B-mode ultrasound images are characterized by speckle artefact, which results from interference effects between returning echoes, and may make the interpretation of images difficult. Consequently, many methods have been developed to reduce this problematic feature.
One widely used method, popular in both medical and non-destructive-testing applications, is a 1D method known as Split Spectrum Processing (SSP), or also as Frequency Diversity. Although this method was designed for speckle reduction applications, the final image experiences a resultant loss of resolution, impinging a trade-off between speckle reduction and resolution loss. In order to overcome this problem, we have developed a new method that is an extension of SSP to 2D data using directive filters, called Split Phase Processing (SPP). Instead of using 1D narrow band-pass filters as in the SSP method, we use 2D directive filters to split the RF ultrasound image in a set of wide band images with different phases.
The use of such filters substantially avoids the resolution loss usually associated with SSP for speckle reduction, because they effectively have the same bandwidth as the original image.
It is concluded that the Split Phase Processing, as introduced here, provides a significant improvement over the conventional Split Spectrum Processing.
A method for reconstructing the image of loss-less object from the measurement of its scattering amplitude is developed for the case of acoustic wave scattering from an inhomogeneous medium consisting of velocity fluctuations (Helmholtz equation). The inversion procedure is exact, and explicitly takes into account all orders of multiple scattering. An interesting feature is that progressively higher resolution of the recovered object is obtained as higher order scattering is progressively incorporated into the inversion procedure. Essentially, the inversion technique, which is based on a consideration of the analyticity properties of the scattering amplitude in the complex scattering angle domain, recovers the Laplace domain representation of the interaction from the angle-scattered data. Although the method described here assumes rotational symmetry, the technique is essentially three-dimensional. However, the method suffers from the disadvantage that the analytic continuation procedures it depends on would be noise sensitive, in practice.
A method for measuring the directivity function of transient fields with a new type of hydrophone that can be located at any convenient distance from the transducer is presented. Fields from planar and focused transducers, for both continuous wave and pulsed excitation, are measured via the new method, and the results compared against conventional measurements as well as against theoretical predictions. The directivity function for pulsed fields is best expressed as a complex directivity spectrum, and images of this fundamental transducer field characteristic are shown to encode a number of unexpected features. The definition and measurement of the directivity function, is not dependent on continuous wave or far-field conditions, and laboratory implementation of the theory is via a new type of hydrophone, with some unusual properties. It is concluded that precise and unambiguous measurement of transducer directivity patterns are straight forward to perform provided a relatively simple, but novel, technique is used. Images of the informative directivity spectrum may be obtained with ease.
Frequency dependent attenuation is a pronounced feature of ultrasound propagation in human tissues. A new technique for calculating that parameter from backscattered echoes is demonstrated, and it is shown how the information may be incorporated into a gray scale image. Analysis of the noisy nature of attenuation estimates from conventional backscattered echo signals suggests a new technique for producing less erratic estimates, by manipulating zeroes in the complex frequency domain. No signal averaging is needed, and the method lends itself to analysis of short data segments, thereby providing suitable input for attenuation imaging. Rf data is required. The new method is found to considerably reduce the variance of the attenuation estimates from short data segments. It is found that attenuation-weighted B-mode images are an informative way to show results. The method of zero manipulation, presented here, for producing less noisy pulse-echo attenuation estimates represents a powerful approach towards the problem.
This communication presents a new method for measuring the thickness of thin membranes with pulse-echo ultrasound. The method is based on a consideration of the structure of the interference of the two echoes from the sides of the membrane. Essentially, it is the interference between these two echoes that gives rise to poor thickness estimation. The new method, which is explained in detail, proceeds from a consideration of the zeros of the echo signal in the complex frequency domain. Measurements with the new technique are compared with two other methods: the time-separation of echo envelopes, and a cross-correlation method. The analysis is presented with both simulated and real (laboratory) data. The effect of noise is taken into account in the laboratory data. This new method is shown to be capable of measuring sub-wavelength membrane thicknesses with excellent precision. The ultrasound rf signal is required, but a substantial improvement over existing techniques is gained.
A new method for computing images of transient ultrasound fields from transducers of arbitrary shape is developed. For simplicity, only transducers with axial symmetry are considered here, but the extension to square and rectangular radiators is straightforward. The more general case may be treated by the same methods. The method is based on the use of the directivity spectrum -- which can be shown to be a generalization of the angular spectrum. It is ideally suited, however, for application to transient fields, and the formalism contains no evanescent waves. Images of pulses over extensive ranges from a variety of transducers are shown. In particular, it is shown that the transient field from a strongly-focused bowl transducer may be readily calculated, without the approximations that are necessary when using the traditional Tupholme-Stepanishen method. The simulation method is powerful and computationally efficient. It is considerably faster than methods used up to now, and may be applied to the computation of fields that are problematic for standard methods. The final output shown here is a high-quality 'snapshot' of the field, at various distances from the transducer face. Phase of the field is shown.
Some aspects of the extremely complex structure of the pulses utilized in medical ultrasound pulse-echo imaging devices are investigated. The following features are examined, in many instances from a fresh point of view: diffraction, Fourier representation, directivity spectrum, propagation of pulse projections, causality, superluminality, and header wave component. Analytical results are underpinned by experimental findings with (single-element) wideband ultrasound transducers. The discussion is conducted within the context of linear fields.
