1 / 34

Magnetic Resonance for BME 458 Francisco (Paco) Martinez

Magnetic Resonance for BME 458 Francisco (Paco) Martinez. MR Principle. Magnetic resonance is based on the absorption and emission of energy in the radio frequency range of the electromagnetic spectrum. Historical Notes.

cameron-mckay
Download Presentation

Magnetic Resonance for BME 458 Francisco (Paco) Martinez

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Magnetic Resonancefor BME 458Francisco (Paco) Martinez Francisco M. Martinez

  2. MR Principle Magnetic resonance is based on the absorption and emission of energy in the radio frequency range of the electromagnetic spectrum. Francisco M. Martinez

  3. Historical Notes • Discovered independently by Felix Bloch (Stanford) and Edward Purcell (Harvard) • Initially used in chemistry and physics for studying molecular structure (spectrometry) and diffusion • In 1973 Paul Lauterbur obtained the 1st MR image using linear gradients • 1970’s MRI mainly in academia • 1980’s Industry joined forces Francisco M. Martinez

  4. MRI Timeline • 1946 MR phenomenon - Bloch & Purcell • 1950 Spin echo signal discovered - Erwin Hahn • 1952 Nobel Prize - Bloch & Purcell • 1950 - 1970 NMR developed as analytical tool • 1972 Computerized Tomography • 1973 Backprojection MRI - Lauterbur • 1975 Fourier Imaging - Ernst (phase and frequency encoding) • 1977 MRI of the whole body - Raymond Damadian Echo-planar imaging (EPI) technique - Peter Mansfield • 1980 MRI demonstrated - Edelstein • 1986 Gradient Echo Imaging NMR Microscope • 1988 Angiography - Dumoulin • 1989 Echo-Planar Imaging (images at video rates = 30 ms / image) • 1991 Nobel Prize - Ernst • 1993 Functional MRI (fMRI) • 1994 Hyperpolarized 129Xe Imaging • 2000? Interventional MRI Francisco M. Martinez

  5. MR Physics • Based on the quantum mechanical properties of nuclear spins • Q. What is SPIN? • A. Spin is a fundamental property of nature like electrical charge or mass. Spin comes in multiples of 1/2 and can be + or -. Protons, electrons, and neutrons possess spin. Individual unpaired electrons, protons, and neutrons each possesses a spin of 1/2 Francisco M. Martinez

  6. Properties of Spin Nuclei with: • Odd number of Protons • Odd number of Neutrons • Odd number of both exhibit a MAGNETIC MOMENT (e.g. 1H, 2H, 3He, 31P, 23Na, 17O, 13C, 19F ) Francisco M. Martinez

  7. Properties of Spin • Two or more particles with spins having opposite signs can pair up to eliminate the observable manifestations of spin. (e.g. 4He, 16O, 12C) • In nuclear magnetic resonance, it is unpaired nuclear spins that are of importance. Francisco M. Martinez

  8. Spins and Magnetic Fields • When placed in a magnetic field of strength B, a particle with a net spin can absorb a photon, of frequency . The frequency depends on the gyromagnetic ratio , of the particle Larmor relationship  =  B  = Resonant Frequency (rad/s)  = Gyromagnetic ratio B = magnitude of applied magnetic field Francisco M. Martinez

  9.  / (2) Nucleus MHz / T • 1H - 42.575 • 13C - 10.705 • 19F-40.054 • 23Na - 11.262 • 31P - 17.235 Francisco M. Martinez

  10. Biological abundances • Hydrogen (H) 63% • Sodium (Na) 0.041% • Phosphorus (P) 0.24% • Carbon (C) 9.4% • Oxygen (O) 26% • Calcium (Ca) 0.22% • Nitrogen (N) 1.5% Calculated from: M.A. Foster, Magnetic Resonance in Medicine and Biology Pergamon Press, New York, 1984. Francisco M. Martinez

  11. Spins and Magnetic Fields • The AVERAGE behavior of many spins (many magnetic moments) results in a NET MAGNETIZATION of a sample (substance/tissue) Bo Net magnetization (Up/Down  0.999993) Randomly oriented Oriented parallel or antiparallel Francisco M. Martinez

  12. Bloch Equation Says that the magnetization M will precess around a B field at frequency  =  B Vs. Francisco M. Martinez

