The Doppler kinetics of microbubble echo contrast

Contrast-Enhanced Ultrasound-Guided Fine-Needle Aspiration for Sentinel Lymph Node Biopsy in Early-Stage Breast Cancer

The purpose of this study was to assess whether translymphatic contrast-enhanced ultrasound (CEUS) combined with fine-needle aspiration (FNA) can be used pre-operatively to assess the status of axillary lymph nodes in early-stage breast cancer patients. Furthermore, we wanted to determine whether this less invasive method could potentially be a pre-operative surgical strategy. One hundred sixty-four sentinel lymph nodes (SLNs) were detected by CEUS after intradermal injection of microbubbles in 126 cases. One hundred twenty of 126 cases (95.24%) were accurately diagnosed with the SLN-FNA method. All 6 false-negative cases were due to micrometastasis or macrometastasis. There were no false-positive results after CEUS-guided FNA biopsy based on post-operative histopathological results. In conclusion, translymphatic CEUS combined with SLN-FNA is a less traumatic approach that has high accuracy in the pre-operative evaluation of axillary lymph node status. It might have the potential to be as reliable an indicator for axillary lymph node dissection as SLN biopsy.

Differential Diagnosis of Arterial Phase Enhanced Hepatic Inflammatory Lesions and Hepatocellular Carcinomas with Contrast-enhanced Ultrasound

We aimed to investigate the enhancement patterns of contrast-enhanced ultrasound (CEUS) with SonoVue and determine the utility of this method for differential diagnosis between hepatic inflammatory lesions with arterial phase enhancement and hepatocellular carcinomas (HCC). Twenty-three patients with arterial-enhanced inflammatory liver lesions and 46 HCC patients were included. These lesions had been subjected to CEUS examination and confirmed by pathologic results or imaging follow-up for at least 1 y. In the arterial phase of CEUS, 65.2% of the inflammatory lesions showed patchy (slight enhancement with poorly defined margins) or centripetal enhancement, whereas 89.1% of the HCC lesions showed homogeneous or heterogeneous enhancement (p < 0.001). Moreover, 82.6% of the inflammatory lesions had poorly defined margins, and 78.3% were irregular in shape at the peak, whereas 87.0% of the HCC lesions had well-defined margins and 76.1% were regular (both p < 0.001). Feeding vessels were more frequently visualized in HCCs (71.7%) than in inflammatory lesions (26.1%, p < 0.001). Additionally, 88.2% of the internal non-perfused areas in inflammatory lesions were regular in shape, while 68.0% of these areas in HCCs had an irregular shape (p < 0.001). CEUS pattern analysis provides important information for differentiating inflammatory liver lesions and HCCs and is helpful for improving diagnostic accuracy.

Contrast-enhanced sonography of thyroid nodules

PURPOSE

To investigate the value of contrast-enhanced ultrasound (CEUS) in the differentiation of benign and malignant nodules.

METHODS

One hundred eighty-nine patients with 213 thyroid nodules were enrolled in this retrospective study. All patients underwent preoperative CEUS and were subsequently scheduled for surgery. The pathology results were obtained after surgery. The time-intensity curves were plotted with TomTec software. The quantitative parameters of the time-intensity curve, such as the maximum intensity of peak (IMAX), the rise time from 10% to 90% of the IMAX, and the time to peak, were compared between the benign and malignant nodules.

RESULTS

There were no significant differences in the rise time or the time to peak among thyroid papillary carcinoma, nodular goiter, and follicular adenoma. However, a significant difference was identified in the IMAX.

CONCLUSIONS

A quantitative evaluation of CEUS is helpful to differentiate benign and malignant thyroid nodules.

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.

Regions of interest and parameters for the quantitative analysis of contrast-enhanced ultrasound to evaluate the anti-angiogenic effects of bevacizumab

The aim of the present study was to identify effective regions of interest (ROIs) and parameters for the quantitative analysis of contrast-enhanced ultrasound (CEUS) to evaluate the anti-angiogenic effects of bevacizumab. Thirty mice were subcutaneously injected with CT26 cells and randomly divided into a bevacizumab‑treated (Bev) group and a control group (normal saline-treated). CEUS and quantitative analysis were performed on days 7, 11, 14 and 21 following tumor establishment. ROItotal, which included the whole tumor, and ROIsmall, which included the most enhanced part of the tumor, were selected and outlined. Parameters including time to peak (TTP), maximum intensity (Imax) and area under the curve (AUC; in addition to rates of AUC1, AUC2, AUCfast and AUCslow) were recorded. The tumors were resected on day 21 for microvessel density (MVD) counting. Our results showed that the MVD in the Bev group was significantly lower compared with that in the control group (4.09 vs. 6.41; P=0.001). Additional parameters of ROIsmall were identified to be significantly different between the two groups, compared with those of ROItotal. No significant differences in TTP and Imax were observed between the two groups at the four time‑points examined (P>0.05). For the AUC parameters in ROIsmall, AUC and the rates of AUC2, AUCfast and AUCslow were lower in the Bev group compared with those in the control group on days 7 and 11 (P<0.05). These findings indicate that ROIsmall and AUC parameters in the quantitative analysis of CEUS may be useful for the evaluation of changes in tumor angiogenesis following bevacizumab treatment.

Improving the sensitivity of high-frequency subharmonic imaging with coded excitation: a feasibility study

PURPOSE

Subharmonic intravascular ultrasound imaging (S-IVUS) could visualize the adventitial vasa vasorum, but the high pressure threshold required to incite subharmonic behavior in an ultrasound contrast agent will compromise sensitivity-a trait that has hampered the clinical use of S-IVUS. The purpose of this study was to assess the feasibility of using coded-chirp excitations to improve the sensitivity and axial resolution of S-IVUS.

