Acoustic Angiography Research Articles

The "Fingerprint" of Cancer Extends Beyond Solid Tumor Boundaries: Assessment With a Novel Ultrasound Imaging Approach.

Abnormalities of microvascular morphology have been associated with tumor angiogenesis for more than a decade, and are believed to be intimately related to both tumor malignancy and response to treatment. However, the study of these vascular changes in-vivo has been challenged due to the lack of imaging approaches which can assess the microvasculature in 3-D volumes noninvasively. Here, we use contrast-enhanced "acoustic angiography" ultrasound imaging to observe and quantify heterogeneity in vascular morphology around solid tumors. Acoustic angiography, a recent advance in contrast-enhanced ultrasound imaging, generates high-resolution microvascular images unlike anything possible with standard ultrasound imaging techniques. Acoustic angiography images of a genetically engineered mouse breast cancer model were acquired to develop an image acquisition and processing routine that isolated radially expanding regions of a 3-D image from the tumor boundary to the edge of the imaging field for assessment of vascular morphology of tumor and surrounding vessels. Quantitative analysis of vessel tortuosity for the tissue surrounding tumors 3 to 7mm in diameter revealed that tortuosity decreased in a region 6 to 10mm from the tumor boundary, but was still significantly elevated when compared to control vasculature. Our analysis of angiogenesis-induced changes in the vasculature outside the tumor margin reveals that the extent of abnormal tortuosity extends significantly beyond the primary tumor mass. Visualization of abnormal vascular tortuosity may make acoustic angiography an invaluable tool for early tumor detection based on quantifying the vascular footprint of small tumors and a sensitive method for understanding changes in the vascular microenvironment during tumor progression.

Phantom evaluation of stacked-type dual-frequency 1–3 composite transducers: A feasibility study on intracavitary acoustic angiography

In this paper, we present phantom evaluation results of a stacked-type dual-frequency 1–3 piezoelectric composite transducer as a feasibility study for intracavitary acoustic angiography. Our previous design (6.5/30MHz PMN–PT single crystal transducer) for intravascular contrast ultrasound imaging exhibited a contrast-to-tissue ratio (CTR) of 12dB with a penetration depth of 2.5mm. For improved penetration depth (>3mm) and comparable contrast-to-tissue ratio (>12dB), we evaluated a lower frequency 2/14MHz PZT 1–3 composite transducer. Superharmonic imaging performance of this transducer and a detailed characterization of key parameters for acoustic angiography are presented. The 2/14MHz arrangement demonstrated a −6dB fractional bandwidth of 56.5% for the transmitter and 41.8% for the receiver, and produced sufficient peak-negative pressures (>1.5MPa) at 2MHz to induce a strong nonlinear harmonic response from microbubble contrast agents. In an in-vitro contrast ultrasound study using a tissue mimicking phantom and 200μm cellulose microvessels, higher harmonic microbubble responses, from the 5th through the 7th harmonics, were detected with a signal-to-noise ratio of 16dB. The microvessels were resolved in a two-dimensional image with a −6dB axial resolution of 615μm (5.5times the wavelength of 14MHz waves) and a contrast-to-tissue ratio of 16dB. This feasibility study, including detailed explanation of phantom evaluation and characterization procedures for key parameters, will be useful for the development of future dual-frequency array transducers for intracavitary acoustic angiography.

Design factors of intravascular dual frequency transducers for super-harmonic contrast imaging and acoustic angiography

Imaging of coronary vasa vasorum may lead to assessment of the vulnerable plaque development in diagnosis of atherosclerosis diseases. Dual frequency transducers capable of detection of microbubble super-harmonics have shown promise as a new contrast-enhanced intravascular ultrasound (CE-IVUS) platform with the capability of vasa vasorum imaging. Contrast-to-tissue ratio (CTR) in CE-IVUS imaging can be closely associated with low frequency transmitter performance. In this paper, transducer designs encompassing different transducer layouts, transmitting frequencies, and transducer materials are compared for optimization of imaging performance. In the layout selection, the stacked configuration showed superior super-harmonic imaging compared with the interleaved configuration. In the transmitter frequency selection, a decrease in frequency from 6.5 MHz to 5 MHz resulted in an increase of CTR from 15 dB to 22 dB when receiving frequency was kept constant at 30 MHz. In the material selection, the dual frequency transducer with the lead magnesium niobate-lead titanate (PMN-PT) 1–3 composite transmitter yielded higher axial resolution compared to single crystal transmitters (70 μm compared to 150 μm pulse length). These comparisons provide guidelines for the design of intravascular acoustic angiography transducers.

