Real-time ultrasound brain perfusion imaging with analysis of microbubble replenishment in acute MCA stroke

Dynamic assessment of brain perfusion in a middle cerebral artery occlusion rat model by contrast-enhanced ultrasound imaging: a pilot study

BACKGROUND

The middle cerebral artery occlusion model (MCAo) is a commonly used animal model for cerebral ischemia studies but lacks accessible imaging techniques for the assessment of hemodynamic changes of the model.

PURPOSE

The study aims to explore the value of contrast-enhanced ultrasound (CEUS) in evaluating brain perfusion in the early stages after MCAo surgery.

MATERIAL AND METHODS

In total, 18 adult male Sprague-Dawley rats were subjected to right MCAo using an intraluminal filament model, and CEUS was performed at the three following timepoints: before (T0), immediately after (T1), and 6 h after permanent MCAo (T2). Twelve rats successfully completed the study, and their brains were removed and stained using 2, 3, 5-triphenyltetrazolium chloride (TTC). CEUS video images were visualized offline, and the time-intensity curves (TICs) were analyzed. Different cerebrovascular patterns and manifestations of the contrast enhancement in rat ischemic hemispheres were observed. Semi-quantitative parameters of TICs in ischemic areas (ROIi) and the surrounding normal- or hypo-perfused areas (ROIn) were calculated and compared between T0, T1, and T2, and also between ROIi and ROIn.

RESULTS

A significant correlation was found between the lesion volume (%) determined by TTC and CEUS parameters (r = -0.691, P = 0.013 for peak intensity; r = -0.742, P = 0.006 for area under the curve) at T2. After the same occlusion, there were differences in contrast perfusion in each group.

CONCLUSION

This study suggests that CEUS could be an effective imaging tool for studying cerebral ischemia and perfusion in small animals as long as the transcranial acoustic window allows it.

A concise guide to transtemporal contrast-enhanced ultrasound in children

Brain contrast-enhanced ultrasound offers insights into the brain beyond the anatomic information offered by conventional grayscale ultrasound. In infants, the open fontanelles serve as acoustic windows. In children, whose fontanelles are closed, the temporal bone serves as the ideal acoustic window due to its relatively smaller thickness than the other skull bones. Diagnosis of common neurologic diseases such as stroke, hemorrhage, and hydrocephalus has been performed using the technique. Transtemporal ultrasound and contrast-enhanced ultrasound, however, are rarely used in children due to the prevalent notion that the limited acoustic penetrance degrades diagnostic quality. This review seeks to provide guidelines for the use of transtemporal brain contrast-enhanced ultrasound in children.

Transtemporal brain contrast-enhanced ultrasound in children: preliminary experience in patients without neurological disorders

AIM

To evaluate the use of transtemporal brain contrast-enhanced ultrasound (CEUS) to assess cerebral blood perfusion in a cohort of children without neurological disorders.

METHODS

We included pediatric patients who were undergoing a clinically-indicated CEUS study. Brain scans were performed with a Siemens Sequoia scanner and a 4V1 transducer, that was placed on the left transtemporal bone. Brain scans were performed simultaneously with the images of the clinically-indicated organ of interest. Qualitative and quantitative analysis was performed to evaluate the hemispherical blood flow at the level of the midbrain during the wash-in and wash-out phases of the time-intensity curve. Clinical charts were reviewed to evaluate post-CEUS adverse events.

RESULTS

Five patients were evaluated (mean age 5.8 ± 5.1 years). Qualitatively, more avid enhancement in the midbrain than the cortex was observed. Structures depicted ranged between the centrum semiovale at the level of the lateral ventricles and the midbrain. A quantitative analysis conducted on four patients demonstrated less avid perfusion on the contralateral (i.e. right) side, with a mean left/right ratio ranging between 1.51 and 4.07. In general, there was a steep positive wash-in slope starting at approximately 10 s after contrast injection, reaching a peak intensity around 15-26 s on the left side, and 17-29 s on the right side. No adverse events were reported.

