Gary X. Shen

The University of Hong Kong, Hong Kong, Hong Kong

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Publications (29)67.5 Total impact

  • Jing Yuan · Juan Wei · Gary X Shen
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    ABSTRACT: Optical glass fiber shows great advantages over coaxial cables in terms of electromagnetic interference, thus, it should be considered a potential alternative for magnetic resonance imaging (MRI) receive coil interconnection, especially for a large number coil array at high field. In this paper, we propose a 4-channel analog direct modulation optical link for a 1.5-T MRI coil array interconnection. First, a general direct modulated optical link is compared to an external modulated optical link. And then the link performances of the proposed direct modulated optical link, including power gain, frequency response, and dynamic range, are analyzed and measured. Phantom and in vivo head images obtained using this optical link are demonstrated for comparison with those obtained by cable connections. The signal-to-noise (SNR) analysis shows that the optical link achieves 6%-8% SNR a improvement over coaxial cables by elimination of electrical interference between cables during MR signal transmission.
    11/2008; 27(10):1432-8. DOI:10.1109/TMI.2008.922186
  • Yong Pang · Gary X. Shen
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    ABSTRACT: With the development of the high-field MRI system, the RF field inhomogeneity becomes a challenging problem in high flip angle excitation. In this study, a new RF pulse design method is proposed, which combines the simulated annealing (SA) algorithm and the k-space method to attain a pulse which can compensate the RF field inhomogeneity effect. The RF pulse envelope is firstly initiated using the k-space method and a cost function which contains the passband error and stopband error is defined. Then the SA is applied to refine the RF pulse shape to achieve low-cost value. Finally, under inhomogeneous RF field an optimized RF pulse with improved flip angle accuracy can be obtained. Simulation results show that when compared with the pulse designed only using the k-space method, the passband error can be decreased by 28%, 40%, and 75% for 90° excitation pulse, refocusing pulse, and 180° inversion pulse, respectively. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 33B: 75–83, 2008
    Concepts in Magnetic Resonance Part B Magnetic Resonance Engineering 04/2008; DOI:10.1002/cmr.b.20113 · 0.95 Impact Factor
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    ABSTRACT: In this paper, a digital wireless transmission system based on 802.11b standard for magnetic resonance imaging (MRI) application is designed and built for the first time to eliminate the interference aroused by coil array cables. The analysis shows that the wireless receiver has a very high sensitivity to detect MRI signals. The modulation technique of differential quadrature phase shift keyed (DQPSK) can be applied to MRI data transmission with rate of 2 Mbps and bandwidth of 2 MHz. The bench test verifies that this wireless link has a dynamic range over 86 dB supporting up to 3 T MRI system data transmission. The 2D spin echo imaging of phantom is performed and the SNR of the image obtained by the wireless transmission can be comparable with that got by the coaxial cables.
    Journal of Magnetic Resonance 07/2007; 186(2):358-63. DOI:10.1016/j.jmr.2007.03.003 · 2.32 Impact Factor
  • Yong Pang · Gary X Shen
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    ABSTRACT: In this study, a Genetic Algorithm (GA) is introduced to optimize the multidimensional spatial selective RF pulse to reduce the passband and stopband errors of excitation profile while limiting the transition width. This method is also used to diminish the nonlinearity effect of the Bloch equation for large tip angle excitation pulse design. The RF pulse is first designed by the k-space method and then coded into float strings to form an initial population. GA operators are then applied to this population to perform evolution, which is an optimization process. In this process, an evaluation function defined as the sum of the reciprocal of passband and stopband errors is used to assess the fitness value of each individual, so as to find the best individual in current generation. It is possible to optimize the RF pulse after a number of iterations. Simulation results of the Bloch equation show that in a 90 degrees excitation pulse design, compared with the k-space method, a GA-optimized RF pulse can reduce the passband and stopband error by 12% and 3%, respectively, while maintaining the transition width within 2 cm (about 12% of the whole 32 cm FOV). In a 180 degrees inversion pulse design, the passband error can be reduced by 43%, while the transition is also kept at 2 cm in a whole 32 cm FOV.
