Implementation and Evaluation of the Single Scatter ...stir.sourceforge.net/MIC2004workshop/STIR-MIC-SSS.pdfSTIR. 2) Reconstruct Emission and Attenuation image (Em, Att) doing the

Implementation and Evaluation of

Implementation and Evaluation of

the Single Scatter Simulation

the Single Scatter Simulation

Algorithm in the

Algorithm in the STIR

STIR

Library

Library

C. Tsoumpas

C. Tsoumpasa, ca, c, P. Aguiar

, P. Aguiar bb, K. Thielemans

, K. Thielemans a, ca, c

aaHammersmith Imanet Limited, Hammersmith Hospital, London, Unite

Hammersmith Imanet Limited, Hammersmith Hospital, London, Unite d Kingdom.

d Kingdom.

bbDept. of Medicine, University of Barcelona, Barcelona, Spain.

Dept. of Medicine, University of Barcelona, Barcelona, Spain.

ccClinical Sciences Center,

Clinical Sciences Center, Dept. of

Dept. of Medicine, Imperial College,

Medicine, Imperial College, United Kingdom.

United Kingdom.

EE-- mail

mail ss: : Charalampos.T

Charalampos.Tsoumpas@

[email protected]

imperial.ac.uk, , [email protected]

[email protected], , [email protected]

[email protected]

STIR Workshop, NSS

STIR Workshop, NSS ––MIC 2004

MIC 2004

Hammersmith

Hammersmith

Imanet

Imanet

2

Overview

Overview

��Scatter in 3D PET

Scatter in 3D PET

��Single Scatter Simulation (

Single Scatter Simulation (SSS

SSS))

��SSS

SSSImplementation in

Implementation in STIR

STIR

��Simulation Characteristics

Simulation Characteristics

••Scanner Sampling

Scanner Sampling

••Simulation Parameters

Simulation Parameters

��Scatter Sinogram Profiles

Scatter Sinogram Profiles

��Evaluation using SimSET

Evaluation using SimSET

��Comparison to Measured Scatter

Comparison to Measured Scatter

��Discussion

Discussion

��Future Work

Future Work

3

SCATTER CASE

Scatter in 3D PET

Scatter in 3D PET

NO SCATTER CASE

Sinogram Radial Profile

Phantom

4

Scatter in 3D PET

Scatter in 3D PET

Scatter Fraction (SF) for a point source at the centre of the

FoV, surrounded by a water filled cylinder (30cm diameter,

20cm length) [Adam et al.]1 :

��SF = (Scattered/Total) Events

SF = (Scattered/Total) Events

��2D PET SF

2D PET SF ≈29%

29%

��3D PET SF

3D PET SF ≈41%

41%

��3D PET Single SF (S

3D PET Single SF (S11F) F)

≈25%

25%

Scatter outside of FoV

Scatter inside of FoV

5

Single Scatter Simulation (SSS)


��Interpolate Attenuation and Emission Images.

Interpolate Attenuation and Emission Images.

��Select Scatter Points Randomly in the

Select Scatter Points Randomly in the

Attenuation Image, above a Threshold.

Attenuation Image, above a Threshold.

��Sample the Detectors using a Sinogram

Sample the Detectors using a Sinogram

Template.

Template.

��Find the Single Scatter Distribution (Sinogram)

Find the Single Scatter Distribution (Sinogram)

using the algorithm proposed by Watson

using the algorithm proposed by Watson22. .

6

. .

..

. .

..



7

SSS

SSSIm

plementation in

Implementation in STIR

STIR (II)

(II)

��Scatter Buildblock

Scatter Buildblock

Create Scatter Viewgram

Create Scatter Viewgram

Sample Scatter Point

Sample Scatter Point

Estimate Scatter from all Scatter Points

Estimate Scatter from all Scatter Points

Estimate Scatter from one Scatter Point

Estimate Scatter from one Scatter Point

Differential and Total Cross Section

Differential and Total Cross Section

Detection Efficiency

Detection Efficiency

Write statistics

Write statistics

Scatter Point to Detector Integrals

Scatter Point to Detector Integrals

Emission and Attenuation

Emission and Attenuation

Cache Factors

Cache Factors

Cache

Cache

Random Points

Random Points

No Cache

No Cache

Reconstructed Image

Reconstructed Image

Attenuation Map

Attenuation Map

Sinogram Template

Sinogram Template

Scatter Sinogram Name

Scatter Sinogram Name

Attenuation Threshold

Attenuation Threshold

Lower & Upper Energy

Lower & Upper Energy

Use of Random Points (?)

Use of Random Points (?)

Use of cache (?)

Use of cache (?)

