Publications

Biomechanics

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Published on Wednesday, 03 March 2010 12:03

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1. Modelling of bone adaptation, healing process and elastic properties.

• The new approach based on application of hypothesis of optimal response of bone was proposed to enable formal derivation of a wide class of mathematical models of functional bone adaptation. Among the other cases the bone adaptation after hip joint implantation, osteoporosis development and intramedullar nail durability were analyzed and compared with the clinical cases (Fig. 6).
• Interaction of bone tissue and metallic implant was considered and the relation between the microstructure of implant surface and tissue differentiation at the interface and its microstructure development was analyzed using computer simulations based on derived bone remodeling models.
• A new model of bone tissue remodeling in case of osteoporosis including biological effects and interactions between specialized cells was proposed and preliminary computational tests were performed (Fig. 5).
• The model of bone tissue healing process was elaborated and computational tests of simplified cases were performed and compared with the clinical observations. The works concerning external stabilizer enabling stimulation of bone healing process are being performed.
• The models of composite material with hierarchical structure were adopted to calculate the anisotropic elastic properties of compact and trabecular bone. The results are in acceptable agreement with experiments.

All of these works were performed and are being performed in a close cooperation with several clinics from Medical Academy and orthopedic departments in hospitals.

2. Wear particles produced during normal functioning of artificial joints.

• The problem was examined on two planes, the in vivo and in vitro experiments. The in vivo experiments were concentrated on the influence of wear particles of polyethylene material on the cell cultures (Fig.1). The biological activity of these cells was determined by measuring levels of different cytokines or enzymes in the supernatants.
• The in vivo studies were conducted on the implants and fibrous tissue samples (tissues surrounded loosened implants) obtained from patients who suffered from aseptic loosening.
• The tissues samples were digested according to specialized procedure and obtained particles were observed under scanning electron microscopy (Fig. 2). From the tissue samples also histological slides were prepared. Those slides demonstrated wear particles inside the tissues, inside the cells (Fig. 3).
• The roughness of the surfaces of the implant heads was examined using the 2D profilmeter. The roughness measurements gave undirected information of the intensity of the wear process in the artificial joint.

3. Heat transfer in soft tissues.

• Mathematical model of arterial tree was constructed.
• Temperature distribution in soft tissues was calculated ( Fig. 4).

VARIATIONAL METHODS IN MECHANICS

1. Effective properties of piezoelectric composite.

• The concept of strength surface of piezoelectric material was introduced
• Design problem of FGM piezoelectric composite with constrains on the level of stresses and electric induction was analyzed.

During the last two years 1 Ph. D. and 2 habilitations works in the form of monographs were finished.

Fig.1. Wear particle of polyethylene produced in vivo on the pin-on-disc tester (SEM image)

Fig.2.Polyethylene particles isolated from tissue samples (SEM image magnification x 3000).

Fig.3.Polyethylene particle inside giant cell in periprosthetic tissue under polarized light (magnification x 200).

Fig. 4 Heat transfer in arterial tree.

• Modelling of functional bone adaptation in application to bone remodelling after hip joint implantation, tissue differentiation at the interface bone - metalic implant and osteoporosis development.

• Mathematical models of coupled fields phenomena inbiomaterials.

Imaging

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Category: Divisions Description

Published on Tuesday, 02 March 2010 14:06

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1.Experimental and numerical studies of nonlinear propagation and detection of the two-tone ultrasound pulses in soft tissues

Preliminary experimental investigations and numerical predictions of scattering of the two-tone acoustic pulses with coded “tones polarization” during their propagation in nonlinear dissipative medium demonstrated that usage of these signals in ultrasonography, acoustic microscopy and other diagnostic applications can cause an increase of the image resolution, signal/noise ratio and a decrease of the mechanical index (MI) in comparison with the pulsed acoustic fields from commonly used transducers driven with a tone burst.The coded transmission method proposed create a possibility to produce more efficient and safe medical diagnostic systems. The experimental and numerical simulation results of the nonlinear waveform distortion and scattering of the two-tone coded ultrasonic pulses werepublished in UMB;. Nowicki A, Wójcik J, Secomski W., Harmonic imaging using multitone nonlinear coding, Ultrasound in Med. & Biol., vol. 33, no.doi:10.1016/j.ultrasmedbio.2007.02.

Further investigations of coded multi-tone beams will make it possible to establish the optimal conditions of their transmission and reception as well as to determine the nonlinear pressure field dependence on the other parameters.

