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Recent progress in medical ultrasonics
Recent progress in medical ultrasonics C. N. Smyth*
The following r e v i e w was p r e s e n t e d as an i n t r o d u c t o r y talk to the Medical and Biophysical U l t r a s o n i c s s e s s i o n at the ULTRASONICS FOR INDUSTRY 1969 C o n f e r e n c e on 7 Octob e r 1969. T e c h n i q u e s which have b e c o m e e s t a b l i s h e d a r e
ESTABLISHED TECHNIQUES The p a s t y e a r has been one of c o n s o l i d a t i o n f o r the body e c h o - s c a n n i n g t e c h n i q u e s and f o r the Doppler principle h e a r t m o v e m e n t d e t e c t o r and blood flow c o m p a r a t o r . Ideas which w e r e e x p e r i m e n t a l a decade to five y e a r s ago have b e c o m e a c c e p t e d as n e c e s s a r y diagnostic m e a s u r e s and wisely made available by industry, s o m e with a m e a s u r e of government support. Although e c h o - m e t h o d s began p r a c t i c a l l y in the UK b e f o r e 1939 and w e r e s u b s e q u e n t l y developed to an advanced f o r m for n o n - d e s t r u c t i v e t e s t i n g in the UK and in the USA, the b a s i c idea s e e m s to have c o m e f r o m Sokolov in 19291, just 40 y e a r s ago. The e x t e n s i o n of this method f r o m n o n - d e s t r u c t i v e t e s t i n g to m e d i c i n e owes much to I. Donald 2 and D. Howry 3. H o w r y ' s p r e m a t u r e death in 1969 l e a v e s a gap in A m e r i c a n r e s e a r c h in u l t r a s o n i c s which we all mourn. C o n s i d e r i n g the Doppler effect, which has found so many useful and beautifully s i m p l e applications, this old principle owes s o m e t h i n g to the J a p a n e s e t e a m 4 who r e s u s c i t a t e d i n t e r e s t in it, and to R u s h m e r 5 and Dean F r a n k l i n who showed the i m p o r t a n c e of p a r t i c u l a t e m a t t e r in the flow for effective action. The i n s t r u m e n t b e c a m e m e d i c a l l y a c c e p t able with the work of M u r r a y Smyth 6 on the safety of the p r o c e d u r e with low p o w e r c o n t i n u o u s - w a v e u l t r a s o u n d . The E n g l i s h can c o n s i d e r t h e m s e l v e s f o r t u n a t e with the p a t e n t - f r e e split probe Doppler 7 and the Scots with t h e i r p u l s e - e c h o contact p r o b e s c a n n e r . The s i m p l i c i t y and e f f e c t i v e n e s s of this p u l s e - e c h o contact p r o b e , a s m a l l f e a t u r e in a c o m p l i c a t e d a p p a r a t u s , has p r o d u c e d a m e d i c a l l y a c c e p t a b l e i n s t r u m e n t which is finding w o r l d - w i d e u s e . The w o r l d r a t e of body s c a n n e r production is said to e x c e e d 50 a month. New p r o d u c t s include f a s t s c a n n e r s , such a s the S i e m e n s rotating p r o b e and p a r a b o l i c m i r r o r , and S p r o u l e ' s (Videoson) r e c i p r o c a t i n g p r o b e s, behind p l a s t i c windows. T h e r e a r e 3000 Doppler foetal h e a r t beat d e t e c t o r s in routine use, 400 in the UK, and s o m e d o z e n s of blood flow c o m p a r a t o r s and blood velocity r e c o r d e r s . During the two y e a r rebuilding of U n i v e r s i t y College Hospital, pneumatic d r i l l s w e r e working much of the t i m e outside the a n t e - n a t a l clinic. F o r t u n a t e l y the Doptones and Sonicades c a m e along just at this t i m e and a l s o enabled us to c o n f i r m foetal life 10 weeks e a r l i e r than by o t h e r methods; and, p e r h a p s , b e s t of all, it gives the m o t h e r s r e a l p l e a s u r e to h e a r t h e i r b a b y ' s h e a r t beat. The Doppler i n s t r u m e n t for flow o r foetal h e a r t m o n i t o r i n g can be u s e d to note venous flow. Some s p e c i a l l y d e s i g n e d at a c a r r i e r of 6MHz give c l e a r s i g n a l s but do not p e n e t r a t e
* University College Hospital, London WC1. Dr. Smyth is one of the Advisory E d i t o r s of ULTRASONICS.
d e s c r i b e d . Most of the r e c e n t e f f o r t in m e d i c a l u l t r a s o n i c s has been devoted to o v e r c o m i n g difficulties on the road to i m p r o v e d t e c h n i q u e s of visualization.
to the d e e p e r v e i n s and vena cava. The 2MHz i n s t r u m e n t s have g r e a t e r depth of action but a r e l e s s d i s c r i m i n a t i n g between a d j a c e n t v e s s e l s . In a m o s t s i g n i f i c a n t contribution, Evans 9 has shown that the method may p r o v i d e advance i n f o r m a t i o n of venous t h r o m b o s i s , a h a z a r d attendant on prolonged abdominal s u r g e r y and difficult childbirth. If the application p r o v e s r e l i a b l e it could lead to the saving of many lives. The m a r k e t f o r this device m u s t be l a r g e , s i n c e t h e r e should be a need f o r Doppler p r i n c i p l e i n s t r u m e n t s in e v e r y m a t e r nity and v a s c u l a r clinic, in s u r g i c a l and o b s t e t r i c w a r d s , in all r e s u s c i t a t i o n units and on all neonatal r e s u s c i t a t i o n t r o l l e y s and in e v e r y ambulance to indicate the need f o r h u r r y and for d e s t i n a t i o n d e c i s i o n . In Japan, a c c i d e n t ambul a n c e s c a r r y 'A' s c o p e s so that p a t i e n t s with c e r e b r a l m i d line shift can go d i r e c t l y to head s u r g e r y units. It may be that a p o r t a b l e foetal doptone which can be s w i t c h e d on i n t e r m i t t e n t l y should be worn by p r e g n a n t m o t h e r s in the l a s t t h r e e months of p r e g n a n c y so that they can r e s t if the foetal h e a r t shows e x t r a s y s t o l i e s o r a b n o r m a l i t i e s of r a t e . In this way the total of 25,000 c h i l d r e n b o r n with mental r e t a r d a t i o n annualty in the UK might be reduced.
