142 research outputs found
7-Tesla-Ultrahochfeld Magnetresonanztomographie im Kopfund Halsbereich mittels 64-Kanal-Signaldetektion und integrierter paralleler 16-Kanal-Sendespule
Die MRT hat sich als wertvolles Diagnosewerkzeug im klinischen Alltag gezeigt und sich seit seiner Einführung konstant weiterentwickelt. So wurde erst kürzlich der erste MRT durch die United States Food and Drug Administration zur klinischen Nutzung freigegeben. Mit einer höheren Magnetfeldstärke ändern sich die physikalischen Effekte und Parameter, die eine positive (kontrastverstärkende Auswirkung), aber auch eine negative (artefaktverstärkende) Auswirkung auf die Bildgebung haben können. So ist es aus technischen Gründen derzeit noch nicht möglich, eine Ganzkörperaufnahme bei einer Feldstärke von 7 T zu generieren, wie es beispielsweise bei 1,5 T möglich ist. Die derzeitige Bildgebung bei 7 T-MRTs beschränkt sich hauptsächlich auf lokale Bereiche wie z.B. das Gehirn, oder das Fuß-, Arm- oder Kniegelenk. Auf Basis der großen Nachfrage aus dem klinischen Bereich, das Bildfeld zu erweitern, ergibt sich die Fragestellung dieser Dissertation:
Ist es möglich, mit dem aktuellen Stand der Technik unter Berücksichtigung der maximal verfügbaren Sende- und Empfangskanäle, die ein derzeitiges kommerzielles 7 T-MRT bietet, ein Bildfeld zu generieren, welches den Kopf- und Halsbereich abdeckt?
Diese Fragestellung wurde durch die Entwicklung von morphologisch angepassten Signalgeneratoren als auch Signaldetektoren gelöst. Das Bildfeld wurde von der Gehirnregion auf den Halsbereich bei 7 T erweitert. Die entwickelte Hardware wurde entworfen, simuliert, konstruiert, getestet und mit einer kommerziell verfügbaren 7 T Gehirnspule verglichen. Ein Fortschritt zum aktuellen Stand der Technik wurde quantifiziert.
Die neu entwickelten Methoden zur Gestaltung und Konstruktion der Sende- und Empfangsstruktur bei 7 T, als auch die zur Prüfung der Funktionalität verwendete Hardware, wurde direkt in abgewandelter Form bei ähnlichen MRT-Forschungsprojekten bei einer Feldstärke von 3 T verwendet und publiziert.
Zusammenfassend wurde mit diesem Projekt sowohl der Grundstein für die klinische Bildgebung als auch für weitere Forschung im kombinierten Kopf- und Halsbereich bei der 7 T gelegt. Der Einfluss dieses Projekts wird sich voraussichtlich in den nächsten Jahren in klinischen Studien zeigen. Limitierende Faktoren wie beispielsweise die SAR können durch Softwaremaßnahmen und der Ansteuerung der Spulen in weiteren Doktorarbeiten optimiert werden, um den Bildgebungsprozess zu optimieren
Recommendations and guidelines from the ISMRM Diffusion Study Group for preclinical diffusion MRI: Part 1 -- In vivo small-animal imaging
The value of in vivo preclinical diffusion MRI (dMRI) is substantial.
Small-animal dMRI has been used for methodological development and validation,
characterizing the biological basis of diffusion phenomena, and comparative
anatomy. Many of the influential works in this field were first performed in
small animals or ex vivo samples. The steps from animal setup and monitoring,
to acquisition, analysis, and interpretation are complex, with many decisions
that may ultimately affect what questions can be answered using the data. This
work aims to serve as a reference, presenting selected recommendations and
guidelines from the diffusion community, on best practices for preclinical dMRI
of in vivo animals. In each section, we also highlight areas for which no
guidelines exist (and why), and where future work should focus. We first
describe the value that small animal imaging adds to the field of dMRI,
followed by general considerations and foundational knowledge that must be
considered when designing experiments. We briefly describe differences in
animal species and disease models and discuss how they are appropriate for
different studies. We then give guidelines for in vivo acquisition protocols,
including decisions on hardware, animal preparation, imaging sequences and data
processing, including pre-processing, model-fitting, and tractography. Finally,
we provide an online resource which lists publicly available preclinical dMRI
datasets and software packages, to promote responsible and reproducible
research. An overarching goal herein is to enhance the rigor and
reproducibility of small animal dMRI acquisitions and analyses, and thereby
advance biomedical knowledge.Comment: 69 pages, 6 figures, 1 tabl
Looking Below the Surface: The Role of Superficial White Matter in Alzheimer’s Disease
Superficial white matter (SWM) contains short fibres and has largely been overlooked in Alzheimer’s disease (AD). Standard MRI approaches face confounding
effects of complex fibre organisation and proximity to the cortex when measuring
SWM in vivo. This thesis investigates SWM in AD by applying advanced MRI
techniques to overcome these limitations.