METHODS

The subharmonic response of Targestar-p(TM), a commercial microbubble ultrasound contrast agent (UCA), to coded-chirp (5%-20% fractional bandwidth) pulses and narrowband sine-burst (4% fractional bandwidth) pulses was assessed, first using computer simulations and then experimentally. Rectangular windowed excitation pulses with pulse durations ranging from 0.25 to 3 μs were used in all studies. All experimental studies were performed with a pair of transducers (20 MHz/10 MHz), both with diameter of 6.35 mm and focal length of 50 mm. The size distribution of the UCA was measured with a Casy(TM) Cell counter.

RESULTS

The simulation predicted a pressure threshold that was an order of magnitude higher than that determined experimentally. However, all other predictions were consistent with the experimental observations. It was predicted that: (1) exciting the agent with chirps would produce stronger subharmonic response relative to those produced by sine-bursts; (2) increasing the fractional bandwidth of coded-chirp excitation would increase the sensitivity of subharmonic imaging; and (3) coded-chirp would increase axial resolution. The experimental results revealed that subharmonic-to-fundamental ratios obtained with chirps were 5.7 dB higher than those produced with sine-bursts of similar duration. The axial resolution achieved with 20% fractional bandwidth chirps was approximately twice that achieved with 4% fractional bandwidth sine-bursts.

CONCLUSIONS

The coded-chirp method is a suitable excitation strategy for subharmonic IVUS imaging. At the 20 MHz transmission frequency and 20% fractional bandwidth, coded-chirp excitation appears to represent the ideal tradeoff between subharmonic strength and axial resolution.

Parametric subharmonic imaging using a commercial intravascular ultrasound scanner: an in vivo feasibility study

OBJECTIVES

The feasibility of visualizing atherosclerotic plaque using parametric subharmonic intravascular ultrasound (IVUS) was investigated in vivo.

METHODS

Atherosclerosis was induced in the aorta of 2 rabbits. Following injection of Definity (Lantheus Medical Imaging, North Billerica, MA), radiofrequency IVUS signals were acquired at 40 MHz with a Galaxy IVUS scanner (Boston Scientific/Scimed, Natick, MA). Subharmonic imaging (SHI; receiving at 20 MHz) was performed offline by applying an 8-order equalization filter. Contrast-to-tissue ratios (CTRs) were computed for the vessel relative to the plaque area over 4 time points. Contrast-to-tissue ratios were also calculated for the plaque-tissue and vessel-tissue from 4 tissue regions of interest at 4 time points. Finally, parametric images showing the cumulative maximum intensity (CMI), time to peak, perfusion (PER), and time-integrated intensity (TII) were generated for the fundamental and subharmonic data sets, and CTR measurements were repeated.

RESULTS

Injection of the contrast agent resulted in improved delineation between plaque and the vessel lumen. Subharmonic imaging resulted in noticeable tissue suppression, although the intensity from the contrast agent was reduced. No significant improvement in the plaque to vessel lumen CTR was observed between the subharmonic and fundamental IVUS (2.1 ± 3.64 versus 2.2 ± 4.20; P = .5). However, the CTR for plaque-tissue was improved (11.8 ± 7.32 versus 9.9 ± 7.06; P < .0001) for SHI relative to fundamental imaging. Cumulative-maximum-intensity and TII maps of both fundamental and subharmonic data provided increased CTRs relative to nonparametric data sets (P < .002). Additionally, the CMI, PER, and TII of SHI IVUS showed significantly improved vessel-plaque CTRs for SHI relative to the fundamental (P < .04).

CONCLUSIONS

Parametric SHI IVUS of atherosclerotic plaque is feasible and improves the visualization of the plaque.

Parametric imaging using subharmonic signals from ultrasound contrast agents in patients with breast lesions

Parametric maps showing perfusion of contrast media can be useful tools for characterizing lesions in breast tissue. In this study we show the feasibility of parametric subharmonic imaging (SHI), which allows imaging of a vascular marker (the ultrasound contrast agent) while providing near complete tissue suppression. Digital SHI clips of 16 breast lesions from 14 women were acquired. Patients were scanned using a modified LOGIQ 9 scanner (GE Healthcare, Waukesha, WI) transmitting/receiving at 4.4/2.2 MHz. Using motion-compensated cumulative maximum intensity (CMI) sequences, parametric maps were generated for each lesion showing the time to peak (TTP), estimated perfusion (EP), and area under the time-intensity curve (AUC). Findings were grouped and compared according to biopsy results as benign lesions (n = 12, including 5 fibroadenomas and 3 cysts) and carcinomas (n = 4). For each lesion CMI, TTP, EP, and AUC parametric images were generated. No significant variations were detected with CMI (P = .80), TTP (P = .35), or AUC (P = .65). A statistically significant variation was detected for the average pixel EP (P = .002). Especially, differences were seen between carcinoma and benign lesions (mean ± SD, 0.10 ± 0.03 versus 0.05 ± 0.02 intensity units [IU]/s; P = .0014) and between carcinoma and fibroadenoma (0.10 ± 0.03 versus 0.04 ± 0.01 IU/s; P = .0044), whereas differences between carcinomas and cysts were found to be nonsignificant. In conclusion, a parametric imaging method for characterization of breast lesions using the high contrast to tissue signal provided by SHI has been developed. While the preliminary sample size was limited, results show potential for breast lesion characterization based on perfusion flow parameters.

