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Diffusion-encoded MRI for assessment of structure and microcirculation : Aspects of q-space imaging and improved IVIM modelling

Scherman Rydhög, Anna LU (2017)
Abstract (Swedish)
Blodet bär med sig syre och näringsämnen som är vitala för vävnadens överlevnad, och därför är karakterisering av hjärnans blodförsörjning av stor betydelse. Blodflödet i kapillärerna kallas för perfusion. Vid MRI-baserad perfusionsmätning i hjärnan utnyttjas ofta signalförändringar orsakade av ett spårämne i blodet. Spårämnet kan antingen skapas internt genom märkning av det arteriella inflödet av blod eller tillföras utifrån via intravenös injektion av ett kontrastmedel. Mätning av signalförändringar när blod med kontrastmedel passerar vävnaden brukar benämnas dynamisk kontrastförstärkt bildtagning. Kontrastmedelsanvändning kan dock vara problematisk under vissa förhållanden, och forskning tyder på att konventionella MR-kontrastmedel kan... (More)
Blodet bär med sig syre och näringsämnen som är vitala för vävnadens överlevnad, och därför är karakterisering av hjärnans blodförsörjning av stor betydelse. Blodflödet i kapillärerna kallas för perfusion. Vid MRI-baserad perfusionsmätning i hjärnan utnyttjas ofta signalförändringar orsakade av ett spårämne i blodet. Spårämnet kan antingen skapas internt genom märkning av det arteriella inflödet av blod eller tillföras utifrån via intravenös injektion av ett kontrastmedel. Mätning av signalförändringar när blod med kontrastmedel passerar vävnaden brukar benämnas dynamisk kontrastförstärkt bildtagning. Kontrastmedelsanvändning kan dock vara problematisk under vissa förhållanden, och forskning tyder på att konventionella MR-kontrastmedel kan stanna kvar länge i vissa vävnader. Det aktuella avhandlingsarbetet fokuserar därför främst på utveckling och utvärdering av helt icke-invasiva metoder för bedömning av mikrocirkulation, samt även i viss mån kvantifiering av mikrostrukturer.
MRI-baserade diffusionsmätningar bygger på mätning av den signalattenuering som sker när vattenmolekyler rör sig slumpmässigt i vävnaden, i närvaro av en magnetfältsgradient, och metoden används framför allt för karakterisering av nervbanornas integritet samt vävnadens mikrostruktur. Vi har använt en diffusionsteknik, s.k. q-space-analys, för att mäta storleken på fibrer i ett biologiskt fantom med en klinisk MR-kamera, som ett steg mot att förstå metoden bättre i denna miljö, då metoden tidigare framför allt använts i NMR-spektrometrar där hårdvaran har högre prestanda.
Med s.k. "intravoxel incoherent motion (IVIM) imaging" mäter man effekter av både diffusion och perfusion i vävnaden, och man kan också separera dessa effekter. Detta är en skonsam och icke-invasiv metod som varken utnyttjar joniserande strålning eller kontrastmedel. I våra studier har IVIM-metoden utvärderats på MR-kameror med olika fältstyrkor, och modellen som används för att beräkna perfusionsrelaterade parametrar har utvecklats för att kunna anpassas till hjärnans olika vävnadstyper och vattenpopulationer på ett mer komplett sätt. I våra studier har vi dragit slutsatsen att IVIM är en teknik som kräver mycket optimering, både vid val av MR-maskinvara, bildtagningsparametrar, bildbehandling, modell och analysmetod. Trots detta kan metoden användas på olika sätt, och ge intressanta resultat. (Less)
Abstract
Diffusion and perfusion MRI are valuable methods for investigating the microstructure and viability of tissue. One pure diffusion study was included in this thesis, with the purpose of studying microstructure and carrying out size estimations with the q-space diffusion imaging method. This is a method that has predominantly been explored with NMR spectrometers. In our study, a biological phantom consisting of asparagus stems was investigated using a clinical MRI unit to gain further knowledge about the q-space methodology in a setting where gradient performance is limited. Even though the q-space method showed limited possibilities, retrieval of some structural information was shown to be feasible.
In the remaining three doctoral... (More)
Diffusion and perfusion MRI are valuable methods for investigating the microstructure and viability of tissue. One pure diffusion study was included in this thesis, with the purpose of studying microstructure and carrying out size estimations with the q-space diffusion imaging method. This is a method that has predominantly been explored with NMR spectrometers. In our study, a biological phantom consisting of asparagus stems was investigated using a clinical MRI unit to gain further knowledge about the q-space methodology in a setting where gradient performance is limited. Even though the q-space method showed limited possibilities, retrieval of some structural information was shown to be feasible.
In the remaining three doctoral thesis projects, the intravoxel incoherent motion (IVIM) imaging concept was explored, allowing for extraction of combined diffusion and perfusion information from a given dataset. IVIM imaging is a non-invasive technique for acquiring diffusivity as well as microvascular and perfusion-related parameters using a diffusion-weighted pulse sequence. The model used in this technique is often limited because no relaxation properties are incorporated, and also because the signal component from blood is contaminated by cerebrospinal fluid/free water.
