Hyungjoon Lim

DOA Estimation for Wideband Signal: Multiple Frequency Bins Versus Multiple Sensors

In conventional direction of arrival (DOA) estimation techniques, the inter-sensor spacing of an array is designed according to the minimal wavelength of an applied signal to avoid spatial ambiguity. This constraint greatly restricts the resolution of array or the performance of DOA estimation for a wide band signal. In this work, a new concept of wideband DOA estimation is described. Compared with multiple signal classification (MUSIC) method, the proposed method in this work uses multiple frequency bins instead of multiple sensors to form the steering vector, which reduces the sensor number for DOA estimation into 2. Besides, in this wideband DOA estimation method, the inter-sensor spacing is determined by the frequency resolution rather than the highest frequency (or minimal wavelength), so this method greatly decreases the restriction on sensor interval. Simulation results show that the proposed method with only two sensors can achieve comparative performance with convtmtional DOA estimation method using more than 20 sensors.

New Adaptive Clutter Rejection Based on Spectral Analysis for Ultrasound Color Doppler Imaging: Phantom and In Vivo Abdominal Study

Effective rejection of time-varying clutter originating from slowly moving vessels and surrounding tissues is important for depicting hemodynamics in ultrasound color Doppler imaging (CDI). In this paper, a new adaptive clutter rejection method based on spectral analysis (ACR-SA) is presented for suppressing nonstationary clutter. In ACR-SA, tissue and flow characteristics are analyzed by singular value decomposition and tissue acceleration of backscattered Doppler signals to determine an appropriate clutter filter from a set of clutter filters. To evaluate the ACR-SA method, 20 frames of complex baseband data were acquired by a commercial ultrasound system equipped with a research package (Accuvix V10, Samsung Medison, Seoul, Korea) using a 3.5-MHz convex array probe by introducing tissue movements to the flow phantom (Gammex 1425 A LE, Gammex, Middleton, WI, USA). In addition, 20 frames of in vivo abdominal data from five volunteers were captured. From the phantom experiment, the ACR-SA method provided 2.43 dB (p <; 0.001) and 1.09 dB ( ) improvements in flow signal-to-clutter ratio (SCR) compared to static (STA) and down-mixing (ACR-DM) methods. Similarly, it showed smaller values in fractional residual clutter area (FRCA) compared to the STA and ACR-DM methods (i.e., 2.3% versus 5.4% and 3.7%, respectively, ). The consistent improvements in SCR from the proposed ACR-SA method were obtained with the in vivo abdominal data (i.e., 4.97 dB and 3.39 dB over STA and ACR-DM, respectively). The ACR-SA method showed less than 1% FRCA values for all in vivo abdominal data. These results indicate that the proposed ACR-SA method can improve image quality in CDI by providing enhanced rejection of nonstationary clutter.Effective rejection of time-varying clutter originating from slowly moving vessels and surrounding tissues is important for depicting hemodynamics in ultrasound color Doppler imaging (CDI). In this paper, a new adaptive clutter rejection method based on spectral analysis (ACR-SA) is presented for suppressing nonstationary clutter. In ACR-SA, tissue and flow characteristics are analyzed by singular value decomposition and tissue acceleration of backscattered Doppler signals to determine an appropriate clutter filter from a set of clutter filters. To evaluate the ACR-SA method, 20 frames of complex baseband data were acquired by a commercial ultrasound system equipped with a research package (Accuvix V10, Samsung Medison, Seoul, Korea) using a 3.5-MHz convex array probe by introducing tissue movements to the flow phantom (Gammex 1425 A LE, Gammex, Middleton, WI, USA). In addition, 20 frames of in vivo abdominal data from five volunteers were captured. From the phantom experiment, the ACR-SA method provided 2.43 dB (p <; 0.001) and 1.09 dB ( ) improvements in flow signal-to-clutter ratio (SCR) compared to static (STA) and down-mixing (ACR-DM) methods. Similarly, it showed smaller values in fractional residual clutter area (FRCA) compared to the STA and ACR-DM methods (i.e., 2.3% versus 5.4% and 3.7%, respectively, ). The consistent improvements in SCR from the proposed ACR-SA method were obtained with the in vivo abdominal data (i.e., 4.97 dB and 3.39 dB over STA and ACR-DM, respectively). The ACR-SA method showed less than 1% FRCA values for all in vivo abdominal data. These results indicate that the proposed ACR-SA method can improve image quality in CDI by providing enhanced rejection of nonstationary clutter.

