Biomedical subjects

Pai-Chi Li

Publications and source records attributed to Pai-Chi Li.

At least 37 records · Page 2Linked to original sources

PubMed2004

A digital ultrasonic system for small animal imaging.

In this paper, a 50 MHz digital ultrasonic imaging system designed for small animals and its applications to mouse embryos and tumors are presented. Utilizing the programmability of a fully digital system, several advanced imaging techniques are developed and implemented. A synthetic aperture focusing technique is implemented to increase the penetration and extend the depth of focus of a mechanical scan system using a high frequency single crystal transducer. An adaptive weighting technique is introduced to further reduce the sidelobes and enhance image contrast. Experimental results demonstrate efficacy of the proposed two-step adaptive focusing technique. For flow imaging, several flow estimation algorithms are evaluated. Simulations and flow phantom experiments show that the butterfly search technique offers superior flow imaging performance particularly when the SNR is low and the pulse bandwidth is high. The system is tested by imaging mouse embryos and microcirculation of a mouse tumor in vivo. It is demonstrated that the system is capable of simultaneously acquiring B-mode, color Doppler mode and power Doppler mode small animal images. The system set-up, adaptive focusing technique, flow estimation algorithms and imaging results are shown.

Algorithms↗

PubMed2004

Computed tomography sound velocity reconstruction using incomplete data.

An approach based on limited-angle transmission tomography for reconstruction of the sound velocity distribution in the breast is proposed. The imaging setup is similar to that of x-ray mammography. With this setup, the time-of-flight data are acquired by a linear array positioned at the top of the compressed breast that both transmits and receives, and a metal plate is placed at the bottom as a reflector. The setup allows acoustic data acquisition for simultaneous B-mode image formation and the tomographic sound velocity reconstruction. In order to improve the sound velocity estimation accuracy, a new reconstruction algorithm based on a convex programming formulation has been developed. Extensive simulations for both imaging and time-of-flight data based on a 5-MHz linear array were performed on tissues with different geometries and acoustic parameters. Results show that the sound velocity error was generally 1-3 m/s, with a maximum of 5.8 m/s. The radii of the objects under investigation varied from 2 to 6 mm, and all of them were detected successfully. Thus, the proposed approach has been shown to be both feasible and accurate. The approach can be used to complement conventional B-mode imaging to further enhance the detection of breast cancer.

Algorithms↗

PubMed2003

Application of the Barton photochemical reaction in the synthesis of 1-dethia-3-aza-1-carba-2-oxacephem: a novel agent against resistant pathogenic microorganisms.

Racemic 7-(phenylacetamido)-1-dethia-3-aza-1-carba-2-oxacephem 3 was synthesized and found to possess antibacterial activity against Staphylococcus aureus FDA 209P, Escherichia coli ATCC 39188, Pseudomonas aeruginosa 1101-75 and Klebsiella pneumoniae NCTC 418 as well as the beta-lactamase producing organisms E. coli A9675 and P. aeruginosa 18S-H and the methicillin-resistant organism S. aureus 95. Formation of the carbacephem 3 originated from the Barton photochemical reaction in the conversion of 8 to 10. Intramolecular cyclization of syn-oximino beta-lactam 10 afforded 7-azido-2-oxa-3-azacephem 11, which was reduced and acylated to 12. Enzymatic removal of the methyl group from 12 gave the target molecule 3.

Anti-Infective Agents↗

PubMed2003

Contrast-specific ultrasonic flow measurements based on both input and output time intensities.

Ultrasonic contrast agents are used to assess perfusion conditions based on evaluation of the time-intensity curve. Such a curve reflects the concentration of microbubbles in the perfused area and the indicator-dilution theory is used to derive the volumetric flow rate from the measured concentration. Previous results have shown that the technique is not reliable in some conditions due to the shadowing effect. To overcome this problem, a contrast-specific technique using both the input and output time-intensity relationships is proposed; this contrasts with conventional techniques that utilize only the relationship directly from the perfused area. The proposed technique is referred to as the input-output time-intensity curve (IOTIC) method. In this work, the shadowing effect was studied experimentally and the efficacy of the IOTIC technique was assessed and compared with conventional techniques. The results indicate that the IOTIC technique eliminates the shadowing effect and provides a good correlation between the actual flow rate and measured flow-related parameters; thus, making quantitative estimation of perfusion feasible. Note that the IOTIC is applicable, based on the assumption that both the input and the output can be positioned within the same image plane; its clinical applications include situations where the perfused area cannot be effectively imaged by ultrasound (US). One example is the assessment of brain perfusion, and it will be used as a target clinical application of the IOTIC technique.

