Because ablation therapy alters the elastic modulus of tissues, emerging strain imaging methods m... more Because ablation therapy alters the elastic modulus of tissues, emerging strain imaging methods may enable clinicians for the first time to have readily available, cost effective, real-time guidance to identify the location and boundaries of thermal lesions. Electrode displacement elastography is a method of strain imaging tailored specifically to ultrasound-guided electrode-based ablative therapies (eg. radiofrequency or microwave ablation). Here tissue
Conference Record of the 2000 IEEE Industry Applications Conference. Thirty-Fifth IAS Annual Meeting and World Conference on Industrial Applications of Electrical Energy (Cat. No.00CH37129), 2000
Abstract - This paper proposes a novel fault detection approach for DC distributed power systems.... more Abstract - This paper proposes a novel fault detection approach for DC distributed power systems. In normal conditions, impedance of well-designed power electronics modules (converterdinverters) in a distributed power system ought to meet their design ...
Abstract In addition to their numerous benefits over magnetic ballasts, electronic ballasts can p... more Abstract In addition to their numerous benefits over magnetic ballasts, electronic ballasts can provide dimming capability. However, there are great difficulties in dimming a fluorescent lamp below 40%. One of the symptoms is that the lamp current becomes unstable, or even ...
Proceedings of the IECON'97 23rd International Conference on Industrial Electronics, Control, and Instrumentation (Cat. No.97CH36066), 1997
This paper investigates the efficiency of an electric vehicle inverter using zero-voltage-transit... more This paper investigates the efficiency of an electric vehicle inverter using zero-voltage-transition (ZVT) topology. The ZVT topology design and operation is explained. Steady state operating points are used to show efficiency improvements. Next, the dynamic performance of the ZVT circuit is measured using drive cycle profiles for city and highway driving. Energy use evaluates the effectiveness of the ZVT inverter
ABSTRACT-In the front-end DC/DC converters, the secondary rectification contributes an important ... more ABSTRACT-In the front-end DC/DC converters, the secondary rectification contributes an important part of loss. This paper investigates using synchronous rectifier working in Quasi-Square-Wave (QSW) mode to reduce both the conduction and reverse recovery loss in the secondary ...
Proceedings of 1994 IEEE Applied Power Electronics Conference and Exposition - ASPEC'94, 1994
The leakage inductance of planar magnetic structures designed for high-frequency power electronic... more The leakage inductance of planar magnetic structures designed for high-frequency power electronics applications is a critical performance parameter that significantly effects circuit operation. This paper presents numerical models for a particular high power (100-500 W) transformer designed for low-profile applications. The impact that secondary winding terminations have on the leakage characteristics of the device is examined using a set of
21st Annual IEEE Conference on Power Electronics Specialists, 1990
... DCI DC CONVERTERS * Wojciech A. Tabisz, Fred C. Lee and Dan Y. Chen Virginia Power Electronic... more ... DCI DC CONVERTERS * Wojciech A. Tabisz, Fred C. Lee and Dan Y. Chen Virginia Power Electronics Center The Bradley Department of Electrical Engineering Virginia Polytechnic Institute and State University Blacksburg, VA 24061 ...
[Proceedings] APEC '92 Seventh Annual Applied Power Electronics Conference and Exposition, 1992
The current stage of development of distributed power systems is presented. Various dc-bus and ac... more The current stage of development of distributed power systems is presented. Various dc-bus and ac-bus distributed power system architectures are discussed. System integration issues related to paralleling and cascading of dc/dc converters are explained. Benefits and challenges of distributed power systems in various applications are summarized. 11 0-7803-0485-3/92 $3.00 0 1992 IEEE
APEC '88 Third Annual IEEE Applied Power Electronics Conference and Exposition, 1988
The characteristics and limitations o f the half-bridge zero-voltageswitclied (ZVS) quasi-resonan... more The characteristics and limitations o f the half-bridge zero-voltageswitclied (ZVS) quasi-resonant converters (QRCs) are described. A novel multi-resonant switch concept is proposed for the half-bridge topology to improve the ZVS-QRC's load range. Experimental results for 300 v, 75 W zero-voltage-switched quasi-resonant and multi-resonant converters operating in the frequency range of 2 MHz to 8 MHz are presented.
