Visualization of brain perfusion with harmonic gray scale and power doppler technology : an animal pilot study

Quantitative Analysis of Brain CT Perfusion in Healthy Beagle Dogs: A Pilot Study

Brain computed tomography (CT) perfusion is a technique that allows for the fast evaluation of cerebral hemodynamics. However, quantitative studies of brain CT perfusion in veterinary medicine are lacking. The purpose of this study was to investigate the normal range of perfusion determined via CT in brains of healthy dogs and to compare values between white matter and gray matter, differences in aging, and each hemisphere. Nine intact male beagle dogs were prospectively examined using dynamic CT scanning and post-processing for brain perfusion. Regional cerebral blood volume (rCBV), regional cerebral blood flow (rCBF), mean transit time, and time to peak were calculated. Tissue ROIs were drawn in the gray matter and white matter of the frontal, temporal, parietal, and occipital lobes; caudate nucleus; thalamus; piriform lobe; hippocampus; and cerebellum. Significant differences were observed between the white matter regions and gray matter regions for rCBV and rCBF (p < 0.05). However, no significant differences were identified between hemispheres and between young and old groups in brain regions. The findings obtained in this study involving healthy beagle dogs might serve as a reference for regional CT perfusion values in specific brain regions. These results may aid in the characterization of various brain diseases in dogs.

Identification of brain structures and blood vessels by conventional ultrasound in rats

BACKGROUND

Ultrasound is a safe, non-invasive and affordable imaging technique for the visualization of internal structures and the measurement of blood velocity using Doppler imaging. However, despite all these advantages, no study has identified the structures of the rat brain using conventional ultrasound.

METHODS

A 13 MHz high frequency transducer was used to identify brain structures in the rat. The enlargement of the transcranial window was performed gradually using the ultrasound directly on the skin of the animal, then against the skull, then through a delimited craniotomy and finally through a complete craniotomy.

RESULTS

Our results showed that ultrasound allowed the identification of cerebral ventricles and subarachnoid cisterns, as well as the analysis of real-time monitoring of cerebral blood flow in the main brain arteries of the rat.

COMPARISON WITH EXISTING METHODS

Ultrasound is a tool with the potential to identify brain structures and blood vessels. In contrast to MRI, transcranial ultrasound is a fast, non-invasive, well tolerated and low-cost method and can be done at the bedside.

CONCLUSION

In the present study, we described an atlas of the main brain structures as well as the main vasculature in the rat using ultrasound. This technique could be applied in animal models of various neurological diseases.

Detecting stripe artifacts in ultrasound images

Brain perfusion diseases such as acute ischemic stroke are detectable through computed tomography (CT)-/magnetic resonance imaging (MRI)-based methods. An alternative approach makes use of ultrasound imaging. In this low-cost bedside method, noise and artifacts degrade the imaging process. Especially stripe artifacts show a similar signal behavior compared to acute stroke or brain perfusion diseases. This document describes how stripe artifacts can be detected and eliminated in ultrasound images obtained through harmonic imaging (HI). On the basis of this new method, both proper identification of areas with critically reduced brain tissue perfusion and classification between brain perfusion defects and ultrasound stripe artifacts are made possible.

Vaskuläre Bildgebung mittels kontrastverstärkter Sonographie in der experimentellen Anwendung

The possibility of employing contrast-enhanced ultrasound for sensitive detection of perfusion has resulted in new forms of application in fundamental medical biological research that go far beyond mere preclinical evaluation of these techniques. This contribution explains the methods for visualization and quantification of perfusion with contrast-enhanced sonography and provides an overview of how these functional examinations have been used to date. The procedure is generally considered indicated when information on tissue perfusion using ultrasound is required. This topic is also gaining increasing clinical interest, e.g., for assessment of myocardial, cerebral, and renal perfusion or for monitoring therapy. Among the various new treatment procedures that have been investigated in animal models with ultrasound, particularly pro-angiogenic and antiangiogenic therapy approaches predict promising new fields for application of contrast-enhanced ultrasound.

