Measurement of thermal and ultrasonic properties of some biological tissues.

The measurement of thermal and ultrasonic properties of biological tissues is essential for the assessment of the temperature rise induced in vivo by diagnostic ultrasound. In this paper, we present measurements of thermal conductivity, thermal diffusivity, speed of sound and ultrasonic attenuation of fresh ex vivo porcine tissue, namely 'muscle' (from abdomen and leg), 'skin with subcutaneous fat' (from abdomen and leg), 'abdominal fat' and 'bone'. The measurements of the thermal properties of biological tissue samples are based on a transient method. Thermal property measurements show that subcutaneous fat has the lowest thermal conductivity (0.23 W m(-1) K(-1)), while muscle gives the highest values (0.46 W m(-1) K(-1)). Thermal diffusivity of muscle tissue recorded the highest value among the studied tissues (0.16 mm(2) s(-1)) while that of skin with subcutaneous fat gave the lowest value (0.11 mm(2) s(-1)). A scanning acoustic macroscope was used to measure attenuation coefficient and speed of sound for the tissue samples. The results for the speed of sound are broadly similar to those reported in the literature. The power law dependence of the attenuation coefficient of the form eta = a f (b) as a function of frequency was found to be more appropriate than the linear fit in this study.

The unravelling of primary myocardial impairment in Marfan syndrome by modern echocardiography.

Use of echocardiography has dramatically changed the way in which patients with Marfan syndrome are diagnosed, monitored and treated. Owing to the lethal nature of aortic complications, priority has been given to the assessment of the aortic root. Echocardiographic studies on patients with Marfan syndrome have also provided data supporting primary myocardial involvement, although this evidence has remained controversial for several years. Use of more sensitive ultrasound techniques has demonstrated mild myocardial impairment in these patients. Biventricular function assessment should be added to the aortic root evaluation, so that appropriate treatment may be offered to support myocardial function.

Nuchal translucency thickness measurement: repeatability using a virtual ultrasound scanner.

AIM: To use a PC-based virtual ultrasound scanner (VirUS) in the investigation of inter- and intraoperator nuchal translucency (NT) thickness measurement repeatability of experienced ultrasound operators. METHODS: Realistic fetal ultrasound images of defined NT thickness were simulated with VirUS with emulation of scanner gain and time-gain compensation and gain-dependent echo size changes. A set of 50 images was generated with uniformly distributed NT thickness (range, 1-5 mm at 1-mm intervals) and translucency angle (mean +/- standard deviation of +/- 2.52 degrees +/- 1.85 degrees about the horizontal). Operators (n = 13) measured NT thickness in the image set on three occasions separated by at least 1 day, giving 150 measurements per operator (total measurements, 1950). RESULTS: Inter- and intraoperator repeatabilities were +/- 0.41 mm and +/- 0.22 mm, respectively (at the 95% confidence level). There were significant correlations between repeatability and mean measured NT thickness (r = - 0.72, P = 0.005 at 4-mm interval), between gain and mean measured NT thickness (P

A rapid and reproducible on line automated technique to determine endothelial function.

OBJECTIVE: To evaluate clinically a new on line, automated technique to measure flow mediated dilatation (FMD) as a marker of endothelial function. DESIGN: Prospective study. PATIENTS: 12 healthy volunteers and 12 patients with significant, angiographically documented coronary artery disease. INTERVENTIONS: Brachial arteries were imaged using a standard vascular ultrasound system with a 5-12 MHz linear transducer. Arterial diameter was measured on line (in real time) by connecting the ultrasound system to a personal computer equipped with a frame grabber and artery wall detection software (VIA) specially developed by the authors' group. By using this new technique, FMD was measured following 4.5 minutes of ischaemia of the proximal forearm in all subjects on two separate days. RESULTS: The mean (SD) day to day variability in FMD measurements was 0.90 (0.48)%,which compares very favourably with current methods. The FMD measurement was available within seconds of completing the scan. CONCLUSIONS: Personal computer based automated techniques to assess FMD involve image acquisition and recording after which a second (off line) image interpretation session is required. The need for off line analysis makes current methods time consuming and increases the variability of measurement. This on line, automated analysis technique for FMD assessment reduces the variability and greatly increases the speed of measurement. Using this system may mean that fewer patients will be required in clinical trials assessing the effects of interventions on endothelial function. Adopting this method may also facilitate the screening of larger numbers of subjects for endothelial dysfunction.

