F.S. Foster

A NEW ULTRASOUND INSTRUMENT FOR IN VIVO MICROIMAGING OF MICE

We report here on the design and evaluation of the first high-frequency ultrasound (US) imaging system specifically designed for microimaging of the mouse. High-frequency US or US biomicroscopy (UBM) has the advantage of low cost, rapid imaging speed, portability and high resolution. In combination with the ability to provide functional information on blood flow, UBM provides a powerful method for the investigation of development and disease models. The new UBM imaging system is demonstrated for mouse development from day 5.5 of embryogenesis through to the adult mouse. At a frequency of 40 MHz, the resolution voxel of the new mouse scanner measures 57 microm x 57 microm x 40 microm. Duplex Doppler provides blood velocity sensitivity to the mm per s range, consistent with flow in the microcirculation, and can readily detect blood flow in the embryonic mouse heart, aorta, liver and placenta. Noninvasive UBM assessment of development shows striking similarity to invasive atlases of mouse anatomy. The most detailed noninvasive in vivo images of mouse embryonic development achieved using any imaging method are presented.

Beyond 30 MHz [applications of high-frequency ultrasound imaging]

Most medical ultrasound imaging systems operate in the frequency range from 3 to 10 MHz and can resolve objects approximately 1 mm in size. In the mid 1980s, new transducer materials led to the development of the first transducers suitable for high-frequency (30-100 MHz) clinical imaging. These high-frequency transducers can provide images of subsurface structures with microscopic resolution. In this article, the authors introduce the basic principles of high-frequency ultrasound imaging and discuss six applications of this new technology: eye imaging, skin imaging, catheter-based intravascular imaging, intra-articular imaging, high-frequency flow imaging, and in-vivo imaging of mouse embryonic development. These examples illustrate a few of the potential applications of high-frequency ultrasound in medicine and biology.

Principles and applications of ultrasound backscatter microscopy

The development of ultrasound backscatter microscopy (UBM) is described together with initial clinical and biological applications. UBM is essentially an extension of the powerful B-mode backscatter methods developed for clinical imaging in the 3-10-MHz frequency range. The development of new high sensitivity transducers in the 40-100-MHz range now permits visualization of tissue structures with resolution approaching 20 mu m and a maximum penetration of approximately 4 mm. The performance characteristics and trade-offs of these new polymer and ceramic devices are reviewed, and the implementation of high-frequency imaging systems is described. Initial clinical applications of UBM include ophthalmic, skin, and intravascular imaging. Examples of images and progress in these areas are presented. The biological application of UBM is illustrated by studies of drug uptake in living tumor spheroids. Significant increases in backscatter levels resulting from drugs targeting oxic and hypoxic cell populations are demonstrated.<<ETX>>

Measurement of the ultrasonic properties of vascular tissues and blood from 35–65 MHz

A 50 MHz ultrasound backscatter microscope has been built to measure the acoustic properties of vascular tissues and blood over the frequency range from 35-65 MHz. High resolution (45 microns) ultrasound backscatter microscope images of nine femoral and eight common carotid human artery samples were made and compared with corresponding histological sections. Individual tissue layers were selected using these images for quantitative measurement of the frequency dependent backscatter. Backscatter measurements were made in each layer of an artery at two different angles of incidence: along the axis of the artery (axial direction) and at 90 degrees to this measurement radially out from the center of the artery (radial direction). Scattering was found to be higher in elastic arteries (carotid) than in the muscular arteries (femoral). The largest difference was found in the media where the average scatter (measured in the radial direction at 50 MHz) increased from 0.002 sr-1 mm-1 in muscular arteries to 0.4 sr-1 mm-1 in elastic arteries. Large differences in scattering between measurements made in the axial and radial direction were also found. Again, the largest differences were found in the media where scattering (at 50 MHz) in carotid arteries increased from 0.003 sr-1 mm-1 measured in the axial direction to 0.4 sr-1 mm-1 measured in the radial direction. The speed of sound and attenuation in the artery wall of each sample were measured. Speed of sound measurements were found to range between 1579-1628 ms-1. The average attenuation in the artery wall increased from 4 dB mm-1 at 30 MHz to 10 dB mm-1 at 60 mHz. This is higher than the attenuation measured in blood which increased from 1.6 dB mm-1 to 5 dB mm-1 over the same frequency range. The backscatter coefficient for flowing blood was measured for flow velocities up to 36 cms-1. At flow velocities below 18 cms-1 a level of scattering of 0.0005 sr-1 mm-1 (at 50 MHz) was found. An increase in scattering of 1.6 times was measured when the flow velocity was increased to 36 cms-1. All measurements were made at 37 degrees C. The relevance of these results to clinical imaging and image interpretation is discussed.

