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Holographic display of data from ultrasonic scanning
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Conference Paper
Holographic display of data from ultrasonic scanning J. D. Redman*, W. P. Walton*, J. E. F l e m i n g * * and A. M. Hall**
A m e d i c a l a p p l i c a t i o n of a h o l o g r a p h i c m u l t i p l e x i n g t e c h n i que i s d e s c r i b e d . T h e t e c h n i q u e i s not l i m i t e d to m e d i c a l a p p l i c a t i o n s and can be e x t e n d e d to r e p r e s e n t c o n t o u r l i n e s o r a f a m i l y of g r a p h s . In i t s u s e with d i a s o n o g r a p h e q u i p m e n t , the h o l o g r a p h i c t e c h n i q u e a l l o w s i n t e r p r e t a t i o n of i m a g e s p r e v i o u s l y o b s c u r e d and a s s i s t s in t r a i n i n g of c l i n i c i a n s in the u s e of the e q u i p m e n t .
H o l o g r a p h i c m u l t i p l e x i n g can be u s e d to d i s p l a y d a t a in a way which m a k e s i n t e r p r e t a t i o n e a s i e r . T h i s p a p e r i s about an a p p l i c a t i o n of one type of m u l t i p l e x i n g . U s i n g it, we have b e e n able to p r o d u c e t h r e e - d i m e n s i o n a l u l t r a s o n i c r e c o n s t r u c t i o n s of the u t e r u s .
What the h o l o g r a m plate h a s done i s to r e c o r d both the a m p l i tude and the p h a s e of the s c a t t e r e d w a v e - f r o n t . What you s e e when you view the r e c o n s t r u c t e d w a v e s i s the object a s though v i e w e d t h r o u g h a window f o r m e d by the p h o t o g r a p h i c p l a t e . You hold the p h o t o g r a p h i c p l a t e up and you look t h r o u g h it. T h r o u g h the plate you s e e the o r i g i n a l object.
HOLOGRAPHIC VISUAL DISPLAYS At the l a s t U l t r a s o n i c s f o r I n d u s t r y C o n f e r e n c e , J. H a l s t e a d of I m p e r i a l C o l l e g e gave an e x c e l l e n t p a p e r on u l t r a s o u n d h o l o g r a p h y and the w r i t t e n v e r s i o n which a p p e a r e d in U l t r a s o n i c s , A p r i l 1968, g i v e s a d e t a i l e d r e v i e w of the t h e o r y of h o l o g r a p h y , in g e n e r a l . I will not a t t e m p t to d u p l i c a t e H a l s t e a d ' s p a p e r but I will r e v i e w , in q u a l i t a t i v e t e r m s , t h o s e a s p e c t s of h o l o g r a p h y which a r e r e l e v a n t to v i s u a l d i s p l a y s . F i r s t l y , optical h o l o g r a p h y is a m e t h o d of m a k i n g t h r e e d i m e n s i o n a l p i c t u r e s . The r e s u l t s a r e so r e a l i s t i c that it i s difficult to d i s t i n g u i s h b e t w e e n a h o l o g r a p h i c r e c o n s t r u c t i o n of an object and the object i t s e l f . To m a k e an optical h o l o g r a m an object or s c e n e i s u s u a l l y i l l u m i n a t e d c o h e r e n t l y by m e a n s of a l a s e r b e a m . The s c a t t e r e d light i s allowed to fall on a p h o t o g r a p h i c p l a t e . S i m u l t a n e o u s l y , a c o h e r e n t r e f e r e n c e b e a m of light f a l l s on the p h o t o g r a p h i c p l a t e . T h i s r e f e r e n c e b e a m is a s i m p l e wave which can e a s i l y be r e p r o d u c e d - - s u c h a s a p l a n e wave or diverging beam. At the p h o t o g r a p h i c p l a t e the light s c a t t e r e d by the object and the light in the r e f e r e n c e b e a m p r o d u c e an i n t e r f e r e n c e p a t t e r n . W h e n the p l a t e i s developed, the r e s u l t i s a v e r y c o m p l i c a t e d type of d i f f r a c t i o n g r a t i n g . When t h i s g r a t i n g i s i l l u m i n a t e d by a r e p l i c a of the r e f e r e n c e wave, a r e p l i c a of the s c a t t e r e d w a v e - f r o n t s f r o m the object i s r e c o n s t r u c t e d and t r a v e l s o u t w a r d s f r o m t h e p h o t o g r a p h i c plate-a l m o s t a s if it had n e v e r b e e n i n t e r r u p t e d . When t h e s e w a v e s fall on the eye, the eye l e n s f o c u s e s and one s e e s the o r i g i n a l object in t r u l y t h r e e - d i m e n s i o n a l f o r m i n c l u d i n g m o n o c u l a r a s well a s b i n o c u l a r p a r a l l a x . By c h a n g i n g o n e ' s viewpoint one c a n s e e d i f f e r e n t a s p e c t s of the object j u s t a s one would if the object w e r e r e a l l y t h e r e .