Ultrasonic evaluation with second harmonic imaging and SonoVue in the assessment of cerebral perfusion in diabetic patients: a case-control study

The purpose was to compare human brain tissue perfusion in diabetic patients and healthy subjects with second harmonic imaging ultrasound and SonoVue to test the hypothesis that brain tissue perfusion differences are present in these two groups of patients. In a prospective case-control study, second harmonic examinations performed in 20 patients with type II diabetes mellitus and in 20 matched control patients were compared. After administration of 2.5 ml of SonoVue, 60 time-triggered images were recorded. Time-intensity curves, including peak intensity and positive gradient normalized to the middle cerebral artery, were calculated to quantify ultrasound intensity in a region of interest. The Mann-Whitney U-test was used to reveal any differences between healthy and diabetic subjects. Mean peak intensity was 0.64+/-0.1 Au in healthy subjects and 0.53+/-0.09 Au in diabetic patients. Mean positive gradient was 0.04+/-0.007 Au/s in healthy subjects and 0.04+/-0.008 Au/s in diabetic patients. Peak intensity and positive gradient were significantly lower in diabetic patients than in healthy subjects (P<0.05). Ultrasound examination with second harmonic imaging and SonoVue administration is able to detect clinically silent, reduced cerebral perfusion in type II diabetic patients. Diabetic patients have reduced cerebral perfusion in comparison to healthy subjects.

Assessment of local changes of cerebral perfusion and blood concentration by ultrasound harmonic B-mode contrast measurement in piglet

This study tested the hypothesis that changes in the blood concentration, and possibly in the perfusion, of different areas in the brain can be assessed by the use of ultrasound contrast agent (CA) and (linear) echo densitometry. The experiments were performed with piglets (n=3) under general anesthesia and artificial ventilation. Ultrasound CA was administered through a femoral vein as a short bolus. First passage wash-in curve was measured from image gray level during continuous low level (mechanical index<0.2) ultrasound imaging. This curve was obtained from 1-cm2 areas of the cortex (surface), the brain stem (inner) and the left carotid artery (vessel). Cerebral hemoglobin concentration changes were measured with near-infrared spectroscopy (NIRS). This approach enabled a cross-validation of these techniques. The measurements were repeated under conditions of normocapnia, mild hypercapnia and deep hypercapnia. Several physiologic signals, as well as the carotid blood flow, were measured simultaneously and related to gray level by linear regression analysis. The most significant results found were a high R2-statistic of the regression of the percentage change of the peak of the surface and inner wash-in curves with the arterial carbon dioxide pressure (R2=0.63 and R2=0.70, respectively), the blood pH (R2=0.79 and R2=0.81), the carotid flow (R2=0.75 and R2=0.72) and the partial arterial oxygen pressure (R2=0.47 and R2=0.55). Finally, a high correlation of peak gray level with total hemoglobin concentration change, independently measured by NIRS, was found (R2=0.69). In conclusion, these experiments show a reasonable intersubject variability of various relative measures derived from gray level ultrasound wash-in curves. High sensitivity to physiologic changes related to hypercapnia was observed for the peak contrast of wash-in curves. For up-slope and area-under-the-curve (first passage) this was lower but still highly significant. The gray-level ultrasound measures are highly correlated to changes in regional hemoglobin concentration in brain tissue assessed by NIRS.

