Use of modulated excitation signals in medical ultrasound. Part II: Design and performance for medical imaging applications

In the first paper, the superiority of linear FM signals was shown in terms of signal-to-noise ratio and robustness to tissue attenuation. This second paper in the series of three papers on the application of coded excitation signals in medical ultrasound presents design methods of linear FM signals and mismatched filters, in order to meet the higher demands on resolution in ultrasound imaging. It is shown that for the small time-bandwidth (TB) products available in ultrasound, the rectangular spectrum approximation is not valid, which reduces the effectiveness of weighting. Additionally, the distant range sidelobes are associated with the ripples of the spectrum amplitude and, thus, cannot be removed by weighting. Ripple reduction is achieved through amplitude or phase predistortion of the transmitted signals. Mismatched filters are designed to efficiently use the available bandwidth and at the same time to be insensitive to the transducer's impulse response. With these techniques, temporal sidelobes are kept below 60 to 100 dB, image contrast is improved by reducing the energy within the sidelobe region, and axial resolution is preserved. The method is evaluated first for resolution performance and axial sidelobes through simulations with the program Field II. A coded excitation ultrasound imaging system based on a commercial scanner and a 4 MHz probe driven by coded sequences is presented and used for the clinical evaluation of the coded excitation/compression scheme. The clinical images show a significant improvement in penetration depth and contrast, while they preserve both axial and lateral resolution. At the maximum acquisition depth of 15 cm, there is an improvement of more than 10 dB in the signal-to-noise ratio of the images. The paper also presents acquired images, using complementary Golay codes, that show the deleterious effects of attenuation on binary codes when processed with a matched filter, also confirmed by presented simulated images.     

Transverse or axial superresolution in a 4Pi-confocal microscope by phase-only filters

A novel procedure to design axial and transverse superresolving pupil filters for the 4Pi-confocal microscope is presented. The method is based on the use of a series of figures of merit developed to describe the effect of inserting two identical filters in the two arms of the illumination path of the microscope. As a practical implementation, we have applied our method to obtain superresolving continuous phase-only filters. Different resolution-improving phase functions are shown for the transverse and the axial direction. These filters provided axial gain up to 1.3 and transverse gain up to 1.4 without an increase in sidelobes.     

Code optimization for direct sequence spread spectrum and SAW-matched filter implementation

This paper introduces a new optimization algorithm for the minimization of the time sidelobes of the correlation function of a pseudonoise (PN) sequence by applying dynamic weighting to the sequence. The resulting optimized time sidelobe level sequences are to be used in direct sequence spread spectrum (DS-SS) systems with digital modulations such as BPSK, DPSK, QPSK, etc. The new optimization algorithm starts with a PN sequence. It first optimizes the correlation time sidelobes for the case where the consecutive data bits are identical (11 or 00). It then optimizes the correlation time sidelobes for the case of alternating consecutive data bits (10 or 01). The suppressed time sidelobe level sequences are derived by iterating these algorithms alternately starting from the initial PN sequence. The derived suppressed time sidelobe sequences show excellent correlation characteristics when compared to conventional PN sequences such as maximal length sequences, Gold sequences and Barker codes. Surface acoustic wave (SAW) devices were used to implement the optimized time sidelobe level sequences in a matched filter pair. The design of the apodized SAW-matched filters and their predicted second order effects are presented. The experimental results for the SAW-matched filters for the optimized time sidelobe level sequences derived from a Barker code were found to be in good agreement with the theoretical predictions from this new algorithm.     

Dichroic rugate filters

We reduce the sidelobes that are present in the bandpass zones of rugate filters, using antireflection coatings to design dichroic rugate filters. Our designs are based on the formalism of equivalent systems that allow us to consider a rugate filter a simple equivalent layer with equivalent optical admittance and equivalent phase thickness functions. Rugate filters with apodization were considered.     

Mismatched filtering of periodic and odd-periodic binary arrays

Arrays with good correlation properties are required for coded-aperture imaging, as well as for other applications of two-dimensional signal processing. Since binary arrays with perfect periodic autocorrelation functions exist for only a few sizes, mismatched filtering is discussed. Mismatched filtering entirely suppresses any sidelobes of the periodic autocorrelation function at the expense of a slightly reduced signal-to-noise ratio. New construction methods for binary arrays are presented for which this loss, with respect to periodic or odd-periodic correlation, converges to zero with an increasing array size.     

Three-dimensional superresolution by three-zone complex pupil filters

Complex pupil filters are introduced to improve the three-dimensional resolving power of an optical imaging system. Through the design of the essential parameters of such filters, the transmittance and radius of the first zone, three-dimensional superresolution is realized. The Strehl ratio and the transverse and axial gains of such filters are analyzed in detail. A series of simulation examples of such filters are also presented that prove that three-dimensional superresolution can be realized. The advantage of such filters is that it is easy to realize three-dimensional superresolution, and the disadvantage is that the sidelobes of the axial intensity distribution are too high. But this can be overcome by the application of a confocal system.     