IEEE MTT-S International Microwave Symposium digest. IEEE MTT-S International Microwave Symposium, 2004
We apply a new triaxial antenna for microwave ablation procedures. The antenna consists of a coax... more We apply a new triaxial antenna for microwave ablation procedures. The antenna consists of a coaxial monopole inserted through an 18-gauge biopsy needle positioned one quarter-wavelength from the antenna base. The biopsy needle creates a triaxial structure, which enhances return loss by more than 10 dB, thus limiting return currents along the feed line. Numerical simulations are used to optimize the antenna design. Numerical and ex-vivo experimental results are presented to quantify the field distribution, heating pattern and return loss of the antenna.
2005 European Conference on Power Electronics and Applications, 2005
This paper proposes a mixed-signal control for Voltage Regulated Modules (VRMs), which is based o... more This paper proposes a mixed-signal control for Voltage Regulated Modules (VRMs), which is based on a digital voltage loop and a peak-modulator digitally controlled by a low-resolution digital-to-analog converter (DAC). Improvement in the dynamic response and elimination of static quantization effects are obtained performing the peak-modulation on a weighted sum of output voltage and inductor current. The control features good
21st Annual IEEE Conference on Power Electronics Specialists, 1990
Page 1. DC ANALYSIS AND DESIGN OF FORWARD MULTI-RESONANT CONVERTER' Wei Tang, Wojcie... more Page 1. DC ANALYSIS AND DESIGN OF FORWARD MULTI-RESONANT CONVERTER' Wei Tang, Wojciech A. Tabisz, Ashraf Lotfi, FC Lee and V. Vorperian ... verter operates in voltage half-wave mode. 5) the active switch is bi-directional, and the con-Fig. ...
Journal of Vascular and Interventional Radiology, 2014
To characterize modified triaxial microwave antennas configured to produce short ablation zones. ... more To characterize modified triaxial microwave antennas configured to produce short ablation zones. Fifty single-antenna and 27 paired-antenna hepatic ablations were performed in domestic swine (N = 11) with 17-gauge gas-cooled modified triaxial antennas powered at 65 W from a 2.45-GHz generator. Single-antenna ablations were performed at 2 (n = 16), 5 (n = 21), and 10 (n = 13) minutes. Paired-antenna ablations were performed at 1-cm and 2-cm spacing for 5 (n = 7 and n = 8, respectively) and 10 minutes (n = 7 and n = 5, respectively). Mean transverse width, length, and aspect ratio of sectioned ablation zones were measured and compared. For single antennas, mean ablation zone lengths were 2.9 cm ± 0.45, 3.5 cm ± 0.55, and 4.2 cm ± 0.40 at 2, 5, and 10 minutes, respectively. Mean widths were 1.8 cm ± 0.3, 2.0 cm ± 0.32, and 2.5 cm ± 0.25 at 2, 5, and 10 minutes, respectively. For paired antennas, mean length at 5 minutes with 1-cm and 2-cm spacing and 10 minutes with 1-cm and 2-cm spacing was 4.2 cm ± 0.9, 4.9 cm ± 1.0, 4.8 cm ± 0.5, and 4.8 cm ± 1.3, respectively. Mean width was 3.1 cm ± 1.0, 4.4 cm ± 0.7, 3.8 cm ± 0.4, and 4.5 cm ± 0.7, respectively. Paired-antenna ablations were more spherical (aspect ratios, 0.72-0.79 for 5-10 min) than single-antenna ablations (aspect ratios, 0.57-0.59). For paired-antenna ablations, 1-cm spacing appeared optimal, with improved circularity and decreased clefting compared with 2-cm spacing (circularity, 0.85 at 1 cm, 0.78 at 2 cm). Modified triaxial antennas can generate relatively short, spherical ablation zones. Paired-antenna ablations were rounder and larger in transverse dimension than single antenna ablations, with 1-cm spacing optimal for confluence of the ablation zone.
Abstract A novel topology, so called active-clamp coupled-buck, is a good candidate for the 12V i... more Abstract A novel topology, so called active-clamp coupled-buck, is a good candidate for the 12V input voltage regulation module. Both DC and AC analysis of this circuit are considered. Poles and zeros are analyzed. A moving left-half-plane zero is identified. Theoretical ...
