A general overview of Chang’s attenuation correction method

1. “Chang attenuation
correction
method on SPECT”
Final Presentation
Nuclear medicine2
Hossein Aslian
July14-2014
Contents:
1.Introduction
2.The definition and purpose
3.Chang method
4.Conclusion
5.References
1

2. Rotating scintillator
camera
 In SPECT similar to
CT-scan, projection
data are acquired
from different view
around patient.
 Although image
processing and
image reconstruction
methods are the
same as CT, there are
some significant
differences between
them.
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1. Introduction

3. One of the most significant differences is that
SPECT images depredate due to:
Photon
Attenuation
Scattering
Detector
Response
Images are reconstructed regardless of
photons attenuation amount
Image reconstruction is done through
attenuation simplification
Importing and registering maps from
another modality
 They are three methods for attenuation
correction from the perspective of image reconstruction
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1. Introduction

4. Degradation caused by attenuation!
Intensity is higher due to lower attenuation
Intensity is lower due to higher attenuation
Consequence of attenuation
Where in the body radioactivity is located
Depends on size of the patient and tissue
Depends on energy of radiation activity
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2. Definition and purpose
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5. From Prof.Bertocchi lecture notes we know that
a general way to compensate attenuation is
conjugate counting
 It refers to acquiring data for directly opposing views and
then combining these data into a single data set.
D
a b
Detector.1 Detector.2
1) Arithmetic mean
2) Geometric mean
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3. Attenuation correction
Basic ways

6. Chang attenuation correction method
Even with geometric mean we cannot have a quantitative accuracy.
The best-known and most widely employed method that
performs AC with FBP is due to Chang (1978).
FBP It is the most commonly used method
for image reconstruction in SPECT
However physical phenomena
cannot be modeled by FBP:
Chang proposed one
post reconstruction
method
In particular effect of
attenuation
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3. Chang Method

7. Chang attenuation correction Steps:
1. An initial image reconstructed by FBP
2. Estimated object geometry and counter
to measure path length
3. Assumed the linear attenuation at given energy
is constant for all body tissue:
4. Using Attenuation Correction Factor for each
pixel
5. Multiplying the reconstructed image by ACF for
each pixel and finding corrected image
=
6. Next slides
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3. Chang Method

8. Chang ACF term:
Projection
Pixel(x1,y1)
d1
d2
Pixel(x2,y2)
M : number of projection
Mu : attenuation coefficient
di : path length
 d1>d2: ACF1>ACF2
 Gray level pixel1< Gray level pixel2
 AFC1 pixel1=more increase
 AFC1 pixel1=less increase
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3. Chang Method

9. 9/14
3. Chang Method
Fig.2: H.Ziadi,”Determination ooff tthhee
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Schematics of Profiles
Pixel number Pixel number Pixel number
Pixel Value
Pixel Value
Pixel Value

10. Complete implementation
of chang method:
Step 6: Finding error projection
6.1: forward projected profile obtained:
Forward projection is the image obtained with the first order correction
6.2 subtract the forward projected from the original measured
projection profile
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3. Chang Method

11. 11/14
4. Conclusion

12. 12/14
4.2 Attenuation correction
methods

13. 1. Ch.Chen, "Emission Tomography",Elsevier
press,2004
2. A.Webb, "Introduction into biomedical imaging
system",Wiely,2005
3. Jennifer Prekeges, "Nuclear Medicine
Instrumentation", Jones & Bartlett Learning,2009
4. Lecture notes by Professor Bertochi and
professor Benuti,MMP2014,ICTP
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5. References

14. Full of thanks
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