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WAVELET NOISE REDUCTION AND VASCULAR WATER TRANSPORT MODELLING : APPLICATIONS TO DIFFUSION AND PERFUSION MRI

Bibic, Adnan LU (2018)
Abstract (Swedish)
Avbildning med hjälp av magnetresonans (MR) är en teknik som används i klinisk diagnostik för att upptäcka och karakterisera en rad sjukdomstillstånd. Metoden baseras på en kombination av statiskt magnetfält, magnetfältsgradienter och pulsade radiofrekventa elektromagnetiska fält i en magnetkamera för avbildning av form och struktur (morfologi) samt vissa funktioner hos olika organ i kroppen.
Diffusions- och perfusionsviktade MR-bilder kan tillhandahålla information utöver vad som återges i konventionella morfologiska MR-bilder. Diffusionsviktade bilder används för avbildning av vattenmolekylers termiska rörelse i vävnaden, och graden av rörelse ger information om underliggande vävnads- och cellstruktur. Tekniken kan därmed bidra till... (More)
Avbildning med hjälp av magnetresonans (MR) är en teknik som används i klinisk diagnostik för att upptäcka och karakterisera en rad sjukdomstillstånd. Metoden baseras på en kombination av statiskt magnetfält, magnetfältsgradienter och pulsade radiofrekventa elektromagnetiska fält i en magnetkamera för avbildning av form och struktur (morfologi) samt vissa funktioner hos olika organ i kroppen.
Diffusions- och perfusionsviktade MR-bilder kan tillhandahålla information utöver vad som återges i konventionella morfologiska MR-bilder. Diffusionsviktade bilder används för avbildning av vattenmolekylers termiska rörelse i vävnaden, och graden av rörelse ger information om underliggande vävnads- och cellstruktur. Tekniken kan därmed bidra till att avslöja en rad sjukdomar, t.ex. tumörer, vissa skelettsjukdomar samt sjukdomar i hjärnans blodkärl. Studier av kapillärt blodflöde (genomblödning, perfusion) är också av stor vikt för diagnostik av cerebrovaskulära sjukdomar, samt vid tumörsjukdomar där ökat blodflöde och förändrad mikrovaskulatur kan relatera till aggressivitetsgrad hos tumören. I detta avhandlingsarbete studeras en perfusionsmätningsteknik som kallas arteriell spinnmärkning (arterial spin labelling, ASL), som är en icke-invasiv MR-metodik för kvantifiering av cerebralt blodflöde. Fördelar med ASL-tekniken, liksom med diffusionsmätningarna, är bl.a. att varken joniserande strålning eller exogena kontrastmedel krävs för undersökningen. I samband med hjärninfarkter kan en kombination av de båda teknikerna bidra till att, i ett tidigt skede, upptäcka de områden i hjärnan som fortfarande kan räddas (den s.k. ischemiska penumbran) om rätt behandling sätts in i tid.
En av begränsningarna med båda de ovan nämnda teknikerna är systemets signal-till-brusförhållande (signal-to-noise ratio, SNR). Kvantifieringen av vissa diffusionsbaserade parametrar försvåras av det så kallade brusgolvet, som är ett systematiskt fel i MR bilderna i situationer där SNR är lågt. Kravet på god signal finns även i ASL-tekniken, som generellt lider av lågt signal-till-brusförhållande, och detta medför att ett stort antal medelvärderingar av signalinsamlingen ofta krävs, med relativt långa undersökningstider som följd.
I detta doktorsavhandlingsarbete presenteras en filtreringsmetod för att minska systemfelet (brusgolvet) i diffusionsviktade bilder samt för att minska antalet medelvärderingar för kvantitativa blodflödesmätningar med ASL-tekniken, inklusive analys av filtreringarnas effekter på bildkvaliteten.