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Now, to t u r n to r e c e n t a d v a n c e s o r a d v a n c i n g s - - f o r t h e r e s e e m s little r e a l l y new and ready f o r use, u n l e s s my e a r has left the ground. Much attention goes on visualization m e t h o d s , m o r e s e n s i t i v e and f a s t e r r e s p o n d i n g d e t e c t o r s of u l t r a s o u n d and holography. Holography a p p e a l s with its t h e o r e t i c a l 3D and l e n s l e s s p e r f e c t i o n . T h e r e a r e p o s s i b i l i t i e s , but t h e r e a r e difficulties too lo. Such difficulties a r e p a r t i c u l a r l y m e d i c a l b e c a u s e the e n e r g y l e v e l s m u s t be kept within safe l i m i t s and b e c a u s e the p a r t u n d e r examination may have to r e m a i n s t a t i o n a r y to a f r a c t i o n of a wavelength of u l t r a s o u n d while the whole p i c t u r e i s r e g i s t e r e d . T h e s e t e c h n i q u e s m u s t show t h e i r p a c e s in e n g i n e e r i n g applications f i r s t . So often one is a s k e d if m e d i c i n e alone can justify the d e v e l o p m e n t of holography? We can only a n s w e r t h e s e questions by saying that if the p i c t u r e can be r e g i s t e r e d i n s t a n t a n e o u s l y all over, that is if scanning s y s t e m s can be avoided, and, if, the incident e n e r g y is a m i c r o - p u l s e of l e s s than l W / c m 2 then a holographic visualization method b e c o m e s p o s s i b l e and may have r e s e a r c h if not c l i n i c a l applications. Visualization i s v e r y valuable when exploring the unknown; when s t r u c t u r e s a r e known v a r i a t i o n s in s i z e , m o v e m e n t of shape can often be r e g i s t e r e d by s i m p l e r t e c h n i q u e s . The B - s c a n method of r e c o r d i n g h e a r t valve m o v e m e n t s is probably m o r e use than a sight of the h e a r t in action. We now have t h e s e t e c h niques in action, f r o m e x t e r n a l p r o b e s . With i n t e r n a l r o t a t ing p r o b e s , Kossoff 1 ~ and his c o l l e a g u e s at Campaign, Illinois ULTRASONICS J a n u a r y 1970
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and D a l l a s , T e x a s , h a v e shown u s the whole c y c l e of v e n t r i c u l a r c o n t r a c t i o n a s a n i n t r a - v e n t r i c u l a r o b s e r v e r would s e e it. In a n o t h e r c o n t e x t P e r r o n e a u 12 f r o m F r a n c e and F l a h e r t y ~3 of C h i c a g o h a v e s h o w n t h a t p u l s e d D o p p l e r m e t h o d s c a n d e t e r m i n e the plane at w h i c h m o v e m e n t s o c c u r and t h u s plot h e a r t c o n t o u r s and blood flow v e l o c i t y p r o f i l e s . Light at the M e d i c a l R e s e a r c h C e n t r e , H a m p s t e a d , London h a s s h o w n the p o s s i b i l i t i e s of a s s e s s i n g t h e a o r t i c flow, and h e n c e c a r d i a c output, by D o p p l e r s h i f t m e a s u r e m e n t s t h r o u g h the c h e s t wall.
HOLOGRAPHY R e v e r t i n g to v i s u a l i z a t i o n t e c h n i q u e s in which the i m a g e i s o b t a i n e d and p o r t r a y e d s p e e d i l y so t h a t m o v e m e n t s of t i s s u e s c a n be followed a s with the Sokolov 14, o r the w r i t e r ' s 15, o r J a c o b s 16 u l t r a s o u n d c a m e r a the r e s e a r c h e r s a r e c u r r e n t l y divided b e t w e e n t h o s e who u s e l e n s e s o r s h a d o w s and the e x p o n e n t s of h o l o g r a p h y . H o l o g r a p h y h a s d a z z l e d all but the i n v e n t o r and p i o n e e r s of the t e c h n i q u e . M e d i c a l l y it b e c a m e f a s c i n a t i n g when s o m e o n e s a i d it could be u s e d f o r s e e i n g t h r o u g h f r o s t e d g l a s s , and f o r g o t to add t h a t one had to c a p t u r e the p a n e of g l a s s to r e c o n s t r u c t the i m a g e . F u r t h e r the d i s p a r i t y b e t w e e n the w a v e l e n g t h of u l t r a s o u n d and of light, 500 to 1, i s so g r e a t t h a t the 3D a d v a n t a g e c a n n o t be c a p t u r e d with o r d i n a r y m a t e r i a l s . U n l e s s the l i f e - s i z e u l t r a s o n i c i m a g e s c a n be r e d u c e d in s i z e by the w a v e l e n g t h r a t i o (which would m a k e a 2 5 c m 2 p i c t u r e 1 / 4 0 0 m m 2, and n e e d and e m u l s i o n and p r o c e s s i n g u n i f o r m i t y to a f r a c t i o n of an o p t i c a l w a v e l e n g t h ) and t h e n view it t h r o u g h a high p o w e r m i c r o s c o p e (the e m u l s i o n being g r a i n l e s s ) the p r o c e s s f a l l s down. T h e b e s t h o l o g r a m s a r e o b t a i n e d by a m o d e s t r e d u c t i o n in s i z e (10 to 1) and n e g l e c t of 3D, b e c a u s e u n d e r t h e s e c i r c u m s t a n c e s the object a p p e a r s to be f a r away and the v i e w i n g a p e r t u r e too s m a l l f o r a s i d e view. The h o l o g r a m c o m p r i s e s a p a t t e r n of wavy c l o s e l y s p a c e d l i n e s . T h e r e s o l u t i o n i s of the o r d e r of line s p a c i n g and