Ultrasonic evaluation of pathological brain perfusion in acute stroke using second harmonic imaging

Ultrasound Perfusion Imaging for the Detection of Cerebral Hypoperfusion After Aneurysmal Subarachnoid Hemorrhage

Background

Delayed cerebral ischemia increases mortality and morbidity after aneurysmal subarachnoid hemorrhage (aSAH). Various techniques are applied to detect cerebral vasospasm and hypoperfusion. Contrast-enhanced ultrasound perfusion imaging (UPI) is able to detect cerebral hypoperfusion in acute ischemic stroke. This prospective study aimed to evaluate the use of UPI to enable detection of cerebral hypoperfusion after aSAH.

Methods

We prospectively enrolled patients with aSAH and performed UPI examinations every second day after aneurysm closure. Perfusion of the basal ganglia was outlined to normalize the perfusion records of the anterior and posterior middle cerebral artery territory. We applied various models to characterize longitudinal perfusion alterations in patients with delayed ischemic neurologic deficit (DIND) across the cohort and predict DIND by using a multilayer classification model.

Results

Between August 2013 and December 2015, we included 30 patients into this prospective study. The left–right difference of time to peak (TTP) values showed a significant increase at day 10–12. Patients with DIND demonstrated a significant, 4.86 times increase of the left–right TTP ratio compared with a mean fold change in patients without DIND of 0.9 times (p = 0.032).

Conclusions

UPI is feasible to enable detection of cerebral tissue hypoperfusion after aSAH, and the left–right difference of TTP values is the most indicative result of this finding.

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.

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.

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.

Assessment of Hepatic Vascular Network Connectivity with Automated Graph Analysis of Dynamic Contrast-enhanced US to Evaluate Portal Hypertension in Patients with Cirrhosis: A Pilot Study

PURPOSE

To test whether graph analysis of vascular images obtained with hepatic dynamic contrast material-enhanced (DCE) ultrasonography (US) allows calculation of the degree of organization of the liver circulation and whether graph properties are correlated to the severity of portal hypertension.

MATERIALS AND METHODS

Institutional review board approval and written informed consent were obtained. Fifteen patients with liver cirrhosis (nine men; mean age ± standard deviation, 55 years ± 8) who underwent DCE US and hepatic venous pressure gradient (HVPG) measurement and four healthy subjects (two men and two women; mean age, 34 years ± 4) were included between January 2007 and December 2008. Individual graph models ("vascular connectomes") were computed on the basis of time series analysis of video sequences of DCE US examinations (conducted with the disruption-reperfusion technique). Graph analysis was performed, and the clustering coefficient C was calculated. Correlations between clustering coefficient and HVPG were assessed.

RESULTS

Healthy subjects had a high clustering coefficient of vascular connectome (C = 0.4447; interquartile range [IQR], 0.3864-0.4679), suggesting a highly organized hepatic vascular network. Conversely, patients with cirrhosis showed a low clustering coefficient, indicating disruption of normal anatomy (C = 0.0288; IQR, 0.0157-0.0861; P = .001 vs healthy subjects). The clustering coefficient decreased as HVPG increased, with a clustering coefficient of 0.0237 (IQR, 0.0066-0.0378) in patients with HVPG of at least 10 mm Hg versus 0.1180 (IQR, 0.0987-0.1414) in those with HVPG of less than 10 mm Hg (P = .006). The correlation between the best model derived from the distribution of the clustering coefficient (10 bins) of vascular connectome and HVPG had a Pearson correlation of 0.977 (root mean squared error, 1.57 mm Hg; P < .0001).

CONCLUSION

This pilot study demonstrates that graph modeling of vascular connectivity based on video processing of liver DCE US examinations and subsequent graph analysis enable calculation of personalized parameters that reflect the degree of organization of the hepatic microvascular network and are correlated to the severity of portal hypertension in cirrhosis.

Intraoperative contrast-enhanced ultrasound for brain tumor surgery

BACKGROUND

Contrast-enhanced ultrasound (CEUS) is a dynamic and continuous modality that offers a real-time, direct view of vascularization patterns and tissue resistance for many organs. Thanks to newer ultrasound contrast agents, CEUS has become a well-established, live-imaging technique in many contexts, but it has never been used extensively for brain imaging. The use of intraoperative CEUS (iCEUS) imaging in neurosurgery is limited.

OBJECTIVE

To provide the first dynamic and continuous iCEUS evaluation of a variety of brain lesions.

METHODS

We evaluated 71 patients undergoing iCEUS imaging in an off-label setting while being operated on for different brain lesions; iCEUS imaging was obtained before resecting each lesion, after intravenous injection of ultrasound contrast agent. A semiquantitative, offline interobserver analysis was performed to visualize each brain lesion and to characterize its perfusion features, correlated with histopathology.

RESULTS

In all cases, the brain lesion was visualized intraoperatively with iCEUS. The afferent and efferent blood vessels were identified, allowing evaluation of the time and features of the arterial and venous phases and facilitating the surgical strategy. iCEUS also proved to be useful in highlighting the lesion compared with standard B-mode imaging and showing its perfusion patterns. No adverse effects were observed.