  13. Nomenclature B0 = External magnetic field normally on the “z” direction Magnetization Longitudinal magnetization Transverse magnetization Magnetic Field M0 = Initial magnetization B0 = Magnitude of main magnetic field B1 = Magnitude of RF field Detected signal Francisco M. Martinez

  14. Solution to Bloch Eq. Jump to Matlab simulations that solve the Bloch Equation • Observe “Rotating Frame of Reference” Francisco M. Martinez

  15. Excitation Recall that the net magnetization (M) is aligned to the applied magnetic field (B0). Q. How can we rotate M so that it becomes perpendicular to B0? A. RF Excitation Rotating magnetic fields (B1) applied in the plane transverse to B0 Francisco M. Martinez

  16. Tip angle Tip angle = Francisco M. Martinez

  17. Resonance If RF = 0 Resonance Excitation is effective If RF  0 Excitation occurs but it is not optimal Matlab simulation Francisco M. Martinez

  18. Relaxation There are thermal processes that will tend to bring M back to its equilibrium state • T1 recovery = Spin-lattice relaxation • T2 relaxation = Spin-Spin relaxation Francisco M. Martinez

  19. T1 - relaxation • Longitudinal magnetization (Mz) returns to steady state (M0) with time constant T1 • Spin gives up energy into the surrounding molecular matrix as heat • Factors • Viscosity • Temperature • State (solid, liquid, gas) • Ionic content • Bo • Diffusion • etc. Francisco M. Martinez

  20. T2 - relaxation • Transverse magnetization (Mxy) decay towards 0 with time constant T2 • Factors • T1 (T2 T1) • Phase incoherence • Random field fluctuations • Magnetic susceptibility • Magnetic field inhomogeneities (RF, B0, Gradients) • Chemical shift • Etc. Matlab simulations of T1 and T2 Francisco M. Martinez

  21. Typical T1’s, T2’s, and Relative Density for brain tissue T1 (sec) T2 (sec) R Distilled Water 3 3 1 CSF 3 0.3 1 Gray matter 1.2 0.06 - 0.08 0.98 White matter 0.8 0.045 0.8 Fat 0.15 0.035 1 Francisco M. Martinez

  22. Bloch Eq. Revised Solution on the rotating frame of reference Francisco M. Martinez

  23. Pulse Sequences • 90° - 90° - 90° - … •  -  -  -  -  - … • 180° - 90° - 180° - 90° … (Inversion recovery) • 90° - 180° - 180° - 180° - 180° … • 90° - 180° - 90° - 180° … (Spin echo) Francisco M. Martinez

  24. Hardware • For the BME458 laboratory RF Amp. PERMANENT Pulse Programmer RF Synthesized Oscillator RF Transmitting coil Oscilloscope Sync. CH1 CH2 Sample Receiver Mixer RF Receiving coil MAGNET RF Amplifier Detector Francisco M. Martinez

  25. Receiver • High gain • Linear • Low noise • Centered at 15 MHz Francisco M. Martinez

  26. Pulse programmer • Pulse generator that creates the pulse sequences. • Pulses can be varied in: • Duration (1 – 30 s) • Spacing (10 s – 9.99 s) • Number of “B” pulses (0 – 99) • Repetition time (1 ms – 10 s) Francisco M. Martinez

  27. 15 MHz Osc/Amp/Mixer • Tunable oscillator • Display • Coarse/fine adjustment • Power amplifier • Amplifies pulses to produce 12 gauss (Max 150W) • Mixer • Multiplies CW-RF with received signal Francisco M. Martinez

  28. 15 MHz Osc/Amp/Mixer • Mixer • Multiplies CW-RF with received signal CW FID Mix Francisco M. Martinez

  29. Imaging • Requires magnetic fields as a function of position • Therefore frequency of oscillation is a function of position Francisco M. Martinez

  30. Gradients Francisco M. Martinez

  31. Gradients • Recall that: • Now: Francisco M. Martinez

  32. Hardware Francisco M. Martinez

  33. Pulse sequences • Spin echo • Gradient echo • EPI • Spiral • … 100’s Francisco M. Martinez

  34. References • http://www.cis.rit.edu/htbooks/mri/ • Principles of magnetic resonance imaging. Dwight G. Nishimura, 1996 Francisco M. Martinez

More Related