CONCLUSION

Transtemporal brain CEUS is feasible and safe in the pediatric population and allows qualitative and quantitative assessment of cerebral perfusion.

Contrast-enhanced ultrasound of the neonatal brain

Cranial US is an integral component of evaluating the neonatal brain, especially in the setting of critically ill infants and in the emergency setting, because cranial US can be performed portably at the bedside, is safe, and can be repeated whenever needed. Contrast-enhanced ultrasound (CEUS) involves intravenously injecting microbubbles to allow for improved visibility of large and small vessels to assess vascularity and is becoming a widespread technique to improve diagnostic performance of US across a broad spectrum of applications. CEUS has the potential to add value to routine brain US and become a useful adjunct to MRI in infants in need of bedside imaging. In this review we describe the basics of US contrast agents and CEUS technique, including safety considerations, and detail the potential clinical uses of brain CEUS.

Very Low Frequency Radial Modulation for Deep Penetration Contrast-Enhanced Ultrasound Imaging

Contrast-enhanced ultrasound imaging allows vascular imaging in a variety of diseases. Radial modulation imaging is a contrast agent-specific imaging approach for improving microbubble detection at high imaging frequencies (≥7.5 MHz), with imaging depth limited to a few centimeters. To provide high-sensitivity contrast-enhanced ultrasound imaging at high penetration depths, a new radial modulation imaging strategy using a very low frequency (100 kHz) ultrasound modulation wave in combination with imaging pulses ≤5 MHz is proposed. Microbubbles driven at 100 kHz were imaged in 10 successive oscillation states by manipulating the pulse repetition frequency to unlock the frame rate from the number of oscillation states. Tissue background was suppressed using frequency domain radial modulation imaging (F-RMI) and singular value decomposition-based radial modulation imaging (S-RMI). One hundred-kilohertz modulation resulted in significantly higher microbubble signal magnitude (63-88 dB) at the modulation frequency relative to that without 100-kHz modulation (51-59 dB). F-RMI produced images with high contrast-to-tissue ratios (CTRs) of 15 to 22 dB in a stationary tissue phantom, while S-RMI further improved the CTR (19-26 dB). These CTR values were significantly higher than that of amplitude modulation pulse inversion images (11.9 dB). In the presence of tissue motion (1 and 10 mm/s), S-RMI produced high-contrast images with CTR up to 18 dB; however, F-RMI resulted in minimal contrast enhancement in the presence of tissue motion. Finally, in transcranial ultrasound imaging studies through a highly attenuating ex vivo cranial bone, CTR values with S-RMI were as high as 23 dB. The proposed technique demonstrates successful modulation of microbubble response at 100 kHz for the first time. The presented S-RMI low-frequency radial modulation imaging strategy represents the first demonstration of real-time (20 frames/s), high-penetration-depth radial modulation imaging for contrast-enhanced ultrasound imaging.

Contrast-enhanced ultrasound of the pediatric brain

Brain contrast-enhanced ultrasound (CEUS) is an emerging application that can complement gray-scale US and yield additional insights into cerebral flow dynamics. CEUS uses intravenous injection of ultrasound contrast agents (UCAs) to highlight tissue perfusion and thus more clearly delineate cerebral pathologies including stroke, hypoxic-ischemic injury and focal lesions such as tumors and vascular malformations. It can be applied not only in infants with open fontanelles but also in older children and adults via a transtemporal window or surgically created acoustic window. Advancements in CEUS technology and post-processing methods for quantitative analysis of UCA kinetics further elucidate cerebral microcirculation. In this review article we discuss the CEUS examination protocol for brain imaging in children, current clinical applications and future directions for research and clinical uses of brain CEUS.

