Transcranial Ultrasound Brain Perfusion Assessment With a Contrast Agent-Specific Imaging Mode

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.

Repeatability of Bolus Kinetics Ultrasound Perfusion Imaging for the Quantification of Cerebral Blood Flow

Ultrasound perfusion imaging (UPI) can be used for the quantification of cerebral perfusion. In a neuro-intensive care setting, repeated measurements are required to evaluate changes in cerebral perfusion and monitor therapy. The aim of this study was to determine the repeatability of UPI in quantification of cerebral perfusion. UPI measurement of cerebral perfusion was performed three times in healthy patients. The coefficients of variation of the three bolus injections were calculated for both time- and volume-derived perfusion parameters in the macro- and microcirculation. The UPI time-dependent parameters had overall the lowest CVs in both the macro- and microcirculation. The volume-related parameters had poorer repeatability, especially in the microcirculation. Both intra-observer variability and inter-observer variability were low. Although UPI is a promising tool for the bedside measurement of cerebral perfusion, improvement of the technique is required before implementation in routine clinical practice.

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.

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.

Pitch-catch phase aberration correction of multiple isoplanatic patches for 3-D transcranial ultrasound imaging

Having previously presented the ultrasound brain helmet, a system for simultaneous 3-D ultrasound imaging via both temporal bone acoustic windows, the scanning geometry of this system is utilized to allow each matrix array to serve as a correction source for the opposing array. Aberration is estimated using cross-correlation of RF channel signals, followed by least mean squares solution of the resulting overdetermined system. Delay maps are updated and real-time 3-D scanning resumes. A first attempt is made at using multiple arrival time maps to correct multiple unique aberrators within a single transcranial imaging volume, i.e., several isoplanatic patches. This adaptive imaging technique, which uses steered unfocused waves transmitted by the opposing, or beacon, array, updates the transmit and receive delays of 5 isoplanatic patches within a 64° x 64° volume. In phantom experiments, color flow voxels above a common threshold have also increased by an average of 92%, whereas color flow variance decreased by an average of 10%. This approach has been applied to both temporal acoustic windows of two human subjects, yielding increases in echo brightness in 5 isoplanatic patches with a mean value of 24.3 ± 9.1%, suggesting that such a technique may be beneficial in the future for performing noninvasive 3-D color flow imaging of cerebrovascular disease, including stroke.

Monitoring of cerebral hemodynamics and oxygenation to detect delayed ischemic neurological deficit after aneurysmal subarachnoid hemorrhage

One of the major goals in the treatment of patients with aneurysmal subarachnoid hemorrhage (aSAH) is early detection and treatment of delayed ischemic neurologic deficits (DINDs) to prevent cerebral infarction and thus poor outcome or even death. The complex changes of cerebral metabolism, hemodynamics, and oxygenation after SAH are underestimated if they are considered exclusively based on angiographic cerebral vasospasm (CVS). The discrepancies on one hand may arise from the heterogeneous and complex pathophysiology of DINDs. On the other hand, the occurrence of DINDs may depend on the relationship between local cerebral oxygen delivery and demand, which can only be determined if cerebral blood flow (CBF) and the cerebral metabolic rate of oxygen (CMRO(2)) can be measured. We briefly review the most relevant methods for monitoring cerebral hemodynamics and oxygenation and discuss the limitations associated with early diagnosis of DINDs in patients with severe aSAH not amenable for clinical neurological examination.

Application of a second-generation US contrast agent in infants and children--a European questionnaire-based survey

BACKGROUND

No US contrast agent (US-CA) is currently licensed for use in children.

OBJECTIVE

To survey the off-label use in children of a second-generation US-CA.

MATERIALS AND METHODS

Questionnaires were e-mailed to European paediatric radiologists, who were asked about their experience with the second-generation US-CA Sonovue® (Bracco, Milan, Italy). Number of examinations per indication and adverse effects were recorded. Examinations were categorised by intravenous or intracavitary use of US-CA.

RESULTS

Out of 146 respondents, 88 stated that they did not perform contrast-enhanced US in children, but 36 of these (44%) would appreciate paediatric approval. Forty-five centres reported 5,079 examinations in children (age mean: 2.9 years; range: birth-18 years, M/F: 1/ 2.8). The majority (4,131 [81%] in 29 centres) were intravesical applications. The minority (948 [19%] in 30 centres) were intravenous applications. No adverse effects had been recorded from intravesical use. Six minor adverse effects (skin reaction, unusual taste, hyperventilation) had been recorded after five intravenous studies (0.52%).

CONCLUSION

Responses suggest a favourable safety profile of this second-generation US-CA in children. It also demonstrates a demand for such US-CA from paediatric radiologists.

