MRI 1

BME 595 - Medical Imaging Applications Part 2: INTRODUCTION TO MRI Lecture 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 References Books covering basics of 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 References Online resources for introductory 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 Timeline of MR Imaging 1972 – Damadian patents idea for large NMR scanner to detect malignant tissue 1924 - Pauli suggests that nuclear particles may have angular momentum (spin) 1920 1973 – Lauterbur publishes method for generating images using NMR gradients 1937 – Rabi measures magnetic moment of nucleus Coins “magnetic resonance” 1930 1940 1985 – Insurance reimbursements for MRI exams begin MRI scanners become clinically prevalent NMR renamed MRI 1950 1946 – Purcell shows that matter absorbs energy at a resonant frequency 1946 – Bloch demonstrates that nuclear precession can be measured in detector coils 1960 1959 – Singer measures blood flow using NMR (in mice) 1970 1980 1973 – Mansfield independently publishes gradient approach to MR 1975 – Ernst develops 2D-Fourier transform for MR 1990 2000 1990 – Ogawa and colleagues create functional images using endogenous, blood-oxygenation contrast Nobel Prizes for Magnetic 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) Magnetic Resonance Techniques Nuclear Spin 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) Equipment 4T magnet RF Coil B0 gradient coil (inside) Magnet Gradient Coil RF Coil Main Components of a 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 Shimmingrf rf gradient coil coil main magnet main magnet Transmit Receive Control Computer Main Magnet Field Bo • Purpose is to align H protons in H2O (little magnets) [Main magnet and some of its lines of force] [Little magnets lining up with external lines of force] RF Excitation Excite Radio Frequency (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 Transverse magnetization B0 B1 Signal Detection via RF coil Signal Detection Signal is damped due to relaxation Relaxation via magnetic field interactions with the local environment Spin-Lattice (T1) relaxation via molecular motion Effect of temperature Effect of viscosity T1 Relaxation efficiency as function of freq is inversely related to the density of states Spin-Lattice (T1) relaxation Spin-Spin (T2) Relaxation via Dephasing Relaxation Relaxation T2 Relaxation Efffective T2 relaxation rate: 1/T2’ = 1/T2 + 1/T2* Total = dynamic + static Spin-Echo Pulse Sequence Spin-Echo Pulse Sequence Multiple Spin-Echo 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