Biomagnetometry Imaging the Hearts Magnetic Field
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B r Heart J 1991;65:61-2 BRITISH HEART JOURNAL Editorial Biomagnetometry: imaging t h e heart's magnetic field Make a fist with your right hand, le t go slightly, a nd stick your thumb in t h e air. Y ou ar e now illustrating the "right hand rule", which states that if t h e thumb points i n th e direction o f electric current flow t h e curl of t h e fingers indicates t h e direction o f t h e magnetic field. Hans Oersted, i n 1819, was t h e first to show that whenever an electric current flows i t produces a magnetic field a t right angles t o the direction o f th e current. T he problem i n trying t o measure t h e magnetic fields produced by electrically active parts ofthe body such as t h e brain a n d t h e heart i s that these fields ar e exceedingly weak. T h e field strength a t th e surface o f t h e body i s between 10-` T a n d IO-`4 T compared with t h e earth's magnetic field o f about I O - T . Research a n d development by basic scientists undaunted b y these problems have resulted in commercial biomag- netometers that ar e completely non-invasive a n d have excellent temporal resolution a n d reasonably good spatial resolution. I n 1963 Baule a nd McFee published a recording of a human magnetocardiogram that they obtained using t wo large induction coils, each consisting o f 2 million windings, positioned over th e chest.' T h e record looked like a noisy derivative o f t h e electrocardiogram. T h e magnetocar- diogram may b e subjected t o filtering a n d signal averaging b u t t h e information obtained i s n ot as good as that i n a signal averaged electrocardiogram because o f t h e inherently smaller signal. Measurements o f magnetic fields can, however,provide better spatial resolution than electric currents because they ar e n ot distorted b y flow through t h e tissues. There i s significant and varying resistance i n t h e different tissues o f t h e body t o electric current flow b u t t h e body is transparent t o l o w frequency magnetic fields. T h e recording head of a biomagnetometer lies i n a plane that i s normal t o th e surface o f t h e body a nd a signal c a n b e measured providing that t h e dipole source o f t h e magnetic field i s eccentric in th e body a n d i s orientated predominan- t l y tangentially t o t h e body surface. T o make a map ofthe heart's magnetic field more recording sites a re necessary, which means that t h e recording heads need t o b e small. T he problem o f h ow t o g e t measurable signals from small detection coils h a s been overcome b y t h e development o f a lo w noise amplifier known as a superconducting quantum interference device ( or SQUID for short). Within t h e recording head t h e detection coils a n d t h e SQUID a re cooled t o 4 'C above absolute zero ( - 269'C) b y liquid helium i n an evacuated container. T h e amplified signals a re digitised a n d processed b y computer. Isomagnetic contour maps of t h e changing magnetic fields under th e sensor heads c a n b e displayed on a video screen a n d printed out. T h e first human magnetocardiogram obtained with a SQUID was recorded i n 1969.2 The SQUID became available commercially i n 1970 a n d t h e next year the first magnetocardiogram m a p w as recorded.3 From these maps o f magnetic field strength i t i s possible to calculate t h e position of a n electric current source that would produce the magnetic contour patterns observed and, depending on th e model used, this point ma y b e called an equivalent current dipole. T h e position of this electrical dipole m a y b e superimposed over a magnetic resonance image (separately obtained) t o show t h e position o f t h e apparent current source that gives rise t o t h e magnetic field. A t this point i t is worth stating th e distinction between magnetic resonance imaging a nd biomagnet- ometry. Magnetic resonance imaging measures proton magnetic resonance i n a n applied magnetic field a n d gives information that i s anatomically useful. Biomagnetometry gives physiological information about th e magnetic fields generated by t h e individual. Indeed a substantial part of t h e very high cost o f a biomagnetometry system goes to housing t h e non-magnetic equipment i n a shielded room to minimise interference from extraneous magnetic an d elec- trical fields and from mechanical vibration. T h e simple shape of t h e human head makes localisation o f current sources i n t h e brain more straightforward than i n theheart an d t h e technique h a s great potential i n neurology f o r th e localisation o f epileptic foci a n d other electrical events that leave n o trace on computed tomograms o r magnetic resonance images. Computer reconstruction o f t h e source(s) o f t h e magnetic signals from th e heart is complicated b y t h e difficulty o f modelling th e heart's moving geometry an d also because th e magnetic signal a t t h e surface o f th e body i s affected b y current flow between individual cardiac myocytes.4 Nevertheless, with a n isomagnetic m a p i t is possible t o derive a n equivalent current dipole corresponding t o, for example, th e onset o f ventricular depolarisation. T h e spatial accuracy o f t h e technique i s illustrated b y finding that i n patients with ventricular pacemaker th e equivalent current dipole i s located within 1 1 c m o f t h e position of t h e electrode tip.5 Similarly, t h e sources o f extrasystoles6 7 and t h e positions of accessory pathways8 c a n b e plotted a n d good correlation h a s been found between t h e sites o f accessory pathways recorded directly a n d those derived from biomag- netometry. T h e spatial resolution of the technique theoretically approaches that o f magnetic resonance imag- ing, which i s approximately 2 mm. Some a r e available from earlier biomagnetometers consisting o f 1 - 7 recording channels a n d these suggest that t h e accuracy of three dimensional localisation o accessory pathways i s i n t h e range 3 mm-5 c m with these machines. T h e latest genera- tion of machines available n 1990 us e 45 recording units, 3 7 o f which a r e for data collection a n d eight for noise 6 1
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1 B a u l e GM , McFee R . D e t e c t i o n o f t h e m a g n e t i c f i e l d o f t h e h e a r t . Am H e a r tJ 1 9 6 3 ; 6 6 : 9 5 - 6 .
