Guang Yang

University of North Carolina at Chapel Hill, North Carolina, United States

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Publications (17)

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    [Show abstract] [Hide abstract] ABSTRACT: The purpose of this study is to investigate the feasibility of increasing the system spatial resolution and scanning speed of Hologic Selenia Dimensions digital breast tomosynthesis (DBT) scanner by replacing the rotating mammography x-ray tube with a specially designed carbon nanotube (CNT) x-ray source array, which generates all the projection images needed for tomosynthesis reconstruction by electronically activating individual x-ray sources without any mechanical motion. The stationary digital breast tomosynthesis (s-DBT) design aims to (i) increase the system spatial resolution by eliminating image blurring due to x-ray tube motion and (ii) reduce the scanning time. Low spatial resolution and long scanning time are the two main technical limitations of current DBT technology. A CNT x-ray source array was designed and evaluated against a set of targeted system performance parameters. Simulations were performed to determine the maximum anode heat load at the desired focal spot size and to design the electron focusing optics. Field emission current from CNT cathode was measured for an extended period of time to determine the stable life time of CNT cathode for an expected clinical operation scenario. The source array was manufactured, tested, and integrated with a Selenia scanner. An electronic control unit was developed to interface the source array with the detection system and to scan and regulate x-ray beams. The performance of the s-DBT system was evaluated using physical phantoms. The spatially distributed CNT x-ray source array comprised 31 individually addressable x-ray sources covering a 30 angular span with 1 pitch and an isotropic focal spot size of 0.6 mm at full width at half-maximum. Stable operation at 28 kV(peak) anode voltage and 38 mA tube current was demonstrated with extended lifetime and good source-to-source consistency. For the standard imaging protocol of 15 views over 14, 100 mAs dose, and 2 × 2 detector binning, the projection resolution along the scanning direction increased from 4.0 cycles/mm [at 10% modulation-transfer-function (MTF)] in DBT to 5.1 cycles/mm in s-DBT at magnification factor of 1.08. The improvement is more pronounced for faster scanning speeds, wider angular coverage, and smaller detector pixel sizes. The scanning speed depends on the detector, the number of views, and the imaging dose. With 240 ms detector readout time, the s-DBT system scanning time is 6.3 s for a 15-view, 100 mAs scan regardless of the angular coverage. The scanning speed can be reduced to less than 4 s when detectors become faster. Initial phantom studies showed good quality reconstructed images. A prototype s-DBT scanner has been developed and evaluated by retrofitting the Selenia rotating gantry DBT scanner with a spatially distributed CNT x-ray source array. Preliminary results show that it improves system spatial resolution substantially by eliminating image blur due to x-ray focal spot motion. The scanner speed of s-DBT system is independent of angular coverage and can be increased with faster detector without image degration. The accelerated lifetime measurement demonstrated the long term stability of CNT x-ray source array with typical clinical operation lifetime over 3 years.
    Full-text Article · Apr 2012 · Medical Physics
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    [Show abstract] [Hide abstract] ABSTRACT: Digital breast tomosynthesis (DBT) is a limited angle computed tomography technique that can distinguish tumors from its overlying breast tissues and has potentials for detection of cancers at a smaller size and earlier stage. Current prototype DBT scanners are based on the regular full-field digital mammography systems and require partial isocentric motion of an x-ray tube over certain angular range to record the projection views. This prolongs the scanning time and, in turn, degrades the imaging quality due to motion blur. To mitigate the above limitations, the concept of a stationary DBT (s-DBT) scanner has been recently proposed based on the newly developed spatially distributed multibeam field emission x-ray (MBFEX) source technique using the carbon nanotube. The purpose of this article is to evaluate the performance of the 25-beam MBFEX source array that has been designed and fabricated for the s-DBT system. The s-DBT system records all the projection images by electronically activating the multiple x-ray beams from different viewing angles without any mechanical motion. The configuration of the MBFEX source is close to the published values from the Siemens Mammomat system. The key issues including the x-ray flux, focal spot size, spatial resolution, scanning time, beam-to-beam consistency, and reliability are evaluated using the standard procedures. In this article, the authors describe the design and performance of a distributed x-ray source array specifically designed for the s-DBT system. They evaluate the emission current, current variation, lifetime, and focal spot sizes of the source array. An emission current of up to 18 mA was obtained at 0.5 x 0.3 mm effective focal spot size. The experimentally measured focal spot sizes are comparable to that of a typical commercial mammography tube without motion blurring. Trade-off between the system spatial resolution, x-ray flux, and scanning time are also discussed. Projection images of a breast phantom were collected using the x-ray source array from 25 different viewing angles without motion. These preliminary results demonstrate the feasibility of the proposed s-DBT scanner. The technology has the potential to increase the resolution and reduce the imaging time for DBT. With the present design of 25 views, they demonstrated experimentally the feasibility of achieving 11 s scanning time at full detector resolution with 0.5 x 0.3 mm source resolution without motion blur. The flexibility in configuration of the x-ray source array will also allow system designers to consider imaging geometries that are difficult to achieve with the conventional single-source rotating approach.
