1. Single Photon Emission Computed Tomography
Lindsey Appenzoller
w/ Kevin Oliver
University of Pennsylvania
Medical Physics

2. Introduction - SPECT
Detect single photons emitted by radionuclide tracers
99mTc emits 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

3. 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

4. Detector Schematic

5. 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

6. 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

7. 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

8. Example of Sinogram Fit: r = -98.9 ± 0.05 mm ϕ0 = 2.5 ± 0.02 °
Position:

9. Initial Position of Source
Scan 1:
Scan 2:
Position of Center Source in Air
Position of Off-Center Source in Air
Position (mm)
σ (mm)
σ (mm) x1
-4.45
0.03 x2
-4.31
0.04 y1
10.31 y2
98.81
0.05
Scan 3:
Scan 4:
Position of Center Source in Water
Position of Off-Center Source in Water
Position (mm)
σ (mm) x3
-4.74
0.05 x4
-7.47
0.20 y3
6.28 y4
91.25
0.23

10. 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

11. 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:

12. 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)

13. Plot of Path Length in Water
Min: ϕ = 180° - Source closest to detector
Max: ϕ = 0° Source furthest from detector

14. 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 = ± cm-1
Accepted value: μ = 0.15 cm-1
Constant attributed to scatter in Lucite shell of phantom

15. Questions?? ? ?