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W D O'Brien

Publications and source records attributed to W D O'Brien.

At least 19 recordsLinked to original sources
PubMed2001

Superthreshold behavior of ultrasound-induced lung hemorrhage in adult mice and rats: role of pulse repetition frequency and exposure duration.

Superthreshold behavior for ultrasound-induced lung hemorrhage was investigated in adult mice and rats at an ultrasound center frequency of 2.8 MHz to assess the role of pulse repetition frequency and exposure duration. One hundred fifty, 6-7-week-old female ICR mice and 150 10-11-week-old female Sprague-Dawley rats were each divided into 15 exposure groups (10 animals per group) for a 3 x 5 factorial design (3 exposure durations of 5, 10, and 20 s and 5 pulse repetition frequencies of 25, 50, 100, 250, and 500 Hz). The in situ (at the pleural surface) peak rarefactional pressure of 12.3 MPa and the pulse duration of 1.42 micros were the same for all ultrasonically-exposed animals. In addition, 15 sham exposed mice and 15 sham exposed rats were included into both studies. In each study of 165 animals, the exposure conditions were randomized. The lesion depth and surface area were measured for each animal, as well as the percentage of animals with lesions per group. The characteristics of the lesions produced in mice and rats were similar to those described in studies by our research group and others, suggesting a common pathogenesis for the initiation and propagation of the lesions at the gross and microscopic levels. The proportion of lesions in both species was related statistically to pulse repetition frequency (PRF) and exposure duration (ED), with the exception that PRF in rats was not quite significant; the PRF x ED interaction (number of pulses) for lesion production was not significant for either species. The PRF, but not ED, significantly affected lesion depth in both species; the PRF x ED interaction for depth was not significant for either species. Both PRF and ED significantly affected lesion surface area in mice, while neither affected area in rats; the PRF x ED interaction for surface area was not significant for either species.

Analysis of Variance↗
PubMed2001

Superthreshold behavior and threshold estimation of ultrasound-induced lung hemorrhage in adult mice and rats.

Threshold estimates and superthreshold behaviors for ultrasound-induced lung hemorrhage were investigated as a function of species (adult mice and rats) and ultrasound frequency (2.8 and 5.6 MHz). A total of 151 6-to-7-week-old female ICR mice and 160 10-to-11-week-old female Sprague-Dawley rats were randomly divided into two ultrasonic frequency groups, and further randomly divided into seven or eight ultrasonic peak rarefactional pressure groups. Each group consisted of about 10 animals. Animals were exposed to pulsed ultrasound at either 2.8-MHz center frequency (1-kHz PRF, 1.42-microsecond pulse duration) or 5.6-MHz center frequency (1-kHz PRF, 1.17-microsecond pulse duration) for a duration of 10 seconds. The in situ (at the pleural surface) peak rarefactional pressure levels ranged between 2.5 and 10.5 MPa for mice and between 2.3 and 11.3 MPa for rats. The mechanical index (MI) ranged between 1.4 and 6.3 at 2.8 MHz for mice and between 1.1 and 3.1 at 5.6 MHz for rats. The lesion surface area and depth were measured for each animal as well as the percentage of animals with lesions per group. The characteristics of the lesions produced in mice and rats were similar to those described in previous studies by our research group and others, suggesting a common pathogenesis in the initiation and propagation of the lesions at the gross and microscopic levels. The percentage of animals with lesions showed no statistical differences between species or between ultrasound frequencies. These findings suggest that mice and rats are similar in sensitivity to ultrasound-induced lung damage and that the occurrence of lung damage is independent of frequency. Lesion depth and surface area also showed no statistically significant differences between ultrasound frequencies for mice and rats. However, there was a significant difference between species for lesion area and a suggestive difference between species for lesion depth. The superthreshold behavior of lesion area and depth showed that rat lung had more damage than mouse lung, and the threshold estimates showed a weak, or lack of, frequency dependency, suggesting that the MI is not consistent with the observed findings.

Animals↗
PubMed2001

Attenuation coefficient estimates of mouse and rat chest wall.

