Biomedical subjects

Tomas Olofsson

Publications and source records attributed to Tomas Olofsson.

3 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

Semi-sparse deconvolution robust to uncertainties in the impulse responses.

The received signal in ultrasonic pulse-echo inspection can be modeled as a convolution between an impulse response, or prototype, and the reflection sequence that is the impulse characteristic of the inspected object. Deconvolution aims at approximately inverting this process to improve the time resolution so that the overlap between echoes from closely spaced reflectors becomes small. For the relatively high contrast reflection sequences often found in non-destructive testing applications, semi-sparse deconvolution algorithms will typically yield better results than the classical Wiener filter solution. However, this requires a prototype that is a good representative for all echo responses found in the signals. Since, in practice, the prototype is often chosen by the operator directly from the inspection data, the prototype may incidentally be a bad representative for modeling the echoes for the remaining part of the object. Because of the sensitivity to deviations in the prototype this can yield deconvolution results with poor reproducibility. This paper presents a new semi-sparse deconvolution algorithm that is robust to deviations in the prototype. The new robust algorithm is based on a modification of an earlier presented non-robust semi-sparse algorithm. The robustness is obtained by including a stochastic model of the variations in the prototypes to the signal model when deriving the algorithm. Experiments performed using simulated data verify that the robust algorithm is less sensitive to deviations in the prototypes compared to the non-robust version of the algorithm and show that the proposed algorithm yields better estimates than its non-robust version and the Wiener filter in scenarios for which the algorithm was derived. Results using real ultrasonic data further show that the algorithm can be useful in practical scenarios where similar deconvolution results are required from slightly different echoes.

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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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