Differentiation between acutely ischemic myocardium and zones of completed infarction in dogs on the basis of frequency-dependent backscatter

The goal of this work was to determine whether the frequency dependence of apparent backscatter coefficient (not corrected for attenuation within the myocardium) could differentiate completed, remote infarction from acute myocardial injury in vivo. Myocardial infarcts were produced in six dogs by coronary artery occlusion. One to 12 months later, acute ischemic injury was induced in each dog by ligation of a coronary artery that supplied a region of myocardium adjacent to the established infarct. Infarct, ischemic, and normal regions were interrogated with a 5-MHz, circular, 0.5-in. diam, broadband, focused, piezoelectric transducer mounted in a water-filled stand-off device placed against the exposed, beating heart. Apparent backscatter coefficients were measured over the range of frequencies from 3-7 MHz. The frequency dependence was obtained from the slope of log apparent backscatter coefficient versus log frequency. No significant difference in frequency dependence was found between normal and acutely ischemic myocardium for periods of up to 2 h of ischemia. In contrast, frequency dependence in regions of remote infarct (1.8 +/- 0.1, mean +/- standard error) was significantly lower than that in acutely ischemic or nonischemic regions (2.3 +/- 0.1) (p less than 0.01). These results suggest that remote myocardial infarction can be differentiated from acutely injured but still potentially salvageable myocardium in vivo on the basis of the frequency dependence of backscatter.     

Identification of human myocardial infarction in vitro based on the frequency dependence of ultrasonic backscatter.

It has been reported previously that acute and mature myocardial infarction in dogs can be differentiated in vitro and in vivo by ultrasonic tissue characterization based on measurement of the frequency dependence of ultrasonic backscatter. To characterize human infarction with an index of the frequency dependence of backscatter that could be obtained in patients, cylindrical biopsy specimens from 7 normal regions and 12 regions of infarction of 6 fixed, explanted human hearts in 2-deg steps around their entire circumference with a 5-MHz broadband transducer were insonified. One to six consecutive transmural levels were studied for each specimen. The dependence of apparent (uncompensated for attenuation or beam width) backscatter, /B(f)/2, on frequency (f) was computed from spectral analyses of radio-frequency data as /B(f)/2 = afn, where from theoretical considerations the magnitude of n decreases as scatterer size increases. Apparent integrated backscatter was computed as the average of /B(f)/2 from 3 to 7 MHz. The average value for n for normal tissue (0.9 +/- 0.1) exceeded that for tissue from regions of infarction (0.6 +/- 0.1; p less than 0.05). Infarct manifested a significant decrease of n from epicardial to endocardial levels (epi----mid----endo: 0.9----0.7----0.2; p less than 0.05) whereas normal tissue manifested similar values for n at each transmural level (0.8----1.1----0.9; p = NS). Average integrated backscatter across all transmural levels for infarct was significantly greater than for normal tissue (-48.3 +/- 0.5 vs -53.4 +/- 0.4 dB, infarct versus normal; p less than 0.05). The presence of fibrosis was associated with smaller values of n and greater integrated backscatter.(ABSTRACT TRUNCATED AT 250 WORDS)     

Anisotropy of the ultrasonic backscatter of myocardial tissue: II. Measurements in vivo

The purpose of this investigation was to determine the angular dependence of the backscatter from canine myocardial tissue in vivo and to compare it with the variation of backscatter over the cardiac cycle that has been recognized and reported previously. The backscatter was measured from regions of left ventricular wall in canine hearts in which the fibers of the muscle lay parallel to the surface of the heart and were oriented predominantly in a circumferential fashion. Because of technical considerations, the angle of insonification was varied systematically through two cycles in which the angle relative to the muscle fiber axes ranged from 60 degrees-120 degrees. Backscatter was maximum at angles of interrogation perpendicular to the myocardial fibers and minimum at those most acute (60 degrees) relative to the orientation of the fibers. The previously observed variation of integrated backscatter over the heart cycle was evident at each angle of interrogation. At end systole, the average maximum-to-minimum angular variation of integrated backscatter as 5.0 +/- 0.4 dB. At end diastole, the average maximum-to-minimum angular variation was 3.2 +/- 0.4 dB. Thus, even though angular dependence of the backscatter from tissues with directionally oriented structures is substantial, the anisotropy does not account for cardiac-cycle-dependent variation of backscatter. Accordingly, the angular dependence should be incorporated in approaches to quantitative tissue characterization with ultrasound.     