First, SWM in a cohort of young-onset AD and healthy controls was investigated using an advanced diffusion MRI model. Findings showed that young-onset
AD participants have lower density, but increased dispersion, of SWM neurites
compared to healthy controls. The second project found that these advanced diffusion MRI metrics are associated with independent quantitative MRI metrics sensitive to microstructure, and indicate previous findings are linked to losses in SWM
myelin and iron. Project three explored the confounding effect of nearby CSF on
cortical diffusion MRI measures and showed that a tissue-weighting approach can
ameliorate CSF’s influence on regional averages. Project four investigated whether
SWM is affected in autosomal dominant AD. Results supported earlier findings of
lower density and higher dispersion of SWM neurites in symptomatic mutation carriers compared to non-carriers. Presymptomatic mutation carriers also had a lower
density of neurites in entorhinal SWM. The final project used high-resolution 7T
MRI to limit the influence of partial volume effects from nearby tissues on SWM
measures in typical and atypical AD participants and healthy controls. An interim
analysis qualitatively supports a loss of myelin and iron, with a potential increase
in dispersion, occurring in the SWM during AD.
In summary, SWM undergoes both degenerative and organisational changes
during AD that coincide with a loss of iron. This extends the literature by overcoming limitations of standard MRI techniques previously used to investigate SWM in
AD. SWM represents an overlooked region of brain changes in AD that may help
detect and characterise the disease
Development of novel magnetic resonance methods for advanced parametric mapping of the right ventricle
The detection of diffuse fibrosis is of particular interest in congenital heart disease patients,
including repaired Tetralogy of Fallot (rTOF), as clinical outcome is linked to the accurate
identification of diffuse fibrosis.
In the Left Ventricular (LV) myocardium native T1 mapping and Diffusion Tensor Cardiac
Magnetic Resonance (DT-CMR) are promising approaches for detection of diffuse fibrosis. In
the Right Ventricle (RV) current techniques are limited due to the thinner, mobile and
complex shaped compact myocardium. This thesis describes technical development of RV
tissue characterisation methods.
An interleaved variable density spiral DT-CMR method was implemented on a clinical 3T
scanner allowing both ex and in vivo imaging. A range of artefact corrections were
implemented and tested (gradient timing delays, off-resonance and T2* corrections). The off-
resonance and T2* corrections were evaluated using computational simulation demonstrating
that for in vivo acquisitions, off-resonance correction is essential.
For the first-time high-resolution Stimulated Echo Acquisition Mode (STEAM) DT-CMR data
was acquired in both healthy and rTOF ex-vivo hearts using an interleaved spiral trajectory
and was shown to outperform single-shot EPI methods.
In vivo the first DT-CMR data was shown from the RV using both an EPI and an interleaved
spiral sequence. Both sequences provided were reproducible in healthy volunteers. Results
suggest that the RV conformation of cardiomyocytes differs from the known structure in the
LV.
A novel STEAM-SAturation-recovery Single-sHot Acquisition (SASHA) sequence allowed the
acquisition of native T1 data in the RV. The excellent blood and fat suppression provided by
STEAM is leveraged to eliminate partial fat and blood signal more effectively than Modified
Look-Locker Imaging (MOLLI) sequences. STEAM-SASHA T1 was validated in a phantom
showing more accurate results in the native myocardial T1 range than MOLLI. STEAM-SASHA
demonstrated good reproducibility in healthy volunteers and initial promising results in a
single rTOF patient.Open Acces
Deep Learning in Medical Image Analysis
The accelerating power of deep learning in diagnosing diseases will empower physicians and speed up decision making in clinical environments. Applications of modern medical instruments and digitalization of medical care have generated enormous amounts of medical images in recent years. In this big data arena, new deep learning methods and computational models for efficient data processing, analysis, and modeling of the generated data are crucially important for clinical applications and understanding the underlying biological process. This book presents and highlights novel algorithms, architectures, techniques, and applications of deep learning for medical image analysis
Investigating the biomechanics and biochemistry underlying MRI measures of neuronal function
This thesis investigates the biophysical mechanisms underlying functional magnetic resonance imaging (fMRI) measures of brain activity. Diffusion-weighted fMRI (DWfMRI) has been suggested as an alternative to the established Blood Oxygenation Level Dependent (BOLD) method. It is speculated to be sensitive to transient microstructural changes within active brain tissue, which could provide a more direct measure of neuronal activity than techniques relying on attendant haemodynamic changes. DWfMRI has yet to become widely accepted however, as the mechanism driving the observed signal is not well understood. Here, experimental and theoretical investigations of the fMRI signal are presented.
As part of this work, a functional MRI study was undertaken to compare BOLD and DWfMRI responses to stimulated brain activity in human volunteers. The effect of different experimental protocols were explored, with an emphasis on stimulus design. Analysis methods and their potential impact on interpretation of the response are explored.
Neuronal activation is accompanied by heamodynamic changes detectable with Optical Imaging Spectroscopy. Additionally, there is a growing base of evidence showing microstructural changes in excited neuronal tissue. This tortuosity change might be observable through the use of Spatial Frequency Domain Imaging (SFDI). These properties can be observed in the animal model and compared with fMRI to aid interpretation. The following work presents the development of in-vivo optical imaging techniques for the measurement of tissue optical property changes during brain activity. This includes theoretical explorations of the analysis pipeline, and of the potential limitations of these techniques and their sensitivity.
A Monte Carlo simulation of light transport through tissue was written to provide calibration data for the optical imaging methods. The simulation was used to explore the impact of tissue parameters on the optical results and inform interpretation. The simulation was extended to explore tissue absorption in the context of biophotomodulation
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