THE ROLE OF SUPERCONDUCTING MAGNETS IN NMR MEDICAL IMAGING

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THE ROLE OF SUPERCONDUCTING MAGNETS IN NMR MEDICAL IMAGING

M. Green, J. Singer

To cite this version:

M. Green, J. Singer. THE ROLE OF SUPERCONDUCTING MAGNETS IN NMR MEDICAL IMAGING. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-685-C1-690.

�10.1051/jphyscol:19841139�. �jpa-00223610�

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Colloque C l , supplkment au n o 1, Tome 45, janvier 1984 page C1-685

THE ROLE OF SUPERCONDUCTING MAGNETS I N NMR M E D I C A L I M A G I N G

M.A. Green and J.R. s i n g e r r

Lawrence BerkeZey Laboratory, 1 Cyclotron Road, BerkeZey, CA 94720, U.S.A.

l ~ e ~ a r t m e n t of E l e c t r i c a l Engineering and Computer Science, U n i v e r s i t y o f C a l i f o r n i a , Berkeley, CA 94720, U.S.A.

Resume

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Au cours des c i n q dernieres annees, une technique d ' i m a g e r i e u t i l i s a n t l a RE!W (Rksonance Magnetique Nucl e a i r e ) a & t e devel oppee. L ' imagerie par RMN produi

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r a des images p r 6 c i s e s de t i s s u s mous sans a v o i r recours aux dangereuses ra- d i a t i o n s i o n i s a n t e s . L ' i m a g e r i e R M N peut &re p a r t i c u l i e r e m e n t u t i l e pour l a d e t e c t i o n de cancers e t de c e r t a i n e s maladies c a r d i o - v a s c u l a i r e s . I 1 se peut que l ' i m a g e r i e RMN c o n s t i t u e l a premiere u t i l i s a t i o n a grande 6 c h e l l e de l a technologie de bobines supraconductrices. Cet a r t i c l e d e c r i t l e r61e des bo- bines supraconductrices dans l ' i m a g e r i e RMII. Les c r i t e r e s de d e f i n i t i o n pour l e s bobines supraconductrices u t i l i s a b l e s pour l ' i m a g e r i e 8 corps e n t i e r s sont presentes. Quelques approches de conception de bobines sont Ggalement discutees.

A b s t r a c t - Modern medicine has become dependent on x-rays and v a r i o u s imaging techniques f o r t h e diagnosis o f diseases. W i t h i n the l a s t f i v e years, an imagi ng technique which uses NMR (Nuclear Magnetic Resonance) has been

developed. NMR imaging w i l l produce d e t a i l e d p i c t u r e s o f s o f t t i s s u e s w i t h o u t the use o f dangerous i o n i z i n g r a d i a t i o n . NMR imaging can be p a r t i c u l a r l y u s e f u l f o r t h e d e t e c t i o n o f cancer and c e r t a i n cardiovascular diseases. NMR imaging may w e l l p r o v i d e the f i r s t l a r g e - s c a l e use o f superconducting magnet technology by s o c i e t y as a whole. This paper describes the r o l e o f

superconducting magnets i n NMR imaging. The design c r i t e r i a f o r super- conducting magnets s u i t a b l e f o r whole-body imaging o f humans are presented.

A couple o f magnet design approaches are discussed.

1) I n t r o d u c t i o n

Medical imaging using Nuclear Magnetic Resonance (NMR) has become a l a r g e s c a l e user of superconducting magnets. To some NMR imaging w i l l r e p l a c e a whole h o s t o f medical d i a g n o s t i c techniques; t o o t h e r s NMR imaging i s o n l y somewhat b e t t e r than CT scanning (computerized tomography using x-rays). Whether o r n o t NMR imaging becomes important t o t h e f i e l d o f s u p e r c o n d u c t i v i t y depends on a number o f f a c t o r s , t e c h n i c a l p o l i t i c a l and economic.

There are many i n the medical community who b e l i e v e t h e NMR imaging w i l l replace, f o r many p a t i e n t s , x - r a y CT scans, P o s i t r o n Emission Tomography (PET) scans, Angigrams, Mylograms, many types of nuclear scans and d i g i t a l s u b s t r a c t i o n radiography. I t has become c l e a r t h a t NMR imaging i s p a r t i c u l a r l y good f o r

studying s o f t t i s s u e s such as the brain, l i v e r , and kidneys.l,2 (Some researchers f e e l t h a t NMR scanning w i l l be good i n t h e heart, lungs and o t h e r organs as we1 1 . ) NMR scanning a l s o shows g r e a t promise i n areas such as t h e spine and limb

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841139

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j o i n t s .