Preoperative diagnosis of ovarian tumors using pelvic contrast-enhanced sonography

OBJECTIVE

The purpose of this study was to assess the feasibility of using a contrast agent for the sonographic examination of adnexal tumors and identify discriminating parameters in the preoperative diagnosis of malignant tumors.

METHODS

We conducted a prospective descriptive monocenter study that analyzed validated echographic criteria and parameters of the enhancement curve obtained by sonographic contrast agent injection. Patients included were referred for a second opinion after the discovery of a suspicious ovarian image. The final diagnosis was reached after surgery and an anatomopathologic examination.

RESULTS

Fifty-two tumors were analyzed. Morphologic and Doppler criteria analyses were conducted as described in the literature. The significant parameters of the enhancement curve were the time-intensity curve total area and the duration of activity of the contrast agent during the first phase of decay (P < .002). The performance of the contrast agent was lower than that of the examiner's subjective diagnosis, with an area under the receiver operating characteristic curve (AUC) of 0.78 versus 0.80. When borderline tumors were eliminated, there was an inversion of this, with an AUC of 0.85 versus 0.73. The inclusion of contrast results in the examiner's diagnosis in the context of a bivariate model comparing malignant and borderline tumors with benign tumors provided an AUC of 0.88.

CONCLUSIONS

Contrast-enhanced sonography improves preoperative diagnosis of ovarian tumors parameters. The significant parameters of the enhancement curve were significantly different for malignant and benign tumors. Borderline tumors contribute to a reduction of the discriminating capacity of the contrast agent.

Acoustic and kinetic behaviour of definity in mice exposed to high frequency ultrasound

Microbubble contrast agents have shown clinical potential for characterising blood flow using 1 to 10 MHz ultrasound; however, scaling their use for similar applications in the mouse with high frequency ultrasound (20 to 60 MHz) has not been addressed. The goal was to determine the utility of microbubbles for mouse imaging with 30 MHz ultrasound by investigating their attenuation and backscatter characteristics as a function of concentration in vitro and dose response in vivo. The agent was exposed to a 30 MHz, 20% bandwidth pulse with a peak negative pressure of 244 kPa. In vitro results showed that the attenuation and backscatter increased linearly for concentrations between 2.8 x 10(6) and 28 x 10(6) bubbles per mL of deionized water. In vivo experiments where performed in the jugular vein of CD-1 mice and time intensity curves were acquired for doses between 10 and 100 microL kg(-1). These doses corresponded to the range of concentrations used in vitro. In vivo results showed that the peak enhancement of the agent increased linearly for doses between 10 and 60 microL kg(-1), the duration of enhancement varied between 200 to 300 s and the integrated enhancement (area under the curve) increased linearly up to 100 microL kg(-1). A maximum enhancement of 13 dB over the blood pool was observed for a dose of 100 microL kg(-1). The intra- and inter-mouse variabilities were 10% to 40% and indicate that further optimisations are required. These results suggest that quantitative contrast flow studies in the mouse using high frequency ultrasound are possible for doses between 10 and 60 microL kg(-1).

Feasibility exploration of blood flow estimation by contrast-assisted Nakagami imaging

The microbubble contrast agent destruction/replenishment technique has been widely applied to ultrasound-based blood flow estimation. The rate of increase of the time-intensity curve (TIC) due to microbubbles flowing into the region of interest as measured from B-mode images closely reflects the flow velocity. In this study, we monitored microbubble replenishment by a proposed new approach called the time-Nakagami-parameter curve (TNC) obtained from Nakagami-parameter images for quantifying the flow velocity. The feasibility of using the TNC to estimate the flow was evaluated in computer simulations of the TIC and TNC for flow velocities from 10 to 30 cm/s under an ultrasound frequency of 5 MHz. The clutter effects on the TIC and TNC were explored in amore realistic situation by carrying out phantom measurements of 25 MHz. The rates of increase of the TIC and TNC were expressed by the rate constants beta1 and betaN of a monoexponential model, respectively. The average beta1 increased from 38 to 110 s(-1) as the flow velocity increased from 10 to 30 cm/s (r = 0.98), and the average betaN increased from approximately 40 to 120 s(-1) for the same increase in flow velocity (r = 0.98). The p-value between the results of beta1 and betaN as a function of flow velocity was 0.77. These results represent that betaN quantifies the flow velocity similarly to the conventional beta1. In particular, both the simulation and experimental results revealed that the TNC method conditionally tolerates the presence of nonperfused areas (e.g., surrounding tissues or vessel walls) in the region of interest without requiring application of an additional wall filter to cancel the influences of clutter echoes on the flow estimation. These findings suggest that the TNC-based technique may be a potential method as a complementary tool for the conventional TIC technique to improve the estimation of blood flow.

In vivo perfusion estimation using subharmonic contrast microbubble signals

OBJECTIVE

The purpose of this study was to quantify perfusion in vivo using contrast-enhanced subharmonic imaging (SHI).

METHODS

A modified LOGIQ 9 scanner (GE Healthcare, Milwaukee, WI) operating in gray scale SHI mode was used to measure SHI time-intensity curves in vivo. Four dogs received intravenous contrast bolus injections (dose, 0.1 mL/kg), and renal SHI was performed. After 3 contrast agent injections, a microvascular staining technique based on stable (nonradioactive) isotope-labeled microspheres (BioPhysics Assay Laboratory Inc, Worcester, MA) was used to quantify the degree of perfusion in 8 sections of each kidney. Low perfusion states were induced by ligating surgically exposed segmental renal arteries followed by contrast agent injections and microvascular staining. Digital clips were transferred to a personal computer, and SHI time-intensity curves were acquired in each section using Image-Pro Plus software (Media Cybernetics, Silver Spring, MD). Subharmonic fractional blood volumes were calculated, and the perfusion was estimated from the initial slope of the fractional blood volume uptake averaged over 3 injections. Subharmonic perfusion data were compared with the gold standard (ie, the microspheres) using linear regression analysis.