Quantification of Microvascular Tortuosity during Tumor Evolution Using Acoustic Angiography

The recent design of ultra-broadband, multifrequency ultrasound transducers has enabled high-sensitivity, high-resolution contrast imaging, with very efficient suppression of tissue background using a technique called acoustic angiography. Here we perform the first application of acoustic angiography to evolving tumors in mice predisposed to develop mammary carcinoma, with the intent of visualizing and quantifying angiogenesis progression associated with tumor growth. Metrics compared include vascular density and two measures of vessel tortuosity quantified from segmentations of vessels traversing and surrounding 24 tumors and abdominal vessels from control mice. Quantitative morphologic analysis of tumor vessels revealed significantly increased vascular tortuosity abnormalities associated with tumor growth, with the distance metric elevated approximately 14% and the sum of angles metric increased 60% in tumor vessels versus controls. Future applications of this imaging approach may provide clinicians with a new tool in tumor detection, differentiation or evaluation, though with limited depth of penetration using the current configuration.

Optimization of Contrast-to-Tissue Ratio Through Pulse Windowing in Dual-Frequency “Acoustic Angiography” Imaging

Early-stage tumors in many cancers are characterized by vascular remodeling, indicative of transformations in cell function. We have previously presented a high-resolution ultrasound imaging approach to detecting these changes that is based on microbubble contrast agents. In this technique, images are formed from only the higher harmonics of microbubble contrast agents, producing images of vasculature alone with 100- to 200-μm resolution. In this study, shaped transmit pulses were used to image the higher broadband harmonic echoes of microbubble contrast agents, and the effects of varying pulse window and phasing on microbubble and tissue harmonic echoes were evaluated using a dual-frequency transducer in vitro and in vivo. An increase in the contrast-to-tissue ratio of 6.8 ± 2.3 dB was observed in vitro using an inverted pulse with a cosine window relative to a non-inverted pulse with a rectangular window. The increase in mean image intensity resulting from contrast enhancement in vivo in five rodents was 13.9 ± 3.0 dB greater for an inverted cosine-windowed pulse and 17.8 ± 3.6 dB greater for a non-inverted Gaussian-windowed pulse relative to a non-inverted pulse with a rectangular window. Implications for pre-clinical and diagnostic imaging are discussed.

On the Relationship Between Microbubble Fragmentation, Deflation and Broadband Superharmonic Signal Production

Acoustic angiography imaging of microbubble contrast agents uses the superharmonic energy produced from excited microbubbles and enables high-contrast, high-resolution imaging. However, the exact mechanism by which broadband harmonic energy is produced is not fully understood. To elucidate the role of microbubble shell fragmentation in superharmonic signal production, simultaneous optical and acoustic measurements were performed on individual microbubbles at transmit frequencies from 1.75 to 3.75 MHz and pressures near the shell fragmentation threshold for microbubbles of varying diameter. High-amplitude, broadband superharmonic signals were produced with shell fragmentation, whereas weaker signals (approximately 25% of peak amplitude) were observed in the presence of shrinking bubbles. Furthermore, when populations of stationary microbubbles were imaged with a dual-frequency ultrasound imaging system, a sharper decline in image intensity with respect to frame number was observed for 1-μm bubbles than for 4-μm bubbles. Finally, in a study of two rodents, increasing frame rate from 4 to 7 Hz resulted in decreases in mean steady-state image intensity of 27% at 1000 kPa and 29% at 1300 kPa. Although the existence of superharmonic signals when bubbles shrink has the potential to prolong the imaging efficacy of microbubbles, parameters such as frame rate and peak pressure must be balanced with expected re-perfusion rate to maintain adequate contrast during in vivo imaging.

