Contrast burst depletion imaging (CODIM): a new imaging procedure and analysis method for semiquantitative ultrasonic perfusion imaging

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.

Molecular Ultrasound Imaging for the Detection of Neural Inflammation: A Longitudinal Dosing Pilot Study

Molecular ultrasound imaging provides the ability to detect physiologic processes noninvasively by targeting a variety of biomarkers in vivo. The current study was performed by exploiting an inflammatory biomarker, P-selectin, known to be present following spinal cord injury. Using a murine model (n = 6), molecular ultrasound imaging was performed using contrast microbubbles modified to target and adhere to P-selectin, prior to spinal cord injury (0D), acute stage postinjury (7D), and chronic stage (42D). Additionally, two imaging sessions were performed on each subject at specific time points, using doses of 30 μL and 100 μL. Upon analysis, targeted contrast analysis parameters were appreciably increased during the 7D scan compared with the 42D scan, without statistical significance. When examining the dose levels, the 30-μL dose demonstrated greater values than the 100-μL dose but lacked statistical significance. These findings provide additional preclinical evidence for the use of molecular ultrasound imaging for the possible detection of inflammation.

Targeted Contrast-Enhanced Ultrasound for Inflammation Detection

Molecular imaging is a form of nanotechnology that enables the noninvasive examination of biological processes in vivo. Radiopharmaceutical agents are used to target biochemical markers, permitting their detection and evaluation. Early visualization of molecular variations indicative of pathophysiological processes can aid in patient diagnoses and management decisions. Molecular imaging is performed by introducing into the body molecular probes, which are often contrast agents that have been nanoengineered to target and tether to molecules, thus enabling their radiologic identification. Through a nanoengineering process, ultrasound contrast agents can be targeted to specific molecules, extending ultrasound’s capabilities from the tissue to molecular level. Molecular ultrasound, or targeted contrast-enhanced ultrasound (TCEUS), has recently emerged as a popular molecular imaging technique due to its ability to provide real-time anatomic and functional information without ionizing radiation. However, molecular ultrasound represents a novel form of molecular imaging and consequently remains largely preclinical. This review explores the commonalities of TCEUS across several molecular targets and points to the need for standardization of kinetic behavior analysis. The literature underscores evidence gaps and the need for additional research. The application of TCEUS is unlimited but needs further standardization to ensure that future research studies are comparable.

Exploring Targeted Contrast-Enhanced Ultrasound to Detect Neural Inflammation

Targeted contrast-enhanced ultrasound (TCEUS) is an innovative method of molecular imaging used for detection of inflammatory biomarkers in vivo. By targeting ultrasound contrast to cell adhesion molecules (CAMs), which are known inflammatory markers within neural tissue, a more direct evaluation of neural inflammation can be made. Due to the novel nature of TCEUS, standardized methods of image analysis do not yet exist. Time intensity curve (TIC) shape analysis is currently used in magnetic resonance contrast imaging to determine temporal behavior of perfusion. Therefore, the presented research attempts to determine TIC shape analysis utility in TCEUS imaging by applying it to TCEUS scans targeted to CAMs present in neural inflammation. This was done in an animal model that underwent a traumatic spinal cord injury to induce inflammation ( n = 31). Subjects were divided into four groups, each receiving a TCEUS targeted to a different CAM seven days after surgery (P-selectin, intracellular adhesion molecule 1 [ICAM-1], vascular cell adhesion molecule 1 [VCAM-1], and control). TICs were generated using average pixel intensity within the injured region of the spinal cord. TIC shape analysis found similar curves were produced while targeting P-selectin and VCAM-1, both demonstrating rapid and sustained enhancement. Control injections demonstrated no enhancement. ICAM-1 injections demonstrated limited enhancement and a shape similar to the control.

