A new ultrasound instrument for in vivo microimaging of mice

Nonlinear contrast imaging with an array-based micro-ultrasound system

The main goal of this study was to determine the optimal strategy for a real-time nonlinear contrast mode for small-animal imaging at high frequencies, on a new array-based micro-ultrasound system. Previously reported contrast imaging at frequencies above 15 MHz has primarily relied on subtraction schemes involving B-mode image data. These approaches provide insufficient contrast to tissue ratios under many imaging conditions. In this work, pulse inversion, amplitude modulation and combinations of these were systematically investigated for the detection of nonlinear fundamental and subharmonic signal components to maximize contrast-to-tissue ratio (CTR) in the 18-24 MHz range. From in vitro and in vivo measurements, nonlinear fundamental detection with amplitude modulation provided optimal results, allowing an improvement in CTR of 13 dB compared with fundamental imaging. Based on this detection scheme, in vivo parametric images of murine kidneys were generated using sequences of nonlinear contrast images after intravenous bolus injections of microbubble suspensions. Initial parametric images of peak enhancement (PE), wash-in rate (WiR) and rise time (RT) are presented. The parametric images are indicative of blood perfusion kinetics, which, in the context of preclinical imaging with small animals, are anticipated to provide valuable insights into the progression of human disease models, where blood perfusion plays a critical role in either the diagnosis or treatment of the disease.

Measurement of absolute blood flow velocity in outflow tract of HH18 chicken embryo based on 4D reconstruction using spectral domain optical coherence tomography

The measurement of blood-plasma absolute velocity distributions with high spatial and temporal resolution in vivo is important for the investigation of embryonic heart at its early stage of development. We introduce a novel method to measure absolute blood flow velocity based on high speed spectral domain optical coherence tomography (OCT) and apply it to measure velocities across the heart outflow tract (OFT) of a chicken embryo (stage HH18). First, we use the OCT system to acquire 4D 
[(x,y,z) + t] images of the OFT in vivo. Second, we reconstruct the 4D microstructural images and obtain the orientation of the OFT at its maximum expansion, from which the centerline of the OFT is calculated based on the OFT boundary segmentation. Assuming flow is parallel to the vessel orientation, the obtained centerline indicates the flow direction. Finally, the absolute flow velocity is evaluated based on the direction given by the centerline and the axial velocity obtained from Doppler OCT. Using this method, we compare flow velocity profiles at various positions along the chicken embryo OFT.

An evaluation of transmitral and pulmonary venous Doppler indices for assessing murine left ventricular diastolic function

With the continued development of genetically engineered mouse models of cardiac disease, further advancement of noninvasive techniques for evaluating cardiac diastolic dysfunction in these models would be valuable. Therefore, we performed comprehensive transmitral and pulmonary venous Doppler echocardiographic studies to devise novel indices of diastolic function in a mouse model with cardiac hypertrophy, which were validated against invasively measured hemodynamic parameters. We examined 10 HopX(Tg) transgenic mice with diastolic dysfunction and 10 age-matched controls sedated with 1% to 2% isoflurane (male, age 14-18 weeks). These studies revealed that the acceleration time of the transmitral Doppler E-wave was the best Doppler parameter for unmasking LV diastolic dysfunction in HopX(Tg) mice. This is the first study to assess the utility of the acceleration time of the E-wave and pulmonary venous Doppler echocardiography as a primary diagnostic modality for assessing murine diastolic function.

Molecular ultrasound imaging: current status and future directions

Targeted contrast-enhanced ultrasound (molecular ultrasound) is an emerging imaging strategy that combines ultrasound technology with novel molecularly-targeted ultrasound contrast agents for assessing biological processes at the molecular level. Molecular ultrasound contrast agents are nano- or micro-sized particles that are targeted to specific molecular markers by adding high-affinity binding ligands onto the surface of the particles. Following intravenous administration, these targeted ultrasound contrast agents accumulate at tissue sites overexpressing specific molecular markers, thereby enhancing the ultrasound imaging signal. High spatial and temporal resolution, real-time imaging, non-invasiveness, relatively low costs, lack of ionising irradiation and wide availability of ultrasound systems are advantages compared to other molecular imaging modalities. In this article we review current concepts and future directions of molecular ultrasound imaging, including different classes of molecular ultrasound contrast agents, ongoing technical developments of pre-clinical and clinical ultrasound systems, the potential of molecular ultrasound for imaging different diseases at the molecular level, and the translation of molecular ultrasound into the clinic.

Effect of microbubble size on fundamental mode high frequency ultrasound imaging in mice

High-frequency ultrasound imaging using microbubble (MB) contrast agents is becoming increasingly popular in pre-clinical and small animal studies of anatomy, flow and vascular expression of molecular epitopes. Currently, in vivo imaging studies rely on highly polydisperse microbubble suspensions, which may provide a complex and varied acoustic response. To study the effect of individual microbubble size populations, microbubbles of 1-2 microm, 4-5 microm and 6-8 microm diameter were isolated using the technique of differential centrifugation. Size-selected microbubbles were imaged in the mouse kidney over a range of concentrations using a Visualsonics Vevo 770 ultrasound imaging system (Visualsonics, Toronto, Ontario, Canada) with a 40-MHz probe in fundamental mode. Results demonstrate that contrast enhancement and circulation persistence are strongly dependent on microbubble size and concentration. Large microbubbles (4-5 and 6-8 microm) strongly enhanced the ultrasound image with positive contrast, while 1-2 microm microbubbles showed little enhancement. For example, the total integrated contrast enhancement, measured by the area under the time-intensity curve (AUC), increased 16-fold for 6-8 microm diameter microbubbles at 5 x 10(7) MB/bolus compared with 4-5 microm microbubbles at the same concentration. Interestingly, 1-2 microm diameter microbubbles, at any concentration, did not measurably enhance the integrated ultrasound signal at tissue depth, but did noticeably attenuate the signal, indicating that they had a low scattering-to-attenuation ratio. When concentration matched, larger microbubbles were more persistent in circulation. However, when volume matched, all microbubble sizes had a similar circulation half-life. These results indicated that dissolution of the gas core plays a larger role in contrast elimination than filtering by the lungs and spleen. The results of this study show that microbubbles can be tailored for optimal contrast enhancement in fundamental mode imaging.