RESULTS

In vivo gray scale SHI clearly showed flow and, thus, perfusion in the kidneys with almost complete suppression of tissue signals. In total, 270 SHI time-intensity curves were acquired, which reduced to 94 perfusion estimates after averaging. Subharmonic perfusion estimates correlated significantly with microsphere results (r = 0.57; P < .0001). The best SHI perfusion estimates occurred for high perfusion states in the anterior of the kidneys (r = 0.73; P = .0001). The corresponding root mean square error was 2.4%.

CONCLUSIONS

Subharmonic perfusion estimates have been obtained in vivo. The perfusion estimates were in reasonable to good agreement with a microvascular staining technique.

Effect of shell type on the in vivo backscatter from polymer-encapsulated microbubbles

This study compared in vivo enhancement from four different polymer-encapsulated ultrasound (US) contrast agents. The agents were produced with a rigid shell composed of the biodegradable block copolymer poly[D,L-lactide-co-glycolide] (PLGA) with the lactic and glycolic acid ratios 50:50, 75:25, 85:15 and 100:0 (i.e., increasingly hydrophobic shell compositions). Approximately the same bubble diameter (1.2 microm) and concentration (0.4 g/mL) were obtained for each agent. In four rabbits, audio Doppler signals were acquired from a 10 MHz cuff transducer placed around a surgically exposed vessel (contrast dose: 0.0125 to 0.15 mL/kg). In vivo dose responses were calculated off-line (in dB). Nine rabbit kidneys were imaged during contrast administration (0.1 mL/kg) in power Doppler and grey-scale pulse inversion harmonic (PIHI) modes using an HDI 5000 scanner (Philips Medical Systems, Bothell, WA). Time-intensity curves were produced and the time-to-peak, peak intensity, slope, area under the curve (AUC) and total duration of enhancement for each agent were compared. All agents produced marked Doppler enhancement with increasing duration from the 50:50 agent (48 +/- 10 s) to the 75:25 agent (166 +/- 46 s), the 85:15 agent (403 +/- 83 s) and with the 100:0 agent (603 +/- 93 s) lasting longest (p < 0.02). No other parameters changed significantly, except the AUC of the 85:15 agent, which was greater than that of the 50:50 agent (190.75 vs. 61.58; p = 0.02). The in vivo dose-response curves were similar for all agents, with mean enhancement up to 20.6 +/- 1.11 dB (p = 0.17). In conclusion, contrast duration increases by an order of magnitude as the lactic acid component in the polymer-encapsulated bubbles increases and the shell, thus, becomes increasingly hydrophobic.

Measurement of radial artery contrast intensity to assess cardiac microbubble behavior

OBJECTIVES

We sought to determine whether analysis of the contrast signal from the radial artery is better able to reflect changes in left ventricular (LV) microbubble dynamics than the signal from the LV itself.

BACKGROUND

Assessment of microbubble behavior from images of the LV may be affected by attenuation from overlying microbubbles and nonuniform background signal intensities. The signal intensity from contrast in a peripheral artery is not affected by these artifacts and may, thus, be more accurate.

METHODS

After injection of a contrast bolus into a peripheral vein, signal intensity was followed simultaneously in the LV and radial artery. The measurements were repeated using continuous, triggered, low and high mechanical index harmonic imaging of the LV.

RESULTS

Peak and integrated signal intensities ranged from 25 dB and 1550 dB/s, respectively, with radial artery imaging to 5.6 dB and 471 dB/s with ventricular imaging. Although differences in microbubble behavior during the different imaging protocols could be determined from both the LV and radial artery curves, analysis of the radial artery curves yielded more consistent and robust differences.

CONCLUSIONS

The signal from microbubbles in the radial artery is not affected by shadowing and is, thus, a more accurate reflection of microbubble behavior in the LV than the signal from the LV itself. This may have important implications for the measurement of myocardial perfusion by contrast echocardiography.

Transfer function analysis of ultrasonic time-intensity measurements

Time-intensity measurements of ultrasonic-contrast microbubbles based on the dilution theory have been used to assist blood flow estimation. The compartment model has been employed to describe the dilution process. Under the linear and time-invariant assumption, the time-intensity curve measured at the output of a compartment (i.e., blood mixing chamber) is the convolution of the input time-intensity curve with the compartment's transfer function. Thus, transfer function analysis is possible using deconvolution when the temporal variations in both the input and the output intensities are available. Note that the linear and time-invariant assumption requires a constant flow rate because, with flow pulsation, the flow rate changes with time and the mixing process becomes time varying. Thus, the purpose of this paper was to study the effects of flow pulsation on time-intensity measurements. In addition, a deconvolution technique based on a recursive least squares approach is used for transfer function analysis. Both simulations and experiments were performed; the results from which indicate that the pulsation generally does not affect the validity of time-intensity-based flow estimation. The proposed deconvolution technique is also effective for both constant and pulsatile flows; thus, permitting transfer function analysis in various flow conditions. One potential application of this transfer function analysis is to remove the effects of a noninstantaneous input function. The results from this paper lead to future work in brain-perfusion estimation based on extracranial time-intensity measurements.

Kinetics of a US contrast agent in benign and malignant adnexal tumors

PURPOSE

To evaluate the effects of a microbubble contrast agent on the power Doppler ultrasonographic (US) examination of adnexal tumors, with a special focus on the timing of the transit of the microbubble bolus.

