Donglin Zu - Academia.edu (original) (raw)

Papers by Donglin Zu

Chinese Physics B, Jul 1, 2010

This paper presents an approach of active ferromagnetic shimming for C-type permanent magnetic re... more This paper presents an approach of active ferromagnetic shimming for C-type permanent magnetic resonance imaging magnet. It is designed to reduce inhomogeneity of magnetostatic field of C-type permanent magnet to meet the stringent requirement for magnetic resonance imaging applications. An optimal configuration (locations and thicknesses) of active ferromagnetic pieces is generated through calculation according to the initial field map and the demanded final homogeneity specifications. This approach uses a minimisation technique which makes the sum of squared magnetic moment minimum to restrict the amount of the active ferromagnetic material used and the maximal thickness of pieces stacked at each hole location in the shimming boards. Simulation and experimental results verify that the method is valid and efficient.

Chinese Physics B, 2010

For a superconducting magnet of magnetic resonance imaging (MRI), the novel approach presented in... more For a superconducting magnet of magnetic resonance imaging (MRI), the novel approach presented in this paper allows the design of cylindrical gradient and shim coils of finite length. The method is based on identification of the weighting of harmonic components in the current distribution that will generate a magnetic field whose z-component follows a chosen spherical harmonic function. Mathematical expressions which relate the harmonic terms in the cylindrical current distribution to spherical harmonic terms in the field expansion are established. Thus a simple matrix inversion approach can be used to design a shim coil of any order pure harmonic. The expressions providing a spherical harmonic decomposition of the field components produced by a particular cylindrical current distribution are novel. A stream function was applied to obtain the discrete wire distribution on the cylindrical-surface. This method does not require the setting of the target-field points. The discussion referring to matrix equations in terms of condition numbers proves that this novel approach has no ill-conditioned problems. The results also indicate that it can be used to design cylindrical-surface shim coils of finite length that will generate a field variation which follows a particular spherical harmonic over a reasonably large-sized volume.

A new 9-cell cavity shape is designed for Tev Superconducting Linear Accelerators (TESLA). The ra... more A new 9-cell cavity shape is designed for Tev Superconducting Linear Accelerators (TESLA). The ratio of the maximal Surface electric field to the accelerating gradient, %/J-L, was lowered down to 2. 024 in the center cell while the cell to cell coupling is 1. 95% for the 1. 3GHz accelerating mode. The properties of the higher order modes were investigated with a "dissipated band model".

IEEE Transactions on Nuclear Science, Feb 1, 1998

A nine-cell superconducting RF accelerating cavity is designed for the TeV electron linear accele... more A nine-cell superconducting RF accelerating cavity is designed for the TeV electron linear accelerator collider in the next century. The ratio of the maximum surface electric field to the accelerating gradient, E pk =E acc , is reduced to 2.024 and the cell-to-cell coupling remains as high as 1.95%. The distribution of the higher-order mode passbands is reasonable. There is no overlap between these bands, therefore no trapped modes. The circle-straight/line-ellipse-type structure provides good mechanical strength in the accelerating cavity. According to the present state of the art of surface processing techniques of Niobium cavities, it is possible to reach an accelerating gradient of 25-30 MV/m with beam load.

Chinese Physics, Oct 1, 2002

The approach of expanding the magnetic scalar potential in a series of Legendre polynomials is su... more The approach of expanding the magnetic scalar potential in a series of Legendre polynomials is suitable for designing a conventional superconducting magnetic resonance imaging magnet of distributed solenoidal configuration. Whereas the approach of expanding the magnetic vector potential in associated Legendre harmonics is suitable for designing a single-solenoid magnet that has multiple tiers, in which each tier may have multiple layers with different winding lengths. A set of three equations to suppress some of the lowest higher-order harmonics is found. As an example, a 4T single-solenoid magnetic resonance imaging magnet with 4×6 layers of superconducting wires is designed. The degree of homogeneity in the 0.5m diameter sphere volume is better than 5.8 ppm. The same degree of homogeneity is retained after optimal integralization of turns in each correction layer. The ratio Bm/B0 in the single-solenoid magnet is 30% lower than that in the conventional six-solenoid magnet. This tolerates higher rated superconducting current in the coil. The Lorentz force of the coil in the single-solenoid system is also much lower than in the six-solenoid system. This novel type of magnet possesses significant advantage over conventional magnets, especially when used as a super-high field functional magnetic resonance imaging magnet.

