Biomedical subjects

S K Alam

Publications and source records attributed to S K Alam.

16 recordsLinked to original sources

PubMed2004

Theoretical derivation of SNR, CNR and spatial resolution for a local adaptive strain estimator for elastography.

Conventional techniques in elastography estimate the axial strain as the gradient of the displacement (time-delay) estimates obtained using cross-correlation of pre- and temporally stretched postcompression radiofrequency (RF) A-line segments. The use of a constant stretch factor for stretching the postcompression A-line is not adequate in the presence of heterogeneous targets that are commonly encountered. This led to the development of several adaptive strain estimation techniques in elastography. Yet, a theoretical framework for the image quality of adaptive strain estimation has not been established. In this work, we develop theoretical expressions for the image quality [measured in terms of the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and spatial resolution] of elastograms obtained using an adaptive strain estimator developed by Alam et al. (1998). We show a linear trade-off between the SNR and axial resolution of the strain elastogram with respect to the window length used for strain estimation. The CNR shows a quadratic tradeoff with the axial resolution with respect to the window length. The SNR, CNR and axial resolution are shown to improve with the ultrasonic bandwidth.

Algorithms↗

PubMed2002

Elastographic imaging using staggered strain estimates.

Conventional techniques in elastography estimate strain as the gradient of the displacement estimates obtained through crosscorrelation of pre- and postcompression rf A-lines. In these techniques, the displacements are estimated over overlapping windows and the strains are estimated as the gradient of the displacement estimates over adjacent windows. The large amount ofnoise at high window overlaps may result in poor quality elastograms, thus restricting the applicability of conventional strain estimation techniques to low window overlaps, which, in turn, results in a small number of pixels in the image. To overcome this restriction, we propose a multistep strain estimation technique. It computes the first elastogram using nonoverlapped windows. In the next step, the data windows are shifted by a small distance (small fraction of window size) and another elastogram is produced. This is repeated until the cumulative shift equals/exceeds the window size and all the elastograms are staggered to produce the final elastogram. Simulations and experiments were performed using this technique to demonstrate significant improvement in the elastographic signal-to-noise ratio (SNRe) and the contrast-to-noise ratio (CNRe) at high window overlaps over conventional strain estimation techniques, without noticeable loss of spatial resolution. This technique might be suitable for reducing the algorithmic noise in the elastograms at high window overlaps.

Computer Simulation↗

PubMed2001

Organochlorine pesticides and heavy metals in muscle and ovaries of Gulf coast striped bass (Morone saxatilis) from the Apalachicola River, Florida, USA.

Eight female Gulf coast striped bass (Morone saxatilis) broodfish collected for induced spawning from the Apalachicola River below the Jim Woodruff Lock and Dam were analyzed for organochlorine pesticides (OCs) and metals in muscle and ovarian tissues. Chemical analyses revealed that muscle and ovaries contained detectable amounts of OCs and metals. Concentrations of p,p'-DDE, a derivative of the pesticide DDT, in muscle and ovary (0.54 and 0.65 microg/g, respectively) were significantly higher than alpha-chlordane, dieldrin, and p,p'-DDD. The presence of p,p'-DDE, an antiandrogenic compound, in females suggests that the compound also may be present in male striped bass. Concentrations of Cr, Hg, Mg, and Mn were higher in muscle than in ovarian tissues. Concentrations of Hg have almost doubled in muscle tissues (0.85 microg/g) and tripled in ovaries (0. 15 microg/g) in our samples, compared with the data from 1986 to 1989. Organochlorine pesticides and metal contaminants were present in muscle and ovarian tissues of adult females and may have been retarding development of eggs leading to low hatching rates.

Animals↗

PubMed2000

Direct strain estimation in elastography using spectral cross-correlation.

