Joaquim Miguel Maia

In Vivo Determination of the Frequency Response of the Tooth Root Canal Impedance versus Distance from the Apical Foramen

Working length (WL) determination is a key factor to the endodontic therapy or root canal treatment success. Almost all therapy procedures depend on this measure and the wrong WL determination may produce severe consequences, like post-therapeutic pain and the need of a new root canal treatment. Electronic foramen locators (EFL) have been replacing the traditional radiographic imaging as they are faster, easier to use and have a higher success rate when measuring WL. EFLs are based on the root canal impedance assessment between two electrodes: one fixed on the endodontic file that is inserted into the root canal, and the other positioned at oral mucosa membrane. There are only few reported studies that qualify or quantify the root canal impedance characteristics. The present work aims to determine the module of tooth root canal frequency response. The preliminary results show the frequency response module variation as a function of endodontic file position inside the root canal and reinforce the methods based on relative impedance over frequency analysis used in modern EFLs.

Comparative study of the performance of the JPEG algorithm using optimized quantization matrices for ultrasound image compression

The compression of medical images is extremely important in medical applications. The compression algorithms allow an efficient way to represent image data, reducing the space required for storage and allow to minimize the demand in transmission through the communication channels. In the JPEG algorithm, the quantization matrix determines the quality of the reconstructed image and thus improved matrices were constructed. This paper makes a comparative study of the performance of the algorithm using JPEG quantization matrices optimized for compression of video frames of ultrasound image. A comparison was made between the traditional quantization matrix of the JPEG algorithm and a set of quantization matrices optimized for this algorithm. This comparison is made using compression measured in bits per [bpp] and criteria of objective fidelity (PSNR, MSE, SSIM and CC). There is coherence between the values of PSNR, MSE, SSIM and CC and the results show that the improved quantization matrices outperform the traditional matrix in terms of performance and can improve the image quality to values of PSNR slightly above 2dB.

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.

A programmable FPGA-based 8-channel arbitrary waveform generator for medical ultrasound research activities

In modern ultrasound imaging systems, digital transmit beamformer module typically generates accurate control of the amplitude of individual elements in a multielement array probe, as well as of the time delays and phase between them, to enable the acoustic beam to be focused and/or steered electronically. However, these systems do not provide the ultrasound researchers access to transmit front-end module. This paper presents the development of a digital transmit beamformer system for generating simultaneous arbitrary waveforms, specifically designed for research purposes. The proposed architecture has 8 independent excitation channels and uses an FPGA (Field Programmable Gated Array) device for electronic steering and focusing of ultrasound beam. The system allows operation in pulse-echo mode, with pulse repetition rate of excitation from 62.5 Hz to 8 kHz, center frequency from 500 kHz to 20 MHz, excitation voltage over 100 Vpp, and individual control of amplitude apodization, phase angle and time delay trigger. Experimental results show that this technique is suitable for generating the excitation waveforms needed for medical ultrasound imaging researches.

Design of a 128-channel FPGA-based ultrasound imaging beamformer for research activities

Medical ultrasound scanners are amongst the most sophisticated signal processing machines in use today. To support the complex analog and digital functions, multiple customized application-specific integrated circuits (ASIC) are typically used for the front-end and back-end processing, with limited programmability. Even with the recent advances in electronic technology, most of these systems do not provide the ultrasound researchers to have access to raw ultrasound radio-frequency (RF) data and several other important transmission and reception functions, which are available only to systems engineers. In this paper, we present the design and progresses of a 128-channel programmable, reconfigurable and modular FPGA-based ultrasound beamformer system specifically designed for medical imaging research. The transmitter beamformer can excite simultaneously 128-channel with arbitrary waveform, with individual enable control, amplitude apodization, phase angle and time delay for dynamic focusing on transmission. The receiver beamformer can handle simultaneous 128-channels acquisition with programmable sampling rate up to 50 MHz and 12-bit resolution. The proposed platform allows connection to linear and convex transducers array, up to 256 elements, or phased array transducers, up to 128 elements, and center frequency between 500 kHz and 20 MHz.

Non-Uniform Piezoelectric Ceramic Polarisation: Minimising Ultrasound Field Diffraction

We have manipulated the poling process of piezoelectric ceramic discs in order to reduce the diffraction effects caused by edge waves in the acoustic field. FEM simulation was used to investigate the poling electric field format and the vibrational behaviour of apodised and non-apodised piezoelectric ceramic discs.

Apodization of piezoelectric ceramics for ultrasound transducers

Our objective in apodizing piezoelectric ceramic discs was to produce discs that vibrate more intensively in the central region than in the region near the edge in order to generate acoustic fields with minimum diffraction effects. A spherical poling electrode was used to format the electrical field across the ceramic disc in order to achieve a polarization stronger in the central region and weaker in the edges. The electrode radius was previously determined by simulation with finite element method. The frequency spectrum of the apodized ceramic discs showed that the resonance and the anti-resonance frequencies shifted to larger values. Ultrasound transducers were constructed with the apodized ceramics and with normal commercial ceramics in order to compare their acoustic fields. The apodized transducers showed an average value of the electromechanical coefficient of 0.576 while for the non-apodized transducers this value was 0.597. Their outputs were measured in a water tank, with a point hydrophone, and showed that the time duration of the pulses generated by the apodized transducers were shorter than the ones generated by the conventional transducers, even though the acoustic pressure output intensities were similar. We have also mapped their apodized and non-apodized transducers. The far field of the apodized transducers showed a smoother decay and extended to larger distances from their face, compared to that of the non- apodized transducers.

Broadband ultrasound attenuation in the calcaneal region: a comparative study of single-position versus scanning systems

This work describes a system developed to measure the broadband ultrasound attenuation (BUA) in the calcaneal region. The patients calcanei were inspected using a microcomputer-controlled X-Y axis displacement unit with two 500-kHz, central-frequency, ultrasound transducers. The transducers facing each other are submerged in a small water tank with a support for the patients foot between them. The system allows data to be collected from a single position or by scanning the calcaneal region to obtain a BUA map. Tests were carried out on 201 patients (110 using the single-position method, and 91 using the scanning method). The results were compared with those of densitometry tests performed using the dual energy X-ray absorptiometry (DEXA) technique (single position: r=0.50; P<0.0001; scanner: r=0.75; P<0.0001). It was concluded that the single position method is more susceptible to errors due to the difficulty in positioning the transducers relative to the calcaneus. The scanning method provides better results and can be used to screen patients before referring them for DEXA.

Influence of backing and matching layers in ultrasound transducer performance

In this work we have investigated the influence of the backing layer composition and the matching layer thickness in the performance of ultrasound transducers constructed with piezoelectric ceramic disks. We have constructed transducers with backing layers of different compositions, using mixtures of epoxy with alumina powder and/or Tungsten powder and with λ/4 or 3λ/4 thickness epoxy matching layers. The evaluation tests were performed in pulse-echo mode, with a flat target, and in transmission/reception mode, with a calibrated PVDF hydrophone. The acoustical field emitted by each transducer was mapped in order to measure the on-axis and transverse field profiles, the aperture size and the beam spreading. The bandwidths of the transducers were determined in pulse-echo mode. Comparing the evaluation tests results of two transducers constructed with the same backing layer, the one constructed with λ/4 thickness epoxy matching layer showed better performance. The results showed that the transducers constructed with epoxy, alumina and Tungsten powders backing layers have larger bandwidth. The larger depth of field was measured for transducers constructed with epoxy and Tungsten powder backing layers. These transducers and those constructed with epoxy, Tungsten and alumina powders backing layers showed the larger field intensities in the measured transverse profiles.