    Full-text Article · Oct 2009 · Medical Physics
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    Ramya Rajaram · Guang Yang · Otto Z. Zhou · [...] · David S. Lalush
    [Show abstract] [Hide abstract] ABSTRACT: We have designed and built a stationary digital breast tomosynthesis (DBT) system containing a carbon nanotube based field emission x-ray source array to examine the possibility of obtaining a reduced scan time and improved image quality compared to conventional DBT systems. There are 25 individually addressable x-ray sources in our linear source array that are evenly angularly spaced to cover an angle of 48°. The sources are turned on sequentially during imaging and there is no motion of either the source or the detector. We present here an iterative reconstruction method based on a modified Ordered-Subset Convex (MOSC) algorithm that was employed for the reconstruction of images from the new DBT system. Using this algorithm based on a maximum-likelihood model, we reconstruct on non-cubic voxels for increased computational efficiency resulting in high in-plane resolution in the images. We have applied the reconstruction technique on simulated and phantom data from the system. Even without the use of the subsets, the reconstruction of an experimental 9-beam system with 960×768 pixels took less than 6 minutes (10 iterations). The projection images of a simulated mammography accreditation phantom were reconstructed using MOSC and a Simultaneous Algebraic Reconstruction technique (SART) and the results from the comparison between the two algorithms allow us to conclude that the MOSC is capable of delivering excellent image quality when used in tomosynthesis image reconstruction.
    Full-text Chapter · Feb 2009
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    Ying Chen · Weihua Zhou · Guang Yang · [...] · Otto Zhou
    [Show abstract] [Hide abstract] ABSTRACT: As a new three-dimensional breast imaging technique, breast tomosynthesis allows the reconstruction of an arbitrary set of planes in the breast from a limited-angle series of x-ray projection images. The breast tomosynthesis technique has been demonstrated as promising to improve early breast cancer detection. This paper represents a preliminary phantom study and computer simulation results of different breast tomosynthesis reconstruction algorithms with a novel carbon nanotube based multi-beam x-ray source. Five representative tomosynthesis reconstruction algorithms, including back projection (BP), filtered back projection (FBP), matrix inversion tomosynthesis (MITS), maximum likelihood expectation maximization (MLEM), and simultaneous algebraic reconstruction technique (SART) were investigated. Tomosynthesis projection images of a phantom were acquired with the stationary multi-beam x-ray tomosynthesis system. Reconstruction results from different algorithms were studied. A computer simulation study was further done to investigate the sharpness of reconstructed in-plane structures and to see how effective each algorithm is at removing out-of-plane blur with parallel-imaging geometries. Datasets with 9 and 25 projection images of a defined 3D spherical object were simulated with a total view angle of 50 degrees. Results showed that the multi-beam x-ray system is capable to generate 3D tomosynthesis images with faster speed compared with current commercial prototype systems. With simulated parallel-imaging geometry, MITS and FBP showed edge enhancement in-plane performance. BP, FBP and MLEM performed better at out-of-plane structure removal with larger number of projection images.