Attenuation coefficients of intercostal tissues were estimated from chest walls removed postmortem (pm) from 41 6-to-7-week-old female ICR mice and 27 10-to-11-week-old female Sprague-Dawley rats. These values were determined from measurements through the intercostal tissues, from the surface of the skin to the parietal pleura. Mouse chest walls were sealed in plastic wrap and stored at 4 degrees C until evaluated, and rat chest walls were sealed in Glad-Lock Zipper sandwich bags, and stored at -15 degrees C. When evaluated, chest wall storage time ranged between 1 and 2 days pm for mice and between 41 and 110 days pm for rats. All chest walls were allowed to equilibrate to 22 degrees C in a water bath prior to evaluation. For both mouse and rat intercostal tissues, the estimated frequency normalized attenuation coefficient was 1.1 dB/cm-MHz. In order to determine if there was an effect of storage time on estimates of attenuation coefficient, an independent experiment was conducted. The intercostal tissues from six mouse chest walls were evaluated at three time points (1, 22, and 144 days pm), and from six rat chest walls were evaluated at four time points (1, 22, 50, and 125 days pm). There was no difference in the estimated intercostal tissue attenuation coefficient as a function of time postmortem.

Animals↗
PubMed2001

Quantifying B-mode images of in vivo rat mammary tumors by the frequency dependence of backscatter.

OBJECTIVE: To evaluate the frequency dependence of ultrasonic backscatter for its ability to differentiate between neoplastic and healthy tissue. METHODS: Standard B-mode images were created of 5 rats with spontaneous mammary tumors, and regions of interest in the lesion and surrounding tissue were parameterized by the slope of the backscatter amplitude versus frequency. RESULTS: In 4 of the 5 rats, the averaged backscatter slope of the regions of interest in the tumor was significantly (P < .05) different from that of the surrounding tissue, and the fifth case had a moderate difference (P = .20). The consistency of the averaged slope values (1.2-1.8 dB/MHz) across all but 1 of the mammary tumors was encouraging for the prospect of identifying a tissue type by its backscatter slope. CONCLUSIONS: This work suggests that characterization and diagnosis of tissue types may be possible by using ultrasonographic images quantified by the frequency dependence of backscatter.

Animals↗
PubMed2000

Determination of biomechanical properties in guinea pig esophagus by means of high frequency ultrasound and impedance planimetry.

Impedance planimetry and high-frequency ultrasound were used to determine circumferential stress and strain from measurements of luminal cross-sectional area and wall thickness during balloon distension of the guinea pig esophagus in vitro (N = 30). The excised esophagus was mounted on two plastic tubes in an organ bath containing oxygenated calcium-free Krebs-Ringer solution with 10(-2) M MgCl2 to abolish smooth muscle contractile activity. One of the plastic tubes was movable in order to stretch the esophagus longitudinally by 15% (elongated state). The impedance planimetry probe was placed with the balloon inside the lumen of the esophagus. A 20-MHz ultrasound transducer was mounted above the esophagus and provided scans in the transverse and longitudinal directions. The luminal cross-sectional area at the highest applied pressure of 2.9 kPa was 13.3 +/- 0.3 mm2 in the resting state. In the elongated state the luminal cross-sectional area at the highest pressure was 12.5 +/- 0.1 mm2 (P < 0.02). The wall thickness decreased from 990 +/- 21 microm at 0 kPa to 640 +/- 9 microm at 2.9 kPa at in vitro length. In the elongated state, the values were 940 +/- 32 microm to 480 +/- 13 microm (P < 0.01). The stress-strain relation was exponential (sigma = alpha(ebetaepsilon - 1), r2 > 0.98, P < 0.01). The circumferential elastic modulus calculated at a Green strain of 0.95 was 44.5 +/- 10.5 kPa in the in vitro state and 81.7 +/- 13.1 kPa in the elongated state. The elastic modulus differed between the resting and elongated states (P < 0.02).

Animals↗
PubMed2000

Ultrasound-induced lung hemorrhage is not caused by inertial cavitation.