Online artery diameter measurement in ultrasound images using artificial neural networks.

An automated online technique is described for measurement of artery diameter in flow-mediated dilation (FMD) ultrasound (US) images, using artificial neural networks to identify and track artery walls. This allows FMD results to be calculated without the inherent delay of current retrospective methods. Two networks were trained to identify artery anterior and posterior walls using over 3200 examples from carotid artery images. Both networks correctly classified approximately 97% of the randomly selected test samples. The technique was verified using a physical model with absolute measurement error of -1.16% +/- 1.04% (mean +/- SD) over the diameter range 2 to 8 mm. Advantages of the technique include: online analysis; wall tracking optimisation before the study proper; measurement of diameter changes over the cardiac cycle; low FMD measurement variance; minimal image degradation; and no unwieldy image store. Measurement of artery diameter changes over the cardiac cycle was explored using simulated image sequences generated with a virtual US scanner.

Measurement of acoustic streaming using magnetic resonance.

Magnetic resonance imaging (MRI) has been used to explore acoustic streaming caused in water under ultrasonic exposure conditions similar to those used for diagnostic applications. Streaming was established in an enclosed tube with acoustically transparent end windows, using a pulsed, weakly-focused transducer of acoustic frequency 3.5 MHz. Phase-detection MRI was used to image and quantify streaming profiles in the region of the acoustic focus. Acoustic powers in the range 0.4 mW to 100 mW were used. The sensitivity of the technique enabled streaming velocities down to 0. 1 mm s(-1) to be measured, generated by acoustic power less than 1 mW. In addition, acoustic streaming generated within open meshes with minimum pore dimensions of 3.0 mm and 2.0 mm was measured. The flow velocity in the coarser mesh reached 0.9 mm s(-1) at 95 mW total acoustic power. These observations demonstrate that acoustic streaming is probably a much more general phenomenon in diagnostic ultrasound (ultrasound) than previously recognised. The combination of magnetic resonance and ultrasound shows promise as a diagnostic method for the differentiation of cystic lesions in vivo, and for their characterisation, with sensitivity significantly greater than using ultrasound alone.

Effects of pulsed ultrasound on embryonic development: an in vitro study.

Whole-embryo culture was used as the model system to study the effects of pulsed ultrasound on embryonic development. Rat embryos (9.5 days old) were exposed to a wide range of ultrasound levels at ultrasound frequencies between 1-4 MHz for 30 min in vitro. After 48 h in culture, absolute control, sham and treatment embryos were assessed for viability, morphology, growth and development. At an ambient temperature of 37 degrees C, no significant effects were observed for spatial peak temporal average intensities below 4 W/cm2 or peak negative pressures below 1.9 MPa. At higher acoustic levels, there was a significant increase in the number of nonviable embryos and the number of morphological abnormalities in viable embryos increased. Abnormal cephalocaudal flexion and abnormal head development were the most common gross morphological abnormalities. Both thermal and nonthermal bioeffect mechanisms are involved.

Validation of a new blood-mimicking fluid for use in Doppler flow test objects.

A blood-mimicking fluid (BMF) suitable for use in Doppler flow test objects is described and characterised. The BMF consists of 5 microns diameter nylon scattering particles suspended in a fluid base of water, glycerol, dextran and surfactant. The acoustical properties of various BMF preparations were measured under uniform flow to study the effects of particle size, particle concentration, surfactant concentration, flow rate and stability. The physical properties, (density, viscosity and particle size), and acoustical properties (velocity, backscatter and attenuation) of the BMF are within draft International Electrotechnical Commission requirements.

