Jerzy Litniewski

Skin imaging with high frequency ultrasound - preliminary results.

This study presents the detailed construction and the principle of performance of high frequency (HF) ultrasound scanner for skin examination. The aim of this study was to show a difference between diseased and healthy skin and to evaluate the usefulness of the scanner in monitoring of therapeutic efficacy of morphea and lichen sclerosus et atrophicus (LSA). We examined 48 patients aged between 15 and 64 years; 25 patients with plaque-type morphea, nine patients with linear morphea and 14 patients with LSA. In the course of 18 months all patients were examined before, during and after treatment. In 29 patients ultrasonographic evidence of regression (decreasing of the skin thickness) was observed, in eight patients ultrasound examination showed progression and in 12 patients no difference in the ultrasound scan before and after treatment could be shown. Our study shows that HF ultrasound scanner is suitable to differentiate between the healthy and diseased skin in morphea and LSA as well as to evaluate the treatment efficacy of these diseases.

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.

Ultrasonic scanner for in vivo measurement of cancellous bone properties from backscattered data

A dedicated ultrasonic scanner for acquiring RF echoes backscattered from the trabecular bone was developed. The design of device is based on the goal of minimizing of custom electronics and computations executed solely on the main computer processor and the graphics card. The electronic encoder-digitizer module executing all of the transmission and reception functions is based on a single low-cost field programmable gate array (FPGA). The scanner is equipped with a mechanical sector-scan probe with a concave transducer with 50 mm focal length, center frequency of 1.5 MHz and 60% bandwidth at -6 dB. The example of femoral neck bone examination shows that the scanner can provide ultrasonic data from deeply located bones with the ultrasound penetrating the trabecular bone up to a depth of 20 mm. It is also shown that the RF echo data acquired with the scanner allow for the estimation of attenuation coefficient and frequency dependence of backscattering coefficient of trabecular bone. The values of the calculated parameters are in the range of corresponding in vitro data from the literature but their variation is relatively high.

Semi-empirical bone model for determination of trabecular structure properties from backscattered ultrasound.

A novel semi-empirical scattering model of trabecular bone facilitating its characterization and allowing optimization of the interrogating pulse-echo transducer performance was developed. The model accounts for spatial density distribution of the trabeculae and includes measurement conditions such as pressure-time waveform of the probing ultrasound wave, the emitted field structure, and the transfer function and limited bandwidth of the acoustic source operating in pulse-echo mode. These measurement conditions are of importance as they modify the scattered echoes, which in turn are linked to the micro-architecture of the bone. The bone was modeled by a random distribution of long and thin cylindrical scatterers having randomly varying diameters and mechanical properties, and oriented perpendicularly to the ultrasound beam axis. To mimic clinically encountered conditions the relevant empirical data obtained at 1 MHz were input to the model. The data included pulse-echo source pressure field distribution in the focal zone and the above mentioned transfer function. With these data the model allowed frequency dependent backscattering coefficient of the simulated bone structure and its statistical properties to be determined. The results obtained indicated that the computer simulation is of particular relevance in studying scattering properties of the cancellous bone and holds promise as a tool to determine the relationship between the physical dimensions and shape of the scatterers and for monitoring of osteoporosis. The results of simulations also indicated that the new bone model proposed is well suited to mimic clinically relevant conditions. In contrast to the existing bone models, which usually assume scatterers to be randomly distributed as infinitely long identical cylinders with a cross-section much smaller than the probing ultrasound wave, the new model includes two populations of scatterers having different physical dimensions and also allows the mechanical properties of the scatterers to be varied.

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.