In animal experiments, the pathogenesis of lung hemorrhage due to exposure to clinical diagnostic levels of ultrasound has been attributed to an inertial cavitation mechanism. The purpose of this article is to report the results of two experiments that directly contradict the hypothesis that ultrasound-induced lung hemorrhage is caused by inertial cavitation. Elevated hydrostatic pressure was used to suppress the involvement of inertial cavitation. In experiment one, 160 adult mice were equally divided into two hydrostatic pressure groups (0.1 or 1.1 MPa), and were randomly exposed to pulsed ultrasound (2.8-MHz center frequency, 1-kHz PRF, 1.42-micros pulse duration, 10-s exposure duration). For the two hydrostatic pressure groups (80 mice each), 8 in situ peak rarefactional pressure levels were used that ranged between 2.82 and 11.8 MPa (10 mice/group). No effect of hydrostatic pressure on the probability of hemorrhage was observed. These data lead to the conclusion that lung hemorrhage is not caused by inertial cavitation. Also, the higher hydrostatic pressure enhanced rather than inhibited the impact of ultrasonic pressure on the severity (hemorrhage area, depth, and volume) of lesions. These counterintuitive findings were confirmed in a second experiment using a 2 x 5 factorial design that consisted of two ultrasonic pressure levels and five hydrostatic pressure levels (100 mice, 10 mice/group). If inertial cavitation were the mechanism responsible for lung hemorrhage, then elevated hydrostatic pressures should have resulted in less rather than more tissue damage at each ultrasonic pressure level. This further supports the conclusion that the pathogenesis of ultrasound-induced lung hemorrhage is not caused by inertial cavitation.

Animals↗
PubMed2000

Localization of multiple sound sources with two microphones.

This paper presents a two-microphone technique for localization of multiple sound sources. Its fundamental structure is adopted from a binaural signal-processing scheme employed in biological systems for the localization of sources using interaural time differences (ITD). The two input signals are transformed to the frequency domain and analyzed for coincidences along left/right-channel delay-line pairs. The coincidence information is enhanced by a nonlinear operation followed by a temporal integration. The azimuths of the sound sources are estimated by integrating the coincidence locations across the broadband of frequencies in speech signals (the "direct" method). Further improvement is achieved by using a novel "stencil" filter pattern recognition procedure. This includes coincidences due to phase delays of greater than 2pi, which are generally regarded as ambiguous information. It is demonstrated that the stencil method can greatly enhance localization of lateral sources over the direct method. Also discussed and analyzed are two limitations involved in both methods, namely missed and artifactual sound sources. Anechoic chamber tests as well as computer simulation experiments showed that the signal-processing system generally worked well in detecting the spatial azimuths of four or six simultaneously competing sound sources.

Adult↗
PubMed2000

Anisotropy of ultrasonic propagation and scattering properties in fresh rat skeletal muscle in vitro.

The anisotropy of frequency-dependent backscatter coefficient, attenuation, and speed of sound is assessed in fresh rat skeletal muscle within 5 h post-mortem. Excised rat semimembranosus and soleus muscles are measured in 37 degrees C Tyrode solution, with the muscle fibers at 90 degrees and 45 degrees orientations to the incident sound beam. Reflected and through transmission signals from either a 6- or 10-MHz focused transducer give frequency dependent information in the 4-14 MHz range. The attenuation coefficient in each muscle is consistently a factor of 2.0 +/- 0.4 lower for propagation perpendicular to the fibers than at 45 degrees, whereas speed of sound shows a much milder anisotropy, and is slightly faster for the 90 degrees orientation. The largest anisotropy is seen in the backscatter coefficient, most notably in the semimembranosus where the magnitude at 90 degrees is over an order of magnitude greater than at 45 degrees, with the frequency dependence in both cases giving a power law between 1.5 and 2.0.

Acoustic Stimulation↗
PubMed2000

Acoustic imaging of objects buried in soil.

In this study, we demonstrate an acoustic system for high-resolution imaging of objects buried in soil. Our goal is to image cultural artifacts in order to assess in a rapid manner the historical significance of a potential construction site. We describe the imaging system and present preliminary images produced from data collected from a soil phantom. A mathematical model and associated computer software are developed in order to simulate the signals acquired by the system. We have built the imaging system, which incorporates a single element source transducer and a receiver array. The source and receiver array are moved together along a linear path to collect data. Using this system, we have obtained B-mode images of several targets by using delay-and-sum beamforming, and we have also applied synthetic aperture theory to this problem.

Acoustics↗
PubMed1999

In vivo ultrasonographic exposimetry: human tissue-specific attenuation coefficients in the gynecologic examination.