Spectral estimation of tissue strain has been performed previously by using the centroid shift of the power spectrum or by estimating the variation in the mean scatterer spacing in the spectral domain. The centroid shift method illustrates the robustness of the direct, incoherent strain estimator. In this paper, we present a strain estimator that uses spectral cross-correlation of the pre- and postcompression power spectrum. The centroid shift estimator estimates strain from the mean center frequency shift, while the spectral cross-correlation estimates the shift over the entire spectrum. Spectral cross-correlation is shown to be more sensitive to small shifts in the power spectrum and, thus, provides better estimation for smaller strains when compared to the spectral centroid shift. Spectral cross-correlation shares all the advantages gained using the spectral centroid shift, in addition to providing accurate and precise strain estimation for small strains. The variance and noise properties of the spectral strain estimators quantified by their respective strain filters are also presented.

Acoustics↗

PubMed2000

Organochlorine, PCB, PAH, and metal concentrations in eggs of loggerhead sea turtles (Caretta caretta) from northwest Florida, USA.

Composite samples of unhatched and physically unaltered loggerhead sea turtle, Caretta caretta, eggs collected from 20 nests along northwest Florida were analyzed for organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and metals. Chemical analyses revealed that turtle eggs contained detectable amounts of metals, PAHs, and PCBs. Only one OCP, p,p'-DDD, was detected, and its presence was restricted to eggs from two nesting sites. None of the PCB concentrations exceeded the Food and Drug Administration's (FDA) action limit. Concentrations of dioxin-like PCB congeners, 105, 118, and 126, and total PCBs were also detected and are contributors to the toxic burden of loggerhead sea turtle eggs. Concentrations of PAHs, 1,2,5,6-dibenzanthracene, 1-methyl naphthalene, C1-naphthalene and naphthalene were variable at nesting sites. Comparison of mean metal burdens in eggs from different beaches suggested that no uniform geographic gradients exist. Presence of OCPs, PCBs, PAHs and metals and their additive or synergistic toxicity is a concern to loggerhead sea turtle eggs; however, additive or synergistic impacts for loggerhead sea turtles are largely undocumented.

Animals↗

PubMed2000

Serological response to immunization with tetanus toxoid.

A total of two hundred women were immunized with tetanus toxoid vaccine. Two batches of toxoid prepared at the Institute of Public Health (IPH), Dhaka and one batch of imported vaccines, were being used by the EPI in Bangladesh for immunization. Each hundred women were immunized by IPH and imported vaccine. Two human doses were given in one month interval. Blood samples from all the study subjects were collected on the day of 1st dose and one month after second dose. Both the preimmunized sera and the sera after vaccination were tested to determine the antibody titre against tetanus toxoid by the haemagglutination method. The preimmunized sera showed the presence of protective antibody in 50(25%) subjects who had the history of previous immunization. Including these initial antitoxin positive cases the seroconversions found among 95% and 96% of the study population respectively after immunization with IPH and imported toxoids, which were 93.05% and 94.87% when these 50 subjects were excluded. No significant difference (p = 1.0) was observed between the immunity of the subjects after receiving IPH and imported vaccine. Antibody titre of initial tetanus-antitoxin positive cases raised eight folds after getting more doses. The result gave fair indication of the antigenicity of all the toxoids used in the study.

Antigens, Bacterial↗

PubMed1999

Power spectral strain estimators in elastography.

Elastography can produce quality strain images in vitro and in vivo. Standard elastography uses a coherent cross-correlation technique to estimate tissue displacement and tissue strain using a subsequent gradient operator. Although coherent estimation methods generally have the advantage of being highly accurate and precise, even relatively small undesired motions are likely to cause enough signal decorrelation to produce significant degradation of the elastogram. For elastography to become more universally practical in such applications as hand-held, intravascular and abdominal imaging, the limitations associated with coherent strain estimation methods that require tissue and system stability, must be overcome. In this paper, we propose the use of a spectral-shift method that uses a centroid shift estimate to measure local strain directly. Furthermore, we also show theoretically that a spectral bandwidth method can also provide a direct strain estimation. We demonstrate that strain estimation using the spectral-shift technique is moderately less precise, but far more robust than the cross-correlation method. A theoretical analysis, simulations and experimental results are used to illustrate the properties associated with this method.