Automatic motion estimation using flow parameters for dynamic contrast-enhanced ultrasound

Dynamic contrast-enhanced ultrasound (DCE-US) sequences are subject to motion which can disturb functional flow quantification. This can make estimated parameters more variable or unreliable. Methods that compensate for motion are therefore desirable. The most commonly used motion correction techniques in DCE-US register the images in the sequence with respect to a user-selected reference image. However, this image may not include all features that are representative of the whole sequence. Moreover, image-based registration neglects pertinent, functional-flow information contained in the DCE-US sequence. An operator-free method is proposed that combines the motion estimation and flow-parameter quantification (M/Q method) in a single mathematical framework. This method is based on a realistic multiplicative model of the DCE-US noise. By computing likelihood in this model, motion and flow parameters are both estimated iteratively. First, the maximization is accomplished by estimating functional and motion parameters. Then, a final registration based on a non-parametric temporal smoothing of the sequence is performed. This method is compared to a conventional (mutual information) registration method where all the images of the sequence are registered with respect to a reference image chosen by an expert. The two methods are evaluated on simulated sequences and DCE-US sequences acquired in patients (N = 15). The M/Q method demonstrates significantly (p < 0.05) lower Dice coefficients and Hausdorff distance than the conventional method on the simulated data sets. On the in vivo sequences analysed, the M/Q methods outperformed the conventional method in terms of mean Dice and Hausdorff distance on 80% of the sequences, and in terms of standard deviation of Dice and Hausdorff distance on 87% of the sequences.

A multiplicative model for improving microvascular flow estimation in dynamic contrast-enhanced ultrasound (DCE-US): theory and experimental validation

Perfusion parameter estimation from dynamic contrast-enhanced ultrasound (DCE-US) data relies on fitting parametric models of flow to curves describing linear echo power as a function of time. The least squares criterion is generally used to fit these models to data. This criterion is optimal in the sense of maximum likelihood under the assumption of an additive white Gaussian noise. In the current work, it is demonstrated that this assumption is not held for DCEUS. A better-adapted maximum likelihood criterion based on a multiplicative model is proposed. It is tested on simulated bolus perfusion data and on 11 sequences acquired in vivo during bolus perfusion of contrast agent in the cortex of healthy murine kidney, an area where the perfusion is expected to be approximately homogeneous. Results on simulated data show a significant improvement (p < 0.05) of the precision and the accuracy for the estimations of perfusion parameters time to peak (TTP), wash-in rate (WiR), and mean transit time (MTT). On the 11 in vivo sequences, the new method leads to a significant reduction (p < 0.05) in the variation of parametric maps for 9 sequences for TTP and 10 sequences for WiR and MTT. The mean percent decreases of the coefficient of variation are 40%, 25%, and 59% for TTP, WiR, and MTT, respectively. This method should contribute to a more robust and accurate estimation of perfusion parameters and an improved resolution of parametric imaging.

Flow quantification with nakagami parametric imaging for suppressing contrast microbubbles attenuation

Flow quantification with contrast-enhanced ultrasound is still limited by the effects of contrast microbubble attenuation. Nakagami parametric imaging (NPI) based on the m parameter, which is related to the statistical property of echo envelope, is implemented to suppress contrast attenuation. Flow velocity (FV) and volumetric flow rate (VFR) are estimated through the least square fitting of burst depletion kinetic model to time m parameter curves (TMCs). A non-recirculating flow phantom is imaged as contrast microbubbles are infused at 10, 15, 20, 25, and 30 mL/min. Contrast microbubbles with two different concentrations are used to generate variations of contrast microbubble attenuation. The results suggest that 4 × 4 mm(2) is the optimal size of a sliding window of NPI for flow quantification under current experiment condition. At a lower microbubble concentration, the FV calculated from TMCs correlates strongly with actual FV in both unattenuated (R(2) = 0.97; p < 0.01) and attenuated regions (R(2) = 0.92; p < 0.01) within phantom. And there is a strong correlation (R(2) = 0.98; p < 0.01; slope = 0.96; intercept = 0.68) between VFR calculated from TMCs and actual VFR within the whole phantom. Similar results are obtained at higher microbubble concentrations. Compared with conventional ultrasound imaging that is intensity dependent, NPI achieves better performance on flow quantification in the presence of contrast microbubble attenuation.