Contrast-Enhanced Ultrasound in Children: Implementation and Key Diagnostic Applications

Contrast-enhanced ultrasound (CEUS) utilization is expanding rapidly, particularly in children, in whom the modality offers important advantages of dynamic evaluation of the vasculature, portability, lack of ionizing radiation, and lack of need for sedation. Accumulating data establish an excellent safety profile of ultrasound contrast agents in children. Although only FDA-approved for IV use in children for characterizing focal liver lesions and for use during echocardiography, growing off-label applications are expanding the diagnostic potential of ultrasound. Focal liver lesion evaluation is the most common use of CEUS, and the American College of Radiology Pediatric LI-RADS Working Group recommends including CEUS for evaluation of a newly discovered focal liver lesion in many circumstances. Data also support the role of CEUS in hemodynamically stable children with blunt abdominal trauma, and CEUS is becoming a potential alternative to CT in this setting. Additional potential applications that require further study include evaluation of pathology in the lung, spleen, brain, pancreas, bowel, kidney, female pelvis, and scrotum. This review explores the implementation of CEUS in children, describing basic principles of ultrasound contrast agents and CEUS technique and summarizing current and potential IV diagnostic applications based on pediatric-specific supporting evidence.

Transcranial contrast-enhanced ultrasound in the rat brain reveals substantial hyperperfusion acutely post-stroke

Direct and real-time assessment of cerebral hemodynamics is key to improving our understanding of cerebral blood flow regulation in health and disease states such as stroke. While a number of sophisticated imaging platforms enable assessment of cerebral perfusion, most are limited either spatially or temporally. Here, we applied transcranial contrast-enhanced ultrasound (CEU) to measure cerebral perfusion in real-time through the intact rat skull before, during and after ischemic stroke, induced by intraluminal filament middle cerebral artery occlusion (MCAO). We demonstrate expected decreases in cortical and striatal blood volume, flow velocity and perfusion during MCAO. After filament retraction, blood volume and perfusion increased two-fold above baseline, indicative of acute hyperperfusion. Adjacent brain regions to the ischemic area and the contralateral hemisphere had increased blood volume during MCAO. We assessed our data using wavelet analysis to demonstrate striking vasomotion changes in the ischemic and contralateral cortices during MCAO and reperfusion. In conclusion, we demonstrate the application of CEU for real-time assessment of cerebral hemodynamics and show that the ischemic regions exhibit striking hyperemia post-MCAO. Whether this post-stoke hyperperfusion is sustained long-term and contributes to stroke severity is not known.

Twenty Years of Cerebral Ultrasound Perfusion Imaging-Is the Best yet to Come?

Over the past 20 years, ultrasonic cerebral perfusion imaging (UPI) has been introduced and validated applying different data acquisition and processing approaches. Clinical data were collected mainly in acute stroke patients. Some efforts were undertaken in order to compare different technical settings and validate results to gold standard perfusion imaging. This review illustrates the evolution of the method, explicating different technical aspects and milestones achieved over time. Up to date, advancements of ultrasound technology as well as data processing approaches enable semi-quantitative, gold standard proven identification of critically hypo-perfused tissue in acute stroke patients. The rapid distribution of CT perfusion over the past 10 years has limited the clinical need for UPI. However, the unexcelled advantage of mobile application raises reasonable expectations for future applications. Since the identification of intracerebral hematoma and large vessel occlusion can also be revealed by ultrasound exams, UPI is a supplementary multi-modal imaging technique with the potential of pre-hospital application. Some further applications are outlined to highlight the future potential of this underrated bedside method of microcirculatory perfusion assessment.

Ultrasonic quantification of cerebral perfusion in acute anterior circulation occlusive stroke-A comparative challenge of the refill- and the bolus-kinetics approach

Purpose

To prospectively evaluate the potential of semi-quantitative evaluation of cerebral perfusion in acute ischemic stroke by comparing two established ultrasound approaches.

Materials and methods

Consecutive inclusion of patients with acute occlusion of middle cerebral artery (MCA) confirmed by either magnetic resonance imaging (MRI) or computed tomography (CT) perfusion imaging qualifying for interventional therapy. Comparison of bilateral high mechanical index (MI) bolus-kinetics (HighMiB) and unilateral low MI refill-kinetics (LowMiR) performed before specific treatment.