    Journal of Magnetic Resonance 06/2007; 186(1):86-93. DOI:10.1016/j.jmr.2007.01.016 · 2.32 Impact Factor
  • Bing Wu · Xiaoliang Zhang · Peng Qu · Gary X Shen
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    ABSTRACT: Microstrip transmission-line loop arrays have been recently proposed for parallel imaging at ultrahigh fields due to their advantages in element decoupling and to their increased coil quality factor. In the microstrip loop array design, interconnecting capacitors become necessary to further improve the decoupling between the adjacent elements when nonoverlapped loops are placed densely. However, at ultrahigh fields, the capacitance required for sufficient decoupling is very small. Hence, the isolations between the elements are usually not optimized and the array is extremely sensitive to the load. In this study, a theoretical model is developed to analyze the capacitive decoupling circuit. Then, a novel tunable loop microstrip (TLM) array that can accommodate capacitive decoupling more easily at ultrahigh fields is proposed. As an example, a four-element TLM array is constructed at 7 T. In this array, the decoupling capacitance is increased to a more reasonable value. Isolation between the adjacent elements is better than -37 dB with the load. The performance of this TLM array is also demonstrated by MRI experiments.
    Magnetic Resonance Imaging 05/2007; 25(3):418-24. DOI:10.1016/j.mri.2006.09.031 · 2.02 Impact Factor
  • Bing Wu · Peng Qu · Chunsheng Wang · Jing Yuan · Gary X. Shen
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    ABSTRACT: Interconnecting L/C components are often applied to decouple the array elements for parallel imaging. Although it has been recognized that interconnecting capacitors and inductors can both be employed for decoupling, quantitative study of this decoupling technique has not yet been presented. In this study, a theoretical analysis for the interconnecting L/C decoupling circuit is provided. The analysis reveals that the required decoupling capacitance decreases with the resonant frequency, whereas the decoupling inductance is independent of the frequency. The inductive decoupling scheme is applied in the design of a four-channel knee coil for 0.5 T open MRI with vertical magnetic field. Experimental results show that good isolations (−19 dB ∼ −45 dB) between coil elements can be achieved and only 5% ∼ 11% degeneration of Q is caused by this decoupling method. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 31B: 116–126, 2007
    Concepts in Magnetic Resonance Part B Magnetic Resonance Engineering 04/2007; 31B(2):116-126. DOI:10.1002/cmr.b.20087 · 0.95 Impact Factor
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    ABSTRACT: In a neonatal rat model of hypoxic-ischemic (HI) brain injury, using T2-weighted imaging (T2WI) and diffusion-weighted imaging (DWI), we aim to determine the best MRI method of lesion quantification that reflects infarct size. Twenty 7-day-old rats underwent MRI 24h after HI brain injury was induced. Lesion size relative to whole brain was measured using T2WI and apparent diffusion coefficient (ADC) maps, applying thresholds of 60%, 70% and 80% contralateral control hemisphere mean ADC, and at day 10 post-HI on pathology with TTC staining. Multiple linear regression analysis was used to study the relationships between lesion size at MRI and pathology. Lesion size measurement using all MRI methods significantly correlated with infarct size at pathology; using T2WI, r=0.808 (p<0.001), using 80% ADC, 70% ADC and 60% ADC thresholds, r=0.888 (p<0.001), 0.761, (p<0.001) and 0.569 (p=0.014), respectively. Eighty percent ADC threshold was found to be the only significant independent predictor of final infarct volume (adjusted R(2)=0.775). At 24h post-HI, lesion size on DWI, using 80% ADC threshold is the best predictor of final infarct volume. Although T2WI performed less well, it has the advantage of superior spatial resolution and is technically less demanding. These are important considerations for experiments which utilize MRI as a surrogate method for lesion quantification in the neonatal rat HI model.
