system was analyzed in serial examinations. Furthermore, stability of noninvasive registration of the patient's head to the IG system was analyzed. Data are presented as mean±SD for descriptive statistics. Twelve patients were included.

RESULTS

Using IG, a Doppler signal was recorded immediately in all cases for middle cerebral artery (MCA) (29 examinations), in 81% for carotid-T (27 examinations), and in 90% for basilar tip (29 examinations). The optimal Doppler signal was found within 2.64±1.15 mm (94 preplanned targets). At serial examinations, a spatial deviation of 2.75±1.20 mm was found (56 trajectories acquired in 19 serial examinations). Examination time did not influence accuracy of noninvasive patient registration.

CONCLUSIONS

Data suggest that image-guided TCD allows for accurate examinations with high intraprocedural and high interprocedural reproducibility. It facilitates identification of specific vessel segments and generation of standardized examination protocols for serial examinations.

Functional ultrasound imaging of the brain: theory and basic principles

Hemodynamic changes in the brain are often used as surrogates of neuronal activity to infer the loci of brain activity. A major limitation of conventional Doppler ultrasound for the imaging of these changes is that it is not sensitive enough to detect the blood flow in small vessels where the major part of the hemodynamic response occurs. Here, we present a μDoppler ultrasound method able to detect and map the cerebral blood volume (CBV) over the entire brain with an important increase in sensitivity. This method is based on imaging the brain at an ultrafast frame rate (1 kHz) using compounded plane wave emissions. A theoretical model demonstrates that the gain in sensitivity of the μDoppler method is due to the combination of 1) the high signal-to-noise ratio of the gray scale images, resulting from the synthetic compounding of backscattered echoes; and 2) the extensive signal averaging enabled by the high temporal sampling of ultrafast frame rates. This μDoppler imaging is performed in vivo on trepanned rats without the use of contrast agents. The resulting images reveal detailed maps of the rat brain vascularization with an acquisition time as short as 320 ms per slice. This new method is the basis for a real-time functional ultrasound (fUS) imaging of the brain.

Transcranial Doppler ultrasound in neurovascular diseases: diagnostic and therapeutic aspects

Albeit no direct anatomical information can be obtained, neurosonological methods provide real-time determination of velocity, and spectral waveform of blood flow in basal intracranial arteries adds significant benefit to the care of the patients with neurovascular diseases. Several features, such as relative simplicity in terms of interpretation and performance, significantly low cost, totally non-invasiveness, portability, and excellent temporal resolution, make neurosonology increasingly popular tool for evaluation, planning, and monitoring of treatment, and for determining prognosis in various neurovascular diseases. Usefulness of transcranial Doppler in diagnosing/monitoring subarachnoid hemorrhage related vasospasm and sickle cell vasculopathy is already well known. Utility in diagnosis of intracranial arterial stenosis, acute occlusion and recanalization, intracranial hemodynamic effect of the cervical arterial pathologies, intracranial pressure increase, and cerebral circulatory arrest are also well established. Neurosonological determination of vasomotor reactivity, cerebral autoregulation, neurovascular coupling, and micro-embolic signals detection are useful in the assessment of stroke risk, diagnosis of right-to-left shunting, and monitoring during surgery and interventional procedures. Transcranial Doppler is also an evolving ultrasound method with a therapeutic potential such as augmentation of clot lysis and cerebral delivery of thrombolytic or neuroprotective agent loaded nanobubbles in neurovascular diseases. The aim of this study is to give an overview of current usage of the different ultrasound modalities in different neurovascular diseases.

Estimating concentration of ultrasound contrast agents with backscatter coefficients: experimental and theoretical aspects

Ultrasound contrast agents (UCAs) have been explored as a means to enhance therapeutic techniques. Because the effectiveness of these techniques relies on the UCA concentration at a target site, it would be beneficial to estimate UCA concentration noninvasively. In this study, a noninvasive method for estimating UCA concentration was developed in vitro. Backscatter coefficients (BSCs) estimated from measurements of Definity(®) UCAs were fitted to a theoretical scattering model in the 15-25 MHz range using a Levenberg-Marquardt regression technique. The model was defined by the UCA size distribution and concentration, and therefore concentration estimates were extracted directly from the fit. Calculation of the BSC was accomplished using planar reference measurements from the back wall of a Plexiglas(®) chamber and an average of 500 snapshots of ultrasonic backscatter from UCAs flowing through the chamber. In order to verify the ultrasonically derived UCA concentration estimates, a sample of the UCAs was extracted from the flow path and the concentration was estimated with a hemacytometer. UCA concentrations of 1, 2, and 5 times the dose recommended by the manufacturer were used in experiments. All BSC-based estimates were within one standard deviation of hemacytometer based estimates for peak rarefactional pressures of 100-400 kPa.

