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«RF PULSE DESIGN FOR PARALLEL TRANSMISSION IN ULTRA HIGH FIELD MAGNETIC RESONANCE IMAGING by Hai Zheng B.S., Xi’an JiaoTong University, 2005 ...»

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RF PULSE DESIGN FOR PARALLEL TRANSMISSION IN ULTRA HIGH

FIELD MAGNETIC RESONANCE IMAGING

by

Hai Zheng

B.S., Xi’an JiaoTong University, 2005

Submitted to the Graduate Faculty of

the Swanson School of Engineering in partial fulfillment

of the requirements for the degree of

Doctor of Philosophy

University of Pittsburgh

UNIVERSITY OF PITTSBURGH

SWANSON SCHOOL OF ENGINEERING

This dissertation was presented by Hai Zheng It was defended on January 23, 2012 and approved by George D. Stetten, M.D., Ph.D., Professor, Department of Bioengineering and Radiology Tamer S. Ibrahim, Ph.D., Associate Professor, Department of Bioengineering and Radiology Douglas C. Noll, Ph.D., Professor, Department of Biomedical Engineering and Radiology, University of Michigan Dissertation Director: Fernando E. Boada, Ph.D., Professor, Department of Radiology and Bioengineering ] ii Copyright © by Hai Zheng iii

RF PULSE DESIGN FOR PARALLEL TRANSMISSION IN ULTRA HIGH

FIELD MAGNETIC RESONANCE IMAGING

Hai Zheng, Ph.D.

University of Pittsburgh, 2012 Magnetic Resonance Imaging (MRI) plays an important role in visualizing the structure and function of the human body. In recent years, ultra high magnetic field (UHF) MRI has emerged as an attractive means to achieve significant improvements in both signal-to-noise ratio (SNR) and contrast. However, in vivo imaging at UHF is hampered by the presence of severe B1 and B0 inhomogeneities. B1 inhomogeneity leads to spatial non-uniformity excitation in MR images. B0 inhomogeneity, on the other hand, produces blurring, distortions and signal loss at tissue/air interfaces. Both of them greatly limit the applications of UHF MRI. Thus mitigating B1 and B0 inhomogeneities is central in making UHF MRI practical for clinical use.

Tailored RF pulse design has been demonstrated as a feasible means to mitigate the effects of B1 and B0 inhomogeneities. However, the primary limitation of such tailored pulses is that the pulse duration is too long for practical clinical applications. With the introduction of parallel transmission technology, one can shorten the pulse duration without sacrificing excitation performance. Prior reports in parallel transmission were formulated using linear, small-tip-angle approximation algorithms, which are violated in the regime of nonlinear largetip-angle excitation.

The overall goal of this dissertation is to develop effective and fast algorithms for parallel transmission UHF RF pulses design. The key contributions of this work include 1) a novel largetip-angle RF pulse design method to achieve significant improvements compared with previous

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current field induced on RF shield for parallel transmission and leading to improved excitation and time efficiency; 3) developing new RF pulse design strategy to restore the lost signal over the whole brain and increase BOLD contrast to brain activation in T2*-weighted fMRI at UHF.

For testing and validation, these algorithms were implement on a Siemens 7T MRI scanner equipped with a parallel transmission system and their capabilities for ultra high field MRI demonstrated, first by phantom experiments and later by in vivo human imaging studies.

The contributions presented here will be of importance to bring parallel transmission technology to clinical applications in UHF MRI.

–  –  –

ACKNOWLEDGEMENT

1.0   INTRODUCTION

1.1   THE AIMS OF THIS DISSERTATION

1.1.1   Ultra High Field MRI

1.1.2   Large-tip-angle RF Pulse Design

1.1.3   Eddy Current Compensation

1.1.4   T2*-weighted BOLD fMRI

1.1.5   Specific Aims

1.2   THE SIGNIFICIANCE OF THIS DISSERTATION

1.2.1   Parallel Transmission Technology

1.2.2   RF Pulse Design for Parallel Transmission

1.2.3   Significance of Aims

1.3   THE STURCTURE OF THIS DISSERTATION

2.0   BACKGROUND

2.1   MAGNETIC RESONANCE IMAGING

2.2   RF PULSE DESIGN

2.2.1   Single Channel Transmission Theory

2.2.2   Multiple Channel Transmission Theory

2.3   PARALLEL TRANSMISSION RF PULSE DESIGN

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2.3.2   Large-tip-angle Parallel Pulse Design