Evaluation of regional hepatic perfusion (RHP) by contrast-enhanced ultrasound in patients with cirrhosis

BACKGROUND & AIMS

Ultrasonographic contrast agents allow the assessment of myocardial and renal perfusion through the analysis of refill kinetics after microbubbles rupture. This study evaluated the feasibility of contrast-enhanced ultrasonographic (CEUS) estimations of regional hepatic perfusion in patients with cirrhosis, and its correlation with clinical and hemodynamic parameters.

METHODS

Fifty-five patients with cirrhosis undergoing hepatic vein catheterization were included. Hepatic perfusion was studied by CEUS (using Contrast Coherent Imaging) during a continuous i.v. infusion of microbubbles (SonoVue®); after their rupture (high insonation power), tissue refill was digitally recorded and time-intensity curves were electronically calculated on a region of interest of the right hepatic lobe. Regional hepatic perfusion (RHP) was calculated as microbubbles velocity×microbubble concentration. During hepatic vein catheterization, we measured hepatic blood flow by indocyanine green (ICG) infusion, hepatic venous pressure gradient (HVPG), and cardiac output (Swan-Ganz catheter).

RESULTS

RHP was higher in patients than in healthy controls (5.1±3.7 vs. 3.4±0.7, p=0.003), and correlated with MELD (R=0.403, p=0.002), Child-Pugh score (R=0.348, p=0.009), and HVPG (R=0.279, p=0.041). RHP inversely correlated with ICG extraction (R=-0.346, p=0.039), ICG intrinsic clearance (R=-0.327, p=0.050), and ICG clearance (R=0.517, p=0.001), and directly correlated with hyperdynamic syndrome markers (cardiac index R=0.422, p=0.003; mean arterial pressure R=-0.405, p=0.004; systemic vascular resistance R=-0.496, p=0.001).

CONCLUSIONS

RHP increases in patients with cirrhosis and correlates with the degree of liver failure and hyperdynamic syndrome. RHP increases along with liver functional reserve decrease, suggesting that RHP increase occurs mainly through anatomical/functional shunts. RHP by CEUS is a feasible novel, objective, quantitative, non-invasive tool, potentially useful for the estimation of hepatic perfusion in patients with cirrhosis.

Assessment of tissue perfusion by contrast-enhanced ultrasound

Contrast-enhanced ultrasound (CEUS) with microbubble contrast agents is a new imaging technique for quantifying tissue perfusion. CEUS presents several advantages over other imaging techniques in assessing tissue perfusion, including the use of microbubbles as blood-pool agents, portability, availability and absence of exposure to radiation or nuclear tracers. Dedicated software packages are necessary to quantify the echo-signal intensity and allow the calculation of the degree of tissue contrast enhancement based on the accurate distinction between microbubble backscatter signals and native tissue background. The measurement of organ transit time after microbubble injection and the analysis of tissue reperfusion kinetics represent the two fundamental methods for the assessment of tissue perfusion by CEUS. Transit time measurement has been shown to be feasible and has started to become accepted as a clinical tool, especially in the liver. The loudness of audio signals from spectral Doppler analysis is used to generate time-intensity curves to follow the wash-in and wash-out of the microbubble bolus. Tissue perfusion may be quantified also by analysing the replenishment kinetics of the volume of microbubbles after their destruction in the imaged slice. This allows to obtain semiquantitative parameters related to local tissue perfusion, especially in the heart, brain, and kidneys.

Microbubbles traversing the blood-brain barrier for imaging and therapy

In the last several years great progress has been made in the field of ultrasound perfusion imaging of the brain. Different approaches have been assessed and shown to be capable of early detection of cerebral perfusion deficits. Real-time low mechanical index imaging simplifies the acquisition of perfusion parameters and alleviates many of the previous imaging problems related to shadowing, uniplanar analysis, and temporal resolution. With the advent of this new, highly sensitive contrast-specific imaging technique new possibilities of real-time visualization of brain infarctions and cerebral hemorrhages have emerged. Microbubbles that traverse the blood-brain barrier (BBB) can also elicit bioeffects that may be used to open the BBB for targeted delivery of macromolecular agents to the brain. Possible ways in which substances cross the BBB after application of this novel approach include transcytosis, passage through endothelial cell cytoplasmic openings, opening of tight junctions, and free passage through injured endothelium. Although relatively little tissue damage occurs at low acoustic intensities capable of opening the BBB, no investigation has demonstrated a total lack of BBB injury when using ultrasound and microbubbles. Further studies are necessary to address the effects of ultrasound and microbubbles upon the various transport mechanisms of the BBB. Moreover, investigations aimed at elucidating how ultrasound and microbubbles interact at the molecular level of the BBB are necessary. Results of such studies will increase our understanding of the mechanisms of BBB opening and also allow a better appraisal of the safety of this technique for future clinical applications.

