Study of 3D Reconstruction

Method for Electron-Tracking

Compton Camera

Annual Report Meeting by GCOE Postdocs

March 7th, 2011

Shigeto Kabuki

Contents

Medical imaging

Electron-Tracking Compton Camera (ETCC)

3D reconstruction method Simple Back Projection

List mode Maximum Likelihood Expectation Maximization

Modified Source Space Tree Algorithm

Imaging results

Summary

Radioisotope drug

Accumulation to affected part

g rayinjection

detector

imaging

Diagnosticimaging

Flow of Medical imaging

X ray CT

Xray

absorption

SPECT(single photon

computed

tomography)

E < 300 keV

PET(Positron Emission

Tomography)

E=511keV

RI distribution

Conventional Imaging Detector

Motivation

PET & SPECT

High quality algorithm for imaging.

It is very important that imaging algorithm decide the image

quality because the detector performance is almost same

as any company product.

ETCC ??

Compton camera has no good imaging algorithm because

reconstruction method is very complex compared with

other gamma camera.

ETCC can perform the one directional 3D reconstruction

Scintillator

μPICGEM

e-

Determination of direction and energy of each incident

gamma ray

Electron-Tracking Compton Camera μTPC(Time Projection Chamber)

μPIC(μ Pixel Chamber)+GEM+Drift cage

10 x 10 x 10 cm^3 TPC

uPIC:10x10cm^2, 400um pitch, Gain x3000

GEM: 10x10cm^2, Gain x 10

TPC spatial resolution ～300um

3D track and energy of Compton-recoil

electron

PSA (Pixel Scintillation Array)

GSO or LaBr3 + PMT(H8500:HPKK)

Pixel size: 6 x 6 x 13 mm,

Energy resolution:11%

Position and energy of scattered gamma

ray

TPC

PSA

180cm

70cm120cm

RI reagent

10cm15cm

10cm

10cm

Developed ETCC

Reconstruction method

Simple Back Projection

(SBP)

Source Space Tree

Algorithm (SSTA)

Reconstruction Filter

(RF)

List mode Maximum

Likelihood Expectation

Maximization (List mode

MLEM)

Parametric Wiener filter

(PWF)

RFSBP

SSTA

ListmodeMLEM

PWF

Reconstruction flow

Back Projection Region

Defined the reconstruction region

Using voxel 32 x 32 x 32

Voxel size is 200mm / 32 = 6.25mm20

20

20

10

(unit: cm)

TPC

PSA

1010

reconstruction

region

10

source

voxel

Simple Back Projection (SBP)

Compton point

absorption point

back projection

region

Defined the reconstruction region

Using voxel 32 x 32 x 32

Voxel size is 200mm / 32 = 6.25mm

Defined the reconstruction

function

Cauchy distribution

Source Space Tree Algorithm (SSTA)

Defined the reconstruction region

Using voxel 32 x 32 x 32

Voxel size is 200mm / 32 = 6.25mm

Defined the reconstruction

function

Cauchy distribution

Defined the weight of voxel

Defined the line

Located the reference points

(sampling length is Δd)

Calculate the weight at the reference

point which is included the voxel. R.C. Rohe, M.M. Sharfi, K.A. Kecevar et al., 1997.

SSTA 3D Images

Reconstruction image

1 event

ETCC

Camera

SSTA 3D Images

Reconstruction image

5 events

SSTA 3D Images

Reconstruction image

6000 events

SSTA problem

Point source (365keV)

XY slice

Spatial Resolution 50.1mm

XZ slice

Spatial Resolution 187.9mm??

Depth peak is corresponded

with the location of source

Depth direction has the tail of

reconstruction distribution.

These spatial resolutions are too

worth to image an affected part.