    Full-text Article · Feb 2009 · Proceedings of SPIE - The International Society for Optical Engineering
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    Guang Yang · Ramya Rajaram · Guohua Cao · [...] · Otto Zhou
    [Show abstract] [Hide abstract] ABSTRACT: A stationary digital breast tomosynthesis (DBT) system using a carbon nanotube based multi-beam field emission x-ray (MBFEX) source has been designed. The purpose is to investigate the feasibility of reducing the total imaging time, simplifying the system design, and potentially improving the image quality comparing to the conventional DBT scanners. The MBFEX source consists of 25 individually programmable x-ray pixels which are evenly angular spaced covering a 48° field of view. The device acquires the projection images by electronically switching on and off the individual x-ray pixels without mechanical motion of either the x-ray source or the detector. The designs of the x-ray source and the imaging system are presented. Some preliminary results are discussed.
    Full-text Article · Apr 2008 · Proceedings of SPIE - The International Society for Optical Engineering
  • Guang Yang · Ramya Rajaram · Guohua Cao · [...] · Otto Zhou
    [Show abstract] [Hide abstract] ABSTRACT: A stationary digital breast tomosynthesis (DBT) system using a carbon nanotube based multi-beam field emission x-ray (MBFEX) source has been designed. The purpose is to investigate the feasibility of reducing the total imaging time, simplifying the system design, and potentially improving the image quality comparing to the conventional DBT scanners. The MBFEX source consists of 25 individually programmable x-ray pixels which are evenly angular spaced covering a 48 degrees field of view. The device acquires the projection images by electronically switching on and off the individual X-ray pixels without mechanical motion of either the x-ray source or the detector. The designs of the x-ray source and the imaging system are presented. Some preliminary results are discussed.
    Conference Paper · Jan 2008
  • Guang Yang · Ramya Rajaram · Guohua Cao · [...] · Otto Zhou
    Article · Jan 2008 · Proc SPIE
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    [Show abstract] [Hide abstract] ABSTRACT: The authors report the effects of Cs doping on the field emission properties of a five-shell single carbon nanotube. The chiral indices of each shell of this carbon nanotube have been determined using nanobeam electron diffraction, which has four semiconducting shells and one metallic shell in the middle. From the Fowler-Nordheim plots, a reduction from 4.8 to 3.8 eV has been observed in the work function of the single carbon nanotube before and after Cs doping. (c) 2006 American Institute of Physics.
    Full-text Article · Dec 2006 · Applied Physics Letters
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    Zejian Liu · Guang Yang · Yueh Z. Lee · [...] · Otto Zhou
    [Show abstract] [Hide abstract] ABSTRACT: Microcomputed tomography is now widely used for in vivo small animal imaging for cancer studies. Achieving high imaging quality of live objects requires the x-ray source to have both high spatial and temporal resolutions. Preliminary studies have shown that carbon nanotube (CNT) based field emission x-ray source has significant intrinsic advantages over the conventional thermionic x-ray tube including better temporal resolution and programmability. Here we report the design and characterization of a CNT based field emission x-ray source that also affords a high spatial resolution. The device uses modified asymmetric Einzel lenses for electron focusing and an elliptical shaped CNT cathode patterned by photolithography. Stable and small isotropic x-ray focal spot sizes were obtained.
    Full-text Article · Sep 2006 · Applied Physics Letters
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    Zejian Liu · Jian Zhang · Guang Yang · [...] · Jianping Lu
    [Show abstract] [Hide abstract] ABSTRACT: We report a detailed study on the design of carbon nanotube based microfocus x-ray tube with one electrostatic focusing electrode. Based on the electron optics simulations, such parameters as geometrical distances and applied voltages among all the electrodes are considered, respectively, in relation to the size of x-ray focal spot. The stability of the x-ray focal spot size is also examined with respect to the variation of gate and anode voltages. Experimental results that agree well with the simulated data are also provided to corroborate the design method. We also discuss the operating stability and limitations when designing a carbon nanotube based microfocus x-ray tube with only one electrostatic focusing electrode. The designed x-ray tube with an isotropic focal spot sees wide applications in in vivo medical imaging studies. © 2006 American Institute of Physics.
