Psy 8960, Fall ‘06 Introduction to MRI1 Introduction to MRI: NMR MRI - big picture –Neuroimaging alternatives –Goal: understanding neurall coding Electromagnetic spectrum and Radio Frequency –X-ray, gamma ray, RF NMR phenomena –History (NMR, imaging, BOLD) –Physics nuclei, molecular environment excitation and energy states, Zeeman diagram precession and resonance quantum vs. classical pictures of proton(s)

Psy 8960, Fall ‘06 Introduction to MRI2 Related readings Huettel, Chapter 1 –History, resonance phenomena described (pp ) –Definitions of contrast and resolution (pp ) –Example (of what I don’t like … pp. 12, 13) Buxton, pgs , Haacke, Ch. 1, 2 & 25

Psy 8960, Fall ‘06 Introduction to MRI3 Neuroimaging LocalizationTiming Human studies Interpretation Electrophysiology Optical imaging EEG electroencephalography MEG magnetoencephalography PET Positron emission tomography fMRI functional MRI

Psy 8960, Fall ‘06 Introduction to MRI4 Nuclei

Psy 8960, Fall ‘06 Introduction to MRI5 Periodic table

Psy 8960, Fall ‘06 Introduction to MRI6 Hydrogen spectrum: electron transitions 1 electron volt = 1.6 × J

Psy 8960, Fall ‘06 Introduction to MRI7 Magnets Dipole-dipole interactionsDipole in a static field B NSNS NSNS Lowest energy Highest energy NSNS Lowest energy NSNS Highest energy NSNS NSNS

Psy 8960, Fall ‘06 Introduction to MRI8 The Zeeman effect The dependence of electronic transition energies on the presence of a magnetic field reveals electron spin (orbital angular momentum)

Psy 8960, Fall ‘06 Introduction to MRI9 Stern-Gerlach experiment Discovery of magnetic moment on particles with spins Electron beam has (roughly) even mix of spin-up and spin-down electrons

Psy 8960, Fall ‘06 Introduction to MRI10 NMR - MRI - fMRI timeline 1922 Stern-Gerlach Electron spin 1936 Linus Pauling Deoxyhemoglobin electronic structure 1937 Isidor Rabi Nuclear magnetic resonance 1952 Nobel prize Felix Bloch, Edward Purcell NMR in solids 1973 Paul Lauterbur, Peter Mansfield NMR imaging 1993 Seiji Ogawa, et al. BOLD effect 1902 Pieter Zeeman Radiation in a magnetic field

Psy 8960, Fall ‘06 Introduction to MRI11 Nucleus in magnetic fieldNucleus in free space B E Single spin-1/2 particle in an external magnetic field All orientations possess the same potential energy Spin-up and spin-down are different energy levels; difference depends linearly on static magnetic field

Psy 8960, Fall ‘06 Introduction to MRI12 Resonant frequency, two ways Spins in static magnetic field precess, with  =  Bor =  B where , = precession frequency (radians, Hz) ,  = gyromagnetic ratio (in rad/T or Hz/T) B = static (external) magnetic field (Tesla) B E Transition from high to low energy state emits radiation with characteristic frequency: Proton gyromagnetic ratio:  = MHz/T  = 2  =267,000,000 rad/T

Psy 8960, Fall ‘06 Introduction to MRI13 Gyromagnetic ratio

Psy 8960, Fall ‘06 Introduction to MRI14 B M: net (bulk) magnetization M MM M || Many spin-1/2 particles in an external magnetic field Equilibrium: ~ 1 ppm excess in spin-up state creates a net magnetization Excitation affects phase and distribution between spin-up and spin-down, rotating bulk magnetization

Psy 8960, Fall ‘06 Introduction to MRI15 Information in proton NMR signal Resonant frequency depends on Static magnetic field Molecule Relaxation rate depends on physical environment Microscopic field perturbations –Tissue interfaces –Deoxygenated blood Molecular environment –Gray matter –White matter –CSF Relaxation Excitation

Psy 8960, Fall ‘06 Introduction to MRI16 Proton NMR spectrum: ethanol /grupper/KS-grp/microarray/slides/drablos/Structure_determination

Psy 8960, Fall ‘06 Introduction to MRI17 Water

Psy 8960, Fall ‘06 Introduction to MRI18 Magnetic Resonance Imaging An MR image is (usually) a map of water protons, with intensity determined by local physical environment Contrast and image quality are determined by –Pulse sequence –Field strength –Shim quality –Acquisition time