Estimation of in situ ultrasound exposure during obstetric examinations.

Layered tissue models are developed to estimate in situ ultrasound intensity during common obstetric examinations by incorporation of measured overlying tissue thicknesses with data on the attenuation properties of tissues. Results are compared with attenuation models recommended by the United States Food and Drug Administration (FDA) and National Council on Radiation Protection and Measurements (NCRP). For abdominal and transvaginal scanning in the first trimester, fixed attenuation models based on attenuation values of 1.2 dB/MHz and 0.6 dB/MHz, respectively, are representative of worst-case exposure conditions. For second and third trimesters, a fixed attenuation value of 0.8 dB/MHz is representative of worst-case exposure conditions. A fixed attenuation value of 1.0 dB/MHz is suggested for common Doppler examinations. The study suggests that the FDA-derating factor of 0.3 dB/cm.MHz may not give a conservative estimate of in situ intensity for certain obstetric examinations.

Ultrasonic study of in vivo kinetic characteristics of human tissues.

A method is described for quantifying tissue movement in vivo from the computation of correlation coefficient between pairs of A-scans with appropriate time separation. The method yields quantifiable and repeatable secondary patterns of soft tissue movement in response to primary cardiac movement in a given subject, shows consistently different results as between normal livers and a variety of abdominal tumours, and is sensitive to either progress or therapeutically-induced regression of malignant disease. While the results reported here have been obtained using somewhat simple and crude equipment, the method is well suited to implementation on a commercial real-time scanner.

The differential and total bulk acoustic scattering cross sections of some human and animal tissues.

This paper describes methods that have been developed for the accurate measurement of total and differential bulk acoustic scattering cross sections. Analysis is made of the sources of uncertainty entailed in measurements of this kind and a complete derivation is developed for the angular variation of the interrogated scattering volume. Experimentally determined values of differential scattering cross sections are reported for specimens of skeletal muscle, blood, liver, and secondary tumors arising in liver, obtained in the frequency range 4-7 MHz for scattering angles in the practically attainable range 60 degrees-150 degrees and also at 180 degrees (backscatter). A basis for extrapolating these results to other scattering angles can be derived from a combination of theoretical and experimental considerations, as presented in a companion paper [J. Acoust. Soc. Am. 79, 2048-2054 (1986)]. Based on such extrapolation, values for total scattering cross sections are estimated for the above tissues, the values are compared with determinations by different techniques as reported by other authors, and corresponding estimates are derived for the contribution of scattering processes to total attenuation in the tissues.

The use of angular acoustic scattering measurements to estimate structural parameters of human and animal tissues.

Theoretical formulations are developed, based on mathematical models of inhomogeneous continua for the expected angular variation of bulk scattering from human and animal tissues. These results are compared with experimental data on angular scattering from liver, muscle, and blood, reported in a companion paper [J. Acoust. Soc. Am. 79, 2034-2047 (1986)], and deductions are drawn as to the appropriateness of the various models for representing the mechanical structure of the different tissues. On this basis, the experimental data and theoretical formulations are used to derive estimates, appropriate to the frequency range of observation (4-7 MHz), of correlation distance (or effective scatterer spacing) d, the local variabilities of density and compressibility, gamma rho = delta rho/rho and gamma kappa = delta kappa/kappa 0, and their ratio gamma rho/gamma kappa. For blood, liver, and skeletal muscle, the values derived at 6 MHz for d are approximately 5, 55, and 75 microns and for gamma rho/gamma kappa are 0.5, 0.15, and 0.28, respectively. These results are, in particular, at variance with the commonly made assumption, based on evidence from low-frequency measurements, that the ratio gamma rho/gamma kappa is sufficiently small that density terms can be ignored in calculations of human tissue scattering.

Tissue characterization from ultrasound B-scan data.

