Theses supervised by Prof. Dr. Hayrettin Köymen
20 theses · İhsan Doğramacı Bilkent University
Çökme modu cmut alıcısının rezonans altı duyarlılık hassasiyet eniyileştirilmesi
Capacitive Micromachined Ultrasonic Transducer (CMUT) has a flexural conductive plate suspended over a fixed substrate with metal electrode deposited over it. This circular suspended plate is fixed at the rim but is free to move under ambient medium loading or applied bias between CMUT plate and bottom electrode. In presence of an acoustic medium, when the bias is applied the center of the plate deflects more towards the substrate owing to the electric field which is established in the cavity structure. This electric field and ambient force are balanced by plate restoring force in stable region of un-collapsed mode of plate operation. As bias is increased further, the electrostatic forces overwhelms the plate restoring force and the center of the plate contacts with the substrate with a thin insulation layer in between. We call this state, collapse mode of CMUT operation as long as the center of the plate stays in contact with the substrate. In this work, we etch small cavities and employ thin CMUT plate which is easily depressed by the atmospheric force over an evacuated cavity to produce a stable contact with the bottom electrode without any bias. CMUTs have widely been used as sensors in a wide range of applications such as underwater flow metering sensing, airborne applications, medical ultrasound imaging where CMUTs have been characterized for wide bandwidth and high sensitivity than piezoelectric ceramics. Commercial scanners using 1-D CMUT arrays are also reported to have produced clinical-quality images. Despite the success of CMUTs in medical ultrasound over the past three decades, to this day no effort has been made to optimize the receive sensitivity of a collapsed-mode CMUT. Traditional practice is to bias the CMUT plate close to collapse voltage to acheive higher coupling coefficent and sense the incoming ultrasound as an open circuit receive voltage (OCRV) signal of the transducer using a voltage pre-amplifier or short circuit receive current (SCRC) employing a transimpedance amplifier. Maintaining plate in the vicinity of collapse threshold is rather difficult as any mechanical disturbance may collapse the plate and compromise the sensitivity of the transducer. In this work we propose ambient pressure collapse CMUT transducer capable of providing a stable performance at off-resonance frequencies. When the plate of a collapsed CMUT is subjected to incoming ultrasound its receive signal becomes a strong function of bias. The electric field sustained between the biased collapsed plate and the substrate is larger than an un-collapsed plate owing to small insulation layer gap at the contact center. This results in an increased input capacitance of collapsed CMUT which together with higher electromechanical turns ratio makes collapsed CMUT a viable choice for a higher acoustic output than its conventional counterpart. First, we derive and use a linear-equivalent circuit model under small signal conditions to assess the performance of collapsed CMUT as a function of bias. We derive both OCRV and SCRC normalized to incident pressure for a collapsed CMUT in terms of lumped circuit elements. The performance curves as a function of DC bias for varying CMUT operating conditions are obtained. We compare SCRC performance with OCRV and show that simulated SCRC performance with a transimpedance amplifier is not impaired by electrical losses. We then optimize the SCRC performance with bias for any given CMUT operating conditions or geometry. To characterize the model sensitivity we design and fabricate CMUT cells based on anodic wafer technology. We employ a transimpedance amplifier to measure and verify experimentally through fabricated CMUT cells, -60 dB V/Pa sensitivity at 100 kHz when the CMUT is biased between 50 to 65 Volts.
