Introduction to MRI (Magnetic Resonance Imaging) Speaker ： Tsung-Hsueh Lee Advisor ： Prof Tzi-Dar Chiueh Date ： March 21, 2005 Outline • • • • • • • Physic phenomena Spatial Encoding Image Construction Fast scanning MRI Hardware Conclusion Reference • • • • • • • Physic phenomena Spatial Encoding Image Construction Fast scanning MRI Hardware Conclusion Reference Spinning • Spinning charged particle creates an electromagnetic field • Spin quantum number S # of energy states = 2S+1 magnetic moment spin M=0 Precession ω = γB0 • Larmor Equation • Why hydrogen nucleus – Large component of human body – Odd number of Z protons B0 J or (unpaired protons) |B0|ã ãt Y B0 field X Energy Level • Energy state is not always the same • For 1H B0=1.5T Energy State=2 Precession frequency= 42.58MHz/T*1.5T=64MHz Nuclei H H 31 P 23 Na 14 N 13 C 19 F Unpaired Protons 1 0 0 Unpaired Neutrons 1 1 1 Net Spin 1/2 1/2 3/2 1/2 1/2 (MHz/T) 42.58 6.54 17.25 11.27 3.08 10.71 40.08 RF Pulse Mo • If the pulse F equals Larmor F Resonance θ • Rf pulse causes a flip angle and also makes protons get in phase • Some protons will change energy state Z Ө Bo = γB1τ Y B1 RF MXY X 900 pulse and 1800 pulse • • • • 900 pulse lets MXY=M0 and Used to excite protons Partial flip 1800 inverts M0 and precession direction T1 Relaxation Time • After RF pulse – Spins go back to the lowest energy state – Spins get out of phase M z (t ) = M (1 − e −t T1 ) • T1 also called spin-lattice relaxation time • Spins give energy to the surrounding lattice T2* Relaxation Time • Due to – Interactions among individual spins – External magnetic field inhomogeneity M0 M(t) || t T2 10 GRE • • • • • Gradient Recalled Echo Why not decrease TR? Partial flip angle 1800 pulse can’t be used Another way to refocus Gx 36 37 EPI • Echo Planar Imaging • One shot and Multishot • Signal decays rapidly because T2* • FOV (Field of View) too big • Requirement is hard to achieve 38 • • • • • • • Physic phenomena Spatial Encoding Image Construction Fast scanning MRI Hardware Conclusion Reference 39 MRI System 40 Magnet and Gradient Coil B0 0.015 – 0.3 Tesla Resistive 0.5 – Tesla Superconducting 41 Conclusion • MRI is a very powerful and complicated system • There are already many advanced techniques 42 Reference • MRI The Basics, Ray H Hashermi, William G Bradley • Principles of Magnetic Resonance Imaging, Zhi-Pei Liang, Paul C Lauterbur • MRI Physics for Radiologist, Alfred L Horowitz • Fundamentals of MAGNETIC RESONANCE IMAGING, Donald W Chakeres, Petra Schmalbrock • “MRI made easy” program, Schering 43 B0 Z |B |•γ •t J or or M |B0|ã ãt B1 |B0|ã ãt Y X 44 Spin Echo RF 00 18 00 slice phase readout echo signal TE 45 46 RF pulse Gz A B Gy D GX E C Coherent detector Complex numbers I + jQ K Space Image Space ⇐ DFT ⇒ ‘Real numbers’ 47 Fourier transform af FID F time F -1 frequency 48 Gradient Echo pulse timing RF α0 slice phase readout echo signal TE 49 50 ... • T2 usually much faster than T1 Mz Loss of Mxy Independent Gain of Mz process Mxy RF OFF: Mxy Mz=0 Mxy=M Mz grows Mz grows Mz maximum Mxy Mxy Mxy=0 decreases decreases 12 Pulse Sequence • TR... protons B0 J or (unpaired protons) |B0|ã ãt Y B0 field X Energy Level • Energy state is not always the same • For 1H B0=1.5T Energy State=2 Precession frequency= 42.58MHz/T*1.5T=64MHz Nuclei H H 31... Conclusion • MRI is a very powerful and complicated system • There are already many advanced techniques 42 Reference • MRI The Basics, Ray H Hashermi, William G Bradley • Principles of Magnetic