    Full-text Article · May 2006 · Review of Scientific Instruments
  • Zejian Liu · Yueh Lee · Guang Yang · [...] · Otto Zhou
    [Show abstract] [Hide abstract] ABSTRACT: We have developed a carbon nanotube based microfocus X-ray source with an isotropic focal spot. Two focusing electrodes were implemented in the design with one electrode harnessing the divergence of field-emitted electrons from gate and the other focusing electrons onto the anode. Isotropic X-ray focal spot was achieved by utilizing an elliptical cathode that forms elliptical electron probe on the anode after electrostatic focusing. Based on the design method, an x-ray source with an isotropic focal spot of 65 mum in diameter was experimentally demonstrated in X-ray projection images. This type of X-ray source sees wide applications in cone-beam tomography imaging studies.
    Article · Mar 2006
  • Jie Tang · Guang Yang · Jian Zhang · [...] · Orlin D. Velev
    [Show abstract] [Hide abstract] ABSTRACT: In this paper we show that dielectrophoresis can be utilized as a general approach for guided-assembly of hierarchical structures of one-dimensional nano-materials. As a demonstration, pre-formed carbon nanotubes (CNTs) were assembled into sub-micron diameter fibrils anchored on a metal tip by dielectrophoresis. This method affords fine control of the fibril length and the potential for parallel processing. The CNTs within the fibril are bonded by van der Waals forces and are aligned along the fibril axis. The fibrils with 1~20 µm in length are sufficiently stiff for use as scanning probe microscope probes. Longer fibrils have also been drawn from the liquid. The fibrils are also stable electron field emitters. © 2005 by World Scientific Publishing Co. Pte. Ltd. All rights reserved.
    Conference Paper · May 2005
  • Jie Tang · Guang Yang · Qi Zhang · [...] · Otto Zhou
    [Show abstract] [Hide abstract] ABSTRACT: The feasibility of fabricating carbon nanotube atomic force microscopes (CNT AFM) tips by dielectrophoresis was investigated. The investigation was carried out by using a commercial Si AFM probe and a small metal ring as the counter electrode. In the dielectrophoresis process, the length of the CNT probe was controlled by the distance that the counter electrode was translated under the ac field. The results show that room-temperature liquid-phase dielectrophoresis provides an efficient method for the fabrication of CNT-based AFM probes with controlled length and orientation.
    Article · Feb 2005 · Nano Letters
  • Jie Tang · Guang Yang · Ben Maynor · [...] · Otto Zhou
    [Show abstract] [Hide abstract] ABSTRACT: Carbon nanotube probes with diameter of 1˜100 nm and large aspect ratios have been demonstrated both theoretically and experimentally that they are quasi-one-dimensional solids with many unique electronic and mechanical properties. We have recently demonstrated the feasibility of fabricating carbon nanotube AFM probes by a solution based dielectrophoresis process. CNT AFM probes can be readily assembled on the apexes of commercial AFM probes with controlled and predetermined length and orientation. In this talk we discuss the effects of the structure and morphology of the cnt AFM probes, the dispersion and stability of the cnt suspension on the quality and reliability of the cnt probes fabricated. The structure and mechanical stability of the probes were also investigated.
    Article · Jan 2005
  • [Show abstract] [Hide abstract] ABSTRACT: For field-induced electron emission, the two factors that enable a high emission current density at low applied voltages are (a) low work function of the emitter and (b) sharpness of the emitter tip. We have developed and applied a chemical vapor deposition method to synthesize single-crystalline LaB6 nanowires for applications as point electron emitters. The crystallographic orientation of the grown nanowires can be controlled by the catalysts used in synthesis and their typical diameter is ranged from below 20 nm to over 100 nm. The nanowires’ tip is either hemispherical or flat top with rectangular cross-section depending on the catalyst being utilized. The field emission properties have also been measured from the single nanowire emitters and the results are discussed for applications as point electron sources used in high performance electron optical instruments such as the transmission and scanning electron microscopes.
    Article · Dec 2004 · MRS Online Proceeding Library Archive
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    Qi Zhang · Jie Tang · Guang Yang · [...] · L C Qin
    Full-text Article · Aug 2004 · Microscopy and Microanalysis
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