Browsing by Subject "EPI"

Diffusion-weighted MRI (DWI) is an essential tool in clinical applications such as detecting ischemic stroke, and in research applications studying neuronal connectivity in the brain. Diffusion-weighted imaging with multi-shot echo-planar acquisition (DWEPI) offers several advantages over single-shot EPI, including improved spatial resolution and reduced off-resonance and susceptibility artifacts. However a major limitation of multi-shot DWEPI is its sensitivity to patient motion during the application of diffusion gradients; the motion generates phase errors which vary from shot to shot, resulting in artifacts in the reconstructed image.

Most current methods for correcting motion-induced artifacts involve the use of navigator echoes to estimate the shot-to-shot phase errors. Accurate navigator information comes at the expense of increased scan times however, which is generally undesirable. The aim of this study is to therefore develop and demonstrate the use of an alternative phase estimation technique, iterative phase-cycling, as a new method to correct motion artifacts in DWI without the use of navigators. The developed method involves an iterative column by column estimation of phase errors in the aliased image and reconstruction of an artifact free image using the estimated errors.

In this study the technique was applied to correct artifacts in simulated images, hybrid-simulated images, and true four-shot DWEPI images. Accuracy of the phase-cycling method was evaluated by computing residual image errors and ghost-to-noise ratios after correction. The efficiency of phase-cycling was evaluated by recording computation times of the correction process. Multiple optimization techniques were developed and used in the experiments, and the accuracy/efficiency of these techniques were also assessed.

Results of the experiments demonstrated the ability of phase-cycling to greatly reduce motion-induced artifacts in multi-shot DWEPI at reasonable computation times. The phase-cycling model used in this study accurately estimated linear and nonlinear errors along the frequency encoding direction and linear errors along the phase encoding direction. An additional mathematical framework is presented illustrating the potential of phase-cycling to correct nonlinear errors along the phase encoding direction in future work.

The study establishes the developed technique as a unique and effective method for correcting motion artifacts in DWEPI without the cost of increased scan times.

Magnetic Resonance Imaging (MRI) image quality is strongly dependent on the homogeneity of the main magnetic field, B0. Inhomogeneities in this magnetic field lead to image artifacts such as: blurring, signal loss, and gross distortions of the imaged anatomy of the brain, degrading the images effectiveness to provide diagnostic information. A new radio-frequency (RF) head coil design with integrated direct-current (DC) shim coils to provide localized B0 shimming of the brain and simultaneously provide parallel excitation of reception is presented in this thesis. This design optimizes both the RF and DC shim coils proximity to the subject thereby maximizing both the signal-to-noise ratio and the shimming efficiency. This coil architecture is termed iPRES (integrated parallel receive, excitation and shimming).

An existing 32 channel receive-only head coil array was modified into an iPRES coil architecture. The coils of the array were modified using RF components to enable the simultaneous flow of both RF and DC currents on the same structure. The RF and DC currents provide concurrent signal reception and localized B0 shimming to the brain, respectively. In this thesis, the techniques, measurements and quality-metrics used to facilitate the iPRES coil array modification will be discussed.

The localized B0 shimming performance is evaluated in the frontal region of the brain which suffers from large susceptibility artifacts at the air/tissue boundary of the brain and the sinus. Axial B0 maps and echo-planar images (EPI) are acquired in vivo with optimized DC shim currents demonstrating a reduction in B0 inhomogeneities in the frontal lobe resulting in improved image EPI image quality. The coils quality factor and signal-to-noise ratio did not suffer as a result of the coil modification. The shimming performance and RF quality metrics are compared to standard whole-body spherical harmonic shimming and are discussed at length in the following chapters.

Finally, initial phantom results from the next-generation iPRES coil array will be presented. This architecture again uses an existing RF head coil array to simultaneously drive RF currents for reception and DC currents for local shimming. However, the shimming is further enhanced by providing additional RF-isolated shim coils which increases the shimming degrees of freedom. This design is useful when fast-changing, asymmetric B0 inhomogeneities are present in the imaged anatomy.