Single Photon Emission
Computed Tomography
Lindsey Appenzoller
w/ Kevin Oliver
University of Pennsylvania
Medical Physics
http://www.medical.siemens.com/
Introduction - SPECT
• Detect single photons emitted by radionuclide tracers
• 99mTc emits 140.5 keV gamma rays as it decays
• Determine the origin and direction of emitted gamma
• Reconstruct 3D images of the
source or anatomy
• Used as a diagnostic tool to image
tumors, disease, and perform bone
scans
99mTc
Source and Phantom
• Aqueous solution of 99mTc with initial
activity ~ 318 μCi placed in a cylindrical phantom
• Two positions for source:
• Axis of phantom
• ~ 9 cm above the axis
• Decay detected by triple-headed gamma camera
Detector Schematic
Data collection
• Four scans performed:
• Center in air
• Off-center in air
• Center in water
• Off-center in water
• Each scan acquires data over 360 °
• Steps of 3 °
• 120 ° rotation of the scanner
• Data collected for 15 seconds at each
detector angle
• Collected data is in the form of counts
Detector Coordinate System
• Detector rotates around the coordinate system of the source and phantom
• Interested in the x’ coordinate in the detector reference frame
• Coordinate system of detector:
• x’: 256 bins
• z’: 128 bins
• Bin dimensions => 1.78 x 1.78 mm
Centroid Determination
Source projections at 0° and 90°
• Determine centroid of projection
for each detector angle using:
where
,
• Uncertainty in centroid position:
where
• Plot centroid position vs. angle to
determine initial source positions
• Traces out sine curve
Example of Sinogram
Fit:
Position:
r = -98.9 ± 0.05 mm
ϕ0 = 2.5 ± 0.02 °
Initial Position of Source
Scan 1:
Scan 2:
Position of Center Source in Air
Position (mm)
σ (mm)
x1
-4.45
0.03
y1
10.31
0.03
Scan 3:
Position of Off-Center Source in Air
Position (mm)
σ (mm)
x2
-4.31
0.04
y2
98.81
0.05
Scan 4:
Position of Center Source in Water
Position (mm)
σ (mm)
x3
-4.74
0.05
y3
6.28
0.05
Position of Off-Center Source in Water
Position (mm)
σ (mm)
x4
-7.47
0.20
y4
91.25
0.23
Backprojection
• Our projections represent the source
distribution in the phantom for a single
slice at every angle of the acquisition
• Transform sinogram into the frequency
domain to filter noise
• Backproject counts (from centroid
position) to get information
about the spatial distribution of the
source
• Regions where backprojection lines
from different angles intersect
represent areas which contain a higher
concentration of 99mTc
Image Reconstruction
• Use inverse radon transform on our total number of counts at each
detector angle to reconstruct an image of the point source
• Inverse radon transform filters noise from back projection
• Example image reconstructions for source in water:
Determine Path Length in Water
• Emitted gamma travels a certain
distance (L) through water to reach
the detector at normal incidence
• Use geometry to determine this
path length (varies with angle)
• Plot path length as a function of
detector angle
• Linear attenuation in water is
governed by:
• Account for continuous decay of
source activity (Half life = 6.01 hrs)
Plot of Path Length in Water
Min: ϕ = 180° - Source closest to detector
Max: ϕ = 0°
- Source furthest from detector
Linear Attenuation Coefficient
• Plot attenuation (N/N0) vs. path length (L) to determine μ
• Fit to linear curve:
• Uncertainty in the attenuation is given by:
• μ = 0.128 ± 0.001 cm-1
• Accepted value: μ = 0.15 cm-1
• Constant attributed to scatter in Lucite shell of phantom
Questions??
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