i s L,/'(sin a) w h e r e ' L ' i s the w a v e l e n g t h and ' a ' the a n g u l a r a p e r t u r e . T h e s a m e f o r m u l a g i v e s the l i m i t i n g r e s o l u t i o n f o r a d i r e c t l e n s i m a g i n g s y s t e m . To o b t a i n a r e a s o n a b l e h o l o g r a m at l e a s t 200 l i n e s o r f r i n g e s a r e n e c e s s a r y and in the r e d u c e d i m a g e f o r viewing in a h e l i u m - n e o n l a s e r t h e r e m u s t be 10 f r i n g e s / m m if the i m a g e i s to d i v e r g e 1 ° f r o m the b e a m a x i s . Such h o l o g r a m s h a v e a depth of f o c u s of about l c m and p l a n e s 5 c m a p a r t c a n be s e p a r a t e d t o m o graphically. If h o l o g r a p h y i s to p r o g r e s s t h e n the whole p i c t u r e m u s t be r e g i s t e r e d at once f r o m a s i n g l e p u l s e e c h o of s o u n d and new t y p e s of c a m e r a a r e n e e d e d . It m a y be that the M u l l a r d c h a n n e l m u l t i p l i e r m a t r i x 12 will find a n a p p l i c a t i o n h e r e . T h i s could give s t r e n g t h to a s o u n d c a m e r a s t r u c t u r e but the a p e r t u r e would s t i l l be l i m i t e d to 24 ° if s h e a r w a v e s w e r e not to c o n f u s e the p i c t u r e . It h a s b e e n s u g g e s t e d t h a t the C h a r l o t t e plate ~7 m i g h t be u s e d a n d h a v e a s e p a r a t e point t r a n s d u c e r on the end of e v e r y w i r e . T h e R u s s i a n s a r e s a i d to h a v e p r o d u c e d an e n o r m o u s tube on t h i s p r i n c i p l e . A n o t h e r m e t h o d u n d e r i n v e s t i g a t i o n in A m e r i c a i s the u s e of f l u o r e s c e n t p o w d e r s on p i e z o - e l e c t r i c p l a t e s . T h e high f r e q u e n c y e l e c t r o l u m i n e s c e n c e p r o v i d e s the i m a g e o r h o l o g r a m 18. Oa-l~e holography T h e r e a r e two o t h e r m e t h o d s of h o l o g r a p h y at p r e s e n t u n d e r d i s c u s s i o n f o r 'on l i n e ' , o r n e a r 'on l i n e ' but n e i t h e r s e e m s u i t a b l e for p u l s e work, and, f o r the living s u b j e c t , s u r e l y p u l s e w o r k it would have to b e ? One of t h e s e i s the liquid c r y s t a l , now o b t a i n a b l e c o m m e r c i a l l y (but not l e s s t h a n 100ft 2 at a t i m e 19. T h e s e c r y s t a l s a r e m i c r o - e n c a p s u l a t e d in 5-30~ s p h e r e s and m o u n t e d behind a t r a n s p a r e n t s h e e t and on a d a r k e n e r g y a b s o r b i n g s h e e t , in s h e e t s about q u a r t o s i z e . If kept at c o n s t a n t t e m p e r a t u r e n e a r the t r a n s i t i o n point, 32°C, r a d i a n t h e a t o r u l t r a s o u n d will c h a n g e the c o l o u r . S e n s i t i v i t y n a t u r a l l y i m p r o v e s with the t e m p e r a t u r e s t a b i l i t y , f o r the c r y s t a l s a r e v e r y p u r e . To e x c e e d the s e n s i t i v i t y of the Sokolov c a m e r a the t e m p e r a t u r e would have to be i m p o s s i b l y c l o s e l y c o n t r o l l e d , s a y 10 4°C! Work is being
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ULTRASONICS J a n u a r y 1970
u n d e r t a k e n to o v e r c o m e t h i s d i s a d v a n t a g e by m o u n t i n g the liquid c r y s t a l s b e t w e e n c o n d u c t i n g l a y e r s and p o l a r i s i n g t h e m e l e c t r i c a l l y . The a c t i o n i s of c o u r s e slow a s a t e m p e r a t u r e c h a n g e h a s to be p r o d u c e d . F o r i n d u s t r i a l h i g h p o w e r a p p l i c a t i o n s , the p r o s p e c t s a r e good, s i n c e t h e s e c r y s t a l s c a n be p e r s u a d e d to r e s o l v e 20 l i n e s to the r a m . F i n a l l y t h e r e i s m u c h i n t e r e s t in the o n - l i n e h o l o g r a p h y f r o m l i q u i d - g a s i n t e r f a c e d e f o r m a t i o n . T h e i m a g e o r holog r a m i s p r o d u c e d by the i m p a c t of the s o u n d on a liquid s u r f a c e f r o m below 2o. If the t a n k i s l a r g e , the s u r f a c e in a s m a l l c e l l i s i n s p e c t e d to p r e v e n t g r o s s wave m o v e m e n t s and the c e l l c a n be filled with a v o l a t i l e liquid with a low s u r f a c e t e n s i o n and s o r e s p o n d to h i g h e r s p a c i a l f r e q u e n c i e s . T h e s u r f a c e i s d i r e c t l y i l l u m i n a t e d with l a s e r light and the r e s u l t i n g d i f f r a c t i o n i n s p e c t e d with a t e l e s c o p e . A p p a r e n t l y the r e s u l t s a r e f a i r and n e a r i n s t a n t a n e o u s (the p r o c e s s i s n o n - l i n e a r e n o u g h to give a good q u a d r a t i c c o m p o n e n t ) . U n f o r t u n a t e l y the m e t h o d i s not s u i t a b l e f o r p u l s e w o r k a s the s u r f a c e d e f o r m a t i o n t a k e s t i m e to build up and the a v e r a g e d e f o r m a t i o n on p u l s e o p e r a t i o n would be n e a r z e r o . To date, the b e s t p i c t u r e s and the b e s t d i s c r i m i n a t i o n in depth by t h e s e t e c h n i q u e s h a s b e e n o b t a i n e d by the u s e