Denna avhandling beskriver också en metod för mätning av vattnets passagetider i mikrovaskulaturen samt genom blod-hjärnbarriären. En anpassning av modellen till klinisk MR-utrusning presenteras, samt en ytterligare vidareutveckling av modellen som validerats i prekliniska experiment. Förhoppningen är att parametrar som beskriver vattnets transporttider, eventuellt i kombination med konventionella diffusions- och perfusionsparametrar, kan bidra till bättre förståelse för sjukdomar som påverkar blod-hjärnbarriären. Exempel på potentiella tillämpningar av intresse är Alzheimers sjukdom, inflammationer, tumörkarakterisering och stroke.
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Abstract
Magnetic resonance imaging (MRI) is a powerful medical imaging technique, used to detect and characterise a range of diseases and conditions. It is based on the use of a strong static magnetic field in combination with magnetic field gradients and pulsed radiofrequency electromagnetic fields to visualise various organs and structures in the body according to their morphology or function.
Diffusion and perfusion MRI are established methods for quantitative measurements, often used in neurological and neurovascular clinical applications. Although these techniques are often used separately to investigate a number of diseases, combined diffusion and perfusion information can provide unique information, e.g. for assessment of whether stroke... (More)
Magnetic resonance imaging (MRI) is a powerful medical imaging technique, used to detect and characterise a range of diseases and conditions. It is based on the use of a strong static magnetic field in combination with magnetic field gradients and pulsed radiofrequency electromagnetic fields to visualise various organs and structures in the body according to their morphology or function.
Diffusion and perfusion MRI are established methods for quantitative measurements, often used in neurological and neurovascular clinical applications. Although these techniques are often used separately to investigate a number of diseases, combined diffusion and perfusion information can provide unique information, e.g. for assessment of whether stroke patients in the acute stage are likely to benefit from reperfusion therapy. This may be accomplished by identification of the so-called ischemic penumbra (i.e. the area surrounding the core of an infarct, exhibiting disturbed microcirculation but still viable and salvageable if the local blood supply is efficiently restored). This identification concept is often referred to as the diffusion–perfusion mismatch. In oncological applications, a combination of diffusion and perfusion MRI is sometimes used in tumour characterisation and in attempts to monitor early treatment response.
Quantitative diffusion MRI may be hampered by a bias induced by the so-called rectified noise floor in areas with low signal-to-noise ratio (SNR), and to address this issue, a wavelet-based filtering method was presented and used for noise reduction in diffusion MRI.
Perfusion images acquired by arterial spin labelling (ASL), which is the technique investigated in the present work, suffer from inherently low SNR, and this is commonly addressed by averaging multiple repetitions, which leads to a prolonged acquisition time. As an alternative approach, wavelet-domain filtering for noise reduction was applied to ASL data, and the performance of the proposed filtering technique was investigated (in terms of accuracy, precision and structural degradation), and a comparison with conventional Gaussian smoothing was also included. Additionally, a quantitative non-compartment modelling approach for assessment of blood water transit time through the microvasculature and the blood–brain barrier (BBB) was investigated. In one study, the model was adapted to a clinical setup and applied to test–retest data from healthy volunteers, and the effects of noise on the model were examined by simulations. In an animal study, the model was further developed by introducing a bolus- tracking ASL solution that included a measured arterial input function (AIF) instead of a theoretical rectangular input function. Furthermore, it was explored whether effects of mildly damaged red blood cells on microvascular parameters were detectable using the proposed modelling approach and by ASL-based CBF quantification.
The extracted water transit time parameters can be used separately or in combination with conventional perfusion and diffusion estimates. Changes in the blood water transit time in the microvasculature may be related to alterations in capillary water permeability, and may thus be useful in the assessment of BBB integrity. Disturbed BBB permeability has been attributed to a number of disease states, and may be relevant to, for example, early diagnosis of Alzheimer's disease, inflammation, tumour grading and ischaemic stroke. (Less)
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author
supervisor
opponent
  • Professor Eklund, Anders, Department of Radiation Sciences, Umeå University
organization
publishing date
type
Thesis
publication status
published
subject
keywords
cerebral blood flow, perfusion, diffusion, arterial spin labelling, wavelets, filtering, denoising, blood-brain barrier, permeability
edition
1
pages
92 pages
publisher
Lunds universitet
defense location
Lecture Hall F3, Centralblocket, Skånes universitetssjukhus, Lund
defense date
2018-12-19 09:00
ISBN
978-91-7753-890-5
978-91-7753-891-2
language
English
LU publication?