Journal of Systems Engineering and Electronics, Mar 1, 2001

Multi-modality medical image fusion has more and more important applications in medical image ana... more Multi-modality medical image fusion has more and more important applications in medical image analysis and understanding. In this paper, we develop and apply a multi-resolution method based on wavelet pyramid to fuse medical images from different morlalities such as PET-MRI and CT-MRI. In particular, we evaluate the different fusion results when applying different selection rules and obtain optimum combination of fusion parameters.

... The electrostatic field is an attractive external force model for snakes (active contour). In... more ... The electrostatic field is an attractive external force model for snakes (active contour). In this paper, a novel approach is proposed to solve the infinite problem induced by using the formula of the electric field intensity. By constructing a template plane above the original image ...

Magnetic Resonance in Medicine, Mar 10, 2011

Amide proton transfer (APT) imaging is capable of detecting mobile cellular proteins and peptides... more Amide proton transfer (APT) imaging is capable of detecting mobile cellular proteins and peptides in tumor and monitoring pH effects in stroke, through the saturation transfer between irradiated amide protons and water protons. In this work, four healthy subjects, eight brain tumor patients (four with high-grade glioma; one with lung cancer metastasis; three with meningioma), and four stroke patients (average 4.3 ± 2.5 days after the onset of the stroke) were scanned at 3T, using different radiofrequency saturation powers. The APT effect was quantified using the magnetization-transfer-ratio (MTR) asymmetry at 3.5 ppm with respect to the water resonance. At a saturation power of 2 μT, the measured APT-MRI signal of the normal brain tissue was almost zero, due to the contamination of the negative conventional MTR asymmetry. This irradiation power caused an optimal hyperintense APT-MRI signal in the tumor and an optimal hypointense signal in the stroke, compared to the normal brain tissue. The results suggest that the saturation power of 2 μT is ideal for APT imaging of these two pathologies at 3T with the existing clinical hardware.

Piers Online, 2007

A new approach has been presented in the paper to gain a uniform magnetic field in NMR system. Fi... more A new approach has been presented in the paper to gain a uniform magnetic field in NMR system. First, the adopted shimming piece is modeled as a magnetic dipole moment to calculate its magnetization effect on the background field within region of interest. Then, sequential quadratic programming method is utilized to determine the ideal solution for shimming work. Finally, the ideal solution is discretized, and quantization error control technique is prepared for special cases. This new method helps to reduce the inhomogeneity in the region of interest from 56.8 ppm to 14.21 ppm, within one hour in practical shimming work.

The present approach to a CESR B-factory is to divide the desired beam current of 1-2 amps into a... more The present approach to a CESR B-factory is to divide the desired beam current of 1-2 amps into a large number of low current bunches, which helps reduce detector backgrounds. To fit 230 bunches into the CESR size ring, they need to be very closely spaced, eliminating room for any conventional separators. Instead, a small (12 mr) crossing angle is considered, which may cause harmful coupling between synchrotron and betatron motion [ll. By rotating the bunches before collision ("crabbing") so they collide head-on, and then rotating them back, so they pass through the arcs normally, this dangerous coupling can be eliminated [2]. We show that the needed transverse kick of-2 MV can be achieved with one single cell superconducting cavity, operating in the TM110 mode at 500 MHz, while keeping the surface electric field below 2.5 MV/m to avoid excessive field emission. The cell design allows all modes higher in frequency than the crab mode to propagate out the beam pipe and be damped outside the cryostat with ferrite beam pipe absorbers. 1.5 GHz niobium cavities have been tested to study the multipacting behavior of the TM1 10 mode.