of u l t r a s o n i c l e n s e s , t h e i r o p e r a t i o n with p u l s e d s o u n d i s not p o o r by p r e s e n t day r e s e a r c h s t a n d a r d s . What the f u t u r e will p r o v i d e f r o m l e n s e s o r h o l o g r a p h y f o r new d i a g n o s t i c i n s t r u m e n t s r e m a i n s to be s e e n . In the State U n i v e r s i t y of Ohio, which quite i n c i d e n t a l l y h a s 4 0 , 0 0 0 s t u d e n t s , and t h e r e f o r e no s h o r t a g e of new i d e a s , a g r a n t of $ 3 0 , 0 0 0 f r o m the US Navy to P r o f e s s o r R. C. M c M a s t e r and Hugh M a i l e r is to d e v e l o p a s o u n d s e n s i t i v e e m u l s i o n c o m p a r a b l e to light p h o t o g r a p h y 21. The p r o p o s a l i s to m a k e a f i l m e m u l s i o n of dye in f r a g i l e c a p s u l e s which will b u r s t u n d e r the a c c e l e r a t i o n s e n c o u n t e r e d and r e a c t with the m a t r i x to p r o d u c e a s t a i n which i s v i s i b l e . To i n c r e a s e the e f f i c i e n c y of the p r o c e s s the m i c r o c a p s u l e s m a y c o n t a i n an e x p l o s i v e s u c h a s a f u l m i n a t e o r l e a d azide; to quote the a r t i c l e m e n t i o n e d 22. At l a s t we h a v e a p r o p o s e d m e d i c a l u s e f o r the m a t e r i a l s of war! When we c o n s i d e r the e n c a p s u l a t i o n of liquid c r y s t a l so a m a z i n g l y a c h i e v e d by NCR, p e r h a p s the f u t u r e of t h i s p r o p o s a l i s not s o f a r away. May t h i s o r s o m e new t h o u g h t p r o v o k e d by t h i s s h o r t s u m m a r y l e a d to new p r o g r e s s in this p r o m i s i n g subject. T h i s s h o r t r e v i e w would not be a d e q u a t e without r e f e r e n c e to J a c o b s 22 who a d a p t e d a c o l o u r t e l e v i s i o n r e c e i v e r to w o r k with the Sokolov u l t r a s o n i c c a m e r a and r e v e a l the p h a s e of the s o u n d d i f f r a c t i o n p a t t e r n s . T h i s w o r k h a s b e e n r e p e a t e d in E n g l a n d at 4 . 4 M H z , the c o l o u r t e l e v i s i o n s u b c a r r i e r f r e q u e n c y , at the City U n i v e r s i t y 23. The i m a g e s are startlingly pretty, they are p e r h a p s m o r e informative t h a n b l a c k and white i m a g e s . T h e v a l u e of t h i s t e c h n i q u e is s t i l l u n d e r r e v i e w . One a p p l i c a t i o n is to the d e s i g n of acoustic lenses. Like all i n t r o d u c t i o n s t h i s one i s i n c o m p l e t e , m a n y new i n v e n t i o n s h a v e b e e n o m i t t e d f r o m i g n o r a n c e o r l a c k of space.
REFERENCES 1
Sokolov, S. Ya. 'On the q u e s t i o n of the p r o p a g a t i o n of high f r e q u e n c y a c o u s t i c v i b r a t i o n s in v a r i o u s b o d i e s ' , Elekt N a c h r T e c h , Vol 6 (1929) p454
2
Donald, I., Mac V i c a r , J. and Brown, T . G . ' I n v e s t i g a t i o n of a b d o m i n a l m a s s e s by p u l s e d u l t r a s o u n d ' , L a n c e t , Vol 1 (1958) p1188
3
Howry, D. ' T e c h n i q u e s u s e d in u l t r a s o n i c v i s u a l i s a t i o n of soft t i s s u e s t r u c t u r e s of the body', IRE C o n v e n t i o n R e c o r d 1955, p a r t 9, p75
4
S a t o m u r a , S. and Kaneto, Z. ' U l t r a s o n i c blood r h e o g r a p h ' , P r o c e e d i n g s of 3rd I n t e r n a t i o n a l C o n f e r e n c e on M e d i c a l E l e c t r o n i c s , p254 (IEE, London, 1960)
5
Rushmer, R . F . , B a k e r , D.W. and Stegall, H.F. 'Nondestructive testing of human cardiovascular responses', Digest of 6th International Conference on Medical Electronics ~ Biological Engineering (Tokyo 1965) pp 70-71
13
Flaherty, J . J . a n d Strauts, E.J. 'Ultrasonic pulse Doppler instrumentation', ibid
14
Sokolov, S. Ya., USSR Patent 49 (31 August 1936)
Rushmer, R. F., Stegall, H. F. and Strandness, E. D. 'Horizons of bioinstrumentation',Digest of 7th International Conference on Medical & Biological Engineering (Stockholm 1967)
15
Smyth, C. N., Poynton, F. Y. and Sayers, J . F . 'Ultrasound image camera', Proceedings of IEE, Vol 110 (1966) pp16-28; Journal of IEE, Part E, Vol 9 (1966) p4030; and ULTRASONICS, Vol 4 (1966) pp15-20
6
Smyth, Murray G., J n r 'Animal toxicity studies with ultrasound', Proceedings of 1st International Conference on Diagnostic Ultrasound (Pittsburg 1965) p296
16
Jacobs, J . E . 'Ultrasonic visualization system', US Patent 3,236,944 (1962)
7
Smyth, C.N. 'Sound ranging apparatus', British Patent 702,534 (1950)
17
Charlotte, H. F. 'A direct reading cathode ray tube', Electronic Engineering, Vol 29, No 345 (1957) p373
8
Sproule, D. 'An ultrasonic imaging system for flaw detection', ULTRASONICS FOR INDUSTRY 1969 Conference Papers, pp56-59 (Iliffe Science and Technology Publications Ltd, Guildford, UK, 1969)
18
Poynton, F. Y., (1956) 'Ultrasonic camera with photoluminescence', British Patent 801,985 (1956)
19
National Cash Register Co., Dayton, Ohio, Capsular Products Division Technical Publication (1969) 'Encapsulated liquid crystals'
9
Evans, D. S. and Cockett, F.B. 'Diagnosis of deep vein thrombosis with an ultrasonic doppler technique', British Medical Journal, P a r t B Vol ii (1969) p 802-804
