Regional variation in the measured apparent ultrasonic backscatter of mid-gestational fetal pig hearts

The goal of this study was to characterize and compare regional backscatter properties of fetal hearts through measurements of the apparent integrated backscatter. Sixteen excised, formalin-fixed fetal pig hearts, representing an estimated 53 to 63 days of gestation, were investigated. Spatially localized measurements of integrated backscatter from these specimens were acquired using a 50 MHz single-element transducer. The apparent integrated backscatter measurements demonstrate different patterns of backscatter from the myocardium of the right ventricle compared with that of the left ventricle. These backscatter measurements appear to be consistent with the anisotropy of the fiber orientation observed in histologic assessment of the same specimens. For each of the 16 hearts, the apparent integrated backscatter from the right ventricular myocardium was larger than that from the left ventricular myocardium, exhibiting mean apparent backscatter values of –35.9 ± 2.0 dB and –40.1 ± 1.9 dB (mean ± standard deviation; n = 16; p < 0.001), respectively. This study suggests that the intrinsic ultrasonic properties of the left and right ventricular myocardium are distinct in fetal pig hearts at mid-gestation. (E-mail: mrh@wuphys.wustl.edu)     

Estimating myocardial attenuation from M-mode ultrasonic backscatter

The objective of this investigation was to determine whether measurements of myocardial attenuation can be obtained from analyses of M-mode images. We exploited the inherent anisotropy of myocardial properties as a means of systematically varying the attenuation to evaluate this M-mode image-based method for myocardial tissue characterization. A commercially available ultrasonic imaging system was used to acquire M-mode images of 24 excised cylindrical specimens from six formalin-fixed sheep hearts that were analyzed using video signal analysis. Data were compensated for the presence of bright intramural myocardial echoes, a potentially significant contributor to uncertainty in measurements of attenuation from backscattered ultrasound. The estimated attenuation coefficient in dB/cm at an effective center frequency of 2.75 MHz as a function of angle of insonification for measurements obtained from analyses of M-mode images is presented. Given a linear frequency-dependence of attenuation in myocardial tissue over frequencies ranging from 1.5 MHz to 8 MHz, as has been previously reported, M-mode image-based analyses were used to estimate the slope of attenuation. Results showed slopes of attenuation (over a -10 dB transmit bandwidth of 1.875 MHz to 3.75 MHz) ranging from 1.00 +/- 0.07 to 1.81 +/- 0.08 dB/(cm.MHz) for perpendicular and parallel insonification, respectively. These values were in good agreement with contemporaneously measured values (0.99 +/- 0.02 to 1.77 +/- 0.04 dB/(cm.MHz)) obtained over a frequency bandwidth of 4 MHz to 7 MHz using a through-transmission radio-frequency-based approach. These data suggest that robust measurements of myocardial attenuation can be obtained from analyses of M-mode images and that this method may be diagnostically feasible in the clinical setting.     

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.     

The acoustic properties, centered on 20 MHZ, of an IEC agar-based tissue-mimicking material and its temperature, frequency and age dependence

