Ricardo G. Dantas

Ultrasound speckle reduction using modified gabor filters

B-mode ultrasound images are characterized by speckle artifact, which may make the interpretation of images difficult. One widely used method for ultrasound speckle reduction is the split spectrum processing (SSP), but the use of one-dimensional (1-D), narrow-band filters makes the resultant image experience a significant resolution loss. In order to overcome this critical drawback, we propose a novel method for speckle reduction in ultrasound medical imaging, which uses a bank of wideband 2-D directive filters, based on modified Gabor functions. Each filter is applied to the 2-D radio-frequency (RF) data, resulting in a B-mode image filtered in a given direction. The compounding of the filters outputs give rise to a final image in which speckle is reduced and the structure is enhanced. We have denoted this method as directive filtering (DF). Because the proposed filters have effectively the same bandwidth as the original image, it is possible to avoid the resolution loss caused by the use of narrow-band filters, as with SSP. The tests were carried out with both simulated arid real clinical data. Using the signal-to-noise ratio (SNR) to quantify the amount of speckle of the ultrasound images, we have achieved an average SNR enhancement of 2.26 times with simulated data and 1.18 times with real clinical data

Measurement of transducer directivity function

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.

Diffractive limited acoustic field of an apodized ultrasound transducer

The diffraction in the acoustic field of an ultrasound transducer can be modeled as the result of the interference of edge and plane waves generated from the periphery and the center of the piezoelectric element, respectively. Our objective in developing ultrasound transducers with apodized piezoelectric ceramic discs was to generate acoustical fields with reduced edge waves interference. Transducers were built with apodized ceramic discs (polarized more intensively in the central region than in the edges) and their mapped acoustic fields showed a distinct pattern when compared to those of conventional transducers. A polynomial equation describing the nonlinear poling field intensity, was used with the Rayleigh equation to simulate the nonuniform vibration amplitude distribution generated by the apodized transducers. Simulated acoustic fields were compared to experimental field mappings. The results of simulations and experimental tests showed reduction in the lateral spreading of acoustic fields produced by apodized transducers, compared to those produced by conventional transducers. The reduced presence of the lateral lobes in the apodized acoustic field is due to the minimized vibration of the disc periphery. The numerical and experimental results were in good agreement and showed that it was possible to reduce acoustic field diffraction through nonlinear polarization of the piezoelectric element.

Phase diversity for speckle reduction

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.

Split Phase Processing — a Single Image, Resolution Loss Free Method for Ultrasound Speckle Reduction

B-mode ultrasound images are characterised by the speckle artefact, which results from interference effects between returning echoes, and may make the interpretation of images difficult (Burckhardt, 1978). Consequently, many methods have been developed to reduce this problematic feature, some requiring many images, while others require only a single image as input. Two widely used methods, for both medical and non-destructive testing applications, are: a) split spectrum processing (SSP), also known as frequency diversity (a single image technique); and b) angular compounding (a multi image method). SSP is traditionally performed in 1D, where each single individual RF A-line is filtered by a set of relatively narrow band pass filters and the outputs are compounded to generate a speckle-reduced envelope that exhibits a significant resolution loss (Shankar and Newhouse, 1985). On the other hand, the angular compounding method does not suffer from this problem, but requires that the analysed structure be imaged from a large number of different directions - a severe practical limitation for medical imaging.

Back-propagation of ultrasound pulses via directivity spectrum method

The directivity spectrum encodes information about the three dimensional field and its magnitude is independent of the distance from the transducer face in a lossless medium. The shape of the ultrasound pulse is determined by the phase of the directivity spectrum, and its phase is related to the position of the pulse in the space-time domain. This feature of the directivity spectrum can be used to predict the shape of the ultrasound pulse at any position by changing the phase of its directivity spectrum. The method for measuring the directivity spectrum proposed here depends on the utilization of a Large Aperture Hydrophone (LAH) which has the unique feature that its output is independent of the distance from the transducer face, in a lossless medium, and it is also diffraction-insensitive. The Fourier transform of the field is related to the directivity spectrum of the transducer and the LAH can measure its angular components. We confirmed these arguments by measurements in a water tank, with pulsed fields, for several orientations of the LAH. Once the directivity spectrum of the pulsed field was known, its phase could be shifted by a specific shift factor, and the spatial distribution of the field at a new position could be obtained by the inverse Fourier transform of the shifted directivity spectrum. We present images corresponding to the predicted (back-propagated) spatial distribution of the ultrasound field of a planar transducer, obtained from the directivity spectrum measured at 6 cm from the transducer face.

Evaluation of osteoporosis using ultrasound

We have developed an equipment using ultrasound transducers to help in the diagnosis of osteoporosis. The equipment consists of an X-Y axes displacement system controlled by a microcomputer and uses two ultrasound transducers in opposite sides to inspect the calcaneus region of the patient. We have used two pairs of transducers with 500 kHz and 1 MHz central frequencies. Each pair of transducers was fixed in the X-Y displacement system submerged in a small water tank with a support for the foot of the patient. The transmitter was excited with pulses of 400 - 600 kHz or 800 - 1200 kHz and the ultrasound waves propagating through the bone in the calcaneus region are received by the opposite transducer, amplified and acquired in a digital oscilloscope. The data are transferred to the microcomputer and the ultrasound attenuation and the ultrasound transmission velocity are determined. The system was tested in patients, selected from a group that had already been diagnosed using a DEXA equipment. The results showed that there is a decrease in the ultrasound transmission velocity and the ultrasound attenuation in osteoporotic patients when compared to healthy patients of the same sex and age group. The conclusion is that ultrasound attenuation and the transmission velocity in the calcaneus region may be used as parameters in the evaluation of osteoporosis using our new system.

Ultrasonic Doppler blood flow meter for extracorporeal circulation

In cardiac surgeries it is frequently necessary to carry out interventions in internal heart structures, and where the blood circulation and oxygenation are made by artificial ways, out of the patients body, in a procedure known as extracorporeal circulation (EC). During this procedure, one of the most important parameters, and that demands constant monitoring, is the blood flow. In this work, an ultrasonic pulsed Doppler blood flowmeter, to be used in an extracorporeal circulation system, was developed. It was used a 2 MHz ultrasonic transducer, measuring flows from 0 to 5 liters/min, coupled externally to the EC arterial line destined to adults perfusion (diameter of 9.53 mm). The experimental results using the developed flowmeter indicated a maximum deviation of 3.5% of full scale, while the blood flow estimator based in the rotation speed of the peristaltic pump presented deviations greater than 20% of full scale. This ultrasonic flowmeter supplies the results in a continuous and trustworthy way, and it does not present the limitations found in those flowmeters based in other transduction methods. Moreover, due to the fact of not being in contact with the blood, it is not disposable and it does not need sterilization, reducing operational costs and facilitating its use.