High-frequency color flow imaging of the microcirculation

Collapse pressure measurement of single hollow glass microsphere using single-beam acoustic tweezer

Microbubbles are widely used in medical ultrasound imaging and drug delivery. Many studies have attempted to quantify the collapse pressure of microbubbles using methods that vary depending on the type and population of bubbles and the frequency band of the ultrasound. However, accurate measurement of collapse pressure is difficult as a result of non-acoustic pressure factors generated by physical and chemical reactions such as dissolution, cavitation, and interaction between bubbles. In this study, we developed a method for accurately measuring collapse pressure using only ultrasound pulse acoustic pressure. Under the proposed method, the collapse pressure of a single hollow glass microsphere (HGM) is measured using a high-frequency (20-40 MHz) single-beam acoustic tweezer (SBAT), thereby eliminating the influence of additional factors. Based on these measurements, the collapse pressure is derived as a function of the HGM size using the microspheres' true density. We also developed a method for estimating high-frequency acoustic pressure, whose measurement using current hydrophone equipment is complicated by limitations in the size of the active aperture. By recording the transmit voltage at the moment of collapse and referencing it against the corresponding pressure, it is possible to estimate the acoustic pressure at the given transmit condition. These results of this study suggest a method for quantifying high-frequency acoustic pressure, provide a potential reference for the characterization of bubble collapse pressure, and demonstrate the potential use of acoustic tweezers as a tool for measuring the elastic properties of particles/cells.

In vivo noninvasive preclinical tumor hypoxia imaging methods: a review

Tumors exhibit areas of decreased oxygenation due to malformed blood vessels. This low oxygen concentration decreases the effectiveness of radiation therapy, and the resulting poor perfusion can prevent drugs from reaching areas of the tumor. Tumor hypoxia is associated with poorer prognosis and disease progression, and is therefore of interest to preclinical researchers. Although there are multiple different ways to measure tumor hypoxia and related factors, there is no standard for quantifying spatial and temporal tumor hypoxia distributions in preclinical research or in the clinic. This review compares imaging methods utilized for the purpose of assessing spatio-temporal patterns of hypoxia in the preclinical setting. Imaging methods provide varying levels of spatial and temporal resolution regarding different aspects of hypoxia, and with varying advantages and disadvantages. The choice of modality requires consideration of the specific experimental model, the nature of the required characterization and the availability of complementary modalities as well as immunohistochemistry.

Power Doppler ultrasound and contrast-enhanced ultrasound demonstrate non-invasive tumour vascular response to anti-vascular therapy in canine cancer patients

Combretastatin A4-phosphate (CA4P) is an anti-vascular agent which selectively shuts down blood supply in tumours, resulting in extensive tumour necrosis. The aim of this study was to assess in vivo, non-invasive ultrasound techniques for the early evaluation of tumour perfusion following CA4P treatment of spontaneous tumours. Eight dogs that bore spontaneous tumours were enrolled and were subsequently treated with a single dose of intravenous CA4P. Perfusion of tumours was evaluated by power Doppler ultrasound (PDUS) pre-treatment (0 h), during the injection (10 min, 20 min, 30 min) and after CA4P infusion (24 and 72 h). Vascularity index (VI) of the tumour tissue was quantitatively analysed and accuracy was verified by correlation analysis with the results of immunohistochemical evaluation of microvessel density (MVD). Central and peripheral perfusion was evaluated by contrast-enhanced ultrasound (CEUS) pre-treatment and at 72 h post-treatment. Post-treatment, PDUS demonstrated a significant decrease in VI within 10 min of CA4P infusion. CEUS parameters demonstrated a significant decrease in blood velocity and volume in the central aspect of the tumour. Histology revealed a 4.4-fold reduction (p < 0.001, 95% CI [2.2,9.4]) in MVD and a 4.1-fold increase (p = 0.003, 95% CI [1.4,11.8]) in necrotic tumour tissue. A strong correlation between PDUS results and immunohistochemical results was found (Pearson R² = 0.957, p < 0.001). Furthermore, the findings of PDUS were supported by the objective results of the CEUS analyses. These data suggest a role for ultrasound in real-time, non-invasive monitoring of tumour vascular response as an early indicator of CA4P treatment efficacy.

Investigation of Microbubble Detection Methods for Super-Resolution Imaging of Microvasculature

Ultrasound super-resolution techniques use the response of microbubble (MB) contrast agents to visualize the microvasculature. Techniques that localize isolated bubble signals first require detection algorithms to separate the MB and tissue responses. This work explores the three main MB detection techniques for super-resolution of microvasculature. Pulse inversion (PI), differential imaging (DI), and singular value decomposition (SVD) filtering were compared in terms of the localization accuracy, precision, and contrast-to-tissue ratio. MB responses were simulated based on the properties of Sonovue and using the Marmottant model. Nonlinear propagation through tissue was modeled using the k-Wave software package. For the parameters studied, the results show that PI is most appropriate for low frequency applications, but also most dependent on transducer bandwidth. SVD is preferable for high frequency acquisition where localization precision on the order of a few microns is possible. PI is largely independent of flow direction and speed compared to SVD and DI, so is appropriate for visualizing the slowest flows and tortuous vasculature. SVD is unsuitable for stationary MBs and can introduce a localization error on the order of hundreds of microns over the speed range 0-2 mm/s and flow directions from lateral (parallel to probe) to axial (perpendicular to probe). DI is only suitable for flow rates >0.5 mm/s or as flow becomes more axial. Overall, this study develops an MB and tissue nonlinear simulation platform to improve understanding of how different MB detection techniques can impact the super-resolution process and explores some of the factors influencing the suitability of each.

3-D In Vitro Acoustic Super-Resolution and Super-Resolved Velocity Mapping Using Microbubbles

Standard clinical ultrasound (US) imaging frequencies are unable to resolve microvascular structures due to the fundamental diffraction limit of US waves. Recent demonstrations of 2-D super-resolution both in vitro and in vivo have demonstrated that fine vascular structures can be visualized using acoustic single bubble localization. Visualization of more complex and disordered 3-D vasculature, such as that of a tumor, requires an acquisition strategy which can additionally localize bubbles in the elevational plane with high precision in order to generate super-resolution in all three dimensions. Furthermore, a particular challenge lies in the need to provide this level of visualization with minimal acquisition time. In this paper, we develop a fast, coherent US imaging tool for microbubble localization in 3-D using a pair of US transducers positioned at 90°. This allowed detection of point scatterer signals in 3-D with average precisions equal to [Formula: see text] in axial and elevational planes, and [Formula: see text] in the lateral plane, compared to the diffraction limited point spread function full-widths at half-maximum of 488, 1188, and [Formula: see text] of the original imaging system with a single transducer. Visualization and velocity mapping of 3-D in vitro structures was demonstrated far beyond the diffraction limit. The capability to measure the complete flow pattern of blood vessels associated with disease at depth would ultimately enable analysis of in vivo microvascular morphology, blood flow dynamics, and occlusions resulting from disease states.