The purpose of this study was to characterize the ultrasonic properties of agar-based tissue-mimicking materials (TMMs) at ultrasound frequencies centered around 20 MHz. The TMM acoustic properties measured are the amplitude attenuation coefficient alpha (dB cm(-1)MHz(-1)), the speed of sound (ms(-1)) and the backscattered power spectral density (distribution of power per unit frequency normalized to the total received power) characteristics of spectral slope (dB MHz(-1)), y-axis intercept (dB) and reflected power (dB). The acoustic properties are measured over a temperature range of 22 to 37 degrees C. An intercomparison of results between two independent ultrasound measurement laboratories is also presented. A longitudinal study of the acoustic properties over a period of two years is also detailed, and the effect of water immersion on the acoustic properties of TMM is measured. In addition, the physical parameters of mass density rho (kg m(-3)) and specific heat capacity C (J kg(-1) K(-1)) are included. The measurement techniques used were based on the substitution technique using both broadband and narrowband pulses centered on 20 MHz. Both the attenuation coefficient and speed of sound (both group and phase) showed good agreement with the expected values of 0.5 dB cm(-1) MHz(-1) and 1540 ms(-1), respectively, with average values over the three-year period of 0.49 dBcm(-1)MHz1 (SD +/- 0.05) and 1540.9 ms(-1) (SD +/- 8.7). These results also showed agreement between the two independent measurement laboratories. Speed of sound and attenuation coefficient were shown to change with temperature with rates of + 2.1 m s(-1) degrees C(-1) and -0.005 dB cm(-1) MHz(-1) degrees C(-1), respectively. Attenuation changed linearly with frequency at the high frequency range of 17 to 23 MHz, and speed of sound was found to be independent of frequency in this range. The spectral slope of relative backscattered power for the material increased with frequency at typically 1.5 dB MHz(-1). This compared favorably with theoretical spectral slope values, calculated for a variety of scatterer sizes, albeit at a lower frequency range. It is also noticed that, on extrapolation back to lower frequencies, the backscatter is comparable with that measured at 7 MHz. Overall, this non-commercial agar-based TMM is shown to perform as expected at the higher frequency range of 17 to 23 MHz and is seen to retain its acoustic properties of attenuation and speed of sound over a three-year period.     

Ultrasonic myocardial integrated backscatter and myocardial wall thickness in animal experiments

The purpose of this study was to distinguish between normal and ischemic myocardium using ultrasonic integrated backscatter (IB) measurements and to relate IB with myocardial wall thickness. IB was measured in 9 open-chested Yorkshire pigs (24-30 kg) before, after 30 minutes of partial occlusion of the proximal left anterior descending coronary artery (LADCA), and after 60 minutes of subsequent reperfusion. The ultrasound transducer (4 MHz) was sutured onto the epicardial surface perfused by the LADCA. IB measurements were made with a repetition rate of 50 times per heart rate simultaneously with a left ventricular pressure signal. Myocardial wall thickness was measured off-line. The measurements of integrated backscatter, left ventricular pressure and wall thickness were based on mean values of ten subsequent cardiac cycles. End-systolic IB measurements were 5.3 dB higher during occlusion as compared to the reference measurements (7.1 +/- 3.2 dB versus 1.8 +/- 2.6 dB; p = 0.002). No statistically significant differences were found in end-systolic IB measurements. End-systolic wall thickness was 5 mm smaller during occlusion as compared to the reference measurements (7.2 +/- 1.4 mm versus 12.2 +/- 1.2 mm; p less than 0.001). Simple linear regression analysis showed a statistically significant inverse relationship between IB measurements and wall thickness in 21 out of the 23 sequences in which wall thickness could be measured. End-systolic IB measurements are favourable to distinguish acute ischemic myocardium from normal myocardium. There is a distinct inverse relationship between IB and myocardial wall thickness.     

Analysis of transmural trends in myocardial integrated backscatter in patients with progressive systemic sclerosis.

Cardiac involvement in progressive systemic sclerosis (PSS) is common and has a strong negative impact on the prognosis, especially when autoantibodies are present. To determine whether ultrasonic tissue characterization can detect early ultrastructural changes in the sclerodermal myocardium, we analyzed the transmural heterogeneity in myocardial integrated backscatter (THIB). "A-THIB" was defined as the absolute difference in integrated backscatter between the left (subendocardial) and right (subepicardial) ventricular halves of the myocardium in the septum and posterior wall, and was measured in 11 patients with PSS and 10 age- and sex-matched healthy participants. A-THIB in patients with PSS was higher than that in healthy participants (1.3 +/- 1.3 vs 4.0 +/- 1.4 dB for the septum and 1.1 +/- 0.7 dB vs 2.8 +/- 0.4 dB for the posterior wall; mean +/- SD, respectively, P <.0005). Septal A-THIB was higher in patients with PSS with than without anti-Scl70 or antinucleolar antibodies (3.2 +/- 1.1 vs 5.0 +/- 1.0 dB, P =.0165). Early changes in the myocardium of patients with PSS, possibly related to increased interstitial collagen deposition, can be detected by quantitative analysis of THIB.     