Beslemesiz ve yarı frekansta sürülen sualtı 16x16-elemanlı 2-B fazlı dizin CMUT
Capacitive micromachined ultrasonic transducers (CMUT) are typically used as arrays consisting of separate cells or interconnected sub arrays, and these cells have several operation modes. Design procedure and measurements have been previously presented for an airborne CMUT cell without a DC bias. In unbiased mode, the plate motion spans the entire gap without collapsing. The frequency of sinusoidal electrical input signal is half of the resulting acoustical output signal. A large plate swing can be obtained at low excitation voltages using the entire remaining gap. In this work, a design procedure of arrays with unbiased operation mode is derived. Designs are validated by means of simulations and measurements on fabricated CMUTs. The problems associated with beamforming are resolved. Use of this operation mode in an array configuration enables larger acoustical power output. A better transmitted signal waveform definition is obtained when pulse width modulation (PWM) is employed. In the design process of CMUTs, large-signal equivalent circuit model is used. In order to have volumetric transmission, a 16×16 (256 elements) phased array configuration is chosen. A design procedure is presented considering the radiation pattern, Rayleigh distance and the parameters of the lumped-element model. This procedure is applied for designing two arrays, with resonance frequencies at 7.5 MHz and at 18.5 MHz. Harmonic balance and transient analyses are carried out with Gaussian enveloped tone burst, transient sinusoidal and PWM signals with different duty cycles. Outputs of these simulations are fed in beamforming toolboxes for further verification. Corresponding experimental measurements are conducted on an ultrasound measurement system. The dimensions of the CMUT elements in the array are determined as 80 µm element radius, 15 µm plate thickness, and 171 nm effective gap height at 7.5 MHz, where the center-to-center inter-element pitch is set at 192 µm. The array is designed such that it has maximum Rayleigh distance of 45.3 mm, while having a maximum sidelobe level of −17.4 dB. According to these specifications, CMUT array is manufactured using wafer scale batch compatible production. Only two lithography masks, which require conventional photolithography steps, are used in production. The vibrating plate is constructed with anodic wafer bonding and the fabricated CMUT array chip is integrated to PCB with flip-chip bonding. Measurements are conducted for this novel device by integrating the CMUT array chip manufactured with MEMS techniques and conventional macro scale manufactured PCB by using impedance analyser.
Yüksek yoğunluklu CMUT iletim dizilerinin ışın yönlendirme uygulamarı için tasarımı, üretimi ve kullanımı
Several studies have reported airborne ultrasound transmission systems focused on achieving beamforming. However, beam steering and beamforming for capacitive micromachined ultrasonic transducers (CMUTs) at high intensity remains to be accomplished. CMUTs, like other ultrasonic transducers, incorporate a loss mechanism to obtain a wide bandwidth. They are restricted to a limited amount of plate swing due to the gap between the radiating plate and the bottom electrode, along with a high dc bias operation. CMUTs can be designed to produce high-intensity ultrasound by employing an unbiased operation. This mode of operation allows the plate to swing the entire gap without collapsing, thus enabling higher intensity. In this study, we use an equivalent circuit-based model to design unbiased CMUT arrays driven at half the mechanical frequency. This model is cross verified using finite element analysis (FEA). CMUT arrays are produced in multiple configurations using a customized microfabrication process that involves anodic wafer bonding, a single lithographic mask, and a shadow mask. We use impedance measurements to characterize the microfabricated devices. We experimentally obtained the highest reported intensity of 144 dB // 20uPa referred to the array surface using a microfabricated 2×2 CMUT array driven at resonance in a pulsed configuration. This array is also capable of beam steering and beamforming at a high intensity such that it can steer the entire half-space. The beam obtained from the array is in excellent agreement with the theoretical predictions. The amplitude and phase compensation for the devices remain constant that makes these arrays attractive for applications involving park assist, gesture recognition, and tactile displays.
7.5MHz'te çalışan kapasitif mikroişlenmiş ultrasonik dönüştürücüler için transempedans amplifikatör tasarımı
Capacitive Micromachined Ultrasonic Transducers (CMUTs) are MEMS devices used in ultrasound imaging, e.g. ultrasound mammography. CMUT proved to be a viable transducer solution in ultrasound mammography without the hazardous effects of conventional X-ray mammography. The CMUTs have a very high electrical impedance, where a transimpedance amplifier (TIA) is most appropriate for preamplification during reception. A TIA with 25MHz bandwidth and 120kΩ transimpedance gain is designed on Cadence Virtuoso using XFAB XC06M3 process. The CMUT small signal model is used for 50μm radius and 7.5MHz operating frequency. This model is incorporated into TIA circuit simulations. Two design options are proposed for the TIA, one with the passive feedback resistor and the other with a MOSFET feedback resistor. The latter enabled us to save space in layout design compare to the former. Transient and noise simulations are conducted and compared for schematic and layout views. The input referred current noise of the TIA is simulated to be 0.5pA/√Hz and the power consumption is simulated to be approximately 3.3mW for both designs.
