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Nuclear Instruments and Methods in Physics Research A 520 (2004) 628–630

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doi:10.1016

Superheated superconducting grains (SSG) in highdensity dielectric matrix (HDDM): feasibility of a

positron emission tomography

Roger Bru"ere Dawsona,b, Jacques Maillardb,c,*, G!erard Maureld,Jorge Silvae,f, Georges Waysanda

aGPS UMR 7588, Universities Paris VI, Campus Boucicaut, 140 rue Lourmel, Paris 75015, Franceb IN2P3, 3 rue Michel Ange, Cedex 16, Paris 75794, France

cCNRS/IN2P3, IDRIS, Bt 506, BP167 Orsay, Cedex 91403, FrancedUniversit !e Paris VI, Facult!e de M!edecine Saint Antoine, 27 rue de Chaligny, Paris 75012, France

eUniversit!e Paris VI, 4 place Jussieu, Paris 75005, FrancefLaboratoire Souterrain Bas Bruit de Rustrel-Pays d’Apt (Universit !e d’Avignon), Rustrel 84400, France

Abstract

We demonstrate the feasibility of a Positron Emission Tomography camera based on superheated superconducting

grains in a high-density matrix. Simulations show that they compete with other advanced systems. Suspensions of

superheated superconducting grains are a detecting composite material. This could be potentially very useful for

positron cameras where two diametrically opposite cells are simultaneously knocked by 511 keV gammas.

r 2003 Elsevier B.V. All rights reserved.

PACS: 85.25.Oj

Keywords: Nuclear medicine; Monte-Carlo simulation; Micro PET; Detector; Superconductivity

1. Introduction

Positron Emission Tomography (PET) is basedon the simultaneous detection of the opposite511 keV gammas emitted by an impinging posi-tron. Advanced systems are usually limited by thelow stopping power of the detecting material, thedead time of the photomultipliers, the size of theelementary pixel. This paper, which estimates whatcan be provided by SSG dispersed in a high density

onding author. Tel.: +33-169-358-587.

ddress: maillard@idris.fr (J. Maillard).

- see front matter r 2003 Elsevier B.V. All rights reserve

/j.nima.2003.11.361

matrix, is based on experimentally demonstratedSSG detectors.

2. Geometry for the simulation of a single pixel

We take the most conservative parameters forthe SSG elementary cell, which is a cylinder0:78 cm long, 0:4 cm in diameter. The operatingtemperature is about 200 mK [1].Each cell is a suspension with a filling factor in

grains of 0.1 containing tin microspheres ofrespectively 7, 8 or 10 mm radius. Our simulation

d.

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R.B. Dawson et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 628–630 629

tool is GEANT 321 [2] which, for each step, givesthe energy loss and the eventual gamma inter-action.We decrease the standard threshold used ingeant for electron ð10 keVÞ down to 100 eV inorder to take into account the stochastic trajectoryof electron in litharge and grains. With such a lowthreshold, the usual energy losses by scatteringappears realistic.

mouse phantom

source

cylindrical pixels

Fig. 1. Micro PET geometry.

3. Response of superheated superconducting

microspheres

The microspheres are randomly immersed in ahigh-density matrix made of litharge (Pb0,density ¼ 9:35). Such a detector irradiated by511 keV gammas has a stopping power of about37%. The simulation relies, in first approximation,on two independent main stochastic phenomena:

* the energy of the gamma when entering into thegrain.

* the value of the equatorial magnetic field on thecorresponding grain.

In a first stage, for each impinging gamma(‘‘event’’) we simulate with GEANT321 the fulltracking of electrons and gammas, which gives ineach volume (each grain is a volume) the energydeposition for all the particles involved in this‘‘event’’. In a second stage, taking the energydeposited during this event in the grains, wesimulate the transition of the grain taking intoaccount equatorial field and probability of transi-tion. In a last step, we take into account thedetection efficiency of the actual read-out electro-nic [3]. Thus, we obtain, as a function of theimpinging photon energy, the intrinsic efficiency ofthe pixel. We assume:

* That at Happlied 20% of the grains are alreadyflipped to the normal state and that in theinterval ½Happlied;Happlied þ DHmax� the numberof grains per unit of DH is constant.

* The present state of art of the electronic read-out allows us to read 1 grain of 10 mm radius or2 grains of 8 mm radius, between 200 and500 mK: Under these conditions we obtainrespectively, by simulation, an intrinsic effi-ciency which is, in case of 511 keV gammas,

0.534 for the grains with 10 mm radius, 0.380 forthe grains with 8 mm radius.

4. Performance required for a small animal PET

The PET requirements for a small animal werediscussed among others by Huber and Moses [4]:One has to achieve simultaneously high efficiencyof direct gamma capture, high rejection of indirectgamma, narrow time coincidence and spatialresolution. In order to evaluate the performanceof the proposed design, one considers a mousephantom of 29 g placed in the centre of a 20 cmlong cylinder, 5 cm in diameter made of two closedcompact layers of elementary cylindrical pixels asdiscussed above (see Fig. 1). Monte Carlo simula-tion gives a materialization efficiency of the orderof 42% for 511 keV irradiation in a pixel. Theefficiency given by simulation of single pixel allowsan estimation of the noise equivalent counting rate(NECR). It is a standard measure of signal tonoise in reconstructed PET images. It takes inaccount the principal sources of noise andinefficiencies. They are the main parameters toevaluate the performance of micro-pet camera.Then, the challenge is to get images as fast aspossible. Naturally, fine analysis must explore eachsource of inefficiency and noise particularly, andalso the efficiency of the reconstruction program.NECR ¼ True2=ðTrueþ Scatterþ k:RandomÞ;

(k ¼ 1 or 2).The NECR for usual system can be small: A

usual microPET has a measured maximum NECRof 22� 103 counts per second (cps) [4]. Amaximum

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R.B. Dawson et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 628–630630

NECR of 107 cps is expected for a future high-precision camera based on LSO scintillator.For a PET camera based on SSG with 10 mm

radius, in high-density matrix, we obtain:NECR ¼ 0:66� 106; and with 8 mm radius,NECR ¼ 0:3� 106:

5. Conclusion: elements of comparison with others

systems

SSG PET is an alternative to classical techniquespushed to their limits. Existing or proposed systemsare generally using scintillators, mosaics of smallcrystal with diodes, big crystals with collimators,and Multi Wire Proportional Chambers. A high-density detector minimizes the diffusion by Comp-ton effect relative to photoelectric absorption [5]and therefore improves spatial resolution. Thereare no technical limitations to using narrowersolenoids, allowing smaller grains to be read, betterenergy resolution and offering shorter dead time.

Acknowledgements

We would like to thank M.M. Bonnierbale forhis technical help and M. Lallemand, Director ofASCI (Application Scientifique du CalculIntensif), for his support with the numericalsimulations.

References

[1] C.Y. Huang al., Physica C 341–348 (3) (2000) 1963.

[2] CERN. GEANT Users Manual, CERN Program Library

Office Long Write-up W 5013, CERN.

[3] R.B. Dawson, et al., Nucl. Instr. and Meth. 277 (1989)

211.

[4] J.S. Huber, W.W. Moses, IEEE Trans. Nucl. Sci. NS-46

(1999) 498.

[5] R.H. Huesman, S.E. Derenzo, W.W. Moses, T.F. Budinger,

Critical instrumentation issues for o 2 mm resolution high

sensitive brain PET, In: K. Uemura et al. (Eds.), Quanti-

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