Search PubMed⌕ Search

Biomedical subjects

F S Foster

Publications and source records attributed to F S Foster.

At least 91 records · Page 5Linked to original sources
PubMed1989

A 100 MHz B-scan ultrasound backscatter microscope.

The construction and operation of a 100 MHz B-mode ultrasound backscatter microscope are described. The powerful B-mode technique is extended into the domain of microscopy allowing the imaging of internal structure in living specimens on a microscopic scale. A frame rate of 5 frames per second is achieved which gives rapid feedback to the operator. Specially designed components of the scanner are described in detail, including the transducer, motion system and scan converter. An f/2 transducer is employed, leading to a scanner resolution of approximately 36 micron in both the lateral and axial directions. The benefits of such high resolution are demonstrated in preliminary images of multicellular spheroids and intact human ocular tissue.

Equipment Design↗
PubMed1989

A ray tracing method for calculating the speed of sound in a nonparallel layered model using pulse echo ultrasound.

A method of calculating the speed of sound in a nonparallel layered model is presented. A focussed ultrasound source and receiver are used to locate the angle and position of an incident pulse and the reflected pulses at the surface of the model. Triangulation is used to calculate the speed of sound in the first layer and points on the first interface for a number of source positions and angles. For subsequent layers, ray tracing is used to eliminate the effect of the overlying layers. A computer simulation in which images of the speed of sound are reconstructed from data generated by ray tracing through two models is presented. Ways in which this method could be implemented are proposed.

Computer Simulation↗
PubMed1988

Ultrasonographically defined parenchymal patterns of the breast: relationship to mammographic patterns and other risk factors for breast cancer.

We have used ultrasound of the breast to define four parenchymal patterns in which increasing proportions of the breast are replaced by densely echogenic tissue. A series of 452 symptomatic women examined by both ultrasound and conventional X-ray mammography was reviewed to determine whether these ultrasonographic images could predict the breast parenchymal pattern defined mammographically. A very strong correlation was demonstrated between the breast pattern on ultrasound and the volume of the breast replaced by either dysplasia (Kendall's tau-b = 0.731 +/- 0.026, p less than 0.0001) or ductal prominence (Kendall's tau-b = 0.641 +/- 0.049, p less than 0.0001). This was seen both on initial reporting and on a blind re-reading of a random sample of 100 cases. The strength of correlation was similar for subgroups defined by family history of breast cancer, age, menopausal status, and history of benign breast disease, and the breast parenchymal pattern assessed by mammography or ultrasound showed similar associations with these variables. Ultrasonographic parenchymal patterns of the breast can predict the tissue patterns defined mammographically and may therefore be useful as a marker of breast cancer risk.

Adult↗
PubMed1986

Frequency dependence of ultrasound attenuation and backscatter in breast tissue.

Attenuation and backscatter of ultrasound in human breast tissues were measured over the frequency range relevant to breast imaging (3-7 MHz). The first step in the measurement process consisted of generating an ultrasound attenuation image of a macroscopic slice of excised breast tissue. This image depicted the various tissue constituents in the samples so that a region of homogeneous tissue could be selected for further investigation. Ultrasound signals used in the computation of the frequency dependent attenuation and backscatter coefficients were then collected from several points within this region of interest. The measured attenuation and backscatter were averaged over all the points in the region of interest and plotted as functions of frequency which ranged from 3 to 7 MHz. The results showed that the attenuation coefficient of homogeneous regions of infiltrating duct carcinoma was higher than that of fat but somewhat less than that of fibrous and parenchymal tissues. On the other hand the backscattering coefficient of ductal carcinoma samples was similar to that of fat but much lower than the backscatter coefficient of parenchymal tissue. The discriminability of the different breast tissue types under these measures, as well as the relevance of the results to clinical imaging are discussed.

Breast↗
PubMed1986

Quantitative contrast measurements in B-mode images comparison between experiment and theory.

Quantitative measurements of image contrast were carried out for B-mode images of anechoic spheres (cysts) embedded in a random scattering medium. Four transducer geometries were used: (a) f/5.7 spherical transducer in pulse echo mode, (b) f/2.4 spherical transducer in pulse echo mode, (c) f/2.4, 30 degrees cone (hybrid transducer), (d) f/5.7, 30 degrees cone (hybrid transducer). The image contrast was also calculated via two methods; (i) a three-dimensional computer simulation and (ii) a relatively simple numerical convolution which relates the image of a small point-like scatterer [point spread function (PSF)] to image contrast. Generally, good agreement was found between the experimental measurements and the values calculated via both theoretical methods. The results indicate that image contrast is not determined solely by the full width at half maximum (FWHM) of the PSF. In particular, the results demonstrate the importance of far off-axis contributions of the ultrasound beam to the significant degradation of contrast in images obtained with high resolution (low f-number) axicons.

Computers↗
PubMed1986

Speckle reduction in pulse echo imaging using phase insensitive and phase sensitive signal processing techniques.

