Design and simulation of micro-SPECT:

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• Design and simulation of micro-SPECT
• A small animal imaging system

Freek Beekman and Brendan Vastenhouw Section
tomographic reconstruction and instrumentation Im
age Sciences Institute University Medical Center
Utrecht
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• PRESENTATION OUTLINE
• Introduction in tomography
• Tomography with labeled molecules (tracers).
• Principles of SPECT
• Image reconstruction
• Ultra-high resolution SPECT for imaging small
  laboratory animals gt Need for high resolution
  gamma detectors

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Computed Tomography

• Cross-sectional images of the local X-ray
  attenuation in an object are reconstructed from
  line integrals of attenuation (projection data)
  using a computer

1979 Hounsfield and Cormack share Nobel Prize..
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Why Computed Tomography ?
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We are curious how we, other people, animals,
etc, look inside...
but we dont like to (be) hurt !
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• Examples of Tomography
• Anatomy
• X-ray Computed Tomography
• Magnetic Resonance Imaging (MRI)
• Molecule distributions
• Positron Emission Tomography (PET)
• Single Photon Emission Computed Tomography
  (SPECT)

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• X-ray CT Cross-sectional images of X-ray
  attenuation provide knowledge about anatomy

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We are also curious how organs...
..are functioning in vivo
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• Molecular imaging
• Emission tomographs (PET and SPECT) are suitable
  in vivo imaging of functions (blood perfusion,
  use of oxygen and sugar, protein concentrations)
  
• Uses low amounts of injected radiolabeled
  molecules

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What area in the brain is responsible for a task?
PET and SPECT imaging enables mapping of of
radiolabeled molecule distributions
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SPECT Single Photon Emission Computed
Tomography

• Patient is injected with a molecule labeled with
  a gamma emitter.
• For determination of travel direction detectors
  are equipped with a lead collimator.

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Collimated gamma-camera
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
lt Lead collimator
Detector gt

• To form an image, the travel direction of
  detected photons must be known.
• The collimator selects ?-quanta which move
  approximately perpendicular to the detector
  surface.

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lt Slice of Tc-99m distribution
Slice of SPECT image gt

• Slices are reconstructed (Filtered Back
  Projection (FBP) or Iterative Reconstruction).
• Resolution in humans 6-20 mm
• Resolution can be much better in small animals (lt
  1 mm)

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SPECT Technetium-99m Cardiac Perfusion Image
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IMAGE RECONSTRUCTION FROM PROJECTIONS Analytical
(Radon Inversion) Discrete (Statistical) Methods
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SPECT reconstruction problem
p M a n b ltgt p j Mjiai nj
bj ai activity in voxel i pj projection data
in pixel j bj back-ground in pixel j (e.g.
scatter) nj noise in pixel j Mji probability
that photon is emitted in voxel I is detected in
pixel j. Attenuation, detector blur and
scatter can be included. Estimate a from above
equation
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• SPECT reconstruction matrix
• is complicated by
• Detector blurring
• Attenuation
• Scatter
• 3D reconstruction

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Iterative Reconstruction illustrated
Object space
Projection space
Estimated projection
Current estimate

• Simulation (or
• re-projection)

Compare e.g. - or /
Measured projection
Update
Error projection

Object error map
Back- projection
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Example iteration process ML-EM reconstruction
brain SPECT
0 iterations
10 iterations
30 iterations
60 iterations
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line integral model
accurate PSF-model
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Small animal molecular imaging using single
photon emitters (micro-SPECT)
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Expected contribution of micro-SPECT to science

• Partly replacement of sectioning, counting and
  autoradiography.
• Reduction of number of animals required
• Dynamic and longitudinal imaging in intact
  animals
• Contribution to understanding of gene functions
• Acceleration of pharmaceutical development
• Breakthroughs in areas like cardiology,
  neurosciences, and oncology
• Extension of micro-SPECT technology to clinical
  imaging (2006)

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In Vivo Nuclear Microscopy

• (Eur J. Nucl. Med and Mol. Im., in press)
• Golden micro-pinholes
• gt Super High Resolution

SEM image of gold alloy pinhole
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Mouse thyroid
Microscopic slide
I-125 pinhole image
1 mm

20 min. acquisition
arrows indicate locations parathyroid glands
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Pinhole imaging geometries for small animal
imagingSPECT(micro-SPECT)
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• Spatial resolution clinical SPECT 15 mm
• Spatial resolution current small animal SPECT
  and PET 1.0-2.5 mm
• Micro-SPECT dedicated small animal SPECT.
• with resolution 0.2-0.4 mm
• Effect of Resolution on Rat Brain phantom

2 mm 1 mm 0.5 mm 0.25 mm
0 mm
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State-of-the-art pinhole SPECT
A-SPECT two pinholes. Mouse rotates in tube
Thyroid of mouse (I-125)
Mouse bone scan (Tc-99m)
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Micro-SPECT
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Simulations A-SPECT vs. Micro-SPECT
 


Truth


A-SPECT
Micro-SPECT
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Finally We need a ready set of detectors plus
associated electronicsSolid state?
30 mm

• SPECIFICATIONS
• Energies of 30-140keV
• Counting mode
• Capture efficiency gt80 _at_140keV
• Spatial resolution 200 microns
• Energy resolution (10-20)

10 mm

• Contact Freek beekman f.beekman_at_azu.nl
• 31 30 250 7779
• We need approx. 40 detector elements.