Experimental orthotopic prostate tumor in nude mice: techniques for local cell inoculation and three-dimensional ultrasound monitoring

OBJECTIVES

Orthotopic prostate cancer models are of great importance for cancer research. Orthotopic models in mice have been described previously. However, these studies lack a detailed methodological description and fail to define standards for local cell inoculation. Herein, we studied the effect of different protocols on tumor growth and report for the first time the use of high resolution ultrasound for monitoring of tumor growth.

MATERIALS AND METHODS

Orthotopic inoculation of DU 145 MN1 prostate cancer cells was performed in 30 nude mice varying (1) the amount of cells (5 × 10(5) vs. 5 × 10(4)), (2) the number of puncture sites, and (3) the addition of matrigel. Surgical complications such as recoil of cells through the injection canal and rupture of the prostatic capsule were monitored. Animals were tracked by ultrasound imaging after 4, 5, and 6 weeks. Autopsy and histology confirmed local tumor growth.

RESULTS

A take rate of 27/30 (90%) was observed. Growth of orthotopic prostate tumors was increased after inoculation of a large amount of cells under the capsule of 1 dorsal prostate lobe, but inoculation of small amounts of cells still induced local tumors. Noninvasive ultrasound examination allowed to identify orthotopic tumor formation and to monitor tumor growth in vivo. Addition of matrigel did not accelerate tumor growth. Complications like recoil (6.8%) or rupture of the prostate capsule (1.4%) were rare.

CONCLUSIONS

Inoculation of DU 145 MN1 cells under the prostate capsule with a defined procedure results in very high take rates. Ultrasound screening is feasible to repetitively monitor tumor growth.

Ultrasonic microscanning

A detailed review is given of the application of high-frequency ultrasound (HFUS) at frequencies of 20 MHz and above for high-resolution, cross-sectional imaging of biological soft tissue. The state of the art of HFUS imaging systems is discussed with respect to the underlying engineering concepts, system designs, and available transducer technology. Furthermore, the dependency of the spatial resolution on the system's parameters is analysed. Skin imaging, eye imaging, small animal imaging for preclinical research, and intravascular ultrasound in coronary arteries for arteriosclerotic disease diagnostics are presented as examples for the application of HFUS imaging in medical diagnostics. It is shown that, in the frame of the indicated applications, ultrasound in the frequency range 20-100MHz gives a good compromise between the contrary demands for a good spatial resolution and a sufficiently large penetration depth of ultrasound waves into the tissue. Scanning schemes for the imaging of tissue morphology are considered, including spatial compounding as a multidirectional imaging technique.

Effect of triangular pillar geometry on high- frequency piezocomposite transducers

Piezocomposite materials are used extensively in biomedical transducer array fabrication. However, developing high-frequency piezocomposite materials for imaging systems is still a challenge due to the extremely small pillar dimensions required to avoid the interference from lateral resonances. The use of triangular pillar piezocomposite material has been shown to suppress lateral resonances that appear in square pillar composite designs. To further understand how the geometry of the pillars affects the lateral resonances, piezocomposite materials with triangular pillars of different angles have been simulated and fabricated. Simulations were performed on composite transducers of 70-microm pitch, 18-microm kerf width, and 100-microm thickness with isosceles triangular pillars in which the isosceles angle varied from 30 degrees to 60 degrees using a finite-element analysis. By varying the pillar geometry, the composite transducers show large differences in lateral resonances. The simulation results demonstrate that the composite with 45 degrees angle pillars has the lowest secondary pulse amplitude. The secondary pulse becomes larger when the pillar angle deviates from 45 degrees . To study whether the pillar height (which determines the resonance frequency) and aspect ratio would change the optimum angle, composites with 40-mum pitch, 15-microm kerf width, and 45-microm thickness were also simulated. Finally, the composite with triangle pillars was compared with composites with square and round pillars. The simulation results show that the 45 degrees triangular pillar geometry is, for high-frequency applications, the best configuration among all investigated in this work. Composite samples have also been fabricated to confirm results from finite-element modeling. Acoustical and electrical measurements were carried out to compare with theoretical predictions. Three composite transducers with pillar angles of 30 degrees, 45 degrees, and 60 degrees were fabricated using a dice-and-fill technique. The measured electrical impedances and one-way pulse responses agreed well with the theoretical predictions and confirm the optimal nature of the 45 degrees design.

Ultrasound Biomicroscopy for In vivo architectural characterization of gastrocnemius muscle from rats

This work applies the Ultrasound Biomicroscopy (UBM) technique to quantify the pennation angle (PA) and muscle thickness (MT) of rats' gastrocnemius muscle and to determine the reliability of these measurements. UBM (40MHz) images of five Wistar female rats were acquired at two ankle positions (neutral and full extension) and in two different days. A total of 320 images were processed to quantify PA and MT and a statistical analysis assessed data variability and reliability. The coefficients of variation were 9.37 and 3.97% for PA and MT, respectively, for the ankle at full extension and 15.41 and 4.99% for the ankle at neutral position. Pearson correlation between two repeated measurements in the same image were 0.93 and 0.99 for PA and MT, respectively. The results indicate that UBM is suitable for quantitative muscle architectural characterization and can be used in future muscle biomechanical studies.

[Ultrasonographic characterization of an experimental model of liver metastases from colon carcinoma in rats]

OBJECTIVE

To determine the ultrasonographic characteristics of liver metastases induced in a rat model; to evaluate the usefulness of ultrasonography in the noninvasive evaluation of tumor infiltration.

