Characterization of LaBr3:Ce scintillator optimized for spatial resolution in low-energy gamma detection

F. Cusanno, E. Cisbani, S. Colilli, R. Fratoni, F. Garibaldi *, F. Giuliani , M. Gricia, M. Lucentini, M.L. Magliozzi, G. Marano, M. Musumeci, S. Palazzesi, F. Santavenere, P.Veneroni Italian National Institute of Health and INFN-Roma 1, gr. Sanita, Rome, Italy

S. Majewski, J. Mckisson, V. Popov, S. Stolin, B. WelchJefferson Lab, newport News, VA, USA

Y. Wang, G.S.P. Mok, B. M. W. TsuiJohns Hopkins University, Baltimore, MD (USA)

MotivationsMotivationsAn important component of molecular medicine is molecular imaging, where the molecular identification of cellular components, receptors and ligands may allow the detection of early or hidden lesions allowing a new approach to diagnoses and cure of diseases. Strong integration is needed between preclinical and clinical studies. In this framework a key role is played by techniques employing radionuclides that allow imaging of biological processes in vivo with very high sensitivity (picomolar level) providing that a suitable detection system is available. The design of imaging systems (especially for small animals) is challenging due to the concurrent requirements of high spatial resolution and sensitivity. Moreover many research have to be performed on mice that have many advantages: small size, rapid gestation period large litter size, low maintenance costs. Moreover, the mouse genome has been extensively characterized. Gene-targeted "knock-out" and transgenic overexpression experiments are performed using mice, rather than rats, so submm spatial resolution and “good” sensitivity are needed. For this reason we focus on the SPECT modality. In fact PET has intrinsic limitations and doesn’t allow multilabeling that allows looking at different biological processes at the same time. This makes the design of the detectors challenging. In fact dedicated detectors have to be designed for this kind of researches. Multimodality is often mandatory For these reasons careful optimization is needed for the different components of the imaging systems. In this paper we In this paper we focusfocus on the on the optimizationoptimization of the of the intrinsic intrinsic propertiesproperties of the detectors, namely with the of the detectors, namely with the choicechoice and the and the designdesign of the of the scintillatorscintillator..

petCompton Cameracollimation Multipinhole or

CsI(Tl)

Bialkali PMT

Important parameters for detectability/visibility

pixel dim/n.of pixels

scintillator

electronics, DAQ

efficiency collimation

time (and modality)

uptake (radiopharmacy)

. Uniformity of p.h.response (affecs the overall en resolution and the energy window seection)

spatial resolution

spatial resolution

fotofraction

Bialkali PMT

detector intrinsic properties

modality (compression)

CsI(Na)

€

SNR =S − BKG

S

€

IC =Max − BKG

Max

€

σX ∝σ X i

Np.e.

⇒ R i ≡ FWHMX ∝FWHMX i

Np.e.

€

X =

c iX i

i=1

Nchannel

∑

c i

i=1

Nchannel

∑

Improving the intrinsic perfrmances. Spatial resolution. Importance of pixel identification

good pixel identification is fundamental for correct digitization affecting spatial resolution and contrast

C8 strips

M16 (4 x 4) mm2

M64 (2 x 2) mm2

STD[Xrec-Xreal] vs Anode Size for CsI(Tl) scintillator

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 5 6 7 8

Anode Size (mm)

STD[Xrec-Xreal] (mm)

pitch 1.0 mmpitch 0.8 mmpitch 0.6 mmpitch 0.4 mm

MCP (Burle) 1.5x1.5 mm2

or LaBr3 continuous

CsI(Tl) 0.4 -1.0 mm pitch

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

SiPm ?

Optimizing intrinsic performances- Layout : 8 detectors around the animal

- Single module detector dimension: 100 x 100 mm2

- Pinhole (or multipinhole) is mandatory, to get submm spatial resolution

With M=3, we get a FOV = 33 x 33 mm2 (significant portion of mouse)

we are looking for spatial

resolution of ~ 0.5 mm, so we

need intrinsic spatial resolution

of ~ 0.5 mm

0.2 0.4 0.6 0.8 1.0 1.2 1.6 1.8

rt (mm)

ri (mm)

CsI(Tl) (3 mm thick, 0.4 mm pitch, BURLE MCP (1.5 x 1.5 mm2 “anode”)

simulations (GEANT4)

CsI(Tl) (3 mm thick, 0.6 mm pitch, H9500 (3 x 3 mm2 “anode”)

CsI(Tl) (3 mm thick, 0.6 mm pitch, BURLE MCP (1.5 x 1.5 mm2 “anode”)

Labr3 Continuous different performances for different window treatment, diffusing (a), absorbing (b)

a

b

0.6 mm pitch Burle

CsI(Na) is an interesting option. Unfortunately arrays with pixels smaller than 0.8 mm pitch cannot be built. It is igroscopic, so a window is needed (with spatial resolution degradation) or the integral line layout has to be used.

