Ihor Trots

Estimation of ultrasonic attenuation in a bone using coded excitation

This paper describes a novel approach to estimate broadband ultrasound attenuation (BUA) in a bone structure in human in vivo using coded excitation. BUA is an accepted indicator for assessment of osteoporosis. In the tested approach a coded acoustic signal is emitted and then the received echoes are compressed into brief, high amplitude pulses making use of matched filters and correlation receivers. In this way the acoustic peak pressure amplitude probing the tissue can be markedly decreased whereas the average transmitted intensity increases proportionally to the length of the code. This paper examines the properties of three different transmission schemes, based on Barker code, chirp and Golay code. The system designed is capable of generating 16 bits complementary Golay code (CGC), linear frequency modulated (LFM) chirp and 13-bit Barker code (BC) at 0.5 and 1 MHz center frequencies. Both in vivo data acquired from healthy heel bones and in vitro data obtained from human calcaneus were examined and the comparison between the results using coded excitation and two cycles sine burst is presented. It is shown that CGC system allows the effective range of frequencies employed in the measurement of broadband acoustic energy attenuation in the trabecular bone to be doubled in comparison to the standard 0.5 MHz pulse transmission. The algorithm used to calculate the pairs of Golay sequences of the different length, which provide the temporal side-lobe cancellation is also presented. Current efforts are focused on adapting the system developed for operation in pulse-echo mode; this would allow examination and diagnosis of bones with limited access such as hip bone.

Direct and Post-Compressed Sound Fields for Different Coded Excitations

Coded ultrasonography is intensively studied in many laboratories due to its remarkable properties: increased depth penetration, signal-to-noise ratio (SNR) gain and improved axial resolution. However, no data concerning the spatial behavior of the pressure field generated by coded bursts transmissions were reported so far. Five different excitation schemes were investigated. Flat, circular transducer with 15 mm diameter, 2 MHz center frequency and 50\% bandwidth was used. The experimental data was recorded using the PVDF membrane hydrophone and collected with computerized scanning system developed in our laboratory. The results of measured pressure fields before and after compression were then compared to those recorded using standard ultrasonographic short-pulse excitation. The increase in the SNR of the decoded pressure fields is observed. The modification of the spatial pressure field distribution, especially in the intensity and shape of the sidelobes is apparent. Coded sequences are relatively long and, intuitively, the beam shape could be expected to be very similar to the sound field of long-period sine burst. This is true for non-compressed distributions of examined signals. However, as will be shown, the compressed sound fields, especially for the measured binary sequences, are similar rather to field distributions of short, wideband bursts.

Synthetic Aperture Method in Ultrasound Imaging

Medical ultrasound imaging is a technique that has become much more prevalent than other medical imaging techniques since it is more accessible, less expensive, safe, simpler to use and produces images in the real time. However, images produced by an ultrasound imaging system, must be of sufficient quality to provide accurate clinical interpretation. The most commonly used image quality measures are spatial resolution and image contrast which can be determined in terms of beam characteristics of an imaging system: beam width and sidelobe level. In the design of an imaging system, the optimal set of system parameters is usually found as a trade-off between the lowest side-lobe peak and the narrowest beam of an imaging system. In conventional ultrasound imaging system, when one transducer (in mechanical wobble) or linear array are used, the quality of images directly depends on the transducer acoustic field. Also in conventional ultrasound imaging the image is acquired sequentially one image line at a time that puts a strict limit on the frame rate that is important in real-time imaging system. Low frame rate means that moving structures (e.g. heart valves) are not easily imaged and diagnosis may be impaired. This limitation can be reduced by employing synthetic aperture (SA) imaging. The basic idea of the SA method is to combine information from emissions close to each other. The synthetic aperture method has previously not been used in medical imaging. This method is a contrast to the conventional beamforming, where only imaging along one line in receiving is used. This means that every image line is visualized as many times as the number of elements used. This will create an equal amount of low resolution images, which are summed up to create one high resolution image. Problems with medical ultrasound include low imaging depth, and high resolution is achieved only in the region where the transducer is focused. Another problem is decreasing SNR with depth. The basic idea with synthetic aperture is to combine information from emissions close to each other. This is a contrast to the conventional beamforming, were only imaging along one line in receiving is used. This means that every image line is visualized as many times as the number of elements used. This will create an equal amount of low resolution images which are summed up to create one high resolution image. One of the important processes in ultrasound imaging systems is beamforming. There are many different beamforming methods. In this work both the synthetic transmit aperture (STA) (Trots, et al. 2009) and the multi-element STA (Trots, et al. 2010) methods for medical ultrasound imaging system are discussed. In the case of the multi-element STA imaging

Cumulative method of image reconstruction in Synthetic Aperture - theory and experimental results

The Synthetic Aperture (SA) method provides a new solution in ultrasound diagnostics. It has particular importance in applications where frame rate and image resolution are crucial. Our new approach named Cumulative Synthetic Transmit Aperture (CSTA) allows optimizing SA in terms of memory size and computational power. The proposed CSTA algorithm requires 25 times less memory than a reference STA method for 64 elements transducer. This makes feasible implementation of CSTA on a low-power embedded GPU.