MATERIAL AND METHODS

We seeded the livers of 46 WAG/RijCrl rats with CC-531 syngeneic colorectal carcinoma cells by intrasplenic inoculation. At 21, 28, 35, 42, 70 days after tumor seeding, we performed a series of ultrasonographic examinations to evaluate tumor induction in different groups of animals: 37 rats were studied with a 10 MHz linear probe and 9 were studied with a 6-18 MHz multifrequency probe. The following signs were considered indicative of tumor development: intrahepatic nodules, sinuate liver borders, lobe enlargement, and extrahepatic masses. Ultrasonographic findings were verified at autopsy. We determined the number of implants, size (less than 3mm, between 3 and 7 mm, or greater than 7 mm), and lobe location for each technique.

RESULTS

Compared to the autopsy results, ultrasonography detected 64% of the animals with disease. All the extrahepatic masses were correctly diagnosed. Metastases were identified in 90% of the rats with lesions greater than 7 mm, in 75% of those with implants between 3 and 7 mm, and in 25% of those with lesions less than 3mm. In the group in which we used the 6-18 MHz probe, we detected 50% of the lesions less than 3mm.

CONCLUSION

Ultrasonography was useful for monitoring the experimental model and enabled the noninvasive oncologic evaluation of the rat liver with reasonable sensitivity.

Combined photoacoustic and ultrasound biomicroscopy

We report on the development of an imaging system capable of combined ultrasound and photoacoustic imaging based on a fast-scanning single-element 25-MHz ultrasound transducer and a unique light-delivery system. The system is capable of 20 ultrasound frames per second and slower photoacoustic frame rates limited by laser pulse-repetition rates. Laser and ultrasound pulses are interlaced for co-registration of photoacoustic and ultrasound images. In vivo imaging of a human finger permits ultrasonic visualization of vessel structures and speckle changes indicative of blood flow, while overlaid photoacoustic images highlight some small vessels that are not clear from the ultrasound scan. Photoacoustic images provide optical absorption contrast co-registered in the structural and blood-flow context of ultrasound with high-spatial resolution and may prove important for clinical diagnostics and basic science of the microvasculature.

A new 15-50 MHz array-based micro-ultrasound scanner for preclinical imaging

Most institutions now have a suite of imaging tools to follow mouse models of human disease. Micro-ultrasound is one of these tools and is second after whole-mouse fluorescence or bioluminescent imaging, in terms of installed systems. We report in this paper the first commercially available array transducer-based ultrasound imaging system that enables micro-ultrasound imaging at center frequencies between 15 and 50 MHz. At the heart of the new scanner is a laser-machined high-frequency 256 element, linear transducer array capable of forming dynamic diffraction limited beams. The power of the linear array approach is embodied in the uniform high resolution maintained over the full field of view. This leads to greatly expanded scope for real-time functional imaging that is demonstrated in this paper. The unprecedented images made with the new imaging system will enable many new applications not previously possible. These include real-time visualization of flow in the mouse placenta, visualization of flow development in the embryo, studies of embryonic to adult cardiac development/disease, and studies of real-time blood flow in mouse models of tumour angiogenesis.

Morphological ultrasound microimaging of thyroid in living mice

The objective of the study was to explore high-frequency ultrasound (HFUS) for noninvasive microimaging of thyroid in living mice. Thyroid examination was performed by HFUS in 10 normal C57BL/6 mice, eight mice treated by propylthiouracil, and 22 Tg-TRK-T1 transgenic mice. The dimension of the gland and the presence of nodules were evaluated. Nodules were classified as malignant (hypoechogenicity, poorly defined margins, internal microcalcification, irregular shapes, and extra glandular extension) or not, and the findings were compared with histological data. Thyroid images were successfully obtained in all the animals analyzed. Normal thyroid reached a volume of 4.92 microl (range 2.11-4.92 microl). Mice with propylthiouracil-induced goiter showed diffuse thyroid enlargement (median volume 6.67 microl, range 4.09-8.82 microl). In 19 of 22 Tg-TRK-T1 mice (86%), HFUS identified a nodular process (the smallest detected nodule had a diameter of 0.46 mm). Eleven nodules were classified as malignant and eight as benign. Compared with histological analysis, HFUS showed a sensitivity of 100% in the detection of thyroid nodules and a specificity of 60% (two of the nodules identified by HFUS were not confirmed at the histology). The specificity and sensitivity of HFUS in predicting the malignancy of the thyroid nodules were 83 and 91%, respectively. Thus, HFUS is an accurate imaging modality that can potentially replace more invasive techniques, and, therefore, it represents a significant advancement in phenotypic assessment of mouse models of thyroid cancer.

Increased intima thickness of the radial artery in patients with coronary heart disease

Ultrabiomicroscopy is a novel high-frequency (55 MHz) ultrasound technique that could be used to non-invasively measure the vessel wall and separate the intima—media complex into measurements of intima and media thickness. Since no previous study has measured intima and media thickness separately in vivo in patients with coronary heart disease (CHD), the aim of the current study was to measure intima and intima—media thickness of the radial and the anterior tibial arteries among patients with CHD and healthy subjects (HS). Thirty-two patients with CHD and 46 HS underwent investigations with ultrabiomicroscopy measurements of the radial and anterior tibial arteries. Patients with CHD showed a 19% increase in intima thickness of the radial artery compared with HS (0.088 ± 0.024 mm versus 0.074 ± 0.015 mm; p < 0.015), whereas no difference was seen in media thickness. There were no differences in intima or media thickness within the anterior tibial arteries. In conclusion, CHD is associated with thickening of the intima of the radial artery whereas media thickness was unchanged compared with HS. Assessment of intima thickness by high-frequency ultrasound may provide a tool for non-invasive early detection of atherosclerosis.

