Topics in Steady-state MRI Sequences and RF Pulse Optimization.

Small-tip fast recovery (STFR) is a recently proposed rapid steady-state magnetic resonance imaging (MRI) sequence that has the potential to be an alternative to the popular balanced steady-state free precession (bSSFP) imaging sequence, since they have similar signal level and tissue contrast, but STFR has reduced banding artifacts. In this dissertation, an analytic equation of the steady-state signal for the unspoiled version of STFR is first derived. It is shown that unspoiled-STFR is less sensitive to the inaccuracy in excitation than the previous proposed spoiled-STFR. By combining unspoiled-STFR with jointly designed tip-down and tip-up pulses, a 3D STFR acquisition over 3-4 cm thick 3D ROI with single coil and short RF pulses (1.7 ms) is demonstrated. Then, it is demonstrated that STFR can reliably detect functional MRI signal and the contrast is driven mainly from intra-voxel dephasing, not diffusion, using Monte Carlo simulation, human experiments and test-retest reliability. Following that another version of STFR using a spectral pre-winding pulse instead of the spatially tailored pulse is investigated, leading to less T2* weighting, easier implementation. Multidimensional selective RF pulse is a key part for STFR and many other MRI applications. Two novel RF pulse optimization methods are proposed. First, a minimax formulation that directly controls the maximum excitation error, and an effective optimization algorithm using variable splitting and alternating direction method of multipliers (ADMM). The proposed method reduced the maximum excitation by more than half in all the testing cases. Second, a method that jointly optimizes the excitation k-space trajectory and RF pulse is proposed. The k-space trajectory is parametrized using 2nd-order B-splines, and an interior point algorithm is used to explicitly solve the constrained optimization. An effective initialization method is also suggested. The joint design reduced the NRMSE by more than 30 percent compared to existing methods in inner volume excitation and pre-phasing problem. Using the proposed joint design, rapid inner volume STFR imaging with a 4 ms excitation pulse with single transmit coil is demonstrated. Finally, a regularized Bloch-Siegert B1 map reconstruction method is presented that significantly reduces the noise in estimated B1 maps.PhDElectrical Engineering: SystemsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111514/1/sunhao_1.pd

Constrained and Spectral-Spatial RF Pulse Design for Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) provides a non-invasive glimpse inside the human body, generates excellent soft tissue contrast, uses non-ionizing radiation, and has become a critical tool in diagnosis of disease in medicine. Radio Frequency (RF) pulses are an integral component of MRI pulse sequences and can be tailored to particular applications. This dissertation explores the MRI physics, convex optimization problems, and experimental methodologies required for the design of tailored RF pulses First, we introduce constrained RF pulse design, a process that incorporates meaningful, physical constraints, such as peak RF amplitude and integrated RF power, and enables efficient RF pulse design. With this process we explore simultaneous multislice (SMS) imaging, a method used to accelerate MRI and combat notoriously long acquisition times. Compared to an SMS pulse designed without constraints, our constrained pulses achieved lower magnitude normalized root mean squared error (NRMSE) for an equivalent RF pulse length, or alternatively, the same NRMSE for a shorter pulse length. Constrained RF pulse design forms a basis for the rest of the dissertation. Second, we show that prewinding pulses, a special class of RF pulses, help reduce signal loss due to intravoxel dephasing generated by magnetic field inhomogeneities. We propose a spectral-spatial prewinding pulse that leverages a larger effective recovery bandwidth than equivalent, purely spectral pulses. In an in vivo experiment imaging the brain of a human volunteer, we designed spectral-spatial pulses with a complex NRMSE of 0.18, which is significantly improved from the complex NRMSE of 0.54 in the purely spectral pulse for the same experiment. Finally, we consider a slab-selective prewinding pulse, that extends spectral and spectral-spatial prewinding pulses to a common 3D imaging method. Here we integrate optimal control optimization to further improve the slab-selective spectral pulse design and see an in vivo improvement of excitation NRMSE from 0.40 to 0.37. In the context of a steady-state sequence small-tip fast recovery (STFR), we also show a major reduction in mean residual transverse magnetization magnitude after the STFR “tip-up” recovery pulse from 0.18 to 0.02 when adding optimal control. This method has the potential to connect prewinding pulse design from the MRI physicist engineering workspace to a clinical application. In summary, we show that constrained RF pulse design provides an efficient way of improving MRI in terms of acquisition speed (via multislice imaging) and image quality (via signal recovery).PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147647/1/sydneynw_1.pd

