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.     

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)     

Ultrasonic characterization of acute renal failure

The attenuation coefficient, propagation velocity and backscattering coefficient were measured in vitro, on freshly excised, normal and functionally impaired rabbit kidneys. Subcutaneous glycerol treatment was used to introduce acute renal failure. Elevated plasma creatinine levels, measured prior to the excision of kidneys, were used as an index of the degree of renal functional impairment. Propagation velocity for the ten kidneys ranged between 1538–1575 m/s with that for the normals being 1540±4 m/s. Velocity was found to increase with increasing renal damage. The attenuation coefficient for all ten kidneys exhibited a linear frequency dependence over the range 3.5–6.5 MHz. The slope of the attenuation coefficient for the glycerol treated kidneys (0.723 dB/cm/MHz) was found to be higher than the slope for the normals (0.449 dB/cm/MHz). The frequency dependence of the backscattering coefficient was not altered by glycerol treatment leading to the postulate that modification of frequency dependent behavior of the attenuation coefficient in this experimental model is primarily due to absorption.     

Evaluation of backscattering coefficients for excised human tissues: results,interpretation and associated measurements

Measurements are reported on the ultrasonic attenuation coefficient, the velocity of sound propagation and the differential scattering coefficient (for 180° scattering) for 3 types of freshly excised normal human tissues: liver, spleen and brain (white matter). The measurements are made as a continuous function of frequency in the range 0.7–7 MHz. The data for attenuation and scattering are consistent with a power law dependence on frequency though the latter are best described by a two-term fit. Comparison of the scattering data with theory suggests that two forms of tissue structure, with characteristic dimensions of approx. 1 and 0.02 mm, are predominantly responsible for scattering in the frequency range 0.5–5 MHz. The absolute value of backscattering for freshly excised human liver tissue at 1 MHz is 3.5 × 10^?4cm^?1Sr^?1 rising to 9.0 × 10^?4cm^?1Sr^?1 at 3 MHz. Extrapolation to total scattering suggests that the contribution of scattering to attenuation varies from approx. 2% at 1 MHz to 5.4% at 3 MHz.     

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.     

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.     

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.     

Measurements of phase velocity and group velocity in human calcaneus

Ultrasonic velocity in calcaneus correlates highly with bone mineral density, which is a good predictor of osteoporotic fracture risk. Several commercial bone sonometers perform a velocity measurement based on the transit time of a broadband pulse to assess skeletal status. This approach is somewhat problematic, however, because several authors have reported ambiguities in measurements in calcaneus. Phase velocity is an alternative that may be less dependent on device spectral characteristics. In addition, dispersion (the frequency-dependence of phase velocity) is a fundamental property worth investigating to increase understanding of interaction between ultrasound and bone. To compare two group-velocity measurement methods and one phase-velocity measurement method, a polycarbonate sample (for method validation) and 24 human calcanei were investigated in vitro. Phase velocity in calcaneus at 500 kHz was 1511 m/s +/- 30 m/s (mean +/- standard deviation). Average phase velocity decreased approximately linearly with frequency (-18 m/s MHz). The two group velocity measurements were comparable and tended to be slightly lower than phase velocity. The magnitude of dispersion showed little correlation with bone mineral density.     

Characterization of Acoustic Properties of PVA-Shelled Ultrasound Contrast Agents: Linear Properties (Part I)

This work examines the linear acoustic behavior of ultrasound contrast agents made of three types of poly (vinyl alcohol) (PVA) shelled microbubbles manufactured at different pH and temperature conditions. Backscattered power, attenuation coefficient and phase velocity of ultrasonic waves propagating through suspensions of PVA contrast agents were measured at temperature values ranging between 24°C and 37°C in a frequency range from 3 MHz to 13 MHz. Enhancement of the backscattered power higher than 20 dB and displaying a weak dependence on temperature was observed. Attenuation and phase velocity, on the other hand, showed higher sensitivity to temperature variations. A modified version of the Church model, which accounts for the dispersion of the dynamic modulus of the PVA shells, was developed to simultaneously fit the attenuation and phase velocity data at 24°C. The frequency dependence of the storage modulus was found to be that of semiflexible polymeric networks. On the other hand, the frequency dependence of the dynamic loss modulus suggests that additional mechanisms, which may be related to the finite dimensions of the shell and/or to its inhomogeneity, may play a significant role in the dissipation of the acoustic energy. For the microbubbles of interest, this model predicts frequency dependent resonance frequency higher than 100 MHz. (E-mail: dmitryg@kth.se)     

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.     