3.0   PERTURBATION ANALYSIS METHOD FOR LARGE-TIP-ANGLE PARALLEL

RF PULSE DESIGN

3.1   INTRODUCTION

3.2   THEORY

3.2.1   Analytic Framework

3.2.2   Numerical Solution

3.3   METHODS

3.3.1   B1 Mapping

3.3.2   Main Field Inhomogeneity (ΔB0) Mapping

3.3.3   Pulse Design

3.3.4   Computer Simulation

3.3.5   Experimental Data Acquisition

3.4   RESULTS

3.4.1   Selective Excitation

3.4.2   Inversion

3.4.3   Refocusing

3.4.4   Scanner Experiment

3.5   DISCUSSION

3.6   CONCLUSIONS

4.0   PARALLEL TRANSMISSION RF PULSE DESIGN FOR EDDY CURRENT

CORRECTOIN IN ULTRA HIGH FIELD





4.1   INTRODUCTION

4.2   THEORY

–  –  –

4.2.2   RF Pulse Design

4.3   METHODS

4.3.1   B1 and ΔB0 Mapping

4.3.2   Eddy current effects on different coils

4.3.3   Model-based method vs trajectory measurement method

4.3.4   Numerical simulations

4.3.5   Experiments

4.3.6   Simulations vs Experiments

4.4   RESULTS

4.4.1   Comparison between eight-channel loop coil and TEM coil

4.4.2   Comparison between model-based method and trajectory measurement method

4.4.3   Comparison between Simulations and Experiments

4.4.4   Excitation Quality

4.5   DISCUSSION

4.6   CONCLUSION

5.0   MULTI-SLICE PARALLEL TRANSMISSION THREE-DIMENSIONAL

TAILORED RF (PTX 3DTRF) PULSE DESIGN FOR SIGNAL RECOVERY IN

ULTRA HIGH FIELD FUNCTIONAL MRI

5.1   INTRODUCTION

5.2   THEORY

5.2.1   Principle of the 3DTRF method

5.2.2   Parallel Transmission RF Pulse Design

5.2.3   Multi-slice RF Pulse Design of PTX 3DTRF

5.3   METHODS

–  –  –

5.3.2   B1+ mapping

5.3.3   Main field frequency offset mapping

5.3.4   Pulse design

5.3.5   Phantom experiment

5.3.6   In vivo experiment

5.3.7   BOLD fMRI

5.3.8   Effect of trajectory design

5.4   RESULTS

5.4.1   Phantom experiment

5.4.2   In vivo experiment

5.4.3   BOLD fMRI

5.4.4   Effect of trajectory design

5.5   DISCUSSION

5.6   CONCLUSION

6.0   PRACTICAL CONSIDERATIONS FOR THE DESIGN OF PARALLEL

TRANSMISSION RF PULSES IN ULTRA HIGH FIELD

6.1   INTRODUCTION

6.2   FAST B1 MAPPING METHOD

6.2.1   Introduction

6.2.2   Theory

6.2.3   Methods

6.2.4   Results and Discussions

6.2.5   Conclusion

6.3   COMPENSATION FOR DISCRETE SAMPLING

–  –  –

6.3.2   Methods

6.3.2.1   Method I (Constant function)

6.3.2.2   Method II (Linear function)

6.3.3   Results and Discussions

6.3.4   Conclusion and Future work

7.0   CONCLUSIONS AND FUTURE WORK

7.1   CONTRIBUTIONS

7.2   FUTURE WORK

APPENDIX

BIBLIOGRAPHY

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Figure 2.1(a)Spiral k-space trajectory obtained by time-reversed integration of the corresponding gradient waveforms in (b).

(c) Echo-planar (EP) trajectory and (d) the corresponding gradient waveforms

Figure 2.2 (a) Spiral k-space trajectory with undersampling or acceleration factor of 4 and (b) the corresponding gradient waveforms.

(c) Echo-planar (EP) trajectory and (d) the corresponding gradient waveforms with the same acceleration factor as spiral trajectory.

Figure 3.1 Magnitude of water phantom B1+ map (a) and field map (b) used in the simulations of eight-channel parallel excitation.

Figure 3.2 2D selective 90o excitation pulses for a spiral trajectory with various acceleration factors R= 2, 4, 6, respectively.

a-c: The transverse magnetization produced by Small Tip Angle (STA) method. d-f: The transverse magnetization produced by Additive Angle (AA) method. g-i: The transverse magnetization produced by Perturbation Analysis (PTA) method.

