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![Main Magnet Field Bo
• Purpose is to align H protons in H2O (little
magnets)
[Little magnets lining up with external lines of force]
[Main magnet and some of its lines of force]](https://image.slidesharecdn.com/mri1-230209112805-90b8830f/75/MRI_1-ppt-10-2048.jpg)
MRI_1.ppt
- 1. BME 595 -Medical Imaging Applications Part 2: INTRODUCTION TO MRI Lecture 1 Fundamentals of Magnetic Resonance Feb. 16, 2005 James D. Christensen, Ph.D. IU School of Medicine Department of Radiology Research II building, E002C jadchris@iupui.edu 317-274-3815
- 2. References Books covering basicsof MR physics: E. Mark Haacke, et al 1999 Magnetic Resonance Imaging: Physical Principles and Sequence Design. C.P. Slichter 1978 (1992) Principles of Magnetic Resonance. A. Abragam 1961 (1994) Principles of Nuclear Magnetism.
- 3. References Online resources forintroductory review of MR physics: Robert Cox’s book chapters online http://afni.nimh.nih.gov/afni/edu/ See “Background Information on MRI” section Mark Cohen’s intro Basic MR Physics slides http://porkpie.loni.ucla.edu/BMD_HTML/SharedCode/MiscShared.html Douglas Noll’s Primer on MRI and Functional MRI http://www.bme.umich.edu/~dnoll/primer2.pdf Joseph Hornak’s Web Tutorial, The Basics of MRI http://www.cis.rit.edu/htbooks/mri/mri-main.htm
- 4. Timeline of MRImaging 1920 1930 1940 1950 1960 1970 1980 1990 2000 1924 - Pauli suggests that nuclear particles may have angular momentum (spin). 1937 – Rabi measures magnetic moment of nucleus. Coins “magnetic resonance”. 1946 – Purcell shows that matter absorbs energy at a resonant frequency. 1946 – Bloch demonstrates that nuclear precession can be measured in detector coils. 1972 – Damadian patents idea for large NMR scanner to detect malignant tissue. 1959 – Singer measures blood flow using NMR (in mice). 1973 – Lauterbur publishes method for generating images using NMR gradients. 1973 – Mansfield independently publishes gradient approach to MR. 1975 – Ernst develops 2D-Fourier transform for MR. NMR renamed MRI MRI scanners become clinically prevalent. 1990 – Ogawa and colleagues create functional images using endogenous, blood-oxygenation contrast. 1985 – Insurance reimbursements for MRI exams begin.
- 5. Nobel Prizes forMagnetic Resonance • 1944: Rabi Physics (Measured magnetic moment of nucleus) • 1952: Felix Bloch and Edward Mills Purcell Physics (Basic science of NMR phenomenon) • 1991: Richard Ernst Chemistry (High-resolution pulsed FT-NMR) • 2002: Kurt Wüthrich Chemistry (3D molecular structure in solution by NMR) • 2003: Paul Lauterbur & Peter Mansfield Physiology or Medicine (MRI technology)
- 6. Magnetic Resonance Techniques NuclearSpin Phenomenon: • NMR (Nuclear Magnetic Resonance) • MRI (Magnetic Resonance Imaging) • EPI (Echo-Planar Imaging) • fMRI (Functional MRI) • MRS (Magnetic Resonance Spectroscopy) • MRSI (MR Spectroscopic Imaging) Electron Spin Phenomenon (not covered in this course): • ESR (Electron Spin Resonance) or EPR (Electron Paramagnetic Resonance) • ELDOR (Electron-electron double resonance) • ENDOR (Electron-nuclear double resonance)
- 7. Equipment Magnet Gradient CoilRF Coil RF Coil 4T magnet gradient coil (inside) B0
- 8. Main Components ofa Scanner • Static Magnetic Field Coils • Gradient Magnetic Field Coils • Magnetic shim coils • Radiofrequency Coil • Subsystem control computer • Data transfer and storage computers • Physiological monitoring, stimulus display, and behavioral recording hardware
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- 10. Main Magnet FieldBo • Purpose is to align H protons in H2O (little magnets) [Little magnets lining up with external lines of force] [Main magnet and some of its lines of force]
- 11. Common nuclei withNMR properties •Criteria: Must have ODD number of protons or ODD number of neutrons. Reason? It is impossible to arrange these nuclei so that a zero net angular momentum is achieved. Thus, these nuclei will display a magnetic moment and angular momentum necessary for NMR. Examples: 1H, 13C, 19F, 23N, and 31P with gyromagnetic ratio of 42.58, 10.71, 40.08, 11.27 and 17.25 MHz/T. Since hydrogen protons are the most abundant in human body, we use 1H MRI most of the time.
