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 MRI uses a combination of Magnetic and Electromagnetic Fields • NMR measures magnetization of atomic nuclei in the presence of magnetic fields • Magnetization can be manipulated by manipulating the magnetic fields (this is how we get images) • Static magnetic fields don’t change (< 0.1 ppm / hr): The main field is static and (nearly) homogeneous • RF (radio frequency) fields are electromagnetic fields that oscillate at radio frequencies (tens of millions of times per second) • Gradient magnetic fields change gradually over space and can change quickly over time (thousands of times per second) Radio Frequency Fields • RF electromagnetic fields are used to manipulate the magnetization of specific types of atoms • This is because some atomic nuclei are sensitive to magnetic fields and their magnetic properties are tuned to particular RF frequencies • Externally applied RF waves can be transmitted into a subject to perturb those nuclei • Perturbed nuclei will generate RF signals at the same frequency – these can be detected coming out of the subject Electromagnetic Radiation Energy X-Ray, CT MRI 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 Why protons interact with a magnetic field? • Moving (spinning) charged particle generates its own little magnetic field • Spinning particles with mass have angular momentum 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 rate of spin coherence T2 relaxation • NMR signal is a combination of the total number of nuclei (proton density), minus the T1 relaxation and T2 relaxation components Different tissues have different relaxation times Relaxation times are important for generating image contrast • T1 - Gray/White matter • T2 - Tissue CSF • T2* - Susceptibility (functional MRI) MRI Scanner Things needed for a typical MRI scanner Strong magnetic field, usually from superconducting magnets RadioFrequency coils and sub-system Gradient coils and sub-system Shimming coils and sub-system Computer(s) that coordinate all subsystems MRI scanner components Using NMR signals for imaging • Need to prolong and amplify the decaying signal • Need to know the spatial location of the tissue generating the signal The decaying NMR signal can be recovered by realigning spins Spin Echo Imaging Gradient Echo Imaging Spatial location is identified by using spatially varying magnetic fields Proton resonance with uniform magnetic field Proton resonance with axial field gradient It is actually spatial frequency, not physical location, that is scanned • Gradients cause spins to spread out and realign at different times • Bands of tissue with uniform spacing will realign together • MRI scanning systematically samples the strength of the signal at different spatial frequencies Horizontal sampling (Kx) Vertical sampling (Ky) MRI scanner collects spatial frequency data (in k-space) Horizontal spatial frequency density Vertical spatial frequency density A 2-dimensional Fourier transform mathematically converts from spatial frequency to reconstructed MR images The versatility of MRI arises from the different types of tissue contrast that can be generated by manipulating parameters • TR – adjusting the time between acquisitions affects T1 relaxation • TE – adjusting the time between refocusing pulses affects T2 and T2* relaxations • Timing of gradients affects sampling in k-space • Additional gradient pulses before the RF pulse can enhance specific tissue properties • Chemical agents can further enhance image contrast ... some of its lines of force] [Little magnets lining up with external lines of force] Ref: www.simplyphysics.com Net Magnetization Bo M Bo M c T Net magnetization • Small B0 produces small net magnetization