20
10
Ed Metherall, et als 'Acoustical Holography'. (Plenum P r e s s NY 1969)
Metherall et al (Ed) 'Acoustical Holography', (Plenum Press, NY 1969)
21
11
Eggleton, R. C., Townsend, C., Kossoff, G., Herrick, Julia and Hunt, R. 'Computerised ultrasonic visualization of dynamic ventricular configurations', Proceedings of 8th International Conference Medical & Biological Engineering (Chicago 1969)
McMaster, R. C. and Mailer, H., (1969) News in Engineering; Publication of the Engineering Division of the State University of Ohio, Columbus, Ohio (January 1969) p21
22
Jacobs, J.E. 'Colour representation of acoustic phase', Materials Evaluation (August 1968)p158
23
Smyth, C. N., 2nd International Conference on Medical Physics (1969). In press. Ed. Webster, Massachusetts General Hospital, Boston, Massachusetts, USA
12
Peronneau, P. A. mud Leger, F. 'Doppler ultrasonic pulsed blood flowmeter', ibid
ULTRASONICS January 1970
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The following r e v i e w was p r e s e n t e d as an i n t r o d u c t o r y talk to the Medical and Biophysical U l t r a s o n i c s s e s s i o n at the ULTRASONICS FOR INDUSTRY 1969 C o n f e r e n c e on 7 Octob e r 1969. T e c h n i q u e s which have b e c o m e e s t a b l i s h e d a r e
ESTABLISHED TECHNIQUES The p a s t y e a r has been one of c o n s o l i d a t i o n f o r the body e c h o - s c a n n i n g t e c h n i q u e s and f o r the Doppler principle h e a r t m o v e m e n t d e t e c t o r and blood flow c o m p a r a t o r . Ideas which w e r e e x p e r i m e n t a l a decade to five y e a r s ago have b e c o m e a c c e p t e d as n e c e s s a r y diagnostic m e a s u r e s and wisely made available by industry, s o m e with a m e a s u r e of government support. Although e c h o - m e t h o d s began p r a c t i c a l l y in the UK b e f o r e 1939 and w e r e s u b s e q u e n t l y developed to an advanced f o r m for n o n - d e s t r u c t i v e t e s t i n g in the UK and in the USA, the b a s i c idea s e e m s to have c o m e f r o m Sokolov in 19291, just 40 y e a r s ago. The e x t e n s i o n of this method f r o m n o n - d e s t r u c t i v e t e s t i n g to m e d i c i n e owes much to I. Donald 2 and D. Howry 3. H o w r y ' s p r e m a t u r e death in 1969 l e a v e s a gap in A m e r i c a n r e s e a r c h in u l t r a s o n i c s which we all mourn. C o n s i d e r i n g the Doppler effect, which has found so many useful and beautifully s i m p l e applications, this old principle owes s o m e t h i n g to the J a p a n e s e t e a m 4 who r e s u s c i t a t e d i n t e r e s t in it, and to R u s h m e r 5 and Dean F r a n k l i n who showed the i m p o r t a n c e of p a r t i c u l a t e m a t t e r in the flow for effective action. The i n s t r u m e n t b e c a m e m e d i c a l l y a c c e p t able with the work of M u r r a y Smyth 6 on the safety of the p r o c e d u r e with low p o w e r c o n t i n u o u s - w a v e u l t r a s o u n d . The E n g l i s h can c o n s i d e r t h e m s e l v e s f o r t u n a t e with the p a t e n t - f r e e split probe Doppler 7 and the Scots with t h e i r p u l s e - e c h o contact p r o b e s c a n n e r . The s i m p l i c i t y and e f f e c t i v e n e s s of this p u l s e - e c h o contact p r o b e , a s m a l l f e a t u r e in a c o m p l i c a t e d a p p a r a t u s , has p r o d u c e d a m e d i c a l l y a c c e p t a b l e i n s t r u m e n t which is finding w o r l d - w i d e u s e . The w o r l d r a t e of body s c a n n e r production is said to e x c e e d 50 a month. New p r o d u c t s include f a s t s c a n n e r s , such a s the S i e m e n s rotating p r o b e and p a r a b o l i c m i r r o r , and S p r o u l e ' s (Videoson) r e c i p r o c a t i n g p r o b e s, behind p l a s t i c windows. T h e r e a r e 3000 Doppler foetal h e a r t beat d e t e c t o r s in routine use, 400 in the UK, and s o m e d o z e n s of blood flow c o m p a r a t o r s and blood velocity r e c o r d e r s . During the two y e a r rebuilding of U n i v e r s i t y College Hospital, pneumatic d r i l l s w e r e working much of the t i m e outside the a n t e - n a t a l clinic. F o r t u n a t e l y the Doptones and Sonicades c a m e along just at this t i m e and a l s o enabled us to c o n f i r m foetal life 10 weeks e a r l i e r than by o t h e r methods; and, p e r h a p s , b e s t of all, it gives the m o t h e r s r e a l p l e a s u r e to h e a r t h e i r b a b y ' s h e a r t beat. The Doppler i n s t r u m e n t for flow o r foetal h e a r t m o n i t o r i n g can be u s e d to note venous flow. Some s p e c i a l l y d e s i g n e d at a c a r r i e r of 6MHz give c l e a r s i g n a l s but do not p e n e t r a t e
* University College Hospital, London WC1. Dr. Smyth is one of the Advisory E d i t o r s of ULTRASONICS.
d e s c r i b e d . Most of the r e c e n t e f f o r t in m e d i c a l u l t r a s o n i c s has been devoted to o v e r c o m i n g difficulties on the road to i m p r o v e d t e c h n i q u e s of visualization.
