907 research outputs found
Doctor of Philosophy
dissertationMagnetic resonance imaging (MRI) techniques are widely applied in various disease diagnoses and scientific research projects as noninvasive methods. However, lower signal-to-noise ratio (SNR), B1 inhomogeneity, motion-related artifact, susceptibility artifact, chemical shift artifact and Gibbs ring still play a negative role in image quality improvement. Various techniques and methods were developed to minimize and remove the degradation of image quality originating from artifacts. In the first part of this dissertation, a motion artifact reduction technique based on a novel real time self-gated pulse sequence is presented. Diffusion weighted and diffusion tensor magnetic resonance imaging techniques are generally performed with signal averaging of multiple measurements to improve the signal-to-noise ratio and the accuracy of diffusion measurement. Any discrepancy in images between different averages causes errors that reduce the accuracy of diffusion MRI measurements. The new scheme is capable of detecting a subject's motion and reacquiring motion-corrupted data in real time and helps to improve the accuracy of diffusion MRI measurements. In the second part of this dissertation, a rapid T1 mapping technique (two dimensional singleshot spin echo stimulated echo planar image--2D ss-SESTEPI), which is an EPI-based singleshot imaging technique that simultaneously acquires a spin-EPI (SEPI) and a stimulated-EPI (STEPI) after a single RF excitation, is discussed. The magnitudes of SEPI and STEPI differ by T1 decay for perfect 90o RF pulses and can be used to rapidly measure the T1 relaxation time. However, the spatial variation of B1 amplitude induces uneven splitting of the transverse magnetization for SEPI and STEPI within the imaging FOV. Therefore, correction for B1 inhomogeneity is critical for 2D ss-SESTEPI to be used for T1 measurement. In general, the EPI-based pulse sequence suffers from geometric distortion around the boundary of air-tissue or bone tissue. In the third part of this dissertation, a novel pulse sequence is discussed, which was developed based on three dimensional singleshot diffusion weighted stimulated echo planar imaging (3D ss-DWSTEPI). A parallel imaging technique was combined with 3D ss-DWSTEPI to reduce the image distortion, and the secondary spin echo formed by three RF pulses (900-1800-900) is used to improve the SNR. Image quality is improved
Integrated and efficient diffusion-relaxometry using ZEBRA
The emergence of multiparametric diffusion models combining diffusion and
relaxometry measurements provide powerful new ways to explore tissue
microstructure with the potential to provide new insights into tissue structure
and function. However, their ability to provide rich analyses and the potential
for clinical translation critically depends on the availability of efficient,
integrated, multi-dimensional acquisitions. We propose a fully integrated
sequence simultaneously sampling the acquisition parameter spaces required for
T1 and T2* relaxometry and diffusion MRI. Slice-level interleaved diffusion
encoding, multiple spin/gradient echoes and slice-shuffling are combined for
higher efficiency, sampling flexibility and enhanced internal consistency.
In-vivo data was successfully acquired on healthy adult brains. Obtained
parametric maps as well as clustering results demonstrate the potential of the
technique regarding its ability to provide eloquent data with an acceleration
of roughly 20 compared to conventionally used approaches. The proposed
integrated acquisition, called ZEBRA, offers significant acceleration and
flexibility compared to existing diffusion-relaxometry studies and thus
facilitates wider use of these techniques both for research-driven and clinical
applications
Towards in vivo g-ratio mapping using MRI: unifying myelin and diffusion imaging
The g-ratio, quantifying the comparative thickness of the myelin sheath
encasing an axon, is a geometrical invariant that has high functional relevance
because of its importance in determining neuronal conduction velocity. Advances
in MRI data acquisition and signal modelling have put in vivo mapping of the
g-ratio, across the entire white matter, within our reach. This capacity would
greatly increase our knowledge of the nervous system: how it functions, and how
it is impacted by disease. This is the second review on the topic of g-ratio
mapping using MRI. As such, it summarizes the most recent developments in the
field, while also providing methodological background pertinent to aggregate
g-ratio weighted mapping, and discussing pitfalls associated with these
approaches. Using simulations based on recently published data, this review
demonstrates the relevance of the calibration step for three myelin-markers
(macromolecular tissue volume, myelin water fraction, and bound pool fraction).