Besides good s o f t t i s s u e r e s o l u t i o n , NMR scanning i s non-invasive i n t h a t

c a t h e t e r s and c o n t r a s t media (dyes i n t h e b l o o d stream o r a i r i n t h e s p i n a l cord) a r e n o t needed. Repeated scans o f a p a t i e n t can be taken d u r i n g treatment because t h e r e i s no i o n i z i n g r a d i a t i o n involved i n t h e process. (The e f f e c t s o f d.c. magnetic f i e l d and t h e r.f. a r e expected t o be minimal.) At h i g h magnetic f i e l d s chemical b i o p s i e s can be taken. The d e t e c t i o n o f some diseases, such as m u l t i p l e s c l o r o s i s , has been g r e a t l y improved w i t h NMR. NMR has been used t o measure blood f l o w which can be important i n d e t e c t i n g strokes.3 The

proponents o f NMR expect t h a t NMR imaging w i l l r e v o l u t i o n i z e d i a g n o s t i c medicine.4,5 F i g u r e 1 shows an NMR scan o f one o f t h e a u t h o r ' s head. The nose, eyes, ears and lower b r a i n are c l e a r l y v i s i b l e .

NMR i s n o t w i t h o u t i t s disadvantages. I t can n o t be used on p a t i e n t s w i t h pacemakers o r ferromagnetic body p a r t s (i.e., a magnetic s t a i n l e s s s t e e l p l a t e i n t h e head). NMR imaging i s expensive. To those who see NMR

imaging as s c a r c e l y b e t t e r than CT scanning, t h e c o s t o f NMR scanning i s n o t j u s t i f i e d . The e x t r a cost o f t h e NMR scanners i s n o t o n l y i n the magnet b u t i t a l s o i n v o l v e s t h e space ( f r e e o f o r shielded from ferromagnetic o b j e c t s ) which must be provided f o r the system.

2) Superconducting NMR Magnets vs Conventional and Permanent Magnets

NMR imaging systems are c u r r e n t l y being designed o r b u i l t by approximately 15 f i r m s . The magnets being used o r proposed i n c l u d e 1 ) room temperature r e s i s t i v e magnets w i t h o u t i r o n , 2) room temperature r e s i s t i v e magnet w i t h an i r o n r e t u r n path, 3) permanent magnets which use r a r e e a r t h c o b a l t m a t e r i a l s and/or other ferromagnetic m a t e r i a l s , 4) superconducting solenoids w i t h o u t iron, 5) v a r i o u s superconducting c o i l designs w i t h e i t h e r i r o n r e t u r n paths o r s h i e l d s .

Each o f t h e v a r i o u s types of NMR magnets has i t s proponents and d e t r a c t o r s . The authors b e l i e v e t h a t t h e superconducting magnets can p l a y a dominant r o l e i n the NMR imaging f i e l d . We see t h e f o l l o w i n g advantages f o r superconducting magnets i n NMR imaging: 1 ) Superconducting magnets are t h e o n l y economical way t o go t o f i e l d s above 0.5 t e s l a ; 2) superconducting magnets w i t h f i e l d s above 0.3 t e s l a w i l l be more economical t o operate (The proponents of permanent magnets c l a i m t h e r e i s no o p e r a t i n g cost b u t none o f them c l a i m t o produce much more than 0.3 t e s l a . ) ; 3) superconducting magnets a r e p o t e n t i a l l y l i g h t e r and smaller than e i t h e r permanent o r conventional magnets; and 4) superconducting magnets have good time s t a b i l i t y . (The r e s i s t a n c e o f the superconducting magnet system does n o t change w i t h time. Changes i n i r o n p r o p e r t i e s are o f t e n n o t a f a c t o r i n superconducting systems. Operation i n p e r s i s t a n t mode i s possible, b u t n o t necessary f o r good q u a l i t y imaging.

3) I s A Higher Magnetic F i e l d B e t t e r ?