The hardware and software design of a nickel-hydrogen (Ni-H2) battery simulator (BS) with applica... more The hardware and software design of a nickel-hydrogen (Ni-H2) battery simulator (BS) with application to the NASA Earth Observation System (EOS) satellite is presented. The battery simulator is developed as a part of a complete testbed for the EOS satellite power system. The battery simulator involves both hardware and software components. The hardware component includes the capability of sourcing and
Because ablation therapy alters the elastic modulus of tissues, emerging strain imaging methods m... more Because ablation therapy alters the elastic modulus of tissues, emerging strain imaging methods may enable clinicians for the first time to have readily available, cost effective, real-time guidance to identify the location and boundaries of thermal lesions. Electrode displacement elastography is a method of strain imaging tailored specifically to ultrasound-guided electrode-based ablative therapies (eg. radiofrequency ablation). Here tissue deformation is achieved by applying minute perturbations to the unconstrained end of the treatment electrode, resulting in localized motion around the end of the electrode embedded in tissue. In this paper, we present a method for 3D elastographic reconstruction from volumetric data acquired using the C7F2 fourSight 4D ultrasound transducer, provided by Siemens Medical Solutions. Lesion reconstruction is demonstrated for a spherical inclusion centered in a tissue-mimicking phantom, which simulates a thermal lesion embedded in a normal tissue background. Elastographic reconstruction is also performed for a thermal lesion created in vitro in canine liver using radiofrequency ablation. Post-processing is done on the acquired raw radiofrequency data to form surface-rendered 3D elastograms of the inclusion. Elastographic volume estimates of the inclusion compare reasonably well with the actual known inclusion volume, with 3D electrode displacement elastography slightly underestimating the true inclusion volume.
Cryosurgery of the prostate presents as an efficient therapy following failed radiation therapy. ... more Cryosurgery of the prostate presents as an efficient therapy following failed radiation therapy. We report on a 7-year retrospective analysis evaluating the morbidity adn biochemical disease-free survival(bDFS) of this therapy. Between 1993 and 2001, 59 patients who had been previously treated with radiation therapy and had rising serum prostate-specific antigen(PSA) values underwent salvage cryoablation of the prostate for localized, histologically proven, recurrent prostate cancer. Serial serum PSA testing was performed, and biopsies were taken at 6, 12, and 24 months, and again at 5 years, and any time the PSA rose above 0.5 ng/mL. Patients were stratified along clinical parameters. The combined postsalvage bDFS rate using a PSA cutoff of 0.5 ng/mL was 59% and 69% with a 1.0 ng/mL PSA cut off. Using a PSA threshold of 0.5 ng/mL as evidence of biochemical recurrence, 61%, 62%, and 50% of patients with <4 ng/mL, 4-10 ng/mL, and > 10 ng/mL PSA, respectively, remain biochemical...
Because ablation therapy alters the elastic modulus of tissues, emerging strain imaging methods m... more Because ablation therapy alters the elastic modulus of tissues, emerging strain imaging methods may enable clinicians for the first time to have readily available, cost effective, real-time guidance to identify the location and boundaries of thermal lesions. Electrode displacement elastography is a method of strain imaging tailored specifically to ultrasound-guided electrode-based ablative therapies (eg. radiofrequency or microwave ablation). Here tissue
Conference Record of the 2000 IEEE Industry Applications Conference. Thirty-Fifth IAS Annual Meeting and World Conference on Industrial Applications of Electrical Energy (Cat. No.00CH37129), 2000
Abstract - This paper proposes a novel fault detection approach for DC distributed power systems.... more Abstract - This paper proposes a novel fault detection approach for DC distributed power systems. In normal conditions, impedance of well-designed power electronics modules (converterdinverters) in a distributed power system ought to meet their design ...
Abstract In addition to their numerous benefits over magnetic ballasts, electronic ballasts can p... more Abstract In addition to their numerous benefits over magnetic ballasts, electronic ballasts can provide dimming capability. However, there are great difficulties in dimming a fluorescent lamp below 40%. One of the symptoms is that the lamp current becomes unstable, or even ...
Proceedings of the IECON'97 23rd International Conference on Industrial Electronics, Control, and Instrumentation (Cat. No.97CH36066), 1997
This paper investigates the efficiency of an electric vehicle inverter using zero-voltage-transit... more This paper investigates the efficiency of an electric vehicle inverter using zero-voltage-transition (ZVT) topology. The ZVT topology design and operation is explained. Steady state operating points are used to show efficiency improvements. Next, the dynamic performance of the ZVT circuit is measured using drive cycle profiles for city and highway driving. Energy use evaluates the effectiveness of the ZVT inverter
ABSTRACT-In the front-end DC/DC converters, the secondary rectification contributes an important ... more ABSTRACT-In the front-end DC/DC converters, the secondary rectification contributes an important part of loss. This paper investigates using synchronous rectifier working in Quasi-Square-Wave (QSW) mode to reduce both the conduction and reverse recovery loss in the secondary ...