The extracellular matrix is an important source of ultrasound backscatter from myocardium

Ultrasound tissue characterization with measurement of backscatter has been employed in numerous experimental and clinical studies of cardiac pathology, yet the cellular components responsible for scattering from cardiac tissues have not been unequivocally identified. This laboratory has proposed a mathematical model for myocardial backscatter that postulates the fibrous extracellular matrix (ECM) as a significant determinant of backscatter. To demonstrate the importance of ECM, this group sought to determine whether measurements of backscatter from the isolated ECM could reproduce the known directional dependence, or anisotropy of backscatter, from intact cardiac tissues in vitro. Segments of left ventricular free wall from ten formalin fixed porcine hearts were insonified at 50 MHz, traversing the heart wall from endo- to epicardium to measure the anisotropy of myocardial backscatter, defined as the difference between peak (perpendicular to fibers) and trough (parallel to fibers) backscatter amplitude. The tissue segments were then treated with 10% NaOH to dissolve all of the cellular components, leaving only the intact ECM. Scanning electron micrographs (SEM) were obtained of tissue sections to reveal complete digestion of the cellular elements. The dimensions of the residual voids resulting from cell digestion were approximately the diameter of the intact myocytes (10-30 microm). These samples were reinsonified after seven days of treatment to compare the anisotropy of integrated backscatter. The magnitude of anisotropy of backscatter changed from 15.4 +/- 0.8 to 12.6 +/- 1.1dB for intact as compared with digested specimens. Because digestion of the myocardium leaves only extracellular sources of ultrasonic scattering, and because the isolated ECM exhibits similar ultrasonic anisotropy as does the intact myocardium, it is concluded that there is a direct association between the ECM and the anisotropy of backscatter within intact tissue. Thus, it is suggested that ultrasonic tissue characterization represents a potentially clinically applicable method for delineating the structure and function of the ECM.     

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.     

Anisotropy of the backscatter coefficient of formalin-fixed ovine myocardium

The objective of this study was to measure the backscatter coefficient of formalin-fixed myocardial tissue as a function of angle of insonification relative to the myocardial fiber direction. Backscatter measurements were performed on eight cylindrical formalin-fixed lamb myocardial specimens and compensated for attenuation and diffraction effects to determine the backscatter coefficient. The backscatter coefficient at 5 MHz was found to be maximum for insonification perpendicular to the predominant myofiber orientation and minimum for parallel insonification, with values of (17+/-14) and (1.2+/-0.7) x 10(-4) cm(-1) sr(-1) (mean+/-standard deviation), respectively.     

Frequency dependence of ultrasonic backscatter from human trabecular bone: theory and experiment

A model describing the frequency dependence of backscatter coefficient from trabecular bone is presented. Scattering is assumed to originate from the surfaces of trabeculae, which are modeled as long thin cylinders with radii small compared with the ultrasonic wavelength. Experimental ultrasonic measurements at 500 kHz, 1 MHz, and 2.25 MHz from a wire target and from trabecular bone samples from human calcaneus in vitro are reported. In both cases, measurements are in good agreement with theory. For mediolateral insonification of calcaneus at low frequencies, including the typical diagnostic range (near 500 kHz), backscatter coefficient is proportional to frequency cubed. At higher frequencies, the frequency response flattens out. The data also suggest that at diagnostic frequencies, multiple scattering effects on the average are relatively small for the samples investigated. Finally, at diagnostic frequencies, the data suggest that absorption is likely to be a larger component of attenuation than scattering.     