Concepts in Magnetic Resonance Part B-magnetic Resonance Engineering, Feb 17, 2010

A novel target-field approach for designing bi-planar gradient and shim coils of restricted radiu... more A novel target-field approach for designing bi-planar gradient and shim coils of restricted radius for use with permanent-magnet MRI systems is presented in this article. The method is based on identification of the weighting of harmonic components in the current distribution that will generate a magnetic field whose z-component follows a chosen spherical harmonic function. Mathematical expressions that relate the harmonic terms in the surface current distribution to spherical harmonic terms in the field expansion are established and then used in a simple matrix inversion approach to design a range of shim coils. The expressions providing a spherical harmonic decomposition of the field components produced by a particular current distribution are novel. The results can be used to design bi-planar shim coils of restricted radius that will generate a field variation that follows a certain spherical harmonic over a reasonably-sized volume. A stream function was utilized to obtain the discrete wire distribution on the coil plane. This method does not require the setting of the target-field points. Through an analysis of the matrix equations in terms of condition numbers, we show that this novel approach has no ill-conditioned problems.

Journal of Physics D, Jul 13, 2007

Based on the target-field method of solving Fredholm integral equations of the first kind, a new ... more Based on the target-field method of solving Fredholm integral equations of the first kind, a new approach is presented in the paper for designing gradient coils that can be used in a permanent-magnet magnetic resonance imaging (MRI) system with biplanar poles. To restrict the current distribution on the coil plane within a finite radius, the current density is pre-expanded into Fourier series by orthogonal basis functions. By setting the target-field points and B z values over the imaging region of interest, corresponding integral equations are derived from the Biot-Savart law to calculate the current densities. They form a matrix equation, in which the unknown elements of the column vector are the Fourier coefficients for the unknown current density. As long as these target-field points are well chosen, the Fourier coefficients can be solved by inverse matrix calculation instead of the regularization method for Fredholm integral equations of the first kind. Then the current density is discretized using the stream-function method to generate the winding patterns. To verify the feasibility of this approach, the gradient magnetic field generated by the current density is calculated via the Biot-Savart law. Optimized parameters are obtained through computer simulations for some shielded and unshielded transverse gradient coils. The performance of this approach has been demonstrated as well.

Progress in Natural Science: Materials International, Apr 1, 2003

ABSTRACT Two parallel methods for magnetic resonance imaging (MRI) using radiofrequency (RF) phas... more ABSTRACT Two parallel methods for magnetic resonance imaging (MRI) using radiofrequency (RF) phased array surface coils, named spatial local Fourier encoding (SLFE) and spatial RF encoding (SRFE), are presented. The MR signals are acquired from separate channels across the coils, each of which covers a sub-FOV (field-of-view) in a parallel fashion, and the acquired data are combined to form an image of entire FOV. These two parallel encoding techniques can accelerate MR imaging greatly, yet associated artifact may appear, although the SLFE is an effective image reconstruction method which can reduce the localized artifact in some degrees. By the SRFE, RF coil array can be utilized for spatial encoding through a specialized coil design. The images are acquired in a snapshot with a high signal-to-noise ratio (SNR) without the costly gradient system, resulting in great saving of cost. Both mutual induction and aliasing effect of adjacent coils are critical to the success of SRFE. The strategies of inverse source problem and wavelet transform (WT) can be employed to eliminate them. The results simulated by MATLAB are reported.

Chinese Physics, 2003

In the problem of nonhomogeneous field magnetic resonance imaging (MRI),echo peaks decay quickly ... more In the problem of nonhomogeneous field magnetic resonance imaging (MRI),echo peaks decay quickly due to traditional design of radio frequency (RF) pulse.In this article,using inverse scattering transform (IST) of the nonlinear dynamics and Shinnar-Le Roux selective excitation pulse design algorithm (SLR Algorithm),we have designed the RF pulse for the nonhomogeneous field MRI.Simulation has been made to test the signal loss induced by the excitation of different types of RF pulses.Simulation results show that the RF pulse sequence optimized by IST and SLR algorithm can improve the signal to noise ratio of MRI considerably