The l e v e l o f magnetic f i e l d chosen f o r NMR imaging i s dependent on who you t a l k t o and what c o r p o r a t e axe t h e y have t o g r i n d . Most of the imager manufacturers see t h a t i n c r e a s i n g t h e magnetic f i e l d can r e s u l t i n sharper images. Opinion becomes d i v i d e d as soon as one t a l k s about f i e l d s above 0.5 t e s l a . For example Technicare, Dyasonics, P i c k e r and others are marketing a magnetic f i e l d o f 0.5 t e s l a o r l e s s whereas General E l e c t r i c i n t h e United States has been marketing a f i e l d o f 1.5 t e s l a . There are o t h e r groups who have been l o o k i n g a t whole body imagers w i t h f i e l d s as high as 3.0 t e s l a .

The arguments i n f a v o r o f h i g h magnetic f i e l d s can be summarized as f o l l o w s : 1) higher magnetic f i e l d s produce b e t t e r q u a l i t y images, o r 2 ) one can produce images f a s t e r . There i s a t r a d e o f f between image q u a l i t y and p a t i e n t through put. (see F i g u r e 2)6 3) Higher magnetic f i e l d s are necessary f o r doing images

F i g . 1 An NMR image o f t h e human head taken a t t h e l e v e l o f t h e eyes. The upper p a r t o f the b r a i n stem and t h e cerebellem can be c l e a r l y seen.

The photo i s from t h e U n i v e r s i t y o f C a l i f o r n i a a t San Francisco and was made a t a f i e l d o f 0.35T.

Fig. 2

Integration t i m e constant rsvs

( seconds )

Obtainable s p a t i a l r e s o l u t i o n d i s t a n c e d (cube edge f o r an image a s i g n a l t o noise r a t i o o f 100 versus t h e image i n t e g r a t i o n time ( p r o p o r t i o n a l t o imaging t i m e ) curve A f o = 4MHZ, Bo = 0.094T;

curve B fo = 10MHZ, Bo = 0.235T; curve C fo = 20MHz, B, = 0.470T; and curve D f o = 40MHZ, B0 = 0.939T (note: f o i s t h e r.f. frequency which appl i e s f o r Hydrogen)

w i t h constant

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and chemistry i n v o l v i n g elements such as phosphorous 31, sodium 23, potassium 39, calcium 43, and n i t r o g e n 14. ( I t should be noted t h a t a l l o f d i a g n o s t i c NMR imaging being done today uses t h e common i s o t o p e o f hydrogen. )

There are a number o f t e c h n i c a l arguments i n f a v o r o f h i g h f i e l d imaging, b u t t h e r e are a l s o arguments against going t o t o o h i g h a magnetic f i e l d . These arguments are: 1 ) Higher magnetic f i e l d s mean h i g h e r r.f. frequencies which means a s m a l l e r s k i n depth f o r p e n e t r a t i o n o f t h e r.f. waves. Imaging o f t h e trunk may be b e t t e r a t lower f i e l d s than f i e l d s used f o r imaging t h e head. There i s c o n t i n u i n g study on ways t o avoid t h e s k i n depth problem. 2) Higher magnetic f i e l d s mean t h a t t h e f a c i l i t y which houses t h e magnet must be l a r g e r , o r considerable i r o n s h i e l d i n g must be used. The c o s t o f housing a h i g h f i e l d NMR magnet may n o t be worth t h e gain. 3) The c o s t purchasing and c o o l i n g a h i g h f i e l d superconducting magnet i s g r e a t e r than f o r a low f i e l d magnet.

The optimum f i e l d l e v e l f o r NMR imaging magnets has n o t been set. It i s q u i t e probable t h a t t h e f i e l d l e v e l w i l l go up from t h e present 0.35 t o 0.5T used by super-conducting NMR imagers today. The a b i l i t y t o image a t more than one magnetic f i e l d c o u l d e a s i l y change t h e philosophy c u r r e n t l y being employed by t h e

imager manufacturers. This c o u l d r e s u l t i n higher magnetic f i e l d requirements f o r a t l e a s t some kinds o f images.