The development of an exact and practicable technique that allows the full time-space complexity of a wideband ultrasound field to be imaged, from a single set of local measurements, is presented. The method is an improvement over the angular spectrum technique, provides a novel approach towards eliminating evanescent waves, and also allows for more efficient computation of transient fields. The new representation of a field has simply propagating components that may be directly measured (via a specifically designed hydrophone) as the time-varying spatial projections of the field. Reconstruction of the transient field is performed by a technique that is a generalization of the Fourier slice theorem. The theory is vindicated by explicit demonstrations with measurements of the fields from typical ultrasound transducers. Visualization of the field is either as a three- dimensional pressure distribution at any temporal instant, or as the time-variation of the pressure over any plane orthogonal to the field propagation direction. This new method for ultrasound field measurement, prediction, and visualization is shown to be eminently practicable and represents a substantial improvement over conventional methods.
The development of a relatively rapid, but accurate, technique for charactensing pulsed fields from array transducers is reported. The approach enables direct confirmation of the effectiveness of many single element Iarray design, construction and activation procedures. The techniques rests on the employment of a PVDF hydrophone of novel design. Whereas conventional approaches derive field characteristics from point measurements, the new hydrophone allows a direct measurement of the fields 'directivity spectrum' — which efficiently generalises the angular spectrum approach to wideband pulses. The directivity spectrum is shown to encapsulate significant features of both near and far field output characteristics, as well as tightness of focus, even though all measurements are conducted at any convenient distance from the transducer face. The new method is demonstrated in the context of measurements of the fields from typical medical ultrasound transducers. The following field and transducer characteristics are shown : directivity, acoustic axis direction, effective transducer/field coherence, tightness of focus, effective radiating area, effective apodisation, and element uniformity. The relative simplicity of the technique is not compromised when measuring angle-emission characteristics. The theoretical basis for the new field measurement technique is presented, and its advantages over the more usual angular spectrum approach with point measurements, are also discussed. Keywords: Ultrasound; Transducer characterisation; Field characterisation; Directivity spectrum; Large aperture hydrophone.
The aim of the investigation reported here is to clarify the way in which spectral-modifying artefacts, such as tissue attenuation, compromise pulse-wave Doppler measurements, and to accurately measure the magnitude of the corrupting influence of attenuation under controlled laboratory conditions. A theoretical description of the structure of the pulse-echo sequence from a moving scatterer field is constructed from first principles by utilizing a time-domain description of the Doppler process. It is demonstrated that the essential features of the echo signal may be rather more accurately described by a wavelet-, rather than by a Fourier-, transform, and that the power spectrum of the Doppler signal does not necessarily encode the range of the scatterer velocities present in the (pulse-echo) sampling volumes. The analysis provides a better understanding of the origins of the significant levels of noise present in pulse-wave Doppler signals, and allows a novel approach towards noise-reduction -- by zero manipulation in complex frequency space -- to be developed.
This study aims to extract quantifiable indices characterizing ultrasound propagation and scattering in skeletal muscle, from data acquired using a real-time linear array scanner in a paediatric muscle clinic, in order to establish early diagnosis of Duchenne muscular dystrophy in young children, as well as to chart the progressive severity of the disease. Approximately 40 patients with gait disorders, aged between 1 and 11 years, were scanned with a real-time linear array ultrasound scanner, at 5 MHz. A control group consisted of approximately 50 boys, in the same age range, with no evidence or history of muscle disease. Results show that ultrasound quantitative methods can provide a tight clustering of normal data, and also provide a basis for charting the degree of change in diseased muscle. The most significant (quantitative) parameters derive from the frequency of the attenuation and the muscle echogenicity. The approach provides a discrimination method that is more sensitive than visual assessment of the corresponding image by even an experienced observer. There are also indications that the need for traumatic muscle biopsy may be obviated in some cases.
The specific problem of compensating for the presence of speckle in ultrasound B-mode imaging systems is addressed. The basic principles underlying current approaches to speckle reduction are first briefly reviewed. For real-time imaging systems, speckle reduction techniques which operate in the context of a single image (or `look') of the object are of more practical importance. A large number of such techniques have been developed which are based on some form of noise model of speckle, and are hence statistical in nature. All such approaches reduce the presence of speckle at the cost of some other factor of image quality, usually resolution. This paper examines an alternative approach to the single image problem, based on an alternative speckle definition, which does not suffer the usual resolution trade-off. The basis of the approach is to reformulate the single image problem in terms of a structure uniqueness problem which underpins a speckle model based on zeros of the analytic continuation of the rf A-line signal into the complex time and frequency planes.
Ultrasound pulses utilized for medical imaging and information-gathering appear to be coherently scattered from the many inhomogeneities within a tissue. The consequent interference effects complicate the spectral (Fourier) domain properties of the received signal: this is particularly troublesome when attempting to estimate tissue attenuation from backscattered data. A novel way to describe interference effects in (ultrasound) pulse-echo data is described. Recognition of their influence is achieved via analysis of the temporal phase of the pulse-echo signal, and correction of the artefact is achieved via novel signal processing techniques which rely on adjusting the locations of dominate zeros of the analytic continuation of short corrupted data segments (as pinpointed by the recognition procedure) into the complex frequency domain. Estimation of the mean frequency of a short pulse-echo data segment by the new method gives a reduction in variance by a factor of > 4 over existing Fourier methods. The technique finds application in an imaging technique which incorporates ultrasound attenuation information in conventional B-mode imaging.