Optimization in the Multi-element Synthetic Transmit Aperture Method for Ultrasound Imaging

The paper deals with optimization of the transmit aperture in the multi-element synthetic transmit aperture method (MSTA) for ultrasound imaging. In contrast to the conventional synthetic transmit aperture (STA) method, MSTA allows to increase the system frame rate and provides the best compromise between penetration depth and lateral resolution.

Sound fields for coded excitations in water and tissue

Coded ultrasonography is intensively studied in many laboratories due to its remarkable properties; increased penetration depth and signal-to-noise ratio (SNR). However, no data on the spatial behavior of the pressure field generated by coded bursts transmissions in the tissue were yet reported. This paper reports the results of investigations of the field structure in water, in degassed beef liver and in pork tissue using five different, 2MHz excitations signals: two and sixteen periods sine bursts, 8 µs chirp, and sinusoidal sequences phase modulated with 13 bits Barker code and 16 bits Golay complementary codes. The results of measured pressure field distributions before and after compression were compared to those recorded using standard ultrasonographic short pulse excitation

Ultrasound Image Improvement by Code Bit Elongation

This letter analyses the influence of the transducer bandwidth on the compression and the axial resolution of an ultrasound image. The distortion of an electrical signal visible in the final image is a major problem in ultrasonography. To solve this problem, the bit length in Golay-complementary sequences was elongated, narrowing the fractional bandwidth of the coded sequences. Therefore, more energy of the burst signal could be transferred through the ultrasound transducer. The experimental results obtained for transmission of the complementary Golay-coded sequences with two different bit lengths—one-cycle and two-cycles—have been compared, and the efficiency of the pulse compression and its influence on the axial resolution for two fractional bandwidths have been discussed. The results are presented for two transducers having a fractional bandwidth of 25% and 80% and operating at a 6-MHz frequency. The results obtained show that the elongation of the Golay single bit length (doubled in our case) compensate for the limited transducer bandwidth. Two-dimensional ultrasound images of a tissue-mimicking phantom are presented and demonstrate the benefits of the use of two-cycle bit length.

Orthogonal Golay Codes With Local Beam Pattern Correction in Ultrasonic Imaging

The goal of this study is to improve the synthetic transmit aperture (STA) imaging method by employing the transducer array element beam pattern correction combined with emission of mutually orthogonal complementary Golay sequences. The transmit-receive scheme based on simultaneous emission of different Golay pairs by adjacent transmit subapertures is implemented to decrease the image reconstruction time. A brief discussion on the fundamentals of the orthogonal Golay complementary sequences is provided and their advantages for the STA imaging method are demonstrated. The performance of the developed approach was tested using FIELD II simulated synthetic aperture data from the point reflectors, which allowed to estimate both; the penetration depth and the lateral resolution. In the work the 128 element, 5 MHz, linear array transducer was used. The obtained results showed that the applying the beam pattern correction leads to the image quality improvement in the vicinity of the transducer face. Specifically, the noise level evaluated between the point reflectors at the depth of 4 mm decreased from - 14.1 dB for the case of omnidirectional source to - 38.7 dB when the element beam pattern correction was implemented. The simulation proved that the overall imaging quality was improved considerably.

Sound field directivity in multi-element synthetic transmit aperture method for ultrasound imaging

A modified multi-element synthetic transmit aperture (MSTA) method for ultrasound imaging with RF echoes correction taking into account the influence of the element directivity is presented. The property is significant as the element width becomes commensurable with the wavelength of the emitted signal. The angular dependence of the radiation efficiency of the transmit/receive aperture is obtained from exact solution of the corresponding mixed boundary-value problem for periodic baffle system, modeling the transducer array. It is evaluated at the nominal frequency of the excitation signal and is implemented in the developed MSTA algorithm as apodization weights calculated for each imaging point and all combinations of the transmit/receive apertures. The performance of developed method is tested using FIELDII simulated synthetic aperture data of the point reflectors to estimate the visualization depth and lateral resolution. Besides, a FIELDII simulated and measurement data of cyst phantom are used for qualitative assessment of the imaging contrast. Comparison of the results obtained by the modified and conventional MSTA algorithms is given which reveals considerable improvement of the image quality in the area neighboring to the transducers aperture, and increase of the visualization depth at the cost of slight degradation of lateral resolution near the transducer face.

Coded Excitation with Directivity Correction in Synthetic Aperture Imaging System

The paper presents the preliminary results of the computer simulations of the synthetic aperture method combined with the coded transmission in a linear transducer array. It exploits the transmission of long waveforms characterized by a particular autocorrelation function and allows to increase the total energy of the transmitted signal by its extension, without increasing the peak pressure (limited by safety reasons). It can also improve signal-to-noise ratio and increase the visualization depth maintaining the ultrasound image resolution.