Advanced echocardiography in adult zebrafish reveals delayed recovery of heart function after myocardial cryoinjury

Translucent zebrafish larvae represent an established model to analyze genetics of cardiac development and human cardiac disease. More recently adult zebrafish are utilized to evaluate mechanisms of cardiac regeneration and by benefiting from recent genome editing technologies, including TALEN and CRISPR, adult zebrafish are emerging as a valuable in vivo model to evaluate novel disease genes and specifically validate disease causing mutations and their underlying pathomechanisms. However, methods to sensitively and non-invasively assess cardiac morphology and performance in adult zebrafish are still limited. We here present a standardized examination protocol to broadly assess cardiac performance in adult zebrafish by advancing conventional echocardiography with modern speckle-tracking analyses. This allows accurate detection of changes in cardiac performance and further enables highly sensitive assessment of regional myocardial motion and deformation in high spatio-temporal resolution. Combining conventional echocardiography measurements with radial and longitudinal velocity, displacement, strain, strain rate and myocardial wall delay rates after myocardial cryoinjury permitted to non-invasively determine injury dimensions and to longitudinally follow functional recovery during cardiac regeneration. We show that functional recovery of cryoinjured hearts occurs in three distinct phases. Importantly, the regeneration process after cryoinjury extends far beyond the proposed 45 days described for ventricular resection with reconstitution of myocardial performance up to 180 days post-injury (dpi). The imaging modalities evaluated here allow sensitive cardiac phenotyping and contribute to further establish adult zebrafish as valuable cardiac disease model beyond the larval developmental stage.

Calibrating Doppler imaging of preterm intracerebral circulation using a microvessel flow phantom

Introduction: Preterm infants are born during critical stages of brain development, in which the adaptive capacity of the fetus to extra-uterine environment is limited. Inadequate brain perfusion has been directly linked to preterm brain damage. Advanced high-frequency ultrasound probes and processing algorithms allow visualization of microvessels and depiction of regional variation. To assess whether visualization and flow velocity estimates of preterm cerebral perfusion using Doppler techniques are accurate, we conducted an in vitro experiment using a microvessel flow phantom.

Materials and Methods: An in-house developed flow phantom containing two microvessels (inner diameter 200 and 700 μm) with attached syringe pumps, filled with blood-mimicking fluid, was used to generate non-pulsatile perfusion of variable flow. Measurements were performed using an Esaote MyLab70 scanner.

Results: Microvessel mimicking catheters with velocities as low as 1 cm/s were adequately visualized with a linear ultrasound probe. With a convex probe, velocities <2 cm/s could not be depicted. Within settings, velocity and diameter measurements were highly reproducible [intra-class correlation 0.997 (95% CI 0.996–0.998) and 0.914 (0.864–0.946)]. Overall, mean velocity was overestimated up to threefold, especially in high velocity ranges. Significant differences were seen in velocity measurements when using steer angle correction and in vessel diameter estimation (p < 0.05).

Conclusion: Visualization of microvessel-size catheters mimicking small brain vessels is feasible. Reproducible velocity and diameter results can be obtained, although important overestimation of the values is observed. Before velocity estimates of microcirculation can find its use in clinical practice, calibration of the ultrasound machine for any specific Doppler purpose is essential. The ultimate goal is to develop a sonographic tool that can be used for objective study of regional perfusion in routine practice.

Gold nanorods as a contrast agent for Doppler optical coherence tomography

Purpose

To investigate gold nanorods (GNRs) as a contrast agent to enhance Doppler optical coherence tomography (OCT) imaging of the intrascleral aqueous humor outflow.

Methods

A serial dilution of GNRs was scanned with a spectral-domain OCT device (Bioptigen, Durham, NC) to visualize Doppler signal. Doppler measurements using GNRs were validated using a controlled flow system. To demonstrate an application of GNR enhanced Doppler, porcine eyes were perfused at constant pressure with mock aqueous alone or 1.0×10¹² GNR/mL mixed with mock aqueous. Twelve Doppler and volumetric SD-OCT scans were obtained from the limbus in a radial fashion incremented by 30°, forming a circular scan pattern. Volumetric flow was computed by integrating flow inside non-connected vessels throughout all 12 scans around the limbus.

Results

At the GNR concentration of 0.7×10¹² GNRs/mL, Doppler signal was present through the entire depth of the testing tube without substantial attenuation. A well-defined laminar flow profile was observed for Doppler images of GNRs flowing through the glass capillary tube. The Doppler OCT measured flow profile was not statistically different from the expected flow profile based upon an autoregressive moving average model, with an error of −0.025 to 0.037 mm/s (p = 0.6435). Cross-sectional slices demonstrated the ability to view anterior chamber outflow ex-vivo using GNR-enhanced Doppler OCT. Doppler volumetric flow measurements were comparable to flow recorded by the perfusion system.

Conclusions

GNRs created a measureable Doppler signal within otherwise silent flow fields in OCT Doppler scans. Practical application of this technique was confirmed in a constant pressure ex-vivo aqueous humor outflow model in porcine eyes.

Skin-scanning technique for superficial blood flow imaging using a high-frequency ultrasound system

In this paper we propose a skin-scanning technique with a high-frequency ultrasound imaging system that enables images to be acquired at the fixed depth of field of a single-element focused transducer along the profile of an object contour by simultaneously moving the transducer in the horizontal and vertical directions. The scanning path, which closely parallels the profile of the object contour, was determined from the intensity difference between an object and the background in a brightness-mode image. The transducer moved along the profile of the object contour while maintaining a constant distance interval between adjacent pairs of ultrasonic signals in the horizontal direction. The image was then reconstructed by applying an alignment process to eliminate the distortion. The performance of skin-scanning technique was verified in vitro experiment using an arc-shaped phantom and the results showed a percentage error of 0.55% for the volumetric blood flow estimates. Moreover, in vivo experiment on a subcutaneous tumor was also performed. The results indicated that the proposed technique can accurately estimate the blood flow information along the profile of the object contour and avoid distortion of the morphology of blood vessels. The skin-scanning technique has potential for assessing superficial blood flows and prognoses in the oncology and dermatology fields.

Ultrasound backscatter microscopy for imaging of oral carcinoma

OBJECTIVES

Ultrasound backscatter microscopy (UBM), or ultrasound biomicroscopy, is a noninvasive, label-free, and ionizing radiation-free technique allowing high-resolution 3-dimensional structural imaging. The goal of this study was to evaluate UBM for resolving anatomic features associated with squamous cell carcinoma of the oral cavity.

METHODS

The study was conducted in a hamster buccal pouch model. A carcinogen was topically applied to cheeks of 14 golden Syrian hamsters. Six additional hamsters served as healthy controls. A high-frequency (41 MHz, 6-mm focal depth, lateral and axial resolutions of 65 and 37 μm, respectively) UBM system was used for scanning the oral cavity after 14 weeks of carcinogen application. Histologic analyses were conducted on scanned regions.

RESULTS

The histologic structure of buccal tissue and microvasculature networks could be visualized from the UBM images. Epithelial and mucosal hypertrophy and neoplastic changes were identified in animals subjected to the carcinogen. In animals with invasive squamous cell carcinoma, lesion development and destruction of the structural integrity of tissue layers were noted.

CONCLUSIONS

In this pilot study, UBM generated sufficient contrast for morphologic features associated with oral carcinoma compared to healthy tissue. This modality may present a practical technique for detection of oral neoplasms that is potentially translatable to humans.

