Functional Magnetic Resonance Imaging fMRI

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Functional Magnetic Resonance Imaging fMRI
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Magnetic Resonance

• Magnetic Resonance is the absorption or emission
  of electromagnetic radiation by electrons or
  atomic nuclei in response to the application of
  magnetic fields.
• Magnetic Resonance is divided into Electron
  Magnetic Resonance or EMR where electrons absorb
  or emit electromagnetic radiation and into
  Nuclear Magnetic Resonance or NMR where nuclei
  (or protons) absorb or emit electromagnetic
  radiation

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• Nuclear Magnetic Resonance (NMR) of protons was
  first observed in the United States in 1946 by
  Felix Bloch, William W. Hansen, and Martin E.
  Packard at Stanford University and independently
  by Edward M. Purcell, Robert V. Pound, and Henry
  C. Torrey at Harvard.
• Felix Bloch and Edward Purcell shared the 1952
  Nobel Prize in physics for it. NMR quickly proved
  an invaluable tool for identifying chemical
  compounds and for studying their molecular
  structures.
• NMR also forms the basis for MRI, which is used
  in medicine since the 1980s.

Felix Bloch
Edward Purcell
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NMR involves three fundamental physical phenomena
related to magnetic fields.
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First Phenomenon

• The movement of charged particles gives rise to a
  magnetic field.
• A nucleus behaves like the spinning (moving) ball
  of positive charge it will therefore create a
  magnetic field.
• The direction of a nucleuss magnetic field is
  closely associated with that of its spin axis
  curl your right hand around the nucleus of a
  hydrogen atom, say, in the direction in which it
  is rotating then your thumb defines the
  direction of the spin, and points out of the
  protons north magnetic pole.

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Second Phenomenon

• A magnetized body placed in a magnetic field
  already in existence will experience a torque, or
  twisting force, and try to align along that
  field.

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Third Phenomenon

• A nucleus has the option to be in a quasi-stable,
  high-energy state, spinning in the wrong
  direction, with its nuclear magnetic field
  pointing anti-parallel to the external field.
• The stronger the magnetic field of the magnetized
  body, the harder it will be to twist it over so
  that it points in the wrong direction (i.e.
  anti-parallel to the external field) the
  greater the effort that is required.
• Likewise, the energy needed to flip over the
  magnetized body will increase with the strength
  of the external magnetic field.

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• One process by which a nucleus can be elevated
  from the lower- to the higher- energy spin state
  (be flipped over such that it is anti-parallel to
  the external field) is through the absorption of
  a photon of the right energy
• The energy difference between the high
  (anti-parallel to the external field) and low
  (parallel to the external field) energy protons
  is measurable and is expressed in the Larmor
  equation
• Resonance is referred to as the property of an
  atom to absorb energy only at the Larmor
  frequency.

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• In NMR the resonance (the energy absorbs or
  emitted to move nuclei from low to high state of
  energy and vise versa, respectively) of
  different material in different environments is
  monitored in order to analyze the molecular
  composition of the material in question.

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Relaxation Times

• The time it takes a nucleus to emit the energy it
  absorbed and to return to its low energy state is
  termed the relaxation time.
• Relaxation time is another, very important,
  measure in NMR and in MRI.

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Magnetic Resonance Imaging - MRI

• Raymond Damadian a physician at Downstate Medical
  Center in Brooklyn, New York, found in 1971 that
  tissues surgically removed from different organs
  in rats have significantly different relaxation
  times and that tumors in some organs tend to have
  measurably longer NMR relaxation times than do
  the corresponding healthy tissues
• That was the first step toward the medical use of
  NMR

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• Soon thereafter Damadian and Paul Lauterbur
  separately suggested ways in which the NMR
  signals coming from different parts of the body
  could be untangled from one another and utilized
  in imaging.
• Damadian completed the first whole-body MRI
  scanner (named indomitable, now on display at
  the Smithsonian Museum) in 1977.
• These early efforts, followed by the work of
  hundreds of others, have led to the development
  of MRI machines that can now map out spatial
  variations in relaxation times in great detail
  and with superb soft-tissue contrast. MRI can
  thus provide useful information not just about
  anatomy, but also about the physiology of cells,
  and even about their health.

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• MRI can produce three-dimensional anatomical
  images of thin tissue slices at any orientation.
  
