F. L. Thurstone

Cardiac imaging using a phased array ultrasound system. II. Clinical technique and application.

A new two-dimensional ultrasound imaging system capable of producing high resolution tomographic images of the heart in real time has been developed. This system relies on phased array principles to rapidly steer the ultrasound beam through the structures under investigation. A hand-held linear array of 16 ultrasound transducers with overall dimensions of 14 mm by 24 mm at the site of contact may be readily manipulated to image various cardiac structures. The resulting images are displayed in a circular sector format, 60° in azimuth and typically 15 cm in range. At this maximum range, image frames consisting of 256 lines are generated at the rate of 20 frames/second. High azimuthal resolution throughout the field of view is assured by a focused transmit beam and by sweeping the focus of the receiver in synchrony with the range of returning echoes. Azimuthal resolution varies from 2 to 4 mm throughout the field of view while range resolution is 1.5 mm. This imaging system has proven particularly useful for the delineation of left ventricular spatial geometry by the identification of endocardium, myocardium, papillary muscles and interventricular septum. High quality images of anterior and posterior mitral leaflets, aortic root and aortic leaflets as well as left atrium and other cardiac structures have been obtained.

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.

Sampled Aperture Techniques Applied to B-Mode Echoencephalography

Whereas A-mode echoencephalography has been found diagnostically useful, B-mode displays for similar object volumes have not yet provided the additional information that might be expected from a two-dimensional presentation. Poor image quality results from a number of causes stemming from the presence of skull bone (White, 1967; 1968). Large differences in specific acoustic impedance between skull bone and soft tissue combined with irregularities of skull bone surfaces result in unfavorable reflections and reverberations of acoustic energy. Severe attenuation of ultrasound at diagnostically useful frequencies further reduces system sensitivity and target acquisition. Additionally, significant differences between acoustic velocities in bone and tissue result in beam degradation and registration errors in the B-mode image. With one possible exception (Somer, 1969), the nature and extent of these aberrations have thus far limited practical utilization of B-mode scanners in clinical examinations of head structure.

Delay Quantization Error in Phased Array Images

Time quantization errors in phased array systems can cause increased sidelobe amplitude, thereby limiting the dynamic range of the ultrasound system. A simulation program predicting the position and amplitude of these anomalous quantization error grating lobes has been developed to study the effects of transmitted pulse length on the ampli- tude of these anomalous lobes. Three parallel processing techniques for decreasing these sidelobe amplitudes while smoothing the speckle pat- tern are investigated.

Thaumascan: Design Considerations and Performance Characteristics

Thaumascan is an acronym for a two-dimensional, high resolution, actual time, ultrasound, multi-element, array scanner. This system substantially reduces or overcomes such limitations as low data acquisition rates, low lateral resolution and problems associated with the large dynamic range of echo information typical of most currently available imaging systems.

Real time ultrasound tomography of the adult brain.

Initial clinical results are reported from a new real time, 2-dimensional ultrasound scanner modified for adult cephalic applications. An optimized transducer design and the use of the dynamically focused phased array imaging system have resulted in ultrasound tomograms of the brain which are significant improvements over previous attempts. Horizontal and coronal images of the ventricles, the corpus callosum and other midline structures are routinely displayed in a 45° sector format. In addition, pulsating cerebral arteries are displayed in real time. Quantitative information can be obtained concerning cerebral vascular patency by using the selectable M-mode feature of this system. The results indicate that real time ultrasound tomography has potential for clinical application.

An Ultrasound Moving Target Indicator System for Diagnostic Use

An ultrasound system using moving target indicator (MTI) techniques has been developed to provide a different approach to the problem of isolating and presenting echo information arising from dynamic tissue interfaces. The basic MTI procedure accomplishes isolation of moving target information by comparing echo information subsequent to a transmit pulse to the echo information arising from the preceding pulse. Stationary target information is effectively eliminated. In addition, the MTI procedure isolates dynamic target information throughout the volume investigated by the ultrasonic transducer. This feature eliminates the need for rangegating.

Cardiovascular Diagnosis with Real Time Ultrasound Imaging

In 1954, Edler and Hertz first used ultrasound to identify cardiac structures. Since that time, this noninvasive technique has become a useful diagnostic tool which is currently enjoying a phase of rapid technological advancement. New developments have primarily been aimed towards the generation of two-dimensional tomographic images of cardiac structures in an effort to overcome the inherent limitations of a one-dimensional imaging technique, the time-motion (T-M) display mode. Currently, this technique is the standard method of clinical cardiac examinations.

Gray Scale Imaging With Complex TGC And Transducer Arrays

Ultrasound imaging of soft tissues places severe demands on the design of imaging systems because of the large range of echo amplitudes which may be encountered at any point in the body. Simple TGC curves are often inadequate in producing optimum gray scale images since the ultrasound traverses a variety of structures with substantially different attenuation coefficients. Phased array systems are shown to suffer from increased image ambiguity as compared to simple transducers, a result of discrete sampling of the aper-ture, which reduces the effective dynamic range which can be presented accurately. For a 16 element 1.7MHz array with 1.76 wavelengths interelement spacing the maximum side-lobes are only 6db below the main lobe. The use of different transmit and receive array geometries can improve the overall system spatial response. Experimental results from a 16 element 1.7 MHz transmit array with .88 wavelength interelement spacing and a receive array of the same frequency with 1.76 wavelength spacing indicated a reduction in the sidelobes to 13db below that of the main beam.

Thaumascan: Improved Image Quality and Clinical Usefulness

During the past year of clinical evaluation, several modifications have been made to the two-dimensional, real time, high resolution ultrasound imaging system known as the Thaumascan system. These modifications were designed to improve the final image quality as well as to enhance the clinical usefulness of this imaging system in assessing various cardiac disorders.