University of Oulu

NMR imaging of flow : mapping velocities inside microfluidic devices and sequence development

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Author: Ahola, Susanna1
Organizations: 1University of Oulu, Faculty of Science, Department of Physics
Format: ebook
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 4.5 MB)
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Language: English
Published: 2011
Publish Date: 2011-12-12
Thesis type: Doctoral Dissertation
Defence Note: Academic dissertation to be presented with the assent of the Faculty of Science of the University of Oulu for public defence in OP-sali (Auditorium L10), Linnanmaa, on 9 December 2011, at 12 noon
Reviewer: Professor Josef Granwehr
Doctor Melanie Britton
Opponent: Professor Doctor Josef Granwehr
Professor Doctor Josef Granwehr
Kustos: Professor Jukka Jokisaari


The subject of this thesis is flow imaging by methods based on the nuclear magnetic resonance (NMR) phenomenon. The thesis consists of three related topics: In the first one the feasibility of measuring velocity maps and distributions inside a microfluidic device by pulsed field gradient (PFG) NMR has been demonstrated. The second topic was to investigate microfluidic gas flow using a combination of a special detection technique and a powerful signal enhancement method. The third topic is related to the unambiguous determination of velocities under challenging experimental conditions and introduces a new, improved velocity imaging sequence.

In the first part, well established imaging methods have been used to study water flow inside a micromixer. A surface coil matching the region of interest of the mixer was home built and used in the measurements in order to gain a better signal-to-noise ratio. Velocities inside the mixer have been measured by phase-encoding velocity, with unprecedented spatial resolution. Two dimensional NMR imaging and velocity maps revealed clogging and different manufacturing qualities of the mixers. In addition to the velocity maps, which display an average velocity for spins within one pixel, complete velocity distributions (so called average propagators) were measured. It was found that in the absence of spatial resolution in the third dimension, the propagator data can provide valuable insight to the flow system by revealing overlapping flow passages.

The next topic was gas flow inside a microfluidic device. It was investigated by time-of-flight flow imaging. The measurement of the weak gas signal was enabled by the use of two signal enhancement techniques: remote detection NMR and parahydrogen induced polarization (PHIP). The results demonstrate that a very significant signal enhancement can be achieved by this technique. In the future it may enable the investigation of interesting chemical reactions inside microreactors.

The third and last topic of the thesis deals with measuring flow by the so called multiecho sequences. When multiecho sequences are used in combination with phase encoding velocity, an error may be introduced: the multiecho sequence may produce a cumulative error to the phase of the magnetization, if it is sensitive to RF pulse imperfections. The problem has been elaborately explained and various solutions discussed, among the newly proposed one. Experimental results demonstrate the performance of the new velocity imaging sequence and show that the new sequence enables the unambiguous determination of velocities even in challenging experimental conditions resulting from inhomogeneous radio frequency fields of the measurement coils.

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Series: Report series in physical sciences
ISSN: 1239-4327
ISSN-L: 1239-4327
ISBN: 978-951-42-9604-8
ISBN Print: 978-951-42-9603-1
Issue: 69
Copyright information: © University of Oulu, 2011. This publication is copyrighted. You may download, display and print it for your own personal use. Commercial use is prohibited.