Measurements of ultrasonic attenuation properties of midgestational fetal pig hearts

The objectives of this study were to measure the relative attenuation properties of the left and right ventricles in fetal pig hearts and to compare the spatial variation in attenuation measurements with those observed in previously published backscatter measurements. Approximately 1.0-mm-thick, short-axis slices of excised, formalin-fixed heart were examined from 15 midgestational fetal pigs using a 50-MHz single-element transducer. Measurements of the attenuation properties demonstrate regional differences in the left and right ventricular myocardium that appear consistent with the previously reported regional differences in apparent integrated backscatter measurements of the same fetal pig hearts. For regions of perpendicular insonification relative to the myofiber orientation, the right ventricular free wall showed larger values for the slope of the attenuation coefficient from 30-60 MHz (1.48 +/- 0.22 dB/(cm x MHz) (mean +/- SD) and attenuation coefficient at 45 MHz (46.3 +/- 7.3 dB/cm [mean +/- SD]) than the left ventricular free wall (1.18 +/- 0.24 dB/(cm x MHz) and 37.0 +/- 7.9 dB/cm (mean +/- SD) for slope of attenuation coefficient and attenuation coefficient at 45 MHz, respectively). This attenuation study supports the hypothesis that intrinsic differences in the myocardium of the left and right ventricles exist in fetal pig hearts at midgestation.     

Ultrasound attenuation measurements of the liver in vivo using a commercial sector scanner

Attenuation measurements of various tissue mimicking phantoms and three different groups of patients were obtained using a modified commercial sector scanner. Estimates of attenuation were made using the spectral shift method with mean frequencies at different depths of a region of interest being obtained by both zero crossing and fast Fourier transform techniques. The accuracy and precision of both techniques was compared in phantoms and it was found that the FFT technique yielded less day-to-day variation (SD=3 percent) than the zero crossing technique (5 percent). For larger regions of interest, the range of variation in both techniques was more similar. Day-to-day variation in livers of normal patients was much larger than that seen in phantoms (10 to 15 percent) suggesting that in vivo measurements may be less precise due to actual daily changes in patients' livers. Attenuation estimates of phantoms were high by approximately 0.16 dB/MHz/cm compared to values obtained by transmission techniques. The attenuation values of livers in a group of 31 normal patients ranged from 0.214 dB/cm/MHz to 0.849 dB/cm/MHz with a mean of 0.627 +/- 0.126 dB/cm/MHz for the zero crossing technique while the mean value using the FFT technique was 0.86 +/- 0.168 dB/cm/MHz. A group of 26 Gauchers disease patients also showed wide variation with a mean attenuation value of 0.768 +/- 0.21 dB/cm/MHz using the FFT technique. This was significantly different than that of the normal group (p less than .05). Also, a group of 22 chronic B hepatitis patients was examined, having a mean attenuation value of 0.823 +/- 0.21 dB/cm/MHz, not significantly different from those of normal patients. Highly significant differences were found between the three groups when the power spectrum bandwidths of signals received were compared. These differences may be due to differences in the dependence of attenuation as a function of frequency between the groups and may represent a useful tissue characterization parameter.     

Anisotropy of apparent backscatter in the short-axis view of mouse hearts

The goals of this investigation were to measure the anisotropy of backscattered ultrasound observed in the short-axis view of mouse hearts in systole and diastole and to compare these measurements with predictions from a computer simulation. Measurements of midmyocardial apparent backscatter were obtained from analyses of the hearts of seven wild-type mice using a clinical imaging system utilizing a linear array with a nominal center frequency of 13 MHz. A computer model simulating the short-axis view was implemented based on previous measurements of the angle of insonification dependence of myocardial backscatter and attenuation. Results demonstrate that the measured backscatter was largest for those myocardial regions corresponding to approximately perpendicular insonification relative to the myofibers and the smallest for regions of approximately parallel insonification, with the minimum to maximum values of apparent backscatter differing by approximately 10 dB. The measured anisotropy of backscatter was similar for end-systole and end-diastole and was in good agreement with the predicted anisotropy obtained from the computer simulations. (E-mail: mrh@wuphys.wustl.edu)     