CONCLUSION

Our study is the first large-scale implementation of iCEUS in neurosurgery as a dynamic and continuous real-time imaging tool for brain surgery and provides the first iCEUS characterization of different brain neoplasms. The ability of CEUS to highlight and characterize brain tumor will possibly provide the neurosurgeon with important information anytime during a surgical procedure.

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.

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

Real-time ultrasound perfusion imaging (rt-UPI) allows visualization of microbubbles flowing through the cerebral microvasculature. We hypothesized that analysis of microbubble tissue replenishment would enable for characterization of perfusion deficits in acute middle cerebral artery (MCA) territory stroke. Twenty-three patients (mean age 70.2 ± 13.2 years, 9 weeks) were included. Sequential images of bubble replenishment were acquired by transcranial rt-UPI at low mechanical index immediately after microbubble destruction. Different parameters were calculated from regions of interest (ROIs): real-time time to peak (rt-TTP), rise rate (β), and plateau (A) of acoustic intensity, and A × β was used as an index of blood flow. Results were compared with diffusion-weighted and perfusion magnetic resonance imaging. Parameters of rt-UPI had lower values in ROIs of ischemic as compared with normal tissue (β=0.58 ± 0.40 versus 1.25 ± 0.83; P=0.001; A=1.44 ± 1.75 versus 2.63 ± 2.31; P=0.05; A × β=1.14 ± 2.25 versus 2.98 ± 2.70; P=0.01). Real-time time to peak was delayed in ischemic tissue (11.43 ± 2.67 versus 8.88 ± 1.66 seconds; P<0.001). From the analysis of receiver operating characteristic curves, β and A × β had the largest areas under the curve with optimal cutoff values of β<0.76 and A × β<1.91. We conclude that rt-UPI with analysis of microbubble replenishment correctly identifies ischemic brain tissue in acute MCA stroke.

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.

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.

Ultrasound perfusion imaging: determination of thresholds for the identification of critically disturbed perfusion in acute ischemic stroke--a pilot study

Ultrasound harmonic imaging of perfusion after ultrasound contrast agent (UCA) bolus injection (BHI) can detect cerebral perfusion deficits. In a pilot study, we evaluated the ability of time-intensity curve (TIC) measurements to differentiate between normal and hypoperfused brain areas in acute ischemic stroke. Ten patients with symptoms of acute middle cerebral artery infarction were investigated (SONOS 5500, Harmonic Imaging 1.6/3.8 MHz, diencephalic plane, 10 cm investigation depth, SonoVue 2.4 mL bolus). Peak signal increase (PSI), time-to-peak intensity (TPI) and area under the curve (AUC) were calculated for 60 regions-of-interest (ROIs) in each patient. Reference methods: Perfusion- and diffusion-weighted MRI (PWI/DWI) within 4 h before/after BHI (PWI threshold: 4 s). Receiver operating characteristics (ROC) analysis defined cut-off values for each TIC variable to distinguish between normal and affected brain areas as defined by PWI/DWI. In five patients, PWI showed a perfusion delay >4 s; seven patients had a DWI lesion. In three patients, both PWI and DWI findings showed pathology; one patient had a normal MRI of the insonation plane. Cut-off values for PWI delay: PSI: 5.53% (sensitivity .98, specificity .89); TPI: 4.04 s (sensitivity .74, specificity .69) and AUC: .63 (sensitivity .94, specificity .58). Referred to the mean value in unaffected brain areas the relative thresholds were 17.6%, 109.5% and 16.1%, respectively. Regarding DWI, only for PSI, a significant cut-off value was defined: 10.86%, sensitivity .84, specificity .60 (34.6% of mean). In conclusion, these thresholds distinguish between normal and affected brain areas in acute ischemic stroke.

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.

Changes of contrast-specific ultrasonic cerebral perfusion patterns in the course of stroke; reliability of region-wise and parametric imaging analysis

Ultrasound perfusion imaging (UPI) reliably detects size and localization of acute stroke. It remains unclear which time window detects, most sensitively and specifically, early changes of cerebral perfusion patterns and whether region-wise analysis is superior to parametric imaging analysis. Bilateral phase inversion harmonic imaging examinations (bolus kinetic, fitted model function) were performed twice (acutely and 28 h later) in 10 patients with acute ischemic stroke (<12 h). Examinations were evaluated using a region-wise analysis of the time-intensity curve and by parametric images of the time-to-peak intensity maps. Results were correlated in-between the ultrasound examinations and to follow-up cranial computed tomography (CCT) scans. Correlation between the early region-wise UPI examination and follow-up CCT was the strongest (Spearman correlation coefficient 0.76, sensitivity 84%, specificity 96%). Spearman coefficient between the late UPI examination and CCT was 0.51; sensitivity and specificity were 71% and 82%. Values in between UPI examinations were 57% and 88%, with a Spearman coefficient of 0.47 (p for all < 0.001). Values of the analysis of the parametric images were less strong. Concordance between both of the UPI methods was 65% in the early examination and 72% in the late examination. Changes of perfusion patterns are most accurately detected in the early course of stroke, when core of infarction can be differentiated from penumbra and viable tissue. Reperfusion phenomena may impair the diagnostic impact in later examinations. Parametric imaging does not yet seem to be as accurate as region-wise analysis.