OBJECTIVE: The purpose of the current study was to determine in vivo, tissue-specific ultrasonic attenuation coefficients for each of the tissue layers comprising the anterior abdominal wall, uterus, and vagina with use of a quantitative multilayer tissue model. We wanted to validate the "homogeneous" tissue model-based Food and Drug Administration derating factor of 0.3 dB/cm-MHz applied to obstetric-use ultrasonography systems. STUDY DESIGN: With use of a 3. 0-MHz mechanical sector scanner and our previously tested exposimetry equipment, we obtained a set of at least 5 separate acoustic pressure waveforms from each test subject by placing a calibrated 7-element linear-array hydrophone in the anterior vaginal fornix while she was undergoing transabdominal ultrasonography. Corresponding sets of reference in vitro acoustic pressure waveforms were also recorded for each test subject in a 37 degrees C water bath. All linear measurements of individual layer thicknesses and total distances were made on-line with use of electronic calipers. A set of multiple and independent insertion loss values, denoted ILn, was calculated for path n between the abdominal surface and the hydrophone from n sonograms for each test subject. Each tissue layer type was identified and its thickness along each path n was measured. The thickness of tissue type m along path n was denoted by dnm. The only unknown quantities left were the attenuation coefficients Am of each of the m tissue layers for that test subject. The overestimated set of equations dnm Am = ILn was solved for Am with use of a nonnegative least-squares solution technique. RESULTS: With use of data from 162 independent insertion loss estimate paths, the overall tissue-specific attenuation coefficients for each of the tissue layer types, expressed as mean value +/- SD, were 2.3 +/- 1.5 dB/cm-MHz for the skin and subcutaneous layer, 3.1 +/- 2.5 dB/cm-MHz for skeletal muscle, 0.6 +/- 0.5 dB/cm-MHz for myometrium, and 3.6 +/- 2.7 dB/cm-MHz for the vaginal wall. The overall insertion loss assuming the "homogeneous" tissue model was 0.7 +/- 0.3 dB/cm-MHz. CONCLUSIONS: We have determined the specific ultrasonic attenuation coefficients for each of the tissue layers comprising the anterior abdominal wall, uterus, and vagina and validated the Food and Drug Administration derating factor of 0.3 dB/cm-MHz applied to obstetric use ultrasonography systems. Of all the models proposed, the "homogeneous" tissue model appears to be the best model for determining ultrasonic exposure risk during reproductive ultrasonographic examinations.

Abdomen↗
PubMed1999

Detection of lines and boundaries in speckle images--application to medical ultrasound.

This paper describes an approach to boundary detection in ultrasound speckle based on an image enhancement technique. The enhancement algorithm works by filtering the image with "sticks," short line segments which are varied in orientation to achieve the maximum projected value at each point. The statistical properties of this approach have been described in an earlier paper; in this work we present three significant extensions to improve the performance of the basic method. First, we investigate the effect of varying the size and shape of the sticks. We show that these variations affect the performance of the algorithm in very fundamental ways, for example by making it more or less sensitive to thinner or more tightly curving boundaries. Second, we present a means of improving the performance of this technique by estimating the distribution function of the orientation of the line passing through each point. Finally, we show that images can be "stained" for easier visual interpretation by applying to each pixel a false color whose hue is related to the orientation of the most prominent line segment at that point. Examples are given to illustrate the performance of the different settings on a single image.

Algorithms↗
PubMed1999

Interlaboratory comparison of ultrasonic backscatter, attenuation, and speed measurements.

In a study involving 10 different sites, independent results of measurements of ultrasonic properties on equivalent tissue-mimicking samples are reported and compared. The properties measured were propagation speed, attenuation coefficients, and backscatter coefficients. Reasonably good agreement exists for attenuation coefficients, but less satisfactory results were found for propagation speeds. As anticipated, agreement was not impressive in the case of backscatter coefficients. Results for four sites agreed rather well in both absolute values and frequency dependence, and results from other sites were lower by as much as an order of magnitude. The study is valuable for laboratories doing quantitative studies.

1-Propanol↗
PubMed1996

Behavioral effects of prenatal exposure to pulsed-wave ultrasound in unanesthetized rats.