Superconductor Science and Technology, Jul 2, 2010

A method is developed for designing a special iron shielded superconducting magnet for MRI in thi... more A method is developed for designing a special iron shielded superconducting magnet for MRI in this paper. The shield is designed as an integral part of the cryostat and high permeability and high saturated magnetization iron material is adopted. This scheme will result in a compact iron shielded magnet. In the presented design, the finite element(FE) method is adopted to calculate the magnetic field which is produced by superconducting coils and. nonlinear iron material. The FE method is incorporated into the simulated annealing method which is employed for corresponding optimization. Therefore, geometrical configurations of both coils and iron shield can be optimized together here. This method can deal with discrete design variables which are defined to describe cable arrangements of coil cross sections. Detail algorithm of presented design is described and an example for designing 1.5T clinical iron shielded magnet for MRI is shown in this paper.

Progress in Natural Science: Materials International, Sep 1, 2004

ABSTRACT

Piers Online, 2007

A new approach has been presented in the paper to optimize the longitudinal gradient coil perform... more A new approach has been presented in the paper to optimize the longitudinal gradient coil performance. First, traditional spherical harmonic target field method is deduced, the relation between the magnetic field and the current distribution is described by a matrix equation. Then, simulated annealing method is introduced to the optimization procedure, and those high order coefficients which are used to vanish become the variables designed for optimization. Finally, stream function method is used to transform the current density into discrete gradient coils. Comparison between traditional method and the optimized method shows that the inhomogeneity in the region of interest can be reduced from 13.59% to 4.9%, the coil efficiency is increased from 11.56 mT/m/A to 18.07 mT/m/A, and the minimum distance of the discrete current wire is raised from 0.95 mm to 2.02 mm.

Magnetic Resonance Imaging, Jul 1, 2010

To shorten scanning time and increase the feasibility of experimental results, we performed right... more To shorten scanning time and increase the feasibility of experimental results, we performed right coronary artery magnetic resonance angiography (CMRA) at 3.0 T using dual acceptance window weighting function in 25 normal subjects. We examined these subjects using conventional navigator with fixed gating window and 6 dual acceptance window weighted gating (DAWG) sequences with different central weighted ratio (CWR). Compared with the conventional navigator sequence, DAWG sequences with CWRs of 20% and 25% increased the scanning efficiency by 30% and 26% respectively (Pb.05), while maintaining good image quality; further the corresponding scanning time decreased from 2.12-1.64 and 1.69 min, respectively (Pb.05). However, CWRs less than 15% caused image degradation to some extent. The coronary artery lengths and diameters did not show statistically significant differences between the two techniques (PN.05). Briefly, to avoid the problems caused by low navigator efficiency and to maintain comparable image quality, the weighted gating parameters of 3 mm width central acceptance window and 15 mm width outer acceptance window with CWR between 20% and 25% are recommended for right CMRA at 3 T.

Progress in Natural Science: Materials International, Oct 1, 2009

Based on the sequential quadratic programming (SQP) method, a new approach is presented in this p... more Based on the sequential quadratic programming (SQP) method, a new approach is presented in this paper to gain a uniform magnetic field for a permanent MRI magnet with biplanar poles. First, the adopted shimming piece is modeled as a magnetic dipole moment to calculate its effect on the background field over the imaging region of interest. Then, the SQP method is utilized to determine the ideal solution for the shimming equation. Finally, the ideal solution is discrete, and the quantization error control technique is used for special cases. This new method helps to reduce the inhomogeneity from 1234.5 ppm to 21.4 ppm over a 36 cm diameter spherical volume (DSV), within hours in practical shimming work.

Based on the target-field method, an ameliorated approach is presented in this paper for designin... more Based on the target-field method, an ameliorated approach is presented in this paper for designing gradient coils that can be used in biplanar pole system of permanent-magnet MRI. To restrict the current on the coil plane within a finite radius, the current density is pre-expanded with a suitable series of orthogonal basis, setting the target field points, solved out the current density by inverse matrix calculation, and then the winding patterns can be gained from dispersing the continuous distribution of current density by using the stream-function method. Applying the method, the computer emulation design of some transverse gradient coils and self-shield coils result in the optimized parameters, and the effect of the approach has been illustrated.

