Biomedical subjects

Raffaella Righetti

Publications and source records attributed to Raffaella Righetti.

6 recordsLinked to original sources

PubMed2006

Resolution of axial shear strain elastography.

The technique of mapping the local axial component of the shear strain due to quasi-static axial compression is defined as axial shear strain elastography. In this paper, the spatial resolution of axial shear strain elastography is investigated through simulations, using an elastically stiff cylindrical lesion embedded in a homogeneously softer background. Resolution was defined as the smallest size of the inclusion for which the strain value at the inclusion/background interface was greater than the average of the axial shear strain values at the interface and inside the inclusion. The resolution was measured from the axial shear strain profile oriented at 45 degrees to the axis of beam propagation, due to the absence of axial shear strain along the normal directions. The effects of the ultrasound system parameters such as bandwidth, beamwidth and transducer element pitch along with signal processing parameters such as correlation window length (W) and axial shift (DeltaW) on the estimated resolution were investigated. The results show that the resolution (at 45 degrees orientation) is determined by the bandwidth and the beamwidth. However, the upper bound on the resolution is limited by the larger of the beamwidth and the window length, which is scaled inversely to the bandwidth. The results also show that the resolution is proportional to the pitch and not significantly affected by the axial window shift.

Algorithms↗

PubMed2005

A method for generating permeability elastograms and Poisson's ratio time-constant elastograms.

The feasibility of imaging the permeability and Poisson's ratio time-constant of porous media was investigated. The study involved the following steps. First, poroelastograms were generated from porous tofu phantoms under sustained compression. The sample materials used for the experiments were previously characterized through independent mechanical measurements. Second, corresponding Poisson's ratio time-constant elastograms were generated by calculating and displaying the decay time-constants of the local Poisson's ratios over the time interval in which the poroelastograms were acquired. Finally, for homogeneous samples, permeability elastograms were generated using the poroelastograms in combination with a previously proposed biphasic theoretical model, under very specific conditions. A comparison between the results obtained using poroelastography and those obtained through independent mechanical measurements suggests that poroelastography may be used for imaging the local time-dependent behavior of poroelastic media.

Computer Simulation↗

PubMed2005

A new method for generating poroelastograms in noisy environments.

Poroelastography has been recently introduced as a new elastographic technique that may be used to describe the spatial and temporal behavior of poroelastic materials. The experimental methodology proposed thus far for phantoms and tissues in vitro requires the acquisition of a precompression rf frame, the application of a unit step strain compression to the sample and the acquisition of subsequent post-compression frames from the material. Elastograms and poroelastograms are generated by cross-correlating the sequentially-acquired postcompression frames with the reference precompression frame. The application of poroelastography to tissues in vivo must address the echo decorrelation problems that are encountered due to uncontrolled tissue motion, which may become significant shortly after the acquisition of the precompression frame. In this paper, we investigate the feasibility of performing poroelastography experiments using an alternative experimental scheme. In the proposed experimental methodology, the reference precompression frame is continuously moved while the time interval between the frames that are correlated is kept short. This allows long data acquisition times with simultaneous minimization of the decorrelation due to undesired tissue motion in vivo. We validated this new method using both a step and a ramp compression functions. We performed poroelastographic simulations and experiments in phantoms and in tissues in vivo. The results were compared to those obtained using the traditional acquisition methodology. This study shows that the two methods yield similar results in vitro and suggests that the new method may be more robust to decorrelation noise in applications in vivo.

Arm↗

PubMed2004

The feasibility of using elastography for imaging the Poisson's ratio in porous media.

The feasibility of using elastography for experimentally estimating and imaging the Poisson's ratio of porous media under drained and undrained conditions was investigated. Using standard elastographic procedures, static and time-sequenced poroelastograms (strain ratio images) of homogeneous cylindrical gelatin and commercially available tofu samples were generated under sustained applied axial strain. The experimental data show similar trends to those that were observed in finite-elements simulations, and to those that were calculated from classical theoretical models proposed for biphasic materials with similar mechanical properties. To demonstrate the applicability of elastography to monitor time-dependent changes in nonhomogeneous porous structures as well, preliminary time-sequenced poroelastograms were obtained from two-layer porous phantoms and porcine muscle samples in vitro. The results suggest that elastography may have significant potential for quantitatively mapping the time-dependent mechanical behavior of poroelastic media, which is related to the dynamics of fluid flow and to the elasticity and permeability parameters of the media.

Animals↗

PubMed2003

Lateral resolution in elastography.

The factors that control the lateral resolution in elastography were investigated using a simulation study. The lateral resolution was estimated from the simulated axial strain elastograms as the smallest measurable distance between two equally stiff lesions embedded in a homogeneously softer background. The lesions were symmetrically positioned lateral to the center of the target, at the focus of the transducer. Ultrasound (US) systems with different transducer frequencies, bandwidths and f-numbers were simulated. The effects of the ultrasonic parameters, the lateral spacing between adjacent echo signals, the cross-correlation window length, the lesion/background elastic contrast and the lateral motion of scatterers on the estimated lateral resolution were investigated. The results show that the lateral resolution in elastography is proportional to the beam width of the US system used to acquire the data, and is on the same order as the sonographic lateral resolution.

Computer Simulation↗

PubMed2002

Axial resolution in elastography.

The limits and trade-offs of the axial resolution in elastography were investigated using a controlled simulation study. The axial resolution in elastography was estimated as the distance between the full widths at half-maximum of the strain profiles of two equally stiff lesions embedded in a softer homogeneous background. The results show that the upper bound of the axial resolution in elastography is controlled by the physical wave parameters of the ultrasound (US) system used to acquire the data (transducer center frequency and band- width). However, an inappropriate choice of the parameters used to process the US data (cross-correlation window length and shift between consecutive windows) may compromise the best resolution attainable. The measured elastographic axial resolution was found to be on the order of the ultrasonic wavelength.

Computer Simulation↗