Elasticity↗

PubMed1999

Elastography: ultrasonic estimation and imaging of the elastic properties of tissues.

The basic principles of using sonographic techniques for imaging the elastic properties of tissues are described, with particular emphasis on elastography. After some preliminaries that describe some basic tissue stiffness measurements and some contrast transfer limitations of strain images are presented, four types of elastograms are described, which include axial strain, lateral strain, modulus and Poisson's ratio elastograms. The strain filter formalism and its utility in understanding the noise performance of the elastographic process is then given, as well as its use for various image improvements. After discussing some main classes of elastographic artefacts, the paper concludes with recent results of tissue elastography in vitro and in vivo.

Animals↗

PubMed1998

Color flow mapping.

Explore the source record for details and available documents.

Echocardiography, Doppler, Color↗

PubMed1997

Reduction of signal decorrelation from mechanical compression of tissues by temporal stretching: applications to elastography.

Elastography is based on the estimation of strain due to tissue compression. Strain is computed from the estimates of time delays between gated precompression and postcompression echo signals. Time delay estimates are obtained from the location of the peak of the crosscorrelation function between gated precompression and postcompression signals. It is of paramount importance to accurately estimate the time delays for good quality elastograms. A main source of time delay estimation (TDE) error in elasticity imaging is the decorrelation of the echo signal as a result of tissue compression (decorrelation noise). The effect of decorrelation on the mean of the crosscorrelation function and the correlation coefficient has been investigated. The expected value of the cross-correlation function between the precompression and postcompression signals was shown to be a filtered version of the autocorrelation function of the precompression signal. In this article, the effect of temporal stretching of the postcompression echo signal on the cross-correlation function will be investigated along the same line. The applied compression is assumed to be uniform; the decorrelations introduced by the lateral and elevational tissue movements are ignored. The theory predicts that if the postcompression echo signals are stretched before the TDE step, then for small strains, the cross-correlation function very closely resembles the autocorrelation function. For larger strains, correlation is improved if temporal stretching is applied. The theory is corroborated by results from simulation and homogeneous phantom experiments. Thus, the decorrelation noise in elastograms can be reduced by temporal stretching of the postcompression signal.

Elasticity↗

PubMed1997

On the use of envelope and RF signal decorrelation as tissue strain estimators.

Bamber and Bush (1995) used the correlation coefficient for freehand elasticity imaging. Varghese and Ophir (1996) found it to be a biased estimator of strain with a large variability. In this study, we systematically investigate the effect of changes in various system and processing parameters on the performance of the correlation coefficient strain estimator, and demonstrate, using simulated data, that noise and frequency-dependent attenuation can introduce variable bias in this estimator.

Computer Simulation↗

PubMed1996

Reduction of computational complexity in the butterfly search technique.

In the butterfly search technique, echoes from repeated firings of a transducer are resampled along a set of predetermined trajectories of constant velocities, called "butterfly lines," because of their intersection and crossing at a reference range. The slope of the trajectory on which the sampled signals satisfy a predetermined criterion appropriate for the type of signal in question, provides an estimate of the velocity of the target. The search for this trajectory is called "butterfly search," which can be carried out efficiently in a parallel processing scheme. The estimator can be based on the radio frequency (RF) A-lines, the envelopes, or the quadrature components. The butterfly search on quadrature components has shown outstanding noise immunity, even with relatively few successive scan lines, and was found to outperform all the common time domain and Doppler techniques in simulations and experiments with strong noise. It can be simply implemented using elementary digital signal processing hardware. However, it is possible to further improve upon its computational complexity to make the technique even simpler to implement, without any complex multipliers in the parallel channels. In this paper, we present some modifications that significantly reduce the computational complexity of butterfly search on quadrature components.

Algorithms↗

PubMed1995

The butterfly search technique for estimation of blood velocity.