High frequency nonlinear B-scan imaging of microbubble contrast agents

It was previously 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

A history of medical and biological imaging with polyvinylidene fluoride (PVDF) transducers

Polyvinylidene fluoride (PVDF) is a ferroelectric polymer with unique properties suitable for use in a wide range of medical and biological imaging applications. Most notable among these is its low acoustic impedance, which matches that of the body reasonably well, and its flexible mechanical properties. This paper traces the exploitation of PVDF as a transducer material from its early beginnings for thyroid and breast imaging to its current well-established applications in ultrasound biomicroscopy. Although PVDFs electromechanical properties fall short of composite ceramic materials in the traditional diagnostic frequency range, it has significant advantages in the 25-to 100-MHz range. Design criteria for high frequency transducers are reviewed, and examples of relevant medical and biological images are used to illustrate the excellent image quality obtained with this remarkable material.

Development and initial application of a fully integrated photoacoustic micro-ultrasound system

Photoacoustic (PA) imaging for biomedical applications has been under development for many years. Based on the many advances over the past decade, a new photoacoustic imaging system has been integrated into a micro-ultrasound platform for co-registered PA¿ultrasound (US) imaging. The design and implementation of the new scanner is described and its performance quantified. Beamforming techniques and signal processing are described, in conjunction with in vivo PA images of normal subcutaneous mouse tissue and selected tumor models. In particular, the use of the system to estimate the spatial distribution of oxygen saturation (sO2) in blood and co-registered with B-mode images of the surrounding anatomy are investigated. The system was validated in vivo against a complementary technique for measuring partial pressure of oxygen in blood (pO2). The pO2 estimates were converted to sO2 values based on a standard dissociation curve found in the literature. Preliminary studies of oxygenation effects were performed in a mouse model of breast cancer (MDA-MB-231) in which control mice were compared with mice treated with a targeted antiangiogenic agent over a 3 d period. Treated mice exhibited a >90% decrease in blood volume, an 85% reduction in blood wash-in rate, and a 60% decrease in relative tissue oxygenation.

Fabrication and characterization of transducer elements in two-dimensional arrays for medical ultrasound imaging

Some of the problems of developing a two-dimensional (2-D) transducer array for medical imaging are examined. The fabrication of a 2-D array material consisting of lead zirconate titanate (PZT) elements separated by epoxy is discussed. Ultrasound pulses and transmitted radiation patterns from individual elements in the arrays are measured. A diffraction theory for the continuous wave pressure field of a 2-D array element is generalized to include both electrical and acoustical cross-coupling between elements. This theory can be fit to model the measured radiation patterns of 2-D array elements, giving an indication of the level of cross-coupling in the array, and the velocity of the acoustic cross-coupling wave. Improvements in bandwidth and cross-coupling resulting from the inclusion of a front acoustic matching layer are demonstrated, and the effects of including a lossy backing material on the array are discussed. A broadband electrical matching network is described, and pulse-echo waveforms and insertion loss from a 2-D array element are measured.<<ETX>>

The design of protection circuitry for high-frequency ultrasound imaging systems

Transmission line lengths in the protection circuitry of a high-frequency (>20-MHz) ultrasound imaging system have an important effect on the frequency, amplitude, and bandwidth of the pulse-echo response of the system. A model that includes the transmission line lengths between the pulser, transducer, and receiver and the electromechanical properties of high-frequency transducers is used to illustrate the importance of correctly choosing these line lengths. An iterative optimization procedure for designing the protection circuitry for a broadband system is proposed. A theoretical and experimental analysis of the validity of this approach is reported for a 45-MHz PVDF transducer.<<ETX>>

A 100-200 MHz ultrasound biomicroscope

The development of higher frequency ultrasound imaging systems affords a unique opportunity to visualize living tissue at the microscopic level. This work was undertaken to assess the potential of ultrasound imaging in vivo using the 100-200 MHz range. Spherically focused lithium niobate transducers were fabricated. The properties of a 200 MHz center frequency device are described in detail. This transducer showed good sensitivity with an insertion loss of 18 dB at 200 MHz. Resolution of 14 /spl mu/m in the lateral direction and 12 /spl mu/m in the axial direction was achieved with f/1.14 focusing. A linear mechanical scan system and a scan converter were used to generate B-scan images at a frame rate up to 12 frames per second. System performance in B-mode imaging is limited by frequency dependent attenuation in tissues. An alternative technique, zone-focus image collection, was investigated to extend depth of field. Images of coronary arteries, the eye, and skin are presented along with some preliminary correlations with histology. These results demonstrate the feasibility of ultrasound biomicroscopy In the 100-200 MHz range. Further development of ultrasound backscatter imaging at frequencies up to and above 200 MHz will contribute valuable information about tissue microstructure.