Basic Physical Principles of MRI James Voyvodic, Ph.D Brain Imaging and Analysis Center Synopsis of MRI 1) Put subject in big magnetic field 2) Transmit radio waves into subject [2~10 ms] 3) Turn off radio wave transmitter 4) Receive radio waves re-transmitted by subject0 5) Convert measured RF data to image Many factors contribute to MR imaging • • • • • Quantum properties of nuclear spins Radio frequency (RF) excitation properties Tissue relaxation properties Magnetic field strength and gradients Timing of gradients, RF pulses, and signal detection What kinds of nuclei can be used for NMR? • Nucleus needs to have properties: – Spin – charge • Nuclei are made of protons and neutrons – Both have spin ½ – Protons have charge • Pairs of spins tend to cancel, so only atoms with an odd number of protons or neutrons have spin – Good MR nuclei are 1H, 13C, 19F, 23Na, 31P Hydrogen atoms are best for MRI • Biological tissues are predominantly 12C, 16O, 1H, and 14N • Hydrogen atom is the only major species that is MR sensitive • Hydrogen is the most abundant atom in the body • The majority of hydrogen is in water (H2O) • Essentially all MRI is hydrogen (proton) imaging Nuclear Magnetic Resonance Visible Nuclei A Single Proton There is electric charge on the surface of the proton, thus creating a small current loop and generating magnetic moment   + + + J The proton also has mass which generates an angular momentum J when it is spinning Thus proton “magnet” differs from the magnetic bar in that it also possesses angular momentum caused by spinning Magnetic Moment B B I L L F F = IBL Force W = IBLW = IBA Torque  max       sin Angular Momentum J = m=mvr J m v r The magnetic moment and angular momentum are vectors lying along the spin axis  =J   is the gyromagnetic ratio  is a constant for a given nucleus Net magnetization is the macroscopic measure of many spins Bo M Bo M c T Net magnetization • Small B0 produces small net magnetization M • Larger B0 produces larger net magnetization M, lined up with B0 • Thermal motions try to randomize alignment of proton magnets • At room temperature, the population ratio of antiparallel versus parallel protons is roughly 100,000 to 100,006 per Tesla of B0 Quantum vs Classical Physics One can consider the quantum mechanical properties of individual nuclei, but to consider the bulk properties of a whole object it is more useful to use classical physics to consider net magnetization effects To measure magnetization we must perturb it • We can only measure magnetization perpendicular to the B0 field • Need to apply energy to tip protons out of alignment • Amount of energy needed depends on nucleus and applied field strength (Larmor frequency) • The amount of energy added (duration of the RF pulse at the resonant frequency) determines how far the net magnetization will be tipped away from the B0 axis A Mechanical Analogy: A Swingset • Person sitting on swing at rest is “aligned” with externally imposed force field (gravity) • To get the person up high, you could simply supply enough force to overcome gravity and lift him (and the swing) up – Analogous to forcing M over by turning on a huge static B1 • The other way is to push back and forth with a tiny force, synchronously with the natural oscillations of the swing – Analogous to using a tiny RF B1 over a period of time to slowly flip M over g Precession  If M is not parallel to B, then it precesses clockwise around the direction of B “Normal” (fully relaxed) situation has M parallel to B, and therefore does not precess This is like a gyroscope Derivation of precession frequency  =  × Bo  = dJ / dt J = / d/dt =  ( × Bo)  (t) = (xocos Bot + yosin Bot) x + (yocos Bot - xosin Bot) y + zoz This says that the precession frequency is the SAME as the larmor frequency Recording the MR signal • Need a receive coil tuned to the same RF frequency as the exciter coil • Measure “free induction decay” of net magnetization • Signal oscillates at resonance frequency as net magnetization vector precesses in space • Signal amplitude decays as net magnetization gradually realigns with the magnetic field • Signal also decays as precessing spins lose coherence, thus reducing net magnetization NMR signal decays in time • T1 relaxation – Flipped nuclei realign with the magnetic field • T2 relaxation – Flipped nuclei start off all spinning together, but quickly become incoherent (out of phase) • T2* relaxation – Disturbances in magnetic field (magnetic susceptibility) increase the rate of spin coherence T2 relaxation • The total NMR signal is a combination of the total number of nuclei (proton density), reduced by the T1, T2, and T2* relaxation components T2* decay • Spin coherence is also sensitive to the fact that the magnetic field is not completely uniform • Inhomogeneities in the field cause some protons to spin at slightly different frequencies so they lose coherence faster • Factors that change local magnetic field (susceptibility) can change T2* decay Different tissues have different relaxation times These relaxation time differences can be used to generate image contrast • T1 - Gray/White matter • T2 - Tissue/CSF • T2* - Susceptibility (functional MRI)