Distributed three-dimensional simulation of B-mode ultrasound imaging using a first-order k-space method

Computational modeling is an important tool in ultrasound imaging research, but realistic three-dimensional (3D) simulations can exceed the capabilities of serial computers. This paper uses a 3D simulator based on a k-space method that incorporates relaxation absorption and nonreflecting boundary conditions. The simulator, which runs on computer clusters, computes the propagation of a single wavefront. In this paper, an allocation algorithm is introduced to assign each scan line to a group of nodes and use multiple groups to compute independent lines concurrently. The computational complexity required for realistic simulations is analyzed using example calculations of ultrasonic propagation and attenuation in the 30-50 MHz band. Parallel efficiency for B-mode imaging simulations is evaluated for various numbers of scan lines and cluster nodes. An aperture-projection technique is introduced to simulate imaging with a focused transducer using reduced computation grids. This technique is employed to synthesize B-mode images that show realistic 3D refraction artifacts. Parallel computing using 20 nodes to compute groups of ten scan lines concurrently reduced the execution time for each image to 18.6 h, compared to a serial execution time of 357.5 h. The results demonstrate that fully 3D imaging simulations are practical using contemporary computing technology.

A method for differentiating targeted microbubbles in real time using subharmonic micro-ultrasound and interframe filtering

This study introduces a new method for differentiating targeted microbubbles in the presence of flowing microbubbles and tissue using micro-ultrasound. The method relies on subharmonic (SH) imaging for segmenting microbubble signals from tissue signals, and low-pass interframe filtering for segmenting bound targeted microbubbles from flowing microbubbles. The method is evaluated with 30 frames per second SH B-mode imaging in vitro, using a wall-less vessel flow phantom. The SH B-mode cineloops were postprocessed using an interframe moving average filter to segment the regions of bound microbubbles on the inner surface of the vessel phantom. The bound bubbles were then disrupted with sufficiently high ultrasound pressures, so that the dynamic process of targeted microbubble binding under flowing conditions could be observed. These preliminary results show that the proposed method is a feasible solution to the challenge of differentiating targeted microbubbles in the presence of tissue and freely flowing microbubbles at high frequencies, which in turn should improve the specificity of targeted microbubble detection.

Use of ultrasound biomicroscopy to image human ovaries in vitro

OBJECTIVES

To test the hypothesis that ultrasound biomicroscopy is an effective tool for imaging human ovaries in vitro.

METHODS

This was a prospective, observational study of 11 women (mean age, 48.9 +/- 2.3 years; range, 40-65 years) undergoing unilateral or bilateral oophorectomy. Ovaries were obtained in the operating room and imaged in a cooled saline bath using conventional two-dimensional (2D) ultrasound. Follicles identified using conventional 2D ultrasound were then imaged in a cooled saline bath using ultrasound biomicroscopy. Ovaries were then placed in 10% neutral-buffered formaldehyde and transported to the Pathology Department for histological evaluation. Digital images of individually identified ovarian follicles using conventional ultrasound, ultrasound biomicroscopy and histology were obtained (n = 22). Thicknesses of the follicle wall, granulosa layer and theca interna layer were measured using standardized techniques.

RESULTS

The imaging sensitivity, defined as the smallest follicle visualized, was greater using ultrasound biomicroscopy (0.33 +/- 0.07 mm) than it was using conventional ultrasound (1.5 +/- 0.21 mm; P < 0.0001). The mean follicle wall thickness was greater using conventional ultrasound (0.675 +/- 0.039 mm) compared with ultrasound biomicroscopy (0.254 +/- 0.017 mm), which in turn, was greater than that measured by histology (0.160 +/- 0.009 mm) (P < 0.0001). The mean granulosa and theca layer thickness measurements, respectively, were greater using ultrasound biomicroscopy (0.067 +/- 0.004 mm; 0.186 +/- 0.014 mm) compared with histology (0.034 +/- 0.002 mm; 0.126 +/- 0.008 mm) (P < or = 0.001). Protrusions from the follicle wall into the antrum, believed to represent cumulus-oocyte complexes, were visualized in five of 22 follicles using ultrasound biomicroscopy.

CONCLUSIONS

The ultrasound biomicroscope was an effective tool to image human ovaries in vitro. Ultrasound biomicroscopy provided greater sensitivity and resolution for imaging ovarian follicles compared with conventional 2D ultrasonographic techniques.

Scanning head with 128-element 20-MHz PVDF linear array transducer

A scanning head has been designed and fabricated that incorporates a 20-MHz, 128-element linear transducer. The scanning head also incorporates -200 V pulsers and a custom 16-channel amplifier. The transducer was constructed with 28 microm PVDF film with an element pitch of 250 microm. The transducer showed an average -20 dB pulse length of 69 ns. The elements of the PVDF array were tested and found to have 7.5 mPa/ radical Hz equivalent noise pressure. The radiated power level for 32 pulsed elements was approximately 1 MPa. An imaging test shows that the system achieves axial and lateral resolutions of 40 microm and 0.2 mm, respectively. The entire scanning head dissipates approximately 1.6 W at a pulse repetition rate of 750 Hz.

Radiotherapy combined with intratumoral dendritic cell vaccination enhances the therapeutic efficacy of adoptive T-cell transfer

Treatment of C57BL/6 mice with cyclophosphamide (100 mg/kg) and fludarabine (200 mg/kg) induced nonmyeloablative lymphodepletion without inhibiting D5 melanoma tumor growth. Using this model, we found that induction of lymphopenia before adoptive transfer of ex vivo anti-CD3/CD28 activated and interleukin-2 expanded D5-G6 tumor draining lymph node cells enhanced the antitumor efficacy of the infused cells in both pulmonary metastases and subcutaneous D5 bearing mice. However, induction of lymphopenia did not promote intratumoral or extratumoral proliferation or accumulation of the infused cells. We have previously shown that radiotherapy enhances the therapeutic efficacy of intratumoral unpulsed dendritic cell vaccination in subcutaneous murine tumor models by augmenting the induction of antitumor cellular immune responses. Here, we confirmed this finding in a murine metastatic melanoma liver tumor model. Furthermore, local tumor irradiation combined with intratumoral dendritic cell administration significantly enhanced the therapeutic efficacy of tumor-reactive T cell adoptive transfer in this lymphodepleted liver tumor model. This was evident by reduced liver tumor size, decreased incidence of spontaneous intra-abdominal metastasis, and prolonged survival, resulting in 46% of mice cured. This enhanced antitumor activity was associated with a selective increase in proliferation, accumulation, and function of CD4+ rather than CD8+ infused cells. This multimodality regimen may have translational applications for the treatment of human cancers.

