Application of Phase Imaging at High Field MR Thermometry at 7 Tesla
Total Page：16
File Type：pdf, Size：1020Kb
Application of Phase Imaging at High Field MR Thermometry at 7 Tesla Von der Fakult¨atf¨urPhysik und Geowissenschaften der Universit¨atLeipzig genehmigte DISSERTATION zur Erlangung des akademischen Grades doctor rerum naturalium Dr. rer. nat. vorgelegt von Diplom-Physiker Markus Nikola Oliver Streicher geboren am 25. Februar 1980 in Ravensburg Gutachter: Prof. Dr. Robert Turner Prof. Dr. Andrew G. Webb Tag der Verleihung 26. Februar 2018 Universit¨atLeipzig Fakult¨atf¨urPhysik und Geowissenschaften Max-Planck-Institut f¨ur Kognitions- und Neurowissenschaften To my parents for all their love and support 7 Bibliographische Beschreibung: Streicher, Markus Application of Phase Imaging at High Field - MR Thermometry at 7 Tesla Universit¨atLeipzig, Dissertation 152 Seiten, 260 Literaturangaben, 58 Abbildungen, 9 Tabellen Referat: The main purpose of this research was to develop improved methods for RF coil characterisation, and for non-invasive spatio-temporal mapping of temperature in the living body, in order to utilise the full potential of magnetic resonance imaging (MRI) at high magnetic fields by ensuring radiofrequency (RF) safety. Current RF power limits are often overly conservative, unnecessarily limiting the full potential of MRI, especially at high field. Thus it is useful to monitor tissue temperature while running MR imaging sequences which may deposit high RF power. Proton resonance frequency (PRF) MR thermometry can employ the phase of the complex MR signal to estimate temperature change over time. However, the shift of the water PRF with temperature is relatively small, making phase-based MR thermometry inherently sensitive to any extraneously caused changes of local frequency or MR phase. A potential source of error to PRF MR thermometry is a change in surround air susceptibility. The considerable impact of air susceptibility changes on PRF MR thermometry was demonstrated and quantified in experiments and magnetic field simulations. One way of correcting MR thermometry is to use a chemically shifted reference substance, in combination with a phase-sensitive chemical shift-selective MR thermometry sequence. The requirement of having a reliable separation of substances based on their resonance frequency was met by a novel frequency-selective phase-sensitive spin-echo (SE) MR thermometry sequence. This sequence was thoroughly tested in phantom and in-vivo experiments as well as in extensive Bloch simulations. The sequence limitations and advantages are discussed in detail. This technique acquires unsaturated water and fat images in rapid succession at the same position in space. The acquisition of a water and fat slice in less than 100 ms allows the correction of rapid field fluctuations in the brain caused by breathing and heartbeat, while still ensuring the correction of long term drift. With no assumptions required regarding temperature distribution in the tissue, this novel MR thermometry technique can measure brain temperature within a single (1:5 mm)3 voxel with a very low standard deviation (SD) of 0.3 K. Using an MRI phantom with a dimethyl sulfoxide reference, heating experiments achieved a MR temperature measurement with an SD of approximately 0.1 K in a single (1:5 mm)3 voxel. In conclusion, the work presented in this thesis assists the development of a real-time in-vivo temperature monitoring system that guarantees patient RF safety at high field. Contents Abstract . .7 Table of Contents . .9 List of Figures . 13 List of Tables . 15 Glossary . 17 1 Introduction 19 2 Background 23 2.1 Nuclear Magnetic Resonance . 23 2.1.1 Nuclear Spins in a Magnetic Field . 23 2.1.2 Macroscopic Magnetisation in a Magnetic Field . 25 2.1.3 Magnetisation in an RF Field . 25 2.1.4 Bloch Equations and Relaxation . 29 2.1.5 Chemical Shift . 31 Hydrogen Bonding . 32 Temperature-dependent Chemical Shift and Hydrogen Bonding 34 2.2 Magnetic Resonance Imaging . 35 2.2.1 Magnetic Field Gradients . 35 2.2.2 Imaging Equation . 36 2.2.3 MRI Sequences . 38 Spin Echo . 38 Gradient-Recalled Echo . 41 Echo Planar Imaging . 42 Discrete and Finite Sampling . 43 2.3 MR Thermometry . 