4) Superconducting Whole Body NMR Imaging Magnet S p e c i f i c a t i o n s

The general s p e c i f i c a t i o n s f o r whole body superconducting NMR imaging magnets are as f o l l o w s :

1 ) The warm diameter r e q u i r e d i s between 1.0 and 1.2 meters. The g r a d i e n t c o i l s and r.f. c o i l s must be housed w i t h i n t h e bore as w e l l as t h e p a t i e n t . 2) Magnet o u t s i d e dimensions and weight are n o t important a t t h i s time. T h i s i s expected t o change as t h e c o s t o f i n s t a l l a t i o n i s f a c t o r e d i n t o t h e c o s t equation.

3) The f i e l d a t t h e center o f t h e magnet should be a t l e a s t 0.3 t e s l a . A number o f NMR imaging magnets w i t h c e n t r a l f i e l d s o f 1.5 t e s l a have been b u i l t . There are other NMR imaging magnets being proposed w i t h f i e l d s as h i g h as 3.OT.

4) The d e s i r e d f i e l d u n i f o r m i t y f o r whole body imaging i s 10 t o 20 p a r t s per m i l l i o n over a sphere which has a diameter of 50 centimeters. For head imaging b e t t e r u n i f o r m i t y i s wanted, b u t over a smaller diameter.

The manufacturers o f NMR imagers are asking f o r higher f i e l d u n i f o r m i t y than they are now g e t t i n g . F i e l d u n i f o r m i t y and f i e l d s t a b i l i t y seem t o be t h e most important requirements demanded by t h e producers o f NMR imaging equipment. I f t h e magnet i s being used f o r NMR spectroscopy and chemistry t h e f i e l d u n i f o r m i t y requirements may be even g r e a t e r .

5) The Design Approaches Employed i n Superconducting Whole Body NMR Imaging Magnets Many of t h e superconducting NMR magnets being b u i l t today use t h e so c a l l e d t h r e e c o i l design where f i e l d u n i f o r m i t y i s g o t t e n by making a t h i n c e n t r a l r e g i o n w i t h t h i c k e r lumped c o i l s a t t h e ends. (There are a number o f f o u r c o i l designs as w e l l b u t these designs u s u a l l y d o n ' t e l i m i n a t e f i e l d e r r o r s as w e l l as t h e t h r e e c o i l designs.) I n a d d i t i o n , superconducting c o i l s are provided f o r f i e l d c o r r e c t i o n by some manufacturers. The method o f winding and assembly o f t h e magnet c o i l s i s considered a t r a d e s e c r e t by those who are i n the magnet business.

F i e l d u n f o r m i t y i s considered t o be v e r y important a t t h i s time (There i s work being done on computer software which can compensate i n p a r t f o r f i e l d i n

homogeni t y problems.) The issues o f f i e l d c o r r e c t i o n and f i e l d s h i e l d i n g go together. Measures taken t o s h i e l d t h e o u t s i d e environment from t h e e f f e c t s o f t h e magnetic f i e l d w i l l c o n t r i b u t e t o f i e l d imhomogenity w i t h i n t h e magnet bore. Shimming o f the c o i l s i s done using pieces o f i r o n or by using a c t i v e c o i l s . The a c t i v e c o i l s can be superconducting o r conventional o r both.

The f i e l d c o r r e c t i o n and s h i e l d i n g i s v e r y much s i t e dependent. On r u r a l s i t e s , one can b u i l d t h e i d e a l magnetic imaging area where space i s t h e s h i e l d i n g ( l i t t l e shimming i s r e q u i r e d on such s i t e s ) . Large h o s p i t a l s i n urban centers such as New York d o n ' t have t h e l u x u r y o f having an i d e a l s i t e . One i s o f t e n faced w i t h s h i e l d i n g p a r t s o f the environment from t h e NMR magnet. (The general p u b l i c can n o t be allowed i n an area w i t h a f i e l d above 0.0005 T because of c a r d i a c pace makers. There are r e s t r i c t i o n s on t h e l o c a t i o n o f CRT screens and computers as w e l l . ) I n an area where s h i e l d i n g i s a problem, t h e magnets must be h e a v i l y shimmed i n order t o meet f i e l d s p e c i f i c a t i o n s i n t h e imaging region.