The present experiment examined the developmental neurotoxicity of pulsed-wave (pw) ultrasound in rats, using an exposure system designed to eliminate restraint or anesthesia from the exposure conditions. Pregnant Sprague-Dawley CD rats trained to remain immobile in a water-filled ultrasound exposure tank were scanned with 3-MHz pw ultrasound at spatial peak temporal average intensities (ISPTA) of 0, 2, 20, or 30 W/cm2 on embryonic days 4-20 for approximately 10 min/day. The data showed that such insonation produced no adverse effects on maternal weight gain or reproductive outcome, nor on the postnatal growth or survival of the offspring. No exposure-related alterations in behavioral development were observed in the offspring of rats scanned with pw ultrasound during gestation. In addition, there was no consistent evidence of an ultrasound-associated change in the adult offspring behaviors tested; i.e., no treatment effects were found on measures of locomotor activity, water maze learning, and acoustic startle reactivity. An effect on tactile startle was observed on some trials in the low exposure group male offspring, but this effect was neither dose dependent nor consistent with any other finding. Overall, these results indicate that the neurobehavioral development of rats was not altered by prenatal exposure to pw ultrasound at ISPTA levels of up to 30 W/cm2.

Animals↗
PubMed1996

Rabbit and pig lung damage comparison from exposure to continuous wave 30-kHz ultrasound.

Previous comparative studies of ultrasound-induced pulmonary hemorrhage in mice and rabbits suggested that sensitivity to damage was species dependent (O'Brien and Zachary 1994b). In order to understand better these differences in species more analogous to the human, 74 pigs and 75 rabbits were each exposed for 10 min at 1 of 6 acoustic pressure levels (0, 145, 290, 340 [rabbits only], 460 and 490 [pigs only] kPa) at an ultrasonic frequency of CW 30 kHz. Eighteen mice were used as positive controls (10-min duration at 145 kPa). Because pig lung has numerous physiological and anatomical similarities to human lung, it was selected as the appropriate animal model for these studies. Pig lung data were compared to rabbit lung data; rabbit lung data have already been compared with mouse lung data (O'Brien and Zachary 1994a). Comparative analyses and extrapolation of these experimental data are intended to provide a better scientific basis for understanding the potential biological effects of ultrasound on human lungs since such studies will probably never be conducted with humans. Under the same exposure conditions and lung assessment criteria, mouse lung was determined to be more sensitive to ultrasound-induced damage than that of the rabbit by a factor of 3.9, the rabbit lung was more sensitive to ultrasound-induced damage than that of the pig by a factor of 3.7, and the mouse lung was more sensitive to ultrasound-induced damage than that of the pig by a factor of 14.4.

Analysis of Variance↗
PubMed1995

Acoustic propagation properties of normal, stunned, and infarcted myocardium. Morphological and biochemical determinants.

BACKGROUND: Identification of viable but stunned myocardium remains a major problem. Since stunned myocardium results in impairment of myocardial function without any structural damage and infarcted myocardium causes major structural disruption, we postulated that acoustic properties could distinguish between the two insults. METHODS AND RESULTS: Anesthetized open-chest dogs underwent a total occlusion of the left anterior descending coronary artery for 15 minutes (stunned, n = 7) and 90 minutes (infarcted, n = 8), followed by reperfusion for 3 hours. Circumflex coronary artery perfusion territory (n = 15) served as normal control tissue. Regions of myocardium were quantitatively evaluated with a scanning laser acoustic microscope operating at 100 MHz and a research ultrasound system operating at 4 to 7 MHz. Four ultrasonic parameters were determined: attenuation coefficient (an index of loss per unit distance), speed of propagation, a spatial variation of propagation speed called the heterogeneity index (HI), and ultrasonic backscatter at 5 MHz (IBR5). Myocardial water, lipid, and protein contents of normal, stunned, and infarcted myocardium were also determined. The attenuation coefficient of normal myocardium (179 +/- 20 dB/cm) was significantly greater than that of stunned (136 +/- 7 dB/cm, P < .001) and infarcted (130 +/- 8 dB/cm, P < .001) myocardium. The propagation speed of normal myocardium (1597 +/- 6 m/s) was similar to that of stunned (1600 +/- 6 m/s) and significantly higher than that of infarcted (1575 +/- 7 m/s, P < .001) myocardium. The HI for specimen thicknesses of 75 to 100 microns showed an increase of 33% between normal (5.0 +/- 0.8 m/s) and stunned (7.5 +/- 2.3 m/s, P < .05) myocardium. However, for the infarcted myocardium (5.8 +/- 2.0 m/s), the HI was essentially the same as that of the normal myocardium (5.0 +/- 0.8 m/s). The IBR5 of normal (-47.1 +/- 1.0 dB) was not significantly different from that of stunned myocardium (-46.8 +/- 0.9 dB). The IBR5 of infarcted myocardium (-42.4 +/- 1.0 dB) was significantly greater than that of normal myocardium. Myocardial water and protein contents were similar in the normal and stunned myocardium. Water content in the infarcted myocardium (80.8 +/- 2%) was significantly greater (P < .05) than in the normal (72.7 +/- 1.3%), and protein content of 18.5 +/- 0.7% was significantly lower (P < .05) than the normal (21.4 +/- 0.8%). Lipid content was increased in the stunned (8.5 +/- 0.5%) and virtually absent in the infarcted myocardium (0.8 +/- 0.3%) compared with normal (5.5 +/- 0.6%). CONCLUSIONS: We conclude that acoustic propagation properties can identify stunned and infarcted myocardium and may be related to biochemical/morphological differences.

