O.T. von Ramm

High-speed ultrasound volumetric imaging system. I. Transducer design and beam steering

Transducer design and phased array beam steering are developed for a volumetric ultrasound scanner that enables the 3-D visualization of dynamic structures in real time. The authors describe the design considerations and preliminary evaluation of a high-speed, online volumetric ultrasound imaging system that uses the principles of pulse-echo, phased array scanning with a 2-D array transducer. Several 2-D array designs are analyzed for resolution and main lobe-side lobe ratio by simulation using 2-D fast Fourier transform methods. Fabrication techniques are described for 2-D array transducer. Experimental measurements of pulse-echo point spread responses for 2-D arrays agree with the simulations. Measurements of pulse-echo sensitivity, bandwidth, and crosstalk are included.<<ETX>>

High-speed ultrasound volumetric imaging system. II. Parallel processing and image display

For pt.I see ibid., vol.38, no.2, p.100-8 (1991). The authors describe the design, application, and evaluation of parallel processing to the high-speed volumetric ultrasound imaging system. The scanner produces images analogous to an optical camera or the human eye and supplies more information than conventional sonograms. Potential medical applications include improved anatomic visualization, tumor localization, and better assessment of cardiac function. The system uses pulse-echo phased array principles to steer a 2-D array transducer of 289 elements in a pyramidal scan format. Parallel processing in the receive mode produces 4992 scan lines at a rate of approximately 8 frames/s. Echo data for the scanned volume is presented online as projection images with depth perspective, stereoscopic pairs, or multiple tomographic images. The authors also describe the techniques developed for the online display of volumetric images on a conventional CRT oscilloscope and show preliminary volumetric images for each display mode.<<ETX>>

A comparison of real-time, two dimensional echocardiography and cineangiography in detecting left ventricular asynergy.

SUMMARYLeft ventricular wall motion was assessed in 105 consecutive patients both invasively, using biplane cineangiography, and noninvasively, by a real-time, phased-array, two-dimensional echocardiography system. Ventricular wall motion in five anatomic areas of the ventricle (anterolateral, posterolateral, apical, septal, and inferior) was analyzed by both methods in a double-blind manner. Twodimensional echocardiographic images were deemed adequate for analysis in 82% of the regions (430 of 525). Fifty-five discrepancies were noted in the comparison of the remaining 430 regions.The reasons for discrepancies in interpretation between the two methods were established for 54 during retrospective review: 33 were due to echocardiography (inadequate target visualization, observer error, or tangential echo views). Fifteen were related to angiography (overlay of silhouettes or observer error), and six were due to other reasons including definition problems or spatial orientation difficulties.Both real-time, two-dimensional echocardiography and cineangiography have advantages and disadvantages. The techniques used together could provide more complete information concerning ventricular wall movement than is now currently available.

Combined doppler and phased-array echocardiographic estimation of cardiac output.

The capability of a pulsed Doppler flowmeter combined with a phased-array imaging system to measure volume flow was tested in vitro and in patients undergoing cardiac catheterization. The Dopplerphased- array system (DPA) was used to determine vessel diameter and a superimposed cursor was used to locate the range and angle of the Doppler sample volume. DPA estimates of continuous flow through tubing in a water tank correlated strongly (r = 0.99) with measured flow corresponding to physiologic ranges from 3-12 1/min. For pulsatile flow in a water tank, a correlation of r = 0.86 with measured flow was obtained, whereas DPA estimates of cardiac output as compared with Fick estimates in the 11 patients produced a correlation of r = 0.83. These data indicate that estimates of cardiac output are possible using the DPA approach.

Two-Dimensional Arrays for Medical Ultrasound:

The design, fabrication and evaluation of two-dimensional transducer arrays are described for medical ultrasound imaging. A 4 x 32, 2.8 MHz array was developed to use new signal processing techniques for improved B-scan imaging including elevation focusing, phase correction and synthetic aperture imaging. Laboratory measurements from typical array elements showed 50 omega insertion loss of -56 dB, -6 dB fractional bandwidth of 43%, interelement crosstalk of -19 dB, and -6 dB pulse-echo angular response of 62 degrees. Simulations of pulse-echo beam plots have shown grating lobes 20 dB below the main lobe at +/- 7 degrees in the elevation direction. The complete 2-D array has been used for measurements of phase aberrations in breast, and the individual 32 element linear arrays have been used to obtain conventional B-scans. Several 16 x 16 arrays have also been developed for high speed volumetric imaging. These include 96 transmit elements and 32 receive channels. With a lambda/4 matching layer, laboratory measurements show 50 omega insertion loss of -72 dB, -6 dB fractional bandwidth of 63%, interelement crosstalk of -29 dB and -6 dB angular response of 25 degrees. Pulse-echo sensitivity was improved by 21 dB through the use of integrated circuit preamplifiers of high impedance mounted in the transducer handle. In vivo cardiac, abdominal, and obstetric B-scans with elevation focusing, as well as high speed C-scans, have been obtained with these 2-D arrays.