Brain perfusion and ultrasonic imaging techniques

Advances in neurosonology have generated several techniques of ultrasonic perfusion imaging employing ultrasound echo contrast agents (ECAs). Doppler imaging techniques cannot measure the low flow velocities that are associated with parenchymal perfusion. Ultrasonic perfusion imaging, therefore, is a combination of a contrast agent-specific ultrasound imaging technique (CAI) mode and a data acquisition and processing (DAP) technique that is suited to observe and evaluate the perfusion kinetics. The intensity in CAI images is a measure of ECA concentration but also depends on various other parameters, e.g. depth of examination. Moreover, ECAs can be destroyed by ultrasound, which is an artifact but can also be a feature. Thus, many different DAPs have been developed for certain CAI techniques, ECAs and target organs. Although substantial progress in ECA and CAI technology can be foreseen, ultrasound contrast imaging has yet to reliably differentiate between normal and pathological perfusion conditions. Destructive imaging techniques, such as contrast burst imaging (CBI) or time variance imaging (TVI), in combination with new DAP techniques provide sufficient signal-to-noise ratio (SNR) for transcranial applications, and consider contrast agent kinetics and destruction to eliminate depth dependency and to calculate semi-quantitative parameters. Since ultrasound machines are widely accessible and cost-effective, ultrasonic perfusion imaging techniques should become supplementary standard perfusion imaging techniques in acute stroke diagnosis and monitoring. This paper gives an overview on different CAI and DAP techniques with special focus on recent innovations and their clinical potential.

Transcranial contrast diminution imaging of the human brain: a pilot study in healthy volunteers

Analysis of contrast diminution kinetics after bubble destruction is a new aspect in harmonic imaging. The purpose of this study was to investigate this approach to human cerebral perfusion. A total of 12 healthy volunteers were investigated transtemporally (Philips SONOS 5500, S4-probe, 1.8 to 3.6 MHz, 10 cm) at two ultrasound (US) contrast agent (UCA) infusion rates (0.5 and 1.0 mL/min of Optison). After achieving a steady-state, a set of 12 US pulses (6.67 Hz, MI 1.6) was applied. Time-intensity plots of three regions-of-interest (ROIs) (thalamus, white matter and cortex) were analyzed, using an exponential curve fit (I((t)) = I(0)e(-betat) + B). A total of 20 of 20 successful investigations showed a signal decrease after pulsed US application. In all cases, it was possible to generate exponential time-intensity curves. Half-life (T(1/2) = ln2/beta) and baseline intensity (B) showed a significant dependence on infusion rate (p = 0.01). At 1.0 mL/min, T(1/2) also depended on investigation depth (p = 0.01). It is possible to assess contrast diminution kinetics in human cerebral microcirculation. This new approach may provide additional information on cerebral perfusion within a short investigation time.

Human cerebral perfusion analysis with ultrasound contrast agent constant infusion: a pilot study on healthy volunteers

With ultrasound (US) contrast agent (UCA) continuous infusion providing a steady state, mean tissue microbubble velocity can be assessed by analyzing the reappearance rate after microbubble destruction with US energy (refill kinetics). In this study, we investigated this new approach for the assessment of human cerebral perfusion. A total of 12 healthy volunteers were investigated transtemporally with increasing pulsing intervals (250, 500, 750, 1000, 1250, 1500, 2000, 3000 and 4000 ms) and two UCA infusion rates (0.5 and 1.0 mL/min of Optison). Intensity vs. pulsing interval curves were analyzed using an exponential curve fit and parameters of the curve (plateau echo enhancement, A, representing the microbubble concentration within the interrogated tissue; rate constant, beta, which is related to blood flow and their product, F = Abeta) were compared. For 20/20 investigations being available for further analysis, it was possible to generate a typical exponential intensity vs. pulsing interval curve from the ipsilateral thalamus. The plateau echo enhancement A showed a significant (p = 0.02), and the beta as well as the F values displayed a nonsignificant (p = 0.06, both), increase with infusion rate. The qualitative analysis of beta and F parameter images displayed the most homogeneous visualisation of perfusion in the ipsilateral thalamus and main territory of the middle cerebral artery. In conclusion, it is possible to display the UCA refill kinetics in human cerebral microcirculation after microbubble destruction by transcranial US. Grey-scale harmonic imaging allows a quantitative approach to cerebral perfusion with a large interindividual variation of the parameters.