Chinese Physics B, Jul 1, 2010

This paper presents an approach of active ferromagnetic shimming for C-type permanent magnetic re... more This paper presents an approach of active ferromagnetic shimming for C-type permanent magnetic resonance imaging magnet. It is designed to reduce inhomogeneity of magnetostatic field of C-type permanent magnet to meet the stringent requirement for magnetic resonance imaging applications. An optimal configuration (locations and thicknesses) of active ferromagnetic pieces is generated through calculation according to the initial field map and the demanded final homogeneity specifications. This approach uses a minimisation technique which makes the sum of squared magnetic moment minimum to restrict the amount of the active ferromagnetic material used and the maximal thickness of pieces stacked at each hole location in the shimming boards. Simulation and experimental results verify that the method is valid and efficient.

Chinese Physics B, 2010

For a superconducting magnet of magnetic resonance imaging (MRI), the novel approach presented in... more For a superconducting magnet of magnetic resonance imaging (MRI), the novel approach presented in this paper allows the design of cylindrical gradient and shim coils of finite length. The method is based on identification of the weighting of harmonic components in the current distribution that will generate a magnetic field whose z-component follows a chosen spherical harmonic function. Mathematical expressions which relate the harmonic terms in the cylindrical current distribution to spherical harmonic terms in the field expansion are established. Thus a simple matrix inversion approach can be used to design a shim coil of any order pure harmonic. The expressions providing a spherical harmonic decomposition of the field components produced by a particular cylindrical current distribution are novel. A stream function was applied to obtain the discrete wire distribution on the cylindrical-surface. This method does not require the setting of the target-field points. The discussion referring to matrix equations in terms of condition numbers proves that this novel approach has no ill-conditioned problems. The results also indicate that it can be used to design cylindrical-surface shim coils of finite length that will generate a field variation which follows a particular spherical harmonic over a reasonably large-sized volume.

A new 9-cell cavity shape is designed for Tev Superconducting Linear Accelerators (TESLA). The ra... more A new 9-cell cavity shape is designed for Tev Superconducting Linear Accelerators (TESLA). The ratio of the maximal Surface electric field to the accelerating gradient, %/J-L, was lowered down to 2. 024 in the center cell while the cell to cell coupling is 1. 95% for the 1. 3GHz accelerating mode. The properties of the higher order modes were investigated with a "dissipated band model".

IEEE Transactions on Nuclear Science, Feb 1, 1998

A nine-cell superconducting RF accelerating cavity is designed for the TeV electron linear accele... more A nine-cell superconducting RF accelerating cavity is designed for the TeV electron linear accelerator collider in the next century. The ratio of the maximum surface electric field to the accelerating gradient, E pk =E acc , is reduced to 2.024 and the cell-to-cell coupling remains as high as 1.95%. The distribution of the higher-order mode passbands is reasonable. There is no overlap between these bands, therefore no trapped modes. The circle-straight/line-ellipse-type structure provides good mechanical strength in the accelerating cavity. According to the present state of the art of surface processing techniques of Niobium cavities, it is possible to reach an accelerating gradient of 25-30 MV/m with beam load.

Chinese Physics, Oct 1, 2002

The approach of expanding the magnetic scalar potential in a series of Legendre polynomials is su... more The approach of expanding the magnetic scalar potential in a series of Legendre polynomials is suitable for designing a conventional superconducting magnetic resonance imaging magnet of distributed solenoidal configuration. Whereas the approach of expanding the magnetic vector potential in associated Legendre harmonics is suitable for designing a single-solenoid magnet that has multiple tiers, in which each tier may have multiple layers with different winding lengths. A set of three equations to suppress some of the lowest higher-order harmonics is found. As an example, a 4T single-solenoid magnetic resonance imaging magnet with 4×6 layers of superconducting wires is designed. The degree of homogeneity in the 0.5m diameter sphere volume is better than 5.8 ppm. The same degree of homogeneity is retained after optimal integralization of turns in each correction layer. The ratio Bm/B0 in the single-solenoid magnet is 30% lower than that in the conventional six-solenoid magnet. This tolerates higher rated superconducting current in the coil. The Lorentz force of the coil in the single-solenoid system is also much lower than in the six-solenoid system. This novel type of magnet possesses significant advantage over conventional magnets, especially when used as a super-high field functional magnetic resonance imaging magnet.