MATERIALS AND METHODS

Seventy patients who were suspected of having ovarian tumors were examined preoperatively with contrast material-enhanced US. Images obtained during a 5-minute examination were stored digitally, and the behavior of the contrast agent was evaluated objectively with measurement of the time-dependent image intensity at the region of interest with a computer program. A time-intensity curve in each case was derived and analyzed. The Mann-Whitney U test was used to compare intensity changes and tumor parameters in benign and malignant adnexal tumors.

RESULTS

Both the baseline and maximum power Doppler intensities, as well as the absolute and relative (percent) rise in intensity, were significantly higher (P <.001) in malignant as compared with benign tumors. The arrival time was shorter (17.5 vs 22.5 seconds; P =.005) and the duration of contrast agent effect was longer (190.4 vs 103.6 seconds; P <.001) in malignant tumors than they were in benign tumors. The area under the time-intensity curve was significantly greater in malignant tumors compared with that in benign tumors (P <.001).

CONCLUSION

After microbubble contrast agent injection, malignant and benign adnexal lesions behave differently in degree, onset, and duration of Doppler US enhancement.

Time-intensity-based volumetric flow measurements: an in vitro study

Ultrasonic contrast agents have been used to assist blood flow measurements. Several contrast-specific flow measurement techniques have been proposed during the last few years. Among them, a method based on relative enhancement of the backscattered signal as a function of time is of particular interest. This method is also known as the time-intensity method. The method is based on the indicator-dilution theory, and the time-intensity curve is used to derive blood flow-related parameters such as the flow rate and the blood mixing volume. Previous in vitro studies done by other research groups were mainly based on a perfusion model or an artery model. Results showed that several parameters derived from the time-intensity curve had a good correlation with the flow rate under certain conditions. However, the studies did not focus on factors such as mixing volume, mixing chamber configuration and different types of mixing chamber. In this paper, dependence of the time-intensity curve is further studied. Specifically, two types of blood-mixing chambers were constructed. One was a spherical compartment phantom with two different sizes (260 and 580 mL) and different inflow/outflow configurations. The other was a perfusion phantom consisting of dialysis cartridges with the volume ranging from 114 to 351 mL. The time intensities were also measured at both the input and the output of the mixing chamber. A commercial agent (Levovist) and a self-made, albumin-based agent were used and the wash-out time constant and the mean transit time were derived for flow rates ranging from 500 to 1300 mL/min. For the perfusion phantom, results showed that the parameters had a good correlation with both the flow rate and the mixing volume. Results from the compartment phantom, on the other hand, indicated that the inflow/outflow configuration and the mixing size significantly affected the derived time constants. Potential applications of new volumetric flow estimation techniques based on both input and output intensities were also discussed.

Enhancement characteristics of the microbubble agent Levovist: reproducibility and interaction with aspirin

We investigated the reproducibility of Doppler enhancement indices following intravenous bolus injections of Levovist (Schering AG, Berlin) microbubbles. We also aimed to determine whether observations from animal studies suggesting that aspirin potentiates microbubble enhancement were reproducible in humans. In five healthy volunteers, time enhancement profiles of Doppler intensity following repeated bolus injections of Levovist were acquired from the common carotid artery, hepatic vein and kidney using spectral and power Doppler before and after oral aspirin (600 mg). Peak enhancement (PE), area under the curve (AUC) and decay slopes (lambda) were calculated. Hepatic vein contrast arrival time (AT) was determined subjectively. Well-defined carotid enhancement was seen in 19/20 injections. Reproducibility was high (r > 0.8). PE and AUG were unaffected by aspirin, but lambda was slightly reduced (P = 0.02). Renal power Doppler profiles were well defined (10/10) with no significant changes of AUC, PE or lambda after aspirin. Our study demonstrates good reproducibility of carotid spectral Doppler time intensitometry with Levovist in man. Aspirin does not have a significant effect on enhancement indices except carotid spectral Doppler decay. We conclude that aspirin is unlikely to potentiate microbubble enhancement, as seen in animal studies.

Quantification of ultrasound contrast agent response: comparison of continuous wave Doppler and power Doppler to backscattered radiofrequency data

Our goal was to compare two quantification methods of ultrasound contrast agents available in clinical practice [continuous wave Doppler intensity (CWDI) and power Doppler intensity (PWDI)] to the reference technique (radio-frequency analysis) with a simple recirculating flow phantom using a renal dialysis cartridge. Measurements were made at different doses of perflenapent emulsion and BR1. Cineloops of power Doppler images were recorded using a clinically available ultrasound unit (HDI 3000). Simultaneously, integrated backscatter (IBS) was measured by analysis of radiofrequency signals, whereas Doppler signal intensity was measured with a continuous wave Doppler device. A linear relationship was found between CWDI and IBS and between PWDI and IBS when R(2) was calculated for each pair of parameters injection-by-injection. Results are summarized by the average R(2) for all injections between CWDI and IBS (BR1: R(2) = 0.93 +/- 0.05, perflenapent emulsion: R(2) = 0.94 +/- 0.03) and between PWDI and IBS (BR1: R(2) = 0.88 +/- 0.07, perflenapent emulsion: R(2) = 0.79 +/- 0.09). However, for all data obtained from all different injected doses and for both contrast agents, there was considerable variation of CWDI and PWDI values measured for a given value of IBS. In conclusion, for a fixed microbubble population, CWDI and PWDI can be proposed for quantification of USCA. However, their important variations observed at each dose make it difficult to link a single value of PWDI or CWDI or IBS to a single microbubble distribution composition.

