POSITRON EMISSION TOMOGRAPHY

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POSITRON EMISSION TOMOGRAPHY
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Nuclear Medicine

• A nuclear medicine picture is made by imaging the
  gamma rays emitted from a radioactive material
  that was introduced into the body and that has
  concentrated in the organ of interest.

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• Atomic nucleus - protons and neutrons.
• The number of protons - the atomic number ? the
  element type of the atom and its mechanical,
  chemical, optical, electrical, X-ray interaction,
  and ordinary magnetic characteristics.
• The behavior of the nucleus the element and the
  isotope.
• Different isotopes of different elements
• The neutron count - radioactivity

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• There are 2,500 known isotopes of the hundred or
  so elements, and only 270 of them are inherently
  stable. The other isotopes are radioactive.
• Radioactivity is a process whereby an excited,
  unstable nucleus drops to a state of lower energy
  and (often) of greater stability.
• gamma emission - release of a high-energy gamma
  ray photon.

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• Nuclear medicine makes use of radiopharmaceutical,
  special radioactive materials that displays two
  key features an injected or inhaled sample of
  such material is taken up preferentially by a
  specific organ and from there, it emits gamma
  rays that can be detected and imaged from outside
  the body.
• A typical radiopharmaceutical is a solution
  containing special molecules or microscopic
  particles made up of two components, a chemical
  agent and a radioactive atom.
• The agents - blood vessel system of the lungs -
  macroaggregated albumin thyroid - radioactive
  iodine - uptake the liver and spleen and bone
  marrow -radio-labeled sulfur - scavenger cells
• Radioactive atom - Technetium-99m most used

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Sensing the Radiation

• The scintillation detector, a crystal of
  fluorescence material (such as sodium iodide,
  NaI) optically coupled to a photomultiplier tube.
  When excited by a gamma-ray photon, the crystal
  produces a burst of light. The photomultiplier
  generates a jolt of electricity whenever it
  registers such a scintillation and the brighter
  the flush, the greater the voltage kick of the
  pulse.

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The Gamma Camera

• A gamma camera detects and record gamma rays much
  as an ordinary camera records visible light.
• Since gamma rays cannot be focused, the role of
  the lens is played by a multihole collimator,
  consisting of hundreds of small diameter
  channels, separated from one another by thin
  walls of lead foil.
• Behind the collimator is a large (up to
  twenty-four inches in diameter), thin, single
  crystal of sodium iodide doped with trace amounts
  of other material to enhance its fluorescence
  characteristics.
• The crystal in turn is observed by a honeycomb
  array of up to 100 separate photomultiplier
  tubes, each of which will sense any nearby
  scintillation event.
• This information is processed by a computer, and
  the composite image is displayed on a monitor.

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• How does the gamma camera ascertain where the
  gamma ray landed?
• A scintillation in the sodium iodide crystal
  elicits a voltage pulse from each of the
  neighboring photomultiplier tubes. The nearer the
  tube is to site of the flush, the greater its
  apparent brightness, and the larger the voltage
  pulse from the tube. The scintillation-location
  logic circuit knows the position of every
  photomultiplier tube and, by inter-comparing the
  voltage pulses from all of them, it can estimate
  where in the sodium iodide crystal the burst of
  light actually occurred, to better than a
  centimeter.

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Positron Emission Tomography

• Positron emission tomography (PET) is a sub-field
  of nuclear medicine that has long been of
  considerate research value. PET was initially
  employed primarily for imaging variations in the
  flow of blood and in the rate of glucose
  metabolism throughout the normal and abnormal
  brain, and in studies of cerebral activity.
• As with conventional nuclear medicine, PET,
  involves the detection of high-energy photons.
  But PET defers notably in three regards
• the photons are not emitted directly by the
  nucleus
• they happen to be of much higher energy than is
  normally detected in nuclear medicine
• they are always detected in pairs.

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• PET scanning exploits the predisposition of a few
  radioisotopes to emit positrons
• Positron
• After it is ejected from a nucleus, the typical
  positron slows down over a millimeter or so in
  tissue and then bumps into an ordinary electron.
• Antiparticles
• Annihilation photons
• The two new photons will be detected at exactly
  the same time by two separate detectors on
  opposite sides of the patient indicating that
  the positron emitter was located somewhere along
  the straight line joining them.
• Positron-emitting nuclei includes isotopes of
  carbon, nitrogen, and oxygen.
• Lately F-18 fluorodeoxyglucose (FDG)
  metabolized like glucose.

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