Cerebrovascular Circulation↗

PubMed2003

Transfer function analysis of ultrasonic time-intensity measurements.

Time-intensity measurements of ultrasonic-contrast microbubbles based on the dilution theory have been used to assist blood flow estimation. The compartment model has been employed to describe the dilution process. Under the linear and time-invariant assumption, the time-intensity curve measured at the output of a compartment (i.e., blood mixing chamber) is the convolution of the input time-intensity curve with the compartment's transfer function. Thus, transfer function analysis is possible using deconvolution when the temporal variations in both the input and the output intensities are available. Note that the linear and time-invariant assumption requires a constant flow rate because, with flow pulsation, the flow rate changes with time and the mixing process becomes time varying. Thus, the purpose of this paper was to study the effects of flow pulsation on time-intensity measurements. In addition, a deconvolution technique based on a recursive least squares approach is used for transfer function analysis. Both simulations and experiments were performed; the results from which indicate that the pulsation generally does not affect the validity of time-intensity-based flow estimation. The proposed deconvolution technique is also effective for both constant and pulsatile flows; thus, permitting transfer function analysis in various flow conditions. One potential application of this transfer function analysis is to remove the effects of a noninstantaneous input function. The results from this paper lead to future work in brain-perfusion estimation based on extracranial time-intensity measurements.

Blood Flow Velocity↗

PubMed2003

Liver fibrosis grade classification with B-mode ultrasound.

B-mode images of 20 fresh postsurgical human liver samples were obtained to evaluate ultrasound ability in determining the grade of liver fibrosis. Image features derived from gray level concurrence and nonseparable wavelet transform were extracted to classify fibrosis with a classifier known as the support vector machine. Each liver sample subsequently underwent histologic examination and liver fibrosis was graded from 0 to 5 (i.e., six grades total). The six grades were then combined into two, three, four and six classes. Classifications with the extracted image features by the support vector machine were tested and correlated with histology. The results revealed that the best classification accuracy of two, three, four and six classes were 91%, 85%, 81% and 72%, respectively. Thus, liver fibrosis can be noninvasively characterized with B-mode ultrasound, even though the performance declines as the number of classes increases. The elastic constants of 16 samples out of a total of 20 were also correlated with the image features. The Pearson correlation coefficients indicated that the image features are more strongly correlated with the fibrosis grade than with the elastic constant.

Humans↗

PubMed2003

Adaptive imaging using the generalized coherence factor.

Sound-velocity inhomogeneities degrade both spatial and contrast resolutions. This paper proposes a new adaptive imaging technique that uses the generalized coherence factor (GCF) to reduce the focusing errors resulting from the sound-velocity inhomogeneities. The GCF is derived from the spatial spectrum of the received aperture data after proper receive delays have been applied. It is defined as the ratio of the spectral energy within a prespecified low-frequency range to the total energy. It is demonstrated that the low-frequency component of the spectrum corresponds to the coherent portion of the received data, and that the high-frequency component corresponds to the incoherent portion. Hence, the GCF reduces to the coherence factor defined in the literature if the prespecified low-frequency range is restricted to dc only. In addition, the GCF is also an index of the focusing quality and can be used as a weighting factor for the reconstructed image. The efficacy of the GCF technique is demonstrated for focusing errors resulting from the sound-velocity inhomogeneities. Simulations and real ultrasound data are used to evaluate the efficacy of the proposed GCF technique. The characteristics of the GCF, including the effects of the signal-to-noise ratio and the number of channels, are also discussed. The GCF technique also is compared with the correlation-based technique and the parallel adaptive receive compensation algorithm; the improvement in image quality obtained with the proposed technique rivals that of the latter technique. In the presence of a displaced phase screen, this proposed technique also outperforms the correlation-based technique. Computational complexity and implementation issues also are addressed.

Algorithms↗

PubMed2003

Pulse-inversion-based fundamental imaging for contrast detection.

Pulse-inversion-based fundamental imaging was experimentally investigated for the enhancement of contrast detection. The pulse-inversion technique involves two firings with inverted waveforms. When the returning echoes from the two firings are summed, the residue signal is limited to even-order harmonics for tissue. However, when the returning echoes are from microbubbles, the fundamental signal is not completely cancelled because the reaction of the bubbles under compression is different from that under rarefaction. Thus, with the application of pulse-inversion technique, the fundamental signal can be used to enhance the contrast-to-tissue ratio. In this paper, B-mode, pulse-inversion-based fundamental images were constructed with various transmit waveforms. Motion artifacts also were studied. The results indicate that the contrast-to-tissue ratio was significantly enhanced compared to that obtained using either conventional, fundamental imaging or second-harmonic imaging. Longer transmit pulses resulted in a better signal-to-noise ratio, but did not noticeably affect the nonlinear response of the bubbles. In addition, the optimal ratio of the magnitude of the positive pulse to that of the negative pulse was unity, in terms of avoiding the uncancelled, third-order response in the fundamental frequency range. It also was found that the pulse-inversion fundamental technique is highly sensitive to tissue motion because the fundamental tissue signal is not cancelled when motion is present.