A study of the adult zebrafish ventricular function by retrospective Doppler-gated ultrahigh-frame-rate echocardiography

The zebrafish (Danio rerio) has become a preferred animal model for studying various human diseases, particularly those related to cardiovascular regeneration; therefore, a noninvasive imaging modality is needed for observing the cardiac function of zebrafish. Because of its high resolution, high-frequency ultrasound B-mode imaging has recently been used successfully to observe the heart of adult zebrafish. However, ultrahigh-frame-rate echocardiography combining Bmode imaging and color flow imaging is still needed to observe the detailed transient motions of the zebrafish ventricle. This study develops an 80-MHz ultrahigh-frame-rate echocardiography system for this purpose, based on retrospective Doppler- gated technology. B-mode and color flow images of the cardiovascular system of the zebrafish were reconstructed by two-dimensional autocorrelation at maximum frame rates of up to 40,000 and 400 fps, respectively. The timings of end diastole (E(D)) and end systole (E(S)) of ventricle can be determined by using this high-resolution image system. Two ventricular function parameters-fractional shortening (FS) and fractional area change (FAC)-were measured for evaluating the ventricular function by using E(D) and E(S) with their corresponding ventricular dimensions. The experimental results indicated that the measured FS values were 42 ± 4% (mean ± standard deviation) and 60 ± 13% for the long axis and short axis of the ventricle, respectively, and that FAC was 77 ± 9%. This is the first report of these ventricular function parameters for a normal adult zebrafish. The results showed that retrospective high-frequency echocardiography is a useful tool for studying the cardiac function of normal adult zebrafish.

A preclinical study to explore vasculature differences between primary and recurrent tumors using ultrasound Doppler imaging

The purpose of this preclinical study was to perform a longitudinal investigation of the function and morphology of the vasculatures of primary and recurrent tumors, because recurrent tumors have lower curability. Thus, elucidating differences in the features of the vasculatures of primary and recurrent tumors could help to improve tumor therapies. The transgenic adenocarcinoma of the mouse prostate tumors were transplanted in nonirradiated and with 25 Gy of preirradiation normal tissues to produce the primary and recurrent tumor models, respectively. The perfusion and branching index of tumor vasculatures were characterized to reveal the function and morphology information, respectively. The blood vessels were more dilated and continuous in recurrent tumors than in primary tumors. During tumor progression, the perfusion increased in primary tumors but did not change significantly in recurrent tumors. The tumor perfusion was lower in recurrent tumors than in primary tumors, whereas branching index in 2-D ultrasound images did not differ between the two tumor models. Furthermore, the introducing 3-D volumetric power Doppler image may have the potential for accurately revealing the morphologic features within tumors. The results of this study suggest that power Doppler imaging is an easily applied and rapid method for noninvasively assessing the vascular features of primary and recurrent tumors and for exploring differences between their vasculature pathways.

Characterization of tumor vasculature distributions in central and peripheral regions based on Doppler ultrasound

PURPOSE

Tumor heterogeneity is a major obstacle to therapy, and thus, how to achieve the maximal therapeutic gain in tumor suppression is an important issue. To accomplish this goal, assessing changes in tumor behaviors before treatment is helpful for physicians to adjust treatment schedules. In this study, the authors longitudinally and spatially investigated tumor perfusion and vascular density by power Doppler imaging and immunohistochemical analysis, respectively. Moreover, the authors developed a method to describe quantitatively the spatial distribution of the vasculature within the central and peripheral regions of tumors.

METHODS

Tumor perfusion was estimated by power Doppler images at an operating frequency of 25 MHz. To avoid the attenuation effect of such high-frequency ultrasound, murine tumors were subcutaneously transplanted into the thighs of mice and then monitored for 11 days. The tumors were removed at various time intervals for immunohistochemical analysis of their vascular density using CD31 staining. The spatial characteristics of the tumor vasculature were quantified by a γ value, which characterizes the rate at which vascular signals increase with the fractional sizes of the peripheral area within the tumor.

RESULTS

During tumor progression, the volume of tumor perfusion in the power Doppler images was strongly correlated with the vascular density determined by immunohistochemical analysis. In addition, the γ value significantly decreased with increased tumor size in the power Doppler images but not in the immunohistochemical analysis.

CONCLUSIONS

Although the tumor perfusion and vascular density estimates showed good temporal correlations during tumor progression, they did not show good spatial correlations due to tumor perfusion patterns changing from homogeneous to heterogeneous. In contrast to the perfusion patterns, the vascular density of the tumor remained uniformly distributed. In the present study, no necrosis regions were found in the tumor experiments. Furthermore, the measurement of γ value is a simple method for assessing the vasculatures of spatial distribution within tumors.

Three-dimensional high frequency power Doppler ultrasonography for the assessment of microvasculature during fracture healing in a rat model

We aimed to establish a novel approach with 3D high frequency power Doppler ultrasonography (3D-HF-PDU) to assess microvasculature at the fracture site in rat femurs by comparing with microCT-based microangiography. Twenty-four 9-month-old ovariectomized (OVX) osteoporotic rats and age-matched sham-ovariectomized (Sham) rats were used for establishing closed fracture models on right femora. At 2, 4, and 8 weeks post-operatively, four rats in each group underwent in vivo 3D-HF-PDU scanning for evaluation of vascularization and blood flow at the fracture site. Then the fractured femora were harvested for ex vivo microangiography, and neovasculatures within the callus were reconstructed for vascular volume analysis. Correlation between the vascular volumes of the two methodologies was examined. Both 3D-HF-PDU and microangiography showed a decline of vascular volume at the fracture site from 2 to 8 weeks and a significantly larger volume in the Sham group than the OVX group. A significant linear positive correlation (r = 0.87, p < 0.001) was detected between the volumes measured by the two methodologies. Osteoporotic rats had a diminished angiogenic response and lower blood perfusion than Shams. We believe 3D-HF-PDU is feasible and reproducible for in vivo assessment of microvasculature during femoral fracture healing in rats.

Assessment of tumor vasculature for diagnostic and therapeutic applications in a mouse model in vivo using 25-MHz power Doppler imaging

OBJECTIVE

The blood flow rate in the microcirculation associated with angiogenesis plays an important role in the progression and treatment of cancer. Since the microvascular status of tumor vessels can yield useful clinical information, assessing changes in the tumor microcirculation could be particularly helpful for tumor evaluation and treatment planning.

METHODS

In this study we used a self-developed 25-MHz ultrasound imaging system with a spatial resolution of 150 μm for assessing tumor-microcirculation development and the pattern of the vasculature in three tumor-bearing mice in vivo based on power Doppler images. The total Doppler power (DP) and color pixel density (CPD) revealed the presence of functional vessels distributed throughout a tumor volume. The vasculature distributions in the core and periphery were compared to the regulation of vasculature function, which facilitated determination of when the tumor grew rapidly.

RESULTS

The data obtained from a quantified analysis of power Doppler images indicated that the tumor vascularity initially increased throughout the tumor. Both DP and CPD increased rapidly in the tumor periphery when the tumor volume exceeded 10mm(3).

CONCLUSION

Our preclinical findings suggest that power Doppler imaging could be useful for detecting the changes in tumor vascular perfusion and for determining the optimal treatment timing when a tumor begins its rapid volumetric growth.

Photoacoustic correlation spectroscopy and its application to low-speed flow measurement

A photoacoustic correlation technique, inspired by its optical counterpart-the fluorescence correlation spectroscopy (FCS)-was tested for the first time, to our knowledge, to demonstrate the feasibility of low-speed flow measurement based on photoacoustic signal detection. A pulsed laser was used to probe the flow of light-absorbing beads. A photoacoustic correlation system of 0.8 s temporal resolution was built and flow speeds ranging from 249 to 14.9 microm/s with corresponding flow times from 4.42 to 74.1 s were measured. The experiment serves as a proof of concept for photoacoustic correlation spectroscopy, which may have many potential applications similar to the FCS.