A two-dimensional CVIB imaging system with a speckle tracking algorithm

Quantitative ultrasound tissue characterization based on integrated backscatter (IB) has shown great potential in detecting myocardial ischemia. The magnitude of the cyclic variation in IB (CVIB) has been considered one promising parameter in assessing regional myocardial contractile performance. This lab has previously developed a novel ultrasonic fusion imaging method based on CVIB. However, the major problem for clinical applications of this technique is that the myocardial tissue could not be tracked effectively without cardiologist’s intervention. This paper introduced a speckle tracking method into the CVIB-weighted imaging system, called speckle tracking algorithm with adaptive window size (STAWAWS), to track myocardial tissue particle automatically. This method provides a way to obtain the particle’s positions frame by frame in a series of B-mode images. Then using the RF signals according to the particle’s positions the IB curve can be calculated to produce CVIB value. The method was applied on the experimental and clinical data cases’s analysis. The results of dog’s data processing showed that this method could eliminate the misunderstanding of myocardial ischemia especially near the endocardium. The results of clinical data suggested that this method had clinical significance in detecting ischemic myocardium. Though the CVIB-weighted images obtained by the use of this auto-tracking method can improve the accuracy of detecting myocardial ischemia, it is not real-time analysis and the clinical data cases are not sufficient. Further clinical validation is still needed in the future’ work.     

Measurement of dependence of backscatter coefficient from cylinders on frequency and diameter using focused transducers--with applications in trabecular bone

A theory for the elastic scattering response from a cylinder insonified by a plane wave was previously derived by Faran. In the present paper, the empirical relationship between Faran's theory and measurements of backscatter coefficient from cylindrical targets using focused transducers is investigated. Experimental measurements of dependence of backscatter coefficient on frequency and diameter for nylon wires are reported. It is found that, under certain conditions (including weak, incoherent scattering), backscatter coefficient measurements from collections of cylindrical scatterers may be meaningfully compared with Faran's model predictions. At low frequencies, the theory and experimental measurements exhibit similar dependences on frequency and diameter, provided that the scatterers are not too densely packed. At higher frequencies, the fine structure of Faran's predictions becomes difficult to reproduce experimentally with a focused transducer. Implications regarding applications to characterization of trabecular bone are discussed.     

Onset of pulsatile waves in the heart walls at end-systole

We have previously developed a novel ultrasonic method, namely, the phased tracking method, for accurately tracking the movement of the heart wall based on both the phase and magnitude of the demodulated signals to determine the instantaneous position of an object. With this method, it is possible to accurately detect small-amplitude velocity signals of less than a few micrometers of the heart wall that are superimposed on the motion of the heart wall due to the heart beat. There are several remarkable pulsatile waves during one cardiac cycle in the resultant velocity signals, some of them being commonly obtained for both healthy subjects and patients. These pulsatile waves cannot be recognized in standard echocardiography M-mode images. In this paper, by focusing on one pulsatile wave that occurs around the end-systole, the physiological meaning of these is considered based on various in-vivo experiments. The pulsatile wave measured by this novel ultrasonic method will offer potential for a quantitative assessment of myocardial viability.     

Acoustic near-field characteristics of a conical,premixed flame

The occurrence of self-excited pressure oscillations routinely plagues the development of combustion systems. These oscillations are often driven by interactions between the flame and acoustic perturbations. This study was performed to characterize the structure of the acoustic field in the near field of the flame and the manner in which it is influenced by oscillation frequency, combustor geometry, flame length and temperature ratio. The results of these calculations indicate that the acoustic velocity has primarily one- and two-dimensional features near the flame tip and base, respectively. The magnitude of the radial velocity components increases with temperature ratio across the flame, while their axial extent increases with frequency. However, the acoustic pressure has primarily one-dimensional characteristics. They also show that the acoustic field structure exhibits only moderate dependencies upon area expansion and flame temperature ratio for values typical of practical systems. Finally, they show that the local characteristics of the acoustic field, as well as the overall plane-wave reflection coefficient, exhibit a decreasing dependence upon the flame length as the area expansion ratio increases.     