A method to expedite data acquisition for multiple spatial-temporal analyses of tissue perfusion by contrast-enhanced ultrasound

For semiquantitative analyses of tissue perfusion using contrast-enhanced ultrasound the acquisition and processing of time intensity curves (TIC) is required. These TICs can be computed for each pixel of an image plane, yielding parametric images of classification numbers like "blood volume" and "flow rate." The expenditure of time for data acquisition and analysis typically limits semiquantitative perfusion imaging to a single image plane in 2-D. 3-D techniques, however, provide a higher diagnostic value since more information (e.g., of an entire lesion) is obtained. Moreover, spatial compounding, being a 2-D-technique where an object is imaged from different viewing angles, is known to improve image quality by reducing artifacts and speckle noise. Both techniques, 3-D and compounding, call for optimized acquisition and processing of TICs in several image planes (3-D) or in several (overlapping) sections of the same image plane (compounding) to decrease the time needed for data acquisition. Here, an approach of interleaved imaging is presented which is applicable, among others, to contrast perfusion imaging using the replenishment method. The total acquisition time is decreased by sequentially scanning image planes twice for short time spans - first, immediately after microbubble destruction to record the initial rise of the TICs, and second, a sufficient time thereafter to assess final values of the TIC. Data from both periods are combined to fit a model function from which parameters are extracted such as perfusion rate and blood volume. This approach was evaluated by in vitro measurements on a perfusion-mimicking phantom for both, individual images such as would be used for volume reconstruction in 3-D and compound images obtained from full angle spatial compounding (FASC, 360 degrees ). An error analysis is conducted to derive the deviation of the extracted parameters of the proposed method compared with the conventional one. These deviations are entailed by a reduction in acquisition time of the proposed method, which can be adjusted by several parameters, depending on the prevailing flow. Optimization strategies are proposed to find optimal values for those settings.

Full angle spatial compounding for improved replenishment analyses in contrast perfusion imaging: in vitro studies

For contrast enhanced perfusion imaging semi-quantitative methods (such as the bolus-, replenishment- or depletion-method) are commonly used to analyze the dynamic changes in concentration of contrast agent induced by insonification. In order to apply these methods and to decrease artifacts from tissue nonlinearity, perfusion imaging is conducted using decreased transmit power. However, echo signals from deeper structures are often too weak to be successfully analyzed. Furthermore, shadowing artifacts may occur as a result of high concentration of contrast agent in the beam path. Thus, those semi-quantitative methods often fail or yield ambiguous diagnoses. Imaging an object (e.g., the female breast) from multiple viewing angles (spatial compounding) may overcome these issues. In addition, spatial compounding achieves an isotropic resolution and reduces speckle and further common artifacts. In this paper we present results obtained from a combination of spatial compounding with contrast enhanced perfusion imaging. Applying the replenishment method, we extracted perfusion-related parameters and compared the conventional parametric images with the compound parametric images. We found that the compounded parametric images outperform the conventional images due to reduced noise and suppression of artifacts.

Contrast-enhanced ultrasound perfusion imaging in acute stroke patients

The field of neurovascular ultrasound is expanding rapidly with exciting new applications. While ultrasound contrast agents were initially used to overcome insufficient transcranial bone windows for identification of the basal cerebral arteries, new-generation microbubbles in combination with very sensitive contrast-specific ultrasound techniques now enable real-time visualization of stroke. This article will provide a review of recent and emerging developments in ultrasound technology and contrast-specific imaging techniques for evaluation of acute stroke patients.

Nanorod-based flow estimation using a high-frame-rate photoacoustic imaging system

A quantitative flow measurement method that utilizes a sequence of photoacoustic images is described. The method is based on the use of gold nanorods as a contrast agent for photoacoustic imaging. The peak optical absorption wavelength of a gold nanorod depends on its aspect ratio, which can be altered by laser irradiation (we establish a wash-in flow estimation method of this process). The concentration of nanorods with a particular aspect ratio inside a region of interest is affected by both laser-induced shape changes and replenishment of nanorods at a rate determined by the flow velocity. In this study, the concentration is monitored using a custom-designed, high-frame-rate photoacoustic imaging system. This imaging system consists of fiber bundles for wide area laser irradiation, a laser ultrasonic transducer array, and an ultrasound front-end subsystem that allows acoustic data to be acquired simultaneously from 64 transducer elements. Currently, the frame rate of this system is limited by the pulse-repetition frequency of the laser (i.e., 15 Hz). With this system, experimental results from a chicken breast tissue show that flow velocities from 0.125 to 2 mms can be measured with an average error of 31.3%.