    International Journal of Developmental Neuroscience 03/2007; 25(1):1-5. DOI:10.1016/j.ijdevneu.2006.12.003 · 2.92 Impact Factor
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    Peng Qu · Jing Luo · Bida Zhang · Jianmin Wang · Gary X. Shen
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    ABSTRACT: The conjugate-gradient (CG)-based non-Cartesian SENSE reconstruction usually exhibits unstable convergence behavior due to the ill conditioning of the generalized encoding matrix (GEM). To overcome this difficulty, an improved iterative SENSE approach is presented. During a so-called Lanczos iteration process, which is equally efficient as CG, the inversion of GEM can be gradually approximated by calculating inversions of a series of small tridiagonal matrices. In this fashion, inner regularization can be incorporated into the reconstruction without touching the iteration process. The degree of regularization can be determined based on the eigenvalue information provided by the Lanczos process. With inner regularization adaptively applied for every iteration vector, the convergence behavior of iterative SENSE can be significantly improved and noise amplification can be avoided. The feasibility of this novel iterative SENSE technique is demonstrated by radial and spiral MRI experiments. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 31B: 44–50, 2007
    Concepts in Magnetic Resonance Part B Magnetic Resonance Engineering 02/2007; 31B(1):44 - 50. DOI:10.1002/cmr.b.20076 · 0.95 Impact Factor
  • Bing Wu · Xiaoliang Zhang · Peng Qu · Gary X Shen
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    ABSTRACT: By independent control of the phases and amplitudes of its elements, the microstrip transmission-line array can mitigate sample-induced RF non-uniformities, and has been widely used as the transceiver in parallel imaging applications. One major challenge in implementing the microstrip array is the reduction of mutual coupling among individual elements. The low-input impedance preamplifier is commonly used for the decoupling purpose. However, it is impractical in the transceiver array design. Although interconnecting capacitors can be utilized to reduce the mutual coupling, they only efficiently work for the neighbor elements. In addition, this approach is impractical at fields higher than 300 MHz, in which the required decoupling capacitance is commonly less than 0.5 pF. We propose a novel decoupling approach by using decoupling inductors in this study. Due to the fact that the decoupling inductance is independent of the resonant frequency, the microstrip arrays can be well decoupled at ultra-high fields. To verify the proposed approach, an eight-channel microstrip array is fabricated and tested at 9.4 T. For this prototype, couplings between elements are significantly reduced by using the interconnecting inductors. The phantom experiment shows that the inductively decoupled microstrip array has good parallel imaging performance.
    Journal of Magnetic Resonance 10/2006; 182(1):126-32. DOI:10.1016/j.jmr.2006.04.013 · 2.32 Impact Factor
  • Chunsheng Wang · Peng Qu · Gary X Shen
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    ABSTRACT: The potential advantages of the higher-order resonant modes of a low-pass birdcage coil for parallel imaging are investigated using FDTD simulations in a spherical phantom. Better parallel imaging performance can be achieved in axial planes than in sagittal planes. If more modes are employed, the average g-factor (gmean) and maximum g-factor (gmax) will be improved for a specific acceleration factor (R). G-factor performance at 3 T (gmean=1.79, gmax=3.55) and 7 T (gmean=1.60, gmax=2.45) can be achieved even with an acceleration factor as high as four when all six order modes of 12-rung low-pass birdcage coil are incorporated for imaging on a mid-axial plane. For a specific number of channels, the optimum combination of corresponding modes can be obtained for different acceleration factors. Based on the g-factor and SNR performance, the total degenerate multi-mode birdcage coil with six order resonant modes has better homogeneous coverage and SENSE performance than the 8-element phased array coil, although requiring fewer channels. In addition, the dielectric effects at high field can improve the parallel imaging performance.