Pulse-Inversion US Imaging of Testicular Ischemia: Quantitative and Qualitative Analyses in a Rabbit Model

PURPOSE

To quantitatively and qualitatively assess perfusion with pulse-inversion (PI) ultrasonography (US) in rabbit model of acute testicular ischemia.

MATERIALS AND METHODS

Institutional animal care committee approval was obtained. After 35 rabbits underwent unilateral spermatic cord occlusion, testicular Doppler US and contrast material-enhanced PI imaging were performed. Enhancement data yielded perfusion measurements including mean value during the first 10 seconds, mean value over entire recorded replenishment curve, and curve slope during the first 5 seconds. Calculated perfusion ratios were compared with radiolabeled microsphere-derived perfusion ratios. Two readers assessed testicular perfusion as none, possible, or definite and relative perfusion as greater to the right testis than to the left, greater to the left testis than to the right, or as equal to both testes. With kappa statistics, interobserver agreement for all imaging methods was determined. Association between qualitative perfusion categories and radiolabeled microsphere-based perfusion measurements was assessed. Quantitative and qualitative determinations of relative perfusion were compared with radiolabeled microsphere-based measurements.

RESULTS

Correlations between calculated and radiolabeled microsphere-based perfusion ratios were determined (r=0.49-0.64). Interobserver agreement for presence of perfusion was excellent (kappa=0.76), and that for relative perfusion assessment was good (kappa=0.55). Neither kappa value varied significantly with imaging method. The percentage of times a testis classified as having definite perfusion had greater perfusion as measured with radiolabeled microspheres than a testis classified as having no perfusion or possible perfusion was higher with PI imaging than with Doppler US (85%-98% vs 72%-89%). Identification of the testis with less perfusion was better with quantitative methods than with qualitative assessment of images by the readers (75%-79% vs 34%-60%, P<.004).

CONCLUSION

PI imaging, compared with conventional Doppler US methods, provides superior assessment of perfusion in the setting of acute testicular ischemia.

Contrast-enhanced ultrasonic parametric perfusion imaging detects dysfunctional tissue at risk in acute MCA stroke

Ultrasonic perfusion imaging predicts size and localization of acute stroke. It is unclear whether irreversibly damaged tissue can be differentiated from tissue at risk. Thirty-four patients (ischemic stroke <12 h) were included (Phase Inversion Harmonic Perfusion Imaging; bolus kinetic; fitted model function). Three patterns of perfusion were defined in 14 prespecified regions of interest (ROI): 'normal', 'hypoperfusion', and 'no perfusion'. Clinical status was assessed using the National Institutes of Health Stroke Scale (NIHSS) (at baseline and at days 2 to 4). Cranial Computed Tomography (CCT) (days 2 to 4) displayed final infarction. The pattern 'hypoperfusion' (ROIs presumably representing tissue at risk) was tested twofold: (i) Functional impairment by correlating their number with baseline NIHSS. (ii) Viability by correlating their recruitment rate to infarction with clinical course (DeltaNIHSS days 2 to 4). In addition, various predictive values were assessed. Twenty-seven patients were eligible for analysis. The sum of ROIs with 'no perfusion' and 'hypoperfusion' correlated highest with baseline NIHSS (rho=0.78, P<0.001). Recruitment of hypoperfused ROIs to infarction highly correlated with clinical course (rho=0.79, P<0.001). Clinical course dichotomized the patients into subgroups A ('stable', DeltaNIHSS>or=-3) and B ('improved', DeltaNIHSS<or=-4). In A, sensitivity and specificity for hypo- and nonperfused tissue being eventually infarcted were 96% and 88% positive predictive value, PPV 89%, negative predictive value, NPV 96%). In B, sensitivity and specificity for nonperfused tissue eventually being infarcted were 81% and 99% (PPV 99%, NPV 84%). Different perfusion patterns (hypoperfusion, no perfusion) and dysfunctional but viable tissue at risk can be reliably detected by ultrasonic perfusion imaging. This method may give Supplementary information in cases illegible for perfusion-weighted magnetic resonance imaging (PW-MRI).

Transcranial contrast imaging of cerebral perfusion in patients with space-occupying intracranial lesions

OBJECTIVE

The aim of this study was to evaluate a deficit in cerebral perfusion after administration of the contrast agent SonoVue (Bracco Altana Pharma, Konstanz, Germany) in patients with intracranial space-occupying lesions.