Computer simulation model on ultrasonic myocardial backscatter

Myocardial ultrasonic tissue characterization with integrated backscatter (IB) has been studied in recent years. To evaluate the dynamic characters of the myocardium, dynamic tracing of each unit volume of myocardium is a key point. Thus, a 2D CVIB imaging algorithm based on an auto-tracing method was developed in our previous work. Where, an auto-tracking method was used in CVIB-weighted images so that the POI in each frame throughout the cardiac cycle is automatically traced to reflect the real myocardial tissue movement. It is necessary to validate the auto-tracing method. However, at present there are no appropriate models to support the process. In order to validate the auto-tracing method, two myocardium models have been established to simulate the short and long axis view of echocardiography. The motions of partially ischemic myocardium have been simulated. The models have been used to validate the 2D CVIB imaging methods in the detection of myocardial ischemia. The simulation results show that the auto-tracing algorithm is effective. The model developed in this work provides a tool for studying and developing technologies in myocardial behavior.     

Frequency dependence of ultrasonic absorption in mammalian testis

The earlier report that the frequency dependence of ultrasonic absorption of mammalian testis memicked a single relaxation process is explained as an effect of ultrasonic beam width upon the transient thermoelectric technique. More detailed measurements show that the frequency dependence of the ultrasonic absorption in testis is much the same as the attenuation in other soft tissues, viz., a proportional to f1.1. The earlier finding that testis exhibits an ultrasonic absorption coefficient significantly lower than reported for other tissues is confirmed.     

Spectral estimation for characterization of acoustic aberration

Spectral estimation based on acoustic backscatter from a motionless stochastic medium is described for characterization of aberration in ultrasonic imaging. The underlying assumptions for the estimation are: The correlation length of the medium is short compared to the length of the transmitted acoustic pulse, an isoplanatic region of sufficient size exists around the focal point, and the backscatter can be modeled as an ergodic stochastic process. The motivation for this work is ultrasonic imaging with aberration correction. Measurements were performed using a two-dimensional array system with 80 x 80 transducer elements and an element pitch of 0.6 mm. The f number for the measurements was 1.2 and the center frequency was 3.0 MHz with a 53% bandwidth. Relative phase of aberration was extracted from estimated cross spectra using a robust least-mean-square-error method based on an orthogonal expansion of the phase differences of neighboring wave forms as a function of frequency. Estimates of cross-spectrum phase from measurements of random scattering through a tissue-mimicking aberrator have confidence bands approximately +/- 5 degrees wide. Both phase and magnitude are in good agreement with a reference characterization obtained from a point scatterer.     

Negative dispersion in bone: the role of interference in measurements of the apparent phase velocity of two temporally overlapping signals

In this study the attenuation coefficient and dispersion (frequency dependence of phase velocity) are measured using a phase sensitive (piezoelectric) receiver in a phantom in which two temporally overlapping signals are detected, analogous to the fast and slow waves typically found in measurements of cancellous bone. The phantom consisted of a flat and parallel Plexiglas plate into which a step discontinuity was milled. The phase velocity and attenuation coefficient of the plate were measured using both broadband and narrowband data and were calculated using standard magnitude and phase spectroscopy techniques. The observed frequency dependence of the phase velocity and attenuation coefficient exhibit significant changes in their frequency dependences as the interrogating ultrasonic field is translated across the step discontinuity of the plate. Negative dispersion is observed at specific spatial locations of the plate at which the attenuation coefficient rises linearly with frequency, a behavior analogous to that of bone measurements reported in the literature. For all sites investigated, broadband and narrowband data (3-7 MHz) demonstrate excellent consistency. Evidence suggests that the interference between the two signals simultaneously reaching the phase sensitive piezoelectric receiver is responsible for this negative dispersion.     