Sidelobeless multiple-object discriminant filters recorded as discrete-type computer-generated holograms

The computer generation of sidelobeless multiple-object discriminant correlation filters has been stressed. We propose to synthesize the filter functions by use of the simulated-annealing algorithm. By this method the filters can be obtained as discrete-type computer-generated holograms. The filters can suppress the sidelobes and provide sharp correlation peaks. A computer simulation and an optical experiment were performed, and the expected correlation responses were obtained.     

Focal spot generator without sidelobes and its application in coronagraphy

Abstract

A combination of two phase filters is described, which together produce a light beam that can be focused to produce a focal spot with an approximately flat top and with much reduced sidelobes (a top-hat function). There are a variety of potential applications, but our interest lies in the use of this device in astronomical coronagraphy and point spread function engineering in order to aid the detection of extra-solar planet.     

Sidelobe suppression in chiral optical filters by apodization of the local form birefringence

Chiral optical filters are characterized by circular Bragg effects, including the preferential reflection and transmission of circular polarization states. The selective response to circularly polarized light is caused by stratified birefringent plates twisted into a helical arrangement, as seen in cholesteric liquid crystals and columnar thin films produced by oblique-angle physical vapor deposition. A refinement of the latter, glancing angle deposition employs substrate rotation to control the optical anisotropy of columnar thin films, and was used in this study to suppress the reflection sidelobes of chiral optical filters by modulating the local birefringence of helically structured thin films using an apodization function. Both theoretical simulations based on Berreman formalism, and experimental results involving evaporated TiO(2) thin films are presented and compared.     

Correlation of sidelobe suppression between rugate filters and multilayer mirrors

We present the study of the correlation between refractive index profiles and the optical response of rugate filters and multilayer mirrors. The conventionally used method in multilayer mirrors for ripple suppression in the passband will be compared with a similar simple method to remove the rugate filter sidelobes without apodization. The resulting layers are compared in performance with a typical quintic matching layer. An example based on silicon oxynitride alloys with refractive indices ranging between 1.47 and 1.83 was designed and deposited.     

Interaction dynamics of Bragg grating solitons in a semilinear dual-core system with dispersive reflectivity

The interactions of quiescent Bragg grating (BG) solitons in a semilinear dual-core system, where one core is nonlinear and is equipped with a BG with dispersive reflectivity and the other core is linear are investigated. It has been previously shown that the model supports stable quiescent solitons which may or may not have sidelobes in their profiles. The interaction of in-phase solitons can lead to various outcomes such as generation of two moving solitons with equal or unequal velocities, merger of solitons into a quiescent one, generation of three solitons (one quiescent and two moving ones) and destruction of solitons. The presence of sidelobes can radically change the interaction dynamics, e.g. in-phase solitons can repel or form a temporary bound state which subsequently splits into two separating solitons. We have identified the outcome regions for in-phase interactions through systematic numerical simulations in the plane of dispersive reflectivity vs. frequency. We have also analysed the effect of initial separation on the outcomes of interactions. It is found that when sidelobes are present in solitons’ profiles, the outcomes of the interactions are strongly dependent on the initial separation.     

Very small IF resonator filters using reflection of shear horizontal wave at free edges of substrate

A conventional surface acoustic wave (SAW) resonator filter requires reflectors consisting of numerous grating fingers on both sides of interdigital transducers (IDTs). On the contrary, it is considered that small-sized and low loss resonator filters without reflectors consisting of grating fingers can be realized by exploiting this characteristic of the shear horizontal (SH) wave or the Bleustein-Gulyaev-Shimizu (BGS) wave. There are two types of resonator filters: transversely coupled and longitudinally coupled. No transversely coupled filters (neither conventional nor edge-reflection) using the SH wave on a single-crystal substrate have been realized until now, because two transverse modes (symmetrical and asymmetrical modes) are not easily coupled. However, the authors have realized small low loss transversely coupled resonator filters in the range of 25 to 52 MHz using edge reflections of the BGS wave on piezoelectric ceramic (PZT: Pb(Zr,Ti)O/sub 3/) substrates for the first time by developing methods by which the two transverse modes could be coupled. Also the authors have realized small low loss longitudinally coupled resonator filters in the range of 40 to 190 MHz using edge reflection of BGS or SH waves on PZT or 36/spl deg/-rotated-Y X-propagation LiTaO/sub 3/ substrates for the first time. Despite being IF filters, their package (3/spl times/3/spl times/1.03 mm/sup 3/) sizes are as small as those of RF SAW filters.     