Fig.1.Improvement of the resolution. Images of three threads immerged in water scanned using  PI and MNC methods. (a) lateral field cross-section for PI (dashed line) and MNC (solid lines)

2.Improvement of numerical methods for prediction of nonlinear acoustic fields from nonaxisymmetric multi-element sources

The paper addressing this topic was published in Ultrasonics, Wójcik, J. Nowicki, A. Lewin, P.A. Bloomfield, P.E. Kujawska, T. Filipczynski,J, Wave envelopes method for description of nonlinear acoustic wave propagation, Ultrasonics. , Volume: 44, 3, pp. 310-329, 2006
On a basis of fundamental equations of nonlinear acoustics the theoretical model describing a 4D propagation (3D space and time) of the finite amplitude pulsed acoustic wave in nonlinear lossy media with arbitrary attenuation law have been developed. The model equations were solved numerically for sources with axisymmetric and nonaxisymmetric geometry similar to those used in clinical practice for diagnostic tissue visualization purposes (including linear phased arrays with beam deflection). For shortening the computational time of the nonlinear field prediction the novel, time efficient method TAWE was developed and presented in the mentioned above journal. The method can be applied to CW and pulsed waves and implemented for single and complex (multi-element) sources of arbitrary geometry by using the computational power of a standard personal computer (PC). For the fast visualization of the calculation results the graphic software was developed.

Fig.2 Nonlinear acoustic field propagation from 128 elements linear array. Spatial distribution of the Fourier spectra components. Fundamental -left and middle. Second harmonic- right. Logarithmic scale (in color).

3.Coded transmission in high frequency ultrasound applications

Coded transmission is an approach to solve the inherent compromise between penetration and resolution required in ultrasound imaging. It is widely acknowledged that this technique gives major improvement in SNR and enables higher contrast imaging without sacrificing the resolution. Our goal was to examine the performance of the coded excitation in HF (20–35 MHz) ultrasound imaging using in-house build imaging system and wide bandwidth thick film transducers. A novel real-time imaging system for research and evaluation of the coded transmission has been developed. The digital programmable coder-digitizer module (fig. 1) based on a FPGA (Field Programmable Gate Array) chip supports arbitrary waveform coded transmission and RF echoes sampling up to 200 MSPS. The module consists of: FPGA the heart of the module that connects and controls all of its parts, ADC (Analog to Digital Converter) 12-bit resolution for digitization ultrasonic RF echoes preconditioned in the analog section, DAC (Digital to Analog Converter) 12-bit resolution for arbitrary waveforms generation and USB interface chip for real-time streaming of RF samples to PC.

All digital RF and image processing was implemented in software. A unique feature of the designed system is the possibility of switching between different excitation waveforms during the experiment/examination. Single element scanning head (wobbler) with thick film focused spherical transducer 25 MHz center frequency, 4 mm in diameter and 75% bandwidth was used in the experiment. The RF echoes were acquired from a perfect reflector located at the focal depth with 1 cm of tissue mimicking material. All experimental datasets for different coded excitations (single burst and the Golay codes) were obtained with the same system settings – i.e. transmitted signal amplitude and analog input gain. Received RF echoes were time compressed using digital matched filter and demodulated using envelope detection. Two parameters were compared for a single line of the video signal: SNR and axial resolution FWHM (Full Width at Half Maximum).

Fig. 3. Block diagram of the coder digitizer module.

Single sinus burst and 16 bits complementary Golay code excitation at two different fundamental frequencies 20 MHz and 35 MHz were evaluated. SNR gain for the Golay codes (referenced to single burst) of 15 dB for 20 MHz and 16 dB for 35 MHz were obtained. The axial resolution measured at half maximum was 35 ns for 20 MHz and 25 ns for 35 MHz for both single burst and the Golay codes. It clearly shows that the Golay codes can perfectly restore resolution while giving respectable SNR gain. The image SNR for 35 MHz the Golay codes was higher by 2 dB as compared to 20 MHz single burst excitation.

Thanks to the wide bandwidth thick film transducer it was possible to increase the axial resolution by almost doubling the fundamental frequency. The overall image quality (both contrast and level of details) using the 35 MHz Golay codes was much better than for 20 MHz short burst while the penetration depth was the same. The real advantage of the coded excitation is the SNR gain which can be used to increase penetration depth or alternatively to increase resolution (at fundamental frequency) while preserving penetration. Our system programmability enables changing both type and frequency of excitation signal during the examination, so the system can be easily adapted to the different scanning requirements.

1. Lewandowski M., Nowicki A., High Frequency Coded Imaging System with Full Software RF Signal Processing, IEEE International Ultrasonics Symposium, Vancouver, Canada, 2006.