An approach to ultrasonic tissue characterization, using textural features of the B-scan image, is described. Portions of a B-scan image, 64 X 64 pixels spatially by 8 bits deep, are acquired from regions of interest and subjected to computer analysis. A systematic approach to defining a set of 93 textural features of a B-scan is described and methods and criteria for selecting optimum combinations of these are discussed. As a test of its power, the approach has been applied to the discrimination between the B-scan textures corresponding to livers and spleens of normal humans and various measures of "success" have been quantified both on a "training set only" and on a "training set plus test set" basis. The overall test probability of success of 82% on a single image and 94% on a subject yielding multiple images indicates the potential of the techniques for conditions where a subtle but uniform change in parenchymal texture may be present.

Effect of gaseous inclusions on the frequency dependence of ultrasonic attenuation in liver.

The greatest variation in published data of the attenuation of ultrasound in mammalian liver in vitro occurs at the lower end of the 0.5 to 7 MHz frequency range and gives rise to some departure from a linear or simple power law dependence of attenuation on frequency. These effects do not appear to be highly dependent on the method of measurement. It is suggested that they are due to a varying presence of small gas bubbles distributed throughout the tissue--a suggestion based on calculated estimates of the attenuation due to microscopic bubbles and on the measured frequency dependence of attenuation in water loaded sponges containing varying amounts and distribution of gas. We now believe that preferred methods of tissue specimen preparation, for in vitro measurement of ultrasonic attenuation or scattering, should involve either pressurization as described elsewhere (Frizzell et al., 1979) or storage under refrigeration.

The diaphragmatic echo complex: an in vitro study.

The brightly echogenic appearance of the diaphragm on routine clinical scans is not easily reconciled with the well-documented echo-poor appearance of muscle elsewhere in the body. A series of specimens of normal human diaphragm freshly excised at autopsy were suspended in a water bath. Articulated arm scans were done varying the angle of the incident beam to the specimen by 5-degree increments and recording the maximum attenuation which allowed visualization (ie, an extinction point). This was accomplished for intact diaphragm, peritoneal membrane alone, and diaphragmatic muscle alone with both membranes stripped. The bright specular echoes seen are due almost exclusively to the membranes (parietal pleura and peritoneum) and the diaphragmatic muscle itself produces only low-level scattered echoes as elsewhere. However, these scattered echoes account for persistent visualization of the diaphragm at steep angles of the incident beam. A considerable portion of the in vivo thickness of the diaphragmatic echo complex is, therefore, produced by diaphragm-lung interface.

B-scan texture classification: a study using physical and theoretical models.

Recent clinical trials, in our own centre and elsewhere, have demonstrated the ability of A- and B-scan texture analysis techniques to provide useful in vivo separation of certain disease conditions. The present paper reports studies, using physical and theoretical models of tissues, aimed at identifying the particular sets of textural features which are most effective in achieving such separations. Using a 3.5 MHz, single element manual scanner, a total of 300 B-scans have been collected from four physical models consisting of gel-embedded 'point' scatterers, with differing mean separations. Additionally, computer modelling has been used to obtain a further 500 B-scans from 14 statistically different scattering situations. The total set of B-scans thus obtained has been subjected to a variety of texture classification procedures which have previously been developed for application in other imaging fields, such as cytology and aerial photography. The results are discussed in terms of: i) the features which are best suited to the classification of B-scans, and ii) the manner in which the resulting classification schemes are sensitive to variables of the ultrasonic scanning instrumentation.

Attenuation of ultrasound in skeletal muscle.

Ultrasonic attenuation in fresh and 5% formalin fixed beef skeletal muscle has been measured, as a continuous function of frequency, in the range 1-8 MHz, for muscle fibre orientations both parallel and normal to the direction of propagation. Good agreement was found in all cases between two independent sets of measurements employing transmission and reflection techniques respectively. The data are consistent with a power law dependence of attenuation coefficient on frequency, with an exponent that is not significantly different from unity. For propagation normal to the fibres attenuation values are found as 1.1 +/- 0.15 and 1.6 +/- 0.15 dB cm-1 MHz-1 for fresh and fixed tissue respectively, the corresponding values for parallel propagation being 2.9 +/- 0.23 and 4.1 +/- 0.25 dB cm-1 MHz-1.