��Scatter

Scatter (EXECUTABLE)

(EXECUTABLE)

��Scatter Inline

Scatter Inline

8

Scanner Characteristics

82

5R

ing

dia

met

er (

mm

)

15

5.2

Sca

nn

er l

eng

th (

mm

)

9.2

9.2

19

.41

9.4

4.8

5R

ing

Dis

tan

ce (

mm

)

16

16

88

32

Nu

mb

er o

f R

ing

s

14

47

21

44

72

57

6D

etec

tors

per

rin

g

IVII

III

IE

XA

CT

HR

+C

ha

ract

eris

tics

Scanner Sampling (I, ..., IV)

BGO crystals resolution: 25% at 511keV

Energy window: 350-650 keV

Wider Energy window (W-SSS): 320-650 keV

9

Phantoms for Evaluation

Phantoms for Evaluation

Simulation

AB

C

A) Point source:Sphere of Emission Voxel Size diameter.

B) Line Source:Emission Voxel Size diameter, Scanner length.

C) Cylinder Source:20cm diameter, Scanner length.

Attenuation Cylinder:20cm diameter, Scanner length.

10

Simulation Parameters

2.7

61

9.5

7/4

32

91

0×

10

×1

07

2×

72

×(1

6+

2)

IV–

(in

)dir

ect

2.8

31

7.5

4/4

32

91

0×

10

×1

07

2×

72

×1

6IV

–d

irec

t

3.3

21

.15

/11

57

10

×2

0×

20

36

×36

×16

III

–d

irec

t

2.7

74

.64

/23

31

20

×1

0×

10

72

×7

2 ×

8II

–d

irec

t

3.2

00

.30

/71

22

0×

20

×2

03

6 ×

36

×8

I –

dir

ect

Rel

ati

ve

Sta

nd

ard

Dev

iati

on

**

(%)

for

B

CP

U T

ime/S

catt

er

Po

ints

*fo

r C

, ~

min

Att

enu

ati

on

/Em

issi

on

Vo

xel

(m

m3)

Sin

og

ram

siz

eS

am

pli

ng

* CPU time approximately needed for W-SSSby a processor AMD Athlon MP 1900+, for the

simulation C.

**Relative Standard Deviation over all views through the centreof a sinogram, for simulation B.

Attenuation / Emission Image Interpolation

and Relative Standard Deviation

11

Comparing Profiles for different schemes of the SSS for simulation A,

along radial direction, through a single view of a direct sinogram, and

along axial direction through the center of a viewgram.

Single Scatter Sinogram Profiles


�Shape of the distribution has significant importance.

�Random scatter points reduce discretisation artefacts.

�Essentially, all radial profiles have the same shape.

Scatter Points

RandomlyIn the center

12



Comparing Profiles for different schemes of the SSS for

simulation C,along radial and axial directions.

Essentially, all profiles axialand radialhave the same shape.

13

��Data acquired on an ECAT EXACT HR

Data acquired on an ECAT EXACT HR++scanner:

scanner:

••NEMA

NEMA ’’94

94 ––Scatter Acceptance Tests.

Scatter Acceptance Tests.

••Uniform cylinder, 20cm diameter, filled with water.

Uniform cylinder, 20cm diameter, filled with water.

••GeGeline source, 2mm diameter.

line source, 2mm diameter.

••Line Source, radial, at ~0mm and ~80mm.

Line Source, radial, at ~0mm and ~80mm.

��Perform

Perform SSS

SSSusing:

using:

••Energy Window (350

Energy Window (350 ––650) keV.

650) keV.

••Wider Energy Window (320

Wider Energy Window (320 ––650) keV (

650) keV (W

W ––SSS

SSS).).

••Scheme I.

Scheme I.

��Perform SimSET simulation using:

Perform SimSET simulation using:

••Energy Window (350

Energy Window (350 ––650) keV

650) keV

••Detector sampling characteristics based on Scheme I.

Detector sampling characteristics based on Scheme I.

Comparison to Measured Data

Comparison to Measured Data

14



�Mean values of the profiles are the same after global scaling.

�The total scatter distribution is wider than the SSS, as expected.

�SSShas excellent agreement in both positions.

�W-SSS has quite good agreement with SimSET, but not excellent.

Comparing Profiles along radial direction for vertical view of t

Comparing Profiles along radial direction for vertical view of t he he

simulation

simulation BBand SimSET (Scheme I)

and SimSET (Scheme I)using Experimental Data.

using Experimental Data.

15



Comparing

Comparing SSS

SSSand

and WW-- SSS

SSSProfiles along radial direction for vertical view of the

Profiles along radial direction for vertical view of the

simulation

simulation BB--Scheme I and normalised measured sinogram, at ~0mm and ~80 mm.