4. Golay codes

Golay complementary codes exhibit the property of canceling the time (range) side-lobes. Complementary codes were introduced by Golay in the 1961. Golay codes are pairs of binary codes, belonging to a family of sequences called complementary pairs, which consist of two sequences of the same length with a auto-correlation (correlation for the case of ultrasonic echoes compression) functions having the side-lobes equal in magnitude but opposite in sign. Summing them up results in a composite auto-correlation (correlation) function equals to 2n and zero side-lobes.During the last year we had conducted intensively experiments using Golay sequences where two-cycles bit length were used instead of one-cycle bit length, usually used in the transferred codes. The reason for such elongation bit length is the fact that one-cycle bit has a fractional bandwidth wider than the one of the transmitting/receiving transducer, which results in not efficient energy transfer. Applying coding method with two-cycle bit length extends the duration of each bit thereby narrowing the fractional bandwidth of each bit of the code and better matching it to the transducer bandwidth. Naturally the average transmitted energy doubles as well. The spectra of the 8-bits Golay sequences with two-cycles bit length and 16-bits Golay sequence with one-cycle bit length at the nominal frequency 1 MHz are shown in Fig. 4.

Fig. 4. Power spectra of the Golay complementary sequences at center frequency 1 MHz with different bits length: 8-bits code with two-cycles bit length (left) and 16-bits code with one-cycle bit length (right).

The effect of transducer bandwidth on the transferred signal can be shown on the echo signal in time domain. Fig. 2 show the measured RF echo signals of the one from the pair of the 16-bits Golay complementary sequences with one-cycle bit length and 8-bits Golay complementary sequences with two-cycles bit length reflected from the plexiglass flat reflector of thickness 1.3 mm normally oriented to the ultrasonic beam immersed in a water tank.The different amplitudes of the echo signals are related to the bit length of the coded sequence and the limited bandwidth of the ultrasonic transducers. In the case of the double bit length, the amplitudes of the echo signals are evidently higher – it is almost double. From the obtained compressed echoes in Fig. 5, it can be shown that the transducer bandwidth has a crucial influence on the transferred energy.

The use of two-cycles bit signal narrows the transmitted frequency spectrum in comparison with the one cycle one and allows optimization of the pulse-echo sensitivity. In the case of 25% fractional bandwidth, extending of the bit length allows to increase the peak-to-peak amplitude of the compressed signal by factor of 1.89. The axial resolution for both, sing cycle and two cycles transmission is shown in Fig. 6.

Fig. 5.Compressed RF echo signals reflected from the plexiglass plate of thickness 1.3 mm obtained with 6 MHz focused transducer, 25% fractional bandwidth, echoes from the anterior and posterior reflector surface are clearly shown.

Fig. 6. Comparison of the envelopes of echoes in case of using 25% fractional bandwidth with nominal frequency 6 MHz focused transducer.

For both cases the axial resolution is very similar. The difference in width of compressed pulse at –6dB level is equal to 2.7% for the transducer under examination.

The analysis of the results proved that increasing the length of the individual bits in the Golay sequences is a good solution of narrowing fractional bandwidth without deteriorating the axial resolution as long as the fractional bandwidth of the code signal is wider that the one of the transducer used.

1. Trots I., Nowicki A., Lewandowski M., Litniewski J.,Secomski W.. Golay complementary codes, double pulse repetition frequency transmission. Archives of Acoustics, vol. 31, 4, pp.35-40, 2006
2. Nowicki A., Klimonda Z., Lewandowski M., Litniewski J, Lewin P., Trots I., Comparison of sound fields generated by different coded excitations – Experimental results, Ultrasonics, 44, pp.121-129, 2006
3. Litniewski J, Nowicki A., Klimonda Z, Lewandowski M, Sound Fields for Coded Excitations in Water and Tissue: Experimental Approach, Ultrasound in Biology and Medicine,Vol. 33, 4, pp. 601-607, 2007

Microscopy

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Category: Divisions Description

Published on Tuesday, 02 March 2010 14:22

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Scanning Acoustic Microscopy Laboratory

• Development of acoustic microscopes, imaging techniques (C-scan, B-scan, amplitude and phase imaging) and measuring procedures in low frequency (35MHz-200MHz) range
• Visualization of surface and interior of solids and biological materials using waves of the frequency up to 1GHz
• 3 dimensional microscopic imaging
• Bones sonography

Low Frequency Scanning Acoustic Microscope

Fig.1.Scanning Acoustic Microscope built in the Division of Acoustic Microscopy. Operating frequency 35MHz - 200MHz.