Self-demodulation of high-frequency ultrasound

High-frequency (>10 MHz) ultrasound is used in, e.g., small animal imaging or intravascular applications. Currently available ultrasound contrast agents (UCAs) have a suboptimal response for high frequencies. This study therefore investigates the nonlinear propagation effects in a high-frequency ultrasound field (25 MHz) and its use for standard UCA and diagnostic frequencies (1-3 MHz). Nonlinear mixing of two high-frequency carrier waves produces a low-frequency wave, known as the self-demodulation or parametric array effect. Hydrophone experiments showed that the self-demodulated field of a focused 25 MHz transducer (850 kPa source pressure) has an amplitude of 45 kPa at 1.5 MHz in water. Such pressure level is sufficient for UCA excitation. Experimental values are confirmed by numerical simulations using the Khokhlov-Zabolotskaya-Kuznetsov equation on a spatially convergent grid.

Critical review of burn depth assessment techniques: Part I. Historical review

The assessment of burn depth, and as such, the estimation of whether a burn wound is expected to heal on its own within 21 days, is one of the most important roles of the burn surgeon. A false-positive assessment and the patient faces needless surgery, a false-negative one and the patient faces increased length of stay, risks contracture, and hypertrophic scar formation. Although many clinical signs can aid in this determination, accurate assessment of burn depth is possible only 64 to 76% of the time, even for experienced burn surgeons. Through the years, a variety of tools have become available, all attempting to improve clinical accuracy. Part 1 of this two-part article reviews the literature supporting the different adjuvants to clinical decision making is, providing a historical perspective that serves as a framework for part 2, a critical assessment of laser Doppler imaging.

Microbubble destruction by dual-high-frequency ultrasound excitation

The efficiency of high-frequency destruction of microbubble-based contrast agent is limited by the high pressure threshold, while the difficulty of spatially confining destruction induced by low-frequency excitation to a small sample volume potentially increases the risk of adverse bioeffects. The dual-frequency excitation method involves the simultaneous transmission of 2 high-frequency sinusoids to produce an envelope signal at the difference frequency. The envelope signal provides the low-frequency driving force for oscillating the contrast-agent microbubbles to improve destruction efficiency, while the destruction sample volume remains small due to the high frequency of the carrier signal. Experimental results indicate that dual-frequency excitation consistently results in destruction of contrast-agent microbubbles that is superior to using a tone burst at the carrier frequency. With 1 micros pulse length, the acoustic pressure threshold for 95% microbubble destruction markedly reduces from 2.6 MPa to 0.9 MPa when the dual-frequency pulse having envelope frequency of 3 MHz is utilized instead of the 10-MHz sinusoidal pulse. In addition, the dual-frequency pulse having lower envelope frequency generally provides more efficient microbubble destruction, especially when the excitation waveform is long enough to guarantee sufficient envelope component.

A method for assessing the microvasculature in a murine tumor model using contrast-enhanced ultrasonography

OBJECTIVE

The purpose of this study was to develop a method for assessing tumor vascularity in a preclinical model of breast cancer using contrast-enhanced ultrasonography.

METHODS

Eight mice were injected with 67NR breast cancer cells on their hind limbs and imaged with ultrasonography 8 days later. Mice were injected with an ultrasound contrast agent (UCA), and a sequence of images of the resultant backscattered echoes was recorded before and after high-power "destruction" pulses for each of multiple parallel planes. From these, data maps of the maximum contrast enhancement (within each time course) were constructed for each pixel, which enabled reconstruction of high-resolution coregistered sections into a 3-dimensional (3D) volume reflecting tumor vascularity. Additional studies were performed to determine the duration and repeatability of image enhancement, and images were correlated with conventional 3D power Doppler measurements.

RESULTS

The lifetime of the UCA in vivo was found to be 4.3 +/- 1.09 minutes (mean +/- SD). The 3D contrast-enhanced ultrasonographic technique produced images that correlated well with power Doppler images in specific regions but also depicted additional regions of flow surrounding the power Doppler signal. The mean correlation coefficient between voxel measurements of the central slice for each animal was 0.64 +/- 0.07 (P < .01). In addition, sequential studies in each animal were reproducible.

CONCLUSIONS

A method producing high-resolution volumetric assessments of tumor vascularity in a preclinical model of breast cancer is shown that correlates with other ultrasonographic measures of blood flow, which may provide greater sensitivity to the microvasculature.

A high-frame rate duplex ultrasound biomicroscopy for small animal imaging in vivo

Much of the current knowledge of human cardiovascular pathologies and treatment strategies has been gained from understanding the cardiac physiologies and functions in small animal models, such as mice, rats, and zebrafish. In this paper, we present the development of a high-frame-rate duplex ultrasound biomicroscopy (UBM) capable of B-mode imaging and pulsed-wave (PW) Doppler measurement for in vivo cardiovascular investigation in small animals. A frame rate of 200 frames per second (fps) was accomplished at a view of 5 mm x 8 mm, using a novel high-speed sector probe and specially designed lightweight transducers. In a reduced lateral view of 1.2 mm, a frame rate of 400 fps was achieved to examine more detailed cardiac motion. The UBM utilized transducers with different center frequencies (40-75 MHz) and geometries, which made it useful for various applications in small animal cardiac imaging. The highest spatial resolution the UBM achieved was 25 microm x 56 microm. In addition, the image-guided PW Doppler implemented in the UBM demonstrated the detection of the velocity of a moving wire as low as 0.1 mm/s, and flow in a polyimide tube as small as 200 microm in diameter. Furthermore, the UBM achieved a 15- microV minimal detectable signal and a 60-dB dynamic range using a low-cost PCB-based design. Finally, sample in vivo cardiac images of mouse and zebrafish hearts were given. These results showed that the UBM integrated with B-mode and PW Doppler is useful to investigate the pathophysiological mechanism in the cardiovascular studies.

Response of contrast agents to ultrasound

Microbubbles are used as ultrasonic contrast agents that enhance the ultrasound signals of the vascular bed. The recent development of site-targeted microbubbles opened up the possibility for molecular imaging as well as localised drug and gene delivery. Initially the microbubbles' physical properties and their response to the ultrasound beam were not fully understood. However, the introduction of fast acquisition microscopy has allowed the observation of the microbubble behaviour in the presence of ultrasound. In addition, acoustical techniques can determine the scatter of single microbubbles. Sonoporation experiments promise high-specificity drug and gene delivery, but the responsible physical mechanisms, particularly for in vivo applications, are not fully understood. An improvement of microbubble technology may address variability related problems in both imaging and drug/gene delivery.

Assessment of burn depth and burn wound healing potential

The depth of a burn wound and/or its healing potential are the most important determinants of the therapeutic management and of the residual morbidity or scarring. Traditionally, burn surgeons divide burns into superficial which heal by rapid re-epithelialization with minimal scarring and deep burns requiring surgical therapy. Clinical assessment remains the most frequent technique to measure the depth of a burn wound although this has been shown to be accurate in only 60-75% of the cases, even when carried out by an experienced burn surgeon. In this article we review all current modalities useful to provide an objective assessment of the burn wound depth, from simple clinical evaluation to biopsy and histology and to various perfusion measurement techniques such as thermography, vital dyes, video angiography, video microscopy, and laser Doppler techniques. The different needs according to the different diagnostic situations are considered. It is concluded that for the initial emergency assessment, the use of telemetry and simple burn photographs are the best option, that for research purposes a wide range of different techniques can be used but that, most importantly, for the actual treatment decisions, laser Doppler imaging is the only technique that has been shown to accurately predict wound outcome with a large weight of evidence. Moreover this technique has been approved for burn depth assessment by regulatory bodies including the FDA.