The relative contributions of myocardial wall thickness and ischemia to ultrasonic myocardial integrated backscatter during experimental ischemia

The purpose of this study was to assess the empirical relationship between myocardial integrated backscatter (IB) and myocardial wall thickness (WT) in normal myocardium. A second object was to estimate the additional contribution to acute ischemic integrated backscatter levels given this relationship. Myocardial IB measurements and simultaneous myocardial WT measurements were made in 16 open-chested pigs with intact coronary circulation (normal myocardium) and 10 min after the flow in the left anterior descending coronary artery had been reduced to 20% of its baseline value (ischemic myocardium). Measurements were made 50 times during one cardiac cycle and averaged over 10 cardiac cycles. IB and WT measurements were normalized with respect to the nonischemic end-diastolic values. The relationship between IB and WT in normal myocardium was estimated in every individual pig by simple linear regression. Estimates of IB during ischemia were calculated on the basis of this relationship and the ischemic WT measurements. Differences of the estimator and the actual measurement made during ischemia depict the actual contribution of the state of acute ischemia, without the influence of WT. The slope of the relationship between IB and WT during normal myocardial contraction ranged from -0.16 to 0.03 dB/% (mean = -0.036 dB/%, SD = 0.06 dB/%). The additional contribution of ischemia ranged from -3.84 to 5.56 dB (mean = 0.31 dB, SD = 2.72 dB). It was concluded that the average contribution of ischemia to IB measurements is insignificant if the IB dependency on WT is removed from the data and that the higher level of ischemic IB measurements can be explained by the decrease in wall thickness during ischemia and not by the ischemia itself.     

In vitro classification of gallstones by quantitative echography.

Gallstones (n = 20) were classified by dual energy Computer Tomography (CT) into three main classes: pure cholesterol stones (I), combination stones (II) and calcium stones (III). Further subclassification was possible by using morphological criteria. The acoustic measurements that were performed were measurements of the velocity of sound, the attenuation coefficient slope with frequency and intercept at 4.5 MHz, the attenuation coefficient slope per unit of time, the backscattering characteristics, and the appearance of B-mode echograms. The velocity of sound in calcium stones (c = 1695 +/- 107 m/s) was distinctly lower than in those containing some cholesterol (c > 2000 m/s). The attenuation coefficient slope ranged from 4.3 to 16.2 dB/cm MHz, the 4.5 MHz intercept from 21 to 66.2 dB/cm. The lowest values were found for the pure cholesterol stones (class IA), the highest values for subclass IIB (combination stones with shell). The attenuation coefficient slope per unit of time was distinctly lower (< 0.50 dB/microseconds MHz) for the cholesterol stones than for the combination and calcium stones (>0.64 dB/microseconds MHz). The backscattering spectrum was approximated by a straight line fit, and the slope for the cholesterol stones was lower than for the combination and calcium stones (<0.75 dB/MHz vs. >1.0 dB/MHz, respectively). The latter two parameters were assessed by in vivo applicable methods. The front echo level was found to be more than 5 dB higher for class IIB as compared to the other classes, while the spectral backscatter level at 4.5 MHz was considerably higher for both classes IIB and III. The B-mode echograms showed that a strong front echo in combination with a strong attenuation of the remaining echo signals was mainly found for stones of classes IIB and III. A significant difference between the group of stones that are suitable for lithotripsy and dissolution treatment (classes IA, IB and IIA) and the nontreatable calcium-containing stones (classes IIB and III) was found for the velocity of sound (p <0.01), the attenuation coefficient slope per time unit (p <0.10), the slope of the backscattering spectrum (p <0.05) and the 4.5 MHz intercept (p <0.01). B-mode classification yielded no complete distinction of these two groups of stones. It can be concluded that in vivo assessment of quantitative characteristics (front reflection, backscattering characteristics, attenuation coefficient slope per unit of time) in combination with the B-mode characteristics might be useful for in vivo gallstone classification.     