to the d e e p e r v e i n s and vena cava. The 2MHz i n s t r u m e n t s have g r e a t e r depth of action but a r e l e s s d i s c r i m i n a t i n g between a d j a c e n t v e s s e l s . In a m o s t s i g n i f i c a n t contribution, Evans 9 has shown that the method may p r o v i d e advance i n f o r m a t i o n of venous t h r o m b o s i s , a h a z a r d attendant on prolonged abdominal s u r g e r y and difficult childbirth. If the application p r o v e s r e l i a b l e it could lead to the saving of many lives. The m a r k e t f o r this device m u s t be l a r g e , s i n c e t h e r e should be a need f o r Doppler p r i n c i p l e i n s t r u m e n t s in e v e r y m a t e r nity and v a s c u l a r clinic, in s u r g i c a l and o b s t e t r i c w a r d s , in all r e s u s c i t a t i o n units and on all neonatal r e s u s c i t a t i o n t r o l l e y s and in e v e r y ambulance to indicate the need f o r h u r r y and for d e s t i n a t i o n d e c i s i o n . In Japan, a c c i d e n t ambul a n c e s c a r r y 'A' s c o p e s so that p a t i e n t s with c e r e b r a l m i d line shift can go d i r e c t l y to head s u r g e r y units. It may be that a p o r t a b l e foetal doptone which can be s w i t c h e d on i n t e r m i t t e n t l y should be worn by p r e g n a n t m o t h e r s in the l a s t t h r e e months of p r e g n a n c y so that they can r e s t if the foetal h e a r t shows e x t r a s y s t o l i e s o r a b n o r m a l i t i e s of r a t e . In this way the total of 25,000 c h i l d r e n b o r n with mental r e t a r d a t i o n annualty in the UK might be reduced.
CURRENT
RESEARCH
AND
DEVELOPMENT
Now, to t u r n to r e c e n t a d v a n c e s o r a d v a n c i n g s - - f o r t h e r e s e e m s little r e a l l y new and ready f o r use, u n l e s s my e a r has left the ground. Much attention goes on visualization m e t h o d s , m o r e s e n s i t i v e and f a s t e r r e s p o n d i n g d e t e c t o r s of u l t r a s o u n d and holography. Holography a p p e a l s with its t h e o r e t i c a l 3D and l e n s l e s s p e r f e c t i o n . T h e r e a r e p o s s i b i l i t i e s , but t h e r e a r e difficulties too lo. Such difficulties a r e p a r t i c u l a r l y m e d i c a l b e c a u s e the e n e r g y l e v e l s m u s t be kept within safe l i m i t s and b e c a u s e the p a r t u n d e r examination may have to r e m a i n s t a t i o n a r y to a f r a c t i o n of a wavelength of u l t r a s o u n d while the whole p i c t u r e i s r e g i s t e r e d . T h e s e t e c h n i q u e s m u s t show t h e i r p a c e s in e n g i n e e r i n g applications f i r s t . So often one is a s k e d if m e d i c i n e alone can justify the d e v e l o p m e n t of holography? We can only a n s w e r t h e s e questions by saying that if the p i c t u r e can be r e g i s t e r e d i n s t a n t a n e o u s l y all over, that is if scanning s y s t e m s can be avoided, and, if, the incident e n e r g y is a m i c r o - p u l s e of l e s s than l W / c m 2 then a holographic visualization method b e c o m e s p o s s i b l e and may have r e s e a r c h if not c l i n i c a l applications. Visualization i s v e r y valuable when exploring the unknown; when s t r u c t u r e s a r e known v a r i a t i o n s in s i z e , m o v e m e n t of shape can often be r e g i s t e r e d by s i m p l e r t e c h n i q u e s . The B - s c a n method of r e c o r d i n g h e a r t valve m o v e m e n t s is probably m o r e use than a sight of the h e a r t in action. We now have t h e s e t e c h niques in action, f r o m e x t e r n a l p r o b e s . With i n t e r n a l r o t a t ing p r o b e s , Kossoff 1 ~ and his c o l l e a g u e s at Campaign, Illinois ULTRASONICS J a n u a r y 1970
31
and D a l l a s , T e x a s , h a v e shown u s the whole c y c l e of v e n t r i c u l a r c o n t r a c t i o n a s a n i n t r a - v e n t r i c u l a r o b s e r v e r would s e e it. In a n o t h e r c o n t e x t P e r r o n e a u 12 f r o m F r a n c e and F l a h e r t y ~3 of C h i c a g o h a v e s h o w n t h a t p u l s e d D o p p l e r m e t h o d s c a n d e t e r m i n e the plane at w h i c h m o v e m e n t s o c c u r and t h u s plot h e a r t c o n t o u r s and blood flow v e l o c i t y p r o f i l e s . Light at the M e d i c a l R e s e a r c h C e n t r e , H a m p s t e a d , London h a s s h o w n the p o s s i b i l i t i e s of a s s e s s i n g t h e a o r t i c flow, and h e n c e c a r d i a c output, by D o p p l e r s h i f t m e a s u r e m e n t s t h r o u g h the c h e s t wall.