Acoustic Angiography: High-Resolution Contrast-Enhanced Ultrasound Imaging of the Microvasculature

Presenting Author: Paul A. Dayton1 Co-authors: Co-authors: Sarah Shelton1, Brooks Lindsey1, Ryan Gessner1, Yueh Z. Lee4, Stephen Aylward2, Mike Lee3, Emmanuel Cherin3, F. Stuart Foster3 Joint Graduate Department of Biomedical Engineering, University of North Carolina and North Carolina State University, NC, USA Kitware Medical Imaging, 101 East Weaver Street, Carrboro, NC Department of Medical Biophysics, Sunnybrook Health Sciences Centre,Toronto, ON, Canada Department of Neuroradiology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599 Contrast enhanced ultrasound is FDA approved in the US for cardiology applications, and has demonstrated substantial utility off-label and in preclinical research studies. However, to date, contrast enhanced ultrasound in patients is largely performed at low frequencies and does not provide sufficient resolution or tissue suppression to visualize structural details of microvasculature blood flow. Through the use of new dual-frequency ultrasound transducer technology, contrast enhanced ultrasound can be performed with unprecedented resolution and contrast-to-tissue ratio, albeit at shallow penetration depths. This imaging technique enables the visualization of microvascular architecture without signal from background tissues, and can simultaneously provide anatomical information for registration. The resulting images are similar to x-ray angiography, and provide a new means to assess microvascular density and structure. Initial results indicate this new imaging modality can provide non-invasive insight into angiogenic processes involved in tumor growth and progression. We demonstrate the application of this imaging technique in evaluation of cancer angiogenesis in pre-clinical models, and provide a report of first in-human use. Presenting Author: Paul A. Dayton1 Co-authors: Co-authors: Sarah Shelton1, Brooks Lindsey1, Ryan Gessner1, Yueh Z. Lee4, Stephen Aylward2, Mike Lee3, Emmanuel Cherin3, F. Stuart Foster3 Joint Graduate Department of Biomedical Engineering, University of North Carolina and North Carolina State University, NC, USA Kitware Medical Imaging, 101 East Weaver Street, Carrboro, NC Department of Medical Biophysics, Sunnybrook Health Sciences Centre,Toronto, ON, Canada Department of Neuroradiology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599 Contrast enhanced ultrasound is FDA approved in the US for cardiology applications, and has demonstrated substantial utility off-label and in preclinical research studies. However, to date, contrast enhanced ultrasound in patients is largely performed at low frequencies and does not provide sufficient resolution or tissue suppression to visualize structural details of microvasculature blood flow. Through the use of new dual-frequency ultrasound transducer technology, contrast enhanced ultrasound can be performed with unprecedented resolution and contrast-to-tissue ratio, albeit at shallow penetration depths. This imaging technique enables the visualization of microvascular architecture without signal from background tissues, and can simultaneously provide anatomical information for registration. The resulting images are similar to x-ray angiography, and provide a new means to assess microvascular density and structure. Initial results indicate this new imaging modality can provide non-invasive insight into angiogenic processes involved in tumor growth and progression. We demonstrate the application of this imaging technique in evaluation of cancer angiogenesis in pre-clinical models, and provide a report of first in-human use.

Design factors of intravascular dual frequency transducers for super-harmonic contrast imaging and acoustic angiography

Design factors of intravascular dual frequency transducers for super-harmonic contrast imaging and acoustic angiography

Quantification of Microvascular Tortuosity during Tumor Evolution Using Acoustic Angiography

Quantification of Microvascular Tortuosity during Tumor Evolution Using Acoustic Angiography

Acoustic characterization of contrast-to-tissue ratio and axial resolution for dual-frequency contrast-specific acoustic angiography imaging.

