Wooyoul Lee

An efficient pulse compression method of chirp-coded excitation in medical ultrasound imaging

Coded excitation can improve the SNR in medical ultrasound imaging. In coded excitation, pulse compression is applied to compress the elongated coded signals into a short pulse, which typically requires high computational complexity, i.e., a compression filter with a few hundred coefficients. In this paper, we propose an efficient pulse compression method of chirp-coded excitation, in which the pulse compression is conducted with complex baseband data after downsampling, to lower the computational complexity. In the proposed method, although compression is conducted with the complex data, the L-fold downsampling is applied for reducing both data rates and the number of compression filter coefficients; thus, total computational complexity is reduced to the order of 1/L2. The proposed method was evaluated with simulation and phantom experiments. From the simulation and experiment results, the proposed pulse compression method produced similar axial resolution compared with the conventional pulse compression method with negligible errors, i.e., >36 dB in signal-to-error ratio (SER). These results indicate that the proposed method can maintain the performance of pulse compression of chirp-coded excitation while substantially reducing computational complexity.

Orthogonal Golay code based ultrasonic imaging without reducing frame rate

Orthogonal Golay codes feature perfect sidelobe cancellation properties, and are simple to implement because they have elements of +1 and -1 only. However, the frame rate is reduced by one half due to the requirement of two consecutive transmissions per scanline. This paper presents an ultrasonic B-mode imaging method using orthogonal complementary Golay codes without reducing the frame rate while achieving good SNR. In the method, the member codes applied to transducer elements are alternated from one transmission to another, with the focal points switched as well. The sidelobe and grating lobe levels from combining four consecutive transmit events are found to be as much as about 10 dB less than those from combining two transmit events.

Development of a carrier for adhesion of nitrifying bacteria using a thermodynamic approach

Biopop, a novel modified cellulose carrier for nitrification was developed through thermodynamic analysis of carrier materials. By measuring contact angles among nitrifying bacteria, solid carrier and broth, the change in free energy of adhesion of nitrifying bacteria on carrier (δG) was calculated. Cellulose gave the most favorable free energy change for adhesion of nitrifying bacteria and all the inoculated microbes were attached on porous carrier made of cellulose within 1 day. The new carrier gave a nitrification rate (800 g N-NH /m day), which was about four times higher than other carriers made of synthetic polymers such as polyurethane or rubber.

P2B-3 A New Motion Estimation and Compensation Method for Real-Time Ultrasonic Synthetic Aperture Imaging

In synthetic aperture imaging (SAI), it is well known fact that focusing quality will be degraded without estimation and compensation of the target movement. Various methods are available for compensation of the target movement. In this paper, simpler and more robust computational method compare to the conventional method will be presented. Presented method combines autocorrelation method of conventional two dimensional- tissue Doppler imaging (2D-TDI) with SAI, only the transmit sequence is appropriately changed. Computer simulation and the phantom experiment are used to show effectiveness of the presented method.

Smartphone-based portable ultrasound imaging system: A primary result

There is a growing need of portable ultrasound imaging systems since it can allow clinicians to access and diagnose a patient at the scene of an accident or at the patients bedside due to their improved accessibility. Portable ultrasound imaging systems have been developed based on application-specific integrated circuits (ASICs). However, this ASIC approach is typically beneficial to well-defined targeted applications such that its usefulness for POC systems would be limited. The recent advance in application processors (APs) and graphics processing units embedded in smartphones can facilitate smartphone-based portable ultrasound imaging systems. In this paper, the feasibility of the smartphone-based portable ultrasound imaging system is demonstrated where the high-end Android smartphone (i.e., Samsungs Galaxy Note II) is used for performing core ultrasound B-mode signal and image processing (e.g., quadrature demodulation and scan conversion). The ultrasound B-mode image reconstructed from the Android smartphone where an 850×800 image is reconstructed and displayed. The total execution time to perform core functional blocks for 128-scanline, 512-sample data is about 520 milliseconds.

Implementation and optimization of ultrasound signal processing algorithms on mobile GPU

A general-purpose graphics processing unit (GPGPU) has been used for improving computing power in medical ultrasound imaging systems. Recently, a mobile GPU becomes powerful to deal with 3D games and videos at high frame rates on Full HD or HD resolution displays. This paper proposes the method to implement ultrasound signal processing on a mobile GPU available in the high-end smartphone (Galaxy S4, Samsung Electronics, Seoul, Korea) with programmable shaders on the OpenGL ES 2.0 platform. To maximize the performance of the mobile GPU, the optimization of shader design and load sharing between vertex and fragment shader was performed. The beamformed data were captured from a tissue mimicking phantom (Model 539 Multipurpose Phantom, ATS Laboratories, Inc., Bridgeport, CT, USA) by using a commercial ultrasound imaging system equipped with a research package (Ultrasonix Touch, Ultrasonix, Richmond, BC, Canada). The real-time performance is evaluated by frame rates while varying the range of signal processing blocks. The implementation method of ultrasound signal processing on OpenGL ES 2.0 was verified by analyzing PSNR with MATLAB gold standard that has the same signal path. CNR was also analyzed to verify the method. From the evaluations, the proposed mobile GPU-based processing method has no significant difference with the processing using MATLAB (i.e., PSNR<52.51 dB). The comparable results of CNR were obtained from both processing methods (i.e., 11.31). From the mobile GPU implementation, the frame rates of 57.6 Hz were achieved. The total execution time was 17.4 ms that was faster than the acquisition time (i.e., 34.4 ms). These results indicate that the mobile GPU-based processing method can support real-time ultrasound B-mode processing on the smartphone.