An automatic respiratory gating method for the improvement of microcirculation evaluation: application to contrast-enhanced ultrasound studies of focal liver lesions

Contrast-enhanced ultrasound (CEUS), with the recent development of both contrast-specific imaging modalities and microbubble-based contrast agents, allows noninvasive quantification of microcirculation in vivo. Nevertheless, functional parameters obtained by modeling contrast uptake kinetics could be impaired by respiratory motion. Accordingly, we developed an automatic respiratory gating method and tested it on 35 CEUS hepatic datasets with focal lesions. Each dataset included fundamental mode and cadence contrast pulse sequencing (CPS) mode sequences acquired simultaneously. The developed method consisted in (1) the estimation of the respiratory kinetics as a linear combination of the first components provided by a principal components analysis constrained by a prior knowledge on the respiratory rate in the frequency domain, (2) the automated generation of two respiratory-gated subsequences from the CPS mode sequence by detecting end-of-inspiration and end-of-expiration phases from the respiratory kinetics. The fundamental mode enabled a more reliable estimation of the respiratory kinetics than the CPS mode. The k-means algorithm was applied on both the original CPS mode sequences and the respiratory-gated subsequences resulting in clustering maps and associated mean kinetics. Our respiratory gating process allowed better superimposition of manually drawn lesion contours on k-means clustering maps as well as substantial improvement of the quality of contrast uptake kinetics. While the quality of maps and kinetics was satisfactory in only 11/35 datasets before gating, it was satisfactory in 34/35 datasets after gating. Moreover, noise amplitude estimated within the delineated lesions was reduced from 62 ± 21 to 40 ± 10 (p < 0.01) after gating. These findings were supported by the low residual horizontal (0.44 ± 0.29 mm) and vertical (0.15 ± 0.16 mm) shifts found during manual motion correction of each respiratory-gated subsequence. The developed technique could be used as a basis for accurate quantification of perfusion parameters for the evaluation and follow-up of patients under antiangiogenic therapies.

The assessment of microvascular flow and tissue perfusion using ultrasound imaging

Imaging microvascular flow is of diagnostic value for a wide range of diseases including cancer, inflammation, and cardiovascular disease. The introduction of microbubbles as ultrasound contrast agents offers significant signal enhancement to the otherwise weakly scattered signal from blood in the circulation. Microbubbles provide maximum impedance mismatch, but are not linear scatterers. Their complex response to ultrasound has generated research on both their behaviour and their scattered-signal processing. Nearly 20 years ago signal processing started with simple spectral filtering of harmonics showing contrast-enhanced images. More recent pulse encoding techniques have achieved good cancellation of tissue echoes. The good quality contrast-only images enabled ultrasound contrast-imaging applications to be established in microvascular measurements in the liver and the myocardium. The field promises to advance the quantification of microvascular flow kinetics.

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.

Ultrasonic contrast agents in transcranial perfusion sonography (TPS) for follow-up of patients with high grade gliomas

PURPOSE

The aim of this study was to evaluate brain perfusion differences in patients with high grade gliomas after partial tumor resection and irradiation/chemotherapy between tumor and non-tumor hemisphere by transcranial perfusion sonography (TPS) employing a contrast burst imaging (CBI) technique.

METHODS

Six patients with glioblastoma (WHO Grade IV) in the temporoparietal region within the defined axial diencephalic scanning plane were examined by TPS during follow-up. All subjects had an adequate acoustic temporal bone window. Transtemporal insonation on brain tumor and non-tumor hemisphere was performed with a bolus-injection of sulphur hexafluoride-based contrast agent (10 mg i.v., 5mg/ml--SonoVue, Bracco, Altana, Switzerland). Recorded images were analysed off-line by Quanticon Software (3D-Echotech, Munich, Germany) and time intensity curve parameters [area under the curve (AUC, dB s), peak intensity (PI, dB), time to peak (TTP, s)] in five regions of interest (ROI) [thalamus anterior, thalamus posterior, nucleus lentiformis, white matter, whole hemisphere] were evaluated. Statistical analyses were performed.