Recently, dual-frequency transducers have enabled high-spatial-resolution and high-contrast imaging of vasculature with minimal tissue artifacts by transmitting at a low frequency and receiving broadband superharmonic echoes scattered by microbubble contrast agents. In this work, we examine the imaging parameters for optimizing contrast-to-tissue ratio (CTR) for dual-frequency imaging and the relationship with spatial resolution. Confocal piston transducers are used in a water bath setup to measure the SNR, CTR, and axial resolution for ultrasound imaging of nonlinear scattering of microbubble contrast agents when transmitting at a lower frequency (1.5 to 8 MHz) and receiving at a higher frequency (7.5 to 25 MHz). Parameters varied include the frequency and peak negative pressure of transmitted waves, center frequency of the receiving transducer, microbubble concentration, and microbubble size. CTR is maximized at the lowest transmission frequencies but would be acceptable for imaging in the 1.5 to 3.5 MHz range. At these frequencies, CTR is optimized when a receiving transducer with a center frequency of 10 MHz is used, with the maximum CTR of 25.5 dB occurring when transmitting at 1.5 MHz with a peak negative pressure of 1600 kPa and receiving with a center frequency of 10 MHz. Axial resolution is influenced more heavily by the receiving center frequency, with a weak decrease in measured pulse lengths associated with increasing transmit frequency. A microbubble population containing predominately 4-μm-diameter bubbles yielded the greatest CTR, followed by 1- and then 2-μm bubbles. Varying concentration showed little effect over the tested parameters. CTR dependence on transmit frequency and peak pressure were confirmed through in vivo imaging in two rodents. These findings may lead to improved imaging of vascular remodeling in superficial or luminal cancers such as those of the breast, prostate, and colon.

A preliminary engineering design of intravascular dual-frequency transducers for contrast-enhanced acoustic angiography and molecular imaging.

Current intravascular ultrasound (IVUS) probes are not optimized for contrast detection because of their design for high-frequency fundamental-mode imaging. However, data from transcutaneous contrast imaging suggests the possibility of utilizing contrast ultrasound for molecular imaging or vasa vasorum assessment to further elucidate atherosclerotic plaque deposition. This paper presents the design, fabrication, and characterization of a small-aperture (0.6 × 3 mm) IVUS probe optimized for high-frequency contrast imaging. The design utilizes a dual-frequency (6.5 MHz/30 MHz) transducer arrangement for exciting microbubbles at low frequencies (near their resonance) and detecting their broadband harmonics at high frequencies, minimizing detected tissue backscatter. The prototype probe is able to generate nonlinear microbubble response with more than 1.2 MPa of rarefractional pressure (mechanical index: 0.48) at 6.5 MHz, and is also able to detect microbubble response with a broadband receiving element (center frequency: 30 MHz, -6-dB fractional bandwidth: 58.6%). Nonlinear super-harmonics from microbubbles flowing through a 200-μm-diameter micro-tube were clearly detected with a signal-to-noise ratio higher than 12 dB. Preliminary phantom imaging at the fundamental frequency (30 MHz) and dual-frequency super-harmonic imaging results suggest the promise of small aperture, dual-frequency IVUS transducers for contrast-enhanced IVUS imaging.

A preliminary engineering design of intravascular dual-frequency transducers for contrast-enhanced acoustic angiography and molecular imaging

Current intravascular ultrasound (IVUS) probes are not optimized for contrast detection because of their design for high-frequency fundamental-mode imaging. However, data from transcutaneous contrast imaging suggests the possibility of utilizing contrast ultrasound for molecular imaging or vasa vasorum assessment to further elucidate atherosclerotic plaque deposition. This paper presents the design, fabrication, and characterization of a small-aperture (0.6 × 3 mm) IVUS probe optimized for high-frequency contrast imaging. The design utilizes a dual-frequency (6.5 MHz/30 MHz) transducer arrangement for exciting microbubbles at low frequencies (near their resonance) and detecting their broadband harmonics at high frequencies, minimizing detected tissue backscatter. The prototype probe is able to generate nonlinear microbubble response with more than 1.2 MPa of rarefractional pressure (mechanical index: 0.48) at 6.5 MHz, and is also able to detect microbubble response with a broadband receiving element (center frequency: 30 MHz, -6-dB fractional bandwidth: 58.6%). Nonlinear super-harmonics from microbubbles flowing through a 200-μm-diameter micro-tube were clearly detected with a signal-to-noise ratio higher than 12 dB. Preliminary phantom imaging at the fundamental frequency (30 MHz) and dual-frequency super-harmonic imaging results suggest the promise of small aperture, dual-frequency IVUS transducers for contrast-enhanced IVUS imaging.