We present a novel, robust and accurate blood velocity estimation technique that is implementable by elementary digital signal processing. In this technique, echoes from repeated firings of a transducer are resampled along a set of predetermined trajectories of constant velocities, called "butterfly lines" because of their intersection at a reference range. The slope of the trajectory on which the sampled signals satisfy a predetermined criterion appropriate for the type of signal in question, provides an estimate of the velocity of the target. The search for this trajectory is called "butterfly search," which can be carried out efficiently in a parallel processing scheme. The estimator can be based on the RF echo, its envelope, or its quadrature components. We present the theory of the butterfly search and some preliminary results. The butterfly search on quadrature components has shown superior noise immunity, with relatively few successive scan lines, and was found to outperform all the common time domain and Doppler techniques in simulations and phantom experiments with strong noise. The butterfly search can overcome many disadvantages faced by the present day techniques, such as the stringent tradeoff criterion between imaging resolution and velocity resolution implicit in Doppler techniques, and the need for computation-intensive operations.

Blood Flow Velocity↗

PubMed1995

Sonoelasticity imaging: theory and experimental verification.

Sonoelasticity is a rapidly evolving medical imaging technique for visualizing hard tumors in tissues. In this novel diagnostic technique, a low-frequency vibration is externally applied to excite internal vibrations within the tissue under inspection. A small stiff inhomogeneity in a surrounding tissue appears as a disturbance in the normal vibration eigenmode pattern. By employing a properly designed Doppler detection algorithm, a real-time vibration image can be made. A theory for vibrations, or shear wave propagation in inhomogeneous tissue has been developed. A tumor is modeled as an elastic inhomogeneity inside a lossy homogeneous elastic medium. A vibration source is applied at a boundary. The solutions for the shear wave equation have been found both for the cases with tumor (inhomogeneous case) and without tumor (homogeneous case). The solutions take into account varying parameters such as tumor size, tumor stiffness, shape of vibration source, lossy factor of the material, and vibration frequency. The problem of the lowest detectable change in stiffness is addressed using the theory, answering one of the most critical questions in this diagnostic technique. Some experiments were conducted to check the validity of the theory, and the results showed a good correspondence to the theoretical predictions. These studies provide basic understanding of the phenomena observed in the growing field of clinical Sonoelasticity imaging for tumor detection.

Acoustics↗

PubMed1995

Sonoelasticity imaging of prostate cancer: in vitro results.

PURPOSE: To compare sonoelasticity imaging versus ultrasound (US) in detection of prostate cancer. MATERIALS AND METHODS: Sonoelasticity imaging and US were performed on 10 prostatectomy specimens in which cancer was detected at previous biopsy. Six patients had no palpable lesions at digital rectal examination. Specimens were imaged axially at the apex, middle, and base of the gland to correlate with location of pathologic sections. All images were interpreted blindly and prospectively, and results were compared with pathologic findings. RESULTS: Sensitivity and specificity with sonoelasticity imaging were 85% and 84%, respectively, and 30% and 100% with standard US when compared with pathologic findings. Sixty-four percent of pathologically confirmed tumors detected at sonoelasticity imaging were isoechoic on conventional US images. CONCLUSION: In this limited study, sonoelasticity imaging was more sensitive for tumor detection and more accurate for assessment of tumor location than was conventional US.

Adenocarcinoma↗

PubMed1994

Detection of intraocular pressure change in the eye using sonoelastic Doppler ultrasound.

We report the first use of sonoelastic Doppler ultrasound for in vivo and in vitro clinical studies of intraocular pressure (IOP). The method involves low-amplitude and low-frequency sonic excitation of the eye, and detection of the resulting vibration using Doppler ultrasound. A dependence of the frequency of resonance of the eye on the IOP has been observed in both in vitro and in vivo experiments. Preliminary in vitro experiments have been performed using eviscerated and enucleated human and pig eyes. As little as 4 mm Hg change in IOP has been found detectable in these experiments. Preliminary in vivo experiments also showed promising results in this regard. We present a simple model for the resonances of the eye, a method to detect the amplitude of vibration using Doppler ultrasound and results from the in vitro and in vivo experiments.

Animals↗