RESULTS

Perfusion differences between brain tumor and non-tumor hemispheres were detected with contrast burst imaging (CBI) technique with a significantly greater mean AUC (5343.69 dB s vs. 4625.04 dB s, p<0.028) and a significantly prolonged TTP (32.72 s vs. 28.91 s, p<0.046) in the tumor hemisphere.

CONCLUSION

Within our study population, TTP and AUC seem to be the most robust parameters for the evaluation of cerebral perfusion differences assessed by transcranial perfusion sonography with CBI technique. We hypothesize that these results correlate with microvascular changes due to treatment regimens, such as microvessel necrosis after irradiation and chemotherapy. Above that, TPS may be of value for the long-term follow-up of brain tumor therapy concept.

An original methodology for quantitative assessment of perfusion in small animal studies using contrast-enhanced ultrasound

The aim of this paper was to validate the combination of two original methods for assessing perfusion in small animal studies using Contrast-Enhanced Ultrasound. Respiratory motion was first removed using a frame selection method. This method, based on a Principal Component Analysis, enabled the definition of two subsequences, corresponding to the end-of-inspiration plane and to the end-of-expiration plane. Attenuation caused by contrast agent microbubbles was then estimated. The developed method, based on the assumption that a linear relationship holds between local attenuation and backscatter, used the time-intensity variation in a highly echogenic distal area without contrast uptake to estimate local attenuation coefficients. Ten murine renal perfusion studies were investigated after a bolus injection of SonoVue. Replenishment kinetics were acquired too and were finally modeled with an exponential function. Preliminary results were promising. Indeed, the SonoVue concentration was more faithfully represented than on native images, which allowed a better assessment of perfusion parameters in the whole field of view.

Terminology for contrast-enhanced sonography: a practical glossary

OBJECTIVE

The purpose of this glossary is to offer an updated guide to the correct terminology for contrast-enhanced sonography.

METHODS

This report was prepared by a panel of radiologists from the Sonography Section of the Italian Association of Medical Radiology. A leading author prepared a list of terms based on a comprehensive literature survey. The draft was analyzed by 3 experts on the topic of contrast-enhanced sonography. These reviewers reached a consensus and prepared the final version.

RESULTS

A list of 137 terms is included. These terms are briefly defined. Their proper application is discussed, with special reference to potential misleading uses.

CONCLUSIONS

Contrast-enhanced sonography is a relatively new diagnostic tool, now entering clinical practice in several countries. Use of appropriate, universal terminology is mandatory in the scientific setting to allow comparison between different published experiences. Additionally, use of clear, standardized terminology is necessary in the clinical setting to facilitate report understanding by the referring physician. Standardized, nonequivocal nomenclature may also help future diffusion of sonographic contrast media in countries where their application is still not approved.

Acoustic techniques for assessing the Optison destruction threshold

OBJECTIVE

The purpose of this study was to identify the pressure threshold for the destruction of Optison (octafluoropropane contrast agent; Amersham Health, Princeton, NJ) using a laboratory-assembled 3.5-MHz pulsed ultrasound system and a clinical diagnostic ultrasound scanner.

METHODS

A 3.5-MHz focused transducer and a linear array with a center frequency of 6.9 MHz were positioned confocally and at 90 degrees to each other in a tank of deionized water. Suspensions of Optison (5-8x10(4) microbubbles/mL) were insonated with 2-cycle pulses from the 3.5-MHz transducer (peak rarefactional pressure, or Pr, from 0.0, or inactive, to 0.6 MPa) while being interrogated with fundamental B-mode imaging pulses (mechanical index, or MI,=0.04). Scattering received by the 3.5-MHz transducer or the linear array was quantified as mean backscattered intensity or mean digital intensity, respectively, and fit with exponential decay functions (Ae-kt+N, where A+N was the amplitude at time 0; N, background echogenicity; and k, decay constant). By analyzing the decay constants statistically, a pressure threshold for Optison destruction due to acoustically driven diffusion was identified.