Four dimensional spectral‐spatial fat saturation pulse design

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109603/1/mrm25076.pd

Design of spectralâ spatial phase prewinding pulses and their use in smallâ tip fast recovery steadyâ state imaging

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141089/1/mrm26794_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141089/2/mrm26794.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141089/3/mrm26794-sup-0001-suppinfo.pd

Field Inhomogeneity Compensation in High Field Magnetic Resonance Imaging (MRI)

This thesis concentrates on the reduction of field (both main field B0 and RF field B1) inhomogeneity in MRI, especially at high B0 field. B0 and B1 field inhomogeneity are major hindrances in high B0 field MRI applications. B1 inhomogeneity will lead to spatially varying signal intensity in the MR images. B0 inhomogeneity produces blurring, distortion and signal loss at tissue interfaces. B0 artifacts are usually termed off-resonance or susceptibility artifacts. None of the existing methods can perfectly correct these inhomogeneity artifacts.This thesis aims at developing three-dimensional (3D) tailored RF (TRF) pulses to mitigate these artifacts. A current limitation in the use of 3D TRF techniques, however, is that pulses are often too long for practical clinical applications. Multiple transmission techniques are proposed to decrease pulse lengths and provide an inherent correction for B1 inhomogeneity. Shorter pulses are also more robust to profile distortions from susceptibility effects.Specifically, slice-selective 3D TRF pulses for multiple (or ¡°parallel¡±) transmitters were designed and validated in uniform phantom and human brain experiments at 3 Tesla. A pseudo-transmit sensitivity encoding (¡°transmit SENSE¡±) method was introduced using a body coil transmitter and multiple receivers to mimic the real parallel transmitter experiment. The kz-direction was controlled by fast switching of gradients in a fashion similar to Echo planar imaging (EPI). The transverse plane (kx-ky) was sampled sparsely with hexagonal trajectories, and accelerated with the transmit SENSE method. The transmit SENSE 3D TRF pulses reduced the B1 inhomogeneity compared to standard SINC pulses in human brain scans. The undersampled transmit SENSE pulses were only 4.3ms long and could excite a 5mm thick slice, which is very promising for clinical applications. Furthermore, these pulses are shown by numerical simulation to have promise in correcting through-plane susceptibility artifacts

Phase Relaxed Localized Excitation Pulses for Inner Volume Fast Spin Echo Imaging

PURPOSE: To design multidimensional spatially selective radiofrequency (RF) pulses for inner volume imaging (IVI) with three‐dimensional (3D) fast spin echo (FSE) sequences. Enhanced background suppression is achieved by exploiting particular signal properties of FSE sequences. THEORY AND METHODS: The CPMG condition dictates that echo amplitudes will rapidly decrease if a 90° phase difference between excitation and refocusing pulses is not present, and refocusing flip angles are not precisely 180°. This mechanism is proposed as a means for generating additional background suppression for spatially selective excitation, by biasing residual excitation errors toward violating the CPMG condition. 3D RF pulses were designed using this method with a 3D spherical spiral trajectory, under‐sampled by factor 5.6 for an eight‐channel PTx system, at 3 Tesla. RESULTS: 3D‐FSE IVI with pulse durations of approximately 12 ms was demonstrated in phantoms and for T(2)‐weighted brain imaging in vivo. Good image quality was obtained, with mean background suppression factors of 103 and 82 ± 6 in phantoms and in vivo, respectively. CONCLUSION: Inner Volume Imaging with 3D‐FSE has been demonstrated in vivo with tailored 3D‐RF pulses. The proposed design methods are also applicable to 2D pulses. Magn Reson Med 76:848–861, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicin

Joint design of trajectory and RF pulses for parallel excitation

We propose an alternating optimization framework for the joint design of excitation k-space trajectory and RF pulses for small-tip-angle parallel excitation. Using Bloch simulations, we show that compared with conventional designs with predetermined trajectories, joint designs can often excite target patterns with improved accuracy and reduced total integrated pulse power, particularly at high reduction factors. These benefits come at a modest increase in computational time. Magn Reson Med 58:598–604, 2007. © 2007 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/56136/1/21262_ftp.pd

