“Chang attenuation correction method on SPECT”

Final PresentationNuclear medicine2Hossein AslianJuly14-2014

Contents:1.Introduction2.The definition and purpose3.Chang method4.Conclusion5.References

1

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

One of the most significant differences is that SPECT images depredate due to:

Photon Attenuation

Scattering

Detector Response

Importing and registering maps from

another modality

Images are reconstructed regardless of photons attenuation

amount

Image reconstruction is done through attenuation simplification

They are three methods for attenuationcorrection from the perspective of image

reconstruction

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1. Introduction

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 locatedDepends on size of the patient and tissueDepends on energy of radiation activity

Therefore we have to reduce the effect of Therefore we have to reduce the effect of attenuationattenuation

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2. Definition and purpose

Fig.1: Fig.1: H.Ziadi,”Determination H.Ziadi,”Determination of the Attenuation Map of the Attenuation Map in Emission in Emission Tomography”,J Nucl Med Tomography”,J Nucl Med , 2003, 2003

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.

Detector.1

Detector.2

Dba

1) Arithmetic mean

2) Geometric mean

So, it depends on total tissue thickness D, but not on source depth So, it depends on total tissue thickness D, but not on source depth a,ba,b

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3. Attenuation correctionBasic ways

Chang attenuation correction method Even with geometric mean we cannot have a quantitative accuracy.The best-known and most widely employed method thatperforms AC with FBP is due to Chang (1978).

FBPIt is the most commonly used

method for image reconstruction in SPECT

However physical phenomena cannot be modeled by FBP:

In particular effect of attenuation

Chang proposed one post reconstruction

method

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3. Chang Method

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

Chang attenuation correction Steps:

=

6. Next slides

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3. Chang Method

Chang ACF term:

Proj

ectio

n

Pixel(x1,y1)

d1

d2Pixel(x2,y2)

M : number of projectionMu : attenuation coefficientdi : 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

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3. Chang Method

Fig.2

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the A

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Schematics of Profiles

Pixel number

Pixel number

Pixel number

Pix

el V

alu

e

Pix

el V

alu

e

Pix

el V

alu

e

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

Final attenuation corrected image by chang Final attenuation corrected image by chang methodmethod

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3. Chang Method

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4. Conclusion

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4.2 Attenuation correction methods

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

Full of thanks

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