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Coronary angiography 2 Intracranial angiography Magnetic Resonance Imaging Physics and Instrumentation Orlando “Lon” Simonetti, PhD Cardiac function and tissue characterization Professor of Internal Medicine and Radiology Director of Cardiac MRI and Cardiac CT Research The Ohio State University Columbus, OH Orthopedic Imaging Myocardial Perfusion Neuro Imaging 3 4 MRI in a nutshell Day One: Origins of the MR Signal & Protons, spin, and magnetization & Precession and the Larmor Equation & Longitudinal and transverse magnetization & The MR signal & Longitudinal and transverse relaxation Cardiovascular MRI : physical principles to practical protocols / Vivian S. Lee ; Lippincott Williams & Wilkins, c2006 5 6 MRI in a nutshell MRI in a nutshell & Step 1 – Magnetization is created % Excess 1:2,000,000 protons align with1.5T B0 field % Alignment takes about 5xT1 to complete & Step 2 - Magnetization is perturbed % RF pulse applied through transmitter coil % The protons in a selected slice of tissue are “excited” by the RF pulse & Step 3 – Magnetization relaxes back to equilibrium state % After excitation, magnetization immediately begins relaxation back to equilibrium. % Signal decay dependent on T2. % Spatial localization by application of magnetic field gradients. % RF signal detection through receiver coil or coils. & Step 4 – Repetition of the process for every line of raw data % Raw data is acquired in “k-space” or Fourier space. & Step 5 – Fourier transform “reconstruction” of the image from raw data % Image contrast depends on tissue properties, as well as specifics of pulse sequence used to excite and acquire signals. Cardiovascular MRI : physical principles to practical protocols / Vivian S. Lee ; Lippincott Williams & Wilkins, c2006 1 7 8 Spin Basis of MRI & Proton has a unit positive charge, equal and opposite to the electron charge, and the nuclear “spin” produces a magnetic dipole. & NMR % = Nuclear Magnetic Resonance & Neutron is electrically uncharged, but subnuclear charge inhomogeneities result in a magnetic field of opposite direction and approximately equal strength to proton. % Atoms with odd total # of protons and neutrons have noninteger nuclear spin with an associated magnetic moment. & MRI & Magnetic moment describes the magnetic field properties of the nucleus. % = Magnetic Resonance Imaging % Based on the phenomenon of NMR % What is imaged: Hydrogen nucleus (single proton) & If total number of protons and neutrons is even, essentially no magnetic moment. % Body: 80% - 95% H2O (a lot of signal source) & If total number of protons and neutrons is odd, the non-integer nuclear spin generates a magnetic moment. & Composition of an H atom % 1 proton + 1 electron 9 10 Spin Spin & Spin: purely quantum mechanical characteristic of atomic particles. & Rotation of a sphere is only an analogy, but a useful one. & MRI: hydrogen atoms in water and hydrocarbons & Natural abundance of 1H makes it a practical signal source for MRI Magnetic field direction m + Nucleus + Electron Hydrogen atom 1 proton + 1 electron Hydrogen proton Sometimes referred to as “a spin” 12 Other MRI Relevant Nuclei N S Spinning proton behaves like a small bar magnet 2 13 Magnetic Moment 14 Spins and Net Magnetization & & & & A spin: one H atom 1cc of H2O: ~ 1019 H atoms or “spins” Aggregate of spins → net magnetization Net magnetization = vector sum of the effect of spins m M A spin An aggregate of spins gives net magnetization 15 16 Effect of External Magnetic Field on Spins Spin Precession & Spins precess like a top about external field Bo & Precession frequency (fL or ωL) is called Larmor frequency & No external magnetic field % Spins are randomly oriented % No net magnetization & With external magnetic field Bo % Small majority of spins align parallel to the field % Minority of spins align anti-parallel to the field % A small net balance parallel to the field Bo Bo = No magnetic field External magnetic field Net Magnetization 17 18 The Larmor Equation & The Larmor Equation states that the resonance frequency is proportional to the magnetic field: ω = γ • B0 ω: Larmor frequency γ: gyromagnetic ratio specific to type of nucleus γ = 42.577MHz/Tesla for Hydrogen B0: magnetic field strength 3 19 20 MRI in a nutshell Proton Precessional Frequency at Different Field Strengths Bo in Tesla 1.0 Precession frequency (MHz) 42.6 1.5 63.9 3.0 127.7 7.0 298 Cardiovascular MRI : physical principles to practical protocols / Vivian S. Lee ; Lippincott Williams & Wilkins, c2006 21 22 How Spin Dynamics Change in an oscillating radio frequency (RF) Field (B1) The Resonance Phenomenon & Spins reacts to magnetic fields oscillating at the precession frequency & The interaction: energy absorbed and precession changed z Bo z’ θ θ M y B1 M y’ B1 x Spin Trajectory Resultant Spin Motion 23 24 About the B1 (RF) Field The “Big” Picture & A time varying magnetic field & Frequency of oscillation = Larmor frequency & B1 strength and duration affects degree to which spins are tipped & Flip angle = angle that spins are tipped over & To generate B1: & Two magnetic fields needed to manipulate spins % Time varying electric current passed through a coil % Electric field converted to magnetic field % Bo aligns the spins % B1 controls how spins tip % B1 << Bo & Definition of terms: % How spins behave when B1 is turned off – relaxation % How spins behave when B1 is turned on – excitation % How to measure signals from spins – signal detection 4 25 26 z Some Notations Mz & Mz = Longitudinal magnetization & Mxy = Transverse magnetization M Mxy Mo Longitudinal magnetization (Mz) is in the z-direction, along the external magnetic field B0. Bo Mz Mz = Mo·cosα Mxy = Mo·sinα α Transverse magnetization (Mxy) is in the x-y plane, perpendicular to the external magnetic field. Mxy Spins must be in the x-y plane in order for us to receive MR signal. 