A constrained two-layer compression technique for ECG waves.

This paper proposes a constrained two-layer compression technique for electrocardiogram (ECG) waves, of which encoded parameters can be directly used for the diagnosis of arrhythmia. In the first layer, a single ECG beat is represented by one of the registered templates in the codebook. Since the required coding parameter in this layer is only the codebook index of the selected template, its compression ratio (CR) is very high. Note that the distribution of registered templates is also related to the characteristics of ECG waves, thus it can be used as a metric to detect various types of arrhythmias. The residual error between the input and the selected template is encoded by a wavelet-based transform coding in the second layer. The number of wavelet coefficients is constrained by pre-defined maximum distortion to be allowed. The MIT-BIH arrhythmia database is used to evaluate the performance of the proposed algorithm. The proposed algorithm shows around 7.18 CR when the reference value of percentage root mean square difference (PRD) is set to ten.

New adaptive clutter rejection based on spectral decomposition and tissue acceleration for ultrasound color Doppler imaging

In ultrasound color Doppler imaging(CDI), effective clutter rejection is essential for estimating flow velocity and power. Since the clutter has time-varying characteristics, it is challenging to suppress it with a static clutter filter. In this paper, a new adaptive clutter rejection method based on spectral decomposition and tissue acceleration (ACR) for suppressing nonstationary clutter is presented. In the proposed method, tissue and flow characteristics are analyzed from singular value decomposition of backscattered Doppler signals to select optimal clutter filter from a bank of clutter filters. To evaluate the ACR method, phantom and in vivo experiments were conducted. For the phantom experiments, 20 frames of complex baseband data were acquired with a commercial ultrasound system (V10, Samsung Medison, Seoul, Korea) using a 3.5-MHz convex array probe by tapping over the flow phantom (Gammex 1425A LE, Gammex, Middleton, WI, USA) surface to mimic tissue movements. Similarly, 20 frames of in vivo liver data from a volunteer were also acquired. The performance of the proposed ACR method was compared with conventional clutter rejection methods, i.e., static (ST) and down-mixing (DM), using a commonly-used flow signal-to-clutter ratio (SCR) and fractional residual clutter area (FRCA). From the phantom experiments, the ACR method provided 2.03 dB and 0.98 dB improvements in SCR over the ST and DM methods. Similarly, ACR showed improvements in fractional residual clutter area (FRCA) compared to the ST and DM methods (i.e., 2.3% vs. 5.4 % and 3.7%, respectively). The consistent results were obtained with the in vivo experiments. The improvement in SCR from the ACR method is 4.90 dB and 3.98 dB, compared to the ST and DM methods. In addition, the ACR method showed less than 1% FRCA values for all 20 frames of in vivo data. These results indicate that the proposed ACR based on spectral decomposition and tissue acceleration can improve image quality in ultrasound color Doppler imaging by effectively removing the clutter.

Feasibility study of blood velocity vector estimation using three types of transducer

Ultrasound color Doppler imaging has been a useful technique for obtaining vascular blood flow information. The conventional method, however, suffers from angle dependency when the blood flow is normal to the ultrasound beam direction so that it is difficult to visualize the blood flow. To overcome this angle dependency, various vector Doppler imaging methods with multiple transmit and receive sub-apertures have been proposed and showed an angle independent vessel flow information. However, most of them are evaluated on a linear array transducer. This paper evaluates a vector Doppler algorithm consisting of one transmit (TX) sub-aperture and two receive (RX) sub-apertures based on the convex and phased array transducer. Vessel flow velocity magnitudes and directions obtained by the vector Doppler using the convex and phased array show, for convex transducer mean direction error of 8.6843 degrees and 0.3860 m/s of mean velocity magnitude error and for phased array 16.1693 degrees and 0.0247 m/s mean errors respectively. These result indicate that convex and phased array also provide an reasonable vessel velocity magnitude and flow direction comparing to the vessel information of linear transducer which are errors of 2.9440 degrees and 0.1710 m/s respectively.