Artifacts↗

PubMed2003

Improved Fourier-transform-based parallel receive beam formation.

A Fourier transform (FT)-based technique for forming parallel receive beams has been previously employed to increase the imaging frame rate in ultrasonic imaging. However, the image quality in FT-based parallel reconstruction is degraded because differences in range focusing delays are ignored and a wide transmit beam needs to be used. In this paper, an adaptive weighting technique based on a focusing-quality index is used to reduce the sidelobes of the FT-derived parallel receive beams. The focusing-quality index is derived from the spatial spectrum of the received aperture data after the receive delays have been applied. Since the spatial spectrum of the baseband aperture data is also used to approximate receive beams in FT-based parallel reconstruction, the adaptive weighting technique can be directly combined with the FT-based technique for forming parallel receive beams with only a slight increase in system complexity. Real ultrasound data are used to demonstrate the efficacy of the proposed technique on both wire targets and speckle-generating objects. The results clearly demonstrate the effectiveness in reducing the sidelobes. In addition, the image background noise is suppressed. The principles, experimental results, and the extension of the proposed technique to 3D ultrasound imaging are described in this paper.

Fourier Analysis↗

PubMed2002

Theoretical study and X-ray determination of bianthronyl [corrected] : long C-C bond length and preferred gauche conformation.

Five X-ray crystallographic structures of bianthronyl (1, 2b-5b) revealed the presence of long central C(sp3)-C(sp3) bonds (> or =1.60 A) and mostly gauche conformation. DFT calculations were carried out to study the structural features of bianthronyl and related model molecules [corrected]. Our B3LYP/6-31G gas-phase calculation results suggest that steric effect plays a role in causing the long C-C sigma bond length (>1.60 A), nonplanarity of the anthracenone moiety (>20 degrees ), and preferred gauche conformation. However, when the gauche preference is weak, environmental effects such as solvation and packing forces can reverse it. As the degree of nonplanarity of the anthracenone moiety is consistently larger in the anti conformation than in the gauche and the ease of achieving nonplanarity is determined by flexibility of the central six-membered rings directly connected to the C(sp3)-C(sp3) bond, one way to control the structure and anti-gauche energy difference of closely related compounds is by changing the clamping group (Z) in the central six-membered ring.

Journal Article↗

PubMed2002

Time-intensity-based volumetric flow measurements: an in vitro study.

Ultrasonic contrast agents have been used to assist blood flow measurements. Several contrast-specific flow measurement techniques have been proposed during the last few years. Among them, a method based on relative enhancement of the backscattered signal as a function of time is of particular interest. This method is also known as the time-intensity method. The method is based on the indicator-dilution theory, and the time-intensity curve is used to derive blood flow-related parameters such as the flow rate and the blood mixing volume. Previous in vitro studies done by other research groups were mainly based on a perfusion model or an artery model. Results showed that several parameters derived from the time-intensity curve had a good correlation with the flow rate under certain conditions. However, the studies did not focus on factors such as mixing volume, mixing chamber configuration and different types of mixing chamber. In this paper, dependence of the time-intensity curve is further studied. Specifically, two types of blood-mixing chambers were constructed. One was a spherical compartment phantom with two different sizes (260 and 580 mL) and different inflow/outflow configurations. The other was a perfusion phantom consisting of dialysis cartridges with the volume ranging from 114 to 351 mL. The time intensities were also measured at both the input and the output of the mixing chamber. A commercial agent (Levovist) and a self-made, albumin-based agent were used and the wash-out time constant and the mean transit time were derived for flow rates ranging from 500 to 1300 mL/min. For the perfusion phantom, results showed that the parameters had a good correlation with both the flow rate and the mixing volume. Results from the compartment phantom, on the other hand, indicated that the inflow/outflow configuration and the mixing size significantly affected the derived time constants. Potential applications of new volumetric flow estimation techniques based on both input and output intensities were also discussed.

Blood Flow Velocity↗

PubMed2002

Elastic modulus measurements of human liver and correlation with pathology.