MRI Excitation Pulse Design and Image Reconstruction for Accelerated Neuroimaging

Excitation pulse design and image reconstruction are two important topics in MR research for enabling faster imaging. On the pulse design side, selective excitations that confine signals to be within a small region-of-interest (ROI) instead of the full imaging field-of-view (FOV) can be used to reduce sampling density in the k-space, which is a direct outcome of the change in the underlying Nyquist sampling rate. On the reconstruction side, besides improving imaging algorithms’ ability to restore images from less data, another objective is to reduce the reconstruction time, particularly for dynamic imaging applications. This dissertation focuses on these two perspectives: Chapter II is devoted to the excitation pulse design. Specifically, we exploit auto-differentiation frameworks that automatically apply the chain rule on complicated computations. We derived and developed a computationally efficient Bloch-simulator and its explicit Bloch simulation Jacobian operations using such frameworks. This simulator can yield numerical derivatives with respect to pulse RF and gradient waveforms given arbitrary sub-differentiable excitation objective functions. The method does not rely on the small-tip approximation, and is accurate as long as the Bloch simulation can correctly model the spin movements due to the excitation pulses. In particular, we successfully applied this pulse design approach for jointly designing RF and gradient waveforms for 3D spatially tailored large-tip excitation objectives. The auto-differentiable pulse design method can yield superior 3D spatially tailored excitation profiles that are useful for inner volume (IV) imaging, where one attempts to image a volumetric ROI at high spatiotemporal resolution without aliasing from signals outside the IV (i.e., outer volume). In Chapter III, we propose and develop a novel steady-state IV imaging strategy which suppresses aliasing by saturating the outer volume (OV) magnetizations via a 3D tailored OV excitation pulse that is followed by a signal crusher gradient. This saturation based strategy can substantially suppress the unwanted aliasing for common steady-state imaging sequences. By eliminating the outer volume signals, one can configure acquisitions for a reduced FOV to shorten the scanning time and increase spatiotemporal resolution for applications such as dynamic imaging. In dynamic imaging (e.g., fMRI), where a time series is to be reconstructed, non-iterative reconstruction algorithms may offer savings in overall reconstruction time. Chapter IV focuses on non-iterative image reconstruction, specifically, extending the GRAPPA algorithm to general non-Cartesian acquisitions. We analyzed the formalism of conventional GRAPPA reconstruction coefficients, generalized it to non-Cartesian scenarios by using properties of the Fourier transform, and obtained an efficient non-Cartesian GRAPPA algorithm. The algorithm attains reconstruction quality that can rival classical iterative imaging methods such as conjugate gradient SENSE and SPIRiT. In summary, this dissertation has proposed and developed multiple methods for accelerating MR imaging, from pulse design to reconstruction. While devoted to neuroimaging, the proposed methods are general and should also be useful for other applications.PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/168085/1/tianrluo_1.pd

RF Pulse Designs for Signal Recovery in T2*-Weighted Functional Magnetic Resonance Imaging.

In blood-oxygenation-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) using T2* contrast, images suffer from loss of signals at brain regions close to the air-filled cavities in the human head. The artifact arises from magnetic field distortion caused by the magnetic susceptibility difference between air and brain tissues, and hampers functional studies of important brain regions such as the orbito-frontal cortex. In this research project, I investigate two methods of designing radio-frequency (RF) pulses that can recover the signal loss. In addition to slice selective excitation, both pulse designs ``precompensate'' the through-plane dephasing that occurs between excitation and data acquisition. One method, which utilizes ``three-dimensional tailored RF pulses'', achieves these goals via three-dimensional spatially selective excitation. The other method uses spectral-spatial selective excitation, and relies on the assumption that through-plane dephasing is correlated with resonance frequency offset. All these sophisticated pulses are numerically designed using the iterative conjugate gradient method. To facilitate those design methods, I also propose new techniques applicable to general pulse designs, such as frameworks for pulse computation acceleration and joint design of excitation k-space trajectory and RF pulse. With phantom and human experiments, I demonstrate that the methods are efficacious in signal recovery, but not without costs and hurdles to overcome.Ph.D.Electrical Engineering: SystemsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/57708/2/chunyuy_1.pd