Column-based micro-beamformer for improved 2D beamforming using a matrix array transducer

In a 3-D medical ultrasound imaging system, a matrix array probe with 2-D positioning of the elements allows high resolution of ultrasound images due to its capability of two-dimensional dynamic focusing. However, the hundreds (up to thousands) of elements in the matrix array make the fabrication of the transducers and cables challenging. In this paper, to achieve high quality of 3-D ultrasound images with low hardware complexity, we introduce a column-based micro-beamformer (CMB) in which a column in the matrix array is considered as a sub-array and then elevational and lateral beamformings are sequentially conducted in the analog and digital stages, respectively. For the performance evaluation of the proposed beamformer, the beam pattern simulations are conducted and point-spread-functions are obtained. In addition, root-mean-square-errors (RMSEs) in round-trip time delays were measured over the depths. Compared to a conventional micro-beamformer, the CMB produced more tightly focused beam patterns and showed almost equivalent performance to that of fully sampled array. In the near field, the mean RMSEs of the proposed and conventional beamformers were 274 ns and 18.1 ns (improved 93.4% of delay accuracy), respectively.

Evaluation of flow estimation methods for 3D color Doppler imaging

In 3D ultrasound color Doppler imaging (CDI), similar to 2D CDI, 8–12 pulse transmissions (ensembles, E) per each scanline are used for clutter rejection and flow estimation, leading to a low volume acquisition rate. This rate could be improved by using a small number of ensembles (e.g., E=4). However, the impact of the use of a small ensemble size on clutter rejection and flow estimation must be investigated. In this paper, we have evaluated three flow estimation methods: autoregression (AR), eigendecomposition (ED), and autocorrelation (AC) for a small size ensemble (E=4). To compare the performance of three methods, raw radio-frequency (RF) data were acquired from the Doppler flow phantom (RMI-1425A, Gammex Inc., USA) by using a commercial ultrasound machine (Accuvix V10, Medison Corp., Korea) equipped with a research package. From the phantom experiment, the AR estimator exhibited a significant improvement in area under receive operating characteristic curves compared to AC and ED methods (i.e., 0.97 ± 0.02 vs. 0.86 ± 0.07 and 0.83 ± 0.05, respectively). There is no statistically significant difference between the AC and ED methods (p>0.24). These results indicate that the AR estimator would be suitable for flow estimation in 3D CDI with a low ensemble size.

Phantom and in vivo evaluation of sound speed estimation methods: Preliminary results

For medical ultrasound imaging, the dynamic receive beamforming is important for improving image quality, i.e., spatial and contrast resolution. In current dynamic receive beamforming, a constant sound speed (e.g., 1540m/s) is assumed. However, the sound speed dispersed in soft tissues leads to defocusing and degradation of image quality. Various methods have been proposed to estimate the proper sound speed with received data, but these methods have not been verified their performance in clinical cases (e.g., breast tissue). In this paper, the five different sound speed estimation methods (i.e., coherent factor (CF), minimum average phase variance (MAPV), minimum average sum-of-absolute difference (MASAD), focus quality spectra (FQS), and modified nonlinear anisotropic difference (MNAD)) are evaluated with the tissue mimicking phantom and the in vivo breast data under the same condition. The pre-beamformed radio-frequency data (RF) for the tissue mimicking phantom and in vivo breast data are acquired using a 7.5-MHz linear array transducer with the SonixTouch research platform connected to the SonixDAQ parallel data acquisition system. In the phantom study, the five methods show considerable performance in estimating the optimal sound speed (i.e., 1450 ± 25 m/s). The CF and FQS methods also show the low errors in the in vivo breast study, but the MAPV, MASAD and MNAD methods have difficulty in estimating the optimal sound speed (1530 m/s) i.e., 25.0 ± 12.9 and 20.0 ± 8.2 vs. 72.5 ± 45.0, 72.5 ± 41.9, 52.5 ± 28.7, respectively. These results indicated that the CF and FQS methods can robustly estimate the optimal sound speed in the homogenous phantom and heterogeneous soft tissues (e.g., breast).

New baseband pulse compression for chirp coded excitation

In ultrasound medical imaging, the signal-to-noise ratio (SNR) improvement can be achieved by utilizing the code excitation. To preserve the axial resolution, a pulse compression (PC) is performed by applying a matched filter based on the transmit code. However, the conventional PC method requires a few hundred filter coefficients when compressing elongated code (e.g., chirp) into a short pulse, yielding to considerably high computational complexity. In this paper, an efficient PC method of chirp coded excitation is proposed to lower the complexity burden. In the proposed method, the PC is conducted with the decimated complex baseband data instead of a beamformed radio-frequency (RF) data. Although compression is applied with the complex data, a total computational complexity is reduced by a factor of 4 L2 since L-fold decimation is performed to reduce both data and the number of filter coefficients. The proposed method was evaluated with the phantom study. For quantitative comparison, the -6dB axial resolution for 15 wire targets and the clutter-energy-to-total-energy ratios (CTRs) at a hypo-echoic region were measured. CTR were 27.6 dB and 27.4 dB for the conventional and proposed PC methods, respectively. These results indicate that the proposed method can maintain the performance of pulse compression of chirp coded excitation while substantially reducing computational complexity.