Studies on frequency-dependent attenuation in the normal liver and spleen and in liver diseases, using the spectral-shift zero-crossing method

This report presents results of studies using the spectral-shift zero-crossing method to measure frequency-dependent attenuation (FDA) in normal liver and spleen and in diseased liver. We developed a new system for attenuation analysis that calculated FDA in dB/cm/MHz according to the following equation: (formula: see text). Data are collected from the region of interest on the scan image. Graphite-gel phantoms of known attenuation value are used to create a high degree of accuracy in this new system. Mean attenuation of normal livers was 0.55 +/- 0.05 dB/cm/MHz, while that of normal spleen was 0.37 +/- 0.06 dB/cm/MHz. No correlation between FDA and age could be seen. FDA was 0.81 +/- 0.17 dB/cm/MHz in fatty liver, 0.63 +/- 0.13 dB/cm/MHz in liver cirrhosis, and 0.64 +/- 0.12 dB/cm/MHz in chronic hepatitis. These values are higher than those obtained from normal liver, while tumor masses in the liver (hepatocellular carcinoma, hepatoblastoma, hemangioma) and diffuse infiltration by malignant lymphoma produced lower than normal values, averaging 0.38 +/- 0.08 dB/cm/MHz.     

Attenuation of ultrasound in normal liver and diffuse liver disease in vivo

Preliminary results of in vivo attenuation measurements of the liver have been obtained in 39 normal patients and in 35 patients with diffuse liver disease. A modified real-time sector scanner incorporating an online attenuation measurement method was used. The value of attenuation in normal liver was estimated as 0.52 +/- 0.03 dB/cm/MHz, measured at 3 MHz. Significantly higher attenuation values were obtained from patients with alcoholic and cardiac cirrhosis, and following hepatic artery infusion with chemotherapeutic agents. Lower values were obtained from patients with biliary cirrhosis, chronic active hepatitis, and diffuse infiltration by lymphoma or leukemia. Fatty infiltrated livers showed a wide range of values of 0.37-0.66 dB/cm/MHz. The results suggest that estimates of attenuation coefficients are useful in detecting the presence of diffuse liver disease.     

Influence of bright intramural echoes on estimates of ultrasonic attenuation from backscattered ultrasound in excised myocardium

The purpose of this study was to quantitate the influence of bright intramural echoes on estimates of myocardial attenuation from analyses of backscattered ultrasound. To achieve this, M-mode image-based measurements of the inherent anisotropic properties of myocardial attenuation were performed on rotating myocardial specimens. The approach was to use a commercially-available ultrasonic imaging system to acquire M-mode images of 24 excised cylindrical specimens from six formalin-fixed lamb hearts for data analysis using a video signal analysis technique. As a control, through-transmission rf-based measurements were performed concurrently using a pair of focused, single-element ultrasonic transducers. We devised an objective approach to compensate M-mode results for the presence of bright intramural echoes that makes use of the rotational symmetry of the measurements. A comparison of the uncompensated and compensated estimates of attenuation shows that the effect of bright intramural echoes under the conditions of this study increases the average error in M-mode results by approximately 240% compared with that observed when such effects are minimized by compensation. For both uncompensated and compensated M-mode results, increased temporal averaging shows only a modest reduction in average error. These data suggest that for measurements of attenuation from backscattered ultrasound using M-mode images, the effects of bright intramural echoes can be a significant source of error despite increased temporal averaging and therefore may require compensation.     

Characterizing acoustic attenuation of homogeneous media using focused impulsive acoustic radiation force

A new method to characterize a material's attenuation using acoustic radiation force is proposed. Comparison of displacement magnitudes generated in a homogeneous material by acoustic radiation force excitations can be used to estimate the material's attenuation when the excitations are applied over a range of focal depths while maintaining a constant lateral focal configuration. Acoustic attenuations are related to the inverse of the excitation focal depth that yields the greatest focal zone displacement for this protocol. Experimental studies in calibrated tissue-mimicking phantoms are presented to demonstrate the feasibility of this method. Attenuations ranging from 0.3-1.5 dB/cm/MHz were characterized over excitation focal depths ranging from 5-30 mm, with an accuracy of 0.1 +/- 0.15 dB/cm/MHz. As currently implemented, this method is limited to characterizing materials that have homogeneous material properties and acoustic attenuations. This method for characterizing acoustic attenuation can be performed using conventional diagnostic scanners without any additional hardware and could also be performed concurrently with acoustic radiation force-based imaging modalities to generate images of mechanical properties and attenuation that are spatially co-registered with B-mode images.     