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.     

Ultrasound backscatter and attenuation in human liver with diffuse disease

Ultrasound backscatter and attenuation in the liver were measured in patients with diffuse liver disease and in 35 volunteers who had no history of liver ailments. Measurements were done using radiofrequency (RF) echo signals derived from a clinical scanner; a reference phantom was scanned to account for effects of gain, transmit-receive frequency response and transducer beam patterns on echo data. The mean backscatter coefficient at 3 MHz in livers of 7 patients with fatty infiltration was 6.8 x 10(-3) cm(-1)sr(-1) compared to a mean of 0.5 x 10(-3) cm(-1)sr(-1) in healthy patients. Mean attenuation at 3 MHz was 2.54 dB/cm in fatty livers compared to 1.66 dB/cm in healthy patients. A total of 7 patients with end-stage liver disease (cirrhosis) had attenuation values similar to those in the healthy group, and their mean liver backscatter was somewhat greater than the mean backscatter for healthy livers. Quantitating both backscatter and attenuation should be considered for detecting fatty infiltration; additional processing methods are needed to differentiate cirrhotic changes on the basis of acoustic signals.     

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.     

Attenuation correction in echocardiography

A commercial two-dimensional echo unit, modified to permit digital acquisition (2.2 MHz) of echo signals prior to time-gain-compensation (TGC), was ratiometrically calibrated. Simulating the TGC in software, we demonstrated improvements in image quality as compared to conventional video methods. A path-dependent attenuation correction (PDAC) algorithm, utilizing the gray level image statistics to assign one of three attenuation coefficients (for chest, myocardium or blood) to each image pixel, was then developed. Using it on left ventricular (LV) short axis images obtained in nine healthy closed chest dogs, we demonstrated that the backscatter intensity at end diastole (ED) was 2.0 +/- 0.5 dB (mean +/- SEM) higher (p less than 0.01) than at end systole (ES), in circumferential segments of the myocardium oriented anteriorly and posteriorly, relative to the transducer. In seven of the dogs, subsequently occluded for one hour in the anterior descending branch of the left coronary artery (LAD), this normal phasic myocardial backscatter (PMB) variation decreased or was reversed in ischemic segments, relative to preocclusion values, suggesting utility of the method for sequential study of myocardial ischemia and its treatment.     

Further studies on ultrasonic properties of blood clots

Two-dimensional echocardiography has been found to be an effective clinical tool in diagnosing intracardiac thrombi. Misdiagnosis may, however, still frequently occur because of the difficulty in differentiating the thrombi from other intracavitary masses based only on the echographic appearance of these structures. Ultrasonic tissue characterization techniques have been used in attempts to minimize this diagnostic uncertainty. Previously, we have shown that all ultrasonic parameters of blood, including ultrasonic backscatter, a quantitative measure of echogenicity, at 7.5 MHz increase rapidly following clotting. In this article, we report recent results on the measurements of attenuation and backscatter of thrombi as a function of time following clotting over the frequency range of 3 MHz to 8 MHz. These results indicate that ultrasonic backscatter from thrombi 12 h old is at least 18 dB higher than that of unclotted blood over the frequency range of 3 MHz to 8 MHz, and the slope of the attenuation coefficient is increased to 0.43 dB/cm-MHz. Comparison with the backscatter of bovine myocardium shows that the myocardium is more echogenic than fresh thrombi and is less echogenic than thrombi 12 to 24 h old. Similar results were also obtained for integrated backscatter measurements over the same frequency range.     

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.     