Proceedings of 1994 IEEE Applied Power Electronics Conference and Exposition - ASPEC'94, 1994
The leakage inductance of planar magnetic structures designed for high-frequency power electronic... more The leakage inductance of planar magnetic structures designed for high-frequency power electronics applications is a critical performance parameter that significantly effects circuit operation. This paper presents numerical models for a particular high power (100-500 W) transformer designed for low-profile applications. The impact that secondary winding terminations have on the leakage characteristics of the device is examined using a set of
21st Annual IEEE Conference on Power Electronics Specialists, 1990
... DCI DC CONVERTERS * Wojciech A. Tabisz, Fred C. Lee and Dan Y. Chen Virginia Power Electronic... more ... DCI DC CONVERTERS * Wojciech A. Tabisz, Fred C. Lee and Dan Y. Chen Virginia Power Electronics Center The Bradley Department of Electrical Engineering Virginia Polytechnic Institute and State University Blacksburg, VA 24061 ...
[Proceedings] APEC '92 Seventh Annual Applied Power Electronics Conference and Exposition, 1992
The current stage of development of distributed power systems is presented. Various dc-bus and ac... more The current stage of development of distributed power systems is presented. Various dc-bus and ac-bus distributed power system architectures are discussed. System integration issues related to paralleling and cascading of dc/dc converters are explained. Benefits and challenges of distributed power systems in various applications are summarized. 11 0-7803-0485-3/92 $3.00 0 1992 IEEE
APEC '88 Third Annual IEEE Applied Power Electronics Conference and Exposition, 1988
The characteristics and limitations o f the half-bridge zero-voltageswitclied (ZVS) quasi-resonan... more The characteristics and limitations o f the half-bridge zero-voltageswitclied (ZVS) quasi-resonant converters (QRCs) are described. A novel multi-resonant switch concept is proposed for the half-bridge topology to improve the ZVS-QRC's load range. Experimental results for 300 v, 75 W zero-voltage-switched quasi-resonant and multi-resonant converters operating in the frequency range of 2 MHz to 8 MHz are presented.
IEEE MTT-S International Microwave Symposium digest. IEEE MTT-S International Microwave Symposium, 2004
We apply a new triaxial antenna for microwave ablation procedures. The antenna consists of a coax... more We apply a new triaxial antenna for microwave ablation procedures. The antenna consists of a coaxial monopole inserted through an 18-gauge biopsy needle positioned one quarter-wavelength from the antenna base. The biopsy needle creates a triaxial structure, which enhances return loss by more than 10 dB, thus limiting return currents along the feed line. Numerical simulations are used to optimize the antenna design. Numerical and ex-vivo experimental results are presented to quantify the field distribution, heating pattern and return loss of the antenna.
2005 European Conference on Power Electronics and Applications, 2005
This paper proposes a mixed-signal control for Voltage Regulated Modules (VRMs), which is based o... more This paper proposes a mixed-signal control for Voltage Regulated Modules (VRMs), which is based on a digital voltage loop and a peak-modulator digitally controlled by a low-resolution digital-to-analog converter (DAC). Improvement in the dynamic response and elimination of static quantization effects are obtained performing the peak-modulation on a weighted sum of output voltage and inductor current. The control features good
21st Annual IEEE Conference on Power Electronics Specialists, 1990
Page 1. DC ANALYSIS AND DESIGN OF FORWARD MULTI-RESONANT CONVERTER&amp;#x27; Wei Tang, Wojcie... more Page 1. DC ANALYSIS AND DESIGN OF FORWARD MULTI-RESONANT CONVERTER&amp;#x27; Wei Tang, Wojciech A. Tabisz, Ashraf Lotfi, FC Lee and V. Vorperian ... verter operates in voltage half-wave mode. 5) the active switch is bi-directional, and the con-Fig. ...