Multiparameter tuning of surface-acoustic-wave filters

An experimental methodology for tuning the frequency response of surface-acoustic-wave (SAW) filters after initial fabrication is demonstrated. The sensitivity of such a filter to perturbations in the length of individual electrodes of the interdigital transducer is determined from highly accurate swept-frequency measurements. Sensitivity measurements from several electrodes are then used to synthesize improved filter responses. Sensitivity measurements, measurement accuracy, and control of frequency-response sidelobes are demonstrated for devices fabricated on (YZ) lithium niobate. The tuning method provides predictions of improvement that closely match actual performance.     

Porous silicon-based rugate filters

We report an experimental study of porous silicon-based rugate filters. We performed filter apodization, following a half-apodization approach, which successfully attenuated the sidelobes at both sides of the photonic stop band. We achieved successful reduction of interference ripples through the insertion of index-matching layers on the first and last interfaces. An apodized dielectric mirror and a rugate filter are compared: Appreciable differences in the harmonic presence and stop-band performance were observed and are commented on. Bandwidth control when index contrast is modified is also demonstrated. Finally, the possibility of combining different rugate filter designs to attain more complex responses is demonstrated by the achievement of a multi-stop-band filter. Numerical calculations for design optimization and comparison with experimental data are reported too.     

Birefringent filter synthesis by use of a digital filter design algorithm

We present an efficient method for designing birefringent filters comprising a number of birefringent sections with equal length and arbitrary orientation between two polarizers and for producing a specified spectral response in transmission. The method uses a digital filter design algorithm (i.e., the Remez algorithm) to determine an optimal polynomial approximation to obtain a specified finite impulse response, and a layer-peeling algorithm to calculate the filter structure parameters. The design procedure is demonstrated for a 14-section bandpass filter with sidelobes below -40 dB. The influence of errors in length and orientation of the birefringent sections on the filter's spectral response is also discussed.     

Low-loss birefringent spectral filters comprising three identical retarders

Basic fan or folded arrangements comprising three identical retarders in cascade between parallel or crossed polarizers are presented for constructing birefringent spectral filters without an internal polarizer. The retarders' axes are oriented with an angle increment of theta or -theta preferably with /theta/ 相似文献   

Scale and translation invariant minimum average correlation energy filter

A new type, to our knowledge, of scale and translation invariant correlation filter is described. Its form in polar coordinates generalizes the forms of the one-decomposition-term filters. That is combined with the minimum-average-correlation-energy optimization method to suppress the sidelobes and achieve correlation peaks. A theoretical analysis as well as a detailed explanation of the computational procedure is provided. The concept is tested on five interferometric images of 256 gray levels without preprocessing. The computed filter gives correlation peaks for reference image scales in the range of 0.3/4.0 (minimal scale: maximal scale = 1:13). The discrimination ability of the filter is investigated-no false peak occurred. The filter works in the background so that there is no need for input image segmentation. A method for extension is described to calculate a filter for more than one reference image.     

Performance analysis of a fiber Bragg grating filter-based strain/temperature sensing system based on a modified Gaussian function approximation method

This paper analyzes the performance of a fiber Bragg grating (FBG) filter-based strain and/or temperature sensing system based on a modified Gaussian function (MGF) approximation method. Instead of using a conventional Gaussian function, we propose the MGF, which can capture the characteristics of the sidelobes of the reflected spectrum, to model the FBG sensor and filter. We experimentally demonstrate that, by considering the contributions of the sidelobes with the MGF approximation method, behaviors of the FBG filter-based FBG displacement and/or temperature sensing system can be predicted more accurately. The predicted behaviors include the saturation, the sensitivity, the sensing range, and the optimal initial Bragg wavelengths of the FBG sensing system.     

Multi-range-resolution radar using inverse filters

The resolution of a radar range is generally fixed regardless of the detection range. The proposed receiving system for radars is configured with multiple inverse filters so that it has different range resolutions depending on the detection range, and the signal is compressed to a narrow arbitrary pulse width of less than or equal to the reciprocal of the spectrum bandwidth of the transmitted signal. Then the proposed receiving system may be able to have no sidelobes. The frequency response function of each inverse filter used in the receiver is expressed as D( f )/S( f ), where D( f ) is the Fourier transform of the desired output waveform d(t) and S( f ) is the Fourier transform of the transmitted waveform s(t). Specific signal examples are used to clarify what sort of D( f ) and S( f ) are suitable for preventing D( f )/S( f ) from diverging. With regard to this proposed receiving system, a theoretical improvement factor is indicated for the signal-to-noise ratio, and simulations and experiments are conducted to confirm the validity of the proposed receiving system.