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Principle of MRI Physics. Shafiee

Aug 21, 2014


Principle of MRI physics: Magnetic Resonance Imaging

  • 26-Jul-14 1 Basic Physical PrinciplesBasic Physical Principles ofof MRIMRI Present by M.Shafiee. MSc RSO at Urmia Nuclear Medicine Center [email protected] 2 Medical Imaging Modalities: HistoryMedical Imaging Modalities: History 1895: X-ray ~1950: Ultrasound ~1955: Radionuclide 1972: CT ~1980: MRI Nobel Prize
  • 26-Jul-14 2 History of NMRHistory of NMR NMR = nuclear magnetic resonance Felix Block and Edward Purcell 1946: atomic nuclei absorb and re-emit radio frequency energy 1952: Nobel prize in physics nuclear: properties of nuclei of atoms magnetic: magnetic field required resonance: interaction between magnetic field and radio frequency Bloch Purcell NMRNMR MRI: Why the name change?MRI: Why the name change? most likely explanation: nuclear has bad connotations less likely but more amusing explanation: subjects got nervous when fast-talking doctors suggested an NMR
  • 26-Jul-14 3 Strengths of MRI Images of softImages of soft--tissue structures of the body, such as the heart,tissue structures of the body, such as the heart, lungs, liver, are clearer and more detailedlungs, liver, are clearer and more detailed MRI can help evaluate theMRI can help evaluate the functionfunction as well as the structureas well as the structure Invaluable tool in early evaluation of tumorsInvaluable tool in early evaluation of tumors MRI contrast materials are less harmful than those used in XMRI contrast materials are less harmful than those used in X--rayray or CTor CT Fast, nonFast, non--invasive angiographyinvasive angiography Exposure to radiation is minimal (nonExposure to radiation is minimal (non--ionizing)ionizing) Compare the detail:Compare the detail: CT (left) vs. MRI TCT (left) vs. MRI T22 (right)(right)
  • 26-Jul-14 4 Risks and Weaknesses:Risks and Weaknesses:Risks and Weaknesses:Risks and Weaknesses: Metal implants may cause problems Problems with claustrophobia MRI is to be avoided during the first 12 weeks of pregnancy Bone is usually better imaged with X-rays MRI typically costs more than CT Necessary Equipment Magnet Gradient Coil RF Coil Source: Joe Gati, photos RF Coil 4T magnet gradient coil (inside)
  • 26-Jul-14 5 x 80,000 = 4 Tesla = 4 x 10,000 0.5 = 80,000X Earths magnetic field Robarts Research Institute 4T The Big Magnet VeryVery strong!strong! Continuously on 1 Tesla (T) = 10,000 Gauss Earths magnetic field = 0.5 Gauss Main field = B0 B0 MRI Clinical Safety Practice Pacemaker Aneurysm clip Metal in the eye Credit card Bone implant Denture X
  • 26-Jul-14 6 Magnet Safety The whopping strength of the magnet makes safety essential. Things fly Even big things! Screen subjects carefully Make sure you and all your students & staff are aware of hazzards Develop stratetgies for screening yourself every time you enter the magnet Open Bore MRI Scanner Avoid claustrophobia Lower image quality
  • 26-Jul-14 7 Orthopedic MRI Orthoscan MRI 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
  • 26-Jul-14 8 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 2 properties: Spin charge Nuclei are made of protons and neutrons Both have spin 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
  • 26-Jul-14 9 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 A Single Proton ++ ++ ++ There is electric chargeThere is electric charge on the surface of the proton,on the surface of the proton, thus creating a small currentthus creating a small current loop and generating magneticloop and generating magnetic momentmoment mm.. The proton also hasThe proton also has mass which generatesmass which generates anan angular momentumangular momentum JJ when it is spinning.when it is spinning. JJ Thus proton magnet differs from the magnetic bar in that itThus proton magnet differs from the magnetic bar in that it also possesses angular momentum caused by spinning.also possesses angular momentum caused by spinning.
  • 26-Jul-14 10 Polarization Spins are normally oriented randomly. In an applied magnetic field, the spins align with the applied field in their equilibrium state. Excess along B0 results in net magnetization. No Applied Field Applied Field B0 Precession Spins precess about applied magnetic field, B0, that is along z axis. The frequency of this precession is proportional to the applied field: B
  • 26-Jul-14 11 Magnetic Moment II BB FF LL F = IBLF = IBL BB LL WW = IBLW = IBA= IBLW = IBA = = maxmax / / = = tt== mm BB = = sinsin ForceForce TorqueTorque Angular Momentum JJ = m= m==mmvvrr mm vv rr JJ
  • 26-Jul-14 12 mm== ggJJ ggis theis the gyromagneticgyromagnetic ratio. unit: MHz/Tratio. unit: MHz/T ggis a constant for a given nucleusis a constant for a given nucleus The magnetic moment and angular momentum are vectors lying along the spin axis How do protons interact with a magnetic field? Moving (spinning) charged particle generates its own little magnetic field Such particles will tend to line up with external magnetic field lines (think of iron filings around a magnet) Spinning particles with mass have angular momentum Angular momentum resists attempts to change the spin orientation (think of a gyroscope)
  • 26-Jul-14 13 The energy difference between the two alignment states depends on the nucleus DE = mz Bo/I DE = h mz = gh I D E= gh Bo = /2p Bo known as Larmor frequency /2p== 4242..5757 MHz / Tesla for protonMHz / Tesla for proton B
  • 26-Jul-14 14 Resonance frequencies of common nuclei Note: Resonance at 1.5T = Larmor frequency X 1.5 MRI X-Ray, CT Electromagnetic Radiation Energy
  • 26-Jul-14 15 MRI uses a combination of Magnetic and Electromagnetic Fields NMR measures the net magnetization of atomic nuclei in the presence of magnetic fields Magnetization can be manipulated by changing the magnetic field environment (static, gradient, and RF fields) Static magnetic fields dont 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)
  • 26-Jul-14 16 Radio Frequency FieldsRadio 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 The Effect of Irradiation to the Spin System Lower Higher Basic Quantum Mechanics Theory of MRBasic Quantum Mechanics Theory of MR
  • 26-Jul-14 17 Spin System After Irradiation Basic Quantum Mechanics Theory of MRBasic Quantum Mechanics Theory of MR Net magnetization is the macroscopic measure of many spins Bo M T B cM o
  • 26-Jul-14 18 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 anti- parallel 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.
  • 26-Jul-14 19 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 S