Microcirculation volumetric flow assessment using high-resolution, contrast-assisted images

To improve the resolution of contrast-assisted imaging systems, we previously developed a 25-MHz microbubbles-destruction/replenishment imaging system with a spatial resolution of 160 X 160 microm. The goal of the present study was to propose a new approach for functionally evaluating the microvascular volumetric blood flow based on this high-frequency, ultrasound imaging system. The approach includes locating the perfusion area and estimating the blood flow velocity therein. Because the correlation changes between before and after microbubble destruction in two adjacent images, a correlated-based approach was introduced to detect the blood perfusion area. We also have derived a new sigmoid-based model for characterizing the microbubbles replenishment process. Two parameters derived from the sigmoid-based model - the rate constant and inflection time - were adopted to evaluate the blood flow velocity. This model was validated using both simulations and in vitro experiments for mean flow velocities ranging from 1 to 10 mm/s, which showed that the model was in good agreement with simulated and measured microbubble-replenishment time-intensity curves. The results indicate that the actual flow velocity is highly correlated with the estimates of the rate constant and the reciprocal of the inflection time. B-mode imaging experiments for mean flow velocities ranging from 0.4 to 2.1 mm/s were used to assess the volumetric flow in the microcirculation. The results indicated the high correlation between the actual volumetric flow rate and the product of the estimated perfusion area and rate constant, and the reciprocal of the inflection time. We also found that the boundary of the microbubble destruction volume significantly affected estimations of the flow velocity. The perfusion area can be located, and the corresponding flow velocity can be estimated simultaneously in a one-stage, microbubble-destruction/replenishment process, which makes the assessment of the volumetric bloo- d flow in the microcirculation feasible using a real-time, high-frequency ultrasound system.

Ultrasound basics

Imaging technologies for in vivo functional and molecular imaging in small animals have undergone a very fast development in the last years with very intense competition to further develop resolution and molecular sensitivity. Among the imaging technologies available, ultrasound-based molecular imaging methods are of particular interest, since the use of ultrasound contrast agents allows specific and sensitive depiction of molecular targets. Together with new developments in quantification methods of targeted microbubbles, sonography represents a dynamic and seminal tool for molecular imaging.

Volumetric high-frequency Doppler ultrasound enables the assessment of early antiangiogenic therapy effects on tumor xenografts in nude mice

The sensitivity of Doppler ultrasound below 10 MHz to assess antiangiogenic therapy effects in tumor xenografts has been shown to be limited. Thus, our aim was to evaluate high-frequency volumetric power-Doppler ultrasound (HF-VPDU) for monitoring antiangiogenic treatments. Squamous cell carcinoma xenografts grown in nude mice were scanned with HF-VPDU at a center frequency of 30 MHz. Images with 200-microm slice thicknesses were recorded and merged into a three-dimensional dataset, of which the relative color pixel density was determined. Animals received either VEGFR2 antibodies or 0.9% NaCl and were examined at days 0, 3 and 6 of treatment. After the last examination, tumors were resected and their vascularization characterized by immunohistology. At day 6, the volumes of treated and untreated tumors were not significantly different yet. In contrast, mean tumor vascularization in treated animals had decreased to 44%, while in the control group it had increased to 152% (P < 0.01). In correspondence, vessel density, as determined by CD31 staining, was 0.19 +/- 0.10% in treated and 0.92 +/- 0.41% in untreated tumors (P < 0.01). Additionally, the fraction of mature (SMA-positive) vessels increased under therapy. Thus, HF-VPDU can be considered as an easily applicable and fast method to screen high animal numbers for antiangiogenic therapy effects.

Computer-aided diagnosis of prostate cancer with emphasis on ultrasound-based approaches: a review

This paper reviews the state of the art in computer-aided diagnosis of prostate cancer and focuses, in particular, on ultrasound-based techniques for detection of cancer in prostate tissue. The current standard procedure for diagnosis of prostate cancer, i.e., ultrasound-guided biopsy followed by histopathological analysis of tissue samples, is invasive and produces a high rate of false negatives resulting in the need for repeated trials. It is against these backdrops that the search for new methods to diagnose prostate cancer continues. Image-based approaches (such as MRI, ultrasound and elastography) represent a major research trend for diagnosis of prostate cancer. Due to the integration of ultrasound imaging in the current clinical procedure for detection of prostate cancer, we specifically provide a more detailed review of methodologies that use ultrasound RF-spectrum parameters, B-scan texture features and Doppler measures for prostate tissue characterization. We present current and future directions of research aimed at computer-aided detection of prostate cancer and conclude that ultrasound is likely to play an important role in the field.

Investigation of micro-ultrasound for microvessel imaging in a model of chronic total occlusion

The aim of the current study is to investigate the ability of micro-ultrasound (microUS) to identify microvasculature in CTOs in vivo. Results are compared with MRI studies. CTOs were developed in nine porcine superficial femoral arteries (SFA) by percutaneous insertion of a dissolvable polymer plug. This model is characterized by acute thrombosis that later organizes into a fibrotic CTO containing abundant microchannels. 3D microUS images with Power Doppler (PD) overlays from the arteries were acquired at two timepoints: one and eight weeks after placement ofthe polymerplug. Phase contrast MRI and contrast enhanced MRI was also performed. Imaging was performed transcutaneously. Microvessels were identified in vivo in six of eight CTOs using microUS, and in three of seven CTO vessels with MRI, compared with five of seven seen histologically. PW Doppler profiles showed pulsatile blood velocities of approximately 2 cm/s. Intraluminal microvessels within CTOs can be consistently identified by 3D microUS. This technique appears to be more sensitive than MRI. MicroUS may play a role in guiding CTO interventions.

Interframe clutter filtering for high frequency flow imaging

This study investigates 'interframe' clutter filtering with a high frequency (HF) flow imaging system with the objective of improving the performance of HF microvascular imaging at high frame rates. An interframe filter exploits the correlation of tissue signals on the time scale of the frame rate and is, therefore, insensitive to tissue spectral broadening induced by sweeping a single element transducer over a region of tissue. In vitro experiments were conducted in a tissue-mimicking flow phantom over a range of mean flow velocities (0.5 to 70.0 mm/s). Power Doppler (PD) imaging and color flow (CF) imaging were performed for both slow (0.25 fps) and fast (20 fps) scanning acquisitions. Flow data acquired at 20 fps and interframe filtered had similar velocity and mean Doppler power values as the 0.25 fps single-frame filtered data sets. In vivo validation experiments were conducted using a 500 microm blood vessel in a human finger and detected blood flow of 2 to 3 mm/s. Further in vivo experiments examining experimental murine tumors demonstrated the feasibility of performing HF PD and CF imaging at high frame rates using interframe filtering.