Ultrasonographic contrast-agent imaging of sub-millimeter vessel structures with spatial compounding:in vitroanalyses / Kontrastmittelgestützte Ultraschallabbildung von Sub-Millimeter-Gefäßstrukturen mittels Spatial Compounding:In-vitroAnalysen

In clinical diagnostics, ultrasonographic contrast-agent imaging gives access to medical parameters such as perfusion and vascularization. In addition to the artifacts that are typical for ultrasonic imaging, e.g., speckle noise and depth-dependent sensitivity and resolution, contrast-agent imaging shows more pronounced depth dependence and may suffer from shadowing artifacts that arise from high attenuation of the ultrasound waves by the contrast agent at high concentrations. By imaging an object from different viewing angles in one 2D image plane and summing the images obtained (spatial compounding), image quality can be increased and artifacts can be suppressed. In the present study, we combined both techniques to overcome the limitations of contrast-agent imaging. We used a commercially available ultrasound scanner and a custom-made high-precision mechanical system to rotate the ultrasound transducer fully around the object under investigation. Using this set-up, ultrasound data were acquired in reflection mode to generate a 360 degrees compound scan of a flow-mimicking phantom supplied with contrast agent.

Intraoperative ultrasound using phase inversion harmonic imaging: first experiences

OBJECTIVE

To study the feasibility of intraoperative ultrasound using the phase inversion harmonic imaging (PIHI) technique.

METHODS

Eight patients with intracranial middle cerebral artery aneurysms and five patients with arteriovenous malformations were studied after written informed consent. A first ultrasound study was performed through the intact dura mater after cranial trepanation to assess the pathology, its feeding artery, and downstream segments. A second ultrasound study was performed immediately after intervention to monitor the success of the procedure. All patients were studied using a Siemens Sonoline Antares ultrasound machine (Siemens Medical Solutions USA, Inc., Malvern, PA) before and after intravenous administration of an ultrasound contrast agent (Optison; GE Healthcare, Milwaukee, WI). Other than conventional brightness mode, PIHI is sensitive to the nonlinear acoustic response of tissue, and especially to ultrasound contrast agent microbubbles. The latter enables contrast-specific vascular imaging.

RESULTS

PIHI provided anatomically detailed information. In combination with an ultrasound contrast agent, angiography-like views of the vascular pathologies, including their surrounding vessels, could be obtained. Flow velocities in afferent and downstream vascular segments, as well as inside the pathology, could be assessed. Flow dynamics inside the aneurysm sac or the arteriovenous malformation could be studied in real-time. Postintervention, contrast-enhanced PIHI could be used to immediately monitor the success of the surgical procedure.

CONCLUSION

PIHI enables intraoperative visualization and morphological assessment of neurovascular pathologies, such as middle cerebral artery aneurysms or arteriovenous malformations. In combination with an ultrasound contrast agent, the flow dynamics of these lesions can be displayed in real-time.

Microbubble ultrasound contrast agents: an update

Microbubble contrast agents for ultrasound (US) have gained increasing interest in recent years, and contrast-enhanced US (CEUS) is a rapidly evolving field with applications now extending far beyond the initial improvements achieved in Doppler US. This has been achieved as a result of the safe profile and the increased stability of microbubbles persisting in the bloodstream for several minutes, and also by the availability of specialized contrast-specific US techniques, which allow a definite improvement in the contrast resolution and suppression of signal from stationary tissues. CEUS with low transmit power allows real-time scanning with the possibility of prolonged organ insonation. Several reports have described the effectiveness of microbubble contrast agents in many clinical applications and particularly in the liver, spleen, and kidneys. CEUS allows the assessment of the macrovasculature and microvasculature in different parenchymas, the identification and characterization of hepatic and splenic lesions, the depiction of septal enhancement in cystic renal masses, and the quantification of organ perfusion by the quantitative analysis of the echo-signal intensity. Other fields of application include the assessment of abdominal organs after traumas and the assessment of vesico-ureteral reflux in children. Finally, tumor-targeted microbubbles make possible the depiction of specific biologic processes.

Intraoperative brain ultrasound: a new approach to study flow dynamics in intracranial aneurysms

The aim was to evaluate the potential of contrast-enhanced ultrasound to visualize the hemodynamics in intracranial aneurysms during neurosurgical intervention and to quantify the ultrasound data using digital particle image velocimetry (DPIV) technique. Aneurysms were scanned through the intact dura mater, preclipping and again postclipping after closure of the dura. After intravenous injection of Optison, angio-like views of the vascular tree surrounding the aneurysm, including the aneurysm sac, were obtained. Single ultrasound contrast agent microbubbles could be visualized in the aneurysm sac and the flow dynamics could be assessed in vivo. Spatial and temporal distributions of the velocity in the aneurysm and in the parent vessels were measured with DPIV using the backscattered signals from the microbubbles. Subsequently, the fluid stresses, vorticity, circulation, etc., were calculated from the velocity fields. We demonstrate in this paper that intraoperative contrast-enhanced ultrasound can be used to quantify the flow dynamics within an aneurysm.