Most o f t h e whole body superconducting imaging magnets being b u i l t today employ a conventional bath c o o l i n g system. The magnet c o i l o r c o i l s are i n a tank o f l i q u i d helium. The tank i s surrounded by gas cooled s h i e l d s which use b o i l o f f helium from t h e tank. The gas cooled s h i e l d s are surrounded by l i q u i d n i t r o g e n cooled s h i e l d s . The magnets are operated i n a p e r s i s t a n t mode w i t h the e l e c t r i c a l lead r e t r a c t e d i n order t o minimize t h e helium b o i l o f f . The r a t e d helium b o i l o f f r a t e o f these magnets i s around 0.5 l i t e r s per hour ( w i t h t h e leads

r e t r a c t e d ) , and t h e r a t e d l i q u i d n i t r o g e n consumption i s around 2 l i t e r s per hour.

The magnets being b u i l t today use d e l i v e r e d l i q u i d helium and l i q u i d n i t r o g e n a t an estimated annual cost Q - t h e h o s p i t a l or imaging c l i n i c o f 40,000-60,000 U.S. D o l l a r s (these p r i c e s apply i n urban areas o f t h e United S t a t e s ) . The estimated c o s t s do not i n c l u d e c o o l i n g t h e magnet down o r o p e r a t i n g t h e magnet w i t h i t s e l e c t r i c a l leads i n . Operation o f these magnets on a r e f r i g e r a t o r appears t o be economically j u s t i f i e d , p a r t i c u l a r l y o u t s i d e t h e United States.

Several types o f r e f r i g e r a t i o n systems a r e now under study. They range from a r e f r i g e r a t o r which w i l l supply l i q u i d helium and r e f r i g e r a t i o n f o r a l l o f t h e s h i e l d s ( a cooldown c a p a b i l i t y c o u l d be supplied by such a r e f r i g e r a t o r ) t o a small r e f r i g e r a t o r which w i l l j u s t cool t h e gas cooled s h i e l d s between t h e helium p o t and t h e l i q u i d n i t r o g e n pot. ( T h i s would g r e a t l y reduce t h e consumption o f l i q u i d helium.)

A number o f design approaches are being proposed by a number o f magnet companies.

At l e a s t one group proposes a conventional bath cooled magnet i n v e r y low heat leak c r y o s t a t t h a t has t o be f i l l e d w i t h l i q u i d helium o n l y once a year. A t t h e o t h e r end o f t h e scale t h e r e i s a proposed magnet which i s cooled w i t h s u p e r c r i t i c a l helium c i r c u l a t e d through t h e magnet d i r e c t l y from a helium r e f r i g e r a t o r . The NMR imaging magnet o f the f u t u r e probably h a s n ' t been b u i l t y e t . One can expect t h e magnet t o come w i l l be d i f f e r e n t from t h e dominant types o f today. Whether o r n o t superconducting NMR imaging magnets dominate t h e market place i n t h e f u t u r e depends on t h e cryogenic design decisions made today and t h e r e d u c t i o n o f c o s t o f these magnets.

ACKNOWLEDGEMENTS

The authors have had i n f o r m a l discussions w i t h numberous NMR imager manufacturers,

N M R imaging e x p e r i m e n t a l i s t s , and magnet designers. We thank a l l o f these people f o r t h e i r comments.

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REFERENCES

1. S c i e n t i f i c Program f o r t h e S o c i e t y o f Magnetic Resonance i n Medicine, 1 s t Annual Meeting, Boston, Mass, August 1982.

2. S c i e n t i f i c Program f o r t h e S o c i e t y o f Magnetic Resonance i n Medicine, 2nd Annual Meeting, San Francisco, CA, August 1983.

3. J. R. Singer and L. E. Crooks, "Nuclear Magnetic Resonance Blood Flow Measure- ments i n t h e Human Brain" Science, Vol. 221, No. 4611, (August 12, 1983), p. 654.

4. E d i t o r i a l i n t h e Annuals o f I n t e r n a l Medicine, Vol. 98, Number 4, A p r i l 1983.

5. P. A. Bottomley, "Nuclear Magnetic Resonance Beyond Physical Imaging", IEEE Spectrum, February 1983, p. 32.

6. J. M. Libove and J. R. Singer, "Resolution and Signal t o Noise R e l a t i o n s h i p s i n NMR Imaging i n t h e Human Body", J. Phys. E; Sci Instrum, 13 (1980) p. 38.