Antitumor effects and blood flow dynamics after photodynamic therapy using benzoporphyrin derivative monoacid ring A in KLN205 and LM8 mouse tumor models

Photodynamic therapy (PDT) using benzoporphyrin derivative monoacid ring A (BPD-MA) induces direct tumor cell damage and microvascular injury. We administered BPD-MA at 3h or 15min before laser irradiation to KLN205 and LM8 tumors in murine models. Tumor growth delay was induced more effectively by 15-min-interval PDT than by 3-h-interval PDT. Vascularity and blood perfusion was significantly decreased by 15-min-interval PDT. We observed death of all tumor cells, except peripheral cells, in the 3-h-interval PDT group, and death of cells around the damaged tumor vasculature in the 15-min-interval PDT group. Thus, 15-min-interval PDT enhanced the antitumor effect by damaging tumor vasculature.

Barker-coded ultrasound color flow imaging: theoretical and practical design considerations

Barker-coded excitation is applied to improve the sensitivity versus resolution tradeoff for ultrasound color flow imaging (CFI). Direct sequence encoding and complex baseband decoding methods that enable flexible combination of demodulation frequency and Barker-chip duration are proposed. Based on a general Wiener decoding filter formulation, three different conditions that pertain to the relationship between the Barker-chip duration and demodulation frequency are found, such that they result in real, complex symmetric and complex asymmetric decoding filter sequences, respectively. It is also shown that the matched filter and the inverse filter represent two extreme cases of the Wiener filter, and the latter is proposed for decoding Barker-coded CFI signals with maximum range sidelobe suppression. Some practical considerations such as coding gain, integrated sidelobe level (ISL) and peak sidelobe level (PSL) for various decoding filter lengths, and the influence of flow rate also are analyzed. Linear array imaging of steady flow in straight tubes is simulated based on a 4-cycle base pulse at 6.25 MHz, a 5-chip Barker code, a 32-tap decoding filter, and standard color flow data processing. The resultant color flow images demonstrate the expected improvements in penetration and axial resolution.

Photoacoustic imaging of the microvasculature with a high-frequency ultrasound array transducer

Visualization of microvascular networks could provide new information about function and disease. We demonstrate the capabilities of a 30-MHz ultrasound array system for photoacoustic microscopy of small (< or = 300 microm) vessels in a rat. 3D images obtained by translating the array in the elevation direction are compared with photographs of excised skin. The system is shown to have 100-microm lateral resolution, 25-microm axial resolution, and 3-mm imaging depth. To our knowledge this is the first report on photoacoustic microscopy of the microvasculature with a high-frequency array transducer. It is anticipated that the system can be used for studying and diagnosing a number of diseases including cancer, atherosclerosis, dermatological disorders, and peripheral microvascular complications in diabetes.

L’échographie haute résolution de la souris

Small-animal ultrasound imaging has been made possible using high-resolution imaging devices. The spatial resolution is therefore sufficient to accurately measure anatomical parameters in mice. This paper reviews some of the main applications of high-resolution ultrasound imaging of the mouse and highlights what could be the forthcoming advances.

Dynamic assessment of antiangiogenic therapy by monitoring both tumoral vascularization and tissue degeneration

Tumor growth is dependent both on endothelial and tumor cells. The aim of this study was to investigate dynamically whether changes in tumor vasculature implicate tumor tissue degeneration during antiangiogenic therapies. In order to quantify intra-tumor vascularization and necrosis, we have used ultrasound technology. This study has identified essential parameters needed to quantify specifically and sensitively the number of microvessels and the extent of necrosis in xenografted human carcinomas during natural tumor evolution, using contrast-enhanced high-frequency ultrasonography with (HFCDUS) or without (HFUS) color Doppler. We showed that quantification of intra-tumor microvessels between HFCDUS and immunohistochemistry is correlated using an anti-CD31 antibody. Furthermore, quantification of tumor necrosis with HFUS was confirmed by histological examination of hematoxylin-eosin-saffranin-stained sections over the observation period. Subsequently, for the assessment of novel angiogenic inhibitors, HFCDUS and HFUS were used to elucidate the underlying dynamics linking vessel inhibition and tumor eradication. We describe a novel application for HFCDUS/HFUS that constitutes an effective, convenient, and non-invasive method for clinical assessment of angiogenic inhibitors. In conclusion, we showed that tumor cells abruptly became necrotic following an antivascular therapy, whereas untreated tumors were protected from degeneration by a significant blood supply.

Contrast Harmonic Intravascular Ultrasound

OBJECTIVE

We sought to investigate feasibility of vasa vasorum imaging using the novel technique of contrast harmonic intravascular ultrasound.

METHODS

Prototype intravascular ultrasound (IVUS) instrumentation was developed for the sensitive detection of micro-bubble contrast agents. The technique, "harmonic" imaging, involves transmitting ultrasound at 20 MHz (fundamental) and detecting contrast signals at 40 MHz (second harmonic). Phantom experiments were conducted to investigate the detection of a small vessel in the wall surrounding a larger vessel. In vivo experiments were conducted in atherosclerotic rabbit abdominal aortas.

RESULTS

The phantom experiments showed improved small vessel detection in harmonic mode relative to fundamental mode. For the in vivo experiments, harmonic imaging enabled the visualization of contrast agent outside the aortic lumen through a statistically significant (P < 0.001) enhancement of image power, consistent with the detection of adventitial microvessels. These microvessels were not detected in fundamental imaging mode.

CONCLUSIONS

These results indicate the feasibility of contrast harmonic intravascular ultrasound as a new technique for vasa vasorum imaging.

Ultrahigh frame rate retrospective ultrasound microimaging and blood flow visualization in mice in vivo

To overcome frame rate limitations in high-frequency ultrasound microimaging, new data acquisition techniques have been implemented for 2-D (B-scan) and color flow visualization. These techniques, referred to as retrospective B-scan imaging (RBI) and retrospective color flow imaging (RCFI) are based on the use of the electrocardiogram (ECG) to trigger signal acquisitions. B-scan and color flow images are reconstructed by retrospectively assembling the processed data on a line-by-line basis. Retrospective techniques are used to produce the first in vivo B-scan and color flow images of mouse carotid arteries at frame rates up to 10,000 fps. Retrospective B-scan images of mouse heart were also produced at frame rates of 1000 fps using a version of RBI implemented on a commercial imaging system (Vevo660, VisualSonics, Toronto, ON, Canada). This technology enables detailed in vivo biomechanical studies of dynamic tissues such as the myocardium of the mouse heart with high temporal resolution.