    Journal of Magnetic Resonance 10/2006; 182(1):160-7. DOI:10.1016/j.jmr.2006.06.015 · 2.32 Impact Factor
  • Chunsheng Wang · Gary X Shen
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    ABSTRACT: To compare the performance of birdcage, transverse electromagnetic (TEM) and microstrip volume coils at 7T under the same geometric conditions. Birdcage, TEM, and microstrip coils are modeled with the same dimensions. The finite difference time domain (FDTD) method is adopted to calculate the electromagnetic fields of the coils. Further, B(1) field, specific absorption rate (SAR) and signal-to-noise ratio (SNR) are calculated for these coils. In the unloaded case, within the central axial plane, the variation of B(1) field magnitude over 18-cm distance is about 15% for the birdcage coil, 23% for the TEM coil, and 38% for the microstrip coil. In the loaded case, the percentages of the samples on the central axial plane, which have B(1) field magnitude within +/-20% of the average B(1) field magnitude, are about 57% for the birdcage, 72% for the TEM, and 59% for the microstrip coil. Average SAR levels are 11.4% and 42.9% higher in the birdcage than those in the TEM and microstrip coils, respectively. The average relative SNR on the central axial plane for the shielded birdcage, TEM, and microstrip coils are 1, 1.07, and 1.48, respectively. The birdcage coil has the best unloaded B(1) field homogeneity, and the TEM coil has the best loaded B(1) field homogeneity and the lowest radiation loss; while the microstrip coil is better in SAR and SNR at 7T than the birdcage and TEM coils.
    Journal of Magnetic Resonance Imaging 08/2006; 24(2):439-43. DOI:10.1002/jmri.20635 · 2.79 Impact Factor
  • Peng Qu · Chunsheng Wang · Gary X Shen
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    ABSTRACT: To develop a novel regularization method for GRAPPA by which the regularization parameters can be optimally and adaptively chosen. In the fit procedures in GRAPPA, the discrepancy principle, which chooses the regularization parameter based on a priori information about the noise level in the autocalibrating signals (ACS), is used with the truncated singular value decomposition (TSVD) regularization and the Tikhonov regularization, and its performance is compared with the singular value (SV) threshold method and the L-curve method, respectively by axial and sagittal head imaging experiments. In both axial and sagittal reconstructions, normal GRAPPA reconstruction results exhibit a relatively high level of noise. With discrepancy-based choices of parameters, regularization can improve the signal-to-noise ratio (SNR) with only a very modest increase in aliasing artifacts. The L-curve method in all of the reconstructions leads to overregularization, which causes severe residual aliasing artifacts. The 10% SV threshold method yields good overall image quality in the axial case, but in the sagittal case it also leads to an obvious increase in aliasing artifacts. Neither a fixed SV threshold nor the L-curve are robust means of choosing the appropriate parameters in GRAPPA reconstruction. However, with the discrepancy-based parameter-choice strategy, adaptively regularized GRAPPA can be used to automatically choose nearly optimal parameters for reconstruction and achieve an excellent compromise between SNR and artifacts.
    Journal of Magnetic Resonance Imaging 07/2006; 24(1):248-55. DOI:10.1002/jmri.20620 · 2.79 Impact Factor
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    Peng Qu · Kai Zhong · Bida Zhang · Jianmin Wang · Gary X Shen
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    ABSTRACT: In non-Cartesian SENSE reconstruction based on the conjugate gradient (CG) iteration method, the iteration very often exhibits a "semi-convergence" behavior, which can be characterized as initial convergence toward the exact solution and later divergence. This phenomenon causes difficulties in automatic implementation of this reconstruction strategy. In this study, the convergence behavior of the iterative SENSE reconstruction is analyzed based on the mathematical principle of the CG method. It is revealed that the semi-convergence behavior is caused by the ill-conditioning of the underlying generalized encoding matrix (GEM) and the intrinsic regularization effect of CG iteration. From the perspective of regularization, each iteration vector is a regularized solution and the number of iterations plays the role of the regularization parameter. Therefore, the iteration count controls the compromise between the SNR and the residual aliasing artifact. Based on this theory, suggestions with respect to the stopping rule for well-behaved reconstructions are provided. Simulated radial imaging and in vivo spiral imaging are performed to demonstrate the theoretical analysis on the semi-convergence phenomenon and the stopping criterion. The dependence of convergence behavior on the undersampling rate and the noise level in samples is also qualitatively investigated.