Aktif sonar sistemlerinde yayılı izge tekniği ile kodlanmış işaretlerin kullanımı
Performance of coded signals in active Sound Navigation and Ranging (SONAR) technology is studied in this work. In this work, the possibility of having covertness and environment friendliness in SONAR systems is investigated. Spread spectrum ping signal is considered to achieve low probability of detection and interception, while maintaining good performance. Direct Sequence Spread Spectrum (DSSS) coded transmitted signals having a sequence type of maximal with lengths from 7 chips to 127 chips and tone burst pulse having single length as a reference signal are used as spread spectrum ping signal. The length of the tone burst pulse can not be increased indefinitely because of the multipath propagation. The problem of detection and localization of the targets and the cross correlation properties of the maximal length sequences are investigated as well. The Signal to Noise Ratio (SNR) and Signal to Interference and Distortion ratio (SINAD) are important parameters in detection and localization of the targets. It is found that as the number of chip length increases, the SNR and SINAD increase but the improvement of the SINAD is comparatively less because of the cross correlation properties of the maximal length sequences. Keywords: Target Detection and Localization, Covertness, Direct Sequence Spread Spectrum Modulation (DSSS), Maximal Length Sequences, Tone Burst Pulse, SNR, SINAD.
Su altı akustik sistemlerde yayılı izge tekniğini kullanarak ortogonal gold kodlar ile huzme kodlama
In this study, the applicability of beam coding approaches enabling coded transmission with direct sequence spread spectrum (DSSS) signals to multiple high resolution beams in underwater acoustic systems is investigated. Performance of (DSSS) signals for beam coding application is theoretically analyzed and experimentally proved. Instead of conventional (DSSS) parameters maximal length sequence and binary phase shift keying (BPSK) modulation, gold code sequences are more appropriate for multi-user applications due to their better cross-correlation function and minimum shift keying (MSK) modulation is more appropriate for constant amplitude signals due to its phase continuity. Beam coding experiments were carried out with 511 gold chips sequences with Minimum Shift Keying (MSK) modulation to spread over the frequency range of 70 - 90 kHz. The results of these experiments indicate that each coded signal can be separately detectable after signal processing in Matlab. And this shows that the beam coding approach is implementable in underwater acoustic systems.
İki boyutlu düzlemsel akustik dizinlerde kare piston elemanlar için karşılıklı empedans değerlendirmeleri
Large acoustic arrays have tremendous value in military and civil applications for producing well-formed beams. In order to realize the potential gains of these applications enabled by large acoustic arrays, it is essential to calculate and take into account the acoustic coupling of the sonar arrays. Although many studies have been conducted on this phenomenon, the practical and theoretical studies in public literature remain inadequate especially on mutual radiation impedance of the acoustic sonar arrays. In order to address this need, self and mutual radiation impedances of square piston elements in two-dimensional planar acoustic arrays are investigated in this thesis. Impedance matrices are formed from the self and mutual radiation impedances of these elements. Operation of arrays and possible mutual radiation impedances are analyzed through the active simulations of planar arrays. Importantly, the mutual impedance data is tested on a generic equivalent circuit model of a piston transducer. Resonance of these transducers and Rayleigh-Bloch waves are also observed as the outcome of these tests. Keywords: Mutual Acoustic Coupling, Self and Mutual Radiation Impedances, Planar Array, Square Piston Elements, Transducer, Rayleigh-Bloch Waves.