It highlights the need to estimate both the slope and offset of the
relationship between these MRI-based markers and the true myelin volume
fraction if we are really to achieve the goal of precise, high sensitivity
g-ratio mapping in vivo. Other challenges discussed in this review further
evidence the need for gold standard measurements of human brain tissue from ex
vivo histology. We conclude that the quest to find the most appropriate MRI
biomarkers to enable in vivo g-ratio mapping is ongoing, with the potential of
many novel techniques yet to be investigated.Comment: Will be published as a review article in Journal of Neuroscience
Methods as parf of the Special Issue with Hu Cheng and Vince Calhoun as Guest
Editor
Respiratory organ motion in interventional MRI : tracking, guiding and modeling
Respiratory organ motion is one of the major challenges in interventional MRI, particularly in interventions with therapeutic ultrasound in the abdominal region. High-intensity focused ultrasound found an application in interventional MRI for noninvasive treatments of different abnormalities. In order to guide surgical and treatment interventions, organ motion imaging and modeling is commonly required before a treatment start. Accurate tracking of organ motion during various interventional MRI procedures is prerequisite for a successful outcome and safe therapy.
In this thesis, an attempt has been made to develop approaches using focused ultrasound which could be used in future clinically for the treatment of abdominal organs, such as the liver and the kidney. Two distinct methods have been presented with its ex vivo and in vivo treatment results. In the first method, an MR-based pencil-beam navigator has been used to track organ motion and provide the motion information for acoustic focal point steering, while in the second approach a hybrid imaging using both ultrasound and magnetic resonance imaging was combined for advanced guiding capabilities.
Organ motion modeling and four-dimensional imaging of organ motion is increasingly required before the surgical interventions. However, due to the current safety limitations and hardware restrictions, the MR acquisition of a time-resolved sequence of volumetric images is not possible with high temporal and spatial resolution. A novel multislice acquisition scheme that is based on a two-dimensional navigator, instead of a commonly used pencil-beam navigator, was devised to acquire the data slices and the corresponding navigator simultaneously using a CAIPIRINHA parallel imaging method. The acquisition duration for four-dimensional dataset sampling is reduced compared to the existing approaches, while the image contrast and quality are improved as well.
Tracking respiratory organ motion is required in interventional procedures and during MR imaging of moving organs. An MR-based navigator is commonly used, however, it is usually associated with image artifacts, such as signal voids. Spectrally selective navigators can come in handy in cases where the imaging organ is surrounding with an adipose tissue, because it can provide an indirect measure of organ motion. A novel spectrally selective navigator based on a crossed-pair navigator has been developed. Experiments show the advantages of the application of this novel navigator for the volumetric imaging of the liver in vivo, where this navigator was used to gate the gradient-recalled echo sequence
Cortical depth dependent functional responses in humans at 7T: improved specificity with 3D GRASE
Ultra high fields (7T and above) allow functional imaging with high contrast-to-noise ratios and improved spatial resolution. This, along with improved hardware and imaging techniques, allow investigating columnar and laminar functional responses. Using gradient-echo (GE) (T2* weighted) based sequences, layer specific responses have been recorded from human (and animal) primary visual areas. However, their increased sensitivity to large surface veins potentially clouds detecting and interpreting layer specific responses. Conversely, spin-echo (SE) (T2 weighted) sequences are less sensitive to large veins and have been used to map cortical columns in humans. T2 weighted 3D GRASE with inner volume selection provides high isotropic resolution over extended volumes, overcoming some of the many technical limitations of conventional 2D SE-EPI, whereby making layer specific investigations feasible. Further, the demonstration of columnar level specificity with 3D GRASE, despite contributions from both stimulated echoes and conventional T2 contrast, has made it an attractive alternative over 2D SE-EPI. Here, we assess the spatial specificity of cortical depth dependent 3D GRASE functional responses in human V1 and hMT by comparing it to GE responses. In doing so we demonstrate that 3D GRASE is less sensitive to contributions from large veins in superficial layers, while showing increased specificity (functional tuning) throughout the cortex compared to GE
Real‐time motion and retrospective coil sensitivity correction for CEST using volumetric navigators (vNavs) at 7T