METHODS

We used transcranial duplex sonography to examine 10 healthy volunteers and 4 patients. Of the patients, one 55-year-old woman had an intracranial glioblastoma; one 54-year-old woman had an intracranial hemorrhage; and one 49-year-old woman and one 69-year-old man had a malignant middle cerebral artery infarction. A decompressive craniectomy was performed in the 2 patients with malignant middle cerebral artery infarction. Triggered images with pulsing intervals of 1000 milliseconds were used for the evaluation of time-intensity curves in several regions of interest. The mechanical index was set at 1.6; in patients with a craniectomy, the mechanical index was set at 1.1.

RESULTS

In all patients, the perfusion deficit could be recognized in the ipsilateral hemisphere. The superimposition of the sonographic images with those from computed tomography or magnetic resonance imaging showed a good correspondence in shape and size in patients with a craniectomy. In patients without a craniectomy, a rough correspondence with findings from magnetic resonance imaging or computed tomography could be recognized.

CONCLUSIONS

By using contrast-enhanced transcranial duplex sonography, it is possible to image the perfusion deficit in cerebral microcirculation in patients with intracranial space-occupying lesions. These results should be confirmed by more pathologic cases and correlated with magnetic resonance imaging and other neuroimaging techniques. Additionally, further technical development in sonographic systems is necessary to improve the diagnostics of cerebral perfusion deficit.

Semiquantitative analysis of ultrasonic cerebral perfusion imaging

The bolus kinetic in ultrasonic cerebral perfusion imaging is the most favored data acquisition and processing technique. However, there has not yet been convincing evidence for the potential to (semi-) quantitatively describe perfusion. Aim of this study was to determine the intraindividual range of relevant perfusion parameters to describe individual physiological cutoff scores. In 20 healthy volunteers, cerebral perfusion was evaluated using the bilateral approach with phase inversion harmonic imaging and the bolus kinetic. Relevant parameters (time-to-peak intensity, TPI; peak width, PW) were derived in 14 regions-of-interest in both hemispheres. The median and quartile deviation (QD) of these values were individually calculated. Within the 20 individuals, the mean QD of TPI was 0.68 s, and there was no case in which any TPI exceeded the mean more than 2 s. With PW, the mean QD was 1.2 s, and the mean was not exceeded by more than 6 s. Intraindividual perfusion parameters, especially TPI, show a considerable small range. Thus, the bolus kinetic derives reliable semiquantitative information once intraindividual comparison can be accomplished. We therefore propose that bilateral examination with the unaffected hemisphere as referential region should be performed in acute stroke. Future studies have to evaluate the potential of this approach of discriminating ischemia and hypoperfusion in the affected hemisphere.

Multiplanar transcranial ultrasound imaging: standards, landmarks and correlation with magnetic resonance imaging

The purpose of this study was to define a standardized multiplanar approach for transcranial ultrasound (US) imaging of brain parenchyma based on matched data from 3-D US and 3-D magnetic resonance imaging (MRI). The potential and limitations of multiple insonation planes in transverse and coronal orientation were evaluated for the visualization of intracranial landmarks in 60 healthy individuals (18 to 83 years old, mean 41.4 years) with sufficient temporal bone windows. Landmarks regularly visualized even in moderate sonographic conditions with identification rates of >75% were mesencephalon, pons, third ventricle, lateral ventricles, falx, thalamus, basal ganglia, pineal gland and temporal lobe. Identification of medulla oblongata, fourth ventricle, cerebellar structures, hippocampus, insula, frontal, parietal and occipital lobes was more difficult (<75%). We hypothesize that multiplanar transcranial US images, with standardized specification of tilt angles and orientation, not only allow comparison with other neuroimaging modalities, but may also provide a more objective framework for US monitoring of cerebral disease than freehand scanning.

Perfusion imaging of high-grade gliomas: a comparison between contrast harmonic and magnetic resonance imaging

✓ Transcranial contrast harmonic (CH) imaging is emerging as a promising tool for the evaluation of brain perfusion. The authors report on two cases of histologically proven high-grade gliomas evaluated using CH imaging in comparison to perfusion magnetic resonance (pMR) imaging. In both cases, pMR imaging results demonstrated a massive decrease in signal intensity and an elevated regional cerebral blood volume (rCBV) in the tumor region; however, signal decrease was less prominent and rCBV was lower in healthy brain tissue. In one patient, the rCBV ratio of tumor/brain was 5.0 and the maximal signal decay occurred 3.1 times deeper in the tumor than in the healthy brain tissue. Results of an ultrasonography examination using CH imaging revealed similar data: the tumor/brain ratio for the area under the curve, a parameter corresponding to rCBV, was 4.1. The maximal signal intensity in the tumor was 3.3 times greater than in adjacent healthy brain. Comparable data were obtained in a second patient. Taken together, these findings indicate that CH imaging may be a valuable alternative to pMR imaging. This new, cost-effective bedside ultrasonic technique could be helpful not only as a means of noninvasive staging of gliomas but also as a follow-up imaging modality to evaluate postoperative tumor recurrence or response to antiangiogenic therapy.

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.