In conventional B-scan imaging, speckle often distorts the true representation of small structures and lesions and hence reduces the diagnostic potential of the technique. Considerable speckle reduction has been achieved with an f/2.4, cone hybrid system by cutting the large aperture transducers into sectors, and using either phase insensitive sector addition (PISA), phase insensitive sector multiplication (PISM), or an rf multiplicative processing technique. We have compared two modes of operation with the hybrid system. In the first, the cone acts as transmitter, and each sector of the f/2.4 spherical aperture is a receiver. In the second mode, the f/2.4 sphere acts as transmitter, and each sector of the cone is a receiver. Quantitative assessment using the contrast to speckle ratio developed by Patterson et al, indicates that processing under both modes results in reduced speckle noise and improved image quality, with the cone sector receive (rx) geometry offering the greatest improvement.

Breast↗
PubMed1984

The ultrasound macroscope: initial studies of breast tissue.

A system called the ultrasound macroscope has been developed to measure the ultrasonic properties of tissues. Based on the concept of the scanning acoustic microscope, it is capable of generating quantitative images of ultrasound velocity, attenuation and backscatter for macroscopic slices of tissue. By maintaining high resolution (approximately 120 microns at 13 MHz) the morphology of complex tissues is well delineated. Data from regions of interest in the tissue are transformed into three dimensional probability distributions representative of the tissue's ultrasonic characteristics. Sets of two dimensional projections of these distributions are used to display the information. As an initial study, the properties of breast tissues are analyzed. Examples of both normal and abnormal pathology are presented and the relevance of the results to clinical imaging are discussed.

Adult↗
PubMed1983

The improvement and quantitative assessment of B-mode images produced by an annular array/cone hybrid.

Hybrid ultrasound imaging systems, which combine spherical focusing on transmit with axicon focusing on receive, provide excellent resolution over a useful depth of field. This paper presents a new hybrid design with improved sensitivity, in which the axicon focusing is achieved by two conical mirrors and a PZT 5A disk out into 8 sectors. We have investigated two methods of processing the signals from the 8 sectors. In the first, phase insensitive sector addition (PISA), the B-scan is formed from the sum of the 8 demodulated signals. In the second, multiplicative processing (MP), the 8 rf waveforms are multiplied and the resultant is demodulated to form the image. Both techniques result in smoothed speckle but degraded lateral resolution. As well, MP decreases the off-axis sensitivity of the system and artifacts characteristic of axicon focusing. Quantitative assessment of the effects of PISA and MP was performed using a new approach called contrast-to-speckle ratio (CSR). The CSR data, which is a measure of the image contrast of cylindrical voids in a random scattering medium relative to contrast fluctuations due to speckle, shows the superiority of PISA and MP. This conclusion is supported by images of in vitro human breast tissue.

Breast↗
PubMed1983

Computer simulations of speckle in B-scan images.

The granularity or speckle in medical ultrasound images tends to mask the presence of small lesions. As well, artifactual filling in of anechoic regions such as cysts, reduces the diagnostic potential of the images. These effects depend not only on the acoustic properties of the tissue but also are strongly influenced by the imaging system, especially the transducer geometry. To study the effect of the transducer on the final B-scan image, a computer model has been developed simulating the interaction of ultrasound with a simple scattering medium. This model, incorporating the position dependence of the point response of the transducer, is based on single scattering from a collection of points positioned randomly in a three-dimensional volume. Using this approach, B-scan images showing speckle have been generated for different transducer geometries. Inclusion of a 2.6 mm void mimicking a cyst within the three-dimensional scattering volume has allowed us to predict the cyst contrast in the image for the different transducer systems. Experimental B-scan images of a scattering phantom were obtained using the different pulse echo systems. Quantitative assessment using first and second order statistics of the images shows good agreement between experiment and theory.

Acoustics↗
PubMedDate not supplied

Breast imaging with a conical transducer/annular array hybrid scanner.

A hybrid conical scanner consisting of a 37.5 degrees, 4 MHz conical receiver and a five element 5 cm dia. 4 MHz annular array transmitter has been developed for breast imaging. The ultrasound fields and imaging characteristics of three interesting geometries are compared. These are: (1) the annular array used in conventional pulse-echo mode, (2) the conical transducer used in combination with the full annular array and (3) the conical transducer used in combination with the center element of the annular array (simple conical scanner). Both conical systems provide extremely narrow (0.3 mm) beamwidths at -6dB but the cone/annular array combination has significantly improved sidelobe characteristics compared with the simple conical scanner. The combination of high resolution and reasonable sidelobe levels for the cone/annular array hybrid results in images with very fine speckle patterns and good definition of small structures. Breast images from young normal volunteers show that this approach has considerable potential for clinical application.

Adult↗
PubMedDate not supplied

Artifactual echoes in B-mode images due to multiple scattering.

In B-mode images, echoes, which appear to arise from inherently anechoic regions due to scattering from neighbouring echogenic regions, can be considered artifactual. We have observed multiple scatter contributions to such artifactual echoes in images of plane boundaries between random scattering phantoms and anechoic regions. For the strongest scattering phantom studied, multiple scatter echoes were 3-9% of the average phantom signal using two sharply focused transducers, an annular array in pulse echo mode, and an annular array/cone hybrid. Multiple scatter was less than 5% for a conventional transducer. Echo amplitudes were also measured in normal excised human liver and breast tissue. It was estimated that artifactual echoes due to multiple scatter would be negligible in B-mode images of liver. However, for breast imaging, the level of artifactual echoes was estimated to be similar to that found for our strongest scattering phantom.

Breast↗