Scheme I and normalised measured sinogram, at ~0mm and ~80 mm.

�Mean values of the profiles are the same, ignoring the peak.

�The y-axis is scaled such that the maximum in the measured data corresponds to 1.

�Estimated single scatter distribution too narrow, as expected.

�The W-SSSovercomes, somehow, this problem.

�Reasonable agreement with the measured data.

16

How scatter is corrected with

How scatter is corrected with

STIR

STIR ??(1st release)

(1st release)

1) Sample the detectors of the scanner using a Sinogram Template,

constructed by STIR.

2) Reconstruct Emission and Attenuation image (Em, Att) doing the

appropriate pre-corrections to the projection data.

3) Rescale the reconstructed images and perform the SSSor W-SSS.

4) Interpolate the Scatter Sinogram (SS) to have the same dimensions

with the Emission Sinogram (ES).

5) Find the sum of the tails, at the SSand ESand define a global scaling

factor (s).

6) Remove the scatter by subtraction and multiply with the Attenuation

Sinogram (AS) to obtain a scatter Corrected Sinogram (CS):

CS = AS �(ES -s�SS)

7) Reconstruct the Corrected Sinogram (CS).

17

Why to use

Why to use STIR

STIR

Scatter Correction ?

Scatter Correction ?

�Easy to use. Package will include Visual C++

scatter project.

�All needed functions/utilities will be included into

the first release.

�Flexible choice of the voxel sizes for the Emission

and Attenuation images and of the detector

sampling characteristics.

�Easy to change between W-SSSto SSS.

�Open Source under Lesser GNU License.

18

Future Work

Future Work ––MIC 2005 (?)

MIC 2005 (?)

��Find a robust scale factor of the scatter

Find a robust scale factor of the scatter

distribution to the emission distribution.

distribution to the emission distribution.

��Evaluate using Brain and whole Body phantoms.

Evaluate using Brain and whole Body phantoms.

��Test if the indirect scatter sinogram has

Test if the indirect scatter sinogram has

significant information.

significant information.

��Integrate the Scatter Estimation into Analytic

Integrate the Scatter Estimation into Analytic

and Iterative Reconstruction Algorithms.

and Iterative Reconstruction Algorithms.

��Include it into

Include it into STIR

STIR55and make it available to

and make it available to

PET community.

PET community.

19

References

References

[1] [1] Adam L E, Karp J S and

Adam L E, Karp J S and Brix

BrixG, "G, "Investigation of scattered radiation in 3D whole

Investigation of scattered radiation in 3D whole-- body positron emission

body positron emission

tomography using Monte Carlo simulations

tomography using Monte Carlo simulations ." ." Phys. Med. Biol.

Phys. Med. Biol. 44 2879

44 2879––95, 1999.

95, 1999.

[2] C. C. Watson, New, faster, image

[2] C. C. Watson, New, faster, image --based scatter correction for 3D PET,

based scatter correction for 3D PET, IEEE Trans. Nuclear Sci.

IEEE Trans. Nuclear Sci.47 (4)

47 (4)

1587

1587-- 1594, 2000.

1594, 2000.

[3] NEMA Standards Publication NU 2, Performance Measurements of

[3] NEMA Standards Publication NU 2, Performance Measurements ofPositron Emission Tomography:

Positron Emission Tomography:

National Electrical Manufacturers Association, 1994.

National Electrical Manufacturers Association, 1994.

[4] SimSET web site:

[4] SimSET web site: http://

http://depts.washington.edu/simset/html/simset_home.html

depts.washington.edu/simset/html/simset_home.html , June 2004.

, June 2004.

[5] [5] STIR

STIR1.3 software library and documentation

1.3 software library and documentation http://stir.

http://stir. HammersmithImanet

HammersmithImanet.com

.com, , September

September2004.

2004.

The authors would like to thankDr Terry Spinks, who provided the experimental data,

Dr Robert Harrison from Imaging Research Lab, at University of Washington Medical

Center and Dr Albert Cot for the assistance during the SimSET simulations,Dr George

Kontaxakis, Dr Oliver Nix,Dr Falk Poenisch and Dr Alexander Werling for the

assistance during the SSS implementation.C. T. also thanks Dr George Loudos and Dr

Sofia Tzimopoulou for many valuable discussions.

Acknowledgments

Acknowledgments

Documents

Implementation and Evaluation of the Single Scatter ...stir.sourceforge.net/MIC2004workshop/STIR-MIC-SSS.pdfSTIR. 2) Reconstruct Emission and Attenuation image (Em, Att) doing the