One study was dedicated to exploring the IVIM parameters at different field strengths and the influence of relaxation and signal-to-noise ratio (SNR) was observed. Although the repeatability was generally better at higher magnetic field strength, it was shown that the relaxation properties and an unexpectedly low SNR at high field strengths resulted in erroneous blood volume estimates. The model commonly used for IVIM data analysis was then modified to compensate for relaxation effects, based on literature values. When this correction was performed, results from the lower field strengths showed lower discrepancy from expected values, while the results from higher field strength were still erroneous, likely due to physiological noise.
In a following project, relaxation times were actually measured during the data collection, and incorporated to compensate for relaxation and improve the fitting procedure. The model was also upgraded to a three-compartment model to better describe the underlying tissue by including the cerebrospinal fluid component. Compared to a non-relaxation-compensated model, the three-compartment model with relaxation-compensated data modified the obtained results and reduced the CSF contamination.
The last IVIM project also included a three-compartment model, but in this case the purpose of the third compartment was to improve the quantification of the fraction of free water. The free water fraction has been established as a source of clinically useful image contrast, pointing at pathologies that affect the extracellular space, for example, atrophy and neuroinflammation. With our model, the bias from microcirculation was reduced in the free water estimate. A model where extracellular and microvascular effects can be separated might enable new diagnostic possibilities. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Docent Starck, Göran, Department of Radiation Physics at Institute of Clinical Sciences, Sahlgrenska University Hospital, University of Gothenburg, Sweden.
organization
publishing date
type
Thesis
publication status
published
subject
pages
130 pages
publisher
Lund University, Faculty of Science, Department of Medical Radiation Physics
defense location
Demonstration Room 10, Department of Medical Imaging and Physiology, Main building, Skåne University Hospital, Lund
defense date
2017-12-15 09:00
ISBN
978-91-7753-481-5
978-91-7753-481-5
language
English
LU publication?
yes
id
b36047c9-bbd0-425b-8162-10bd4f84c0ff
date added to LUP
2017-11-21 11:42:32
date last changed
2017-11-29 15:55:29
@phdthesis{b36047c9-bbd0-425b-8162-10bd4f84c0ff,
  abstract     = {Diffusion and perfusion MRI are valuable methods for investigating the microstructure and viability of tissue. One pure diffusion study was included in this thesis, with the purpose of studying microstructure and carrying out size estimations with the q-space diffusion imaging method. This is a method that has predominantly been explored with NMR spectrometers. In our study, a biological phantom consisting of asparagus stems was investigated using a clinical MRI unit to gain further knowledge about the q-space methodology in a setting where gradient performance is limited. Even though the q-space method showed limited possibilities, retrieval of some structural information was shown to be feasible.<br/>In the remaining three doctoral thesis projects, the intravoxel incoherent motion (IVIM) imaging concept was explored, allowing for extraction of combined diffusion and perfusion information from a given dataset. IVIM imaging is a non-invasive technique for acquiring diffusivity as well as microvascular and perfusion-related parameters using a diffusion-weighted pulse sequence. The model used in this technique is often limited because no relaxation properties are incorporated, and also because the signal component from blood is contaminated by cerebrospinal fluid/free water.<br/>One study was dedicated to exploring the IVIM parameters at different field strengths and the influence of relaxation and signal-to-noise ratio (SNR) was observed. Although the repeatability was generally better at higher magnetic field strength, it was shown that the relaxation properties and an unexpectedly low SNR at high field strengths resulted in erroneous blood volume estimates. The model commonly used for IVIM data analysis was then modified to compensate for relaxation effects, based on literature values. When this correction was performed, results from the lower field strengths showed lower discrepancy from expected values, while the results from higher field strength were still erroneous, likely due to physiological noise.<br/>In a following project, relaxation times were actually measured during the data collection, and incorporated to compensate for relaxation and improve the fitting procedure. The model was also upgraded to a three-compartment model to better describe the underlying tissue by including the cerebrospinal fluid component. Compared to a non-relaxation-compensated model, the three-compartment model with relaxation-compensated data modified the obtained results and reduced the CSF contamination.<br/>The last IVIM project also included a three-compartment model, but in this case the purpose of the third compartment was to improve the quantification of the fraction of free water. The free water fraction has been established as a source of clinically useful image contrast, pointing at pathologies that affect the extracellular space, for example, atrophy and neuroinflammation. With our model, the bias from microcirculation was reduced in the free water estimate. A model where extracellular and microvascular effects can be separated might enable new diagnostic possibilities.},
  author       = {Scherman Rydhög, Anna},
  isbn         = {978-91-7753-481-5},
  language     = {eng},
  pages        = {130},
  publisher    = {Lund University, Faculty of Science, Department of Medical Radiation Physics},
  school       = {Lund University},
  title        = {Diffusion-encoded MRI for assessment of structure and microcirculation : Aspects of q-space imaging and improved IVIM modelling},
  year         = {2017},
}