Ultrasound Microbubble Destruction Imaging in Acute Middle Cerebral Artery Stroke

Background and Purpose— Cerebral perfusion imaging in acute stroke assists in determining the subtype and the severity of ischemia. Recent studies in perfusion models and in healthy volunteers have shown that ultrasound perfusion imaging based on microbubble destruction can be used to assess tissue perfusion. We applied ultrasound microbubble destruction imaging (MDI) to identify perfusion deficits in patients with acute middle cerebral artery (MCA) territory stroke.

Methods— Fifteen acute MCA stroke patients with sufficient transtemporal bone windows were investigated with ultrasound MDI and perfusion-weighted MRI (PWI). MDI was performed using power pulse-inversion contrast harmonic imaging. Thirty seconds after a bolus injection of the echo contrast agent SonoVue, microbubbles were destroyed using a series of high-energy pulses. Local perfusion status was analyzed in selected regions of interest by destruction curves and acoustic intensity differences (ΔI) before and after microbubble destruction. Local perfusion status was then compared with perfusion compromise as identified on PWI.

Results— The mean differences of acoustic intensity from the ischemic MCA territory were significantly diminished compared with the normal hemisphere (ΔI=2.52±1.75 versus ΔI=13.79±7.31; P <0.001), resulting in lower slopes of microbubble destruction. PWI confirmed perfusion changes in the selected anatomical regions on time-to-peak maps in all 15 patients.

Conclusions— MDI is a qualitative method that can rapidly detect perfusion changes in acute stroke. When combined with other ultrasound techniques and PWI, it may well be valuable in the care of stroke unit patients, eg, as a screening method and for follow-up assessments of perfusion deficits.

Ultrasound perfusion imaging in acute middle cerebral artery infarction predicts outcome

BACKGROUND AND PURPOSE

Initial reports indicate that transcranial harmonic imaging after ultrasound contrast agent bolus injection (BHI) can detect cerebral perfusion deficits in acute ischemic stroke. We evaluated parametric images of the bolus washout kinetics.

METHODS

Twenty-three patients with acute internal carotid artery infarction were investigated with perfusion harmonic imaging after SonoVue bolus injection < or =40 hour after the onset of symptoms. The findings were compared with those of cranial computed tomography (CCT) and clinical course 4 months after stroke.

RESULTS

Images of pixel-wise peak intensity (PPI) and time to peak intensity could be calculated for all patients. Spearman rank correlations of r=0.772 (P<0.001) and r=0.572 (P=0.008) between area of PPI signal decrease and area of infarction in the follow-up CCT as well as outcome after 4 months were obtained, respectively.

CONCLUSIONS

In the early phase of acute ischemic stroke, BHI after SonoVue bolus injection is a useful ultrasound tool for analyzing cerebral perfusion deficits at the patient's bedside. BHI data correlate with the definite area of infarction and outcome after 4 months.

Comparison of Transcranial Brain Tissue Perfusion Images Between Ultraharmonic, Second Harmonic, and Power Harmonic Imaging

Background and Purpose— To clarify optimal brain tissue perfusion images visualized by transcranial ultrasound harmonic imaging, we compared gray-scale integrated backscatter (IBS) images of new ultraharmonic imaging (UHI) and conventional second harmonic imaging (SHI) with power harmonic imaging (PHI) (harmonic B-mode with harmonic power Doppler images) in 10 patients with and 4 without a temporal skull.

Methods— Using a SONOS 5500 (Philips), we evaluated transient response images taken after a bolus Levovist injection at a horizontal diencephalic plane via temporal windows. Based on transmitting/receiving frequencies (MHz), 4 imaging procedures using an S3 transducer (SHI2.6 [1.3/2.6], UHI [1.3/3.6], PHI2.6 [1.3/2.6], and PHI3.2 [1.6/3.2]) and 2 imaging procedures using an S4 transducer (SHI3.6 [1.8/3.6] and PHI3.6 [1.8/3.6]) were compared in terms of size and location, peak intensity (PI), contrast area demarcation, and background image quality.

Results— In intact skull cases, gray-scale imaging tended to show larger contrast areas than PHI. A large contrast area was most frequently observed in SHI2.6 images, despite there being more high-PI cases in UHI. No contrast area with unclear background was observed in a few cases. In craniectomized cases, all contrast images tended to have large and high PI compared with the intact skull cases. PHI, particularly PHI3.6, demonstrated sharper demarcation and a clearer background than gray-scale imaging.

Conclusions— Transcranial gray-scale SHI using a low receiving frequency of 2.6 MHz is the superior method. PHI identifies contrast area localization better than gray-scale imaging and is particularly suitable for intraoperative and postoperative cases.

Parametric Perfusion Imaging With Contrast-Enhanced Ultrasound in Acute Ischemic Stroke

Background and Purpose— Color-coded perfusion maps can be calculated from ultrasound harmonic gray-scale imaging data after ultrasound contrast agent bolus injection to analyze brain tissue perfusion. First reports indicate that this method can display cerebral perfusion deficits in acute ischemic stroke. We performed a prospective patient study to evaluate this approach.

