Seth D. Silverstein

Ultrasound scattering model: 2-D cross-correlation and focusing criteria-theory, simulations, and experiments

A microscopic scattering model is developed to expedite simulation studies of ultrasound imaging in soft tissue using multichannel transducer probes. The model fully accounts for the physics of broadband signals, propagating wave packets, and time delay focusing. Analytical results are presented for 2-D transducer arrays; 1-D results can be trivially extracted by setting the number of rows equal to unity. The 2-D cross-correlation and the 2-D form of the Mallart-Fink (MF) focusing factor are calculated. It is demonstrated that the scattering model reduces to the 2-D form of the monochromatic van Cittert Zernike (VCZ) analysis. Simulation results for the focusing factor are presented, and comparisons are given between the values obtained from simulation, analytical theory, and actual water tank experiments. The comparative results are ail in close accord with each other.

Autofocusing in medical ultrasound: the scaled covariance matrix algorithm

This work develops a class of ultrasound phase aberration correction/autofocusing algorithms that are based upon the properties of the covariance matrix of the channel signals for time-delay focused resolution/speckle cells. The scaled covariance matrix SCM algorithms are designed to blindly estimate and correct focusing timing errors due to thin layers of unanticipated fatty tissue located in the near field of the transducer array. An important aspect of the algorithm is that the scaling of the covariance matrix elements fundamentally establishes a channel independent phase reference relative to which the aberrant channel phases are estimated. The model development involved the combination of a rigorous mathematical analysis of the scattering of ultrasound in random scattering media and extensive statistical simulation studies with phase aberrations imposed upon both the transmit and received channel signals. Under the assumption of a near field aberration model, the statistical simulation analyses showed that the SCM algorithms in simulation are capable of accurately estimating relative time delay channel errors with RMS timing errors up to /spl sim/62 ns, with interchannel correlation lengths as short as 1.4 mm.

A new use of ESPRIT for detection and model order estimation

This work studies the distribution of the ESPRIT roots in the complex z plane. Real source roots are tightly clustered about their true values, while pseudoroots corresponding to overestimated nonexisting sources are distributed broadly with a phase angle bias away from the real source roots. The ESPRIT roots distributions can be coupled with Neyman-Pearson tests for detection/system model order estimation. A mathematical derivation of the asymptotic distribution of the pseudoroots is given.<<ETX>>

Synthetic aperture radar image signatures of rotating objects

This work develops the theory of image signatures of rotating objects in synthetic aperture radar images. The mathematical results are similar in form to the expressions describing vibrating objects. The differences lie mainly in the fact that the expansion parameters in the phase for rotating objects are typically too large to be expanded to first order, as is done in the micro-Doppler vibrational image analysis. These results have potential application in target recognition systems, as rotating components of larger targets cause specific types of identifiable image aberrations.

A robust auto-focusing algorithm for medical ultrasound: consistent phase references from scaled cross-correlation functions

This work features a robust auto-focusing phase aberration correction algorithm for digitally sampled coherent imaging systems such as medical ultrasound. The robust nature is attributed to the establishment of a consistent phase reference relative to which the aberrant channel phases are estimated. The algorithm has its foundation in detailed theoretical analyses of the scattering of ultrasound wave packets in a random scattering medium. Illustrated Monte Carlo simulations exhibit excellent algorithmic performance.

Algorithms for remote calibration of active phased array antennas for communication satellites

This article describes two algorithms that are effective for the remote calibration of an N/sub e/ element active phased array antenna. These algorithms involve transmission of orthogonal encoded signals. The unitary transform encoding (UTE) algorithm is most suited for digital beamforming as it requires additional encoding hardware for an analog implementation. The control circuit encoding (CCE) algorithm is ideally suited for analog beamformers as it requires no additional encoding hardware. The CCE method encodes phased array elemental signals using a Hadamard matrix to control the switching of intrinsic phase shifter delay circuits. The UTE and CCE algorithms can reduce the average measurement integration times for the complete set of calibration parameters by /spl sim/N/sub e/ relative to the corresponding values for single-element calibration procedures.

Increasing the Depth of Focus in Medical Ultrasound B-Scan

Obtaining hight quality ultrasound images at high frame rates has great medical importance, especially in applications where tissue motion is significant (e.g. the beating heart). Dynamic focusing and dynamic apodization can improve image quality significantly, and they have been implemented on the receive beam in state-of-the-art medical ultrasound systems. However implementing dynamic focusing and dynamic apodization on the transmit beam compromises frame rate. We present a novel transmit apodization scheme where a continuum of focal points can be obtained in one transmission, and uniform sensitivity and uniform point spread function can be achieved over very large range without reducing frame rate. Preliminary simulations demonstrate significant promises of the new technique.

Quadratic equalization: a method for producing extended uniform depth of focus in high frame rate medical ultrasound B-scans

This work describes a new and novel method for achieving the beneficial imaging effects that can be obtained from the coherent superposition of multiple firings with a uniform distribution of transmit foci, with a single transmit firing. It is demonstrated that a sine amplitude modulation of the transmit windowing function with an argument quadratic in the transducer coordinates is essentially equivalent to a synthetic image generated from the coherent superposition of an infinite number of firings with their transmit foci uniformly covering the full range of an extended B-scan. Using these methods, simulation results give range compensated peaks of the range energy impulse response (measure of image brightness) that are flat within /spl sim/ /spl plusmn/0.2 dB over the range scan of the B-mode image at fractional bandwidths commensurate with range resolutions down to /spl sim/ 0.25 mm.

Near-field three-dimensional coherent imaging: Theory and simulations

This work presents a rigorous mathematical derivation of an effective approximate solution to the three-dimensional inverse scattering/imaging problem that is applicable for all imaging zones ranging from the near to the far field. Simulation results for the point spread function illustrate the range and cross-range resolution as a function of the optical f number. The model system operates in a synthetic aperture type mode, where the coherent signals are transmitted, and the scattered signals are subsequently received at individual transmitters and receivers. Potential applications of this technology include: Medical ultrasound, foliage penetrating synthetic aperture radar, ground penetrating radar for land mine detection, and electromagnetic millimeter-wave scanning for concealed weapon detection.

Emulating Dynamic Target in Compact Radar Ranges. Part I: Theory

Analytical expressions are developed that describe the artifacts encountered when translating, rotating, and vibrating point sources are imaged by compact radar ranges (CRRs) emulating airborne synthetic-aperture radar (SAR). The approach starts with coherent-aperture imaging basics and develops a general solution for imaging using the Born approximation. We show that moving-target artifacts on CRRs are similar to the artifacts encountered with SAR moving targets, suggesting that CRRs may be suitable for such emulations.