P.N.T. Wells

Ultrasonic Doppler studies of the breast

The growth of a malignant tumour depends on vascularisation. The ultrasonic Doppler method can detect the blood flow associated with malignant breast tumours, the signals differing qualitatively from those due to benign lesions. Several descriptors of the Doppler signals were tested; benign and malignant lesions are best separated by the difference between the maximum systolic frequencies from suspect and contralateral sites. Corresponding main arterial sites are reliably coincident in normal breast pairs. Consideration of the powers and frequency spectra of Doppler signals leads to the rejection of models of tumour vascularisation giving Doppler signals based on capillary perfusion and on a single feed artery. The data are compatible with a multiple feed artery model, and this is supported by a contrast angiogram. Doppler ultrasound may be useful as a preliminary screening method, in the management of patients with radiologically dense breasts or diffuse dysplasia, and for monitoring unexcised tumours undergoing hormone therapy.

Ultrasonic imaging of the human body

Ultrasonic imaging is a mature medical technology. It accounts for one in four imaging studies and this proportion is increasing. Wave propagation, beam formation, the Doppler effect and the properties of tissues that affect imaging are discussed. The transducer materials and construction of the probes used in imaging are described, as well as the methods of measuring the ultrasonic field. The history of ultrasonic imaging is briefly reviewed. The pulse-echo technique is used for real-time grey-scale imaging and the factors that limit the spatial and temporal resolutions are considered. The construction and performance of transducer arrays are discussed, together with the associated beam steering and signal processing systems. Speckle and scattering by blood are introduced, particularly in the context of the observation of blood flow by means of the Doppler effect and by time-domain signal processing. Colour flow imaging, and the colour coding schemes used for velocity and power imaging, are explained. The acquisition and display of three-dimensional images are discussed, with particular reference to speed and segmentation. Specialized imaging methods, including endoluminal scanning, synthetic aperture imaging, computed tomography, elasticity imaging, microscanning, contrast agents, and tissue harmonic imaging, are reviewed. There is a discussion of issues relating to safety. Conclusions are drawn and future prospects are considered.

Speckle in ultrasonic imaging

Abstract The resolution of an ultrasonic pulse-echo imaging system is determined by the dimensions of the resolution cell. The image is characterized by a granular pattern, or ‘speckle’, which varies from place to place in the tissue; it can be explained in terms of the coherent formation of the echo from many small scatterers within the resolution cell. These scatterers are randomly distributed, and the reflected amplitude fluctuation becomes larger as the size of the resolution cell is reduced. Speckle can be smoothed either by small movements of the tissue during scanning, or by compound scanning.

Absorption and dispersion of ultrasound in biological tissue

Abstract Absorption of ultrasound is the process of conversion of vibrational energy into heat. In biological soft tissues, the absorption coefficients are roughly proportional to the frequency; typically α = 1 dB cm −1 MHz −1 . The velocities in soft tissues are similar, being about 1,500 msec − . Classical viscosity theory cannot explain this form of absorption. A relaxation process is associated with a range of frequency over which there is dispersion in velocity—from a low value at low frequencies to a high value at high frequencies—and a maximum in the absorption per wavelength. Experimental data, particularly for haemoglobin solutions, indicate that absorption and dispersion in biological materials are due to relaxation processes distributed over a range of frequencies. The dispersion is small, and usually negligible in relation to variations and uncertainties of measurement. The natures of the relaxation processes which are involved have yet to be resolved; possibilities include solvent-solute interactions and disturbances in H-bonding equilibria. Lung has a lower velocity than that of solid tissues, whereas that of bone is higher; both have higher values of absorption.

Tumour detection by ultrasonic Doppler blood-flow signals

Ultrasonic Doppler blood-flow signals which seem to be associated with malignant tumour neovascularization have been detected in the female breast. No such signals have been detected from cysts. This discovery may lead to the development of a highspeed ultrasonic Doppler scanner which might make breast screening for cancer practicable.