• MRI can reveal valuable information on the
  metabolic and physiological status of soft
  tissues.
• In MRI there are no radiation risks to the
  patient there are no X rays, MRI involves only
  stable, non-radioactive nuclei and a variety of
  magnetic fields.

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• Approximately 70 of the body is made up of water
  which contains two hydrogen atoms and one oxygen
  atom.
• The nucleus of the hydrogen atom consists of one
  proton, and no neutrons.
• The single proton in the nucleus of the hydrogen
  atom gives it a positive charge and creates a
  large magnetic moment (i.e. magnetic field).
• All the above make hydrogen atoms extremely
  sensitive to magnetic resonance.
• Indeed, it is the hydrogen atoms that are focused
  on to produce a MR image.

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Precession

• Hydrogen atoms do not actually align directly
  with the direction of the magnetic field, but
  rather rotate or wobble around the axis of the
  external magnetic field (B0). What is actually
  aligned with the axis of B0 is the axis around
  which the hydrogen proton wobbles.
• The term to describe this secondary spin is
  precession. Protons actually precess at an angle,
  spinning a cone-shape around the direction of the
  external magnetic field. The speed at which the
  protons precess is referred to as the
  precessional frequency and is measured in
  megahertz.

Spinning Proton
Precession
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MRI utilize three different magnetic fields

• The principal or external field
• The gradient magnetic fields
• The magnetic component of radio frequency fields

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The Principal or External Field

• The external field generated by the principal
  magnet, is designed to be very strong ,
  homogenous (uniform) throughout the volume of the
  patients body being imaged, and constant over
  time.
• Magnetic field strength is measured in a unit
  called the Tesla (T). The field strength of the
  earth itself, viewed as a huge bar magnet, is
  about 0.00005 T.
• MRI is commonly carried out with the principal
  field in the 0.1 T to 1.5 T range.

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The Gradient Magnetic Fields

• The gradient magnetic fields, generated by the
  gradient magnets, are intentionally made to vary
  in strength from place to place, and are rapidly
  switched on and off intermittently from time to
  time.
• This allows a better localization of the signal
  in MRI

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The Magnetic Component of Radio Frequency Fields

• The magnetic component of radio frequency fields,
  generated by the radio wave transmission and
  reception equipment, oscillate millions of times
  a second.
• In MRI the radio wave transmission equipment
  broadcasts a radiofrequency (RF) pulse toward the
  human body. This pulse frequency is the same as
  precess frequency of the hydrogen protons (i.e.
  at the hydrogen Larmor frequency) and
  consequently it elevates them to a higher energy
  state. In other words, the hydrogen protons
  "resonate" to the pulse frequency.

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T1 T2 Relaxation Times

• T1 is the time it takes the orientation of the
  spin to be again parallel to the magnetic field
  created by the large magnet.
• T2 is the time it takes the radius of the spin
  itself to become less wobbly and like the
  original spin.

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• The values of T1 and T2 are strongly influenced
  by the precise manner in which the water
  molecules are moving around and bumping into
  other molecules within the cells.
• Those interactions, in turn, are highly sensitive
  to the fine-tuning of the physiology of the
  tissues.
• T1 and T2 have differential sensitivity to
  different tissues.
• Therefore, MRI can produce images that clearly
  distinguish among the various parts of brain
  tissue and other organs.

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fMRI

• In 1990, Seiji Ogawa of ATT's Bell Laboratories
  reported that, in studies with animals,
  deoxygenated hemoglobin, when placed in a
  magnetic field, would increase the strength of
  the field in its vicinity, while oxygenated
  hemoglobin would not.
• Ogawa showed in animal studies that a region
  containing a lot of deoxygenated hemoglobin will
  slightly distort the magnetic field surrounding
  the blood vessel, a distortion that shows up in a
  magnetic resonance image.

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• Functional MRI is based on the increase in blood
  flow to the local blood vessels that accompanies
  neural activity in the brain.
• This results in a corresponding local reduction
  in deoxyhemoglobin because the increase in blood
  flow occurs without an increase of similar
  magnitude in oxygen extraction
• Deoxyhemoglobin alters the T2 weighted magnetic
  resonance image signal.
• Thus, deoxyhemoglobin is sometimes referred to as
  an endogenous contrast enhancing agent, and
  serves as the source of the signal for fMRI.

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