Pharmacokinetics of echocontrast agent infusion in a dog model

Ultrasound contrast agents (UCAs) have a distinct diagnostic impact on transcranial Doppler (TCD) and duplex sonography. In addition to the properties of the UCA and ultrasound imaging modes, the duration of contrast enhancement depends on the administration mode. Infusion of UCAs may be appropriate for prolonging the diagnostically useful time of elevated Doppler intensity. Five sedated dogs were investigated by TCD during infusion with SonoVue, a new UCA consisting of sulfur hexafluoride microbubbles. The infusion rate was varied, and the time-intensity curves were analyzed. Infusion rate of 70 ml/h provided a stable mean level of increased Doppler intensity up to 24 to 26 dB over baseline, whereas a rate of 35 ml/h did not result in a stable plateau (range 8-19 dB over baseline [5 minutes after starting time]). The maximum increases in Doppler mean intensity (18.2 dB [35 ml/h] and 25.6 dB [70 ml/h]) were significantly different (P = .025). Pharmacokinetic analysis of SonoVue during inflow (by exponential functional fitting of the time-mean intensity curves) and elimination (by linear regression analysis) revealed no dose-related differences. This study demonstrated a dose-dependent level of increased Doppler mean intensity within the brain circulation during infusion of SonoVue. Unlike the bell-shaped course of Doppler signal enhancement seen after bolus injection, infusion generates a stable plateau, which is an important prerequisite for more advanced contrast applications.

An in vitro comparison of ultrasonic contrast agents in solutions with varying air levels

The performance, in particular, the stability of ultrasound (US) contrast agents has yet to be assessed. An in vitro system has been set up to investigate the properties of ultrasonic contrast agents under different suspension conditions. This is designed to contribute to the optimal use of agents in clinical practice. In this study, the contrast agents were introduced into solutions of different oxygen concentration levels, as might be encountered in blood, and their relative performance was assessed in terms of decay in the solution environment. The partial pressures of oxygen in those solutions ranged between 1.5 and 26 kPa. Three IV and one arterial contrast agents were used: Levovist, DMP115, Quantison and Myomap. Levovist showed the highest sensitivity to oxygen concentration in the solution, and the other three proved tolerant for the above values of oxygen concentrations.

Contrast harmonic color Doppler left ventriculography: machine-interpreted left ventricular ejection fraction compared with equilibrium-gated radionuclide ventriculography

BACKGROUND

Multi-gated acquisition (equilibrium-gated radionuclide ventriculography) (MUGA) is considered the gold standard for measuring left ventricular ejection fraction (LVEF) because it is accurate, machine interpreted, and reproducible. Echocardiographic LVEF measurements are subject to variability in image acquisition and interpretation and to the limitations of 2-dimensional (2D) versus 3-dimensional imaging.

GOAL

The shortcomings of traditional echocardiography may be addressed by combining multiplane 2D harmonic imaging, echocardiographic contrast, color Doppler ultrasonography, and digital image processing to create a new imaging modality: contrast harmonic color Doppler left ventriculography.

METHODS

We compared the accuracy of a new method for measuring LVEF that allows for machine interpretation and uses contrast-enhanced intermittent harmonic color Doppler ultrasonography (CHCD). Quantitative LVEF measurements by hand-traced harmonic 2D echocardiography, contrast-enhanced harmonic 2D echocardiography, CHCD, and machine-interpreted CHCD were compared with MUGA in 35 patients.

RESULTS

Contrast-enhanced intermittent harmonic color Doppler provided images with vivid endocardial definition in all patients, but hand-traced harmonic 2D echocardiography and contrast-enhanced harmonic 2D echocardiography had inadequate images in 9% of patients. The MUGA LVEF range was 0. 09 to 0.70. All echocardiographic methods showed excellent correlation with the MUGA LVEF (R (2) > 0.96), but the CHCD method had the best limits of agreement.

CONCLUSIONS

Contrast-enhanced intermittent harmonic color Doppler LVEF correlates with MUGA at least as well as traditional noncontrasted echocardiography, but it provides diagnostic images in a greater proportion of patients. The CHCD images have vivid endocardial delineation and can be machine interpreted.

Infusion versus bolus of an ultrasound contrast agent: in vivo dose-response measurements of BR1

RATIONALE AND OBJECTIVES

To determine the efficacy of an ultrasound contrast agent infusion using Doppler intensitometry estimation of backscatter enhancement in blood.

METHODS

Multiple intravenous injections of BR1 (SonoVue) were performed in chronic dog studies, using bolus (0.05-2 mL) and infusion (3-40 mL/h during 6 minutes) administration. The pulsed Doppler signal from the femoral artery was recorded and analyzed for mean Doppler power and integrated fractional enhancement.

RESULTS

For bolus injection, time-intensity curves exhibited a rapid first pass (peak 30 dB for 0.45 mL) followed by a slower washout. Integrated fractional enhancement exhibited a linear relation with the dose (R2 = 0.99). For infusion administration, peak enhancement increased with the infusion rate from 8 to 22 dB. At rates exceeding 30 mL/h, the enhancement was stable with a plateau-like pattern.

CONCLUSIONS

Infusion of BR1 is easily achieved and allows the duration of enhancement to be increased as long as desired. Stable enhancement is obtained for rates greater than 30 mL/h.