Viral hepatitis causes fibrosis in the liver and may change mechanical properties of the liver. To evaluate the impact of fibrosis on elastic properties of human liver and to investigate potential benefits of ultrasonic elasticity imaging, 19 fresh human liver samples and 1 hepatic tumor (focal nodular hyperplasia) sample obtained during operations were studied. Simple 1-D estimates based on the cyclic compression-relaxation method were performed. Elastic modulus values were derived from the predetermined strain (controlled by a step motor system) and the stress values (measured by an electronic balance). Each specimen subsequently received histologic examination and a grade of liver fibrosis was scored from 0 to 5. Results show that the elastic modulus values were on the order of several hundreds to thousands of Pascals. The elastic modulus generally increased with the fibrosis grade, although some discrepancies existed at the middle grades of fibrosis (scores 1 to 3). The correlation between the fibrosis score and the elastic modulus was significant (p < 0.01) based on the statistical analysis using the Pearson correlation method. In addition, the relation between the elastic modulus and the fibrosis grade generally exhibited a quadratic trend. It was concluded that severity of fibrosis had a good correlation with stiffness of the liver. Results also indicated that the elasticity imaging of the liver may provide significant clinical values if the elastic modulus can be accurately measured.

Elasticity↗

PubMed2002

Strain compounding: a new approach for speckle reduction.

A new compounding technique for reducing speckle brightness variations is proposed. This method exploits the decorrelation between signals under different strain states. The different strain states can be created using externally applied forces such as the ones used in sonoelastography. Such forces produce three-dimensional tissue motion. By correcting only the in-plane (i.e., axial and lateral) motion, the images under different strain states have similar characteristics except for speckle appearance caused by the uncorrected out-of-plane (i.e., elevational) motion. Additional speckle decorrelation is also introduced through tissue motion correction caused by the change of effective in-plane sample volume geometry. Therefore, these images can be combined for speckle reduction with less degradation in in-plane spatial resolution than conventional approaches. In this paper, three-dimensional tissue motion under various strain conditions were simulated. It was found that significant speckle decorrelation existed at strains achievable in some clinical situations. Experiments were also conducted to test efficacy of this approach. Pulse-echo data from a gelatin-based phantom were acquired using a 5-MHz, single crystal transducer, and both conventional and compound B-mode images were formed. Results indicated that speckle brightness variations were reduced, and detectability of low contrast objects was enhanced. Performance limitations and fundamental differences between the proposed technique and existing techniques are discussed.

Acoustics↗

PubMed2002

Doppler angle estimation using AR modeling.

The transit time spectrum broadening effect has long been explored for Doppler angle estimation. Given acoustic beam geometry, the Doppler angle can be derived based on the mean Doppler frequency and the Doppler bandwidth. Spectral estimators based on the fast Fourier transform (FFT) are typically used. One problem with this approach is that a long data acquisition time is required to achieve adequate spectral resolution, with typically 32-128 flow samples being needed. This makes the method unsuitable for real-time two-dimensional Doppler imaging. This paper proposes using an autoregressive (AR) model to obtain the Doppler spectrum using a small number (e.g., eight) of flow samples. The flow samples are properly selected, then extrapolated to ensure adequate spectral resolution. Because only a small number of samples are used, the data acquisition time is significantly reduced and real-time, two-dimensional Doppler angle estimation becomes feasible. The approach was evaluated using both simulated and experimental data. Flows with various degrees of velocity gradient were simulated, with the Doppler angle ranging from 20 degrees to 75 degrees. The results indicate that the AR method generally provided accurate Doppler bandwidth estimates. In addition, the AR method outperformed the FFT method at smaller Doppler angles. The experimental data for Doppler angles, ranging from 33 degrees to 72 degrees, showed that the AR method using only eight flow samples had an average estimation error of 3.6 degrees, which compares favorably to the average error of 4.7 degrees for the FFT method using 64 flow samples. Because accurate estimates can be obtained using a small number of flow samples, it is concluded that real-time, two-dimensional estimation of the Doppler angle over a wide range of angles is possible using the AR method.

Algorithms↗

PubMed2002

Efficient dynamic focus control for three-dimensional imaging using two-dimensional arrays.