Ultrasonic absorption in polymer gel dosimeters

Ultrasonic absorption in polymer gel dosimeters was investigated. An ultrasonic interferometer was used to study the frequency (f) dependence of the absorption coefficient (alpha) in a polyacrylamide gel dosimeter (PAG) in the frequency range 5-20 MHz. The frequency dependence of ultrasonic absorption deviated from that of an ideal viscous fluid. The presence of relaxation mechanisms was evidenced by the frequency dependence of alpha/f(2) and the dispersion in ultrasonic velocity. It was concluded that absorption in polymer gel dosimeters is due to a number of relaxation processes which may include polymer-solvent interactions as well as relaxation due to motion of polymer side groups.The dependence of ultrasonic absorption on absorbed dose and formulation was also investigated in polymer gel dosimeters as a function of pH and chemical composition. Changes in dosimeter pH and chemical composition resulted in a variation in ultrasonic dose response curves. The observed dependence on pH was considered to be due to pH induced modifications in the radiation yield while changes in chemical composition resulted in differences in polymerisation kinetics.     

Thickness dependence of temperature coefficient of resistivity of polycrystalline bismuth films

Results for the temperature coefficient of resistivity (TCR) of polycrystalline bismuth films deposited on to glass substrate are reported for the thickness range 30–300 nm. The film TCR is found to be negative for all thicknesses studied and its absolute value exhibits a maximum of 3.70×10^–3 K^–1 near 72.5 nm. The variation of charge carrier density with film thickness has been estimated from the presence of surface states. To include the thickness dependence of charge carrier density, a modified theory has been used to explain the observed behaviour of the TCR. The experimental results for the TCR of Bi films are found to be consistent with the theoretical values. The existence of the extremum is theoretically verified. From the analysis, the specularity parameter p is about 0.44 and the reflection coefficient R is 0.1.     

Fundamental precision limitations for measurements of frequency dependence of backscatter: applications in tissue-mimicking phantoms and trabecular bone.

Various models for ultrasonic scattering from trabecular bone have been proposed. They may be evaluated to a certain extent by comparison with experimental measurements. In order to appreciate limitations of these comparisons, it is important to understand measurement precision. In this article, an approach proposed by Lizzi and co-workers is adapted to model precision of estimates of frequency-dependent backscatter for scattering targets (such as trabecular bone) that contain many scatterers per resolution cell. This approach predicts uncertainties in backscatter due to the random nature of the interference of echoes from individual scatterers as they are summed at the receiver. The model is validated in experiments on a soft-tissue-mimicking phantom and on 24 human calcaneus samples interrogated in vitro. It is found that while random interference effects only partially explain measured variations in the magnitude of backscatter, they are virtually entirely responsible for observed variations in the frequency dependence (exponent of a power law fit) of backscatter.     

Frequency-dependent ultrasonic differentiation of normal and diffusely diseased liver

The attenuation coefficient in 38 pathologically graded in vitro liver specimens was measured over a frequency range from 1.25-8 MHz and fitted to the power law model. The attenuation in the normal group (n = 17) exhibited a frequency dependence of the form 0.399f1.139; in the mild disease group (n = 13), it exhibited a dependence of the form 0.395f1.212; and in the moderate/severe disease group (n = 8), it exhibited a dependence of the form 0.391f1.325. Using a Student's t test, it is shown that, due to these differences in the frequency dependence, the statistical significance level at which the null hypothesis regarding the difference between the mean attenuation slopes of any two of these categories is rejected, is a strong function of frequency in the range of 1-4 MHz. The significance level relating to the difference between the normal and moderate/severe disease group is more than one order of magnitude better than the other categories. In all cases, no substantial improvement occurs beyond 4 MHz. It is also shown that attenuation slope values at 3 MHz confirm in vivo literature results obtained via different techniques.