Journal of Vascular and Interventional Radiology, 2014
To characterize modified triaxial microwave antennas configured to produce short ablation zones. ... more To characterize modified triaxial microwave antennas configured to produce short ablation zones. Fifty single-antenna and 27 paired-antenna hepatic ablations were performed in domestic swine (N = 11) with 17-gauge gas-cooled modified triaxial antennas powered at 65 W from a 2.45-GHz generator. Single-antenna ablations were performed at 2 (n = 16), 5 (n = 21), and 10 (n = 13) minutes. Paired-antenna ablations were performed at 1-cm and 2-cm spacing for 5 (n = 7 and n = 8, respectively) and 10 minutes (n = 7 and n = 5, respectively). Mean transverse width, length, and aspect ratio of sectioned ablation zones were measured and compared. For single antennas, mean ablation zone lengths were 2.9 cm ± 0.45, 3.5 cm ± 0.55, and 4.2 cm ± 0.40 at 2, 5, and 10 minutes, respectively. Mean widths were 1.8 cm ± 0.3, 2.0 cm ± 0.32, and 2.5 cm ± 0.25 at 2, 5, and 10 minutes, respectively. For paired antennas, mean length at 5 minutes with 1-cm and 2-cm spacing and 10 minutes with 1-cm and 2-cm spacing was 4.2 cm ± 0.9, 4.9 cm ± 1.0, 4.8 cm ± 0.5, and 4.8 cm ± 1.3, respectively. Mean width was 3.1 cm ± 1.0, 4.4 cm ± 0.7, 3.8 cm ± 0.4, and 4.5 cm ± 0.7, respectively. Paired-antenna ablations were more spherical (aspect ratios, 0.72-0.79 for 5-10 min) than single-antenna ablations (aspect ratios, 0.57-0.59). For paired-antenna ablations, 1-cm spacing appeared optimal, with improved circularity and decreased clefting compared with 2-cm spacing (circularity, 0.85 at 1 cm, 0.78 at 2 cm). Modified triaxial antennas can generate relatively short, spherical ablation zones. Paired-antenna ablations were rounder and larger in transverse dimension than single antenna ablations, with 1-cm spacing optimal for confluence of the ablation zone.
Abstract A novel topology, so called active-clamp coupled-buck, is a good candidate for the 12V i... more Abstract A novel topology, so called active-clamp coupled-buck, is a good candidate for the 12V input voltage regulation module. Both DC and AC analysis of this circuit are considered. Poles and zeros are analyzed. A moving left-half-plane zero is identified. Theoretical ...
The hardware and software design of a nickel-hydrogen (Ni-H2) battery simulator (BS) with applica... more The hardware and software design of a nickel-hydrogen (Ni-H2) battery simulator (BS) with application to the NASA Earth Observation System (EOS) satellite is presented. The battery simulator is developed as a part of a complete testbed for the EOS satellite power system. The battery simulator involves both hardware and software components. The hardware component includes the capability of sourcing and
Because ablation therapy alters the elastic modulus of tissues, emerging strain imaging methods m... more Because ablation therapy alters the elastic modulus of tissues, emerging strain imaging methods may enable clinicians for the first time to have readily available, cost effective, real-time guidance to identify the location and boundaries of thermal lesions. Electrode displacement elastography is a method of strain imaging tailored specifically to ultrasound-guided electrode-based ablative therapies (eg. radiofrequency ablation). Here tissue deformation is achieved by applying minute perturbations to the unconstrained end of the treatment electrode, resulting in localized motion around the end of the electrode embedded in tissue. In this paper, we present a method for 3D elastographic reconstruction from volumetric data acquired using the C7F2 fourSight 4D ultrasound transducer, provided by Siemens Medical Solutions. Lesion reconstruction is demonstrated for a spherical inclusion centered in a tissue-mimicking phantom, which simulates a thermal lesion embedded in a normal tissue background. Elastographic reconstruction is also performed for a thermal lesion created in vitro in canine liver using radiofrequency ablation. Post-processing is done on the acquired raw radiofrequency data to form surface-rendered 3D elastograms of the inclusion. Elastographic volume estimates of the inclusion compare reasonably well with the actual known inclusion volume, with 3D electrode displacement elastography slightly underestimating the true inclusion volume.
Cryosurgery of the prostate presents as an efficient therapy following failed radiation therapy. ... more Cryosurgery of the prostate presents as an efficient therapy following failed radiation therapy. We report on a 7-year retrospective analysis evaluating the morbidity adn biochemical disease-free survival(bDFS) of this therapy. Between 1993 and 2001, 59 patients who had been previously treated with radiation therapy and had rising serum prostate-specific antigen(PSA) values underwent salvage cryoablation of the prostate for localized, histologically proven, recurrent prostate cancer. Serial serum PSA testing was performed, and biopsies were taken at 6, 12, and 24 months, and again at 5 years, and any time the PSA rose above 0.5 ng/mL. Patients were stratified along clinical parameters. The combined postsalvage bDFS rate using a PSA cutoff of 0.5 ng/mL was 59% and 69% with a 1.0 ng/mL PSA cut off. Using a PSA threshold of 0.5 ng/mL as evidence of biochemical recurrence, 61%, 62%, and 50% of patients with <4 ng/mL, 4-10 ng/mL, and > 10 ng/mL PSA, respectively, remain biochemical...
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Papers by Fred Lee