RESULTS

The decay constants determined from quantified 3.5-MHz radio frequency data and B-mode images were in good agreement. The peak rarefactional pressure threshold for Optison destruction due to acoustically driven diffusion at 3.5 MHz was 0.15 MPa (MI=0.08). Furthermore, the rate of Optison destruction increased with increasing 3.5-MHz exposure pressure output.

CONCLUSIONS

Optison destruction was quantified with a laboratory-assembled 3.5-MHz ultrasound system and a clinical diagnostic ultrasound scanner. The pressure threshold for acoustically driven diffusion was identified, and 3 distinct mechanisms of ultrasound contrast agent destruction were observed with acoustic techniques.

Angiogenesis: noninvasive quantitative assessment with contrast-enhanced functional US in murine model

PURPOSE

To evaluate quantitative functional ultrasonography (US) in a murine gel model by using microbubble destruction kinetics to determine whether parametric indices provided with US could help assess angiogenesis.

MATERIALS AND METHODS

Institutional Animal Subjects Committee approved experiments and procedures. In 36 normal mice, two 0.4-mL gel implants were placed subcutaneously on either side of spine. One implant contained 0.5, 1.0, or 1.5 microg human basic fibroblast growth factor (bFGF) per milliliter of gel. Functional US quantitative analysis of angiogenesis with microbubble contrast agent was performed on days 3, 6, 9, and 12; histologic data were collected. Time-intensity curve of implant was fitted to mathematic decay model to calculate fractional blood volume and fraction of blood replaced per unit of time. Microvascular density (MVD) and percentage of microvascular area (MVA) were measured after anti-CD31 staining. Spearman rank order correlation was used in analyses.

RESULTS

bFGF-containing implants induced MVD of eight, 35, 42, and 42 vessels per square millimeter on days 3, 6, 9, and 12, respectively; in controls, MVD was four vessels/mm2 (P<.05 on days 6, 9, and 12). bFGF-containing implants induced percentage MVA of 2%, 5%, 20%, and 27%, respectively; in controls, it was 0.5% (P<.05). Maximum enhancement was significantly increased in bFGF implants (23.3 gray level+/-14.1 [standard deviation]) compared with controls (11.0+/-5.5, P<.001). Implants containing bFGF showed poor correlations between fractional blood volume and MVD (r2=0.42) or percentage MVA (r2=0.51) at US. There was no correlation between microbubble velocity and MVD (r2<0.05) or percentage MVA (r2<0.13).

CONCLUSION

Functional US perfusion parameters do not correlate with current histologic indices for quantifying angiogenesis. MVD, as a histologic quantitative measurement of angiogenesis, may not be an appropriate standard for contrast-enhanced imaging that relies on perfused neovessels.

Ultrasonic contrast agent shell rupture detected by inertial cavitation and rebound signals

Determining the rupture pressure threshold of ultrasound contrast agent microbubbles has significant applications for contrast imaging, development of therapeutic agents, and evaluation of potential bioeffects. Using a passive cavitation detector, this work evaluates rupture based on acoustic emissions from single, encapsulated, gas-filled microbubbles. Sinusoidal ultrasound pulses were transmitted into weak solutions of Optison at different center frequencies (0.9, 2.8, and 4.6 MHz), pulse durations (three, five, and seven cycles of the center frequencies), and peak rarefactional pressures (0.07 to 5.39 MPa). Pulse repetition frequency was 10 Hz. Signals detected with a 13-MHz, center-frequency transducer revealed postexcitation acoustic emissions (between 1 and 5 micros after excitation) with broadband spectral content. The observed acoustic emissions were consistent with the acoustic signature that would be anticipated from inertial collapse followed by "rebounds" when a microbubble ruptures and thus generates daughter/free bubbles that grow and collapse. The peak rarefactional pressure threshold for detection of these emissions increased with frequency (e.g., 0.53, 0.87, and 0.99 MPa for 0.9, 2.8, and 4.6 MHz, respectively; five-cycle pulse duration) and decreased with pulse duration. The emissions identified in this work were separated from the excitation in time and spectral content, and provide a novel determination of microbubble shell rupture.