Novel use of ultrasound to examine regional blood flow in the mouse kidney

Conventional methods used for measuring regional renal blood flow, such as laser-Doppler flowmetry, are highly invasive, and each measurement is restricted to a discrete location. The aim of this study was to determine whether ultrasound imaging in conjunction with enhanced contrast agent (microbubbles; Vevo MicroMarker, VisualSonics) could provide a viable noninvasive alternative. This was achieved by determining changes in renal cortical and medullary rate of perfusion in response to a bolus injection of endothelin-1 (ET-1; 0.6, 1.0, or 2.0 nmol/kg) and comparing these responses to those observed in separate groups of mice with conventional laser-Doppler methods. Intravenous infusion of ET-1 in anesthetized male C57bl/6 mice resulted in a dose-dependent increase in mean arterial pressure and a dose-dependent decrease in total renal blood flow as measured by pulse-wave Doppler. ET-1 infusion resulted in a dose-dependent decrease in regional kidney perfusion as measured by both ultrasound with enhanced contrast agent and laser-Doppler measurements, verifying the use of ultrasound to measure regional kidney perfusion. Noted limitations of ultrasound imaging compared with laser-Doppler flowmetry included a lower degree of sensitivity to changes in tissue perfusion and the inability to assess rapid or transient changes in tissue perfusion. In conclusion, ultrasound represents an effective and noninvasive method for the measurement of relatively short-term, steady-state changes in regional blood flow in the mouse kidney.

High-frequency ultrasound to grade disease progression in murine models of Duchenne muscular dystrophy

OBJECTIVE

This study used high-frequency ultrasound (HFU) imaging to assess muscle damage noninvasively in a longitudinal study of 2 transgenic murine models of Duchenne muscular dystrophy (DMD): mdx, which has mutated cytoskeletal protein dystrophin; and udx, which has mutated dystrophin and lacks another cytoskeleton protein, utrophin. The mdx group was further subdivided into exercised and nonexercised subgroups to assess exercise-induced damage.

METHODS

Muscle damage was assessed with HFU imaging (40 MHz) at biweekly intervals for 16 weeks. The assessment was based on the number of hyperechoic lesions, the lesion diameter, and muscle disorganization, giving a combined grade according to a 5-point scale.

RESULTS

High-frequency ultrasound discriminated the severity of muscle damage between wild-type and transgenic models of DMD and between mdx and udx models. Qualitative comparisons of 3-dimensional HFU images with serial histologic sections of the skeletal muscle showed the ability of ultrasound to accurately depict changes seen in the muscle architecture in vivo.

CONCLUSIONS

High-frequency ultrasound images soft tissue in mice at high contrast and spatial resolution, thereby showing that this microimaging modality has the capability to assess architectural changes in muscle fibers due to myotonic dystrophy-related diseases such as DMD.

In vitro ultrasound biomicroscopic imaging of colitis in rats

OBJECTIVE

The purpose of this study was to show the feasibility of 50-MHz ultrasound biomicroscopy (UBM) to image the rat colon.

METHODS

B-mode images were obtained from ex vivo colon samples (n = 4) collected from Rattus norvegicus (Berkenhout, 1769) rats, with 2,4,6-trinitrobenzene sulfonic acid-induced colitis in 3 of them. Left colon rectangular fragments (5 x 5 mm) were obtained after necropsy, and UBM images were acquired with the samples immersed in saline at 37 degrees C. All layers of the normal intestinal wall were analyzed according to their thickness and the presence of uneven bowel mucosa (ulcers). The folds and layers detected by UBM were correlated with histopathologic analysis.

RESULTS

The 4 layers of the normal colon were identified on the UBM images: the mucosa (hyperechoic), muscularis mucosae (hypoechoic), submucosa (hyperechoic), and muscularis externa (hypoechoic). On 2 UBM images, superficial ulcers were detected, approximately 0.5 mm in size, with intestinal involvement limited to the mucosa. The histopathologic analysis verified enlargement of submucosa layers due to an edema associated with sub-mucosa leukocyte infiltration. On 1 UBM image, it was possible to detect a deep ulcer, which was confirmed by the light microscopic analysis.

CONCLUSIONS

An ultrasound imaging system was scaled and optimized to visualize the rat colon. Ultrasound biomicroscopy provided axial and lateral resolutions close to 25 and 45 mum, respectively, and adequate penetration depth to visualize the whole thickness of an inflamed colon. The system identified the colon layers and was able to detect mural changes and superficial ulcers on the order of 500 mum.

Ovarian imaging in the mouse using ultrasound biomicroscopy (UBM): a validation study

The mouse is a well accepted model for studies of human reproduction despite little being known about follicle dynamics in this species. Longitudinal studies of mouse folliculogenesis have been hampered by the lack of an appropriate imaging tool. Ultrasound biomicroscopy (UBM) may overcome this obstacle as it confers near-microscopic resolution through the use of high-frequency ultrasound waves. The objective of the present study was to determine whether UBM could be used to count and measure ovarian follicles and corpora lutea (CL) reliably in mice. Ovaries of 25 adult CD-1 mice were imaged using a 55-MHz transducer and then excised and processed for histology. Follicles and CL were counted and measured from digitally stored UBM cine-loops and photographed histological sections. Differences between techniques were assessed by Bland-Altman agreement analyses. Follicle counts yielded by the two techniques varied by only +/-1 follicle when follicles ranged between 300 and 499 microm. Perfect agreement among counts was evident when follicles were >500 microm. The total number of CL was accurately estimated using UBM; however, the number of 350-699 microm CL was underestimated and the number of CL>or=700 microm was overestimated. In conclusion, UBM can be used reliably to count and measure follicles in mice.