Kısa menzil, çoklu kullanıcılı, sualtı akustik iletişimi için ofdm kullanımı analizi
Acoustic waves are being used in several underwater applications, suchas SONARs, underwater communication systems. Most of already developedand deployed underwater communication systems use narrow bandcommunication and lacks layered communication approach. In this thesis, wepropose a spread spectrum, layered architecture for underwater communicationsystem, such as for SCUBA divers. The communication device shall bedesigned such that divers can communicate with each other in shallow water,short range in a multi-user fashion and provide not only voice communicationbut also data transmission as well. The device shall use Orthogonal FrequencyDivision Multiplexing (OFDM) as a spread spectrum technique. The OFDMtechnique is selected from other spread spectrum techniques due to it?s inherentability to combat the channel impairments and flexibility of implementing thecommunication system using software defined radio (SDR). The spreadspectrum system shall operate in 100 kHz to 300 kHz frequency band usingwideband acoustic transducers. In this work, we studied a layered architecturefor the communication device. We mainly studied the application layer, dataiiilink layer and physical layer in order to analyze the achievable data rate andperformance. In this work, we tried to find the optimal communicationparameters to achieve guaranteed communication performance for possiblescenarios. The communication parameters are set in order to achieve bestperformance for the worst condition. Using the optimal parameters, the systemshall occupy 5 users voice and data communication at the same time using theentire frequency band at the same time, however with certain Grade of Service(GOS) the capacity shall be increased. The capacity of the system shall furtherbe increased if the system uses adaptive communication parameters that areadapted to changing channel and user conditions. The system using adaptivecommunication parameters shall provide at most 16 users? voice and datacommunication using the entire frequency band at the same time.Keywords: Underwater Acoustic Communications, Spread SpectrumCommunications, Orthogonal Frequency Division Multiplexing (OFDM),Coded OFDM (COFDM), Layered Communication Architectureiv
İstanbul'da CDMA-PMR ağ planlaması ve optimizasyonu
The aim of this work is to design a complete PMR (Professional Mobile Ra-dio) network based on CDMA 450 in Istanbul metropolitan region. Coverageand capacity analysis are performed by the developed simulation software tool.Trade oï¬ between several system parameters such as quality of service, user mo-bility, traï¬c distribution, data rate and bandwidth are considered to optimizethe capacity and coverage of the network. Since, managing system interferenceby precise power control is a very critical concept for CDMA; forward and re-verse link interference analysis, and their eï¬ects are also studied. Coverage andhandoï¬ areas are shown on the map by the aid of GIS (Geographical InformationSystems) tool. System capacity and coverage are presented as the main resultsof the proposed network.Keywords: CDMA 450, Cell Design, Network Optimization, PMR.iii
Sualtı iletişimi için geniş bandlı ve çift yönlü akustik çevirici modellemesi
A two ceramic layer stacked transducer structure for short range underwatercommunications at high frequencies is studied in this work. The structure has awide bandwidth of one octave and operates at 350 kHz center frequency.Transducer structure inherently has two electrical and two acoustic ports. Ceramiclayers are matched to water load through quarter wavelength thick matching layerson each radiating face. Using electrical ports separately to compensate for the largeacoustic length of the structure in water is also investigated. It is shown that thewide bandwidth operation can be maintained. The beamwidth of the structure isnarrow due to end - fire effect of two back-to-back radiating elements.
Çoğul FM radyo kanalı kullanılarak pasif tutarlı konumlama radar sistemlerinde uzaklık çözünürlüğü artırma
Passive coherent location (PCL) radar systems that use single FM radio channelsignal as illuminator of opportunity have reasonable Doppler resolution, butsuffer from limited range resolution due to low modulation bandwidth and highdependence on the content that is being broadcasted from the FM station. Animprovement in range resolution is obtained by using multiple adjacent FMchannels, emitted from co-sited transmitters, which is often the case in largetowns. The proposed scheme computes the autocorrelation function of the signaldirectly received from the co-located FM transmitter, and compares it to thecross-ambiguity function, obtained from direct and target scattered signals. Thegeometry of the problem is like in the case of monostatic radar. The rangeinformation is obtained by the delay between the cross-ambiguity function andthe autocorrelation function. When a single FM channel that has a modulationbandwidth of 25 kHz is employed the range resolution is 6 km. It is shown thatdown to â33dB signal to noise ratio (SNR), which corresponds to a distance of110 km from the receiver, targets that are 3 km separated from each other canbe detected with 3 adjacent FM channels. It is possible to detect targets that are100 m separated from each other with 7 FM channels. The detection of the timedelays is a linear estimation problem.Keywords: Passive coherent location (PCL), range resolution, modulationbandwidth, FM radio, cross-ambiguity.
