Biomedical subjects

Tadeusz Stepinski

Publications and source records attributed to Tadeusz Stepinski.

4 recordsLinked to original sources

PubMed2006

An iterative synthetic aperture imaging algorithm with correction of diffraction effects.

In this paper we present an iterative version of the synthetic aperture imaging algorithm extended synthetic aperture technique (ESAFT) proposed recently. The algorithm is based on a linear model that accounts for the distortions effects of an imaging system used for acquisition of ultrasonic data. Improved resolution (both lateral and temporal) in the reconstructed image is obtained as a result of minimizing the reconstruction mean square error. In this work, the minimization is extended to parameters that characterize expected amplitudes of each image element in the area of interest. An iterative optimization scheme is proposed, which in each step performs minimization of the reconstruction error based on the parameter matrix found in the previous step. Comparing to ESAFT, the proposed approach yields a significant improvement in resolution and a high degree of robustness with regard to initial choice of the parameter matrix. Performance of the proposed algorithm is evaluated using both real and simulated ultrasonic data.

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PubMed2004

Trapping of microparticles in the near field of an ultrasonic transducer.

We are investigating means of handling microparticles in microfluidic systems, in particular localized acoustic trapping of microparticles in a flow-through device. Standing ultrasonic waves were generated across a microfluidic channel by ultrasonic microtransducers integrated in one of the channel walls. Particles in a fluid passing a transducer were drawn to pressure minima in the acoustic field, thereby being trapped and confined at the lateral position of the transducer. The spatial distribution of trapped particles was evaluated and compared with calculated acoustic intensity distributions. The particle trapping was found to be strongly affected by near field pressure variations due to diffraction effects associated with the finite sized transducer element. Since laterally confining radiation forces are proportional to gradients in the acoustic energy density, these near field pressure variations may be used to get strong trapping forces, thus increasing the lateral trapping efficiency of the device. In the experiments, particles were successfully trapped in linear fluid flow rates up to 1mm/s. It is anticipated that acoustic trapping using integrated transducers can be exploited in miniaturised total chemical analysis systems (microTAS), where e.g. microbeads with immobilised antibodies can be trapped in arrays and subjected to minute amounts of sample followed by a reaction, detected using fluorescence.

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PubMed2003

Synthetic aperture imaging using sources with finite aperture: deconvolution of the spatial impulse response.

A method for ultrasonic synthetic aperture imaging using finite-sized transducers is introduced that is based on a compact, linear, discrete model of the ultrasonic measurement system developed using matrix formalism. Using this model a time-domain algorithm for deconvolution of the transducer's spatial impulse responses (SIRs) is developed that is based on a minimum mean square error (MMSE) criterion. The algorithm takes the form of a spatiotemporal filter that compensates for the SIRs associated with a finite-sized transducer at every point of the processed image. A major advantage of the proposed method is that it can be used for any transducer, provided that its associated SIRs are known. This is in contrast to the synthetic aperture focusing technique (SAFT), which treats the transducer as a point source. The performance of the method is evaluated with simulations and experiments, performed in water using a linear phased array. The results obtained using the proposed method are compared to those obtained with a classical time-domain SAFT algorithm. For a finite aperture source, it is clearly shown that the resolution obtained using the proposed method is superior to that obtained using the SAFT algorithm.

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PubMed2002

A spatial impulse response based method for determining effective geometrical parameters for spherically focused transducers.

This paper proposes a novel method for determining effective geometrical parameters (GPs) of spherically focused transducer. Theoretical basis of the method is the spatial impulse response method (SIRM) that is a powerful tool to calculate transient fields from a piston-like transducer. Experimentally, the method is implemented by measuring arrival times of the pulse signals (either transmission or pulse echo signals) and using them to determine the time intervals between the direct and edge wave pulses. Since the time intervals for a given measurement position depend on the transducer's GPs, then the effective GPs can be obtained. The measurements are carried out in the near-field so that the time intervals can be resolved. The proposed method has been applied to a spherically focused transducer operating in pulse echo mode. The results have shown that the effective GPs used in the SIRM give a closer agreement between the theory and the measurements than the nominal GPs. The method is best suited for characterizing broadband transducers with short pulse excitations. With certain modification it can be also applied to narrowband transducers. The method can be also applied to transducers of other shapes, e.g., planar circle, and planar or curved rectangle provided the piston approximation is appropriate.

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