Harmonic imaging of the brain parenchyma in a dog model following NC100100 (Sonazoid) bolus injection

BACKGROUND AND PURPOSE

NC100100 (Sonazoid) is a new perfluorocarbon-based ultrasound contrast agent (UCA) that has not been introduced to transcranial harmonic imaging.

METHODS

In an animal study on 6 beagle dogs, the authors performed harmonic power Doppler and gray-scale imaging (SONOS 5500, S4 probe) after bolus injection of 2 different doses of Sonazoid. Time intensity curves for brain parenchyma, masticatory muscle, and contralateral skull were generated, and the peak increase from baseline was computed.

RESULTS

With harmonic gray-scale imaging, a dose-dependent homogeneous increase in acoustic intensity of the brain parenchyma was observed. Evaluation of the contralateral base of the skull showed a moderate signal decrease. In harmonic power Doppler sonography, signal increase was dose dependent also, but the signal pattern was inhomogeneous, with stronger enhancement in the anterior part of the brain.

CONCLUSION

The new UCA Sonazoid is suitable for displaying brain perfusion. As already observed for other UCAs, gray-scale harmonic imaging with Sonazoid leads to a more homogeneous contrast increase in cerebral parenchyma compared to harmonic power Doppler imaging. Sonazoid produces a moderate shadowing effect, with signal attenuation in the underlying deeper regions of interest.

Harmonic imaging--a new method for the sonographic assessment of cerebral perfusion

In this review, methodological aspects of cerebral perfusion imaging with ultrasound signal enhancing agents are described. The various experimental bases, contributing to the understanding of the phenomena are summarised and the resulting human investigation techniques are illustrated. By means of harmonic imaging technology, human cerebral perfusion can be depicted as a two-dimensional scan. The two major principles of contrast measurement are analysis of the bolus kinetics and analysis of the refill kinetics. Using the bolus method, hypoperfused areas in stroke patients can be visualised and parameter images of wash-in and wash-out curves can be generated off-line. The recently developed theory on the refill kinetics of UCA enables us to calculate quantitative parameters for the description of the cerebral microcirculation, being less affected by the depth dependence of the contrast effect. These parameters, too, can be visualised as parameter images. The ultrasound methods described in this article represent new minimal-invasive bedside techniques for analysing brain perfusion. Although their development is still in an early state, the potential of these ultrasound technologies to compete with perfusion-CT, perfusion-MRI or single-photon emission computed tomography in the diagnostic arsenal of brain imaging techniques is becoming evident.

Evaluation of blood flow in the cerebral microcirculation: analysis of the refill kinetics during ultrasound contrast agent infusion

By means of harmonic imaging, it is possible to display brain perfusion qualitatively using ultrasound (US) contrast agent (UCA) bolus injection. With UCA continuous infusion reaching a steady state, mean microbubble velocity can be measured, analyzing the reappearance rate after microbubble destruction by US (refill kinetics). We performed an animal pilot study to investigate this new method for the assessment of brain perfusion. Using harmonic grey-scale imaging, five sedated male beagle dogs were investigated through the intact skull with increasing pulsing intervals (250 to 8000 ms) and three UCA infusion rates (0.5, 1.0 and 1.5 mL/min of Optison). Cerebral blood flow was increased by acetazolamide (30 mg/kg BW). Intensity vs. pulsing interval curves were analyzed using an exponential curve fit [I(t) = A(1-e(-beta t))] and parameters of the curve were compared. We found that increasing the pulsing interval above 4000 ms led to no further increase of echo enhancement for infusion rates. Mean beta values were not influenced by infusion rate (p = 0.25 and p = 0.55). Mean F values increased nonsignificantly with rising infusion rate (p = 0.25 and p = 0.86). Acetazolamide led to an increase of mean beta and F values (p = 0.18 and p = 0.025, respectively). It is possible to evaluate changes in brain perfusion through the intact skull by analyzing the UCA refill kinetics after US-induced microbubble destruction.