Angle independent ultrasonic blood flow detection by frame-to-frame correlation of B-mode images

We have previously reported initial clinical results of a novel blood velocity imaging technique utilizing a two-dimensional correlation search applied to consecutively acquired echoes. In this paper, we describe both the physical principles underlying this technique and test tank experiments which define its performance under a variety of conditions. The results indicate that, unlike Doppler flow imaging systems, this technique defines the flow velocity vector in two dimensions and is not subject to aliasing.

Phased array ultrasound imaging through planar tissue layers

Conventional ultrasound imaging devices are designed based on the assumption of a homogeneous tissue medium of constant acoustic velocity = 1540 m/sec. However, the body consists of tissue layers of varying thicknesses and velocities which range from 1470 m/sec in fat to 3200 m/sec in skull bone. Refraction effects from these layers degrade ultrasound image quality. In this paper, pulse-echo ultrasound imaging is modeled as imaging an organ of interest through an intervening planar tissue layer, such as liver through fat in the abdomen or brain through skull bone in the adult head. Refraction effects from planar tissue layer interfaces are analyzed using Snells law and measured using phantoms. We also introduce an on-line phased array correction technique based on planar tissue layers to restore ultrasound image quality. We conclude that fat/organ planar interfaces do not degrade image quality significantly. However, refraction effects at a skull/brain planar interface degrades resolution and target acquisition and introduces geometric distortion. Our plane layer phased array correction technique significantly improves image quality in phantoms through lucite aberrators and improves adult cephalic ultrasound image quality when used through the top of the adult skull. The correction technique is robust even in the presence of inaccurate estimates of skull thickness.

A New Ultrasound Imaging Technique Employing Two-Dimensional Electronic Beam Steering

In recent years, ultrasound imaging based on B-mode echosonography has become an accepted and useful technique in medical diagnosis. However, the application of this technique has been limited by the time required to obtain an adequate image, the resolution that can be obtained in the image and problems related to the dynamic range of the echo information. A new ultrasound imaging system which removes or substantially reduces many of these limitations has been developed in the hope that ultrasound tomography may find even more widespread application and greater diagnostic value.

Theory and operation of 2-D array piezoelectric micromachined ultrasound transducers

Piezoelectric micromachined ultrasound transducers (pMUTs) are a new approach for the construction of 2-D arrays for forward-looking 3-D intravascular (IVUS) and intracardiac (ICE) imaging. Two-dimensional pMUT test arrays containing 25 elements (5 times 5 arrays) were bulk micromachined in silicon substrates. The devices consisted of lead zirconate titanate (PZT) thin film membranes formed by deep reactive ion etching of the silicon substrate. Element widths ranged from 50 to 200 mum with pitch from 100 to 300 mum. Acoustic transmit properties were measured in de-ionized water with a calibrated hydrophone placed at a range of 20 mm. Measured transmit frequencies for the pMUT elements ranged from 4 to 13 MHz, and mode of vibration differed for the various element sizes. Element capacitance varied from 30 to over 400 pF depending on element size and PZT thickness. Smaller element sizes generally produced higher acoustic transmit output as well as higher frequency than larger elements. Thicker PZT layers also produced higher transmit output per unit electric field applied. Due to flexure mode operation above the PZT coercive voltage, transmit output increased nonlinearly with increased drive voltage. The pMUT arrays were attached directly to the Duke University T5 phased array scanner to produce real-time pulse-echo B-mode images with the 2-D pMUT arrays.

3D Ultrasound Tissue Motion Tracking Using Correlation Search

Several methods for ultrasound tissue motion tracking and blood velocity estimation have been proposed and clinically applied. While providing valuable information to the clinician that was not obtainable from B-mode anatomical imaging, these methods still suffer from various fundamental and practical limitations that compromise their performance in certain clinical situations. A significant limitation is the inability of most of these methods to estimate the complete 3D motion or velocity vectors. With the introduction of ultrasound volumetric imaging, the need for a method that is capable of obtaining the complete motion vector is even more pressing. In this paper, we investigate the implementation of a correlation search scheme to estimate the 3D motion vectors using successive volumetric ultrasound scans. We present tracking results for motion along different axes and discuss the advantages and limitations of performing the correlation search in 3D.