G X Shen

The University of Hong Kong, Hong Kong, Hong Kong

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Publications (44)78.23 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.
    IEEE transactions on medical imaging. 11/2008; 27(10):1432-8.
  • 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 01/2008; · 0.71 Impact Factor
  • Jing Yuan, Juan Wei, G X Shen
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    ABSTRACT: Optical glass fiber is a promising alternative to traditional coaxial cables for MRI RF receive coil interconnection to avoid any crosstalk and electromagnetic interference between multiple channels. A direct modulated optical link is proposed for MRI coil interconnection in this paper. The link performances of power gain, frequency response and dynamic range are measured. Phantom and in vivo human head images have been demonstrated by the connection of this direct modulated optical link to a head coil on a 0.3T MRI scanner for the first time. Comparable image qualities to coaxial cable link verify the feasibility of using the optical link for imaging with minor modification on the existing scanners. This optical link could also be easily extended for multi-channel array interconnections at high field of 1.5 T.
    Journal of Magnetic Resonance 12/2007; 189(1):130-8. · 2.30 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. · 2.30 Impact Factor
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    Jing Yuan, G X Shen
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    ABSTRACT: Bi-2223 tape is a low-cost, long-length high temperature superconducting material. The new application of Bi-2223 tape for gradient coils for magnetic resonance imaging (MRI) is studied in this paper. Because Bi-2223 tape has a much higher critical current density and a much lower power loss than copper, a Bi-2223 high temperature superconductor (HTS) gradient coil shows great advantages in high gradient strengths, long continuous gradient rating, and free of cooling system over a copper gradient coil. The power losses of Bi-2223 tapes at working frequencies of gradient coils are compared to copper. The critical current degradation of tapes is also discussed. A prototype of HTS tape gradient coil is fabricated and its gradient field distributions are measured. Technical issues of resistance, gradient strength, continuous gradient rating, and cryostat are also discussed.
    Medical Engineering & Physics 06/2007; 29(4):442-8. · 1.78 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. · 2.30 Impact Factor
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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. · 2.06 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. · 2.69 Impact Factor
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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. · 0.71 Impact Factor
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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 01/2007; · 0.71 Impact Factor
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    ABSTRACT: Electron paramagnetic resonance (EPR) spin trapping technology is a sensitive and unambiguous method for detection of nitric oxide (NO). Due to the short lifetime, NO must be trapped before EPR measurement. There are two EPR spin trapping techniques used currently, including the detections of EPR signals of diethyldithiocarbamate-iron-nitric oxide (DETC2-Fe2+-NO) and nitrosyl hemoglobin (HbNO). In this study, we firstly investigated the kinetics of the EPR signal of DETC2-Fe2+-NO in normal and ischemia-reperfused kidneys. In normal rat kidneys, the signal of DETC2-Fe2+-NO was found at 5 min after the spin trappers Fe2+/DETC were administrated, the peak concentration was at 15 min and the period with relatively stable signal intensity was at the time range from 15 to 70 min. In the ischemia-reperfused rat kidneys, the signal of DETC2-Fe2+-NO was increased at 30 min of ischemia and decreased at 60 min of ischemia after the occlusion of renal artery (corresponding to the time course of 60 and 90 min after Fe2+/DETC injection respectively). We then investigated the EPR signal of HbNO in blood. No characteristic HbNO signal was found in the rats of the sham control and 30 min of ischemia. An HbNO signal occurred in the rats exposed to 60 min of ischemia and it became pronounced with increased duration of reperfusion. The signal intensity reached a plateau at 150 min of reperfusion. The results suggest that the DETC2-Fe2+-NO signal can be only suitable for the NO measurement in the short-term ischemia-reperfusion model, whereas the HbNO signal can be applied to represent NO in the relatively long-term ischemia-reperfusion model. In addition, NG-nitro-L-arginine (L-NAME) and allopurinol were used to identify the source of NO. By detecting the HbNO signal, we demonstrated that the activation of xanthine oxidase is an important source of NO formation at the long-term period of ischemia and reperfusion.
