Longwood SAIF Series: MicroSPECT IV of VI

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Longwood SAIF SeriesMicroSPECT (IV of VI)

• Joanne T. Vannah
• Longwood Small Animal Imaging Facility
• Beth Israel Deaconess Medical Center
• Boston, MA

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MicroSPECT

• S Single
• P Photon
• E Emission
• C Computed
• T Tomography

Amyloid spleen in a mouse model
Awaiting permission Oak Ridge National
Laboratory http//www.ornl.gov/sci/ismv/research_b
io_spect.shtml
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How SPECT works

• Gamma ray emissions are the source for
  information
• Information labeled by radiotracers,
  radiopharmaceutical decays, emits a gamma ray
• Information is collected through a gamma camera
• Image is reconstructed through data analysis

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MicroSPECT basics
Parallel Hole Lead/ Tungsten Collimator

Photon Energies 30-300keV
Radio- atom
Gamma Camera
Energy Resolution 14
3-D Similar to CT, rotate collimator 180-360
and perform reconstruction algorithm
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Radioisotopes

• A radioisotope is an atom with an unstable
  nucleus, which is characterized by elevated
  energy which may be used to produce a
  newly-created radiation particle within the
  nucleus, or else to an atomic electron . The
  radioisotope, in this process, undergoes
  radioactive decay, and emits a gamma ray(s)
  and/or subatomic particles. These particles
  constitute ionizing radiation.

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Radioactive decay is an exponential function

• The time for radioactive material to decline to
  one half its initial value is termed the half
  life
• Activity does not fall at a steady rate, but is
  an exponential function.
• After two half lives the activity falls to ¼ of
  the initial value, after three half lives, 1/8 of
  the initial value
• This pattern will continuously repeat as the
  activity falls more slowly towards zero without
  ever reaching it

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Gamma emitting radionuclides for microSPECT
Readily available radioisotopes for use in the
Longwood SAIF
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Functions of radiolabeled molecules in microSPECT

• Molecules may function as a substrate for
  metabolic processes (e.g., 18F)
• Molecules may function as a reporter for
  physiological processes such as perfusion or
  excretion (e.g., 99mTc)
• Radionuclide itself molecule of interest,
  iodine
• Receptor directed molecules are useful for
  imaging areas that have increased expression of
  the receptor compared to other tissues (e.g.,
  somatostatin receptors )

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Small animal dosing

• Information on dose using the Medical Internal
  Radiation Dose (MIRD) incorporates
• Specific absorbed fractions (SAF) of energy
• S-values (SAF summed over the energies and
  respective yields of the radionuclide decay
  spectrum)
• Whole body dose in small animals (30 g mouse, 300
  g rat)
• 6 cGy 90 cGy (mice)
• 1 cGy 27 cGy (rats)

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Whole body dose very high compared to lethal dose

• Lethal dose 50/30 7 Gy
• Small animals should be carefully monitored when
  used in longitudinal radioactive studies

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Limitations of microSPECT

• Collimator attenuates most incident photons and
  reduces the sensitivity of camera system
• Large amounts of radiation needed to provide
  enough exposure for camera system to detect
  scintillation dots to form image
• Transmission of photons decay through living
  tissue
• Results in substantial losses in signal

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Computed tomography

• Analytical vs iterative algorithms
• Analytical
• Filtered back projection, fast but produces
  significant noise
• Iterative
• Requires more processing steps but can reduce
  reconstruction artifacts
• No ideal methods exist

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Siemens Inveon (MicroSPECT/PET/CT)
Pixelated NaI(Tl) 2.2 mm detector pitch 1
detector head Max spatial resolution lt1
mm Energy resolution 14 Detectable energy
range 30-300 keV
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Combining structure and functionMicro CT/SPECT
Anatomical detail of microCT in concert with
physiological detail of microSPECT maximizes data
analysis
Amyloid spleen in a mouse model
Awaiting permission Oak Ridge National
Laboratory http //www.ornl.gov/sci/ismv/research
_bio_spect.shtml
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MicroSPECT applications

• Monitor physiologic functions
• Track metabolic processes
• Complement structural information with
• physiological information
• Determine cerebral specificity stroke, receptor
  densities

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MicroSPECT pros and cons

• Pros
• Relatively long half life tracers, monitor slow
  biological processes
• Image physiologic functional components
• Good spatial resolution, lt 1mm
• Energy resolution, lt 14
• Cons
• Radiotracers required
• Sensitivity loss through collimators and tissue

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Summary

• Radiotracers label information of interest
• Signal measured through gamma camera when gamma
  rays are emitted
• Valuable for capturing the functional details of
  physiologic processes