Molecular imaging of antiangiogenic agents

Many novel antiangiogenic agents are currently in various phases of clinical testing. These agents tend to be cytostatic, and therefore few responses are observed with conventional imaging by computerized tomography. Furthermore, toxicity with these agents is seen when the maximum-tolerated dose is combined with chemotherapy. Hence, there is a need to develop imaging strategies that can determine the minimum and optimum biologically active doses. There is increasing awareness of the need to obtain evidence of drug activity through the use of surrogate markers of the biologic mechanism of action during early clinical trials, in addition to determining the pharmacokinetics, toxicity profile, and maximum-tolerated dose. One of the major impediments to the rapid development of antiangiogenic agents in the past has been the lack of validated assays capable of measuring an antiangiogenic effect directly in patients. Recently, dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) has emerged as a useful technique for noninvasive imaging of tumor vasculature in preclinical and clinical models. The problem of tumor heterogeneity remains to be addressed. The major challenge is the standardization of the technique worldwide for the purpose of early clinical studies that are likely to be multicenter. Convincing data on correlations between changes observed through molecular imaging and changes in tumor angiogenesis, and hence tumor biology, are still lacking. Whether this would translate into a survival advantage remains to be seen. The ultimate test of the surrogate biological end points determined by molecular imaging will occur in randomized phase III trials. Results of the first randomized trial that showed a survival advantage in favor of antiangiogenic agents were released at the American Society of Clinical Oncology meeting in 2003. There it was reported that the combination of 5-fluorouracil, leucovorin, and irinotecan (Camptosar; Pfizer Pharmaceuticals; New York, NY) with anti-vascular endothelial growth factor antibody (bevacizumab-Avastin; Genentech, Inc.; South San Francisco, CA) was superior to the chemotherapy regimen alone when used to treat patients with metastatic colorectal cancer. However, until further phase III clinical trials confirm these results, surrogate end points of clinical efficacy of the newer agents are urgently needed so that development of ineffective drugs can be halted early. This review briefly discusses the role of molecular imaging in general, and DCE-MRI in particular, in relation to treatment with antiangiogenic agents and highlights some of the difficulties encountered in this area.

Quantitative assessment of tumor vasculature and response to therapy in kaposi's sarcoma using functional noninvasive imaging

Two noninvasive methods, thermography and laser Doppler imaging (LDI), were assessed for their ability to quantitatively assess parameters of vascularity in lesions of HIV-associated Kaposi's sarcoma (KS). Thermography and LDI images of a representative KS lesion were recorded in 16 patients and compared to normal skin either adjacent to the lesion or on the contralateral side. Eleven of the 16 patients had greater than 0.5 degrees C increased temperature and 12 of the 16 patients had increased flux (measured by LDI) as compared to normal skin. There was a strong correlation between these two parameters (R = 0.81, p < 0.001). In ten patients, measurements were obtained prior to therapy and after receiving a regimen of liposomal doxorubicin and interleukin-12. After 18 weeks of therapy, temperature and blood flow of the lesions were significantly reduced from the baseline (p = 0.004 and 0.002 respectively). These techniques hold promise to assess physiologic parameters in KS lesions and their changes with therapy.

High-frequency, nonlinear flow imaging of microbubble contrast agents

It has been shown that nonlinear scattering can be stimulated from microbubble contrast agents at high-transmit frequencies (14-32 MHz). This work was extended to demonstrate the feasibility of nonlinear contrast imaging through modifications of existing ultrasound biomicroscopy linear B-scan imaging instrumentation. In this study, we describe the development and evaluation of prototype coherent flow imaging instrumentation for nonlinear microbubble imaging using transmit frequencies from 10 to 50 MHz. Phantom validation experiments were conducted to demonstrate color and power flow imaging using nonlinear 10 MHz (subharmonic) scattering induced by a 20 MHz transmit frequency. In vivo flow imaging of a rabbit ear microvessel was successfully performed. This work indicates the feasibility of performing flow imaging at high frequencies using nonlinear scattering from microbubbles.

High frequency nonlinear B-scan imaging of microbubble contrast agents

It previously was shown that it is possible to produce nonlinear scattering from microbubble contrast agents using transmit frequencies in the 14-32 MHz range, suggesting the possibility of performing high-frequency, nonlinear microbubble imaging. In this study, we describe the development of nonlinear microbubble B-scan imaging instrumentation capable of operating at transmit center frequencies between 10 and 50 MHz. The system underwent validation experiments using transmit frequencies of 20 and 30 MHz. Agent characterization experiments demonstrate the presence of nonlinear scattering for the conditions used in this study. Using wall-less vessel phantoms, nonlinear B-scan imaging is performed using energy in one of the subharmonic, ultraharmonic, and second harmonic frequency regions for transmit frequencies of 20 and 30 MHz. Both subharmonic and ultraharmonic imaging modes achieved suppression of tissue signals to below the noise floor while achieving contrast to noise ratios of up to 26 and 17 dB, respectively. The performance of second harmonic imaging was compromised by nonlinear propagation and offered no significant contrast improvement over fundamental mode imaging. In vivo experiments using the subharmonic of a 20 MHz transmit pulse show the successful detection of microvessels in the rabbit ear and in the mouse heart. The results of this study demonstrate the feasibility of nonlinear microbubble imaging at high frequencies.

High-resolution functional vascular assessment with ultrasound

In order to improve the resolution of contrast-assisted imaging systems, we have created a high-frequency destruction/contrast replenishment imaging system with a spatial resolution of 160 microm x 160 microm. The system utilizes a 1-MHz cylindrically focused transducer for destruction and a 25-MHz spherically focused transducer for pulse/echo imaging. Speckle tracking and a clutter filter are applied across frames to remove the challenging physiologic motion artifacts that are obtained when imaging with a mechanically scanned transducer. Using a new estimation technique, flow constants proportional to absolute flow rate were estimated from B-mode time-intensity curves (TICs). The in vitro results indicate a correlation between the actual flow velocity and the estimated rate constant. In vivo images are presented showing blood perfusion in the ciliary processes and iris of the rabbit eye. The regions of interest (ROIs) from the ciliary processes yielded slower perfusion compared with the iris, as expected from vascular casts of the microcirculation in this region. Potential applications of this system include high-resolution perfusion assessment in small animals.

Transit time kinetics in ordered and disordered vascular trees

Imaging modalities exploit tracer-dilution methods to measure bulk haemodynamic parameters such as blood flow and volume at the level of the microcirculation. Here, we ask the question of whether the kinetics of a tracer can reveal morphological information about the vessels through which the tracers flow. The goal is to relate the acquired time-intensity characteristic to details of the vascular structure that lies below the imaging resolution. Two fractal vascular models are developed that represent organized 'kidney-like' and disorganized 'tumour-like' structures. The models are generated using simple rules of branching and fractal geometry in two dimensions. Blood flow and tracer kinetics are simulated using fundamental laws of haemodynamics. The flow conditions are matched in the two models. The fractal box dimensions of the kidney (D(B) = 1.67 +/- 0.01) and the tumour (D(B) = 1.80 +/- 0.01) vasculatures fall in the range given in the literature (D(B) = 1.61 +/- 0.06 and D(B) = 1.84 +/- 0.04, respectively). The tracer kinetic curves of the kidney and the tumour vasculatures have the same initial slope and final asymptote, corresponding to the same flow rate and vascular volume, but have different forms. The difference in the two curves is related to the distribution function of transit times of the vascular models, and is a consequence of the randomness introduced in vessel diameter and length. In principle, the form of the tracer kinetic curve from a contrast imaging study may offer information relating not only to vascular volume and flow rate, but also to the organization of a microvascular network.

Radiosensitivity of rat mammary tumors correlates with early vessel changes assessed by power Doppler sonography

OBJECTIVE

To investigate the changes occurring in the vascularization of tumors during irradiation, we used a model of autochthonous mammary tumors in rats and assessed early vascular changes after irradiation by power Doppler sonography.

METHODS

Mammary tumors were induced in 24 female Sprague Dawley rats by a single subcutaneous injection of N-nitroso N-methyl urea. After tumor areas reached 1 cm2, the animals received a single fraction of 18-Gy radiation or intraperitoneal saline injection. Power Doppler sonographic quantification of detected vessels was performed 1 day before irradiation and 7 days after the use of a power Doppler index of 5 different tumor imaging planes. Final tumor shrinkage was compared with early changes in the power Doppler index. Not all tumors regressed in a similar fashion. Radiosensitive tumors were defined as tumors with a greater than 50% decrease in baseline area 28 days after irradiation, whereas radioresistant tumors were tumors with a less than 50% decrease in baseline area. Statistical analysis was performed by the Mann-Whitney U test.