Dairesel sonlu engel üzerinde geniş bant geniş hüzme açıklıklı piston tipi akustik dönüştürücü tasarımı
The design of a high power piezoelectric underwater transducer operating at frequency range 42 kHz-78 kHz with acoustic power capability in excess of 250W is described. The transducer consists of two back-to-back elements. Each element is formed by stacked PZT-4 ceramic rings, a matching and a steel backing layer, and placed in a finite rigid circular baffle. We investigate the dependence of bandwidth and beamwidth to the combination of piston and baffle radii, a and b, respectively. With ka of 2.45 (k is the wave number) at resonance and a b/a ratio of 2, the transducer resonates at 60kHz with 60% bandwidth and has a beamwidth of 60º at each half space. We show that when two transducers are placed at right angles spatially and driven in parallel, we can obtain an omnidirectional beam pattern in the lower frequency band. The beam pattern exhibits two dips in each quadrant at the higher end of the frequency band, which are within 8 dB. We also investigate power handling capability of the transducer from thermal point of view using finite element analysis. The input impedance measurements agree well with the numerical results within the pass band.
31-modu açık sclcndcr tüp yapılı transdüser tasarımı
In this work, design and electromechanical analysis of radially polarized 31-Mode free-flooded ring transducers which have high power capability in deep submergence are explained. 31-Mode free-flooded ring transducers have Helmholtz resonance caused by the water inside, besides the radial resonance. By adjusting the dimensions of the ring, these resonance frequencies can be changed to the preferred values and desired band characteristics can be obtained.Transducer is designed using circuit theory techniques on the electrical equivalent circuit of free-flooded ring. The ring transducer is modeled in ANSYS, a Finite Element Modeling tool. The results obtained by analyzing the electrical equivalent circuit and the ANSYS outputs are compared. Electrical equivalent circuit parameters are updated considering the result obtained in finite element analysis. Effects of mounting end tubes are investigated by finite element analysis. New components related to the end tubes are inserted to the electrical equivalent circuit.
1. sınıf gerilim ile bükülen akustik dönüştürücü tasarımı
This thesis describes the design of low frequency, high power capability class-I flextensional, otherwise known as the barrel-stave, flextensional transducer. Piezoelectric ceramic rings are inserted inside the shell. Under an electric drive, ceramic rings vibrate in the thickness mode in the longitudinal axis. The longitudinal vibration of the rings is transmitted to the shell and converted into a flexural motion. Low amplitude displacements on its axis create high total displacement on the shell, acting as a mechanical transformer.Equivalent circuit analysis of transducer is performed in MATLAB and the effects of structural variables on the resonance frequency are investigated. Critical analysis of the transducer is performed using finite element modeling (FEM). Three dimensional transducer structure is modeled in ANSYS, and underwater acoustical performance is investigated. Acoustical analysis is performed by applying a voltage on piezoelectric material both in vacuum and in water for the convex shape barrel-stave transducer. Effects of transducer structural variables, such as transducer dimensions, shell thickness, shell curvature and shell material, on the electrical input impedance, electro-acoustical transfer function, resonance frequency and quality factor are investigated. Thermal analysis of designed transducer is performed in finite element analysis. Measured results of the transducer are compared with the theoretical results.Keywords: Underwater Acoustic Transducer, Barrel-Stave, Flextensional Transducer, Low Frequency, High Power, Piezoelectric.