In vivo breast tissue backscatter measurements with 7.5- and 10-MHz transducers

Measurements of the ultrasound (US) backscatter coefficient (BSC) of fibroglandular and fatty breast tissues in vivo from 5.25 through 13 MHz, using the reference phantom method, are presented. Radiofrequency echo data were collected at a series of locations in the left breasts of 16 adults, age 46 to 84, and in a custom-built phantom calibrated for backscatter and attenuation. Matched regions of interest (ROIs) were then selected in these images, from which the backscatter coefficient and the backscatter frequency dependence were ratiometrically estimated, after compensation for attenuation. The mean results in fibroglandular tissues were 78.9 x 10(-3)/cm, sr at 7.2 MHz (n(ROI) = 43, n = 13) and 146 x 10(-3)/cm, sr at 10.3 MHz (n(ROI) = 19, n = 10) with frequency dependencies of f(2.28) and f(3.25). The corresponding results in subcutaneous fat were 2.59 x 10(-3)/cm, sr at 7.2 MHz (n(ROI) = 56, n = 16) and 7.08 x 10(-3)/cm, sr at 10.3 MHz (n(ROI) = 57, n = 16) with frequency dependencies of f(3.49) and f(3.43). These findings are discussed and compared to similar measurements in the literature.     

High-frequency ultrasonic attenuation and backscatter coefficients of in vivo normal human dermis and subcutaneous fat.

In vivo attenuation and backscatter coefficients of normal human forearm dermis and subcutaneous fat were determined in the ranges 14 to 50 MHz and 14 to 34 MHz, respectively. Data were collected using three different transducers to ensure that results were independent of the measurement system. Attenuation coefficient was obtained by computing spectral slopes vs. depth, with the transducers axially translated to minimize diffraction effects. Backscatter coefficient was obtained by compensating recorded backscatter spectra for system-dependent effects and, additionally, for one transducer using the reference phantom technique. Good agreement was seen between the computed attenuation and backscatter results from the different transducers/methods. The attenuation coefficient of the forearm dermis was well described by a linear dependence with a slope that ranged between 0.08 to 0.39 (median = 0.21) dB mm(-1) MHz(-1). The backscatter coefficient of the dermis was generally in the range 10(-3) to 10(-1) Sr(-1) mm(-1) and showed an increasing trend with frequency. No significant differences in attenuation coefficient slope between the forearm dermis and fat were noted. Within the range of 14 to 34 MHz, the ratio of integrated (average) backscatter of dermis to that of fat ranged from 1.03 to 87.1 (median = 6.45), indicating significantly higher backscatter for dermis than for fat. Data were also recorded at the fingertip where the attenuation coefficient slope of the dermis was seen to be higher than that at the forearm.     

Three-dimensional characterization of human ventricular myofiber architecture by ultrasonic backscatter.

Normal human left ventricular architecture comprises a highly aligned array of cardiac myofibers whose orientation depends on transmural location. This study was designed to determine whether measurement of integrated backscatter could be used detect the progressive transmural shift of myofiber alignment that occurs from epicardium to endocardium in human ventricular wall segments. Integrated backscatter was measured at 32 transmural levels in seven cylindrical biopsy specimens (1.4 cm diam) sampled from normal regions of six explanted fixed human hearts by insonification of samples at 180 independent angles in 2 degrees steps around their entire circumference with a 5-MHz broadband piezoelectric transducer. Histologic analysis was performed to determine fiber orientation. Integrated backscatter varied approximately as a sinusoidal function of the angle of insonification at each transmural level. Greater integrated backscatter was observed for insonification perpendicular as compared with parallel to fibers (difference = 14.5 +/- 0.6 dB). Ultrasonic analysis revealed a progressive transmural shift in fiber orientation of approximately 9.2 +/- 0.7 degrees/mm of tissue. Histologic analysis revealed a concordant shift in fiber orientation of 7.9 +/- 0.8 degrees/mm of tissue. Thus, human myocardium manifests anisotropy of ultrasonic scattering that may be useful for characterization of the intramural fiber alignment and overall three-dimensional organization of cardiac myofibers.     

