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Submitted on 1 Jan 1978
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THE DEVELOPMENT OF A SUPERCONDUCTING
GAMMA CAMERA
J. Behar, J. Cardi, B. Herszberg, D. Hueber, C. Valette, G. Waysand
To cite this version:
THE DEVELOPMENT OF A SUPERCONDUCTING GAMMA CAMERA
J. Behar, J.M. Cardi, B. Herszberg, D. Hueber, C. Valette and G. Waysand
Laboratoive de Physique des Solides, Universite Paris-Sud, 91405 Ovsay, France
Abstract.- We described the development of first gamma camera, the detector of which is a suspension of superheated superconducting granules. This device has been designed in view of clinical utilization. It allows the obtention of 81 x 81 mm pictures with a spa-tial resolution of 1.27 mm.
In spite of the successful introduction of scanners for medical exploration, the need of gamma-cameras remains. In a scanner, the "view" of the internal of the body is provided by differences of absorption of X rays emitted by a tube diametrally opposed to a detector (scintillator Nal or semi-conductor) . The rotation of the tube and its asso-ciated detector provides "slices" of the body. The quality image is good but one get only anatomical informations. The spatial resolution is about 10mm.
On the contrary a gamma-camera is a very large detector (usually Nal up to 40 cm in diame-ter) . A radionuclide is injected to the patient and the diffusion of the gamma emitter can be fol-lowed dynamically. The optics is provided by lead collimators, the quality image is bad : 3 mm of spatial resolution in the optimum conditions blur-red by Compton diffusion but one can deduce flow measurements, dynamic behavior, etc... The need for better resolution is urgently felt but the pro-gress of Nal cameras are now very slow. Furthermore the introduction of medical cyclotron providing short-live radio nuclides of high intensity requi-res high counting rates, which are obtained only by a degradation of the image quality : in Nal came-ras the gamma is detected by a set of 19, 37 or 64 photomultiplier (PM) looking at the fluorescence spot. Each PM has to be tuned on the photoelectric peak and their intensities are normalised to the total of all the output currents before a localisa-tion can be made.
In front of this situation we felt that the feasibility of a gamma camera based on a super-heated superconducting detector should be tested.
It is well known that superheated supercon-ducting metastable states can be easily observed in small spherical granules of type I superconduc-tor (Hg, In, Sn under 50u). The metastability is broken under irradiation by an X or a y ray if the energy loss of the photon inside the granule is higher than the energy barrier between the metas-table and the normal state. This difference depends on the difference between the superheated critical magnetic field H , and the external field applied
sn
to the granule. In other words, a collection of superheated superconducting granules is a detector with the unique property to locate, by principle, the materialization of the photon within a discre-te granule, furthermore its average energy barrier can be monitored by an external magnetic field. To detect the transition of the granules from super-heated to normal state we used the disparition of the Meissner effect. At LT 14, two of us (C.V. and G.W.) reported the very first detection in real
time of the transition of one granule under irra-diation (in that case by transition rairra-diation pho-non created by a 6 GeV electron interacting with the granule) /1,2/. In a coil, 140 y in diameter surrounding a 20 u mercury grain a tiny current is generated during the penetration of the external magnetic field in the grain. This tiny current is detected by a low impedance charge preamplifier /3/. Since that time we have improved this preamplifier and we can detect the same grain in a 1 cm loop. This progress allows us to realize a large area de-tector. The detector we built has a matrix organisa-tion by analogy with wire chambers. A slice of gra-nules is sandwiched between two arrays of U shaped
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-120I
Résumé. Nous décrivons l e développement de l a première gamma caméra dont l e détecteur
est une suspension de grains supraconducteurs en é t a t de surchauffe. Cette r é a l i s a t i o n
conçue en vue d ' u t i l i s a t i o n c l i n i q u e permet l ' o b t e n t i o n d'images de 81 x 81 mm avec une
r é s o l u t i o n s p a t i a l e de 1,27 mm.
i n d u c t i v e loops. I n e a c h a r r a y t h e loops a r e p a r a l - l e l t o t h e same d i r e c t i o n b u t t h e two a r r a y s a r e p e r p e n d i c u l a r one t o t h e o t h e r . Thus, t h e f l i p p i n g of g r a n u l e i s d e t e c t e d on one loop of each a r r a y . The two s i g n a l s a r e s e n t t o a c o i n c i d e n c e d i s c r i m i - n a t o r and t h e a d d r e s s o n t h e e v e n t g i v e n by t h e iden- t i f i c a t i o n of t h e two loops i s s e n t t o a computer
(Figure 1 ) .