Journal of Systems Engineering and Electronics, Mar 1, 2001

Multi-modality medical image fusion has more and more important applications in medical image ana... more Multi-modality medical image fusion has more and more important applications in medical image analysis and understanding. In this paper, we develop and apply a multi-resolution method based on wavelet pyramid to fuse medical images from different morlalities such as PET-MRI and CT-MRI. In particular, we evaluate the different fusion results when applying different selection rules and obtain optimum combination of fusion parameters.

... The electrostatic field is an attractive external force model for snakes (active contour). In... more ... The electrostatic field is an attractive external force model for snakes (active contour). In this paper, a novel approach is proposed to solve the infinite problem induced by using the formula of the electric field intensity. By constructing a template plane above the original image ...

Magnetic Resonance in Medicine, Mar 10, 2011

Amide proton transfer (APT) imaging is capable of detecting mobile cellular proteins and peptides... more Amide proton transfer (APT) imaging is capable of detecting mobile cellular proteins and peptides in tumor and monitoring pH effects in stroke, through the saturation transfer between irradiated amide protons and water protons. In this work, four healthy subjects, eight brain tumor patients (four with high-grade glioma; one with lung cancer metastasis; three with meningioma), and four stroke patients (average 4.3 ± 2.5 days after the onset of the stroke) were scanned at 3T, using different radiofrequency saturation powers. The APT effect was quantified using the magnetization-transfer-ratio (MTR) asymmetry at 3.5 ppm with respect to the water resonance. At a saturation power of 2 μT, the measured APT-MRI signal of the normal brain tissue was almost zero, due to the contamination of the negative conventional MTR asymmetry. This irradiation power caused an optimal hyperintense APT-MRI signal in the tumor and an optimal hypointense signal in the stroke, compared to the normal brain tissue. The results suggest that the saturation power of 2 μT is ideal for APT imaging of these two pathologies at 3T with the existing clinical hardware.

Piers Online, 2007

A new approach has been presented in the paper to gain a uniform magnetic field in NMR system. Fi... more A new approach has been presented in the paper to gain a uniform magnetic field in NMR system. First, the adopted shimming piece is modeled as a magnetic dipole moment to calculate its magnetization effect on the background field within region of interest. Then, sequential quadratic programming method is utilized to determine the ideal solution for shimming work. Finally, the ideal solution is discretized, and quantization error control technique is prepared for special cases. This new method helps to reduce the inhomogeneity in the region of interest from 56.8 ppm to 14.21 ppm, within one hour in practical shimming work.

The present approach to a CESR B-factory is to divide the desired beam current of 1-2 amps into a... more The present approach to a CESR B-factory is to divide the desired beam current of 1-2 amps into a large number of low current bunches, which helps reduce detector backgrounds. To fit 230 bunches into the CESR size ring, they need to be very closely spaced, eliminating room for any conventional separators. Instead, a small (12 mr) crossing angle is considered, which may cause harmful coupling between synchrotron and betatron motion [ll. By rotating the bunches before collision ("crabbing") so they collide head-on, and then rotating them back, so they pass through the arcs normally, this dangerous coupling can be eliminated [2]. We show that the needed transverse kick of-2 MV can be achieved with one single cell superconducting cavity, operating in the TM110 mode at 500 MHz, while keeping the surface electric field below 2.5 MV/m to avoid excessive field emission. The cell design allows all modes higher in frequency than the crab mode to propagate out the beam pipe and be damped outside the cryostat with ferrite beam pipe absorbers. 1.5 GHz niobium cavities have been tested to study the multipacting behavior of the TM1 10 mode.

