Young-Wook Choi

Pulse forming lines for square pulse generators

The designing of square pulse generators using pulse-forming lines (PFLs) made up of identical L and C, was reviewed in this study. 14 different types of PFL were analyzed utilizing PSpice simulation results. These PFLs were characterized with respect to their distinct features: the number of forming lines (single or double), the circuit relationship of the PFL and load (parallel or series), and the types of energy storage (current source, voltage source or a combination of both). The characteristic impedance, output parameters such as pulse width, voltage and current magnitudes, and power were derived for each scheme. The merits and demerits of the output parameters were also included.

Computer-aided detection system for masses in automated whole breast ultrasonography: development and evaluation of the effectiveness

Purpose: The aim of this study was to evaluate the performance of a proposed computer-aided detection (CAD) system in automated breast ultrasonography (ABUS). Methods: Eighty-nine two-dimensional images (20 cysts, 42 benign lesions, and 27 malignant lesions) were obtained from 47 patients who underwent ABUS (ACUSON S2000). After boundary detection and removal, we detected mass candidates by using the proposed adjusted Otsus threshold; the threshold was adaptive to the variations of pixel intensities in an image. Then, the detected candidates were segmented. Features of the segmented objects were extracted and used for training/testing in the classification. In our study, a support vector machine classifier was adopted. Eighteen features were used to determine whether the candidates were true lesions or not. A five-fold cross validation was repeated 20 times for the performance evaluation. The sensitivity and the false positive rate per image were calculated, and the classification accuracy was evaluated for each feature. Results: In the classification step, the sensitivity of the proposed CAD system was 82.67% (SD, 0.02%). The false positive rate was 0.26 per image. In the detection/segmentation step, the sensitivities for benign and malignant mass detection were 90.47% (38/42) and 92.59% (25/27), respectively. In the five-fold cross-validation, the standard deviation of pixel intensities for the mass candidates was the most frequently selected feature, followed by the vertical position of the centroids. In the univariate analysis, each feature had 50% or higher accuracy. Conclusion: The proposed CAD system can be used for lesion detection in ABUS and may be useful in improving the screening efficiency.

Optimization of configuration parameters in a newly developed digital breast tomosynthesis system

The purpose of the present work was to investigate the effects of variable projection-view (PV) and angular dose (AD) distributions on the reconstructed image quality for improving microcalcification detection. The PV densities at central and peripheral sites were varied through the distribution of 21 PVs acquired over ±25° angular range. To vary the AD distribution, 7 PVs in the central region were targeted with two, four and six times the peripheral dose, and the number of central PVs receiving four times the peripheral dose was increased from 3 to 11. The contrast-to-noise ratio (CNR) for in-focus plane quality and the full width at half maximum (FWHM) of artifact spread function (ASF) for resolution in the z-direction were used. Although the ASF improved with increasing PV densities at two peripheral sites, the CNRs were inferior to those obtained with other subsets. With increasing PV density in the central area, the vertical resolution decreased but the CNR increased. Although increasing the central PV or AD concentrations improved image quality, excessive central densities reduced image quality by increasing noise in peripheral views.

A constant current high voltage capacitor charging power supply for pulsed power applications

This paper describes a power supply for a rapid pulsed power charging system designed for charging a 0.25 /spl mu/F capacitor up to 20 kV in approximately 3 ms. It is capable of charging the load capacitor at repetition rate of 300 pps. This power supply is based on a series resonant three phase inverter followed by step-up transformers. Experimental results carried out at different repetition rates and charging voltage are included along with the system descriptions.

Development of a magnetic pulse compression modulator for flue gas treatment

A pulse generator of a one-stage magnetic pulse compression (MPC) modulator, which has an average power of 7.5 kW, was developed. The modulator consists of a constant-current capacitor charger (max. 21.7 kV, 300 Hz), an initial charge capacitor (0.2 /spl mu/F, 40 kV), a thyratron switch, a pulse transformer with a turn ratio of 1 to 8, ceramic capacitors (2.7 nF, 160 kV), and a magnetic switch. The output voltage, current and the pulsewidth of the modulator were measured to be 85.6 kV, 1.88 kA, and 200 ns (full width half maximum) respectively, with the initial charging voltage of 20 kV. This modulator system can be operated at the maximum repetition rate of 300 Hz and the energy delivery efficiency from the initial charge capacitor to the dummy load is estimated as 55%. The pulse generator is applied to a reactor of pin-to-hole type to investigate the impedance matching between the pulse generator and the reactor, and the experimental results are presented.

Fast high-voltage pulse generation using nonlinear capacitors

Many pulsed power applications require short high-voltage pulses with a high-repetition rate. Conventional high-voltage discharge pulse-switches such as thyratrons, spark gap switches, and vacuum tube switches have a short lifetime, whereas the semiconductor switches have a long lifetime and high reliability. The semiconductor switches, however cannot be directly applied to fast high-voltage pulsed power generation due to their limited operating voltage ratings despite their relatively long switching times. Therefore, they are used with voltage amplification and a pulse compression stage. This paper describes two pulse generators that use the semi-conductor switches and nonlinear capacitors: one is based on an opening switch (IGBT) and inductive energy storage, the other is a combination of a closing switch (RSD) and capacitive energy storage.