An integrated optoacoustic transducer combining etalon and black PDMS structures

An integrated optoacoustic transducer combining etalon and black polydimethylsiloxane (PDMS) structures has been designed and developed. The device consists of an 11-μm-thick black PDMS film confined to a 2-mm-diameter circular region acting as an optoacoustic transmitter, surrounded by a 5.9-μm Fabry-Perot polymer etalon structure serving as an optoacoustic detector array. A pulsed laser is focused onto a 30-μm spot on the black PDMS film, defining the transmit element, while a CW laser probes a 20-μm spot on the etalon for ultrasound detection. Pulse-echo signals display center frequencies of above 30 MHz with bandwidths of at least 40 MHz. A theta-array is formed for 3-D ultrasound imaging by mechanically scanning the generation laser along a 1-D array and the detection laser around an annular array. Preliminary images with 3 metal wires as imaging targets are presented. Characterization of the device’s acoustical properties, as well as preliminary imaging results, suggest that all-optical ultrasound transducers are potential alternatives to piezoelectric techniques for high-frequency 2-D arrays enabling 3-D high-resolution ultrasound imaging.

In vivo spectral domain optical coherence tomography volumetric imaging and spectral Doppler velocimetry of early stage embryonic chicken heart development

Progress toward understanding embryonic heart development has been hampered by the inability to image embryonic heart structure and simultaneously measure blood flow dynamics in vivo. We have developed a spectral domain optical coherence tomography system for in vivo volumetric imaging of the chicken embryo heart. We have also developed a technique called spectral Doppler velocimetry (SDV) for quantitative measurement of blood flow dynamics. We present in vivo volume images of the embryonic heart from initial tube formation to development of endocardial cushions of the same embryo over several stages of development. SDV measurements reveal the influence of heart tube structure on blood flow dynamics.

Use of ultrasound imaging for early diagnosis of pregnancy and determination of litter size in the mouse

Precocity and efficiency of ultrasonography for pregnancy diagnosis and determination of litter size in mice were assessed on a total of 46 adult mice of different lines (19 BALB/c, 15 C57BL/6 and 12 CD1) from Day 4.5 after vaginal plug. Different commercial ultrasound machines and probes (linear versus sectorial; 7.5 MHz versus 10 MHz) were compared. The best images were obtained by the use of 10 MHz linear transducers. The first visualization compatible with pregnancy, specifically with the implantation sites, was observed at Day 4.5 in three animals. Presence of embryonic vesicles was differentiated at Day 5.5 in all the females. The embryos and remaining gestational structures inside the vesicles were clearly distinguished at Day 8.5. Data were validated not only after delivery but also by comparison with postmortem findings on crucial days (Days 4.5 and 8.5). The efficiency for counting the exact number of embryos was very low, due mostly to underestimation in highly prolific females. Conversely, the estimation of the range of the number of conceptuses, instead of the total number, was more accurate. Sensitivity, specificity and total efficiency reached 100% at Day 8.5. Ultrasonography can be accurately used as an alternative non-invasive technique for pregnancy diagnosis and determination of litter size in the mouse from very early stages of gestation, replacing other procedures currently used and increasing efficiency in animal management and research.

Thin polymer etalon arrays for high-resolution photoacoustic imaging

Thin polymer etalons are demonstrated as high-frequency ultrasound sensors for three-dimensional (3-D) high-resolution photoacoustic imaging. The etalon, a Fabry-Perot optical resonator, consists of a thin polymer slab sandwiched between two gold layers. It is probed with a scanning continuous-wave (CW) laser for ultrasound array detection. Detection bandwidth of a 20-microm-diam array element exceeds 50 MHz, and the ultrasound sensitivity is comparable to polyvinylidene fluoride (PVDF) equivalents of similar size. In a typical photoacoustic imaging setup, a pulsed laser beam illuminates the imaging target, where optical energy is absorbed and acoustic waves are generated through the thermoelastic effect. An ultrasound detection array is formed by scanning the probing laser beam on the etalon surface in either a 1-D or a 2-D configuration, which produces 2-D or 3-D images, respectively. Axial and lateral resolutions have been demonstrated to be better than 20 microm. Detailed characterizations of the optical and acoustical properties of the etalon, as well as photoacoustic imaging results, suggest that thin polymer etalon arrays can be used as ultrasound detectors for 3-D high-resolution photoacoustic imaging applications.

Realtime photoacoustic microscopy of murine cardiovascular dynamics

Non-invasive visualization of cardiovascular dynamics in small animals is challenging due to their rapid heart-rates. We present a realtime photoacoustic imaging system consisting of a 30-MHz ultrasound array transducer, receive electronics, a high-repetition-rate laser, and a multicore-computer, and demonstrate its ability to image optically-absorbing structures of the beating hearts of young athymic nude mice at rates of approximately 50 frames per second with 100 microm x 25 microm spatial resolution. To our knowledge this is the first report of realtime photoacoustic imaging of physiological dynamics.

Imaging of the calf vocal fold with high-frequency ultrasound

OBJECTIVES/HYPOTHESIS

High-frequency ultrasound imaging offers the potential for assisting in the diagnosis and treatment of vocal fold pathology if it allows aspects of vocal fold microstructure to be visualized noninvasively. The objective of this study was to assess the ability of high-frequency ultrasound to image vocal fold anatomy and injected biomaterials.

STUDY DESIGN

The vocal folds of two excised calf larynges were imaged ex vivo and compared with corresponding histological sections.

METHODS

High-frequency ultrasound imaging was performed under saline submersion using 40 and 50 MHz transducers, and corresponding cryostat cross-sections were stained with H&E, Trichome, and Verhoeff's Van Gieson stains.

