Longwood SAIF Series: MicroSPECT (IV of VI)

							Longwood SAIF Series:
MicroSPECT (IV of VI)
      Joanne T. Vannah
Longwood Small Animal Imaging
           Facility
 Beth Israel Deaconess Medical Center
              Boston, MA
                  MicroSPECT


•   S                                               Single
•   P                                               Photon
•   E                                               Emission
•   C                                               Computed
                 Amyloid spleen in
•   T            a mouse model                      Tomography
Awaiting permission: Oak Ridge National Laboratory
http://www.ornl.gov/sci/ismv/research_bio_spect.shtml

                   Longwood Small Animal Imaging Facility
                    Beth Israel Deaconess Medical Center         2
                            www.LongwoodSAIF.org
        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


             Longwood Small Animal Imaging Facility
              Beth Israel Deaconess Medical Center    3
                      www.LongwoodSAIF.org
       MicroSPECT basics

                                                   Parallel
                                                   Hole
                                                   Lead/
                                                   Tungsten
                                                   Collimator

            Photon Energies 30-300keV
   Radio-                                                       Gamma
  atom                                                          Camera
             Energy Resolution ~ 14%


3-D: Similar to CT, rotate collimator 180-360° and perform
reconstruction algorithm

                Longwood Small Animal Imaging Facility
                 Beth Israel Deaconess Medical Center                    4
                         www.LongwoodSAIF.org
          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.

             Longwood Small Animal Imaging Facility
              Beth Israel Deaconess Medical Center    5
                      www.LongwoodSAIF.org
      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
                 Longwood Small Animal Imaging Facility
                  Beth Israel Deaconess Medical Center          6
                          www.LongwoodSAIF.org
Gamma emitting radionuclides for
       microSPECT
   Readily available radioisotopes for use in the Longwood SAIF
    Common              Chemical                                         Research
      Name                Name                Modality                       Use
   99m
       Tc-        Technetium-99m           micro              Bone scanning,
   MDP            methylene                SPECT/CT           microcalcification in breast
                  diphosphonate                               cancer animal models,
                                                              osteoblastic metastases in
                                                              prostate cancer animal models
   99m
       Tc-MIBI Technetium-99m-             micro              Perfusion and viability imaging
               Sestamibi hexakis (1-       SPECT/CT           of tumors
               isocyano-2-methoxy2-
               methylpropane
   201
       Tl      Thallium-201                microSPECT/CT Perfusion/viability measurments
                                                         of tumors where 18F-FDG is
                                                         insensitive due to uptake by
                                                         normal brain, or prostate cancer
                                                         when mouse models are not
                                                         FDG-avid
   111
         In-Oxine In-111 oxyquinoline      microSPECT/CT microSPECT/CT tracking of
                                                         living cells in vivo over days to
                                                         weeks
                            Longwood Small Animal Imaging Facility
                             Beth Israel Deaconess Medical Center                               7
                                     www.LongwoodSAIF.org
   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., 99m Tc)
• 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 )
             Longwood Small Animal Imaging Facility
              Beth Israel Deaconess Medical Center    8
                      www.LongwoodSAIF.org
         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)




                Longwood Small Animal Imaging Facility
                 Beth Israel Deaconess Medical Center        9
                         www.LongwoodSAIF.org
  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


           Longwood Small Animal Imaging Facility
            Beth Israel Deaconess Medical Center    10
                    www.LongwoodSAIF.org
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


             Longwood Small Animal Imaging Facility
              Beth Israel Deaconess Medical Center    11
                      www.LongwoodSAIF.org
    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


             Longwood Small Animal Imaging Facility
              Beth Israel Deaconess Medical Center    12
                      www.LongwoodSAIF.org
             Siemens Inveon
          (MicroSPECT/PET/CT)
Pixelated NaI(Tl)
2.2 mm detector pitch
1 detector head
Max spatial resolution:
<1 mm
Energy resolution
~14%
Detectable energy
range:
30-300 keV


                          Longwood Small Animal Imaging Facility
                           Beth Israel Deaconess Medical Center    13
                                   www.LongwoodSAIF.org
Combining structure and function
       Micro 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

                     Longwood Small Animal Imaging Facility
                      Beth Israel Deaconess Medical Center    14
                              www.LongwoodSAIF.org
   MicroSPECT applications
• Monitor physiologic functions
• Track metabolic processes
• Complement structural information with
  physiological information
• Determine cerebral specificity: stroke,
  receptor densities


           Longwood Small Animal Imaging Facility
            Beth Israel Deaconess Medical Center    15
                    www.LongwoodSAIF.org
  MicroSPECT pros and cons
• Pros
  Relatively long half life tracers, monitor slow
   biological processes
  Image physiologic functional components
  Good spatial resolution, < 1mm
  Energy resolution, < 14%
• Cons
  Radiotracers required
  Sensitivity loss through collimators and tissue


              Longwood Small Animal Imaging Facility
               Beth Israel Deaconess Medical Center    16
                       www.LongwoodSAIF.org
               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


           Longwood Small Animal Imaging Facility
            Beth Israel Deaconess Medical Center    17
                    www.LongwoodSAIF.org