Results

In 16/31 patients HighMiB was eligible, in 8/31 patients LowMiR was eligible. In six out of these eight patients both HighMiB and LowMiR were eligible for direct comparison. In MR/CT perfusion imaging of the 16 patients eligible for HighMiB, 29/48 cortical regions of interest (ROIs) (60%) displayed hypoperfusion or ischemia, areas inadequately accessible by LowMiR. These ROIs made up 49% of the 59 ROIs displaying hypoperfusion or ischemia, altogether. Matching of parameters in normal and impaired ROIs between LowMiR and MRI/CT perfusion imaging was significantly poorer than in HighMiB.

Conclusion

LowMiR using refill-kinetics potentially has the advantage of real time imaging and better resolution. The diagnostic impact, however, proves inferior to HighMiB both with respect to imaging quality and semi-quantitative evaluation.

Potential of Contrast-Enhanced Ultrasound as a Bedside Monitoring Technique in Cerebral Perfusion: a Systematic Review

Contrast-enhanced ultrasound (CEUS) has been suggested as a new method to measure cerebral perfusion in patients with acute brain injury. In this systematic review, the tolerability, repeatability, reproducibility and accuracy of different CEUS techniques for the quantification of cerebral perfusion were assessed. We selected studies published between January 1994 and March 2017 using CEUS to measure cerebral perfusion. We included 43 studies (bolus kinetics n = 31, refill kinetics n = 6, depletion kinetics n = 6) with a total of 861 patients. Tolerability was reported in 28 studies describing 12 patients with mild and transient side effects. Repeatability was assessed in 3 studies, reproducibility in 2 studies and accuracy in 19 studies. Repeatability was high for experienced sonographers and significantly lower for less experienced sonographers. Reproducibility of CEUS was not clear. The sensitivity and specificity of CEUS for the detection of cerebral ischemia ranged from 75% to 96% and from 60% to 100%. Limited data on repeatability, reproducibility and accuracy may suggest that this technique could be feasible for use in acute brain injury patients.

Is ultrasound perfusion imaging capable of detecting mismatch? A proof-of-concept study in acute stroke patients

In this study, we compared contrast-enhanced ultrasound perfusion imaging with magnetic resonance perfusion-weighted imaging or perfusion computed tomography for detecting normo-, hypo-, and nonperfused brain areas in acute middle cerebral artery stroke. We performed high mechanical index contrast-enhanced ultrasound perfusion imaging in 30 patients. Time-to-peak intensity of 10 ischemic regions of interests was compared to four standardized nonischemic regions of interests of the same patient. A time-to-peak >3 s (ultrasound perfusion imaging) or >4 s (perfusion computed tomography and magnetic resonance perfusion) defined hypoperfusion. In 16 patients, 98 of 160 ultrasound perfusion imaging regions of interests of the ischemic hemisphere were classified as normal, and 52 as hypoperfused or nonperfused. Ten regions of interests were excluded due to artifacts. There was a significant correlation of the ultrasound perfusion imaging and magnetic resonance perfusion or perfusion computed tomography (Pearson's chi-squared test 79.119, p < 0.001) (OR 0.1065, 95% CI 0.06-0.18). No perfusion in ultrasound perfusion imaging (18 regions of interests) correlated highly with diffusion restriction on magnetic resonance imaging (Pearson's chi-squared test 42.307, p < 0.001). Analysis of receiver operating characteristics proved a high sensitivity of ultrasound perfusion imaging in the diagnosis of hypoperfused area under the curve, (AUC = 0.917; p < 0.001) and nonperfused (AUC = 0.830; p < 0.001) tissue in comparison with perfusion computed tomography and magnetic resonance perfusion. We present a proof of concept in determining normo-, hypo-, and nonperfused tissue in acute stroke by advanced contrast-enhanced ultrasound perfusion imaging.