Physiological interpretation of Doppler-shift waveforms-III. Clinical results

Abstract The blood-velocity/time waveforms over the cardiac cycle obtained from the common femoral arteries of 44 limbs, are defined using Fourier transform and curve-fitting techniques. This results in a third order Laplace transform whose coefficients can be related to distal impedance, proximal lumen diameter and stiffness. The sensitivity of this technique has been investigated in the study of aorto-iliac disease, and compared with single plane arteriography and Pulsatility Index. The results show that the Laplace transform method is a sensitive technique for determining the presence of minor stenoses in the aorto-iliac segment, for determining changes in elastic modulus of the proximal vessels, and for studying changes in distal impedance. Pulsatility Index, is sensitive to the presence of proximal disease but it does not seem able to differentiate between stenoses of less than or greater than 50%. PI is sensitive also to changes in distal impedance, but insensitive to changes in arterial elasticity.

Methods of measuring the performance of ultrasonic pulse-echo diagnostic equipment.

Abstract Methods are outlined whereby the main diagnostic performance parameters exhibited by ultrasonic pulse-echo equipment may be measured and reported. Particular aspects of performance that are considered are: acoustic frequency, echo detection capability, gain characteristics, display characteristics, geometrical resolution and geometrical alignment accuracy. Specifically excluded from consideration is the measurement of acoustic output of the equipment, since it is intended that this subject will be dealt with separately elsewhere.

Ultimate limits in ultrasonic imaging resolution

According to elementary theory, the resolution of an ultrasonic imaging system increases with the ultrasonic frequency. However, frequency is limited by frequency-dependent attenuation. For imaging at any required depth, resolution improvement beyond the limit imposed by ultrasonic frequency can be obtained by increasing the ultrasonic intensity. This is itself, however, dependent on safety considerations and the effects of nonlinearity. In homogeneous media, image resolution increases with decreasing f-number. Particularly at low f-numbers, however, tissue inhomogeneity leads to a deterioration in image quality. Inhomogeneity may also be considered in terms of phase aberration. It has been found that for a given aperture, image degradation due to phase aberration is worse at higher frequencies. Schemes have been proposed for correction of this problem, but so far model systems do not lend themselves to clinical application. Deconvolution is unsatisfactory, speed correction is impracticable and synthetic aperture scanning and holography are virtually useless in biological tissues. Ultrasound-computed tomography has had only limited success. Speckle reduction can improve target detectability, but at the expense of resolution. Time-frequency control provides a useful partial solution to the problem of resolution reduction resulting from attenuation. It is clear that improved resolution would result in significant clinical benefits. An optimisation system for aperture size and ultrasonic frequency is proposed with signal averaging for resolution enhancement of a defined object area. This would have a compact ultrasonic beam and would allow frame rate to be traded for resolution, by means of signal averaging.

PULSATILITY INDEX OF SUPERIOR MESENTERIC ARTERY BLOOD VELOCITY WAVEFORMS

A transcutaneous Doppler ultrasound technique was used to investigate superior mesenteric artery blood velocity waveforms in normal subjects. The shape of the waveforms was quantified by means of the pulsatility index (PI). The mean value +/- standard error of the mean of the PI measured in 82 normal subjects in the resting and fasting state was 3.57 +/- 0.11. There was no difference in the PI between sexes nor correlation between the PI and age. Following the ingestion of a meal in a group of 15 subjects the PI decreased by 46% (p less than 0.001). A significant fall persisted over the next two hours.

A new approach to the noninvasive measurement of cardiac output using an annular array doppler technique—I. Theoretical considerations and ultrasonic fields

This paper describes the development of a Doppler flowmeter capable of measuring blood volume flow rate without the need to measure the vessel lumen area or beam-vessel angle. It requires the production of a uniform wide ultrasound beam to encompass the whole vessel and thus to produce a Doppler spectrum which corresponds to all the flowing blood, and a narrow reference beam placed within the lumen to compensate for various unknown quantities, such as tissue attenuation. The general definition of volume flow rate is described and applied to a new flowmeter, which allows an absolute value of volume flow rate to be measured independently of vessel size, beam-vessel angle, and tissue attenuation. By electronically apodising an annular array transducer in transmission and reception, a uniform wide beam and a narrow reference reception beam are produced. Theory to predict these beam patterns is developed and a computer simulation is made. The ultrasonic fields obtained from an annular array transducer in water are compared with the theoretical fields.