Biomedical subjects
Publications and source records attributed to J F Greenleaf.
We have investigated the properties of a recursive process in which the output signal from a given RF excitation pulse may be used as the input (excitation) pulse of a subsequent iteration. This recursive excitation technique increases contrast and improves feature segmentation for the purpose of motion tracking.
Conventional ultrasonic transducers generate beams that diffract as they travel. This phenomenon causes images produced in B-mode to be degraded in the far-field of the transducers. Focused transducers are used to improve image quality. Unfortunately, focused transducers have short depth of field. Although multiple pulse transmissions focused at several depths are used to increase the effective depth of field, imaging frame rate is reduced dramatically leading to blurred images of moving objects such as the heart. We present a family of transducers that produce nondiffracting beams of large depth of field. Therefore, uniformly high resolution throughout the imaging area can be obtained without sacrificing the imaging frame rate. In addition, the nondiffracting property of these beams makes the correction for beam diffraction negligible in tissue characterization. This paper reports the results of computer simulations as well as in vitro and in vivo pulse-echo imaging experiments with a nondiffracting transducer. Images are compared to those obtained by conventional focused Gaussian shaded beam transducers and a commercial ACUSON 128 B-scanner. The new transducer has much longer depth of field with higher sidelobes than conventional transducers of the same aperture. Sidelobes can be reduced using the new transducer to transmit and the dynamically focused transducer to receive.
This paper presents a method for imaging the nonlinear parameter B/A. It is based on measuring phase shift and frequency shift of a high frequency acoustic probe wave by a low frequency acoustic pump wave. These measurements yield the real and the imaginary parts of the Fourier component of the distribution B/A at the wave number of the pump wave. We also derive the necessary set of wave numbers of the pump wave which must be employed to accurately image the B/A parameter. We show that these wave numbers are a function of sampling period and sampling number. Simulations of the imaging method show that the proposed method in this paper leads to more accurate imaging of B/A than the earlier method.
A general equation is derived that describes the behavior of a piezoelectric transducer with a nonuniform distribution of piezoelectric coefficient within its bulk, when submitted to an arbitrary distribution of acoustic pressure. Based on this equation, an expression for the receiving transfer function of the transducer is calculated. The results demonstrate the dependence of the transfer function on the distribution of piezoelectric coefficient, and that it is possible to benefit from a nonuniform distribution to optimize the transfer function. The general equation also describes the influence of the external electric circuit loading the transducer, which leads to another independent means of optimizing the transfer function. The proposed model combines effects of piezoelectric material characteristics, acoustic backing, and electric loading, without resorting to Mason or other equivalent circuits for the transducer.
This paper reports sound speed co, adiabatic acoustic nonlinear parameter B/A, and isothermal nonlinear parameter (B/A)' of 33 samples of excised human and animal fats and some simple mixtures, e.g., skim milk and powdered milk. Tissues were obtained from different parts of the body and the measurements were made in the temperature range of 20 degrees C to 37 degrees C. All the fats were highly nonlinear. Although there was a considerable overlap in the values of acoustic nonlinearity of different body fats, in general their values followed the order: mesenteric fat greater than subcutaneous fat greater than omentum fat greater than breast fat. Temperature coefficients of co and B/A, and the difference between adiabatic and isothermal nonlinearity, correlated highly with water/fat content of tissues. A reciprocal relationship between B/A and co, similar to that proposed for liquids, was observed for soft tissues but not for solutions of powdered milk in skim milk. No significant correlation was observed between acoustic measurements and the age of human subjects from which fat tissues were obtained.
This study deals with the relationship between the magnitude of acoustic nonlinearity, sound speed of tissues and the amount of fat present in them. A two-component model, i.e., nonfat and fat, has been assumed and equations that relate the nonlinear parameter (B/A) of a medium to the properties of its components have been derived for two cases. In the first case, the density and sound speed are the same for the two components; B/A is a linear function of fat concentration. To represent this case, mixtures of egg white and yolk were studied. Even though the differences in density and sound speed of the two egg components were within 1 percent of each other, B/A showed a deviation from linearity. In the second case, i.e., when the density and sound speed of the two components are not equal, a quadratic equation has been derived using mixture laws. Livers, fats, egg mixtures, and oil data were used to represent this case. B/A increased quadratically with fat concentration. The presence of proteins on the binary aqueous/fat mixtures modified the quadratic rule.
Ultrasonic and nuclear magnetic resonance properties of media rich in water are investigated. The chemical shift of the water proton in pure water, aqueous solutions of tertiary butanol, and sodium chloride is shown to be linearly correlated to the reciprocal of sound speed in these media. A new method of determining the self-diffusion coefficient of water by using acoustic nonlinearity and sound speed is proposed. The method is tested on a variety of media that include pure water, aqueous solutions of glycerol, serum albumin, egg constituents, plant tissues, frog muscle and liver, and excised human tissues. In all the cases the results are found to compare closely to diffusion coefficients measured by magnetic resonance. The results presented here indicate that the acoustic and magnetic resonance modalities, though inherently different in their origin, can provide closely related information on the properties of water.
The acoustic nonlinearity parameter B/A and sound speed c have been determined for excised normal and abnormal human livers at 20-37 degrees C. These values are compared with analytic measurements of fat and water content of tissues. The results show that normal liver containing 71.0% water and 2.9% fat by weight has a B/A value of 6.75 and sound speed of 1592 m/s at 37 degrees C. Both these parameters increase at an average rate of 0.026 degrees C and 1.5 m/s/degrees C, respectively, as the temperature is raised from 20 to 37 degrees C. Fatty liver (24% fat by weight) exhibits highest B/A (9.12) and lowest c (1522 m/s) of all the livers studied. In contrast to normal livers sound speed in such a liver was found to decrease with temperature. Based on the acoustic and composition measurements, quantitative correlations of B/A and c with fat-water composition have been developed. Inversion of these relationships provide a simple method to determine composition of a tissue sample from B/A and c measurements.