Suyla yüklenmiş kapasitif mikroişlenmiş ultrasonik çeviricilerin harmonik denge analizi için doğrusal olmayan modeli
Finite element method (FEM) is used for transient dynamic analysis of capacitive micromachined ultrasonic transducers (CMUT), which is particularly useful when the membranes are driven in the nonlinear regime. A transient FEM analysis shows that CMUT exhibits strong nonlinear behavior even at very low AC excitation under DC bias. One major disadvantage of FEM is the excessive time required for simulation. Harmonic Balance (HB) analysis, on the other hand, provides an accurate estimate of the steady-state response of nonlinear circuits very quickly. It is common to use Mason's equivalent circuit to model themechanical section of CMUT. However, it is not appropriate to terminate Mason's mechanical LC section by a rigid piston's radiation impedance, especially, for an immersed CMUT. We studied the membrane behavior using a transient FEM analysis and found out that for a wide range of harmonics around the series resonance, the membrane displacement can be modeled as a clamped radiator. We considered the root mean square of the velocity distribution on the membrane surface as the circuit variable rather than the average velocity. With this definition the kinetic energy of the membrane mass is the same as that in the model. We derived the force and current equations for a clamped radiator and implemented them in a commercial HB simulator. We observed much better agreement between FEM and the proposed equivalent model, compared to the conventional model.
OFDM tabanlı bir sualtı akustik modeminin gerçeklenmesi
In this thesis we designed and implemented an underwater acoustic (UWA) communicationsystem employing multicarrier modulation in the form of orthogonal frequencydivision multiplexing (OFDM). UWA communication became more popular as there hasbeen a growing interest in transmitting real-time data, such as video and sonar images. There are many applications where these transmissions are used. These applications are underwater wireless sensor networks(UWSN) and unmanned underwater vehicles (UUVs) for military and scientifc purposes. Therefore, building an efficient UWA communication system which has a high data rate can improve these applications' performance significantly. Currently, many underwater communication systems use single carrier modulation which have limited data rate due to complexity of their receivers, as frequency selectivity of the channel increases when the symbol rate increases, so we preferred to use multicarriermodulation in UWA communication in order to increase data rate of our system. In this thesis, we considered a system that uses zero-padded (ZP) OFDM modulation. Based on ZP-OFDM, we used a receiver model that performs pilot-tone based channel estimation, carrier frequency offset compensation based on least squares (LS) fitting error or null subcarriers if they occur and data demodulation for each OFDM block individually. We used MATLAB environment for implementing our system. The MATLAB scripts generate a data burst that contains OFDM blocks, and then it is transmitted to the hardware from a laptop by using a Data Acquisition (DAQ) Card. At the other side of the system, the receiver laptop gets the data by using a DAQ Card. As the data is received, MATLAB scripts are used for demodulating it. As we built our system, we performed underwater experiments at Bilkent Lake Facility to investigate its performance in a real UWA channel. In our test, a data rate of 13.92 kbps has been achieved with quadrature phase shift keying (QPSK) modulation while the bit-error-rate (BER) was less then 9x10^-2 without using any coding.