Methods— Thirty consecutive patients suffering from acute middle cerebral artery infarction who presented to our department within 12 hours after symptom onset were investigated with ultrasound perfusion harmonic imaging (PHI) after Levovist bolus injection. Color-coded perfusion maps were calculated from the ultrasound data. In addition, the original gray-scale images were analyzed in cine mode. Findings were compared with those of cranial CT.

Results— All 30 patients suffered from acute ischemic stroke of the middle cerebral artery territory (median National Institutes of Health Stroke Scale score, 16 points). Twenty-three of the 30 patients (76.7%) had sufficient PHI insonation conditions. In 19 of these 23 patients (82.6%), a marked deficit in contrast enhancement could be visualized by initial PHI with the color-coded parameter images and cine-mode images. In 17 of the 23 (73.9%), the perfusion deficit was found on the parameter images. The area of hypoperfusion in the initial PHI investigation corresponded to the definite area of infarction in follow-up cranial CT. In 3 of 23 patients (13.0%), a perfusion deficit could be demonstrated in PHI, although the supplying artery was found patent by transcranial color-coded duplex sonography.

Conclusions— With PHI, it is possible to display cerebral perfusion deficits in acute ischemic stroke. PHI yields additional information on the perfusion state of the human brain compared with extracranial and transcranial color-coded duplex sonography.

Parameters of cerebral perfusion in phase-inversion harmonic imaging (PIHI) ultrasound examinations

The aim was to evaluate phase-inversion harmonic imaging (PIHI) with respect to brain perfusion imaging using a novel "bilateral approach" (depth of examination: 150 mm) and established unilateral approach (100 mm). After bolus injection of two contrast agents (CA, Optison and SonoVue), perfusion-related parameters (time-to-peak intensity, Itpk, peak intensity, Ipk, and peak width, Wpk) were extracted by fitting a model function to time-intensity curves for different regions-of-interest (ROI) in 14 volunteers. In 207 (92%) of 224 ipsilateral ROIs and in 165 (98%) of 168 contralateral ROIs (372 or 95% of 392 altogether), parameters could be derived. Itpk and Wpk of gray matter ROIs did not vary in or between both CA groups (18.1-21.9 s and 7.9-14.2 s). ROIs within arteries showed significantly shorter Itpk (16.1-16.7 s) and longer Wpk (12.8-28.3 s). Level of significance was 0.05 (two-sided). Newer CAs are usable for nonlinear imaging over a wider range of acoustic intensities, so that sensitivity of PIHI is sufficient to image the brain bilaterally. This approach proves to be reliable in patients with adequate bone windows. For acute stroke patients, this implies that both hemispheres can be compared in one instead of two examinations, reducing time of examination by 50%. Furthermore, evaluation of regions close to the probe becomes possible. Thus, the "bilateral approach" should be considered as a new standard approach of acute ultrasonic perfusion imaging.

Perfusion harmonic imaging in acute middle cerebral artery infarction

Initial reports indicate that cerebral perfusion deficits in acute ischemic stroke might be detectable by means of transcranial harmonic imaging after an ultrasound contrast agent (UCA) bolus injection. Twenty-four patients with acute middle cerebral artery (MCA) infarction were investigated twice with perfusion harmonic imaging (PHI) after Levovist (Schering, Berlin, Germany) bolus injection no longer than 12 h after symptom onset. The findings were compared with those of cranial computed tomography (CCT). All 24 patients suffered from acute ischemic stroke of the MCA territory (median National Institutes of Health Stroke Scale score: 15 points). Corresponding to the area of infarction in follow-up CCT, a marked contrast deficit was visualized in 19 of 24 patients by initial PHI, which had a sensitivity and specificity of 86.4% and 96.2%, respectively, for predicting the occurrence and localization of a definite infarction in the midthalamic plane. The area of hypoperfusion in the initial PHI investigation correlated with the definite area of infarction in follow-up CCT (r=0.66, p<0.01). When time-intensity curves of both hemispheres were compared, the areas under the curve were significantly less in the symptomatic brain regions (p=0.01). With PHI and UCA bolus injection, it is possible to assess cerebral perfusion deficits that correlate with the definite area of infarction in acute ischemic stroke patients.

Second harmonic imaging: a new ultrasound technique to assess human brain tumour perfusion

BACKGROUND

Second harmonic imaging is a new ultrasound technique that allows evaluation of brain tissue perfusion after application of an ultrasound contrast agent.

OBJECTIVE

To evaluate the potential of this technique for the assessment of abnormal echo contrast characteristics of different brain tumours.