Changes in ultrasonic attenuation and backscatter of muscle with state of contraction

We have previously demonstrated that backscatter (uncompensated for attenuation) of canine myocardium varies systematically throughout the cardiac cycle and in relation to regional contractile performance. To elucidate these phenomena under conditions independent of blood flow and complex myofibrillar architecture, we measured attenuation with a phase-insensitive receiver and backscatter over a wide range of frequencies in an intermittently tetanized (10 stimulations), isolated frog gastrocnemius preparation (n = 12 muscles). Muscle contraction, as compared with relaxation, was associated with increased values of slope of attenuation (0.78 +/- 0.04 vs 0.58 +/- 0.03 dB/(cm MHz); p less than 0.001) and increased values of integrated backscatter (-29.5 +/- 0.9 vs -35.5 +/- 0.8 dB; p less than 0.005). Differences in attenuation and backscatter diminished with the number of muscle stimulations (as the muscle fatigued). Thus, quantitative ultrasonic indices of skeletal muscle vary systematically with the contractile performance of the tissue. Extrapolation of these results to cardiac muscle suggests that the sensitivity of these indices to contractile function of muscle may provide an approach for noninvasive assessment of intrinsic properties of myocardium that determines its performance.     

Acoustic speed and attenuation coefficient in sheep aorta measured at 5-9 MHz

B-mode ultrasound (US) images from blood vessels in vivo differ significantly from vascular flow phantom images. Phantoms with acoustic properties more closely matched to those of in vivo arteries may give better images. A method was developed for measuring the speed and attenuation coefficient of US over the range 5 to 9 MHz in samples of sheep aorta using a pulse-echo technique. The times-of-flight method was used with envelope functions to identify the reference points. The method was tested with samples of tissue-mimicking material of known acoustic properties. The tissue samples were stored in Krebs physiologic buffer solution and measured over a range of temperatures. At 37 degrees C, the acoustic speed and attenuation coefficient as a function of frequency in MHz were 1600 +/- 50 ms(-1) and 1.5 +/- 4f(0.94 +/- 1.3) dB cm(-1), respectively.     

Characterization of anisotropic myocardial backscatter using spectral slope, intercept and midband fit parameters

The specific myocardial structural components that contribute to the observed level of backscatter from the heart and its dependence on the angle of insonification have not been completely identified: The objectives of this study were to measure the anisotropy of backscatter from myocardium using the approach first introduced by Lizzi et al. [J Acoust Soc Am 73, 1366-1373 (1983)] and to use the extracted spectral parameters (spectral slope, intercept and midband fit) to characterize changes in the apparent scatterer size, spatial concentration and acoustic impedance properties as functions of the angle of insonification. Backscatter measurements were performed in vitro on eight cylindrical formalin-fixed lamb myocardial specimens using a 5 MHz focused transducer. The backscattered spectral data as a function of angle of insonification relative to the myocardial fiber direction were analyzed over the frequency range of 4 to 6 MHz. The spectral parameters describing features of backscatter were determined by applying a linear fit to attenuation-compensated normalized spectra. Results show that values of the spectral slope do not exhibit a significant dependence on the angle of insonification within uncertainties; however, the zero-frequency intercept showed clear anisotropy and was found to be a maximum for insonification perpendicular to the predominant myofiber orientation and a minimum for parallel insonification. A comparison of midband fit values at 5 MHz with attenuation-compensated integrated backscatter values showed excellent agreement for all angles of insonification. These data suggest that measurements of spectral slope, intercept, and midband fit can provide insights regarding aspects of tissue microstructure underlying the observed anisotropy of myocardial scattering properties. Measurements of the slope parameter suggest a very modest change in effective scatterer size with angle of insonification. However, the observed anisotropy in intercept and midband fit and apparent absence of anisotropy in the spectral slope suggests an angle of insonification dependence of acoustic concentration, the combination of effective spatial scatterer concentration and acoustic impedance properties, without a significant contribution from changes in effective scatterer size.     