RESULTS

The epithelial surface, lamina propria, and underlying muscle were easily identified with the high-frequency ultrasound as verified with histological sections representing each imaged region. The arytenoid cartilage vocal process can also be clearly distinguished from the surrounding tissue, as can the full extent of injected biomaterials within the superficial lamina propria. Useful ultrasound resolution was obtained to depths of at least 10 mm within the tissue with the 40 MHz transducer.

CONCLUSIONS

This preliminary study demonstrates the capability of high-frequency ultrasound to image the layered anatomy of the calf vocal fold and to discern materials injected into the superficial lamina propria, indicating that this technology holds a strong potential for use in phonosurgery.

In vivo small-animal imaging using micro-CT and digital subtraction angiography

Small-animal imaging has a critical role in phenotyping, drug discovery and in providing a basic understanding of mechanisms of disease. Translating imaging methods from humans to small animals is not an easy task. The purpose of this work is to review in vivo x-ray based small-animal imaging, with a focus on in vivo micro-computed tomography (micro-CT) and digital subtraction angiography (DSA). We present the principles, technologies, image quality parameters and types of applications. We show that both methods can be used not only to provide morphological, but also functional information, such as cardiac function estimation or perfusion. Compared to other modalities, x-ray based imaging is usually regarded as being able to provide higher throughput at lower cost and adequate resolution. The limitations are usually associated with the relatively poor contrast mechanisms and potential radiation damage due to ionizing radiation, although the use of contrast agents and careful design of studies can address these limitations. We hope that the information will effectively address how x-ray based imaging can be exploited for successful in vivo preclinical imaging.

Pulse inversion sequences for mechanically scanned transducers

Mechanically scanned transducers are currently used for tissue harmonic imaging (THI) and nonlinear microbubble imaging at high frequencies. The pulse inversion (PI) technique is widely used for suppressing the fundamental signal, but its effectiveness is reduced by relative tissue/transducer motion. In this paper, we investigate multipulse inversion (MPI) sequences that achieve a significant improvement on the fundamental suppression for mechanically scanned single-element transducers. MPI was subsequently applied on simulated and measured RF-data and relative fundamental suppression was compared with the 2-pulse PI technique. Simulations showed, for example, an increased fundamental suppression of 6 and 10 dB for MPI-sequences that combined 3 and 7 pulses, respectively, for a rotating intravascular ultrasound transducer with an interpulse angle of 0.15 degrees. Initial application of MPI sequences on RF-data from in vivo acquisitions resulted in similar fundamental suppression levels. The investigated MPI technique will help to reduce relative tissue/transducer motion effects and might lead to improved sensitivity and spatial resolution in nonlinear tissue imaging and improved microbubble detection in contrast imaging for mechanically scanned transducers.

Effects of rosuvastatin on cardiovascular morphology and function in an ApoE-knockout mouse model of atherosclerosis

This study investigated the effects of rosuvastatin on plaque progression and in vivo coronary artery function in apolipoprotein E-knockout (ApoE-KO) mice, using noninvasive high-resolution ultrasound techniques. Eight-week-old male ApoE-KO mice (n = 20) were fed a high-fat diet with or without rosuvastatin (10 micromol.kg(-1).day(-1)) for 16 wk. When compared with control, rosuvastatin reduced total cholesterol levels (P < 0.05) and caused significant retardation of lesion progression in the brachiocephalic artery, as visualized in vivo using an ultrasound biomicroscope (P < 0.05). Histological analysis confirmed the reduction of brachiocephalic atherosclerosis and also revealed an increase in collagen content in the statin-treated group (P < 0.05). Coronary volumetric flow was measured by simultaneous recording of Doppler velocity signals and left coronary artery morphology before and during adenosine infusion. The hyperemic flow in response to adenosine was significantly greater in left coronary artery following 16 wk of rosuvastatin treatment (P < 0.001), whereas the baseline flow was similar in both groups. In conclusion, rosuvastatin reduced brachiocephalic artery atherosclerotic plaques in ApoE-KO mice. Coronary artery function assessed using recently developed in vivo ultrasound-based protocols, also improved.

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.

In vivo quantification of embryonic and placental growth during gestation in mice using micro-ultrasound

BACKGROUND

Non-invasive micro-ultrasound was evaluated as a method to quantify intrauterine growth phenotypes in mice. Improved methods are required to accelerate research using genetically-altered mice to investigate the interactive roles of genes and environments on embryonic and placental growth. We determined (1) feasible age ranges for measuring specific variables, (2) normative growth curves, (3) accuracy of ultrasound measurements in comparison with light microscopy, and (4) weight prediction equations using regression analysis for CD-1 mice and evaluated their accuracy when applied to other mouse strains.

METHODS

We used 30-40 MHz ultrasound to quantify embryonic and placental morphometry in isoflurane-anesthetized pregnant CD-1 mice from embryonic day 7.5 (E7.5) to E18.5 (full-term), and for C57Bl/6J, B6CBAF1, and hIGFBP1 pregnant transgenic mice at E17.5.

RESULTS

Gestational sac dimension provided the earliest measure of conceptus size. Sac dimension derived using regression analysis increased from 0.84 mm at E7.5 to 6.44 mm at E11.5 when it was discontinued. The earliest measurement of embryo size was crown-rump length (CRL) which increased from 1.88 mm at E8.5 to 16.22 mm at E16.5 after which it exceeded the field of view. From E10.5 to E18.5 (full term), progressive increases were observed in embryonic biparietal diameter (BPD) (0.79 mm to 7.55 mm at E18.5), abdominal circumference (AC) (4.91 mm to 26.56 mm), and eye lens diameter (0.20 mm to 0.93 mm). Ossified femur length was measureable from E15.5 (1.06 mm) and increased linearly to 2.23 mm at E18.5. In contrast, placental diameter (PD) and placental thickness (PT) increased from E10.5 to E14.5 then remained constant to term in accord with placental weight. Ultrasound and light microscopy measurements agreed with no significant bias and a discrepancy of less than 25%. Regression equations predicting gestational age from individual variables, and embryonic weight (BW) from CRL, BPD, and AC were obtained. The prediction equation BW = -0.757 + 0.0453 (CRL) + 0.0334 (AC) derived from CD-1 data predicted embryonic weights at E17.5 in three other strains of mice with a mean discrepancy of less than 16%.