FWHM

50.1 mm

Depth peak

96.9 mm

Y

X

Z

X

List mode MLEM

n

k

l

knk

nnm

m

l

ml

mc

yc

S

1

m,k :Image bin

n :event

l : iteration

λml+1 : Image bin m at iteration level l

Sm : Probability that an event emitted in image bin m is detected

Cn m : Position of the event that intersects the image

yn : Probability that an event emitted in image bin m is detected as event bin n

List mode Maximum Likelihood Expectation Maximization

Imaging results & Iteration level

Level 0 (mSSTA) Level 10 Level 100Level 5

Y

X

Z

X

Point source 365keV

Bin 32

Event 1000

Quantities of MLEM

Quantities of

MLEM depends

on number of

events.

Quantities is less

than 5%

(>200events).

5% is same level

as PET and

SPECT

Point source 365keV

Bin 32

Iteration 30

Multi heads 3D study

Multi heads ETCC

has the 2 big merits

Effective area

Depth

information

3heads and 30

iterations condition

is enough to image

the point source

Level

5

Level

10

Level

30

2 heads 3 heads 5 heads1 head

Z

X

List mode MLEM problem

MLEM problem

Big memory size

Number of Events x Num of X bin x Num of Y bin x Num of Z bin.

Long reconstruction time

Computer power

Parametric Wiener filter

Parametric Wiener filter (PWF)

M(u,v,w) : filter function

H(u,v,w) : Fourier transform of a PSF function

H*(u,v,w): complex conjugate of H (u,v,w)

Γ : parameter to reduce the effect of noise

Imaging Result (Point source)

Number of Bin is 128.

PWF does not need

the big memory

size.

The resolution of XY

slice is 16 mm and

XZ is 26 mm

Depth resolution is

better than MLEM

Y

X

Z

X

SSTA PWF

Point source 365keV

Bin 128

Event 6000

Quantitiesof PWF

Quantities of

MLEM depends

on number of

events.

Quantities is less

than 5%

(>1000events).

5% is same level

as PET and

SPECT

Point source 365keV

Bin 32

PWF problem

200 events 1000 events 2000 events

PWF

MLEM

Y

X

Imaging power for

low statistics is

very important

part for medical

gamma camera.

PWF has many

artifact at low

statistics image

PWF is used a

deconvolution

algorithm and need

precise data.

Comparison of algorithm

Advantage & disadvantage

Comparison of List mode MLEM and PWF

Spatial

Resolution

At 365 keV

Statistics Reconstru

ction time

Memory

size

3D

reconstruc

tion

List mode

MLEM

XY : 11mm

XZ ; 21mm

5%(>200events )

X X 〇(Multi head)

PWF XY: 16mm

XZ: 26mm

5%(>1000events )

〇 〇 〇

One directional imaging thyroid phantom

3D imaging of

Thyroid grand

phantom

6M events

Human phantom

I-131 (365keV)

Other area was

filled with neck

bone and water

Amount of material

same as human

PWF

45mm

I-131 Cross linked micelle imaging

Cross linked micelle

imaging

Nagasaki lab. Tsukuba Univ

365keV

4hours data

Accumulation of the micelle in

the tumor.

PWF

Comparison of medical gamma

cameras

Spatial

Resolution

Placement

(FOV)

Energy

Dynamic

Range

Effective

area

3D

reconstructi

on of one

direction.

SPECT 10mm

@365keV

Ring or rotate

(diameter)

<365keV 〇 X

PET 5mm

@511keV

Ring

(diameter)

511keV 〇 X

ETCC 11mm

@365keV

8mm

@511keV

free

(1str)

300～1500keV

X

? (Multi

heads)

〇

Imaging detector

分子プローブ

Molecular

Probe

Visualize

Imaging

detector

Drug design・Clinicapplication

Clinic

29

Medical Imaging

Summary

Development of reconstruction algorithm SSTA

List mode MLEM

Parametric wiener filter

Spatial resolution 11 mm @ 365keV and quantities < 5%

Phantom & Animal 3D reconstruction

Future work Multi heads

Data acquisition algorithm of TPC

New RI reagent