Relationship between ultrasonic attenuation, size and axial strain parameters for ex vivo atherosclerotic carotid plaque

Many ultrasonic parameters, primarily related to attenuation and scatterer size, have been used to characterize the composition of atherosclerotic plaque tissue. In this study, we combine elastographic (axial strain ratio) and ultrasonic tissue characterization parameters, namely the attenuation coefficient and a scattering parameter associated with an "equivalent" scatterer size to delineate between fibrous, calcified, and lipidic plaque tissue. We present results obtained from 44 ex vivo atherosclerotic plaque specimens obtained after carotid endarterectomy on human patients. Our results in the frequency range 2.5 - 7.5 MHz indicate that softer plaques (with higher values of the strain ratio) are usually associated with larger equivalent scatterer size estimates (200 - 500 microm) and lower values of the attenuation coefficient slope (<1 dB/cm/MHz). On the other hand, stiffer plaques (with lower strain ratio values) are associated with smaller equivalent scatterer size estimates (100 - 200 microm) and higher values of the attenuation coefficient slope (1 - 3 dB/cm/MHz). These results indicate that ultrasonic tissue characterization and strain parameters have the potential to differentiate between different plaque types. These parameters can be estimated from radio-frequency data acquired under in vivo conditions and may help the clinician decide on appropriate interventional techniques.     

High-frequency estimation of the ultrasonic attenuation coefficient slope obtained in human skin: simulation and in vivo results.

In vivo ultrasonic characterization of the skin was performed at 40 MHz by estimating the slope of the attenuation coefficient in the human dermis. The centroid algorithm was first tested on simulated backscattered RF lines with a second-order autoregressive model to carry out the spectral analysis. A relative error of less than 8.5% and a relative precision of less than 6% were predicted for a 2-mm tissue thickness and for temporal window sizes ranging from 0.25 to 0.45 micros. In vivo measurements performed on 138 healthy volunteers yielded values of the attenuation coefficient slope ranging from 0.8 to 3.6 dB/cm MHz. A decrease was observed with advancing age, but no significant difference appeared between men and women. The results from this study suggest that this acoustic parameter shows the effect of the ageing process on normal skin tissue in vivo.     

Acoustic measurements in a tissue mimicking liquid.

A liquid has been developed that mimics soft tissue in terms of propagation speed, attenuation, and nonlinearity parameter B/A. Since the slope of the attenuation coefficient is nearly constant up to at least 18 MHz and the value can be anywhere in the range 0.1 through 0.7 dB/(cm MHz), the material is ideal for assessing the effectiveness of attenuation derating of output acoustic intensities and pressures measured in water. A clinical ultrasound system was used to transmit into the tissue mimicking liquid. The pulse intensity integral and rarefactional pressure at various field locations in the tissue mimicking liquid were computed and compared to the corresponding values measured in water with attenuation derating applied. The slope of the attenuation coefficient in the tissue mimicking liquid was used for the derating. From this data it is seen that the present practice of modeling in vivo exposure using linearly derated measurements made in water is not always conservative.     

A new method for attenuation coefficient measurement in the liver: comparison with the spectral shift central frequency method.

OBJECTIVE: To assess the feasibility of a new method of measuring the attenuation coefficient in the liver, which offers less variability of results than the conventional method. METHODS: The attenuation coefficient was evaluated on the basis of the following equation with sound field correction: [log(M0 (z)) - tau(z)]. In our system, the attenuation coefficient was also evaluated by the spectral shift central frequency method at the same time. We used 44 cases of normal liver, 40 cases of fatty liver, and 20 cases of cirrhotic liver in the system. RESULTS: With this new method, attenuation coefficient values were 0.59+/-0.10 dB x cm(-1) x MHz(-1) in normal livers, 0.80+/-0.12 dB x cm(-1) x MHz(-1) in fatty livers, and 0.62+/-0.09 dB x cm(-1) x MHz(-1) in cirrhotic livers. In both methods we recorded a statistically significant difference between normal and fatty livers and between fatty and cirrhotic livers (P < .0001). Only in the fatty liver was any significant difference (P < .0001) found between attenuation coefficients in the new method and those in the spectral shift central frequency method (0.70+/-0.05 dB x cm(-1) x MHz(-1)). CONCLUSIONS: This new method, which was more sensitive in detecting fatty infiltration than the spectral shift central frequency method, was considered usable for evaluating the attenuation coefficient of the liver in vivo.     