Ultrasound imaging: principles and applications in rodent research

Ultrasound imaging utilizes the interaction of sound waves with living tissue to produce an image of the tissue or, in Doppler-based modes, determine the velocity of a moving tissue, primarily blood. These dynamic, real time images can be analyzed to obtain quantitative structural and functional information from the target organ. This versatile, noninvasive diagnostic tool is widely used and accepted in human and veterinary medicine. Until recently its application as a research tool was limited primarily to larger, nonrodent species. Due to advances in ultrasound imaging technology, commercially available ultrasound systems now have the spatial and temporal resolution to obtain accurate images of rat and mouse hearts, kidneys, and other target tissues including tumor masses. As a result, ultrasound imaging is being used more frequently as a research tool to image rats and mice, and particularly to evaluate cardiac structure and function. The developing technology of ultrasound biomicroscopy has even greater spatial resolution and has been used to evaluate developing mouse embryos and guide site-specific injections into mouse embryos. Additional ultrasound imaging technologies, including contrast-enhanced imaging and intravascular ultrasound transducers adapted for transesophageal use, have been utilized in rats and mice. This paper provides an overview of basic ultrasound principles, equipment, and research applications. The use of noninvasive ultrasound imaging in research represents both a significant refinement as a potential replacement for more invasive techniques and a significant advancement in research techniques to study rats and mice.

Harmonic imaging of the brain parenchyma using a perfluorobutane-containing ultrasound contrast agent

We evaluated the signal-enhancing effect of the novel perfluorobutane-based ultrasound contrast agent BR 14 (Bracco Research, Switzerland) in grey-scale harmonic imaging of the brain parenchyma. Six sedated male beagle dogs were investigated with transcranial grey-scale harmonic imaging (SONOS 5500, 1.8/3.6 MHz). After bolus injection of two different doses of BR 14, acoustic densitometry was performed to quantify changes in regional contrast intensity. In the dogs' brain parenchyma, the mean relative peak increase in acoustic intensity was +61% after administration of 0.05 ml/kg BW of BR 14 and +24% after 0.2 ml/kg BW. In the masticatory muscle, application of the higher dose resulted in a stronger increase in contrast intensity compared to the lower dose. Evaluation of the contralateral base of the skull showed a dose-dependent decrease in acoustic intensity. Bolus injection of BR 14 produces an increase in acoustic intensity, which can be used for the visualization of contrast agent in the brain parenchyma. Using high dosages, a strong signal-enhancing effect in the regions near the ultrasound probe leads to a consecutive attenuation of signals from structures being located beyond ("shadowing-effect"). This is the explanation for the paradoxical result that the higher dose leads to a lower peak signal increase in the brain parenchyma.

Quantification of flow rates using harmonic grey-scale imaging and an ultrasound contrast agent: an in vitro and in vivo study

It is unclear if the dye-dilution theory and its corresponding parameters are capable of measuring brain perfusion using harmonic grey-scale imaging. We performed a study on a flow phantom using a SONOS 5500 (1.8--3.6-MHz harmonic imaging) and Levovist as the ultrasound (US) contrast agent (UCA). We applied the UCA in six different doses (0.1 to 3.0 mL) and used eight different flow-rates (180 to 540 mL/min). Additionally, we performed a study on dog brain using Levovist boluses of 1.5 mL and 3 mL. We evaluated the influence of dose and flow-rate on the parameters of the time-intensity curve: peak signal intensity (PSI), area under the curve (AUC) and mean transit time (MTT). Along with an increase of the Levovist dose, the AUC and the PSI increased only in the dose range between 0.1 and 0.5 mL Levovist; further increase led to no change of parameters. Flow-rate showed no influence on AUC, MTT or PSI. The dye-dilution theory is not a useful theoretical model for the analysis of perfusion using harmonic grey-scale imaging. A possible explanation for this effect is the bubble saturation.