OFDM kullanan su altı akustik modem
This thesis is about design, simulation and testing of an underwater acoustic modemusing OFDM. The thesis work combines a theoretical part, whose objective is to understandthe appropriate techniques to deal with the characteristics of the targeted channel, simulationsand a practical part regarding the system deployment and experimental tests. Therehas been a great growing interest in transmitting real-time data and video. Unmanned underwatervehicles (UUVs) for military and scientific applications have become important.Building distributed and scalable underwater wireless sensor networks (UWSN) that willbring significant advantages and benefits to underwater applications, such as ocean observationfor scientific exploration, commercial exploitation, coastline protection and targetdetection in military events has been in the scope of researchers. Based on these, designinga concrete system with high data rate will benefit many underwater acoustic (UWA) applications.The existing systems in literature use single carrier transmission and rely on linearor non-linear equalization techniques to suppress inter-symbol interference (ISI), howeverthis requires complex equalizers and results in low data rates. Therefore we concentrate onmulticarrier modulation. In this thesis ZP-OFDM (Zero Padded-Orthogonal Frequency DivisionMultiplexing) receiver is built, where CFO (Carrier Frequency Offset) compensation,pilot-tone based channel estimation, and data demodulation are carried out on the basisof each OFDM block. The implemented OFDM system has been developed in MATLAB.MATLAB scripts generate a data burst that contained OFDM blocks, and then they aretransmitted to the hardware from a laptop by using a Data Acquisition (DAQ) Card. Atthe other side of the system, the receiver laptop gets the data by using a DAQ Card. As thedata is received, MATLAB scripts are demodulated and data is detected. Simulations aimto provide correct implementation of all the algorithms by coupling the generated OFDMsignal to a channel using Bellhop underwater channel model and noise addition algorithm,that artificially introduces some of the real channel effects into the signal. The method istested in a shallow-water experiment at Bilkent Lake. Over a bandwidth of 12 kHz, thedata rate is 13.92 kb/s with QPSK modulation, when the number of subcarriers was 1024.Bit-error-rate (BER) is less than 9x10^-2 without any coding.
Su altı akustiğinde doğrudan dizili yayılı İzge tekniğini kullanarak düşük güç ile mesafe ölçümü
Performance of direct sequence spread spectrum modulation (DSSS) in underwater acoustical range fi nding is investigated in this thesis. Range estimation using low power DSSS codes is both analyzed theoretically and implemented at 400 kHz for experimental assessment, where the chip duration is 20 us and sequence type is maximal. The eff ect of sequence length on the required transmit power is measured for sequence lengths from 7 chips to 127 chips. The performance of this method is compared to the widely used tone burst pulse range estimation technique. It is found that at a sequence of 127 chips and a pulse sequence length of 2,54 ms, range is estimated with 1,5 cm resolution using source level of 132,8 dB re 1 uPa rms @ 1 m source level , while it is 190,5 cm for the same length and magnitude tone burst modulation, at a reference test range of 4.5 m. Moreover, spectral height of received DSSS signal is well below the ambient noise level so that signal to noise ratio (SNR) for received DSSS signal is -14,8 dB, while it is 12,2 dB for received tone burst pulse. Keywords: Echo Sounding, Direct Sequence Spread Spectrum, Modulation,Range Estimation, Tone Burst Pulse
Fotoakustı̇k görüntüleme ı̇çı̇n sıkıştırılmış algılama tabanlı görüntü oluşturma metodunun performansı
Photoacoustic Imaging (PAI) is an emerging imaging modality that provides high resolution and image contrast. PAI employs Delay-and-Sum (DAS) beamforming in which data on all array elements are processed for image reconstruction. In order to decrease system cost in PAI, it is desired to use fewer samples in image reconstruction. Reducing the number of active elements of the array without changing the aperture decreases the computational cost. However, spatial undersampling results in poor image resolution and contrast, and causes spatial artifacts called grating lobes. Compressed Sensing (CS) is a data completion scheme that alleviates the effects of undersampling using a priori knowledge of the signal of interest and the measurement scheme. We performed simulations and experiments to compare the performances of the CS image reconstruction algorithm and conventional DAS beamforming. We used Full-Width Half Maximum (FWHM) resolution and image contrast ratio (CR) as performance metrics. Simulation results offer improved image resolution and contrast when CS is used. Lateral resolution in DAS beamformed images deteriorates with depth. A lateral resolution of 150 µm is obtained regardless of depth using only a quarter of the transducer elements. However, the resolution of DAS beamformed images ranges between 255 µm to 508 µm as the depth increases. It is also shown that CS suppresses the effects of grating lobes and improves the contrast ratio up to 14 dB. We also presented the experimental verification of the results. We used an ultrasound research scanner, a tunable laser system, and an optoacoustic phantom in the experiments. We experimentally showed that the CS method mitigates the effects of spatial undersampling and outperforms the DAS beamforming method in terms of contrast by 10.81 dB on average. CS method also offers an improved lateral resolution of approximately 350 µm compared to 750 µm in DAS beamforming.