Properties of ultrasonic waves in bovine bone marrow

We investigated the properties of ultrasonic waves in bovine bone marrow. Six bone marrow samples were obtained from different parts (proximal, middle and distal) of the shafts of two bovine femora without destruction. The measured frequency range was 3 to 10 MHz, and the temperature range was 15 to 40°C. Both wave velocity and attenuation coefficient in bone marrow always decreased as temperature increased. The velocity ranged from 1400 to 1610 m/s and attenuation coefficient ranged from 4 to 16 dB/cm. Wave velocities in bone marrow were similar to those in water, whereas the temperature dependences were different, and the wave attenuation coefficients were much higher than those in water. The dependence of velocity on temperature changed slightly around 23–24°C, where a transition from soft gel to oily liquid occurred. The transition temperature was confirmed by differential scanning calorimetry (DSC). Below this transition temperature, positive velocity dispersion was observed.     

Ultrasonic attenuation and propagation speed in mammalian tissues as a function of temperature

Ultrasonic attenuation in the frequency range 1–7 MHz, and the speed of sound, were determined experimentally in both fresh and fixed bovine and human soft tissues for various temperatures in the range 5–65°C. At temperatures below 40°C the attenuation coefficient behaves similarly for fixed and fresh tissues where, at high frequencies, it has a negative dependence on temperature, the value at 20°C being about 21% higher than that at 37°C. As the frequency is reduced, the temperature coefficient of attenuation progressively decreases until, after passing a transition frequency (this varies with the tissue specimens but is around 1–2 MHz), a positive dependence on temperature may be observed. At temperatures above about 40°C, the attenuation coefficient of freshly excised tissues increases with temperature, whereas for fixed tissues the attenuation coefficient continues to decrease. These observations help to resolve a possible discrepancy evident in previous reports of the temperature dependence of attenuation. The speed of sound in non-fatty tissues increases with temperature and exhibits a maximum at around 50°C, while for fatty tissues a negative dependence is observed. The implications of this result for improved diagnostic procedures is discussed.     

Influence of histochemical preparation on acoustic parameters of liver tissue: A 5-MHz study

In this study the influence of various histological techniques on the acoustic parameters of liver tissue was investigated. Radiofrequency (RF) echographic data were obtained in vitro from 21 liver samples taken from 8 white New Zealander rabbits. The samples were measured in four different subsequent histological tissue processing conditions (freshly excised, 4% buffered formalin fixed, after it went through a paraffin cycle and after staining with hematoxylin and eosin). The acoustic parameters that were obtained from the rf data were velocity of sound, slope of the attenuation coefficient versus frequency between 1.9 and 6.9 MHz, attenuation coefficient at 4.4 MHz, slope of the backscattering spectrum between 1.9 and 6.9 MHz, and intercept of the backscattering spectrum. It was found that fixation by formalin preserves the acoustic properties of the tissue to a reasonable extent. Embedding in paraffin and deparaffinizing induces large changes in the acoustic properties of the tissue. As an alternative, freezing prior to cutting, rather than the paraffin cycle, was investigated also in 10 liver samples obtained from 4 New Zealander rabbits. This method produced no significant changes of the acoustic parameters and should therefore be preferred in acoustic microscopy.     

Broadband measurements of the frequency dependence of attenuation coefficient and velocity in amniotic fluid, urine and human serum albumin solutions

The frequency dependence of attenuation coefficient in amniotic fluid, urine and 4.5% and 20% human serum albumin solutions over the frequency range 5 MHz to 25 MHz was measured at both room temperature and physiological temperature using a variable path length technique. A 15 MHz (13 mm diameter) transducer was used to produce a broadband single-cycle pulse and a 4 mm diameter bilaminar polyvinylidene difluoride membrane hydrophone was used to detect the attenuated pulse. Standard time-of-flight measurement techniques were used to measure the acoustic velocity in the same fluid samples. At physiological temperature, the attenuation coefficients in amniotic fluid, urine and 4.5% and 20% human albumin solution were found to be 0.0053 f(1.65), 0.0047 f(1.67), 0.019 f(1.57) and 0.167 f(1.27) dB cm(-1), respectively, where f is in MHz. The velocities in amniotic fluid, urine and 4.5% human albumin solution at physiological temperature were found to be 1541.1 m s(-1) +/- 1.3 m s(-1), 1551.3 m s(-1) +/- 1.3 ms(-1) and 1547.3 m s(-1) +/- 1.0 m s(-1), respectively. The results provide unique data over the diagnostic and therapeutic ultrasonic frequency range that can be used as input data for theoretical models that attempt to simulate nonlinear pressure fields and temperature rises from medical ultrasonic transducers.     