Imaging of perfusion using ultrasound

Ultrasound can be used to image perfusion in two ways: the traditional one using Doppler and the more recent using microbubble contrast agents. Doppler is simple to use and inexpensive but is limited to larger vessels with faster flow rates. It cannot interrogate the microvasculature because bulk tissue movement is faster than capillary flow. It has been used for liver and tumour flow. Contrast studies are much richer and can assess both the macro- and microcirculation. One approach analyses the time-intensity curves in a region of interest, e.g. a tumour, myocardium, brain, following bolus i.v. injection. Another approach measures the time taken for the microbubbles to cross a vascular bed of interest. These arrival times can be useful for the liver in both diffuse and focal diseases and for the kidney. Features derived from time-intensity curves following bolus i.v. injections of microbubbles form sensitive early indicators of the vascular response of tumours to antivascular drugs. This approach, known as dynamic contrast-enhanced ultrasound (DCE-US), has been accepted as a valid technique for monitoring tumour response by several authorities.

Full experimental modelling of a liver tissue mimicking phantom for medical ultrasound studies employing different hydrogels

Tissue mimicking phantoms have been widely reported to be an important tool for development, optimisation and performance testing of ultrasound-based diagnostic techniques. In particular, modern applications of tissue mimicking phantoms often include characterisation of the nonlinear behaviour of experimental ultrasound contrast agents. In such cases, the tissue-mimicking materials should be chosen not only based on the values of their density, speed of sound and attenuation coefficient, but also considering their effect on the appearance of "native harmonics" due to nonlinear distortion of ultrasound signal during propagation. In a previous paper it was demonstrated that a cellulose-based hydrogel is suitable to simulate nonlinear acoustical behaviour of liver tissue for thicknesses up to 8 cm. In this paper we present the experimental characterisation of the nonlinear acoustical behaviour of a different polyethylene glycol diacrylate (PEGDA)-based hydrogel, in order to assess whether and how it can improve the performances and overcome some limitations of the cellulose-based hydrogel as liver tissue-mimicking material. Samples of pig liver tissue, cellulose-based hydrogel and PEGDA-based hydrogel were insonified in a through-transmission set-up, employing 2.25-MHz pulses with different mechanical index (MI) values. Second harmonic and first harmonic amplitudes were extracted from the spectra of received signals and their difference was then used to compare sample behaviours. Obtained results show how a new more accurate and combined experimental model of linear and nonlinear acoustical behaviour of liver tissue is feasible. In fact, a further confirmation of the cellulose-based hydrogel effectiveness to precisely simulate the liver tissue for penetration depths up to 8 cm was provided, and it was also shown that the employment of the PEGDA-based hydrogel can extend the range of useful tissue-mimicking material thicknesses up to 11 cm, moreover allowing a considerable improvement of the time stability and behaviour reliability of the corresponding manufactured phantoms.

Ultrasound targeted microbubble destruction for drug and gene delivery

BACKGROUND

Gas-filled microbubbles have been used as ultrasound contrast agents for some decades. More recently, such microbubbles have evolved as experimental tools for organ- and tissue-specific drug and gene delivery. When sonified with ultrasound near their resonance frequency, microbubbles oscillate. With higher ultrasound energies, oscillation amplitudes increase, leading to microbubble destruction. This phenomenon can be used to deliver a substance into a target organ, if microbubbles are co-administered loaded with drugs or gene therapy vectors before i.v. injection.

OBJECTIVE

This review focuses on different experimental applications of microbubbles as tools for drug and gene delivery. Different organ systems and different classes of bioactive substances that have been used in previous studies will be discussed.

METHODS

All the available literature was reviewed to highlight the potential of this non-invasive, organ-specific delivery system.

CONCLUSION

Ultrasound targeted microbubble destruction has been used in various organ systems and in tumours to successfully deliver drugs, proteins, gene therapy vectors and gene silencing constructs. Many proof of principle studies have demonstrated its potential as a non-invasive delivery tool. However, too few large animal studies and studies with therapeutic aims have been performed to see a clinical application of this technique in the near future. Nevertheless, there is great hope that preclinical large animal studies will confirm the successful results already obtained in small animals.

Can early neurosonology predict outcome in acute stroke?: a metaanalysis of prognostic clinical effect sizes related to the vascular status

BACKGROUND AND PURPOSE

Prediction of short- and long-term prognosis is an important issue in acute stroke care. This metaanalysis explores the prognostic value of initial bed-side transcranial ultrasound in acute stroke.

METHODS

All studies prospectively applying TCCS or TCD within 24 hours of symptom onset in acute stroke, with a minimal cohort size of 20 patients, and reporting clinical outcome variables in relation to the vascular findings were included into this metaanalysis. Study quality was assessed by 2 independent reviewers.

RESULTS

Twenty-five studies with 1813 included patients identified by electronic and manual search fulfilled the inclusion criteria. Middle cerebral artery (MCA) occlusion was associated with a significantly increased risk for a fatal course of stroke (OR 2.46, 95% CI 1.33 to 4.52). Patients with patent MCA were more likely to clinically improve within 4 days than patients with MCA occlusion (OR 11.11, 95% CI 5.44 to 22.69). Full recanalization within 6 hours after symptom onset was highly significantly associated with clinical improvement within 48 hours (OR 5.64, 95% CI 3.82 to 8.31) and functional independence after 3 months (OR 6.07, 95% CI 3.94 to 9.35).

CONCLUSIONS

Transcranial ultrasound provides important information on prognosis in patients with acute stroke.