ROLE OF TRANSCRANIAL COLOUR-CODED DUPLEX SONOGRAPHY IN STROKE MANAGEMENT - REVIEW ARTICLE

The development of transcranial colour-coded duplex sonography (TCCS) has resurrected the hope of safe, real time bedside brain imaging beyond childhood. This review article provides an overview of the role of TCCS in the management of patients with stroke. The objective is to stimulate interest in the field of neurosonology as a potential means of improving neurological outcome for stroke patients and a area for stroke research endeavors in Africa. Literature search was done on MEDLINE, Cochrane library, and Google Scholar databases with the following keywords: transcranial colour Doppler, Transcranial duplex sonography, transcranial colour-coded Doppler sonography, stroke, infarct and haemorrhage. We also identified relevant articles from the references section of studies produced by our literature search. We discussed the roles of TCCS to discriminate ischaemic from haemorrhagic forms; unravel the mechanism of stroke; monitor temporal evolution of stroke and predictors of stroke outcome; and promote better understanding of the epidemiology of stroke. Its emerging role as a potent point-of-care imaging modality for definitive treatment in ischaemic stroke within and outside the hospital setting is also highlighted. Comparison of TCCS with alternative modalities for neuroimaging in stroke is also discussed. A root cause analysis of the untenable high cost of neuroimaging for stroke patients in Africa is presented vis-à-vis the potential economic relief which widespread adoption of TCCS may provide. We advocate capacity building for TCCS and suggest some action plans required to achieve safe, cheap, affordable and reliable ultrasound based neuroimaging for stroke patients in resource limited areas of Africa.

Contrast-enhanced ultrasound imaging for the detection of focused ultrasound-induced blood-brain barrier opening

The blood-brain barrier (BBB) can be transiently and locally opened by focused ultrasound (FUS) in the presence of microbubbles (MBs). Various imaging modalities and contrast agents have been used to monitor this process. Unfortunately, direct ultrasound imaging of BBB opening with MBs as contrast agent is not feasible, due to the inability of MBs to penetrate brain parenchyma. However, FUS-induced BBB opening is accompanied by changes in blood flow and perfusion, suggesting the possibility of perfusion-based ultrasound imaging. Here we evaluated the use of MB destruction-replenishment, which was originally developed for analysis of ultrasound perfusion kinetics, for verifying and quantifying FUS-induced BBB opening. MBs were intravenously injected and the BBB was disrupted by 2 MHz FUS with burst-tone exposure at 0.5-0.7 MPa. A perfusion kinetic map was estimated by MB destruction-replenishment time-intensity curve analysis. Our results showed that the scale and distribution of FUS-induced BBB opening could be determined at high resolution by ultrasound perfusion kinetic analysis. The accuracy and sensitivity of this approach was validated by dynamic contrast-enhanced MRI. Our successful demonstration of ultrasound imaging to monitor FUS-induced BBB opening provides a new approach to assess FUS-dependent brain drug delivery, with the benefit of high temporal resolution and convenient integration with the FUS device.

3-D transcranial ultrasound imaging with bilateral phase aberration correction of multiple isoplanatic patches: a pilot human study with microbubble contrast enhancement

With stroke currently the second-leading cause of death globally, and 87% of all strokes classified as ischemic, the development of a fast, accessible, cost-effective approach for imaging occlusive stroke could have a significant impact on health care outcomes and costs. Although clinical examination and standard computed tomography alone do not provide adequate information for understanding the complex temporal events that occur during an ischemic stroke, ultrasound imaging is well suited to the task of examining blood flow dynamics in real time and may allow for localization of a clot. A prototype bilateral 3-D ultrasound imaging system using two matrix array probes on either side of the head allows for correction of skull-induced aberration throughout two entire phased array imaging volumes. We investigated the feasibility of applying this custom correction technique in five healthy volunteers with Definity microbubble contrast enhancement. Subjects were scanned simultaneously via both temporal acoustic windows in 3-D color flow mode. The number of color flow voxels above a common threshold increased as a result of aberration correction in five of five subjects, with a mean increase of 33.9%. The percentage of large arteries visualized by 3-D color Doppler imaging increased from 46% without aberration correction to 60% with aberration correction.