Effect of filling gases on the backscatter from contrast microbubbles: theory and in vivo measurements

Two surfactant-based contrast agents, ST44 and ST68, were produced according to US Patent # 5,352,436 and filled with either air, C4F10 (perfluorobutane) or SF6 (sulfur hexaflouride). Ten rabbits received i.v. injections of each agent/gas combination with 5 repetitions of each dose (range: 0.005-0.13 mL/kg). A custom-made 10-MHz cuff transducer was placed around the surgically exposed distal aorta and audio Doppler signals were acquired in vivo. Quantitative in vivo dose responses were calculated off-line using spectral power analysis and compared to a theoretical model of microbubble dissolution and enhancement. For qualitative comparisons, 10 rabbits were imaged pre- and postcontrast administration (dose: 0.1 mL/kg) in gray-scale and colour. All agent/gas combinations produced marked Doppler enhancement with air bubbles enhancing least of all (p < 0.0001) and ST68-SF6 best of all (maximum: 27.6 +/- 2.04 dB; p < 0.012). There were no significant differences between other agent/gas combinations (0.30 < p < 0.70). Theoretical enhancement was within 1 order of magnitude of the experimental observations (i.e., deviations of up to 10 dB). The duration of contrast enhancement was 1-2 min for air-filled bubbles, 3-5 min for SF6-filled bubbles and more than 7 min for C4F10-filled bubbles. In conclusion, ST68-SF6 microbubbles produced most in vivo enhancement of the agent/gas combinations studied. Theory matched the measurements within an order of magnitude.

Safety and ultrasound-enhancing potentials of a new sulfur hexafluoride-containing agent in the cerebral circulation

Insufficient ultrasound penetration through the temporal bone is a serious limitation of transcranial ultrasound diagnostics. In a phase I study, the authors studied safety and ultrasound enhancing potentials of the new transpulmonary ultrasound contrast agent SonoVue, which contains sulfur hexafluoride gas microbubbles stabilized by a phospholipid shell. Twelve healthy volunteers received four different doses of SonoVue (0.3 ml, 0.6 ml, 1.2 ml, and 2.4 ml) intravenously. The duration of ultrasound contrast enhancement was measured by transcranial Doppler sonography (TCD) and transcranial color-coded sonography (TCCS). Safety and tolerability was monitored during the study and for 24 hours after contrast agent administration. TCD: Duration of spectral enhancement (signal intensity of 5 dB over baseline) was observed dose-related (p < 0.0001; Friedman-test) for (0.3 ml) 136 +/- 63.4 seconds; (0.6 ml) 191 +/- 63.3 seconds; (1.2 ml) 314 +/- 88 seconds; (2.4 ml) 434 +/- 168 seconds [mean +/- SD]. Dependent on dosage, the peak signal amplification in TCD was significantly different (p < 0.001; Friedman-test) as well: (0.3 ml) 24.5 +/- 2.0 dB; (0.6 ml) 26.0 +/- 1.6 dB; (1.2 ml) 27.6 +/- 2.2 dB; (2.4 ml) 28.4 +/- 2.2 dB (mean +/- SD). TCCS: Mean time of optimal enhancement increased from 214 +/- 73 seconds (0.3 ml) to 356 +/- 14 seconds (2.4 ml) in a dose-dependent manner. In TCCS, signal amplification appeared to be stronger with increasing doses. Adverse events were not observed during the study. This investigation describes the ultrasound enhancing potential of SonoVue in the intracranial cerebral circulation. SonoVue proved to be well tolerated and provided a long-lasting ultrasound contrast enhancement that supports an optimal transcranial ultrasound diagnostic.

The influence of Doppler system settings on the clearance kinetics of different ultrasound contrast agents

OBJECTIVE

To evaluate the influence of different Doppler system settings on time-intensity curves after ultrasound contrast agent (UCA) bolus injection. This is important for the comparison of different UCAs.

METHODS

Six sedated dogs were investigated with a transcranial Doppler system and Doppler power, sample volume size and high pass filter settings were modified during the procedure. Mean time intensity curves were determined and peak values of mean intensity as well as the decrease in Doppler intensity were compared for the different system settings. Three different UCAs were used (SonoVue(TM), BY963 and Levovist(TM)).

RESULTS

The Doppler time intensity curves showed a typical two phase decrease with a distribution phase alpha and an elimination phase beta with all three UCAs. Altering the system settings had a significant effect on the mean peak Doppler intensity for SonoVue(TM) (P=0.02) but not for BY963 or Levovist(TM) (P=0.07 and P=0.39, respectively), due to high variation of the Levovist(TM) and BY963 intensity values. There were no significant differences between the alpha slopes of BY963 and Levovist(TM) (P=0.96), or the beta slope of Levovist(TM) and SonoVue(TM) (P=0.62), when the results of all system settings were combined.

CONCLUSION

Different Doppler system settings show no significant influence on the decrease of mean Doppler intensity, but have a significant effect on peak intensity.

A mathematical model for the assessment of hemodynamic parameters using quantitative contrast echocardiography

A mathematical model for the assessment of hemodynamic parameters using quantitative echocardiography is presented. The method involves the intravenous injection of an ultrasonic echo contrast agent. The relative enhancement of the backscattered ultrasound intensity is measured as a function of time (the time-intensity curve). From this measurement, the volume flow rate (cardiac output) and the mixing volume are calculated. Relevant acoustic properties of the ultrasound contrast agent are discussed. An in vitro experiment is performed to corroborate the theory presented.