Detection of bone disease with ultrasound : comparison with bone densitometry

A system for ultrasonic in-vivo examination of a heel bone (calcaneus) was developed. When operating in transmission mode, the system can measure broadband ultrasonic attenuation-BUA, speed of sound--SOS and thickness of bone. BUA and SOS are measured by comparing the pulses transmitted through the heel with the reference pulse transmitted through water. In our approach, we operate in the backscattered mode in addition to transmission. The backscattered ultrasonic technique for bone characterization is very promising because the magnitude of backscattered waves depends on the scattering cross-section of a trabecular structure that, to some extent, describes the microarchitecture of a calcaneus. Additionally, when the backscattered and transmitted signals are compared, some of the signal distortions caused by tissue and bone interfaces are reduced. A set of data representing signals transmitted through the heel and reflected inside a calcaneus for patients with osteoporosis was collected. Several signal-processing techniques were applied in order to smooth the backscattered signal and to calculate a trabecular structure cros-section (TSC) function. Results obtained by these approaches along with a spectral shift method and a standard BUA measurement are presented and compared to X-ray bone mineral density determination results.

High frequency imaging using coded golay transmission

The issue of maximizing penetration depth with concurrent retaining or enhancement of image resolution constitutes one of the time invariant challenges in ultrasound imaging. Concerns about potential and undesirable side effects set limits on the possibility of overcoming the frequency dependent attenuation effects by increasing peak acoustic amplitudes of the waves

Statistics of the envelope of ultrasonic backscatter from human trabecular bonea)

The paper describes the investigations intended to compare the results of experimental measurements of backscattering properties of the trabecular bone with the results of computer simulations. Ultrasonic RF echoes were collected using two bone scanners operating at 0.58 and 1.3 MHz. The simulations of the backscattered RF echoes were performed using the scattering model of the trabecular bone that consisted of cylindrical and spherical elements uniformly distributed in water-like medium. For each measured or simulated RF backscatter the statistical properties of the signal envelope were determined. Experimental results suggest deviations of the backscattering properties from the Rayleigh distribution. The results of simulation suggest that deviation from Rayleigh distribution depends on the variation of trabeculae diameters and the number of thin trabeculae. Experimentally determined deviations corresponded well to the deviations calculated from simulated echoes assuming trabeculae thickness variation equaled to the earlier published histomorphometric study results.

Acoustic Velocity Determination in Cytoplasm by V(Z) Shift

The scanning acoustic microscope (SAM) working in the reflection mode allows to determine mechanical properties of biological samples1,2. At GHz frequencies the resolving power of the microscope is sufficient to investigate cells on glass or plastic in culture3,4. Previous studies revealed the possibility to measure the distribution of specific impedance and attenuation coefficients of living or fixed cells from the acoustic microscope image 5. Assuming a constant density of the cytoplasm we have obtained velocity distributions and the surface profile (thickness) of the cell. We have also observed that an increase of acoustic velocity in the cytoplasm depends on the amount of polymerized actin. Thus, the velocity is directly connected with the mechanical state of the cell S. The method presented here is also assigned for velocity determination. Comparing to the previous method another physical effect is used - the shift of V(z) curve. No preliminary assumptions for the cell parameters are required.

Modeling and analysis of multiple scattering of acoustic waves in complex media: Application to the trabecular bone

The integral equations that describe scattering in the media with step-rise changing parameters have been numerically solved for the trabecular bone model. The model consists of several hundred discrete randomly distributed elements. The spectral distribution of scattering coefficients in subsequent orders of scattering has been presented. Calculations were carried on for the ultrasonic frequency ranging from 0.5 to 3 MHz. Evaluation of the contribution of the first, second, and higher scattering orders to total scattering of the ultrasounds in trabecular bone was done. Contrary to the approaches that use the μCT images of trabecular structure to modeling of the ultrasonic wave propagation condition, the 3D numerical model consisting of cylindrical elements mimicking the spatial matrix of trabeculae, was applied. The scattering, due to interconnections between thick trabeculae, usually neglected in trabecular bone models, has been included in calculations when the structure backscatter was evaluated. Influence of the absorption in subsequent orders of scattering is also addressed. Results show that up to 1.5 MHz, the influence of higher scattering orders on the total scattered field characteristic can be neglected while for the higher frequencies, the relatively high amplitude interference peaks in higher scattering orders clearly occur.