Çoklu saçılmalı, soğurmalı ve kırınımlı ortamlarda ıraksayan ultrasonik dalgaların sinyal gürültü oranının maksimizasyonu
Diverging wave imaging is an unfocused imaging method in which a diverging beam is transmitted to insonify the entire region of interest. This diverging beam is formed by applying appropriate time delays to each transducer array element. It provides a higher data acquisition rate and thus a higher temporal resolution, quantified as a higher frame rate. Therefore, diverging wave imaging is widely used in fast ultrasound imaging applications where rates above 1000 frames per second are required. Diverging wave imaging is generally implemented with phased array transducers having a smaller aperture than their counterparts to increase the field of view. Although diverging wave imaging allows for a high frame rate, it has a decreased spatial resolution and limited SNR due to the broader unfocused beam transmission compared to conventional focused imaging techniques. Conventional focused imaging techniques employ focused narrow beam transmissions for every image line resulting in a higher spatial resolution and SNR in the focal region. However, it offers approximately 30 frames per second, and thus it is not used in fast ultrasound imaging applications. There is a trade-off between frame rate, image quality, and SNR in diverging wave imaging. Therefore, fast imaging with high SNR and resolution while maintaining a high frame rate remains a practical problem in medical ultrasound. This thesis focuses on SNR maximization of diverging waves in weakly and multiple scattering, attenuating, and diffracting media. The primary outcome is that the SNR improves at deeper regions if the transmitted burst duration or the chip signal duration in the case of coded transmission is decreased when diverging waves are used. The maximum SNR is obtained in diverging wave transmission when the transmitted burst or the chip signal is as short-duration as the array permits. This result does not comply with the expectation implying that more transmitted energy results in higher SNR. The analytical foundation for diverging wave propagation in weakly and multiple scattering media is not sufficient at the level required to derive analytical results. In order to understand this counter-intuitive result, either finite element analysis (FEA) or semi-analytical simulation tools can be utilized. FEA can predict this counter-intuitive result, but detailed modeling of the medium is quite involved and results in very long simulation times, which renders the use of FEA impossible. Unlike the other imaging modalities, the wavelength is on the order of hundred micrometers in medical ultrasound imaging; thus, the simulation of a reasonable tissue volume is impossible. Semi-analytical simulation tools based on linear spatial impulse response produce erroneous results because scatterers are modeled as monopole sources, and multiple scattering is not modeled. As there is no analytical and simulation-based solution for this problem, the experimental verification of the results is presented. The transmitted ultrasound energy spreads over a broader region in diverging wave imaging. The energy spreading further aggravates due to diffraction and multiple scattering, which may cause energy loss. Keeping the transmitted ultrasound energy within the region of interest prevents this energy loss in diverging wave imaging. Therefore, we determined the optimum diverging wave profile to confine the transmitted ultrasound energy in the imaging sector. Using this optimized profile contributes to the SNR maximization. Complementary Golay sequences and Binary Phase Shift Keying modulation are used to code the transmitted signal. We used an ultrasound research scanner, a tissue-mimicking phantom, and a 128-element phased array transducer with 70% bandwidth at 7.5 MHz center frequency for data acquisition. The SNR in speckle and pin targets is maximized with respect to chip signal length and code length. The SNR performances of the optimized coded diverging wave and conventional single-focused phased array imaging are compared on a single frame basis. The focal region in the focused scheme is used as a reference. For the 90° imaging sector, the SNR of an 8-bit coded signal is maximum when the chip signal duration is one cycle of the center frequency. The SNR of the optimized coded diverging wave is higher than that of the conventional single-focused phased array imaging at all depths and regions. One frame of diverging wave data is acquired in 200 microseconds, equivalent to 5000 frames/s, whereas the time required for single-focused phased array imaging is 181 times more.