To evaluate whether myocardial texture changes resulting from acute ischemia can be visualized with satisfactory spatial resolution, short axis compound echo images (CEI) (B-scan) were obtained from 12 excised canine hearts in vitro. Seven had myocardial ischemia produced by open chest ligation of the left anterior descending coronary artery (LAD) for 15-30 min prior to excision. The CEI were constructed by compounding 60 simple linear B-scans. Hearts were sectioned after scanning, and gross morphological changes were recorded. Microscopic comparison between grossly abnormal and normal regions were recorded. The CEI from the ischemic group revealed altered myocardial texture seen as bright coarsely granular echoes in the regions normally perfused by the ligated LAD artery. Corresponding anatomic sections revealed increased redness in these regions. Microscopically these regions revealed interstitial and intercellular edema as compared to the normal regions. Acute myocardial ischemia can be visualized in CEI and these regions have significantly increased backscatter, decreased attenuation, and decreased speed of ultrasound relative to normal regions in the same hearts. Myocardial edema is probably responsible for these changes.
The relationship between Fourier transforms of the refractive index of objects and the Fourier transform of the scattered ultrasonic waves are reviewed. Both the Born and Rytov approximations in the first order are used to linearize the Helmholtz wave equation. Both forward and backward scatter geometries are illustrated. The relationship between these coherent wave considerations and the clinically used echo and transmission modalities is discussed.
A study of the statistics of ultrasound speckle indicates that the noise-to-signal ratio (NSR) of the echo envelope peaks (EEP) in a B scan is monotonically related to the variance of the mean power of the backscattered echoes. If it is assumed that the backscattered echoes all have the same mean power, the NSR of the EEP's in a B scan will be expected to reach a theoretical minimum value NSR0 (0.40 approximately 0.42). In practical situations, the variance of the mean power of the echoes is increased due to the presence of both attenuation (including beam spreading) and the spatial variation in the backscattering cross section (BCS) of the tissue. Accordingly, the measured value of the NSR of the EEP's is expected to be greater than the NSR0. In principle, the effects of beam pattern and the attenuation can be compensated for by system calibration and a depth-related gain function, respectively. The attenuation coefficient of the tissue may then be obtained by adjusting the gain function to minimize the NSR of the EEP's. Due to the random nature of the heterogeneity of diseased tissue, the effect of the variation in the BCS, however, cannot be compensated for by the depth-related gain function. Therefore, the minimum value of the NSR indicates the spatial variation in the BCS and may finally correlate to the disease state of the tissue.
The validity of wave equations employed as system models in acoustical diffraction tomography is investigated using simulations and measurements of the scattering of plane ultrasound waves by cylinders. It is demonstrated by simulation and experiment that it can be appropriate to neglect density fluctuations and shear waves, implying that the commonly used form of the wave equation suitably describes scattering by fluctuations of acoustic speed and absorption. Diffraction tomographic reconstructions of simulated data reveal the importance of absorption, the behavior of the real and imaginary parts of the reconstructed refractive index, and the relative advantages and limitations of the Born and Rytov approximate transformations.
Global and local attenuation coefficient estimates were calculated using three methods: the envelope peak method, the central-frequency-downshift method, and the zero-crossing method. The results show that for each method the error of the local estimate with respect to the global estimate is increased as the sample volume is decreased. While the performances of the latter two methods are comparable, the performance of the envelope peak method is significantly superior.
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Regional displacements of lung parenchyma due to respiratory movements at 1 G and 7 Gy were studied in anesthetized dogs in the left decubitus position in a water-filled respirator that provided control of respiratory volumes and rate and minimized inertial shifts in position and shape of the thorax and abdominal contents and related effects on the lungs. Inspiratory movements at 1 G were relatively uniform, although regional volume increased more in the nondependent (right) lung than in the dependent (left) lung. Regional functional residual capacity (FRC) increased in the nondependent lung and decreased in the dependent lung during exposures to 7 Gy. The greatest inspiratory increase in volume occurred near the midlung, where regional FRC changed the least during acceleration. The decrease in dependent and increase in nondependent lung volumes during acceleration are attributed to the increased weight and consequent downward displacement of the higher specific gravity mediastinal contents concomitantly with upward displacement of pulmonary gas, producing an exaggeration of the dependent-to-nondependent gradient in alveolar size.
Spatial distribution of pulmonary blood flow (SDPBF) during 2- to 3-min exposures to 6-8 Gy acceleration was studied, using radioactive microspheres in dogs, and compared to previously reported 1 Gy control distributions. Isotope distributions were measured by scintiscanning individual 1-cm-thick cross sections of excised, fixed lungs. Results indicate: 1) the fraction of cardiac output traversing left and right lungs did not change systematically with the duration and magnitude of acceleration; but 2) the fraction is strongly affected by the occurrence or absence of fast deep breaths, which cause an increase or decrease, respectively, in blood flow through the dependent lung; and 3) Gy acceleration caused a significant increase in relative pulmonary vascular resistance (PVR) in nondependent and dependent regions of the lung concurrent with a decrease in PVR in the midsagittal region of the thorax. Result 3 may be mediated primarily by changes in regional alveolar volume and geometry in the nondependent hemithorax conbined with hydrostatic effects of extravascular fluid and active hypoxic response in the dependent region and is superimposed on, and may override, hydrostatic effects of perfusion pressures on SDPBF during acceleration.