An exposimetry system using tissue-mimicking liquid

Acoustic output measurements of diagnostic ultrasound scanners are currently performed in water and derated to approximate in situ values. The derating scheme ignores nonlinear propagation of sound waves and has been shown in previous numerical and experimental studies to tend to underestimate relevant pressure and intensity values in tissue mimicking media. This work describes an alternative method, which uses a tissue-mimicking liquid with attenuation coefficient slope of 0.3 dB/cm/MHz, speed of sound of 1,540 m/s and nonlinearity parameter B/A of 7.5. The acoustic properties of this liquid are stable for at least 2 y after production. Initial results using a single M-mode configuration are presented. These results confirm that derating can significantly underestimate the pulse intensity integral and peak rarefactional pressure.     

Age-related alterations of cardiac tissue microstructure and material properties in Fischer 344 rats

The cardiac aging process is accompanied by global mechanical dysfunction that reflects increased myocardial stiffness. Accordingly, age-related changes in microscopic material properties of myocardium were delineated with high-frequency ultrasound (US) (30 to 44 MHz) tissue characterization methods for aging Fischer 344 rats at 6 (adult), 18 (aged), and 24 (senescent) months of age. The excised lateral wall of the left ventricle of rats (n = 10 per group) was insonified with a 50-MHz acoustic microscope for determination of integrated backscatter, backscatter coefficient and attenuation coefficient. Histological and biochemical analyses for collagen content and cardiac myocyte diameter were performed. Collagen concentration increased progressively with age, with the greatest increments occurring from 6 to 18 months (38.0 +/- 6.3 to 53.0 +/- 7.1 mg/g dry wt), and leveling off at 24 months (60.0 +/- 7.4 mg/g dry wt). Tissue microscopic material properties also changed progressively from 6 to 24 months of age, as determined by US methods: integrated backscatter increased (-44.7 +/- 1.8 vs. -40.8 +/- 1.9 dB, p < 0.05), attenuation increased (47.1 +/- 5.9 to 65.3 +/- 7.8 dB/cm, p < 0.05), and the backscatter coefficient increased (0.73 +/- 0.16 x 10(-5) to 3.76 +/- 1.6 x 10(-5) cm(-1), p < 0.05), from 6 to 24 months of age in each case. Age-related alterations in indices of cardiac microscopic material properties were closely correlated with the changes in cardiac microstructure. Ultrasonic tissue characterization may prove to be a sensitive tool to monitor changes in the cardiac microstructure, such as increased collagen deposition, that occur within age-related diastolic dysfunction.     

Experimental Measurements of Ultrasound Attenuation in Human Chest Wall and Assessment of the Mechanical Index for Lung Ultrasound

Knowledge of the acoustic attenuation characteristics of the chest wall is necessary to estimate the acoustic exposure at the pleural surface during lung ultrasound and is useful in the prediction of bio-effects (e.g., pulmonary capillary hemorrhage) and the development of safe, effective lung imaging. Currently, this property is not well characterized in humans. The aim of this work was to characterize ultrasonic attenuation in human chest wall such that the ultrasound exposures of the lung can be estimated for clinically relevant conditions. In this study, we experimentally measured ultrasound transmitted through the intercostal tissue of 15 human cadaver chest wall samples relative to ultrasound transmitted through saline to determine attenuation coefficients for each sample. A GE Vivid 7 diagnostic ultrasound machine (GE Vingmed, Horten, Norway) and 3 S and 5 S phased array probes were used at center frequencies from 1.6 to 5 MHz. The chest wall samples varied in thickness from 2.3–5.5 cm with a median thickness of 3.8 cm. The frequency-normalized attenuation coefficient was approximately 1.44 dB/cm/MHz based on a linear best fit through all attenuation measurements. Attenuation characteristics varied appreciably between samples, and the sample-averaged linear attenuation coefficient was 1.43 ± 0.32 (mean ± standard deviation) dB/cm/MHz. This attenuation is higher than that previously measured in mammalian chest wall samples (1.1–1.3 dB/cm/MHz for mice and rats) and is much greater than that used by the mechanical index (0.3 dB/cm/MHz). Mechanical index values calculated using saline values de-rated by 0.3 dB/cm/MHz were up to 1.2 MPa/MHz^1/2 greater than those calculated using the measured through-tissue ultrasound waves. We conclude that the mechanical index overestimates exposures for lung ultrasound and thus may not be an appropriate dosimetry metric for pulmonary ultrasound.     