1.5 K helium v e s s e l . This v e s s e l i s f e d i n p a r a l l e l by a c a p i l l a r y p i p e and, f o r i n i t i a l f i l l i n g , a l a r g e r p i p e i n c l u d i n g a c o l d v a l v e t o a
6
l i t e r helium v e s s e l a t atmospheric p r e s s u r e . A common vacuum chamber a l l o w s d i r e c t a c c e s s t o t h e d e t e c t o r and i t s 256 w i r e s t h e r m a l i z e d when t h e o u t s i d e jac- k e t i s removed. The a t t e n u a t i o n introduced by t h e c r y o s t a t i s completely n e g l e g i b l e i n t h e p r a c t i c a l medical r a n g e of y r a y s (60 t o 140 keV). T h i sc r y o s t a t h a s t e n hours of autonomy. I t can b e k e p t i n o p e r a t i o n d u r i n g t r a n s p o r t a t i o n t o t h e bed of t h e e v e n t u a l p a t i e n t because i t h a s an i n t e r n a l cryopump. I t can b e s l i g h t l y t i l t e d
( 2
30"). The p r e a m p l i f i e r s o u t p u t a r e - i n t e r f a c e d w i t h a miaicomputer f u l f i l l i n g two t a s k s : 1) t h e management of t h e d a t a flow t o g e t h e r w i t h t h e magnetic f i e l d monitoring.2) a l l t h e s o f t w a r e imaging techniques : zooming, 3D p r o j e c t i o n , i s p c o n t o u r , homogeneity c o r r e c t i o n s , e t c . . . a v a i l a b l e on t h e most advanced y cameras.
The we would l i k e t o show a t t h e conference a r e obthined w i t h a d e t e c t o r of t i n g r a n u l e s i r r a d i a t e d by cs131 r a d i o n u c l i d e .
F u r t h e r s t u d i e s w i l l b e continued s p e c i a l l y r e g a r d i n g t h e energy s e l e c t i o n b u t we can a l r e a d y s t r e s s t h a t such a gamma camera o f f e r s a much b e t - t e r s p a t i a l r e s o l u t i o n independent of any t u n i n g and a h i g h e r counting r a t e .
Fig.The 81 x 81 mm m a t r i x d e t e c t o r .
Since we have i n mind c l i n i c a l u t i l i s a t i o n we l i m i t e d o u r s p a t i a l r e s o l u t i o n t o 1.27 mm which
i,s t h e p e r i o d of each a r r a y . However w i t h no more problems we c a n have a 1 5 0 ~ r e s o l u t i o n . The s i z e of t h e m a t r i x i s , 81 x 81 mm and h a s been r e s t r i c - t e d o n l y by economical c o n s i d e r a t i o n ( i n s m a l l s c a l e p r o d u c t i o n t h e p r i c e of o n e p r e s e n t ampli- f i e r i s a b o u t 300 F ) . We e s t i m a t e t h a t f o r b r a i n and c a r d i a c s t u d i e s 20 cm i n diameter i s enough f b r such a d e t e c t o r . Four p r e a m p l i f i e r s can b e housed i n a 1.5 x 10 x 2Q cm volume. An h y b r i d v e r s i o n cheaper and f o u r times s m a l l e r of a simi- l a r p r e a m p l i f i e r i s a l r e a d y under development. Thus t h e 128 p r e a m p l i f i e r s a r e housed i n a 6" x 19" r a c k .
A d e d i c a t e d c r y o s t a t was b u i l t . The detec- t o r i s cooled b y a cold p l a t e l i n k e d t o a small
References
/ I / V a l e t t e , C. and Waysand G., Proceedings of LT 14, - 2 (1975) 183.
/ 2 / D r u k i e r , A.K., V a l e t t e , C., Waysand, G., Yuan, L.C.L. and P e t e r s , F., Proceedings of LT 14,
4 (1975) 278.
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/3/ Chase, R.L., Griihn, Ch., Hrisoho,A., V a l e t t e , C, and Waysand, G . , Proceedings o f 2nd I s p r a n u c l e a r e l e c t r o n i c s symposium S t r e s a , p.29
