Biomedical subjects

Seshadri Srinivasan

Publications and source records attributed to Seshadri Srinivasan.

8 recordsLinked to original sources

PubMed2005

Enhancing the performance of model-based elastography by incorporating additional a priori information in the modulus image reconstruction process.

Model-based elastography is fraught with problems owing to the ill-posed nature of the inverse elasticity problem. To overcome this limitation, we have recently developed a novel inversion scheme that incorporates a priori information concerning the mechanical properties of the underlying tissue structures, and the variance incurred during displacement estimation in the modulus image reconstruction process. The information was procured by employing standard strain imaging methodology, and introduced in the reconstruction process through the generalized Tikhonov approach. In this paper, we report the results of experiments conducted on gelatin phantoms to evaluate the performance of modulus elastograms computed with the generalized Tikhonov (GTK) estimation criterion relative to those computed by employing the un-weighted least-squares estimation criterion, the weighted least-squares estimation criterion and the standard Tikhonov method (i.e., the generalized Tikhonov method with no modulus prior). The results indicate that modulus elastograms computed with the generalized Tikhonov approach had superior elastographic contrast discrimination and contrast recovery. In addition, image reconstruction was more resilient to structural decorrelation noise when additional constraints were imposed on the reconstruction process through the GTK method.

Algorithms↗

PubMed2005

Comparative evaluation of strain-based and model-based modulus elastography.

Elastography based on strain imaging currently endures mechanical artefacts and limited contrast transfer efficiency. Solving the inverse elasticity problem (IEP) should obviate these difficulties; however, this approach to elastography is often fraught with problems because of the ill-posed nature of the IEP. The aim of the present study was to determine how the quality of modulus elastograms computed by solving the IEP compared with those produced using standard strain imaging methodology. Strain-based modulus elastograms (i.e., modulus elastograms computed by simply inverting strain elastograms based on the assumption of stress uniformity) and model-based modulus elastograms (i.e., modulus elastograms computed by solving the IEP) were computed from a common cohort of simulated and gelatin-based phantoms that contained inclusions of varying size and modulus contrast. The ensuing elastograms were evaluated by employing the contrast-to-noise ratio (CNR(e)) and the contrast transfer efficiency (CTE(e)) performance metrics. The results demonstrated that, at a fixed spatial resolution, the CNR(e) of strain-based modulus elastograms was statistically equivalent to those computed by solving the IEP. At low modulus contrast, the CTE(e) of both elastographic imaging approaches was comparable; however, at high modulus, the CTE(e) of model-based modulus elastograms was superior.

Algorithms↗

PubMed2004

Comparing elastographic strain images with modulus images obtained using nanoindentation: preliminary results using phantoms and tissue samples.

Conventional elastography involves quasistatic mechanical compression (external or internal) of the tissue under ultrasonic insonification to obtain radiofrequency (RF) A-lines before and after compression. Cross-correlation of the pre- and postcompression A-lines results in displacement images with axial gradients that produce the strain images (strain elastograms). Though the strain elastograms show structural similarities to the modulus images, they are not related in a simple way to the modulus images because the strains depend on both modulus and geometry of the materials being deformed. Therefore, a quantification of the similarities between the strain and modulus images may enhance the interpretation confidence of strain elastograms in depicting tissue structure. To demonstrate similarities between modulus images and strain elastograms, a feasibility study of using nanoindentation to obtain modulus images of thin slices of tissue and tissue-mimicking phantoms (agar-gelatin mixtures) was performed first, with encouraging results. This was followed by a comparison of modulus images and strain elastograms obtained from the same sample slices. The experimental results indicated that, under certain experimental conditions, it is feasible to perform quantitative comparisons between strain images (using elastography) and modulus images. A good visual, as well as quantitative, correspondence between structures in the modulus and strain images could be obtained at a 3-mm scale.

Agar↗

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↗

PubMed2003

Visualisation of HIFU lesions using elastography of the human prostate in vivo: preliminary results.

An imaging system was developed for prostate elastography in vivo using a transrectal ultrasound (US) probe to guide high-intensity focused US (HIFU) therapy of prostate cancer. Uniform compression was applied using a balloon, while a sector image was acquired. Strain was calculated from the gradient of the displacements obtained from the ultrasonic signal using the cross-correlation technique. Elastograms were acquired on a total of 31 patients undergoing HIFU therapy for localised prostate cancer. For two patients, only part of the prostate was treated and posttherapy magnetic resonance imaging (MRI) confirmed the size and position of the HIFU lesions seen in the elastograms as low strain areas, with a strain contrast ratio between 1.6 and 3.2. The whole prostate was treated for the next 29 patients. After treatment, the whole prostate appeared to be stiff in the elastograms and a 40% to 60% (mean 50%) decrease in average strain was observed when compared to strains measured before HIFU application. Tumours identified by biopsies and sonograms could occasionally be seen in the preoperative elastograms. Decorrelation effects occurred mainly because of low sonographic signal-to-noise ratio (SNR) and of out-of-plane motion induced by respiration.

Adenocarcinoma↗

PubMed2003

Elastography imaging of small animal oncology models: a feasibility study.

To test the feasibility of applying ultrasonic elastography on small animal oncology models, experiments were performed in vitro and in situ on murine mammary lesions induced exogenously by tumor cell line 66.3. In vitro studies involved three 1-week-old excised tumors embedded in a phantom block with ultrasonic properties similar to those of soft biologic tissues. In situ studies involved five mice whose bodies were embedded in pure gelatin blocks. The data were acquired from the blocks with a clinical scanner modified to have an automated compressor assembly and processed to construct the elastograms at various imaging planes within each block. The results were analyzed both qualitatively and quantitatively to assess the merits of the elastographic imaging and its limitations for in vivo serial studies of tumors in small animal oncology models.

Animals↗

PubMed2003

Differences in the compressive stress-strain response of infiltrating ductal carcinomas with and without lobular features--implications for mammography and elastography.

In a previous study, it was noted that in some cases when compressive strains greater than about 5% were applied to tumors removed from the breast, there was an abrupt and irreversible change in the tissue stiffness. The data from that study were further analyzed and infiltrating ductal carcinomas with and without lobular features were selected for additional testing to explore their behavior under compressive strains from 0-10%. Fresh tumor samples were tested using a servo-hydraulic Instron testing machine to apply ramp type displacement loads to the samples. The results show that when strains greater than 5% are applied to the tumor tissue without lobular features, there is an irreversible decrease in the stiffness of the tissue while no such change is noted in the other tumor tissue. The implications for this behavior in making mammographic and elastographic images of the breast were then explored using finite element simulations to determine under what compression conditions could the critical strain threshold be reached in the tumor tissue.

Breast Neoplasms↗