    Magnetic Resonance in Medicine 10/2005; 54(4):1040-5. DOI:10.1002/mrm.20648 · 3.40 Impact Factor
  • Peng Qu · Gary X Shen · Chunsheng Wang · Bing Wu · Jing Yuan
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    ABSTRACT: The generalized auto-calibrating partially parallel acquisition (GRAPPA) is an auto-calibrating parallel imaging technique which incorporates multiple blocks of data to derive the missing signals. In the original GRAPPA reconstruction algorithm only the data points in phase encoding direction are incorporated to reconstruct missing points in k-space. It has been recognized that this scheme can be extended so that data points in readout direction are also utilized and the points are selected based on a k-space locality criterion. In this study, an automatic subset selection strategy is proposed which can provide a tailored selection of source points for reconstruction. This novel approach extracts a subset of signal points corresponding to the most linearly independent base vectors in the coefficient matrix of fit, effectively preventing incorporating redundant signals which only bring noise into reconstruction with little contribution to the exactness of fit. Also, subset selection in this way has a regularization effect since the vectors corresponding to the smallest singular values are eliminated and consequently the condition of the reconstruction is improved. Phantom and in vivo MRI experiments demonstrate that this subset selection strategy can effectively improve SNR and reduce residual artifacts for GRAPPA reconstruction.
    Journal of Magnetic Resonance 06/2005; 174(1):60-7. DOI:10.1016/j.jmr.2005.01.015 · 2.32 Impact Factor
  • Chunsheng Wang · Gary X Shen · Jin Yuan · Peng Qu · Bing Wu
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    ABSTRACT: The specific absorption rates (SAR) of three tissues (muscle, brain, bone) are investigated both theoretically and experimentally for MRI at the first time. Finite difference time domain (FDTD) analysis is used to simulate the average SAR of three tissues at three magnetic field strengths (0.5 T, 1.5 T, 3T). Simulations show that the SAR of muscle, brain and bone increase 7.49 folds, 10.87 folds and 12.92 folds respectively when the magnetic field strength increases from 0.5 T to 3 T. Experiments are carried out to measure SRAS of different phantoms which simulate the three human tissues at 1.5T and 3T. The experiment results agree with the simulation data very well and within only 11% difference.
    Physica Medica 04/2005; 21(2):61-4. DOI:10.1016/S1120-1797(05)80020-1 · 1.85 Impact Factor
  • Hui Pan · Gary X. Shen
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    ABSTRACT: This article presents a new method of simulating high-temperature superconducting (HTS) RF coils using an electromagnetically trained artificial neural network (EM-ANN). This design is based on a spiral planar coil with distributed capacitance fabricated with Y1Ba2Cu3O7 (YBCO) films. Simulation time with this new method can be reduced to only one millionth of the time required by the commercial electromagnetic software programme HP Momentum. The new method can also exploit the properties of an artificial neural network by providing an inverse algorithm based on a resonant frequency input to derive other properties of an RF coil. This inverse algorithm using EM-ANN is easier, faster, and more interactive than the traditional “moment method.” The simulation results also show excellent agreement with experimental measurements, with a margin of error of less than 3%. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 18B: 9–14, 2003
    Concepts in Magnetic Resonance Part B Magnetic Resonance Engineering 07/2003; 18B(1):9 - 14. DOI:10.1002/cmr.b.10076 · 0.95 Impact Factor
  • Gary X. Shen · Jiangfeng Wu · Fernando E. Boada
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    ABSTRACT: A new interface combining phased arrays and echo-planar imaging (EPI) technologies was developed for two channel breast MR EPI applications. A detailed design for a dual-channel, EPI-compatible, phased array breast coil is described. EPI digital data multiplexing, signal controlling and sampling schemes are also presented. Results from breast phantoms and patients demonstrate a 55% improvement in signal-to-noise ratio when compared to a conventional two-loop, single channel coil configuration. This method can be easily expanded to a four or more channel, EPI-compatible, phased array system to improve field-of-view coverage and signal-to-noise ratio.
    MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 01/2001; 11(3):138-43. DOI:10.1007/BF02678477 · 2.87 Impact Factor
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    ABSTRACT: In this work, we present the first triple quantum filtered (TQF) sodium MR images of the human knee joint in vivo. A 3D TQF data set of 16 slices was obtained in 20 min using a TQF pulse sequence preencoded to a twisted projection imaging readout. Images clearly demarcate patellar cartilage and also demonstrate fluid signal suppressed by the triple quantum filter. Biexponential transverse relaxation times were calculated by fitting the TQF free induction decay to a theoretical signal expression. The average values from three healthy volunteers were T(2fall)(*) = 9.59 +/- 0.35 ms and T(2rise)(*) = 0.84 +/- 0.06 ms. Application of TQF imaging in biological tissues is discussed.
    Journal of Magnetic Resonance 01/2000; 141(2):286-90. DOI:10.1006/jmre.1999.1923 · 2.32 Impact Factor
  • Ileana Hancu · Fernando E. Boada · Gary X. Shen
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    ABSTRACT: A scheme for the generation of three-dimensional, triple-quantum-filtered (TQ) sodium images from normal human brain is presented. In this approach, a three-pulse, six-step, coherence transfer filter was used in conjunction with a fast twisted projection imaging sequence to generate spatial maps of the TQ signal across the entire brain. It is demonstrated, theoretically as well as experimentally, that the use of the three-pulse coherence filter leads to TQ sodium images in which the dependence of the image intensity on the spatial variation of the flip angle is less pronounced than it is in the "standard," four-pulse, TQ filter. Correction for the variation of the TQ signal intensity across the field of view because of radio-frequency (RF) inhomogeneity is straightforward with this approach. This imaging scheme allows the generation of RF inhomogeneity-corrected, TQ, sodium images from human brain at moderate field strength (3.0 T) in times acceptable for routine clinical examinations (20 minutes). Magn Reson Med 42:1146-1154, 1999.
    Magnetic Resonance in Medicine 12/1999; 42(6):1146-54. DOI:10.1002/(SICI)1522-2594(199912)42:6<1146::AID-MRM20>3.0.CO;2-S · 3.40 Impact Factor
  • Keith R. Thulborn · Gary X. Shen
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    ABSTRACT: A flexible quadrature radiofrequency coil that maximizes the signal-to-noise ratio over the field of view of the human brain has been integrated into a head immobilization and visor system for fMRI at 1.5 T. Head motion is reduced by the visor that incorporates a head clamp and a simple visual sighting system that provides feedback on head position. This system is demonstrated in serial images by correction of deliberate head motions. The sensitivity at the cortical surface of fMRI using blood oxygenation level dependent contrast is increased significantly above that of the commercial rigid volume RF coil under the same acquisition conditions. This improved performance is demonstrated using visual activation and eye movement paradigms.
    Journal of Magnetic Resonance 08/1999; 139(1):26-34. DOI:10.1006/jmre.1999.1748 · 2.32 Impact Factor

Publication Stats

717 Citations
67.50 Total Impact Points

Institutions

  • 2001–2008
    • The University of Hong Kong
      • Department of Electrical and Electronic Engineering
      Hong Kong, Hong Kong
  • 2007
    • Chinese Academy of Sciences
      • Graduate School
      Beijing, Beijing Shi, China
  • 1997–2000
    • University of Pittsburgh
      • • Department of Radiology
      • • MR Research Center
      Pittsburgh, Pennsylvania, United States