Refraction correction in 3D transcranial ultrasound imaging

We present the first correction of refraction in three-dimensional (3D) ultrasound imaging using an iterative approach that traces propagation paths through a two-layer planar tissue model, applying Snell's law in 3D. This approach is applied to real-time 3D transcranial ultrasound imaging by precomputing delays offline for several skull thicknesses, allowing the user to switch between three sets of delays for phased array imaging at the push of a button. Simulations indicate that refraction correction may be expected to increase sensitivity, reduce beam steering errors, and partially restore lost spatial resolution, with the greatest improvements occurring at the largest steering angles. Distorted images of cylindrical lesions were created by imaging through an acrylic plate in a tissue-mimicking phantom. As a result of correcting for refraction, lesions were restored to 93.6% of their original diameter in the lateral direction and 98.1% of their original shape along the long axis of the cylinders. In imaging two healthy volunteers, the mean brightness increased by 8.3% and showed no spatial dependency.

Real-time ultrasound perfusion imaging in acute stroke: assessment of cerebral perfusion deficits related to arterial recanalization

We investigated whether real-time ultrasound perfusion imaging (rt-UPI) is able to detect perfusion changes related to arterial recanalization in the acute phase of middle cerebral artery (MCA) stroke. Twenty-four patients with acute territorial MCA stroke were examined with rt-UPI and transcranial color-coded duplex ultrasound (TCCD). Ultrasound studies were consecutively performed within 24 h and 72-96 h after stroke onset. Real-time UPI parameters of bolus kinetics (time to peak, rt-TTP) and of refill kinetics (plateau A and slope β of the exponential replenishment curve) were calculated from regions of interest of ischemic versus normal brain tissue; these parameters were compared between early and follow-up examinations in patients who recanalized. At the early examination, there was a delay of rt-TTP in patients with MCA occlusion (rt-TTP [s]: 13.09 ± 3.21 vs. 10.16 ± 2.6; p = 0.01) and a lower value of the refill parameter β (β [1/s]: 0.62 ± 0.34 vs. 1.09 ± 0.58; p = 0.01) in ischemic compared with normal brain tissue, whereas there were no differences of the parameters A and Axβ. At follow-up, the delay of rt-TTP was reversible once recanalization of an underlying MCA obstruction was demonstrated: rt-TTP [s], 13.09 ± 3.21 at 24 h versus 10.95 ± 1.5 at 72-96 h (p = 0.03). Correspondingly, β showed a higher slope than at the first examination: β [1/s]: 0.55 ± 0.29 at 24 h versus 0.71 ± 0.27 at 72-96 h (p = 0.04). We conclude that real-time UPI can detect hemodynamic impairment in acute MCA occlusion and subsequent improvement following arterial recanalization. This offers the chance for bedside monitoring of the hemodynamic compromise (e.g. during therapeutic interventions such as systemic thrombolysis).

Microvascular imaging in acute ischemic stroke

BACKGROUND

Microvascular imaging (MVI), a new ultrasound technology, is used to analyze brain perfusion at the patient's bedside. This study aims to evaluate the diagnostic and prognostic value of MVI in patients with acute ischemic stroke (AIS).

METHODS

Nineteen patients suffering from AIS (mean age, 70.9 ± 12.2 years; 47% female; mean NIHSS-score, 12 ± 8) were investigated within the first 12 hours after symptom onset. We used the iU22 (Philips) system (S5-1 probe; low-mechanical index; depth, 13 cm), and 2 bolus injections of an ultrasound contrast agent (2.4 mL SonoVue per injection). The area of maximal perfusion deficit (AMPD) was compared with infarction on follow-up cranial computed tomography (CT) and NIHSS score 24 hours after stroke onset.

RESULTS

Of 19 patients, 15 patients (79%) had sufficient insonation conditions. Of these patients, 12 had infarctions. The sensitivity and specificity of detecting infarctions with ultrasound perfusion imaging were 91% and 67%, respectively. A significant correlation existed between the AMPD and NIHSS score at 24 hours after symptom onset (P= .023), and with occlusion of the internal carotid artery (P= .005).

CONCLUSION

Performing bedside MVI in the early phase of AIS provides information on brain parenchyma perfusion and prognosis of AIS.