Dynamic receive focusing in ultrasonic array imaging involves extensive real-time computations and data communication. Particularly for three-dimensional imaging, using fully sampled, two-dimensional arrays, implementation of dynamic focusing can be extremely complicated because of the large channel count. In this paper, an efficient dynamic focus control scheme for a delay-and-sum-based beamformer is proposed. The scheme simplifies dynamic focus control by exploiting the range-dependent characteristics of the focusing delay. Specifically, the overall delay is divided into a range-independent steering term and a range-dependent focusing term. Because the focusing term is inversely proportional to range, approximation can be made to simplify dynamic focus control significantly at the price of minimal degradation in focusing quality at shallow depths. In addition, the aperture growth controlled by a constant f/number can also be utilized to devise a non-uniform quantization scheme for the focusing delay values. Efficacy of the proposed scheme is demonstrated using simulated beam plots of a fully sampled, two-dimensional array. Design procedures are also described in detail in this paper. One design example shows that, with the proposed dynamic focus control scheme, a 4096-element array only requires 227 independent controllers for the range-dependent focusing term. Moreover, only 28 non-uniform quantization levels are required to achieve the same focusing quality as that of a conventional scheme with 784 uniform quantization levels. The beam plots of a fully sampled array show that sidelobes are slightly increased below the -30 dB level for imaging depths less than 3 cm. At greater depths, there is no observable degradation.

Journal Article↗

PubMed2002

Motion artifacts of pulse inversion-based tissue harmonic imaging.

Motion artifacts of the pulse inversion technique were studied for finite amplitude distortion-based harmonic imaging. Motion in both the axial and the lateral directions was considered. Two performance issues were investigated. One is the harmonic signal intensity relative to the fundamental intensity and the other is the potential image quality degradation resulting from spectral leakage. A one-dimensional (1-D) correlation-based correction scheme also was used to compensate for motion artifacts. Results indicated that the tissue harmonic signal is significantly affected by tissue motion. For axial motion, the tissue harmonic intensity decreases much more rapidly than with lateral motion. The fundamental signal increases for both axial and lateral motion. Thus, filtering is still required to remove the fundamental signal, even if the pulse inversion technique is applied. The motion also potentially decreases contrast resolution because of the uncancelled spectral leakage. Also, it was indicated that 1-D motion correction is not adequate if nonaxial motion is present.

Artifacts↗

PubMed2002

Tissue motion and elevational speckle decorrelation in freehand 3D ultrasound.

3D positioning is essential for quantitative volume analysis in 3D ultrasound. Particularly for freehand scanning without an external positioning device, such information must be estimated by analyzing the original 2D images prior to 3D reconstruction. Previous work on freehand 3D positioning has focused on elevational displacement estimation using speckle decorrelation for linear scans. However, the effects of other types of motion have been ignored. Given that all types of motion potentially introduce speckle decorrelation, the accuracy of the elevational displacement estimation is likely to be diminished by the presence of motions of other types. In the present study, simulations were performed to probe the effects of various motions on elevational displacement estimation. In particular, the effects of rotational motions on image correlation are investigated in detail. It is found that these motions significantly affect the estimation results and thus should be taken into account when reconstructing the 3D image. In addition, speckle variations also affect the estimation even when only elevational motion is present. Finally, full motion analysis in freehand scanning may not be possible by only using speckle correlation analysis unless speckle variations can be reduced and the correlation distribution under complex motion can be obtained.

Computer Simulation↗

PubMed2002

Doppler angle estimation of pulsatile flows using AR modeling.

In quantitative ultrasonic flow measurements, the beam-to-flow angle (i.e., Doppler angle) is an important parameter. An autoregressive (AR) spectral analysis technique in combination with the Doppler spectrum broadening effect was previously proposed to estimate the Doppler angle. Since only a limited number of flow samples are used, real-time two-dimensional Doppler angle estimation is possible. The method was validated for laminar flows with constant velocities. In clinical applications, the flow pulsation needs to be considered. For pulsatile flows, the flow velocity is time-varying and the accuracy of Doppler angle estimation may be affected. In this paper, the AR method using only a limited number of flow samples was applied to Doppler angle estimation of pulsatile flows. The flow samples were properly selected to derive the AR coefficients and then more samples were extrapolated based on the AR model. The proposed method was verified by both simulations and in vitro experiments. A wide range of Doppler angles (from 3o degrees to 78 degrees) and different flow rates were considered. The experimental data for the Doppler angle showed that the AR method using eight flow samples had an average estimation error of 3.50 degrees compared to an average error of 7.08 degrees for the Fast Fourier Transform (FFT) method using 64 flow samples. Results indicated that the AR method not only provided accurate Doppler angle estimates, but also outperformed the conventional FFT method in pulsatile flows. This is because the short data acquisition time is less affected by the temporal velocity changes. It is concluded that real-time two-dimensional estimation of the Doppler angle is possible using the AR method in the presence of pulsatile flows. In addition, Doppler angle estimation with turbulent flows is also discussed. Results show that both the AR and FFT methods are not adequate due to the spectral broadening effects from the turbulence.

Blood Flow Velocity↗