Correlation of tissue constituents with the acoustic properties of skin and wound

The purpose of this study was to compare measurements of ultrasound properties of skin and wound tissue with measurements of material properties such as total collagen concentration, acetic acid soluble collagen concentration, water concentration, and morphologic properties. Using a scanning laser acoustic microscope (SLAM), both ultrasonic speed and attenuation coefficient values were obtained for control skin (2-3 cm from the wound), for skin immediately adjacent to wounds (within 0.3 mm), as well as for wound tissue itself. The attenuation coefficient and speed measurements were lowest for wound tissue followed by adjacent skin and then control skin. As the wounds healed there appeared to be an increase in both speed and attenuation coefficient although the wound age at which these increases started and the length of time for which they continued varied from one dog to the next. The precision of duplicate sample measurement of wave speed was +/- 1.7% for control skin, whereas that for attenuation coefficient it was +/- 16%. Both ultrasonic speed and attenuation coefficient were directly correlated with tissue collagen concentration and inversely correlated with tissue water concentration (p less than 0.001). Attenuation coefficient correlated best (r = 0.73) with acetic acid soluble collagen concentration which reflects the changes in collagen taking place during the repair process. These attenuation measurements made at 100 MHz using the SLAM were compared for control skin and wound samples with measurements made at 10-40 MHz using backscatter acoustic techniques (BAT). The tissue samples analyzed by each ultrasound technique were from adjacent locations on the animals.     

Ultrasonic attenuation and absorption in liver tissue

A large range of values for ultrasonic attenuation and absorption coefficients of tissues are reported in the literature. An important distinction both practically and theoretically is the magnitude of the true absorption, which characterizes the rate of conversion of ultrasonic to thermal energy, as compared with the total attenuation of the ultrasonic signal as it propagates through tissue.The magnitudes of these quantities were studied in bovine liver. Total attenuation was measured, in the range of 1–6 MHz, by both phase sensitive and phase insensitive insertion loss techniques. Ultrasonic absorption was determined by two thermal techniques. The standard “transient thermoelectric” or rate-of-heating method, and a new measurement technique based on the temperature decay following a short ultrasonic pulse were employed for the determination of the ultrasonic absorption coefficient.The results demonstrate that the ultrasonic amplitude attenuation and absorption coefficients at low megahertz frequencies are not significantly different in liver. The mean values cluster around 0.05 nepers/cm/MHz (0.4 dB/cm/MHz). The sample-to-sample variation is indicated by the standard deviation in the measurements of 0.01 nepers/cm/MHz (0.09 dB/cm/MHz) or less.The results show that in liver tissue, absorption is the dominant feature of attenuation over this frequency range.     

The frequency dependence of ultrasonic velocity and the anisotropy of dispersion in both freshly excised and formalin-fixed myocardium

The objectives of this study were to measure the frequency dependence of the ultrasonic velocity in myocardium and to quantify the frequency dependence of phase velocity as a function of the insonification angle relative to the predominant direction of the myofibers. Broadband phase spectroscopy data were acquired, spanning a frequency range of 3 to 8 MHz. Measurements were made on 36 tissue specimens cored from 12 freshly excised lamb hearts and were repeated after fixation with formalin. Measured phase velocities were found to be well characterized by a logarithmic fit. For freshly excised myocardium, the dispersion over the 3 to 8 MHz bandwidth was dependent on the direction of insonification, ranging from 1.2 m/s change for perpendicular insonification (across the myofibers) to 3.7 m/s for parallel insonification (along the myofibers). The effects of formalin-fixation resulted in a significant increase in dispersion for perpendicular insonification, but did not appreciably alter the dispersion for parallel insonification.