Comparison of different mathematical models to analyze diminution kinetics of ultrasound contrast enhancement in a flow phantom

Ultrasound (US) energy leads to intensity- and frequency-dependent destruction of US contrast agent (UCA) microbubbles. When applying repeated US pulses, this phenomenon can be detected as contrast diminution over time. Contrast diminution kinetics depend on the replenishment of UCA into the sample volume. Thus, it is related to organ perfusion. To analyze the contrast diminution kinetics following pulsed harmonic US application (SONOS 5500, 1.8-3.6 MHz, MI: 1.6, frame rates: 2, 4, and 6.67 Hz), we performed an in vitro study using SonoVue continuous infusion. Seven flow rates (4.5, 9, 13.5, 18, 22.5, 27 and 36 mL/min) were tested. Based on our results, three mathematical models (linear intensity decrease, exponential decay, and an exponential destruction/reperfusion model) describing diminution kinetics were compared. In 113 (89.7%) of 126 trials, a signal decrease was observed after US application. At higher flow rates (18 to 36 mL/min), curve fitting was not possible for the exponential models. For the linear model, intensity decrease depended significantly on the flow rate (p < or = 0.005, n = 7). A logistic model was fitted to the data, defining the slope in the dynamic range of quasilinear dependence for the different frame rates, as well as the inflection point: The higher the frame rate, the higher the flow rate at the point of inflection. For the exponential model, the contrast half-life was dependent on the flow rate (r = 0.95, p = 0.03, n = 6) only at the highest frame rate (6.67 Hz). The perfusion coefficient derived from the destruction/reperfusion model was not significantly related to the flow rate. In conclusion, the linear intensity decrease correlates well with the flow rate (i.e., flow velocity) and defines optimum frame rates for diminution imaging at different flow velocities. The exponential models, which required curve-fitting procedures, were determined to be inappropriate to describe flow in our phantom.

Quantitative evaluation of cerebrovascular reactivity in brain tissue by a refill kinetic method of transcranial ultrasonic perfusion imaging: a comparison with Doppler sonography

To confirm the reliability of a refill kinetic method of ultrasonic harmonic perfusion imaging (HPI) capable of quantifying separate parameters of microvascular blood flow velocity and volume in brain tissue, we evaluated acetazolamide (ACZ) cerebrovascular reactivity by transcranial HPI in comparison with Doppler sonography (TCD). Methods. HPI during continuous Levovist infusion with changing pulsing intervals (t) and TCD time-averaged maximum velocity (TAMX) in the middle and posterior cerebral arteries were evaluated before and after ACZ administration in 12 patients, 8 without and 4 with a temporal skull defect. Plateau value (A) and rise rate (beta) of intensity (I) represented by HPI time-intensity curves of I(t) = A(1 - e(-beta*t)) were analyzed on the axial diencephalic plane.

RESULTS

1) A significantly decreased in proportion to the region of interest location depth only in the intact skull cases. 2) Despite inter- and intra-individual data scattering, in correspondence with TAMX increases after ACZ, significant beta increases were more frequently identified than increases of A.

CONCLUSIONS

Cerebral vasoreactivity analysis utilizing refill kinetics of transcranial HPI can potentially provide separate quantification based on microvascular blood velocity and volume (capillary patency) with consideration of depth-dependant ultrasound attenuation. This should be suitable for bedside evaluation of neurointensive care patients.

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.

