Sensitivity Study of the RPC–PET whole-body scanner

Sensitivity Study of the RPC–PET whole-body scanner

M. Couceiro1,2, A. Blanco1, Nuno C. Ferreira3,

R. Ferreira Marques1,4, P. Fonte1,2, L. Lopes1

1 LIP, Laboratório de Instrumentação e Física Experimental de Partículas,Coimbra, Portugal

2 ISEC, Instituto Superior de Engenharia de Coimbra, Coimbra, Portugal3 IBILI-FMUC, Instituto Biomédico de Investigação da Luz e Imagem, Faculdade de

Medicina, Coimbra, Portugal4 Departamento de Física, Universidade de Coimbra, Coimbra, Portugal

RPC-PETJornadas do LIP - 2008

Summary

Resistive Plate Chambers (RPCs)

Simulation Results of Sensitivity for Human Full-Body Axial Field Of View PET Systems

Conclusions


XY readout plane

Y-strips

RC

passive netw

ork

RC passive network

X-strips

RPCs for PET

EResistive Cathode

Resistive Anode

At least one resistive electrode

e-

Photon

For 511 keV photons, and commonly used materials

~300 ps FWHM for photon pairs < 0.4% efficiency per gap for singles No energy resolution

Time signalHVSensitive Area

(precise small Gas Gap ~300 µm)


Stacked RPCs

..........

e-

e-

e-

Improving Detection Efficiency

Efficiency depends both on Photon interaction probability in the

converter plate Electron extraction probability from

the converter plate

Converter plates with high interaction probabilityfor 511 keV photons

Optimize plate thickness for optimum electron extraction


Plate Thickness (m)

Pho

ton

(511

keV

)D

etec

tion

Pro

babi

lity

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

10 100 1000

Glass

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

10 100 1000

Lead-Glass

Detection Efficiency: 511 keV Photons (GEANT4 Simulation)

Optimum efficiency is balanced by beam absorption (thicker plates) and extraction probability (thinner plates)

Optimum thickness depends on the number of plates and on the material. Optimum Thickness (µm)

N P

late

s

N Plates

Lead

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

10 100 1000

21

41

61

81

101

121

141

161

181

201

1001

Bateman et al.5 plates of 0.4 mm glass:Our measurements: 0.8% GEANT4 Simulation: 1.0%


Detection Efficiency: Energy Dependence (GEANT4 Simulation)

Pho

ton

Det

ectio

n P

roba

bilit

y

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

100 200 300 400 500

Glass Lead-Glass

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

100 200 300 400 500

Incident Photon Energy (keV)

Lead

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

100 200 300 400 500

21

41

61

81

101

121

141

161

181

201

1001

N Plates

Strong ENERGY SENSITIVY scattered photons statistically rejected

Material

max Glass Lead-Glass Lead

N Plates 101 17% 29% 31%

201 29% 47% 50%

@ optimumthickness

@ optimumthickness

@ optimumthickness


Disadvantages Much smaller detection efficiency: 20% to 50%. No energy resolution, although energy sensitivity.

Advantages

Increased position accuracy Sub-millimetric spatial resolution Full 3D detection

Increased system sensitivity Inexpensive Large areas possible

large solid angle coverage Excellent timing resolution (300 ps FWHM for 511 keV

photon pair) TOF-PET Optimum randoms rejection

Comparison With the Standard PET Technology

Possible specialized

PET applications

Full-BodyHuman

TOF-PET


Human Full Body FOV RPC–PET: Sensitivity Advantage

Main Goals Study the sensitivity gain for large Axial Field Of View human PET systems (> 200 cm)

Crystal based RPC based

Comparison with commercial scanners (e.g. GE Advance)

Methods Simple analytical model for the sensitivity to true coincidences GEANT4 simulations of photon transport through sensitivity phantoms (NEMA NU2 1994

and an extended version) Photon interaction within detectors not simulated Post processing of detection and coincidences (detection efficiency, packing fraction,

energy blurring, energy window, coincidence window, etc.): Scintillator based detectors RPC based detectors

3D true sensitivity computation followed the NEMA NU2 1994 procedure


d

d

x1

x2

1 2

2( ) / cos( ( ))

20 0

1( )

dn x x Z zT Z z e d d

d

Emitting Point

Human Full Body PET Sensitivity: Simplified Analytical Model

2

1 1

202 2

32 22

2

2

z

Z z

R T Z zS d dz dZ

R Z z

1 max 2 , tanz L Z R

2 min 2 , tanz L Z R

1 max 2, 2Z L

L

Water Phantom

Z

z

d

R

2L Z 2L Z

2 2

r

ds

Tomograph


Human Full Body PET Sensitivity: Simulation Setups

(a) 15 cm AFOV by 92.7 cm ØNEMA NU 2 1994 sensitivity phantom

(b) 15 cm AFOV by 92.7 cm Øwith an extended phantom

(c) 240 cm AFOV by 92.7 cm Øwith an extended phantom

(a)