METHODS

27 patients with brain tumours were studied. These were divided into four groups: gliomas, WHO grade III-IV (n = 6); meningiomas (n = 9); metastases (n = 5); and others (n = 7). Patients were examined by second harmonic imaging in a transverse axial insonation plane using the transtemporal approach. Following intravenous administration of 4 g (400 mg/ml) of a galactose based echo contrast agent, 62 time triggered images (one image per 2.5 seconds) were recorded and analysed off-line. Time-intensity curves of two regions of interest (tumour tissue and healthy brain tissue), including peak intensity (PI) (dB), time to peak intensity (TP) (s), and positive gradient (PG) (dB/s), as well as ratios of the peak intensities of the two regions of interest, were derived from the data and compared intraindividually and interindividually.

RESULTS

After administration of the contrast agent a marked enhancement of echo contrast was visible in the tumour tissue in all patients. Mean PI and PG were significantly higher in tumour tissue than in healthy brain parenchyma (11.8 v 5.1 dB and 0.69 v 0.16 dB/s; p < 0.001). TP did not differ significantly (37.1 v 50.2 s; p = 0.14). A tendency towards higher PI and PG as well as shorter TP was apparent in malignant gliomas. When comparing different tumour types, however, none of these variables reached significance, nor were there significant differences between malignant and benign tumours in general.

CONCLUSIONS

Second harmonic imaging not only allows identification of brain tumours, but may also help in distinguishing between different tumour types. It gives additional and alternative information about tumour perfusion. Further studies are needed to evaluate the clinical potential of this technique in investigating brain tumours-for example in follow up investigations of patients undergoing radiation or chemotherapy-especially in comparison with neuroradiological and neuropathological findings.

Acetazolamide vasoreactivity evaluated by transcranial harmonic perfusion imaging: relationship with transcranial Doppler sonography and dynamic CT

To establish the reliability and clinical significance of transcranial ultrasonic harmonic perfusion imaging (HPI), we evaluated HPI's relationships with transcranial Doppler (TCD) and with dynamic CT (DCT), during acetazolamide (ACZ) vasoreactivity tests.

METHODS

The subjects were 12 neurological patients. Time-averaged maximum velocity (TAVMX) in the middle (MCA) and posterior cerebral arteries was measured by TCD. Time-intensity (-density) curves of HPI (DCT) after bolus intravenous contrast injections were created in 3 regions of interest (ROI) on the axial plane involving the temporal lobe, basal ganglia, and thalamus on both sides. Assessments of vasoreactivity were based on comparisons conducted before and after ACZ administration in terms of: a) relative changes (%delta) of the TCD TAVMX, b) HPI contrast area enlargement, c) %delta of calculated cerebral blood volume and flow of the HPI and DCT.

RESULTS

1) TCD vasoreactivity decrease in the left MCA tended to correlate with lower frequency of HPI contrast area enlargement on the left side. 2) HPI and DCT vasoreactivity tended to be disturbed in the same side ROIs.

CONCLUSIONS

Transcranial HPI achieves repeatable non-invasive bedside evaluation of cerebrovascular reserve capacity through qualitative and quantitative measurements of brain tissue perfusion, and will have clinical value in pathophysiological follow-up and therapeutic effectiveness determination of neurointensive care patients.

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.

Comparison between echo contrast agent-specific imaging modes and perfusion-weighted magnetic resonance imaging for the assessment of brain perfusion

BACKGROUND AND PURPOSE

Contrast burst imaging (CBI) and time variance imaging (TVI) are new ultrasonic imaging modes enabling the visualization of intravenously injected echo contrast agents in brain parenchyma. The aim of this study was to compare the quantitative ultrasonic data with corresponding perfusion-weighted MRI data (p-MRI) with respect to the assessment of brain perfusion.

METHODS

Twelve individuals with no vascular abnormalities were examined by CBI and TVI after an intravenous bolus injection of 4 g galactose-based microbubble suspension (Levovist) in a concentration of 400 mg/mL. Complementary, a dynamic susceptibility contrast MRI, ie, p-MRI, of each individual was obtained. In both ultrasound (US) methods and p-MRI, time-intensity curves were calculated offline, and absolute time to peak intensities (TPI), peak intensities (PI), and peak width (PW) of US investigations and TPI, relative cerebral blood flow (CBF) and relative cerebral blood volume (CBV) of p-MRI examinations were determined in the following regions of interest (ROIs): lentiform nucleus (LN), white matter (WM), posterior (PT), and anterior thalamus (AT). In addition, the M(2) segment of the middle cerebral artery (MCA) was evaluated in the US, and the precentral gyrus (PG) was examined in the p-MRI examinations. In relation to a reference parenchymal ROI (AT), relative TPIs were compared between the US and p-MRI methods and relative PI of US investigations with the ratio of CBF (rCBF) of p-MRI examinations in identical ROIs.