PURPOSE: To explore the impact of temporal motion-induced coil sensitivity changes on CEST-MRI at 7T and its correction using interleaved volumetric EPI navigators, which are applied for real-time motion correction. METHODS: Five healthy volunteers were scanned via CEST. A 4-fold correction pipeline allowed the mitigation of (1) motion, (2) motion-induced coil sensitivity variations, Δ B 1 - , (3) motion-induced static magnetic field inhomogeneities, ΔB0 , and (4) spatially varying transmit RF field fluctuations, ΔB 1 + . Four CEST measurements were performed per session. For the first 2, motion correction was turned OFF and then ON in absence of voluntary motion, whereas in the other 2 controlled head rotations were performed. During post-processing Δ B 1 - was removed additionally for the motion-corrected cases, resulting in a total of 6 scenarios to be compared. In all cases, retrospective ∆B0 and - ΔB 1 + corrections were performed to compute artifact-free magnetization transfer ratio maps with asymmetric analysis (MTRasym ). RESULTS: Dynamic Δ B 1 - correction successfully mitigated signal deviations caused by head motion. In 2 frontal lobe regions of volunteer 4, induced relative signal errors of 10.9% and 3.9% were reduced to 1.1% and 1.0% after correction. In the right frontal lobe, the motion-corrected MTRasym contrast deviated 0.92%, 1.21%, and 2.97% relative to the static case for Δω = 1, 2, 3 ± 0.25 ppm. The additional application of Δ B 1 - correction reduced these deviations to 0.10%, 0.14%, and 0.42%. The fully corrected MTRasym values were highly consistent between measurements with and without intended head rotations. CONCLUSION: Temporal Δ B 1 - cause significant CEST quantification bias. The presented correction pipeline including the proposed retrospective Δ B 1 - correction significantly reduced motion-related artifacts on CEST-MRI
Real-time motion and retrospective coil sensitivity correction for CEST using volumetric navigators (vNavs) at 7T
Purpose To explore the impact of temporal motion-induced coil sensitivity changes on CEST-MRI at 7T and its correction using interleaved volumetric EPI navigators, which are applied for real-time motion correction. Methods Five healthy volunteers were scanned via CEST. A 4-fold correction pipeline allowed the mitigation of (1) motion, (2) motion-induced coil sensitivity variations, Delta B1-, (3) motion-induced static magnetic field inhomogeneities, Delta B-0, and (4) spatially varying transmit RF field fluctuations, Delta B1+. Four CEST measurements were performed per session. For the first 2, motion correction was turned OFF and then ON in absence of voluntary motion, whereas in the other 2 controlled head rotations were performed. During post-processing Delta B1- was removed additionally for the motion-corrected cases, resulting in a total of 6 scenarios to be compared. In all cases, retrospective increment B-0 and -Delta B1+ corrections were performed to compute artifact-free magnetization transfer ratio maps with asymmetric analysis (MTRasym). Results Dynamic Delta B1- correction successfully mitigated signal deviations caused by head motion. In 2 frontal lobe regions of volunteer 4, induced relative signal errors of 10.9% and 3.9% were reduced to 1.1% and 1.0% after correction. In the right frontal lobe, the motion-corrected MTRasym contrast deviated 0.92%, 1.21%, and 2.97% relative to the static case for Delta omega = 1, 2, 3 +/- 0.25 ppm. The additional application of Delta B1- correction reduced these deviations to 0.10%, 0.14%, and 0.42%. The fully corrected MTRasym values were highly consistent between measurements with and without intended head rotations. Conclusion Temporal Delta B1- cause significant CEST quantification bias. The presented correction pipeline including the proposed retrospective Delta B1- correction significantly reduced motion-related artifacts on CEST-MRI.Peer reviewe
Three-dimensional echo-shifted EPI with simultaneous blip-up and blip-down acquisitions for correcting geometric distortion
Purpose: Echo-planar imaging (EPI) with blip-up/down acquisition (BUDA) can
provide high-quality images with minimal distortions by using two readout
trains with opposing phase-encoding gradients. Because of the need for two
separate acquisitions, BUDA doubles the scan time and degrades the temporal
resolution when compared to single-shot EPI, presenting a major challenge for
many applications, particularly functional MRI (fMRI). This study aims at
overcoming this challenge by developing an echo-shifted EPI BUDA (esEPI-BUDA)
technique to acquire both blip-up and blip-down datasets in a single shot.
Methods: A three-dimensional (3D) esEPI-BUDA pulse sequence was designed by
using an echo-shifting strategy to produce two EPI readout trains. These
readout trains produced a pair of k-space datasets whose k-space trajectories
were interleaved with opposite phase-encoding gradient directions. The two
k-space datasets were separately reconstructed using a 3D SENSE algorithm, from
which time-resolved B0-field maps were derived using TOPUP in FSL and then
input into a forward model of joint parallel imaging reconstruction to correct
for geometric distortion. In addition, Hankel structured low-rank constraint
was incorporated into the reconstruction framework to improve image quality by
mitigating the phase errors between the two interleaved k-space datasets.
Results: The 3D esEPI-BUDA technique was demonstrated in a phantom and an fMRI
study on healthy human subjects. Geometric distortions were effectively
corrected in both phantom and human brain images. In the fMRI study, the visual
activation volumes and their BOLD responses were comparable to those from
conventional 3D echo-planar images. Conclusion: The improved imaging efficiency
and dynamic distortion correction capability afforded by 3D esEPI-BUDA are
expected to benefit many EPI applications.Comment: 8 figures, peer-reviewed journal pape
- …