Validation of the depletion kinetic in semiquantitative ultrasonographic cerebral perfusion imaging using 2 different techniques of data acquisition

OBJECTIVE

To validate the potential of ultrasonographic depletion imaging for semiquantitatively visualizing cerebral parenchymal perfusion with contrast burst depletion imaging (CODIM) in comparison with phase inversion harmonic depletion imaging (PIDIM) in healthy volunteers.

METHODS

Thirteen healthy adults were examined with both CODIM and PIDIM in accordance with previously described criteria. In addition to the perfusion coefficient, the time to decrease image intensity to 10% above equilibrium intensity from the initial value and the relative error (deviation of measured data from the fitted model) were evaluated to compare the reliability of both techniques in 3 different regions of interest.

RESULTS

Perfusion coefficient values did not show significantly differing values in both groups (1.57-1.64 * 10(-2) s(-1) for CODIM and 1.42-1.58 * 10(-2) s(-1) for PIDIM). The relative error was significantly smaller in the PIDIM group (0.38-0.53 for CODIM and 0.18-0.25 for PIDIM; P < .002).

CONCLUSIONS

Phase inversion harmonic depletion imaging proved to be more reliable than CODIM because values of the relative error were significantly lower in PIDIM even in this relatively small cohort. This is of interest because the underlying technique, phase inversion harmonic imaging, is more widely available than contrast burst imaging.

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.

Low mechanical index contrast-enhanced ultrasound better reflects high arterial perfusion of liver metastases than arterial phase computed tomography

RATIONALE AND OBJECTIVES

We investigated whether observing the arterial vascularization of liver metastases by contrast-enhanced ultrasound with low mechanical index (low-MI) imaging offers additional diagnostic information for the characterization of the liver lesions.

METHODS

Twenty nine patients with untreated liver metastases of different primaries were examined. Measurements were performed using a low frame rate, low-MI pulse inversion technique after injection of 2.4 mL SonoVue. The relative maximum signal intensity of the liver lesions related to the normal liver tissue was quantified. Ultrasound findings were compared with contrast-enhanced, dual-phase computed tomography (CT) using a pattern-based classification scheme.

RESULTS

Compared with contrast-enhanced CT, this modality better detects arterial perfusion. Metastases, even those usually considered hypovascularized, often showed homogeneous enhancement (66%) and higher arterial vascularization than normal liver tissue. CT did not show a comparable vascularization pattern (P < 0.001) or any similarly early signal intensity (P < 0.001).

CONCLUSIONS

Contrast-enhanced CT may not be able to visualize short-lasting but large differences of the arterial perfusion of liver metastases, as does contrast-enhanced low-MI ultrasound. This offers new methods for their characterization and for monitoring of therapeutic effects.

Validation of ultrasound contrast destruction imaging for flow quantification

Our purpose was to validate in vitro a kinetic flow model based on microbubble signal decay curve. Using a 3.5 MHz transducer and phase-inversion (1.8 MHz central transmit frequency), a renal dialysis cartridge oriented vertically was imaged in the transverse plane as 1:1000 dilution of AF0150 was infused at 50, 100, 200, 300 and 400 mL/min. Ten gray-scale images were acquired at each infusion rate using 2.5, 5 and 10 frames/s at 100%, 40%, 15% or 1% of maximum transmit power. Video-intensity measured on each 10 images was fit to a kinetic model using Sigma Plot that yielded microbubble concentration, velocity and destruction per frame. These were correlated with the experimental conditions. At 100% power, video-intensity on the first frame (microbubble concentration at equilibrium) was similar for all flow and frame rates. The model fit the experimental data for all flows at 10 frames/s and for flows lower than 400 and 100 mL/min at 5 frames/s and 2.5 frames/s, respectively. The calculated flow was similar to the experimental flow rates, regardless of technique (r(2) = 0.98). Microbubble fraction destroyed per frame was similar for all flow and frame rates and increased linearly with transmit power (r(2) > 0.98). These results suggest that using appropriate power and frame rate for a given flow rate, estimates of fractional blood volume, flow and destruction fraction can be calculated from the decay curve using 10 frames that can be acquired in 1 to 4 s.

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.

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.