2 C o h e n D , E d e l s a c k A , Zimmerman J E . M a g n e t o c a r d i o g r a m s t a k e n i n s i d e as h i e l d e d r o o m w i t h a s u p e r c o n d u c t i n g p o i n t - c o n t a c t m a g n e t o m e t e r . Ap pP h y s L e t t 1 9 7 0 ; 1 6 : 2 7 8 - 8 0 .
3 C o h e n D , McCaoughan D . M a g n e t o c a r d i o g r a m s a n d t h e i r v a r i a t i o n o v e r t h ec h e s t i n n o r m a l s u b j e c t s . Am J C a r d i o l 1 9 7 2 ; 2 9 : 6 7 8 - 8 5 .
4 W i l l i a m s o n S J , R o m a n i GL , Kaufman L , Modena I , e d s . B i o m a g n e t i s m : a ni n t e r d i s c i p l i n a r y a p p r o a c h . New Y o r k : P l e n u m , 1 9 8 3 .
5 F e n i c i R R , M e l i l l o G , C a p p e l l i A , De L u c a C , M a s s e l l i M. M a g n e t o c a r -d i o g r a p h i c l o c a l i z a t i o n o f a p a c i n g c a t h e t e r . I n : B i o m a g n e t i s m ' 8 9 . T o k y o :T o k y o D e n k i U n i v e r s i t y P r e s s , 1 9 9 0 ( i n p r e s s ) .
6 F e n i c i RR, M a s s e l l i M, L o p e z L , M e l i l l o G . M a g n e t o c a r d i o g r a p h i cl o c a l i z a t i o n o f a r r h y t h m o g e n i c t i s s u e . I n : A t s u m i K , K o t a n i M, Ueno S , .K a t i l a T , W i l l i a m s o n S J , e d s . B i o m a g n e t i s m ' 8 7 . T o k y o : T o k y o D e n k iU n i v e r s i t y P r e s s , 1 9 8 8 : 2 8 2 - 5 .
7 S c h m i t z L , O e f f M, E r n e S N . L o c a l i z a t i o n o f a r r h y t h m o g e n i c a r e a s i n t h ehuman h e a r t . I n : A t s u m i K , K o t a n i M, Ueno S , K a t i l a T , W i l l i a m s o n S J ,e d s . B i o m a g n e t i s m ' 8 7 . T o k y o : T o k y o D e n k i U n i v e r s i t y P r e s s , 1 9 8 8 :2 8 6 - 9 .
8 Nomura M, N a k a y a Y , W a t a n a b e K , e t a l . D e t e c t i on o f a c c e s s o r y p a t h w a y i np a t i e n t s w i t h WPW s y n d r o m e b y m e a n s o f t h e i s o m a g n e t i c map a n d MRI.I n : B i o m a g n e t i s m ' 8 9 . T o k y o : T o k y o D e n k i U n i v e r s i t y P r e s s , 1 9 9 0 ( i np r e s s ) .
9 S c h m i t z L , B r o c k m e i e r K , Trahms L , E r n e S N . M a g n e t o c a r d i o g r a p h y i np a t i e n t s w i t h c a r d i o m y o p a t h y a n d o p e r a t e d c o n g e n i t a l h e a r t d i s e a s e . I n :B i o m a g n e t i s m ' 8 9 . T o k y o : T o k y o D e n k i U n i v e r s i t y P r e s s , 1 9 9 0 ( i n p r e s s ) .
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