Evaluation of lens hardness in cataract surgery using high-frequency ultrasonic parameters in vitro

Ultrasonic parameters of sound velocity and frequency-dependent attenuation ranging from 25 to 45 MHz were measured for the purpose of evaluating the hardness of lenses in cataract surgery (phacoemulsification). Measurements were performed with a 35-MHz ultrasonic transducer on porcine lenses in which artificially cataracts were induced. The hardness of the cataractous lens was also evaluated by mechanical measurement of its elastic properties. The results indicated that the ultrasonic attenuation coefficients in normal porcine lenses were approximately 4.49 +/- 0.05 (mean +/- SD) and 6.32 +/- 0.04 dB/mm at 30 and 40 MHz, respectively. The development progression of the cataracts resulted in the attenuation coefficient increasing linearly to 7.36 +/- 0.25 and 11.1 +/- 0.92 dB/mm, respectively, corresponding to an increase of Young's modulus from 2.6 to 101.2 kPa. The sound velocity concomitantly increased from 1639.8 +/- 4.2 to 1735.6 +/- 10.4 m/s. Evaluation of the relationship between the phacoemulsification energy level and ultrasonic parameters in vitro by surgeons revealed that both the attenuation coefficient and sound velocity were linearly correlated with the phacoemulsification energy (r = 0.941 and 0.915, respectively). These results showed that measuring high-frequency ultrasonic parameters provides surgeons with good capability and reproducibility for selecting the optimal energy level for phacoemulsification.     

Ultrasonic characterization of maturation of fetal lung microstructure: an animal study

Scattering parameters and histological properties of the ewe fetal lung microstructure were examined in vitro. Four groups of lamb fetuses were chosen at 80, 100, 130 and 145 days of gestation (10 lambs in all). The acoustic parameters of the lung (backscatter coefficient, integrated backscatter coefficient and effective scatterer size), texture parameter of the lung (effective density of scatterers) and a relative lung/liver maturation parameter (the integrated spectral ratio) were measured at 19 degrees +/- 2 degrees C within the frequency range 3-11 MHz. Values of integrated backscatter coefficient significantly decreased between 80 and 130 days gestation (from -73.0 +/- 5.7 dB to -84.0 +/- 0.9 dB between 3 to 7 MHz and from -70.0 +/- 0.8 dB to -81.3 +/- 0.5 dB between 5 to 11 MHz). Values of the integrated spectral ratio indicate that the echogenicity of the lung is greater than that of the liver. These values also decreased between 80 and 130 days gestation (from 10.5 +/- 1.1 dB to 5.1 +/- 0.9 dB between 3 to 7 MHz and from 10.0 +/- 0.8 dB to -6.4 +/- 0.6 dB between 5 to 11 MHz). Texture analysis showed that the K distribution is a good model to describe the envelope of the backscattered signals from the lung and the values of the effective density of scatterers that decrease between 80 and 130 days. These trends seem to be linked to the maturation of the microstructure of the lung and, particularly, to the development of respiratory terminal structures.     

Influence of attenuation on measurements of ultrasonic myocardial integrated backscatter during cardiac cycle (an in vitro study).

The purpose of this study was to investigate the dependence of ultrasonic integrated backscatter (IB) and attenuation in myocardium on wall thickness in a state of acute ischemia. Therefore, an in vitro experiment was set up in which attenuation, IB and wall thickness of a piece of freshly excised myocardium could be measured almost simultaneously. The myocardium was taken from 11 Yorkshire pigs (25–30 kg) that were killed less than 45 min before the experiment. The myocardium was placed in the far field of an ultrasound transducer (3.2–7.2 MHz) and then compressed by a stainless steel sphere. Data were processed off-line. Backscatter and attenuation were also measured as a function of frequency at 100% and 75% wall thickness, respectively. Both attenuation and IB varied during compression. Attenuation had an initial value of 2.19 ± 0.76 dB/cm and a slope of 0.015 ± 0.017 dB/cm% wall thickness. IB had an initial value of −76.9 ± 2.7 dB and a slope of −0.12 +- 0.07 dB/% wall thickness. After subtracting the influence of the attenuation from the IB the initial value of IB was −74.0 ± 2.7 dB and the slope −0.08 ± 0.07 dB/% wall thickness. Attenuation appeared to have a linear dependency on frequency. Backscatter appeared not to increase with increasing frequency without correction of the spectrum for the frequency dependent insonified volume.