Biomedical subjects

S Mensah

Publications and source records attributed to S Mensah.

3 recordsLinked to original sources

PubMed2004

Diffraction tomography: a geometrical distortion free procedure.

Ductal echography (DE) is a recent anatomically-led method of investigation of internal mammary structures that provides direct observation of ductolobular structures in mammary lobes. Indeed, breast cancer initiates from the epithelium (hypoechogenic) and develops first in ductolobular structures. Our research aims at developing an ultrasonic scanner (hemispherical antenna) that will allow three-dimensional measurement of the field diffracted by the gland. The present reconstruction procedure is an alternative solution to the standard elliptic back-projection (EBP) technique based on the Born approximation (weak scattering assumption). The latter implicitly assumes that the sound speed is (almost) constant within the breast. This unrealistic a priori is inconsistent with our heterogeneous media characterization purpose, and leads both to geometrical distortion in the reconstruction and to poor focusing of the backpropagated waves (low contrast imaging). However, the EBP technique retains a two-fold advantage: firstly it does not make any other assumption concerning the distribution of mechanical parameters apart from low gradients. Secondly, it shows great robustness, a high resolving power, and is easy to implement. Thus, in order to account for strong wavefront distortions, we integrate temporal compensation of the scattered signals acquired into the scattering EBP technique. The adjustment relies on a time of flight estimation based on a "layer stripping" approach. Numerical tests based on finite difference time domain simulations of data scattered by a random tissue-like phantom are proposed.

Algorithms↗

PubMed2002

Circular antenna for breast ultrasonic diffraction tomography.

Compared to echography, which exploits only the reflected field, ultrasonic diffraction tomography improves image resolution by combining the total diffracted field. For breast cancer imaging, this improvement reinforces contrast between various breast tissues and structures by eliminating some interference phenomena such as speckle and then allowing parameterization of the images. Our work concerns the development of an experimental set-up for fast acquisition of the diffracted field and construction of two-dimensional tomographic images. For this purpose, we developed a multichannel ultrasound circular antenna with eight focused transducers.

Algorithms↗

PubMed1997

High resolution low frequency ultrasonic tomography.

Ultrasonic reflection tomography results from a linearization of the inverse acoustic scattering problem, named the inverse Born approximation. The goal of ultrasonic reflection tomography is to obtain reflectivity images from backscattered measurements. This is a Fourier synthesis problem and the first step is to correctly cover the frequency space of the object. For this inverse problem, we use the classical algorithm of tomographic reconstruction by summation of filtered backprojections. In practice, only a limited number of views are available with our mechanical rig, typically 180, and the frequency bandwidth of the pulses is very limited, typically one octave. The resolving power of the system is them limited by the bandwidth of the pulse. Low and high frequencies can be restored by use of a deconvolution algorithm that enhances resolution. We used a deconvolution technique based on the Papoulis method. The advantage of this technique is conservation of the overall frequency information content of the signals. The enhancement procedure was tested by imaging a square aluminium rod with a cross-section less than the wavelength. In this application, the central frequency of the transducer was 250 kHz so that the central wavelength was 6 mm whereas the cross-section of the rod was 4 mm. Although the Born approximation was not theoretically valid in this case (high contrast), a good reconstruction was obtained.

Computer Simulation↗