    Applied Magnetic Resonance 01/2007; 32(3):243-255. · 0.83 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. · 2.30 Impact Factor
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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. · 2.30 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 09/2006; 24(2):439-43. · 2.57 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 08/2006; 24(1):248-55. · 2.57 Impact Factor
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    ABSTRACT: By using a neonatal rat hypoxia-ischemia (HI) model, we studied the relationship between lesion volume-measured by diffusion-weighted imaging (DWI) and T2-weighted imaging (T2WI) at an early time point-and irreversible infarct volume. We also evaluated the optimal apparent diffusion coefficient (ADC) threshold that provides the best correlation with irreversible infarct size. Twenty-three neonatal rats underwent right common carotid artery ligation and hypoxia. MR imaging was performed 1-2 hours post-HI by using DWI and T2WI and at day 4 post-HI by using T2WI. Lesion volumes relative to whole brain (%LV) were measured on ADC maps by using different relative ADC thresholds 60%-80% of mean contralateral ADC and T2WI. Pearson correlation and multiple linear regression analysis were used to study the relationships between ln(%LV) at MR imaging and %LV at histopathology. At 1-2 hours post-HI, all lesion volume measurements on DWI were significantly correlated with the infarct volume on histopathology, with the best correlation attained at the 80% ADC threshold (r = 0.738; P < .001). The estimated regression formula was %LV on histopathology = 20.60 + 3.33 ln(%LV on 80% ADC threshold) (adjusted R(2) = 0.523; P < .001). Lesion volume at 1-2 hours post-HI tended to underestimate the final infarct volume. Early post-HI MR imaging by using DWI correlates moderately well with the size of irreversible infarct, especially when measured by using a relative ADC threshold of 80% mean contralateral ADC.
    American Journal of Neuroradiology 02/2006; 27(1):51-4. · 3.17 Impact Factor
  • Juan Wei, Jing Yuan, G. X. Shen
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    ABSTRACT: Mutual coupling is an important issue in RF coil array design for MRI. In this study, the dependence of mutual coupling behavior among RF coil array elements on the tuning capacitor placement is specially investigated at different field strengths. For phased arrays with different tuning capacitor configurations, the degree of mutual coupling is estimated by measuring parameters S11 and S12 using Advanced Design System (ADS) simulation and bench measurements. The simulation agrees well with experiment results and shows that the mutual coupling behavior of different configurations varies notably with the resonant frequency. Specifically, at frequencies lower than 64 MHz, the placement of one or two concentrated capacitors suffers less mutual coupling than that of four evenly distributed capacitors around the coil loop; whereas at frequencies higher than 128 MHz, the opposite is observed. This interesting phenomenon provides guidance on minimizing the mutual coupling for coil array design © 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 29B: 50–54, 2006
    Concepts in Magnetic Resonance Part B Magnetic Resonance Engineering 01/2006; · 0.71 Impact Factor
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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 11/2005; 54(4):1040-5. · 3.27 Impact Factor
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    ABSTRACT: The use of Bi-2223 tapes as the material for gradient coils in Magnetic Resonance Imaging (MRI) is promising because Bi-2223 tapes have very high critical current and very low power loss. Frequency dependent AC loss in Bi-2223 tapes was measured at the operating frequencies of gradient coil from 200Hz up to 2000Hz. It is found that AC loss decreases with frequency below 800Hz and then increases from 800Hz to 2000Hz. The contribution of hysteresis loss, eddy current loss and resistive loss to the total AC loss are investigated individually. The power-law E–J characteristic of tapes and position dependent current distribution are considered as the major reasons for the frequency dependence below 800Hz. Eddy current loss should account for the increasing power loss with frequency above 800Hz.
    Physica C Superconductivity 08/2005; 424(1):72-78. · 0.72 Impact Factor
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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. · 2.30 Impact Factor

Publication Stats

637 Citations
78.23 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
  • 1999–2000
    • University of Pittsburgh
      • Department of Radiology
      Pittsburgh, PA, United States