yes
id
2e1f50ab-0316-45d7-94f9-bf5281c700e1
date added to LUP
2018-11-12 21:56:57
date last changed
2018-11-26 11:01:06
@phdthesis{2e1f50ab-0316-45d7-94f9-bf5281c700e1,
  abstract     = {Magnetic resonance imaging (MRI) is a powerful medical imaging technique, used to detect and characterise a range of diseases and conditions. It is based on the use of a strong static magnetic field in combination with magnetic field gradients and pulsed radiofrequency electromagnetic fields to visualise various organs and structures in the body according to their morphology or function.<br/>Diffusion and perfusion MRI are established methods for quantitative measurements, often used in neurological and neurovascular clinical applications. Although these techniques are often used separately to investigate a number of diseases, combined diffusion and perfusion information can provide unique information, e.g. for assessment of whether stroke patients in the acute stage are likely to benefit from reperfusion therapy. This may be accomplished by identification of the so-called ischemic penumbra (i.e. the area surrounding the core of an infarct, exhibiting disturbed microcirculation but still viable and salvageable if the local blood supply is efficiently restored). This identification concept is often referred to as the diffusion–perfusion mismatch. In oncological applications, a combination of diffusion and perfusion MRI is sometimes used in tumour characterisation and in attempts to monitor early treatment response.<br/>Quantitative diffusion MRI may be hampered by a bias induced by the so-called rectified noise floor in areas with low signal-to-noise ratio (SNR), and to address this issue, a wavelet-based filtering method was presented and used for noise reduction in diffusion MRI.<br/>Perfusion images acquired by arterial spin labelling (ASL), which is the technique investigated in the present work, suffer from inherently low SNR, and this is commonly addressed by averaging multiple repetitions, which leads to a prolonged acquisition time. As an alternative approach, wavelet-domain filtering for noise reduction was applied to ASL data, and the performance of the proposed filtering technique was investigated (in terms of accuracy, precision and structural degradation), and a comparison with conventional Gaussian smoothing was also included. Additionally, a quantitative non-compartment modelling approach for assessment of blood water transit time through the microvasculature and the blood–brain barrier (BBB) was investigated. In one study, the model was adapted to a clinical setup and applied to test–retest data from healthy volunteers, and the effects of noise on the model were examined by simulations. In an animal study, the model was further developed by introducing a bolus- tracking ASL solution that included a measured arterial input function (AIF) instead of a theoretical rectangular input function. Furthermore, it was explored whether effects of mildly damaged red blood cells on microvascular parameters were detectable using the proposed modelling approach and by ASL-based CBF quantification.<br/>The extracted water transit time parameters can be used separately or in combination with conventional perfusion and diffusion estimates. Changes in the blood water transit time in the microvasculature may be related to alterations in capillary water permeability, and may thus be useful in the assessment of BBB integrity. Disturbed BBB permeability has been attributed to a number of disease states, and may be relevant to, for example, early diagnosis of Alzheimer's disease, inflammation, tumour grading and ischaemic stroke.},
  author       = {Bibic, Adnan},
  isbn         = {978-91-7753-890-5},
  keyword      = {cerebral blood flow,perfusion,diffusion,arterial spin labelling,wavelets,filtering,denoising,blood-brain barrier,permeability},
  language     = {eng},
  pages        = {92},
  publisher    = {Lunds universitet},
  school       = {Lund University},
  title        = {WAVELET NOISE REDUCTION AND VASCULAR WATER TRANSPORT MODELLING : APPLICATIONS TO DIFFUSION AND PERFUSION MRI},
  year         = {2018},
}