MRI magnetic resonance imaging1

Magnetic Resonance Imaging (MRI) Naomi Kim Outline of Presentation • • • • • • • Background & Brief Description Sample MRI Images Understanding Technology (Procession, Larmor Frequency, Radio Frequency, Gradient Magnets) Advantages & Disadvantages Distortions Figures of Merit Current Research & Future Works What is MRI? • Imaging technique used primarily in medical settings to produce high quality images of the inside of the human body • application of nuclear magnetic resonance (NMR) • Produce anatomical images (organs and soft tissues ) • physical-chemical state of tissues • flow diffusion (blood vessel) • motion information (brain) How Does MRI look like? • a giant cube • horizontal tube running through the magnet from front to back “bore of the magnet.” • The patient, lying on his or her back, slides into the bore on a special table MR Image Sagittal section through a normal human face Sagittal section through a normal human knee MR Image Sagittal section through a normal human What are its components? MRI system consists of: • Radio Frequency (RF) Transmitter • Main Magnet 0.5 to 2.0-tesla or 5,000 to 20,000 gauss (Resistive, Permanent, or Superconducting) • RF coils • Gradient Magnets 18 to 27-millitesla or 180 to 270 gauss • RF amplifier • Fourier Transformer • Computer How Does it Work? – Brief Outline Orient all the nuclear spins in a patient’s body in parallel with a strong magnetic field (Main Magnet) Locate the point to be examined (Gradient Magnets) and flip the spins of the hydrogen atom in the point in the other direction with a strong pulse of radio frequency of exactly the right frequency Collect the electromagnetic signal when the spins relax to their original state and transform the signal to produce image Spin (Precession) • Hydrogen Atom – Magnetic Dipole Moment (MDM) • High water content of non-bony tissues • A symmetric body with spin angular momentum and some torque that is perpendicular to the angular momentum precesses Spin • All of the hydrogen protons will align with the magnetic field in one direction or the other • Vast majority of these protons will cancel each other out • The excess nuclei in the lower energy state give a net MDM component along the field Larmor (Resonance) Frequency • Frequency of procession • Depends on Magnetic field and Gyromagnetic ratio – ratio of the MDM to the nuclear spin angular momentum  L = H/2 where L = Larmor frequency,  = gyromagnetic ratio, and H = applied magnetic field • Unique value for each type of nucleus - each type of nucleus will precess at a unique frequency in a given magnetic field -> we can distinguish between nuclear types! • exactly equal to the frequency of radiation absorbed in a transition from one spin state to another Radio Frequency Magnetic Field H1 • We want to displace M (tiny magnet: Hydrogen atom) from its direction along H and watch M as it tries to go back to its alignment along H • Apply a second magnetic field H to displace M • Little dipole magnets realigning themselves and beginning to precess about the net magnetic field (vector sum of H and H) • H1 turned on and off quickly (90º pulse) -> get a small displacement of M • M precessing about H and gradually realigning itself along H Free Induction Decay (FID) & Fourier Transformation • FID: signal produced by the free return of M to H direction • MR signal (the FID) -> amplitude vs time • FT of the FID -> signal strength vs frequency BUT How we select a “slice”? • Selection: apply gradient magnets so that protons in one slice precess at a unique Larmor frequency, different from all other protons in the imaging field • Apply a range of frequencies to the RF coil to observe slightly different Larmor frequencies in selected slice NOTE: A proton precessing at a certain Larmor frequency will respond to an RF pulse only if the RF pulse frequency is exactly the same as the Larmor frequency Local environmental magnetic fields cause the dipoles to precess at slightly different frequencies (L = H/2) Basis of NMR - identical nuclei in slightly different magnetic fields having different Larmor frequency Advantages • Non-Invasive: MRI does not depend on potentially harmful ionizing radiation, as standard x-ray and CT scans • MRI scans are not obstructed by bone, gas, or body waste, which can hinder other imaging techniques • Can see through bone (the skull) and deliver high quality pictures of the brain's delicate soft tissue structures • Images of organs and soft tissues Drawbacks • • • • Pacemakers Claustrophobic Tremendous amount of noise during a scan MRI scans require patients to hold very still for extended periods of time MRI exams can range in length from 20 minutes to 90 minutes or more • Orthopedic hardware (screws, plates, artificial joints) in the area of a scan can cause severe artifacts • High cost Distortions • • • • • • • • Poor magnetic field homogeneity Imperfect gradient coil design Radio frequency coil inhomogeneity Pulsatile flow artifact – blood flowing perpendicular to the slice direction Respiratory motion Cardiac motion – need motion correction Random bulk motion Chemical shift effects Current Research & Future Works • Still in its infancy - in widespread use for less than 20 years (compared with over 100 years for X-rays) • Very small scanners for imaging specific body parts are being developed • Functional brain mapping • Imaging ventilation dynamics of the lungs through the use of hyperpolarized helium-3 gas • development of new, improved ways to image strokes in their earliest stages is ongoing • MRA (Angiography) was developed to study blood flow Figures of Merit • Producing and holding H constant – most difficult job • H-field value should be as large as possible because the signal-to-noise ratio (S:N) of the output information depends on H • Repetitive pulsing improves the S:N ratio • Detection Limit? – How large a tumor has to be to be detected by MRI? • A point (horizontal points build up slice) is a cube that is about half a millimeter on each side Reference • • • • • • • • • • • • • Bankman, Isaac N Handbook of Medical Imaging: Processing and Analysis The United States of America: Academic Press, 2000 Curry, Thomas S III et al Christensen’s Physcis of Diagnostic Radiology The United States of America: Lea & Febiger, 1990 Mattson, James and Simon Merrill The Pioneerrs of NMR and Magnetic Resonance in Medicine: The Story of MRI The United States of America: Bar-Ilan University Press, 1996 Rubinson, Kenneth A and Rubinson, Judith F Contemporary Instrumental Analysis The United States of America: Prentice-Hall, Inc., 2000 Gould, Todd A