HOLOGRAPHY R e v e r t i n g to v i s u a l i z a t i o n t e c h n i q u e s in which the i m a g e i s o b t a i n e d and p o r t r a y e d s p e e d i l y so t h a t m o v e m e n t s of t i s s u e s c a n be followed a s with the Sokolov 14, o r the w r i t e r ' s 15, o r J a c o b s 16 u l t r a s o u n d c a m e r a the r e s e a r c h e r s a r e c u r r e n t l y divided b e t w e e n t h o s e who u s e l e n s e s o r s h a d o w s and the e x p o n e n t s of h o l o g r a p h y . H o l o g r a p h y h a s d a z z l e d all but the i n v e n t o r and p i o n e e r s of the t e c h n i q u e . M e d i c a l l y it b e c a m e f a s c i n a t i n g when s o m e o n e s a i d it could be u s e d f o r s e e i n g t h r o u g h f r o s t e d g l a s s , and f o r g o t to add t h a t one had to c a p t u r e the p a n e of g l a s s to r e c o n s t r u c t the i m a g e . F u r t h e r the d i s p a r i t y b e t w e e n the w a v e l e n g t h of u l t r a s o u n d and of light, 500 to 1, i s so g r e a t t h a t the 3D a d v a n t a g e c a n n o t be c a p t u r e d with o r d i n a r y m a t e r i a l s . U n l e s s the l i f e - s i z e u l t r a s o n i c i m a g e s c a n be r e d u c e d in s i z e by the w a v e l e n g t h r a t i o (which would m a k e a 2 5 c m 2 p i c t u r e 1 / 4 0 0 m m 2, and n e e d and e m u l s i o n and p r o c e s s i n g u n i f o r m i t y to a f r a c t i o n of an o p t i c a l w a v e l e n g t h ) and t h e n view it t h r o u g h a high p o w e r m i c r o s c o p e (the e m u l s i o n being g r a i n l e s s ) the p r o c e s s f a l l s down. T h e b e s t h o l o g r a m s a r e o b t a i n e d by a m o d e s t r e d u c t i o n in s i z e (10 to 1) and n e g l e c t of 3D, b e c a u s e u n d e r t h e s e c i r c u m s t a n c e s the object a p p e a r s to be f a r away and the v i e w i n g a p e r t u r e too s m a l l f o r a s i d e view. The h o l o g r a m c o m p r i s e s a p a t t e r n of wavy c l o s e l y s p a c e d l i n e s . T h e r e s o l u t i o n i s of the o r d e r of line s p a c i n g and i s L,/'(sin a) w h e r e ' L ' i s the w a v e l e n g t h and ' a ' the a n g u l a r a p e r t u r e . T h e s a m e f o r m u l a g i v e s the l i m i t i n g r e s o l u t i o n f o r a d i r e c t l e n s i m a g i n g s y s t e m . To o b t a i n a r e a s o n a b l e h o l o g r a m at l e a s t 200 l i n e s o r f r i n g e s a r e n e c e s s a r y and in the r e d u c e d i m a g e f o r viewing in a h e l i u m - n e o n l a s e r t h e r e m u s t be 10 f r i n g e s / m m if the i m a g e i s to d i v e r g e 1 ° f r o m the b e a m a x i s . Such h o l o g r a m s h a v e a depth of f o c u s of about l c m and p l a n e s 5 c m a p a r t c a n be s e p a r a t e d t o m o graphically. If h o l o g r a p h y i s to p r o g r e s s t h e n the whole p i c t u r e m u s t be r e g i s t e r e d at once f r o m a s i n g l e p u l s e e c h o of s o u n d and new t y p e s of c a m e r a a r e n e e d e d . It m a y be that the M u l l a r d c h a n n e l m u l t i p l i e r m a t r i x 12 will find a n a p p l i c a t i o n h e r e . T h i s could give s t r e n g t h to a s o u n d c a m e r a s t r u c t u r e but the a p e r t u r e would s t i l l be l i m i t e d to 24 ° if s h e a r w a v e s w e r e not to c o n f u s e the p i c t u r e . It h a s b e e n s u g g e s t e d t h a t the C h a r l o t t e plate ~7 m i g h t be u s e d a n d h a v e a s e p a r a t e point t r a n s d u c e r on the end of e v e r y w i r e . T h e R u s s i a n s a r e s a i d to h a v e p r o d u c e d an e n o r m o u s tube on t h i s p r i n c i p l e . A n o t h e r m e t h o d u n d e r i n v e s t i g a t i o n in A m e r i c a i s the u s e of f l u o r e s c e n t p o w d e r s on p i e z o - e l e c t r i c p l a t e s . T h e high f r e q u e n c y e l e c t r o l u m i n e s c e n c e p r o v i d e s the i m a g e o r h o l o g r a m 18. Oa-l~e holography T h e r e a r e two o t h e r m e t h o d s of h o l o g r a p h y at p r e s e n t u n d e r d i s c u s s i o n f o r 'on l i n e ' , o r n e a r 'on l i n e ' but n e i t h e r s e e m s u i t a b l e for p u l s e work, and, f o r the living s u b j e c t , s u r e l y p u l s e w o r k it would have to b e ? One of t h e s e i s the liquid c r y s t a l , now o b t a i n a b l e c o m m e r c i a l l y (but not l e s s t h a n 100ft 2 at a t i m e 19. T h e s e c r y s t a l s a r e m i c r o - e n c a p s u l a t e d in 5-30~ s p h e r e s and m o u n t e d behind a t r a n s p a r e n t s h e e t and on a d a r k e n e r g y a b s o r b i n g s h e e t , in s h e e t s about q u a r t o s i z e . If kept at c o n s t a n t t e m p e r a t u r e n e a r the t r a n s i t i o n point, 32°C, r a d i a n t h e a t o r u l t r a s o u n d will c h a n g e the c o l o u r . S e n s i t i v i t y n a t u r a l l y i m p r o v e s with the t e m p e r a t u r e s t a b i l i t y , f o r the c r y s t a l s a r e v e r y p u r e . To e x c e e d the s e n s i t i v i t y of the Sokolov c a m e r a the t e m p e r a t u r e would have to be i m p o s s i b l y c l o s e l y c o n t r o l l e d , s a y 10 4°C! Work is being
32
ULTRASONICS J a n u a r y 1970
u n d e r t a k e n to o v e r c o m e t h i s d i s a d v a n t a g e by m o u n t i n g the liquid c r y s t a l s b e t w e e n c o n d u c t i n g l a y e r s and p o l a r i s i n g t h e m e l e c t r i c a l l y . The a c t i o n i s of c o u r s e slow a s a t e m p e r a t u r e c h a n g e h a s to be p r o d u c e d . F o r i n d u s t r i a l h i g h p o w e r a p p l i c a t i o n s , the p r o s p e c t s a r e good, s i n c e t h e s e c r y s t a l s c a n be p e r s u a d e d to r e s o l v e 20 l i n e s to the r a m . F i n a l l y t h e r e i s m u c h i n t e r e s t in the o n - l i n e h o l o g r a p h y f r o m l i q u i d - g a s i n t e r f a c e d e f o r m a t i o n . T h e i m a g e o r holog r a m i s p r o d u c e d by the i m p a c t of the s o u n d on a liquid s u r f a c e f r o m below 2o. If the t a n k i s l a r g e , the s u r f a c e in a s m a l l c e l l i s i n s p e c t e d to p r e v e n t g r o s s wave m o v e m e n t s and the c e l l c a n be