Concepts in Magnetic Resonance Part B-magnetic Resonance Engineering, Feb 17, 2010

A novel target-field approach for designing bi-planar gradient and shim coils of restricted radiu... more A novel target-field approach for designing bi-planar gradient and shim coils of restricted radius for use with permanent-magnet MRI systems is presented in this article. The method is based on identification of the weighting of harmonic components in the current distribution that will generate a magnetic field whose z-component follows a chosen spherical harmonic function. Mathematical expressions that relate the harmonic terms in the surface current distribution to spherical harmonic terms in the field expansion are established and then used in a simple matrix inversion approach to design a range of shim coils. The expressions providing a spherical harmonic decomposition of the field components produced by a particular current distribution are novel. The results can be used to design bi-planar shim coils of restricted radius that will generate a field variation that follows a certain spherical harmonic over a reasonably-sized volume. A stream function was utilized to obtain the discrete wire distribution on the coil plane. This method does not require the setting of the target-field points. Through an analysis of the matrix equations in terms of condition numbers, we show that this novel approach has no ill-conditioned problems.

Journal of Physics D, Jul 13, 2007

Based on the target-field method of solving Fredholm integral equations of the first kind, a new ... more Based on the target-field method of solving Fredholm integral equations of the first kind, a new approach is presented in the paper for designing gradient coils that can be used in a permanent-magnet magnetic resonance imaging (MRI) system with biplanar poles. To restrict the current distribution on the coil plane within a finite radius, the current density is pre-expanded into Fourier series by orthogonal basis functions. By setting the target-field points and B z values over the imaging region of interest, corresponding integral equations are derived from the Biot-Savart law to calculate the current densities. They form a matrix equation, in which the unknown elements of the column vector are the Fourier coefficients for the unknown current density. As long as these target-field points are well chosen, the Fourier coefficients can be solved by inverse matrix calculation instead of the regularization method for Fredholm integral equations of the first kind. Then the current density is discretized using the stream-function method to generate the winding patterns. To verify the feasibility of this approach, the gradient magnetic field generated by the current density is calculated via the Biot-Savart law. Optimized parameters are obtained through computer simulations for some shielded and unshielded transverse gradient coils. The performance of this approach has been demonstrated as well.

Progress in Natural Science: Materials International, Apr 1, 2003

ABSTRACT Two parallel methods for magnetic resonance imaging (MRI) using radiofrequency (RF) phas... more ABSTRACT Two parallel methods for magnetic resonance imaging (MRI) using radiofrequency (RF) phased array surface coils, named spatial local Fourier encoding (SLFE) and spatial RF encoding (SRFE), are presented. The MR signals are acquired from separate channels across the coils, each of which covers a sub-FOV (field-of-view) in a parallel fashion, and the acquired data are combined to form an image of entire FOV. These two parallel encoding techniques can accelerate MR imaging greatly, yet associated artifact may appear, although the SLFE is an effective image reconstruction method which can reduce the localized artifact in some degrees. By the SRFE, RF coil array can be utilized for spatial encoding through a specialized coil design. The images are acquired in a snapshot with a high signal-to-noise ratio (SNR) without the costly gradient system, resulting in great saving of cost. Both mutual induction and aliasing effect of adjacent coils are critical to the success of SRFE. The strategies of inverse source problem and wavelet transform (WT) can be employed to eliminate them. The results simulated by MATLAB are reported.

Chinese Physics, 2003

In the problem of nonhomogeneous field magnetic resonance imaging (MRI),echo peaks decay quickly ... more In the problem of nonhomogeneous field magnetic resonance imaging (MRI),echo peaks decay quickly due to traditional design of radio frequency (RF) pulse.In this article,using inverse scattering transform (IST) of the nonlinear dynamics and Shinnar-Le Roux selective excitation pulse design algorithm (SLR Algorithm),we have designed the RF pulse for the nonhomogeneous field MRI.Simulation has been made to test the signal loss induced by the excitation of different types of RF pulses.Simulation results show that the RF pulse sequence optimized by IST and SLR algorithm can improve the signal to noise ratio of MRI considerably