A preliminary engineering design of intravascular dual-frequency transducers for contrast-enhanced acoustic angiography and molecular imaging

A preliminary engineering design of intravascular dual-frequency transducers for contrast-enhanced acoustic angiography and molecular imaging

Acoustic angiography ultrasound imaging can distinguish vascular differences in tumor cell lines

Acoustic Angiography is a novel ultrasound imaging technology for visualizing intravascular microbubble contrast agents. It uses a prototype dual-frequency transducer to transmit ultrasound pulses at a low frequency near microbubble resonance, and confocally receives higher frequency signal from the broadband microbubble response. By utilizing two widely separated frequencies (transmit at 4 MHz, receive at 30 MHz), it is possible to form a high-resolution image of the vascular structure without any tissue background. Both Acoustic Angiography and standard amplitude-modulation, nonlinear contrast perfusion imaging were used to compare the vascularity of xenograft tumors composed of two tumor cell sub-types with known differences in vascularity. In both image types, tumors were manually segmented in three dimensions and the percent of pixels containing contrast signal was computed. Acoustic Angiography imaging proved to be more sensitive to the vascular differences than standard nonlinear perfusion imaging. For a population of 16 animals, the sensitivity of Acoustic Angiography was 0.73, whereas the sensitivity of non-linear perfusion imaging was 0.13, given a significance level of 0.05. Additionally, two-sided t-tests showed a statistically significant difference between tumor types for Acoustic Angiography images (p = 0.0025), but not for standard nonlinear contrast imaging (p = 0.39).

Effects of contrast agents and high intensity focused ultrasound on tumor vascular perfusion

High intensity focused ultrasound (HIFU) is a treatment modality which causes localized tissue ablation and is further enhanced when microbubble or nanodroplet contrast agents are present. While it is known that HIFU ablation results in diminished blood perfusion, these effects have never been mapped with high-resolution 3-D imaging. We aimed to longitudinally and volumetrically evaluate the effects of microbubbles and nanodroplets on tumor vessel perfusion following HIFU treatment. Rats bearing flank fibrosarcoma tumors were injected with 1.5 × 109 microbubbles (n = 3), nanodroplets (n = 3), or control/no-injection (n = 3) during targeted HIFU (125W/cm2, 1 MHz, 2 MPa, 10% duty cycle, 1 Hz PRF, 3 min). This HIFU power is ~99% less than clinical norms. Tumor perfusion was assessed by contrast enhanced 3-D acoustic angiography before, immediately following, and 72 h after HIFU application. No significant changes in tumor perfusion were observed when HIFU was applied with nanodroplet or control injections. However, HIFU in the presence of microbubbles resulted in a 66.2±15.2% (p < 0.05) reduction in perfused tumor volume. These results suggest that HIFU, when combined with microbubbles, can dramatically remodel tumor vasculature networks even at the low pressure used in this study. This could potentially enhance the safety of HIFU therapy.

WE‐E‐134‐02: High Resolution Acoustic Angiography for Early Assessment of Tumor Response to Radiation Treatment in Rat Tumor Model

Purpose: To test the hypothesis that acoustic angiography would enable early assessment of tumor response to radiation therapy at multiple timepoints after a single‐fraction radiotherapy treatment. Methods: 15 fibrosarcoma tumor‐bearing rats were irradiated (field‐size:1.8×1.8cm) by 6MV photon beam from a clinical linac. Tumors were implanted subcutaneously in the lower flank area and treated 14 days later (Mean‐diameter: 7.03+/−1.62mm). Animals were randomized into three groups (doses of 0, 5 or 20 Gy). Tumors were imaged prior to treatment (Day 0) using both acoustic angiography (microbubble contrast‐based microvessel imaging) and standard high frequency b‐mode, and then again at Days 3, 6, 9, 12, and 18. Tumor volume, and perfused volume ratio (PVR ‐ the ratio of tumor volume with perfused voxels, to total tumor volume) were quantified at each timepoint. Results: The tumors in the 0 or 5 Gy groups continued to grow exponentially after treatment. All animals in the 20 Gy group experienced an inflection point in their growth curve at Day 3, and tumor volume reduced exponentially until Day 12. Three of these tumors are non‐responders(20Gy‐NRs) and had second inflection point at Day 12, and proceeded to increase in volume, on average to their original Day 0 tumor size by Day 18. The other two tumors in the 20 Gy group are responders(20Gy‐R' s) and reduced to undetectable tumor volumes by Day 18. The PVR metric for vascularization revealed differences in the 20Gy‐NR and 20Gy‐R groups at Day 12, which correlated with tumor volumetric differences. Conclusion: Our preliminary data indicate that the acoustic angiography‐derived PVR metric can detect rat fibrosarcoma tumors responded (N=2) and those which did not (N=3) to 20 Gy radiation six days before there is a detectable difference in tumor volumes.