Figure 3.3 2D selective 90o excitation pulses for an EP trajectory with various acceleration factors R= 2, 4, 6, respectively.

a-c: The transverse magnetization produced by Small Tip Angle (STA) method. d-f: The transverse magnetization produced by Additive Angle (AA) method. g-i: The transverse magnetization produced by Perturbation Analysis (PTA) method.

Figure 3.4 Transverse magnetization profiles taken through the center of the excited patterns in Figure 3.

2 for spiral trajectory (a) and in Figure 2.3 for EP trajectory (b) with acceleration factor R=4 using Small Tip Angle method, Additive Angle method and Perturbation Analysis method, respectively.

Figure 3.5 Comparison of excitation accuracy (a) and peak RF magnitude (b) between 90o excitation pulses designed Additive Angle method and Perturbation Analysis method at various acceleration factors R=2, 3, 4, 5, 6

Figure 3.6 Sum of magnitudes of eight channel 90o excitation RF pulses at acceleration factor R=4 (a) in the spiral case and (b) in the EP case.

Figure 3.7 2D selective 180o inversion pulses for spiral and EP trajectories with acceleration factor R=4.

a-f: Mx, My and Mz components of magnetization profiles produced by

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Figure 3.8 2D selective 180o refocusing pulses for a spiral trajectory with acceleration factor R=4

designed using Additive Angle method (a) and Perturbation Analysis method (b). c:

Transverse magnetization profiles taken through the center of the excited patterns from (a) and (b)

Figure 3.9 Experimental results of magnitude images of the water phantom excited via Small Tip Angle method (a, e, i), Additive Angle method (b, f, j) and Perturbation Analysis method (c, g, k) for a spiral trajectory with corresponding transverse magnetization profiles (d, h, l) taken through the midline of the excited patterns with acceleration factors R=2, 4, 6, respectively.

Figure 4.1 Pulse sequence diagram to characterize the system constants.

Data are acquired from multiple slice locations and multiple gradient amplitudes at each slice location. Slice selection and readout are performed on the same physical gradient and then measured for each physical gradient direction. The solid and dashed lines indicate the nominal and actual gradient waveforms, respectively.

Figure 4.2 Different coils to illustrate different eddy current effects between unshielded and shielded RF coils.

(a) Siemens commercial birdcage coil (unshielded RF coil) and (b) TEM coil (shielded RF coil).

Figure 4.3 Excitation patterns produced by the Siemens birdcage coil using spiral and EP trajectories with an acceleration factor of 2.

(a) Excitation obtained with RF pulses designed using the nominal gradient waveforms. (b) Excitation obtained with RF pulses designed using the model-corrected gradient waveforms. Note that no significant differences are observed, which implies that the eddy currents produced on the coil are negligible

Figure 4.4 Gradient waveforms, k-space trajectory and resulting RF pulses for an acceleration factor of 2 with the use of the TEM coil.

(a) Nominal vs actual (model-based correction) gradient waveforms. (b) Corresponding k-space trajectories. (c) Sum of amplitudes of all RF pulses obtained with the nominal (uncorrected) and actual (corrected) trajectories. Note that the deviation between the nominal and actual gradient waveforms leads to significant difference between the uncorrected pulses and corrected pulses

Figure 4.5 Comparison of gradient waveforms and excitation patterns between model-based method and trajectory measurement method.

(a) Gradient waveforms obtained from the nominal trajectory, the model-based method and the trajectory measurement method. The zoom-in panel clearly presents that the gradient waveforms derived from the model-based method and the trajectory measurement methods are very close. By contrast, these gradient waveforms are both significantly different from the nominal

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Figure 4.6 Comparison of the RF performance for the spiral trajectory design over a range of acceleration factors for both simulations (a-b) and experiments (c-d).

(a) Simulated excitation patterns obtained with the RF pulses designed using the nominal gradient waveforms. (b) Simulated excitation patterns obtained with the RF pulses designed using the model-corrected gradient waveforms. (c) Experimental excitation patterns obtained with the RF pulses designed using the nominal gradient waveforms. (d) Experimental excitation patterns obtained with the RF pulses designed using the model-corrected gradient waveforms. Significant improvements are observed when the model-corrected RF pulses are used for both simulations and experiments......... 79 Figure 4.7 Comparison of the RF performance for the EP trajectory over a range of acceleration factors for both simulations (a-b) and experiments (c-d). (a) Simulated excitation patterns obtained with the RF pulses designed using the nominal gradient waveforms.



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