Careful evaluation of the different options has to be done validating the simulations by measurements first.

Then, decision to be taken on the SNR evaluation base looking at the specific application

Tradeoff

spatial resolution/efficiency/

energy resolution

summarizing

1024 Ch. ~ 2 kHz

8192 Ch. (20 kHz)

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

role of readout

resistive chain projectivecoordinates

individual channels

LaBr3 3 mm thick (white) coupled to two H8500

(6 x 6 mm2)

Active area

0.75 mm FWHM

Dead area

1.4 mm FWHM

FOV = 40 mm !

LaBr3 3 mm thick 80% black FWHM = 0.55 mm M64 (2 x 2 mm2 anode)

white vs black entrance windowmeasurements

simulation vs measurement

black scintillator entrance window improves spatial resolution and FOV but worsens energy resolution

Labr3 continuous, 3 mm thick, coupled to 4 H9500

CsI(Tl) 0.8 mm pitch coupled to

PSPMT H9500 (raw image)

LaBr3 continuous vs CsI(Tl pixellated

LaBr3: good sampling of light also in the dead area. Spatial resolution ~ 0.8 mm (see simulations).

CsI(Tl) (preliminary), raw image: The image would improve significantly using smaller anode size PSPMTs probably loosing few pixels in the dead area)

Cardiovascular diseases - Stem cell therapy?

we should try to understand better……….

10 cm

2.5 cm

High Resolution Upper Head:Pin Hole: 0.5 mmScintillator: NaI(Tl), 1.5 pitch

6 mm thickPhotodetector: H8500 (2×2)Rt ~ 0.8 mm, Eff ~ 0.5 cps/μCi M ~ 3, FoV ~ 33 mm

High Sensitivity Lower Head:Pin Hole: 1.0 mmScintillator: NaI(Tl), 1.2 pitch

6 mm thickPhotodetector: H8500 (4+4)Rt ~ 1.4 mm, Eff ~ 1.7 cps/μCi M ~ 2.7, FoV ~ 37 mm

DUAL HEAD DETECTOR

only one used for the measurement

Let’s start- evaluating the effects of the therapy:

1. perfusion on “normal” mice

2. infarction model (multilabeling)2.1 perfusion (mibi-99Tc)

2.2 monitoring of the injected stem cells diffusion (111In)

Preliminary measurement of perfusion on normal mouse with 1 detector module:

- Pin Hole: 0.5 mm

- Scintillator: NaI(Tl),

- 1.5 pitch,

- 6 mm thick

- Photodetector: (2x2) H8500

- M ~ 3

- FoV ~ 33 mm

- Rt ~ 0.8 mm,

- Eff ~ 0.5 cps/Ci

Reconstruction Images of Mouse Perfusion Scan (I)

OS-EM, 6 subsets, 2 iterations, post-smoothed by Butterworth filter (cutoff=0.12, order=8), voxel size = 0.25 mm, image dimension 90x90.

Trans-axial Axial Trans-axial Axial

Reconstruction Images of Mouse Perfusion Scan (II)

Extrapolation to different number of detectors (maximum 8)

Spatial resolution ~ 300 - 400 possible (to be traded with sensitivity)

- sensitivity ~ 0.2 %

- spatial resolution ~ 0.4 mm

1 detector 8 detectors with multipinhole (factor 32)

Conclusions- molecular imaging: crucial role of radionuclides techniques- compact high resolution flexible system

- optimization needed for all the components

- role of the scintillator

LaBr3 continuous seems the best candidate allowing both good

spatial and energy resolution and (multilabeling possible) butbut problems with fragility, dimension, reduction of FOV , cost

- CsI(Tl) (or CsI(Na) pixellated (0.8 mm pitch) is the alternative butbut problems with sampling in the dead area (to be carefully evaluated)

Outlook

- stem cells studies (multilabeling): - selecting “right” cells- monitoring diffusion, differentiation,

grafting etc- looking at the effects of the therapy

===> multimodality (optical, SPECT, MRI, )

- the challenge: 120 pixel/100 mm, 8 modules

150 X 100 mm2 -> FOV 50 x 33 (M=3)) “ideal”

- multidisciplinary approach mandatory

---> “new” photosensors (SiPm?)