(b) (c)


Human Full Body PET Sensitivity: Validation for Crystal Based Systems

3D True Sensitivity (kcps/(µCi/cc))

Lewellen et al. This Simulation

Ring Difference NEMA NU2 1994 PhantomExtended Phantom

11 1020 1013 (-0.7%) 1032 (+1.2%)

17 1240 1160 (-6.5%) 1180 (-4.8%)

Data processing Photons assigned to a Module/Block/Crystal according to the GE Advance segmentation Gaussian energy blurring with 20% FWHM at 511 keV 300 – 650 keV energy window Detection efficiency adjusted to obtain reasonable agreement with published data.


Human Full Body PET Sensitivity: Results for Crystal Based Systems

~100-fold sensitivity advantage for standard PET without TOF? ~300-fold sensitivity advantage for LSO based PET with TOF ?

0

20

40

60

80

100

120

0 30 60 90 120 150 180 210 240

AFOV (cm)

Rel

ativ

e 3D

Tru

e S

ensi

tivi

ty

Simulated Data - 5.7 deg

Simulated Data - 15 deg



Simulated Data - Full Acceptance

Analitical Model - 5.7 deg

Analitical Model - 15 deg



Analitical Model - Full Acceptance

0

5

10

15

0 15 30 45 60

+ Badawi et al.normalized

Normalized to the GE Advance sensitivityring difference of 11 (axial acceptance angle ~5.7 deg)


Human Full Body AFOV PET

Full Body AFOV Crystal Based PET Scanners Maintaining crystal thickness for full body AFOV PET scanners

unaffordable Keeping overall crystal volume, reducing crystal thickness

reduce detection efficiency, and sensitivity

RPC TOF-PET Scanners?


Human Full Body RPC TOF–PET Sensitivity: Selected Efficiencies

y = -1,96E-16 x6 + 3,39E-13 x5 - 2,29E-10 x4 + 7,62E-08 x3 - 1,19E-05 x2 + 8,95E-04 x - 7,05E-03

R2 = 1,00E+00

y = -1,62E-16 x6 + 2,86E-13 x5 - 1,97E-10 x4 + 6,56E-08 x3 - 1,02E-05 x2 + 6,99E-04 x - 5,15E-03

R2 = 1,00E+00

0,00

0,05

0,10

0,15

0,20

0 100 200 300 400 500

60 Plates

120 Plates

Polinómio(60 Plates)


Pho

ton

Det

ectio

n E

ffic

ienc

y

Efficiencies of 60 and 120 stacked RPCsbased on 0.4 mm glass plates

19.4%

11.0%


Human Full Body RPC TOF–PET Sensitivity: Simulation Results

200~ 4.4

( / 2) 45TOF

L mmg

c t mm

t = 300 ps FWHM

With TOF information

~20-fold sensitivity increase

Axial Field of View (cm)

3D T

rue

Sen

sitiv

ity (

kcps

/(µ

Ci/c

c))

0

5000

10000

15000

20000

25000

30000

0 30 60 90 120 150 180 210 240

GE Advance - 11 Rings (Lewellen et al)

GE Advance - 11 Rings (simulation)

GE Advance - 17 Rings (Lewellen et al)

GE Advance - 17 Rings (simulation)

60 Plates - 5.7 degrees

60 Plates - 15 degrees



60 Plates - Full Acceptance

120 Plates - 5.7 degrees




120 Plates - Full Acceptance

GE Advance(max. ~1240 kcps/(µCi/cc))


Human Full Body RPC TOF–PET Scatter Fraction: Simulation Results

Moderate excess of scatter over most

standard PET systems

Scatter Important source of

image noise; Typically rejected by

energy discrimination.

0%

10%

20%

30%

40%

50%

0 50 100 150 200 250

GE Advance

60 Plates

120 Plates

Sca

tter

Fra

ctio

n in

Ful

l A

ccep

tanc

e M

ode

Axial Field of View (cm)

RPC Energy Sensitivity


Eff

icie

ncy

0%

5%

10%

15%

20%

25%

30%

0 100 200 300 400 500

20 4060 80100 120140 160180 200

Number of 0.4 mm Glass Plates


Conclusions

Full-body AFOV sensitivity ~20 fold sensitivity gains for 240 cm AFOV RPC TOF-PET may be attainable Scatter is partially rejected by detector energy sensitivity

RPC application to PET seems possible in Full-body human PET, offering larger throughput - hopefully without extra cost

Comprehensive study of a full system in progress A first detector has been assembled and Luís Lopes in his talk will show the

detector details, and present some preliminary results obtained with it

Sensitivity Study of the RPC–PET whole-body scanner

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