Scanning Acoustic Microscope (SAM) operates at the frequencies of 35MHz, 100MHz and 200MHz and The corresponding lateral resolution of 40μm, 15μm and 7μm for the surface imaging can be achieved. The image is created by amplitude or phase of the reflected signal. The microscope enables surface, subsurface and surface layer imaging.

Amplitude imaging

Operating in amplitude mode the microscope can be used for non-destructive micro-defectoscopy of materials as well as for imaging of soft biological samples or hard materials (bone samples). Acoustic images of the soft unstained tissues enable differentiation of various tissue types that differ in acoustic impedance values less than 1%. We have a good experience with subsurface imaging in metals up to several millimeters beneath the surface. The amplitude inspection of the surface layer is very sensitive to impedance changes and it can provide information about the quality of adhesion of diffusion bonds, welds and laminates because any lack of coupling between materials changes dramatically the reflection coefficient.

Fig.2.Scanning Acoustic Microscope (SAM) C-scan images of biological samples obtained at 100MHz frequency. Successively from the left: eye section, artery section and eyeball cross-section (retina).

Fig.3.SAM C-scan image of the one-cent coin surface (left) and SAM B-scan image of flaws beneath the surface of a turbine blade (right).

Phase imaging

Phase imaging is very powerful and sensitive method for topography imaging. At 100MHz the distance changes less than 0.1μm can be detected. Our experience with phase imaging of the thin gold layers deposited on quartz samples show that this technique is sensitive to any cracks, voids and other defects of the layer. At lower frequencies the topography of the big objects (spherical transducer for example) can be examined.

Fig.5.SAM phase-images. Left – topography of a coin surface, right – gold layer, 0.07mm thick, deposited on the part of quartz surface.

Imaging with surface waves

The microscope can generate surface waves that may be used for imaging and measurements. The contrast of the surface waves-images depends on the velocity of these waves. Thus, the quality or homogeneity of material can be determined from the images that reflect the velocity distributions. The surface waves-imaging technique may be also applied for investigations of the grained materials like metals or alloys. Also, some stress introduced in samples can be investigated due to elasto-acoustic effect (ultrasonic wave velocity changes due to stress variation). So, in very homogeneous samples the stress distribution can be visualized and determined.

Fig.6.SAM amplitude images obtained with surface waves.Left – grains of ALNICO alloy, right – trabeculae of cancellous bone.

High Frequency Scanning Acoustic Microscope

Acoustic Microscopy Laboratory is equipped with commercial acoustic microscope (KSI) that operates in GHz range with the resolution up to 1 micron.

Fig.7.KSI Scanning Acoustic Microscope.

Fig.8.SAM images of integrated circuit structures obtained at 400MHz (left) and at 1GHz frequency (right).

Fig.9.XTH-2 living cell image

Acoustic microscope enables investigations of living cells. Above, XTH-2 living cell image (1GHz) is presented with characteristic fringe-structure that results from interference of waves reflected from the bottom and upper surface of the cell. Additionally, the amplitude of the received signal along the marked line is shown.

3 Dimensional Scanning Acoustic Microscope

A new Acoustic Microscope operating at the frequency up to 200MHz intended for internal structures visualization was developed in the Division of Acoustic Microscopy. The system was built basing on the commercially available components (transmitter, receiver, scanner and 1GHz sampling board) and the self-constructed acoustic lenses. The dedicated software was developed to control the process of 3D RF-data acquisition, processing and presenting the data in 2D cross-sections or 3D surface rendering mode.

Inside ceramic package

Bones sonography

Development of instrumentations and techniques for the complex characterization of the cancellous (trabecular) bone, including acoustic microscopy of a single trabeculae and investigations of the trabecular bone structure by sound transmission and scattering.

Acoustic microscopy of trabecular bone

Fig.12.Cancellous bone structure imaged with the microscope at 100MHz. Resolution = 7μm. Image size = 3mm x 3mm.

The SAM image can be converted into parametric images that reflect the distribution of acoustic impedance (product of density and velocity) or attenuation coefficient within the bone sample.

Fig.13.Successively from the left: Single trabeculae and cortical bone image, parametric-impedance image and parametric-attenuation coefficient image. Bone investigation at low frequency

BUA = 0 dB/MHz BUA = 160 dB/MHz

Parametric image (obtained in vivo) of a heel bone. Image brightness is coded with values of Broadband Ultrasonic Attenuation (BUA) coefficient.