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Biomedical subjects

T K Song

Publications and source records attributed to T K Song.

11 recordsLinked to original sources
PubMed2005

Polarization relaxation induced by a depolarization field in ultrathin ferroelectric capacitors.

Time-dependent polarization relaxation behavior induced by a depolarization field E(d) was investigated on high-quality ultrathin SrRuO3/BaTiO3/SrRuO3 capacitors. The E(d) values were determined experimentally from an applied external field to stop the net polarization relaxation. These values agree with those from the electrostatic calculations, demonstrating that a large E(d) inside the ultrathin ferroelectric layer could cause severe polarization relaxation. For numerous ferroelectric devices of capacitor configuration, this effect will set a stricter size limit than the critical thickness issue.

Journal Article↗
PubMed2002

An efficient motion estimation and compensation method for ultrasound synthetic aperture imaging.

This paper describes a method for overcoming motion artifacts in synthetic aperture imaging. The method is based on a computer simulation study on the influence of target motion on synthetic aperture techniques. A region-based motion compensation approach is used in which only the axial motion is estimated and compensated for a given region of interest under the assumption that the whole ROI moves uniformly. The estimated axial motion is calculated with a crosscorrelation method at the point where the focused signal has the maximum energy within the ROI. We also present a method for estimating axial motion using the autocorrelation method that is widely used to estimate average Doppler frequency. Both computer simulations and in vivo experiments show that the proposed crosscorrelation-based method can greatly improve the spatial resolution and SNR of ultrasound imaging by implementing SA techniques for two-way dynamic focusing without motion artifacts. In addition, the autocorrelation-based motion compensation method provides almost the same results as the crosscorrelation-based method, but with a dramatically reduced computational complexity.

Adult↗
PubMed2001

A study of the display pixel-based focusing method in ultrasound imaging.

This paper presents a new beamforming technique, in which ultrasound waves are focused at the display pixels on the Cartesian coordinates, rather than sampling points located on the uniformly-separated scanlines on either the polar or Cartesian coordinates. Consequently, this method does not require a digital scan converter (DSC) and therefore completely eliminates the distortions due to the DSC. A new imaging system based on the proposed method is also presented, which is advantageous over conventional systems in hardware complexity, especially when high-density imaging is required. The new method can also be used effectively to achieve high frame rates for real-time 3D imaging and cardiac applications. To verify the proposed method, we modified a commercial ultrasound scanner and performed experiments with a 3.5 MHz convex array and a 7.5 MHz linear array. The experimental results with in vivo and in vitro data show that the proposed technique provides much smoother and finer images than the conventional dynamic focusing methods.

Humans↗
PubMed1999

Realization of sinc waves in ultrasound imaging systems.

It is known that the transmit pulse waveforms of a limited-diffraction beam in a linear array transducer should be varied according to transducer element location, dictating the use of sophisticated hardware. In order to overcome this disadvantage while achieving the same field response, we propose a method of synthesizing limited-diffraction beams by combined signal processing of pulsed plane waves propagating in distinct directions over several consecutive insonification time intervals. The method is capable of achieving both higher transmit power and better lateral resolution over a larger depth of field. Although its field response is not uniform throughout the imaging points, this is not a major problem since the response is quite uniform within a region of interest. The proposed method requires the use of multiple insonifications for transmit focusing, and, therefore, can be applied in imaging slowly-moving or still objects. Both simulation and experimental results corroborate its superiority in terms of the lateral resolution at all imaging depths.

Computer Simulation↗
PubMed1996

Synthesis and structure-activity relationships of retinoid X receptor selective diaryl sulfide analogs of retinoic acid.

Retinoids exert their biological effects by binding to and activating nuclear receptors that interact with responsive elements on DNA to promote gene transcription. There are two families of retinoid receptors, the retinoic acid receptor (RAR) family and the retinoid X receptor (RXR) family, which are each further divided into three subclasses: RAR alpha, beta, gamma and RXR alpha, beta, gamma. Herein we describe the synthesis and structure-activity relationships of a new series of diaryl sulfide retinoid analogs that specifically bind and transactivate the RXRs. Furthermore, the sulfoxide and sulfone derivatives of these analogs are partial agonists which activate the RXRs only at high concentrations. Thus, these compounds possess a potential site of metabolic deactivation and may have less prolonged systemic effects than other compounds with arotinoid-like structures. We show also that these compounds have activity in nontransfected cells as demonstrated by their ability to induce TGase activity in HL-60 cells. Finally, we corroborate our earlier report that RXR-specific agonists may possess reduced teratogenic toxicity compared to RAR-specific agonists since these compounds are much less potent inhibitors of chondrogenesis than RAR-specific agonists such as TTNPB.