- 12. Angular Momentum J =mw=mvr m v r J magnetic moment m = g J where g is the gyromagnetic ratio, and it is a constant for a given nucleus
- 13. A Single Proton + + + Thereis electric charge on the surface of the proton, thus creating a small current loop and generating magnetic moment m. The proton also has mass which generates an angular momentum J when it is spinning. J m Thus proton “magnet” differs from a magnetic bar in that it also possesses angular momentum caused by spinning.
- 14. Protons in aMagnetic Field Bo Parallel (low energy) Anti-Parallel (high energy) Spinning protons in a magnetic field will assume two states. If the temperature is 0o K, all spins will occupy the lower energy state.
- 15. Protons align withfield Outside magnetic field randomly oriented • spins tend to align parallel or anti-parallel to B0 • net magnetization (M) along B0 • spins precess with random phase • no net magnetization in transverse plane • only 0.0003% of protons/T align with field Inside magnetic field Mz Mxy = 0 longitudinal axis transverse plane Longitudinal magnetization Transverse magnetization M
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- 17. Larger B0produces larger net magnetization M, lined up with B0 Thermal motions try to randomize alignment of proton magnets At room temperature, the population ratio is roughly 100,000 to 100,006 per Tesla of B0 The Boltzman equation describes the population ratio of the two energy states: N-/N+ = e –E/kT
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- 19. Energy Difference BetweenStates D E h n D E = 2 mz Bo n g/2p Bo known as Larmor frequency g/2p = 42.57 MHz / Tesla for proton Knowing the energy difference allows us to use electromagnetic waves with appropriate energy level to irradiate the spin system so that some spins at lower energy level can absorb right amount of energy to “flip” to higher energy level.
- 20. Spin System BeforeIrradiation Bo Lower Energy Higher Energy Basic Quantum Mechanics Theory of MR
- 21. The Effect ofIrradiation to the Spin System Lower Higher Basic Quantum Mechanics Theory of MR
- 22. Spin System AfterIrradiation Basic Quantum Mechanics Theory of MR
- 23. Precession – QuantumMechanics Precession of the quantum expectation value of the magnetic moment operator in the presence of a constant external field applied along the Z axis. The uncertainty principle says that both energy and time (phase) or momentum (angular) and position (orientation) cannot be known with precision simultaneously.
- 24. Precession – Classical =m × Bo torque = dJ / dt J = m/g dm/dt = g (m × Bo) m(t) = (mxocos gBot + myosin gBot) x + (myocos gBot - mxosin gBot) y + mzoz
- 25. A Mechanical Analogyof Precession • A gyroscope in the Earth’s gravitational field is like magnetization in an externally applied magnetic field
- 26. Equation of Motion:Block equation T1 and T2 are time constants describing relaxation processes caused by interaction with the local environment
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- 28. RF Excitation Excite RadioFrequency (RF) field • transmission coil: apply magnetic field along B1 (perpendicular to B0) • oscillating field at Larmor frequency • frequencies in RF range • B1 is small: ~1/10,000 T • tips M to transverse plane – spirals down • analogy: childrens swingset • final angle between B0 and B1 is the flip angle B1 B0 Transverse magnetization
- 29. Signal Detection viaRF coil
- 30. Signal Detection Signal isdamped due to relaxation
- 31. Relaxation via magneticfield interactions with the local environment
- 32. Spin-Lattice (T1) relaxationvia molecular motion T1 Relaxation efficiency as function of freq is inversely related to the density of states Effect of temperature Effect of viscosity
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- 34. Spin-Spin (T2) Relaxationvia Dephasing
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- 38. T2 Relaxation Efffective T2relaxation rate: 1/T2’ = 1/T2 + 1/T2* Total = dynamic + static
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- 42. HOMEWORK Assignment #1 1)Why does 14N have a magnetic moment, even though its nucleus contains an even number of particles? 2) At 37 deg C in a 3.0 Tesla static magnetic field, what percentage of proton spins are aligned with the field? 3) Derive the spin-lattice (T1) time constant for the magnetization plotted below having boundary conditions: Mz=M0 at t=0 following a 180 degree pulse; M=0 at t=2.0 sec.