44 2.3.1 Diffusion . 45 2.3.2 T1 Relaxation . 46 2.3.3 T2 Relaxation . 46 2.3.4 Proton Resonance Frequency change . 47 PRF MR Phase Thermometry . 48 Spectroscopic Imaging of the PRF shift . 50 Effects of pH on PRF MR thermometry . 51 2.4 Fat Suppression Techniques . 52 9 Contents 10 2.4.1 Water/Fat Imaging Methods . 52 2.4.2 Spin-Echo Frequency-selective Sequences . 53 2.5 Electric Properties of Matter . 55 2.5.1 Dielectric Relaxation . 56 2.5.2 Direct Current (dc) Conductivity . 59 2.5.3 Dielectric Properties of Body Tissue . 60 2.5.4 Phantom Dielectric Properties . 61 2.5.5 Wave Propagation in Dielectrics . 63 2.6 RF Heating and SAR . 64 2.7 Bloch Simulation . 67 + 3 Measuring the transmit RF field - B1 Mapping 70 + 3.1 Double Angle B1 Mapping . 71 3.2 Actual Flip-Angle Imaging . 73 3.3 Pre-Saturation Method . 76 3.4 Phase-Sensitive Method of FA Mapping . 79 + 3.5 B1 Mapping by Bloch-Siegert Shift . 82 4 Influence of Air Susceptibility on PRF MR Thermometry 86 4.1 Experimental Set-ups . 87 4.1.1 Hot Air Experiment . 87 Air Heating Set-up . 87 Imaging and Evaluation . 87 4.1.2 Gas Composition Experiment . 88 Gas Composition Set-up . 88 4.2 Magnetic Field Simulation . 89 4.3 Results . 90 4.3.1 Hot Air Experimental Results . 90 4.3.2 Gas Composition Results . 92 4.4 Discussion . 94 4.5 Conclusion . 96 5 Spin-echo MR Thermometry 98 5.1 Methods & Materials . 98 5.1.1 The Phase-sensitive Frequency-selective SE Sequence . 99 5.1.2 DMSO as a Reference Substance . 102 5.1.3 Temperature change calculation . 104 5.1.4 Bloch Simulation . 105 5.2 Experiments . 106 5.2.1 RF-heating Coil Experiment . 106 Set-up . 106 Results . 108 5.2.2 RF-heating without Heating Coil Experiment . 109 Contents 11 Set-up . 109 Results . 110 5.2.3 In-vivo MR Thermometry . 112 Set-up . 112 Results . 112 5.2.4 Bloch Simulation Results . 115 5.3 Discussion . 118 5.3.1 Spin-Echo Phase . 120 5.3.2 Referenced Asymmetric SE MR Thermometry . 121 5.3.3 Further Points to Discuss . 122 Fat Spectrum . 122 N/2 Ghosting Reference Line Acquisition . 123 6 Summary & Conclusion 125 Bibliography 128 Appendix 149 Acknowledgement . 149 Articles during this PhD project . 150 Conference talks and invited talks during this PhD project . 151 Conference posters during this PhD project . 151 List of Figures 2.1 Spin distribution in a magnetic field . 24 2.2 Spin distribution in an RF field . 27 2.3 Hydrogen bond model in water . 33 2.4 Chemical shift of water as a function of temperature . 34 2.5 Spin echo sequence . 39 2.6 Spin echo isochromats magnitude and phase . 40 2.7 Echo planar imaging sequence . 43 2.8 Frequency selective spin-echo sequence . 54 2.9 Dielectric spectrum of water . 58 2.10 Cole-Cole plot of water . 58 2.11 Dielectric spectrum of an aqueous NaCl solution . 59 2.12 Dielectric spectrum with conductivity of an aqueous NaCl solution . 59 2.13 Dielectric spectrum of human grey matter . 61 2.14 Conductivity of human GM and WM . 62 2.15 Dielectric spectrum of the water/DMSO/NaCl phantom . 63 2.16 Wave propagation at 7 T in GM . 65 2.17 Simulated on-resonant GR SE slice excitation time series . 69 3.1 Relative transverse signal of double angle method . 71 + 3.2 Monte Carlo simulation of double angle B1 method . 72 3.3 AFI sequence diagram . 73 + 3.4 Relative signal amplitude of AFI B1 mapping method . 74 + 3.5 Monte Carlo simulation of AFI B1 mapping sequence . 75 + 3.6 Sequence diagram of the pre-saturation pulse B1 mapping method . 77 3.7 Relative transverse signal of pre-saturation method . 78 + 3.8 Monte Carlo simulation of the pre-saturation B1 method . 79 + 3.9 Phase-sensitive B1 sequence diagram (by Morrell) . 79 + 3.10 Complex signal of phase-sensitive B1 mapping sequence . 80 + 3.11 Phase difference of the phase-sensitive B1 mapping sequence . 81 + 3.12 Monte Carlo simulation of phase-sensitive B1 mapping sequence . 82 + 3.13 Bloch-Siegert shift B1 mapping sequence . 82 + 3.14 Complex signal of the Bloch-Siegert B1 mapping sequence . 84 + 3.15 Phase difference of Bloch-Siegert B1 mapping sequence . 84 13 List of Figures 14 + 3.16 Monte Carlo simulation of Bloch-Siegert B1 mapping sequence . 85 4.1 Hot air set-up . 87 4.2 Hot air temperature maps . 90 4.3 Hot air temperature time course ..