Transcript Effects of truncation of transmission projections on

Design and simulation of micro-SPECT:
A small animal imaging system
Freek Beekman and Brendan Vastenhouw
Section tomographic reconstruction and instrumentation
Image Sciences Institute
University Medical Center
Utrecht
PRESENTATION OUTLINE
• Introduction in tomography
• Tomography with labeled molecules (“tracers”).
• Principles of SPECT
• Image reconstruction
• Ultra-high resolution SPECT for imaging small
laboratory animals => Need for high resolution gamma
detectors
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…..
Why Computed Tomography ?
We are curious how we, other people,
animals, etc, look inside…...
… but we don’t like to (be) hurt !
Examples of Tomography
Anatomy
• X-ray Computed Tomography
• Magnetic Resonance Imaging (MRI)
Molecule distributions
• Positron Emission Tomography (PET)
• Single Photon Emission Computed Tomography (SPECT)
X-ray CT: Cross-sectional images of X-ray
attenuation provide knowledge about anatomy
We are also curious
how organs...
…..are functioning
in vivo
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
What area in the brain is responsible for a task?
PET and SPECT imaging enables mapping of
of radiolabeled molecule distributions
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.
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
<= Lead
collimator
Detector >
Collimated gamma-camera
• 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.
<= Slice of Tc-99m distribution
Slice of SPECT image =>
• Slices are reconstructed (Filtered Back Projection (FBP) or
Iterative Reconstruction).
• Resolution in humans: 6-20 mm
• Resolution can be much better in small animals (< 1 mm)
SPECT Technetium-99m Cardiac Perfusion Image
IMAGE RECONSTRUCTION FROM
PROJECTIONS
Analytical (Radon Inversion)
Discrete (Statistical) Methods
SPECT reconstruction problem
p=Ma+n+b
<=>
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
SPECT reconstruction matrix
is complicated by
• Detector blurring
• Attenuation
• Scatter
• 3D reconstruction
Iterative Reconstruction illustrated
Object space
Current
estimate
Projection space
Simulation (or
“re-projection”)
Measured
projection
Update
Object error
map
Estimated
projection
“Backprojection”
“Error”
projection
“Compare”
e.g. - or /
Example iteration process:
ML-EM reconstruction brain SPECT
0 iterations
10 iterations
30 iterations
60 iterations
line integral model
accurate PSF-model
Small animal molecular imaging
using single photon emitters
(micro-SPECT)
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)
In Vivo Nuclear Microscopy
(Eur J. Nucl. Med and Mol. Im., in press)
Golden micro-pinholes
=> Super High Resolution
SEM image of gold alloy pinhole
Mouse thyroid
I-125 pinhole image
Microscopic slide
20 min. acquisition
arrows indicate locations parathyroid glands
Pinhole imaging geometries for
small animal imaging
SPECT
(micro-SPECT)
• 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
State-of-the-art pinhole SPECT
A-SPECT: two pinholes.
Mouse rotates in tube
Thyroid of mouse
(I-125)
Mouse bone scan
(Tc-99m)
Micro-SPECT
Simulations: A-SPECT vs. Micro-SPECT
Truth
A-SPECT
Micro-SPECT
Finally: We need a ready set of
detectors plus associated electronics
Solid state?
~30 mm
SPECIFICATIONS
• Energies of 30-140keV
• Counting mode
– Capture efficiency >80%
@140keV
– Spatial resolution: 200 microns
– Energy resolution (10-20%)
~10 mm
•Contact Freek beekman:
[email protected]
+31 30 250 7779
We need approx. 40 detector elements.