Attenuation and De-focusing During High-Intensity Focused Ultrasound Therapy Through Peri-nephric Fat

High-intensity focused ultrasound (HIFU) is an attractive therapy for kidney cancer, but its efficacy can be limited by heat deposition in the pre-focal tissues, notably in fat around the kidney (peri-nephric fat), the acoustic properties of which have not been well characterized. Measurements of attenuation were made using a modified insertion-loss technique on fresh, unfixed peri-nephric fat obtained from patients undergoing kidney surgery for cancer. The de-focusing effect of changing the position of the fat layers was also investigated using fresh subcutaneous fat from euthanized pigs. The mean attenuation of human peri-nephric fat was found to be 11.9 ± 0.9 Np/m (n = 10) at 0.8 MHz, the frequency typically used for HIFU ablation of kidney tumors, with a frequency dependence of f^1.2. A typical 2- to 4-cm thickness of peri-nephric fat would result in a de-rated intensity of 3%–62% at 0.8 MHz compared with a hypothetical patient with no peri-nephric fat. Through the use of freshly excised porcine subcutaneous fat, the presence of fat 100 mm in front of the focus was found to have a de-focusing effect of approximately 1 mm in both transverse directions, which corresponds to a full HIFU beam width off-target. Peri-nephric fat may significantly affect both the intensity and accuracy of HIFU fields used for the ablation of kidney cancer.     

Ultrasound properties of lung tissue and their measurements

In-vitro measurements have been carried out for the group velocity and the attenuation, in dB/cm, in cow lung tissue. The velocity and the attenuation were measured both as a function of air content, from 32 to 68%, and as a function of frequency, from 10 to 800 KHz. The group velocity was determined from measurement of the phase difference between the transmitted and incident acoustic signal while the attenuation was measured from the insertion loss with correction for the water-lung interface transmission losses. The measured group velocities indicate only a small degree of dispersion, but a strong dependence on air content. The attenuation is very high and exhibits strong frequency dependence, but weak air content dependence.     

Ultrasonic characterization of the nonlinear properties of contrast microbubbles

The nonlinear properties of microbubble contrast agents have been used to create contrast-specific imaging modalities such as harmonic imaging and subharmonic imaging. Thus, a better understanding of the nonlinear performance of contrast microbubbles may enhance the diagnostic capabilities of medical ultrasound (US) imaging. The first and second harmonic, the 1/2 order subharmonic and the 3/2 order ultraharmonic components in spectra of scattered signals from Optison microbubbles insonified at 2 and 4 MHz have been investigated using an in vitro laboratory pulse-echo system. The development of these signal components over time is quite different for 2-MHz insonification compared to 4-MHz insonification. Scattered subharmonic and ultraharmonic signals are much more time-dependent than first and second harmonic echoes. The dependence of the first and second harmonic, subharmonic and ultraharmonic components on acoustic pressure for 2-MHz insonification is similar to that for 4-MHz insonification. The first and second harmonic components increase linearly with acoustic pressure (in double logarithmic scales) and the subharmonic and ultraharmonic amplitudes undergo rapid growths in the intermediate acoustic pressure range and much slower increases at both lower and higher acoustic pressures.     

Sonic velocity and attenuation in wet compact cow femur for the frequency range 5 to 100 MHz.

Ultrasonic studies on bone have not been done as intensively as other tissues, probably because there is less opportunity to examine bone in vivo by this modality. There is considerable interest in using ultrasonics to learn about the material properties of bone for which purpose the sonic spectrum may be useful. The longitudinal sonic velocity and attenuation have been found for several sections from one sample of wet cow femur for both the axial and radial directions over the frequency range of 5 to 100 MHz. The basic technique was solid-to-solid contact using a fused quartz buffer rod coupled to the bone specimen. There is clear evidence of velocity dispersion which Lakes et al. (1986) did not find over a frequency range up to 16 MHz. While there seems to be a peak in both velocity and attenuation at 70 MHz, it is necessary to obtain measurements at higher frequencies to make sure this is not an artifact. The axial sonic velocity varied from 4.29 km/s at 5 MHz to 4.447 km/s at 50 MHz, while the radial velocity was 3.45 km/s at 5 MHz and increased to 3.62 km/s at 50 MHz. The attenuation coefficient axially started at a mean value of 3.5 db/mm at 5 MHz and increased to a mean of 19 db/mm at 100 MHz. The corresponding radial attenuation coefficients are 5.2 and 26, respectively.     

Measurement of speed of sound dispersion in soft tissues using a double frequency continuous wave method

We introduce a method for measuring the speed of sound dispersion. It combines a short pulse transmission followed by a long burst comprised of two frequencies, one being double that of the other. The method allows the determination of the speed of sound dispersion using a single transmission. To validate the method, the dispersion was first measured in plastic samples and then in in vitro soft tissues samples. The results obtained for Perspex samples are in excellent agreement with values reported in the literature. The dispersion index in soft tissues ranged for a bovine heart from 0.63 +/- 0.24 (m/s.MHz) at 1.5 MHz to 0.27 +/- 0.05 (m/s.MHz) at 4.5 MHz and for a turkey breast from 1.3 +/- 0.28 (m/s.MHz) at 1.75 MHz to 0.73 +/- 0.1 (m/s.MHz) at 3.8 MHz. The significant difference in the speed of sound dispersion index between the studied materials indicates that dispersion may be used as a new index for soft tissue characterization by ultrasound.     

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.