Animals↗
PubMed1993

Modified X waves with improved field properties.

A method to obtain a good compromise between the depth of field and the lateral resolution of "X waves" is proposed. The original X waves are theoretically nondiffracting beams generated by a specially phased infinite transmit aperture. When generated by a finite aperture, X waves are diffracting beams but have a large depth of field, maintaining uniform lateral field profiles. The proposed modification of the wave equation solution for X waves replaces a constant parameter representing the propagation angle of ultrasound with a function of radial distance at the aperture surface, and results in modified X waves that have a larger depth of field than the original X waves. Computer simulations show that a proper choice of the modification function can produce a new beam with improved field properties compared with the original X waves, promising images with higher lateral resolution and increased contrast over a large depth of field in high frame rate medical imaging. Experimental results are presented to verify the simulation results of the proposed method.

Humans↗
PubMed1990

A new digital phased array system for dynamic focusing and steering with reduced sampling rate.

In order to enhance the resolution of the ultrasonic B-mode image, digital techniques for dynamic focusing and steering have been receiving a great deal of attention for a long time. In this paper, we present the theory and preliminary experimental results for a new architecture of a fully digital B-mode imaging system with this desired feature. The system employs the Pipelined-Sampled-Delay-Focusing (PSDF) scheme to eliminate the bulky memory addressing and interpolation circuits which are needed in conventional digital imaging systems. To reduce hardware requirements and the bandwidth required for digital processing, we describe an efficient method to generate the sampling clocks for dynamic focusing and steering and some modified bandwidth sampling techniques to reduce the sampling rate for signal digitization. Using these methods, it is possible to achieve dynamic focusing and steering in real-time sector imaging using a linear phased array.

Humans↗
PubMed1989

Optimum focusing in an ultrasonic annular array imaging system using a nonspherical lens.

In this paper, a new focusing method for ultrasonic annular array imaging systems is presented. The technique minimizes the lateral resolution for a specified sidelobe suppression over the depth of interest, using a nonspherical lens for line focusing in transmit in combination with continuous dynamic focusing in receive. For this purpose, an effective analysis model is developed for calculating the impulse response of an annulus with an arbitrarily-shaped lens in the whole imaging plane. Using this analysis method, an iterative technique is then applied to obtain an optimum transmit delay profile when continuous receive focusing is employed at all imaging points along the depth of view. The delay profile is optimum in the sense that the lateral beam width is minimized under a specified sidelobe suppression at three depths of the imaging field. The continuous receive focusing is realized in a new digital focusing system which enables arbitrary control of transmit time delays and dynamic focusing in the receive mode. In the proposed system, focusing is achieved by adjusting the sampling time for each array element according to the arrival time differences among array elements for a given focal point. Experimental results show the validity of the analysis model and the optimum lens design.

Humans↗
PubMed1988

Ultrasound attenuation estimation in soft tissue using the entropy difference of pulsed echoes between two adjacent envelope segments.

In this paper, we describe a new method of estimating the ultrasound attenuation coefficient of soft tissue based on the entropy difference between two adjacent envelope segments of narrowband ultrasound pulse echoes. Assuming uniform attenuation in the region under investigation, the attenuation is estimated by minimizing the difference of entropies in the two segments as the attenuation is continuously compensated for. The simulation and experimental results with a tissue equivalent (TE) phantom show that the proposed estimator is quite robust to receiver noise and requires considerably less data length as compared with conventional methods. At present, an experimental result with a TE phantom shows that a resolution of 10 mm x 10 mm can be attained with echoes from a depth of 3 cm using a 2.25 MHz transducer.

Computer Simulation↗
PubMed1987

Pipeline sampled-delay focusing in ultrasound imaging systems.

A sampled-delay focusing technique was recently proposed by the authors which completely eliminates the use of analog L-C delay lines for beam focusing in completely eliminates the use of analog L-C delay lines for beam focusing in ultrasound B-mode imaging systems. With this approach, the product of sampling rate and maximum time delay is required to be less than unity. To remove this constraint, we propose in this paper a first-in-first-out pipelining technique. This allows one to perform beam steering and dynamic focusing simultaneously on a resolution-cell basis and in a completely digital fashion without the use of analog L-C delay lines.

Humans↗