Animals↗
PubMed1995

Relation of ultrasonic backscatter and acoustic propagation properties to myofibrillar length and myocardial thickness.

BACKGROUND: Ultrasonic backscatter demonstrates a cardiac cycle-dependent modulation. The exact mechanism of the modulation is under debate. The objective of the present study was to test the hypothesis that a change in size and configuration of myofilaments from systole to diastole alters acoustic propagation properties and backscatter. METHODS AND RESULTS: In vivo measurements were made of integrated backscatter at 5 MHz (IBR5), followed by in vitro measurements of ultrasonic attenuation, speed, and heterogeneity index using a scanning laser acoustic microscope at 100 MHz. Studies were performed in canine hearts (16) arrested in systole (8) with calcium chloride or arrested in diastole (8) with potassium chloride. Sarcomere length was measured with a calibrated eyepiece on a Ziess microscope. Wall thickness was measured with calipers. The attenuation coefficient of 220 +/- 34 dB/cm during systole was significantly higher than the coefficient of 189 +/- 24 dB/cm during diastole (P < .01); the IBR5 of -44.7 +/- 1.2 dB during systole was significantly greater than the IBR5 of -47.0 +/- 1.0 dB during diastole (P < .01); the ultrasonic speed of 1591 +/- 11 m/s during systole was higher than the speed of 1575 +/- 4.2 m/s during diastole (P < .01); and the heterogeneity index of 7.4 +/- 1.8 m/s during systole was significantly lower than the index of 9.0 +/- 2.0 m/s during diastole (P < .02). The sarcomere length of 1.804 +/- 0.142 microns during diastole was significantly higher than the length of 1.075 +/- 0.177 micron during systole (P < .01). Wall thickness was significantly greater during systole than during diastole (20 +/- 3 versus 9 +/- 3 mm, P < .01). CONCLUSIONS: Ultrasonic backscatter and propagation properties are directly related to sarcomere length and myocardial thickness and may be responsible for cardiac cycle-dependent variation in backscatter.

Animals↗
PubMed1995

In situ human obstetrical ultrasound exposimetry: estimates of derating factors for each of three different tissue models.

A specialized in vivo exposimetry system was developed to acquire transabdominal in situ ultrasound exposure quantities in obstetric patients. Under surgical conditions, the sterilized 7-element calibrated linear array hydrophone was introduced into the uterus under direct ultrasound guidance and placed in direct contact with the products of conception, usually in the saggital midplane of the uterine cavity. Twenty-five patients with empty bladders and 10 patients with full bladders were studied at gestational ages between 7 and 20 weeks. In the empty bladder condition, the sound beam traversed the anterior abdominal wall, uterus, amniotic fluid and fetal parts and in the full bladder condition, the sound beam also traversed the fluid-filled bladder. Each study was conducted with a 3 MHz, mechanical sector transducer in combination with an ATL Ultramark 4 diagnostic ultrasound imaging system. Calibration data were recorded after completion of each in vivo patient study. The acquired exposimetry data from the 35 obstetric patients were used to evaluate the appropriateness of three tissue attenuation models, viz., fixed path, homogeneous and overlying. All three tissue models yield a mean attenuation coefficient value of about a factor of 3 to 4 greater than their respective minimum values. In the case of the overlying and homogeneous tissue models, there was a statistically significant correlation between their calculated attenuation coefficients and total distance for the combined data set whereas there was no such dependency for the calculated fixed-path tissue model. In summary, any one of the three tissue models may be used to estimate in utero acoustic quantities during the first and second trimesters of human pregnancy based on this study.

Adult↗