The influence of different gases on acoustic properties of a spherosome-based ultrasound contrast agent (BY963). A transcranial Dopplersonography study

Ultrasound contrast agents improve the signal-to-noise ratio of reflected ultrasound, enhancing the diagnostic value of transcranial Doppler (TCD). In dog studies, we investigated the time course of TCD signal amplitude after application of a phospholipid-containing ultrasound contrast agent (BY963) filled with different gases. The median time of Doppler amplitude enhancement exceeding 5 dB was determined using isoflurane-, isopentane-, trichlortrifluoroethane-, air-, argon-, and perfluoropentane-filled BY963 (69, 72, 75, 78, 88, and 245 seconds respectively). The decrease of time-intensity curve and the duration of signal enhancement showed significant differences comparing the different gases (p = 0.04 and 0.03, respectively). The time course of in vitro stability of BY963 agitated with the different gases measured by absorbance of light (500 nm) showed a retarded decay for perfluoropentane, a rapid decrease for air, isopentane, trichlortrifluoroethane, and argon, and a very rapid decrease using isoflurane. The time course of the different gases depended on the physiochemical properties (lipophilicity and the solubility in water) of the gas encoated in the phospholipid shell. Perfluoropentane-filled BY963 showed the highest in vitro stability and the longest duration of TCD enhancement compared with the other gases used.

Characteristics of transcranial Doppler signal enhancement using a phospholipid-containing echocontrast agent

BACKGROUND AND PURPOSE

Ultrasound attenuation caused by the skull is a major limitation of transcranial Doppler. Echocontrast agents (EAs) may solve this problem. The aim of the present study was to investigate the characteristics of a new echocontrast agent (BY963) containing air bubbles stabilized by phospholipids.

METHODS

Nine healthy volunteers received three different doses (2.5, 5.0, and 10 mL) of BY963 at an injection rate of 0.25 mL/s. The Doppler signal amplitude obtained from the middle cerebral artery was recorded with a 2-MHz pulsed-wave Doppler system. After complete decay of the signal enhancement, upward stroking of the veins of the upper arm was performed to evaluate the stability of the EA in the venous system.

RESULTS

A dose-dependent increase of at least 30 dB in the Doppler signal amplitude lasted 19 to 47, 35 to 64, and 48 to 126 heart cycles (68% range) after 2.5, 5.0, and 10 mL EA, respectively. In 6 cases, there was a biphasic increase in EA enhancement. Upward stroking of the forearm, in general 12 to 18 minutes after administration, caused a Doppler signal enhancement of at least 30 dB in 6 cases.

CONCLUSIONS

Each injection of BY963 caused a diagnostically relevant Doppler signal enhancement. A considerable amount of EA remained stable in the venous system for at least 12 minutes. The biphasic dose-response fits to models of dilution-indicator theory and indicates free recirculation, as well as a nonlinear washout curve.

Quantitative acoustic characterization of a new surfactant-based ultrasound contrast agent

The acoustic properties of a new ultrasound contrast agent, ST68, have been investigated. ST68 is a sonicated mixture of nonionic surfactants (Span-type and Tween-type) consisting of stabilized microbubbles with a mean diameter of 3.8 microns and a concentrations of 7.1 x 10(8) bubbles/mL. A pulsatile flow system was used to acquire data in vitro. The acoustic properties of ST68, as a function of time, frequency and dose, were calculated. Enhancement changed nonlinearly with contrast agent dose; maximum was 13.1 dB +/- 1.0 dB for a dose of 0.30 microL/mL of suspending medium. Attenuation reached approximately 11 dB/cm for dosages above 0.27 microL/mL and for frequencies between 2.5 and 6.0 MHz. In vivo, i.v. injections of ST68 were given to 4 rabbits (doses from 0.01 to 0.23 mL/kg). A clear increase in flow signal intensity was observed for 1 to 2 min. An in vivo dose-response curve was calculated from audio Doppler signals obtained with a 10-MHz cuff transducer placed around the distal aorta. Maximum enhancement was 18.3 dB +/- 3.13 dB for a 0.13 mL/kg dose. Moreover, ST68 appears to follow a simple relationship between in vivo enhancement and dose. In conclusion, ST68 is capable of producing marked vascular enhancement. Its acoustic properties have been characterized in vitro and in vivo.

Harmonic imaging with Levovist

Our purpose was to test the hypothesis that second harmonic imaging preferentially detects backscatter from microbubbles compared with tissue structural components. A prototype second harmonic scanner was used to image a flow channel in a tissue-mimicking rubber phantom (liver density). Video time-intensity curves were calculated from repeated bolus injections of microbubble echocardiographic contrast material under the same fluid dynamic conditions but with three different imaging modes: (1) fundamental imaging at 2.5 MHz (transmit and receive at 2.5 MHz), (2) fun damental imaging at 5.0 MHz (transmit and receive at 5.0 MHz), and (3) second harmonic imaging (transmit at 2.5 MHz and receive at 5.0 MHz). Each video time-intensity curve was calibrated-such that quantitative backscatter intensity was measured relative to the tissue phantom (0 dB). The peak increase in backscatter from the contrast material in the channel relative to the tissue phantom and the intensity in the channel before the contrast effect (the noise floor) was measured along with the area under the calibrated time-intensity curve relative to the phantom. When referenced to the noise floor in the flow channel, all imaging modes produced approximately 25 dB of enhancement. However, when referenced to the tissue phantom, second harmonic imaging produced a 22.3 +/- 1.8 dB peak enhancement, which was greater than either fundamental imaging at 2.5 MHz (15.5 +/- 0.8 dB; p < 0.001) or fundamental imaging at 5.0 MHz (15.3 +/- 1.5 dB; p < 0.001). The area under the time-intensity curves confirmed that harmonic imaging has approximately 7 dB of relative enhancement to the phantom compared with fundamental imaging at either frequency. Second harmonic imaging specifically enhances backscatter from microbubbles compared with a tissue-mimicking phantom. This specificity for microbubbles is due to a decrease in backscatter for the tissue phantom, rather than an increase in backscatter for the microbubbles. These data support the hypothesis that second harmonic imaging may be able to detect microbubbles in the tissue vascular space by preferentially decreasing the backscatter from tissue structural components.