Acoustic Angiography: A New Imaging Modality for Assessing Microvasculature Architecture

The purpose of this paper is to provide the biomedical imaging community with details of a new high resolution contrast imaging approach referred to as “acoustic angiography.” Through the use of dual-frequency ultrasound transducer technology, images acquired with this approach possess both high resolution and a high contrast-to-tissue ratio, which enables the visualization of microvascular architecture without significant contribution from background tissues. Additionally, volumetric vessel-tissue integration can be visualized by using b-mode overlays acquired with the same probe. We present a brief technical overview of how the images are acquired, followed by several examples of images of both healthy and diseased tissue volumes. 3D images from alternate modalities often used in preclinical imaging, contrast-enhanced micro-CT and photoacoustics, are also included to provide a perspective on how acoustic angiography has qualitatively similar capabilities to these other techniques. These preliminary images provide visually compelling evidence to suggest that acoustic angiography may serve as a powerful new tool in preclinical and future clinical imaging.

Imaging tortuosity: the potential utility of acoustic angiography in cancer detection and tumor assessment

Imaging tortuosity: the potential utility of acoustic angiography in cancer detection and tumor assessment

Mapping Microvasculature with Acoustic Angiography Yields Quantifiable Differences between Healthy and Tumor-bearing Tissue Volumes in a Rodent Model

To determine if the morphologies of microvessels could be extracted from contrast material-enhanced acoustic angiographic ultrasonographic (US) images and used as a quantitative basis for distinguishing healthy from diseased tissue. All studies were institutional animal care and use committee approved. Three-dimensional contrast-enhanced acoustic angiographic images were acquired in both healthy (n = 7) and tumor-bearing (n = 10) rats. High-spatial-resolution and high signal-to-noise acquisition was enabled by using a prototype dual-frequency US transducer (transmit at 4 MHz, receive at 30 MHz). A segmentation algorithm was utilized to extract microvessel structure from image data, and the distance metric (DM) and the sum of angles metric (SOAM), designed to distinguish different types of tortuosity, were applied to image data. The vessel populations extracted from tumor-bearing tissue volumes were compared against vessels extracted from tissue volumes in the same anatomic location within healthy control animals by using the two-sided Student t test. Metrics of microvascular tortuosity were significantly higher in the tumor population. The average DM of the tumor population (1.34 ± 0.40 [standard deviation]) was 23.76% higher than that of the control population (1.08 ± 0.08) (P < .0001), while the average SOAM (22.53 ± 7.82) was 50.73% higher than that of the control population (14.95 ± 4.83) (P < .0001). The DM and SOAM metrics for the control and tumor populations were significantly different when all vessels were pooled between the two animal populations. In addition, each animal in the tumor population had significantly different DM and SOAM metrics relative to the control population (P < .05 for all; P value ranges for DM, 3.89 × 10(-)(7) to 5.63 × 10(-)(3); and those for SOAM, 2.42 × 10(-)(12) to 1.57 × 10(-)(3)). Vascular network quantification by using high-spatial-resolution acoustic angiographic images is feasible. Data suggest that the angiogenic processes associated with tumor development in the models studied result in higher instances of vessel tortuosity near the tumor site.

WE-C-218-01: Ultrasound Contrast Agents.

1. Understand the basic principles of ultrasound contrast agents a. What are microbubble contrast agents? b. Properties of microbubbles c. Safety concerns and biological effects 2. Understand basic contrast imaging techniques a. Harmonic and suharmonic imaging techniques b. Pulse inversion techniques 3. Understand the use of contrast agents in various vascular applications a. Traditional methods (Cardiovascular, Abdominal) b. Advanced perfusion imaging techniques 4. Understand the role of contrast agents in preclinical applications a. Ultrasound molecular imaging b. Gene therapy c. Drug delivery d. Acoustic angiography.