27 28 z z H 90° 180° Flip Angle (α) is the total amount of deflection of the magnetization after the end of an applied RF pulse. A 90°-pulse flips the magnetization into the x-y plane. The stronger the energy of the applied RF pulse, the greater the flip angle. A 180°-pulse flips the magnetization into exactly the opposite direction. 29 30 The MR Signal Signal Detection • Only & & & & spins tipped into the transverse plane can induce a signal. • RF coils are used to receive the MR signal. z • The simplest example of an MR signal is the free induction decay (FID). B1 field on/off creates a change of Mxy Change of Mxy is time varying (at Larmor frequency) Change of Mxy detected by a coil Time varying Mxy → Electromagnetic induction → time varying electrical signal y RF coil S x S z N M N θ Detector coil y x B1 Free Induction Decay 5 31 Tissue Contrast in MRI Magnetic Relaxation •Proton Density & Bo is always on & When RF current to coil is turned off % B1 field is off •T1 % Mz goes back to Mo Each tissue has a unique set of T1, T2 and PD Mo Mz •T2 α α Mxy MT Mo Mz M0 % Mxy reduces to 0 Mxy 33 34 Longitudinal Magnetization Relaxation Time Constants T1 (Spin-Lattice) Relaxation Time - The characteristic time for the longitudinal spins to align themselves in the main magnetic field. The longitudinal magnetization vector will regain 63% of its maximum value in the time T1. T2 (Spin-Spin) Relaxation Time - The characteristic time for the loss of phase coherence among the spins oriented in the transverse direction. The transverse magnetization vector will loss 63% of its value in the time T2. T2* Relaxation Time - The characteristic time for the loss of phase coherence among the spins oriented in the transverse direction. This time constant accounts for such non-idealities as field inhomogeneities. The T2* time constant is always smaller than the T2 time constant (i.e. the signal decays faster). 35 Longitudinal Relaxation 36 Longitudinal Relaxation in Tissues & In body, T1 is between 100ms and 2s & T1 fat < T1 myocardium < T1 blood Mz 1.2 T1 relaxation for different tissues at 1.5T Mo 1.0 0.8 0.63Mo 0.6 blood fat myocardium 0.4 0.2 0.0 Mz = 1-exp(-T1/t) 0 500 1000 1500 2000 2500 3000 Time (ms) 6 37 38 Transverse Relaxation Transverse Relaxation: T2 and T2* & & & & & & Also called spin-spin relaxation Cause: mutual interaction among spins Local magnetic field of spins differ Spin precession frequencies differ from each other Spins disperse or “dephase” on the transverse plane Relaxation time constant = T2 Spins on transverse plane (at resonant freq) Net magnetization Time course 39 40 Transverse relaxation Transverse Relaxation: T2 & & & & T2 relaxation is due to spin - spin interaction. T2 decay is the destruction of transverse magnetization. T2 relaxation is faster than T1 relaxation. In body, T2 is between 20ms to 1.5s Mxy Mo M xy (t ) = M 0e− t / T2 0.63Mo 0.37Mo time T2 B1 field off 41 Field inhomogeneity: changes spins’ precession frequency More dispersion of spins over the transverse plane Faster decay of transverse magnetization Relaxation time constant = T2* T2* < T2 & Transverse relaxation due to spin dephasing & T2 irreversible dephasing & T2/ reversible dephasing & Combined effect M x(t)/M x(0) Transverse Relaxation: T2* & & & & & 42 Transverse Relaxation: T2* Mxy Mo T2 decay M xy (t ) = M 0e − t / T2 T2* decay time * 0.8 0.6 0.4 0.2 1 1 1 = + T2* T2 T2/ M ⊥ (t ) = M ⊥ (0)e 1.0 0.0 0 1 2 * t/T2 3 4 5 −t / T2* B1 field off 7 43 44 The MR Signal T1 and T2 in tissues • Only spins tipped into the transverse plane can induce a signal. • RF coils are used to receive the MR signal. z • The simplest example of an MR signal is the free induction decay (FID). y RF coil x S N Tissue T1 T2 T2/T1 Blood 1250 msec 250msec (arterial) 1/5 Myocardium 900 msec 50 msec 1/18 Fat 180 msec 90 msec 1/2 Muscle 900 msec 40 msec 1/22 Free Induction Decay 46 Contrast dependent on T1 and T2 The “Big” Picture & & & & Bo field aligns spins B1 field tips spins to transverse plane Signal from spins detected by a coil that picks up Mxy Spins return to original state by: % T1 relaxation – magnetization relaxes back to longitudinal equilibrium % T2 & T2* relaxation – coherent spins dephased 47 Day Two: Echoes and Image Encoding & Spin echoes & Echo Time (TE) and Repetition Time (TR) & Image contrast & Gradients & Slice selection & In-plane image encoding 8