Optimization of the key imaging parameters for detection of microcalcifications in a newly developed digital breast tomosynthesis system

PURPOSE The purpose of this study was to investigate the effect of different acquisition parameters and to characterize their relationships in order to improve the detection of microcalcifications using digital breast tomosynthesis (DBT). MATERIALS AND METHODS DBT imaging parameters were optimized using 32 different acquisition sets with 6 angular ranges (± 5°, ± 10°, ± 13°, ± 17°, ± 21°, and ± 25°) and 8 projection views (PVs) (5, 11, 15, 21, 25, 31, 41, and 51 projections). To investigate the effects of variable angular dose distribution, the acquisition sets were evaluated with delivering more dose toward the central views. RESULTS Our results show that a wide angular range improved the reconstructed image quality in the z-direction. If a large number of projections are acquired, then electronic noise may dominate the contrast-to-noise ratio (CNR) due to reduced radiation dose per projection. With delivering more dose toward the central views, it was found that the vertical resolution was reduced with increasing dose in the central PVs. On the other hand, the CNR clearly increased with increasing concentration of dose distribution in central views. CONCLUSIONS Although it was found that increasing angular range improved the vertical resolution, it was also found that the image quality of microcalcifications in the in-focus plane did not improve by increasing the noise due to greater effective breast thickness. Angular dose distributions, with more dose delivered to the central views, generally yielded a higher quality factor than uniform dose distributions.

Fast high voltage pulse generation using nonlinear capacitors

Many pulsed power applications require short high voltage pulses with high repetition rate. Conventional high-voltage discharge pulse-switches such as thyratrons, spark gap switches, vacuum tube switches have short lifetime, whereas the semiconductor switches have long lifetime and high reliability. The semiconductor switches, however, can not be directly applied to fast high voltage pulsed power generation due to their limited operating voltage ratings despite their relatively long switching times. Therefore they are used with a voltage amplification and a pulse compression stages. This paper describes two pulse generators using the semiconductor switches and nonlinear capacitors: one is based on an opening switch (IGBT) and an inductive energy storage. The other is a combination of a closing switch (RSD) and a capacitive energy storage.

Characterizing X-ray detectors for prototype digital breast tomosynthesis systems

The digital breast tomosynthesis (DBT) system is a newly developed 3-D imaging technique that overcomes the tissue superposition problems of conventional mammography. Therefore, it produces fewer false positives. In DBT system, several parameters are involved in image acquisition, including geometric components. A series of projections should be acquired at low exposure. This makes the system strongly dependent on the detectors characteristic performance. This study compares two types of x-ray detectors developed by the Korea Electrotechnology Research Institute (KERI). The first prototype DBT system has a CsI (Tl) scintillator/CMOS based flat panel digital detector (2923 MAM, Dexela Ltd.), with a pixel size of 0.0748 mm. The second uses a-Se based direct conversion full field detector (AXS 2430, analogic) with a pixel size of 0.085 mm. The geometry of both systems is same, with a focal spot 665.8 mm from the detector, and a center of rotation 33 mm above the detector surface. The systems were compared with regard to modulation transfer function (MTF), normalized noise power spectrum (NNPS), detective quantum efficiency (DQE) and a new metric, the relative object detectability (ROD). The ROD quantifies the relative performance of each detector at detecting specified objects. The system response function demonstrated excellent linearity (R2>0.99). The CMOS-based detector had a high sensitivity, while the Anrad detector had a large dynamic range. The higher MTF and noise power spectrum (NPS) values were measured using an Anrad detector. The maximum DQE value of the Dexela detector was higher than that of the Anrad detector with a low exposure level, considering one projection exposure for tomosynthesis. Overall, the Dexela detector performed better than did the Anrad detector with regard to the simulated Al wires, spheres, test objects of ROD with low exposure level. In this study, we compared the newly developed prototype DBT system with two different types of x-ray detectors for commercial DBT systems. Our findings suggest that the Dexela detector can be applied to the DBT system with regard to its high imaging performance.

Development of a 30-kW Plasma Gun System With a Long Lifetime

A hollow cathode that has extremely stable discharge characteristics has been developed. It is composed of two separated lanthanum hexaboride (LaB6) disks in the tube as the electron emitters. The design accomplishes great advantages by extending the surface discharge area of LaB6, an extension that is also useful for optimal fixing of LaB6. The hollow cathode is capable of producing 30 kW (100 V and 300 A) of power continuously. The plasma beam generated by the hollow cathode passes through holes in the centers of two intermediate electrodes covered with tungsten, which is water cooled due to the high plasma temperature entailed (greater than 3000degC ). The combination of the hollow cathode and the two intermediate electrodes is practically useful for an ion plating plasma beam source.