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Imaging

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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.

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.

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
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
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