RESULTS

Mean TPIs varied from 18.3+/-5.0 (AT) to 20.1+/- 5.8 (WM) to 17.2+/-4.9 (MCA) seconds in CBI examinations and from 19.4+/-5.3 (AT) to 20.4+/-4.3 (WM) to 17.3+/-4.0 (MCA) seconds in TVI examinations. Mean PIs were found to vary from 581.9+/-342.4 (WM) to 1522.9+/-574.2 (LN) to 3400.9+/- 621.7 arbitrary units (MCA) in CBI mode and from 7.5+/-4.6 (WM) to 17.5+/-4.9 (LN) to 46.3+/-7.1 (MCA) arbitrary units in TVI mode. PW ranged from 7.3+/-4.5 (AT) to 9.1+/-4.0 (LN) to 24.3+/-12.8 (MCA) seconds in CBI examinations and from 7.1+/-3.9 (AT) to 8.7+/-3.5 (LN) to 26.7+/-18.2 (MCA) seconds in TVI examinations. Mean TPI was significantly shorter and mean PI and mean PW were significantly higher in the MCA compared with all other ROIs (P<0.05). Mean TPI of the p-MRI examinations ranged from 22.0+/-6.9 (LN) to 23.0+/-6.8 (WM) seconds; mean CBF ranged from 0.0093+/- 0.0041 (LN) to 0.0043+/-0.0021 (WM). There was no significant difference in rTPI in any ROI between US and p-MRI measurements (P>0.2), whereas relative PIs were significantly higher in areas with lower insonation depth such as the LN compared with rCBF.

CONCLUSIONS

In contrast to PI, TPI and rTPI in US techniques are robust parameters for the evaluation of cerebral perfusion and may help to differentiate physiological and pathological perfusion in different parenchymal regions of the brain.

Targeted imaging using ultrasound

The discipline of medical imaging is expanding to include both traditional anatomic modalities and new techniques for the functional assessment of the presence and extent of disease. Current FDA-approved ultrasound contrast agents are micron-sized bubbles with a stabilizing shell. Microbubble contrast agents can be used to estimate microvascular flow rate in a manner similar to dynamic contrast-enhanced magnetic resonance imaging (MRI). The concentration of these agents within the vasculature, reticulo-endothelial, or lymphatic systems produces an effective passive targeting of these areas. Liquid-filled nanoparticles and liposomes have also demonstrated echogenicity and are under evaluation as ultrasound contrast agents. Actively targeted ultrasound relies on specially designed contrast agents to localize the targeted molecular signature or physiologic system. These agents typically remain within the vascular space, and therefore possible targets include molecular markers on thrombus, endothelial cells, and leukocytes. The purpose of this review is to summarize the requirements, challenges, current progress, and future directions of targeted imaging with ultrasound.

Second harmonic imaging of the human brain: the practicability of coronal insonation planes and alternative perfusion parameters

BACKGROUND AND PURPOSE

Second harmonic imaging (SHI) is a novel ultrasound technique that allows the evaluation of brain tissue perfusion. The purpose of this study was to assess normal cerebral echo contrast characteristics in 3 regions of interest (ROIs) in the transverse axial and coronal insonation planes through the temporal bone window. Materials and Methods- SHI examinations were performed in 25 patients without cerebrovascular disease (aged 50+/-19 years) in a transverse axial and a coronal diencephalic insonation plane through the temporal bone window. After intravenous administration of 2.5 g (400 mg/mL) of a galactose-based echo contrast agent, 62 time-triggered images with a transmission rate of 1 frame per 2.5 seconds were recorded for offline analysis. Time-intensity curves, including peak intensity (PI) (dB) and positive gradient (PG) (dB/s), were calculated to quantify ultrasound intensity in 3 different ROIs in both planes of the following sections: the thalamus (ROI(thal)), the lentiform nucleus (ROI(ncl)), and the area supplied by the middle cerebral artery (ROI(mca)).

RESULTS

Characteristic time-intensity curves with high PIs and steep PGs were recorded in each ROI. Statistical analysis of the aforementioned parameters showed no significant difference for comparison of the 3 ROIs in the transverse axial versus the coronal insonation plane. Comparison of different ROIs in the transverse axial insonation plane revealed that PI was significantly higher in ROI(thal) than in ROI(mca) (7.8 versus 5.5 dB; P<0.05) and significantly higher in ROI(ncl) than in ROI(thal) (9.3 versus 7.8 dB; P<0.05). In contrast, PG was comparable in ROI(thal) and in ROI(mca) (0.21 versus 0.25 dB/s; P=0.42).

CONCLUSIONS

SHI is a promising technique for the evaluation of cerebral parenchymal perfusion. Comparison of the transverse axial and coronal insonation planes shows similar time-intensity curves with comparable values for PIs and PGs. Coronal insonation allows the evaluation of perfusion abnormalities near the vertex and skull base, areas that cannot be depicted in the transverse axial plane. Comparison of the different ROIs indicates that the PG is a more robust and reliable parameter than the PI.

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.

Clinical application of transcranial colour-coded duplex sonography--a review

Transcranial colour-coded duplex sonography (TCCS) is a new and non-invasive ultrasound application that combines both imaging of intracranial vessels and parenchymal structures at a high spatial resolution. This manuscript reviews the clinical applications of TCCS with focus on its diagnostic abilities in acute stroke patients. Furthermore, new experimental imaging techniques are discussed.

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.