Superconductor Science and Technology, Jul 2, 2010

A method is developed for designing a special iron shielded superconducting magnet for MRI in thi... more A method is developed for designing a special iron shielded superconducting magnet for MRI in this paper. The shield is designed as an integral part of the cryostat and high permeability and high saturated magnetization iron material is adopted. This scheme will result in a compact iron shielded magnet. In the presented design, the finite element(FE) method is adopted to calculate the magnetic field which is produced by superconducting coils and. nonlinear iron material. The FE method is incorporated into the simulated annealing method which is employed for corresponding optimization. Therefore, geometrical configurations of both coils and iron shield can be optimized together here. This method can deal with discrete design variables which are defined to describe cable arrangements of coil cross sections. Detail algorithm of presented design is described and an example for designing 1.5T clinical iron shielded magnet for MRI is shown in this paper.

Progress in Natural Science: Materials International, Sep 1, 2004

ABSTRACT

Piers Online, 2007

A new approach has been presented in the paper to optimize the longitudinal gradient coil perform... more A new approach has been presented in the paper to optimize the longitudinal gradient coil performance. First, traditional spherical harmonic target field method is deduced, the relation between the magnetic field and the current distribution is described by a matrix equation. Then, simulated annealing method is introduced to the optimization procedure, and those high order coefficients which are used to vanish become the variables designed for optimization. Finally, stream function method is used to transform the current density into discrete gradient coils. Comparison between traditional method and the optimized method shows that the inhomogeneity in the region of interest can be reduced from 13.59% to 4.9%, the coil efficiency is increased from 11.56 mT/m/A to 18.07 mT/m/A, and the minimum distance of the discrete current wire is raised from 0.95 mm to 2.02 mm.

Magnetic Resonance Imaging, Jul 1, 2010

To shorten scanning time and increase the feasibility of experimental results, we performed right... more To shorten scanning time and increase the feasibility of experimental results, we performed right coronary artery magnetic resonance angiography (CMRA) at 3.0 T using dual acceptance window weighting function in 25 normal subjects. We examined these subjects using conventional navigator with fixed gating window and 6 dual acceptance window weighted gating (DAWG) sequences with different central weighted ratio (CWR). Compared with the conventional navigator sequence, DAWG sequences with CWRs of 20% and 25% increased the scanning efficiency by 30% and 26% respectively (Pb.05), while maintaining good image quality; further the corresponding scanning time decreased from 2.12-1.64 and 1.69 min, respectively (Pb.05). However, CWRs less than 15% caused image degradation to some extent. The coronary artery lengths and diameters did not show statistically significant differences between the two techniques (PN.05). Briefly, to avoid the problems caused by low navigator efficiency and to maintain comparable image quality, the weighted gating parameters of 3 mm width central acceptance window and 15 mm width outer acceptance window with CWR between 20% and 25% are recommended for right CMRA at 3 T.

Progress in Natural Science: Materials International, Oct 1, 2009

Based on the sequential quadratic programming (SQP) method, a new approach is presented in this p... more Based on the sequential quadratic programming (SQP) method, a new approach is presented in this paper to gain a uniform magnetic field for a permanent MRI magnet with biplanar poles. First, the adopted shimming piece is modeled as a magnetic dipole moment to calculate its effect on the background field over the imaging region of interest. Then, the SQP method is utilized to determine the ideal solution for the shimming equation. Finally, the ideal solution is discrete, and the quantization error control technique is used for special cases. This new method helps to reduce the inhomogeneity from 1234.5 ppm to 21.4 ppm over a 36 cm diameter spherical volume (DSV), within hours in practical shimming work.

Based on the target-field method, an ameliorated approach is presented in this paper for designin... more Based on the target-field method, an ameliorated approach is presented in this paper for designing gradient coils that can be used in biplanar pole system of permanent-magnet MRI. To restrict the current on the coil plane within a finite radius, the current density is pre-expanded with a suitable series of orthogonal basis, setting the target field points, solved out the current density by inverse matrix calculation, and then the winding patterns can be gained from dispersing the continuous distribution of current density by using the stream-function method. Applying the method, the computer emulation design of some transverse gradient coils and self-shield coils result in the optimized parameters, and the effect of the approach has been illustrated.