CONCLUSION

Micro-ultrasound provides a feasible tool for in vivo morphometric quantification of embryonic and placental growth parameters in mice and for estimation of embryonic gestational age and/or body weight in utero.

Characterization of a broadband all-optical ultrasound transducer-from optical and acoustical properties to imaging

A broadband all-optical ultrasound transducer has been designed, fabricated, and evaluated for high- frequency ultrasound imaging. The device consists of a 2-D gold nanostructure imprinted on top of a glass substrate, followed by a 3 microm PDMS layer and a 30 nm gold layer. A laser pulse at the resonance wavelength of the gold nanostructure is focused onto the surface for ultrasound generation, while the gold nanostructure, together with the 30 nm thick gold layer and the PDMS layer in between, forms an etalon for ultrasound detection, which uses a CW laser at a wavelength far from resonance as the probing beam. The center frequency of a pulse-echo signal recorded in the far field of the transducer is 40 MHz with -6 dB bandwidth of 57 MHz. The signal to noise ratio (SNR) from a 70 microm diameter transmit element combined with a 20 microm diameter receive element probing a near perfect reflector positioned 1.5 mm from the transducer surface is more than 10 dB and has the potential to be improved by at least another 40 dB. A high-frequency ultrasound array has been emulated using multiple measurements from the transducer while mechanically scanning an imaging target. Characterization of the device's optical and acoustical properties, as well as preliminary imaging results, strongly suggest that all-optical ultrasound transducers can be used to build high-frequency arrays for real-time high-resolution ultrasound imaging.

Needle size and injection rate impact microbubble contrast agent population

The most common type of ultrasound contrast agents are encapsulated microbubbles, typically 1 to 5 microns in diameter. These microbubbles are injected into the bloodstream to provide image enhancement during an ultrasound examination. Because of their compressibility, these microbubbles are inherently sensitive to changes in pressure. For imaging, this is beneficial in that these microbubbles oscillate in an acoustic field and allow imaging systems to detect their response uniquely from tissue. However, this sensitivity also means that microbubbles can be readily destroyed by significant hydrostatic pressure. Injection of these microbubbles through a small-gauge catheter, such as is sometimes performed in small animal imaging studies, can result in microbubble destruction. In this manuscript, the effects of microbubble injection through catheters of varying diameter are examined. Our results indicate that the concentration and size distribution of microbubbles can be substantially altered in cases of rapid injection through small-gauge needles.

High-frequency subharmonic pulsed-wave Doppler and color flow imaging of microbubble contrast agents

A recent study has shown the feasibility of subharmonic (SH) flow imaging at a transmit frequency of 20 MHz. This paper builds on these results by examining the performance of SH flow imaging as a function of transmit pressure. Further, we also investigate the feasibility of SH pulsed-wave Doppler (PWD) imaging. In vitro flow experiments were performed with a 1-mm-diameter wall-less vessel cryogel phantom using the ultrasound contrast agent Definity and an imaging frequency of 20 MHz. The phantom results show that there is an identifiable pressure range where accurate flow velocity and power estimates can be made with SH imaging at 10 MHz (SH10), above which velocity estimates are biased by radiation force effects and unstable bubble behavior, and below which velocity and power estimates are degraded by poor SNR. In vivo validation of SH PWD was performed in an arteriole of a rabbit ear, and blood velocity estimates compared well with fundamental (F20) mode PWD. The ability to suppress tissue signals using SH signals may enable the use of higher frame rates and improve sensitivity to microvascular flow or slow velocities near large vessel walls by reducing or eliminating the need for clutter filters.

High frequency ultrasound imaging detects cardiac dyssynchrony in noninfarcted regions of the murine left ventricle late after reperfused myocardial infarction

Cardiac dyssynchrony in the left ventricles of murine hearts late (> or =28 d) after reperfused myocardial infarction (post-MI) was assessed using high frequency 30 MHz B-mode ultrasound imaging. Nine post-MI and six normal C57Bl/6 mice were studied in both short- and long-axis views. Regional time to peak displacement (T(peak_d)) and time to peak strain (T(peak_s)) were calculated in 36 sectors along the myocardial circumference; then their standard deviations (SD_T(peak_d) and SD_T(peak_s)) were computed among noninfarcted myocardial regions for each mouse and were compared between the normal and post-MI mouse groups with Student's t-test. The comparison revealed that SD_T(peak_d) and SD_T(peak_s) were significantly larger in the post-MI hearts than in the normal hearts. The displacement uniformity ratio was determined to be 0.97 +/- 0.01 and 0.85 +/- 0.07 for radial and circumferential displacements in the normal hearts, respectively; and 0.59 +/- 0.17 and 0.64 +/- 0.24 in the post-MI hearts. In conclusion, this high resolution ultrasound image tracking method provides for the detection of cardiac dyssynchrony in the noninfarcted regions in the murine left ventricles late after MI by identifying the temporal and spatial disparity of regional myocardial contraction.

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.

Low-noise wideband ultrasound detection using polymer microring resonators

Polymer microring resonators for low-noise, wideband ultrasound detection are presented. Using a nanoimprinting technique, we fabricated polymer microring resonators with a quality factor of 6000 resulting in high sensitivity to ultrasound. A noise-equivalent pressure of 0.23 kPa over 1-75 MHz and a detection bandwidth of over 90 MHz at -3 dB were measured. These results demonstrate the potential of polymer microring resonators for high-frequency ultrasound and photoacoustic imaging. For a typical photoacoustic imaging test case, the high sensitivity demonstrated in these devices would increase imaging depth by a factor of 3 compared to state-of-the-art polyvinylidene fluoride detectors.