Magnetic Resonance Imaging

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Magnetic Resonance Imaging

• Basic principles of MRI
• This lecture was taken from Simply Physics
• Click here to link to this site

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Introduction

• Magnetic resonance imaging (MRI) is an imaging
  technique used primarily in medical settings to
  produce high quality images of the soft tissues
  of the human body.
• It is based on the principles of nuclear magnetic
  resonance (NMR), a spectroscopic technique to
  obtain microscopic chemical and physical
  information about molecules
• MRI has advanced beyond a tomographic imaging
  technique to a volume imaging technique

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Tomographic Imaging

• Started out as a tomographic imaging modality for
  producing NMR images of a slice though the human
  body.
• Each slice is composed of several volume elements
  or voxels.
• The volume of a voxel is 3 mm3.
• The computer image is composed of several picture
  elements called pixels. The intensity of each
  pixel is proportional to the NMR signal
  intensity.

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Microscopic Principles

• The composition of the human body is primarily
  fat and water
• Fat and water have many hydrogen atoms
• 63 of human body is hydrogen atoms
• Hydrogen nuclei have an NMR signal
• MRI uses hydrogen because it has only one proton
  and it aligns easily with the MRI magnet.
• The hydrogen atoms proton, possesses a property
  called spin
• A small magnetic field
• Will cause the nucleus to produce an NMR signal

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Magnetic Principles

• The spinning hydrogen protons act like small ,
  weak magnets.
• They align with an external magnetic field (Bø).
• There is a slight excess of protons aligned with
  the field. (for 2 million , 9 excess)
• 6 million billion/voxel at 1.5T
• The of protons that align with the field is so
  very large that we can pretty much ignore quantum
  mechanics and focus on classical mechanics.

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More Magnetic Principles

• The spinning protons wobble or precess about
  that axis of the external Bø field at the
  precessional, Larmor or resonance frequency.
• Magnetic resonance imaging frequency
• n g Bo
• where g is the gyromagnetic
  ratio
• The resonance frequency n of a spin is
  proportional to the magnetic field, Bo.

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More Principles

• Now if an electromagnetic radio frequency (RF)
  pulse is applied at the resonance (Larmor,
  precession, wobble) frequency, then the protons
  can absorb that energy, and (at the quantum
  level) jump to a higher energy state.
• At the macro level, the magnetization vector, Mø,
  (6 million billion protons) spirals down towards
  the XY plane.

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Stages in Magnetic Resonance

• Once the RF transmitter is turned off three
  things happen simultaneously. 1. The absorbed RF
  energy is retransmitted (at the resonance
  frequency). 2. The excited spins begin to return
  to the original Mz orientation. (T1 recovery to
  thermal equilibrium).3. Initially in phase, the
  excited protons begin to dephase (T2 and T2
  relaxation)

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Electromagnetism

• Once Mz (a magnetization vector) has been tipped
  away from the Z axis, the vector will continue to
  precess around the external Bø field at the
  resonance frequency wø. A rotating magnetic field
  produces electromagnetic radiation. Since wø is
  in the radio frequency portion of the
  electromagnetic spectrum the rotating vector is
  said to give off RF waves.

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Magnetization

• The RF emission is the net result of the Z
  component (Mz) of the magnetization recovering
  back to Mø
• The time course whereby the system returns to
  thermal equilibrium, or Mz grows to Mø, is
  mathematically described by an exponential curve.
  This recovery rate is characterized by the time
  constant T1, which is unique to every tissue.
  This uniqueness in Mz recovery rates is what
  enables MRI to differentiate between different
  types of tissue.

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Imaging Hardware

• Hardware Overview
• Magnet
• Gradient Coils
• RF Coils
• Safety

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Clinical Images

• Knee
• Spine
• Brain

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The End This lecture was taken from the web site
Simply Physics Click here to link to this site
14
A schematic representation of the major systems
on a magnetic resonance imager
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The Magnet

• The most expensive component of the imaging
  system.
• Most magnets are of the superconducting type.
  This is a picture of a 1.5 Tesla
• A superconducting magnet is an electromagnet made
  of superconducting wire.
• Superconducting wire has a resistance close to
  zero when it is cooled to a zero temperature
  (-273.15o C or 0 K, by emersion in liquid
  helium).
• Once current flows in the coil, it will continue
  to flow as long as the coil is kept at liquid
  helium temperatures.

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Gradient Coils
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Gradient Coils Priciples

• These are room temperature coils
• A gradient in Bo in the Z direction is achieved
  with an antihelmholtz type of coil.
• Current in the two coils flow in opposite
  directions creating a magnetic field gradient
  between the two coils.
• The B field at one coil adds to the Bo field
  while the B field at the center of the other coil
  subtracts from the Bo field
• The X and Y gradients in the Bo field are created
  by a pair of figure-8 coils. The X axis figure-8
  coils create a gradient in Bo in the X direction
  due to the direction of the current through the
  coils.
• The Y axis figure-8 coils provides a similar
  gradient in Bo along the Y axis.

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RF Coils
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R F Coils contd

• RF coils create the B1 field which rotates the
  net magnetization in a pulse sequence.
• RF coils can be divided into three general
  categories
• 1) transmit and receive coils
• 2) receive only coils
• 3) transmit only coils

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Safety
The patient's arm was against the wall of a body
coil being operated in a transmit mode with a
surface coil as the receiver. The burn first
appeared as a simple blister and progressed to a
charring that had to be surgically removed.
A third degree RF burn
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Knee
Coronal
Sagittal
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Spine in Sagittal Plane
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Brain MRI
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