filled with a v o l a t i l e liquid with a low s u r f a c e t e n s i o n and s o r e s p o n d to h i g h e r s p a c i a l f r e q u e n c i e s . T h e s u r f a c e i s d i r e c t l y i l l u m i n a t e d with l a s e r light and the r e s u l t i n g d i f f r a c t i o n i n s p e c t e d with a t e l e s c o p e . A p p a r e n t l y the r e s u l t s a r e f a i r and n e a r i n s t a n t a n e o u s (the p r o c e s s i s n o n - l i n e a r e n o u g h to give a good q u a d r a t i c c o m p o n e n t ) . U n f o r t u n a t e l y the m e t h o d i s not s u i t a b l e f o r p u l s e w o r k a s the s u r f a c e d e f o r m a t i o n t a k e s t i m e to build up and the a v e r a g e d e f o r m a t i o n on p u l s e o p e r a t i o n would be n e a r z e r o . To date, the b e s t p i c t u r e s and the b e s t d i s c r i m i n a t i o n in depth by t h e s e t e c h n i q u e s h a s b e e n o b t a i n e d by the u s e of u l t r a s o n i c l e n s e s , t h e i r o p e r a t i o n with p u l s e d s o u n d i s not p o o r by p r e s e n t day r e s e a r c h s t a n d a r d s . What the f u t u r e will p r o v i d e f r o m l e n s e s o r h o l o g r a p h y f o r new d i a g n o s t i c i n s t r u m e n t s r e m a i n s to be s e e n . In the State U n i v e r s i t y of Ohio, which quite i n c i d e n t a l l y h a s 4 0 , 0 0 0 s t u d e n t s , and t h e r e f o r e no s h o r t a g e of new i d e a s , a g r a n t of $ 3 0 , 0 0 0 f r o m the US Navy to P r o f e s s o r R. C. M c M a s t e r and Hugh M a i l e r is to d e v e l o p a s o u n d s e n s i t i v e e m u l s i o n c o m p a r a b l e to light p h o t o g r a p h y 21. The p r o p o s a l i s to m a k e a f i l m e m u l s i o n of dye in f r a g i l e c a p s u l e s which will b u r s t u n d e r the a c c e l e r a t i o n s e n c o u n t e r e d and r e a c t with the m a t r i x to p r o d u c e a s t a i n which i s v i s i b l e . To i n c r e a s e the e f f i c i e n c y of the p r o c e s s the m i c r o c a p s u l e s m a y c o n t a i n an e x p l o s i v e s u c h a s a f u l m i n a t e o r l e a d azide; to quote the a r t i c l e m e n t i o n e d 22. At l a s t we h a v e a p r o p o s e d m e d i c a l u s e f o r the m a t e r i a l s of war! When we c o n s i d e r the e n c a p s u l a t i o n of liquid c r y s t a l so a m a z i n g l y a c h i e v e d by NCR, p e r h a p s the f u t u r e of t h i s p r o p o s a l i s not s o f a r away. May t h i s o r s o m e new t h o u g h t p r o v o k e d by t h i s s h o r t s u m m a r y l e a d to new p r o g r e s s in this p r o m i s i n g subject. T h i s s h o r t r e v i e w would not be a d e q u a t e without r e f e r e n c e to J a c o b s 22 who a d a p t e d a c o l o u r t e l e v i s i o n r e c e i v e r to w o r k with the Sokolov u l t r a s o n i c c a m e r a and r e v e a l the p h a s e of the s o u n d d i f f r a c t i o n p a t t e r n s . T h i s w o r k h a s b e e n r e p e a t e d in E n g l a n d at 4 . 4 M H z , the c o l o u r t e l e v i s i o n s u b c a r r i e r f r e q u e n c y , at the City U n i v e r s i t y 23. The i m a g e s are startlingly pretty, they are p e r h a p s m o r e informative t h a n b l a c k and white i m a g e s . T h e v a l u e of t h i s t e c h n i q u e is s t i l l u n d e r r e v i e w . One a p p l i c a t i o n is to the d e s i g n of acoustic lenses. Like all i n t r o d u c t i o n s t h i s one i s i n c o m p l e t e , m a n y new i n v e n t i o n s h a v e b e e n o m i t t e d f r o m i g n o r a n c e o r l a c k of space.
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Rushmer, R . F . , B a k e r , D.W. and Stegall, H.F. 'Nondestructive testing of human cardiovascular responses', Digest of 6th International Conference on Medical Electronics ~ Biological Engineering (Tokyo 1965) pp 70-71
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Flaherty, J . J . a n d Strauts, E.J. 'Ultrasonic pulse Doppler instrumentation', ibid
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Smyth, C. N., Poynton, F. Y. and Sayers, J . F . 'Ultrasound image camera', Proceedings of IEE, Vol 110 (1966) pp16-28; Journal of IEE, Part E, Vol 9 (1966) p4030; and ULTRASONICS, Vol 4 (1966) pp15-20
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Smyth, C.N. 'Sound ranging apparatus', British Patent 702,534 (1950)
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Charlotte, H. F. 'A direct reading cathode ray tube', Electronic Engineering, Vol 29, No 345 (1957) p373
8
Sproule, D. 'An ultrasonic imaging system for flaw detection', ULTRASONICS FOR INDUSTRY 1969 Conference Papers, pp56-59 (Iliffe Science and Technology Publications Ltd, Guildford, UK, 1969)
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Poynton, F. Y., (1956) 'Ultrasonic camera with photoluminescence', British Patent 801,985 (1956)
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National Cash Register Co., Dayton, Ohio, Capsular Products Division Technical Publication (1969) 'Encapsulated liquid crystals'
9
Evans, D. S. and Cockett, F.B. 'Diagnosis of deep vein thrombosis with an ultrasonic doppler technique', British Medical Journal, P a r t B Vol ii (1969) p 802-804
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Eggleton, R. C., Townsend, C., Kossoff, G., Herrick, Julia and Hunt, R. 'Computerised ultrasonic visualization of dynamic ventricular configurations', Proceedings of 8th International Conference Medical & Biological Engineering (Chicago 1969)
McMaster, R. C. and Mailer, H., (1969) News in Engineering; Publication of the Engineering Division of the State University of Ohio, Columbus, Ohio (January 1969) p21
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Jacobs, J.E. 'Colour representation of acoustic phase', Materials Evaluation (August 1968)p158
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Smyth, C. N., 2nd International Conference on Medical Physics (1969). In press. Ed. Webster, Massachusetts General Hospital, Boston, Massachusetts, USA
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Peronneau, P. A. mud Leger, F. 'Doppler ultrasonic pulsed blood flowmeter', ibid
ULTRASONICS January 1970
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