RESULTS

Tumor area changes were similar in radioresistant and radiosensitive tumors 7 days after irradiation (-41% and -35%, respectively; P > .05, not significant), whereas reduction in the power Doppler index was significantly greater in radiosensitive tumors (mean value, -63%) than in radioresistant tumors (mean value, -12%) (P = .001). Late tumor regrowth was correlated with day 7 power Doppler index changes (P = .009). A 40% reduction in the power Doppler index at day 7 distinguished 8 of 9 radiosensitive tumors and 8 of 9 radioresistant tumors (P = .003).

CONCLUSIONS

This study suggests that early changes in tumor perfusion as assessed by power Doppler sonography after tumor irradiation may precede the long-term tumor regression.

Comparison of intermittent-bolus contrast imaging with conventional power Doppler sonography: quantification of tumour perfusion in small animals

Replenishment kinetics of microbubbles were adapted to a single bolus injection to investigate tumour angiogenesis in small animals with intermittent imaging, and to compare vascularisation parameters from this new approach with conventional power Doppler ultrasound (US). A reformulation of the imaging protocol and the derivation of perfusion parameters was necessary, taking into account the time-dependence of the systemic microbubble concentration after single bolus injection. Using this new method, tumour vascularisation was evaluated in 13 experimental murine tumours. Furthermore, parameters calculated with intermittent imaging after bolus injection of 100 microl Levovist were compared with parameters from the signal intensity-time curve. The results showed that quantifying tumour perfusion, blood volume and flow, as well as the assessment of the mean blood velocity (in m/s), is possible in tumours with a volume of more than 0.1 mL. In larger tumours, a lower perfusion was calculated than in smaller ones (k = -0.88; p < 0.001). Only limited correlations were found between conventional power Doppler US quantities and parameters of intermittent sonography: Perfusion correlated with the maximum signal intensity (k = 0.61, p < 0.05) and the gradient to maximum (k = 0.82, p < 0.01), full width-half maximum was associated with blood volume (k = 0.62, p < 0.05). We conclude that intermittent bolus contrast sonography allows the quantification of tumour perfusion, even in small animals, and the monitoring of basic antiangiogenic studies with perfusion parameters shows a higher significance than conventional power Doppler US.

Diagnostic value of high-resolution B-mode and power-mode sonography in the follow-up of thyroid cancer

OBJECTIVE

Ultrasonography is an established diagnostic modality in the follow-up of thyroid cancer. Color flow Doppler has been proposed by some authors as an additional tool for differentiating benign from malignant cervical lesions in various types of head and neck cancer. Over the last few years, a new generation of high-resolution ultrasound platforms with the "power-mode" feature has become available, that also enables the imaging of small vessel blood flow. The objective of our study was to find ways of optimizing the differentiation of benign and malignant cervical tumors in thyroid cancer follow-up by means of sonography.

METHODS

Hundred and twelve cervical lesions in 90 patients with thyroid cancer were evaluated by high-end ultrasonography (Sonoline Elegra, Siemens) using a small-part transducer (7.5 L 40, Siemens). B-mode sonography was performed at a frequency of 8 MHz. The Solbiati index (SI= ratio of largest to smallest diameter), configuration, echogenicity, intranodular structures, and margins were assessed. Perinodular and intranodular blood flow was evaluated by color flow Doppler (PRF 1250 Hz for conventional color flow Doppler, 868 Hz for power-mode Doppler). Possible malignancy was validated by histology, cytology, scintigraphy, and follow-up. Thirty five lesions were benign (diameter 0.4-3.0 cm) and 77 were malignant (0.4-5.4 cm). The patients were randomized into a test group and a learning group to determine the diagnostic value of various ultrasound criteria by means of statistical analysis. In the learning group, decision rules based on the dichotomized criteria were developed using a logistic regression model. Sensitivity and specificity of these decision rules were then evaluated in the test group.

RESULTS

The presence of an echocomplex pattern or irregular hyperechoic small intranodular structures (criterion A) and the presence of an irregular diffuse intranodular blood flow (criterion B) are the best indicators of malignancy, whereas an SI >>2 is highly indicative of benign changes. Color flow Doppler is a useful addition to B-mode scanning for distinguishing benign and malignant neoplasms in the follow-up of thyroid cancer. Power-mode Doppler sonography significantly improves imaging of perinodular and intranodular blood flow when compared with conventional color flow Doppler.

CONCLUSION

We propose the following decision rules based on a combination of the criteria above: (A) and (B) fulfilled: malignant, if SI< or =4; (B) but not (A) fulfilled: malignant, if SI< or =3; (A) but not (B) fulfilled: malignant, if SI< or =2; neither (A) nor (B) fulfilled: malignant, if SI approximately equal to 1 (sensitivity: 90%; specificity: 82%; accuracy 88%).

High-frequency 3-D color-flow imaging of the microcirculation

High-frequency (> 20 MHz) ultrasound (US) flow imaging has the potential to be an important tool for assessing microvascular blood flow in superficial tissues noninvasively. In this paper, we describe the development and evaluation of a 3-D US flow imaging system capable of operating at center frequencies in the 20- to 50-MHz range. Flow images are made for tissue volumes of sizes up to 10 mm laterally and 5 mm in depth, permitting a range of scientific and clinical applications. To acquire data sets in a reasonable time, the 2-D sections were derived from data collected with a transducer that was scanning continuously in a direction perpendicular to the beam axis. Due to spectral broadening effects induced by scanning tissue, significant tradeoffs must be made between frame rate, lateral resolution and the minimum detectable blood velocity. 3-D flow images were reconstructed with flow data acquired from a series of adjacent planes. The system was evaluated at a center frequency of 50 MHz, using two PVDF transducers with lateral resolutions of 43 microm and 65 microm and axial resolutions of 66 microm to 72 microm, respectively. Velocity ranges were from below 1 mm/s to 25 mm/s. In vivo validation experiments using the mouse ear demonstrated the ability to follow branching patterns of closely spaced microvessels from 30 microm to 100 microm in diameter. Experiments conducted on mouse tumors successfully imaged microvessel morphology in the tumor microcirculation.

A new high resolution color flow system using an eigendecomposition-based adaptive filter for clutter rejection

We present a new signal processing strategy for high frequency color flow mapping in moving tissue environments. A new application of an eigendecomposition-based clutter rejection filter is presented with modifications to deal with high blood-to-clutter ratios (BCR). Additionally, a new method for correcting blood velocity estimates with an estimated tissue motion profile is detailed. The performance of the clutter filter and velocity estimation strategies is quantified using a new swept-scan signal model. In vivo color flow images are presented to illustrate the potential of the system for mapping blood flow in the microcirculation with external tissue motion.

Functional ultrasound methods in oncological imaging

The real time nature of ultrasound and functional methods such as Doppler ultrasound mean that ultrasound can claim to have always been a functional imaging method, but recent developments in quantitation, dramatic improvement in Doppler performance and now microbubbles have created many exciting new applications. These include methods for assessing the neovascularity of tumours, for following the effects of therapy and for predicting the likelihood of development of metastatic disease at the staging of primary tumours.