* Atomic Weapons Research Establishment, Aldermaston, Berkshire, England ** Q u e e n M o t h e r ' s H o s p i t a l , G l a s g o w , Scotland
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ULTRASONICS J a n u a r y 1969
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F i g 1 P h o t o g r a p h s of the h o l o g r a p h i c r e c o n s t r u c t i o n of a toy b u s (a) with c a m e r a f o c u s s e d on the f r o n t of the reconstruction (b) with c a m e r a f o c u s s e d on the back of the reconstruction F i g 1 s h o w s two p h o t o g r a p h s of the h o l o g r a p h i c r e c o n s t r u c t i o n of a toy b u s . T h e s e p h o t o g r a p h s i l l u s t r a t e the t h r e e - d i m e n s i o n a l i t y of the r e c o n s t r u c t i o n b e c a u s e in o n e of t h e m t h e c a m e r a w a s f o c u s e d at the f r o n t of the r e c o n s t r u c t i o n and in the o t h e r the c a m e r a w a s f o c u s e d on the back. I l l u s t r a t i o n s of t h i s type abound in the p u b l i s h e d l i t e r a t u r e on h o l o g r a p h y . F o r a t i m e , h o l o g r a p h y s e e m e d to be e x c l u s i v e l y c o n c e r n e d with a w o r l d of dinky t o y s , c h e s s m e n and the like. It w a s r e l a t i v e l y e a s y to m a k e h o l o g r a m s of s m a l l o b j e c t s which could be h e l d s t a t i o n a r y to within a f r a c t i o n of a w a v e l e n g t h of light duririg the e x p o s u r e . S m a l l o b j e c t s w e r e u s e d so t h a t the light f r o m the l a s e r could be c o n c e n t r a t e d into a r e l a t i v e l y h i g h i n t e n s i t y b e a m which w a s s t a t i o n a r y to within a f r a c t i o n of a w a v e l e n g t h . T h i s c o n t r o l m e a n t t h a t the i n t e r f e r e n c e p a t t e r n at the h o l o g r a m plate would not be s m e a r e d out d u r i n g the e x p o s u r e . One could not m a k e h o l o g r a m s of people and outdoor s c e n e s . N e v e r t h e l e s s , s p e a k e r s on h o l o g r a p h y f r e q u e n t l y i l l u s t r a t e d
Ul=~l~i=l~Sl~llC8 F O R ~ their lectures using illustrations such as the one in Fig 2 which is a photograph of a holographic reconstruction. In order to m a k e a hologram of this object, one simply takes a photograph of the person in ordinary light with an ordinary camera. This photograph is then m a d e into a transparency. This transparency is then illuminated diffusely from behind and the light it transmits is allowed to fall on a hologram plate. Simultaneously a reference b e a m falls on the plate. W h e n the plate is developed and illuminated by a replica of the reference b e a m a replica of the light transmitted by the transparency is reconstructed. And when you look through the hologram plate you see 'the diffusely illuminated transparency. Of course no matter how m u c h you change your viewpoint in looking at the holographic reconstruction you cannot see round people in the transparency. However, b e c a u s e the t r a n s p a r e n c y was p l a c e d at a c e r t a i n d i s t a n c e behind the h o l o g r a m plate when the h o l o g r a m was being made, the r e c o n s t r u c t i o n of the t r a n s p a r e n c y a p p e a r s to be at that d i s t a n c e . So a s you move your head about, a s you a r e looking at the t r a n s p a r e n c y , you see p a r a l l a x between it and the h o l o g r a m plate. It s e e m s to move about r e l a t i v e to the window you a r e looking at it through. It i s a s if you a r e looking through a window at a t r a n s p a r e n c y which i s being held up s o m e d i s t a n c e behind it. J u s t a s in o r d i n a r y photography one can e x p o s e s e v e r a l v i e w s on the s a m e f r a m e so one can e x p o s e s e v e r a l holographic v i e w s on the s a m e hologram. In the c a s e of o r d i n a r y photography, h o w e v e r , t h e e x p o s u r e of s e v e r a l v i e w s on the s a m e f r a m e often l e a d s to confusion and it is u s e d only in v e r y s p e c i a l i s e d applications. In holography, one can use the t h r e e - d i m e n s i o n a l i t y of the r e c o n s t r u c t i o n to avoid t h i s confusion. We can s t o r e many t r a n s p a r e n c i e s in one h o l o g r a m plate in such a way that they r e c o n s t r u c t at d i f f e r e n t d i s t a n c e s behind the h o l o g r a m plate. The s t o r a g e of many w a v e - f r o n t s in the s a m e h o l o g r a m plate i s known a s multiplexing--by analogy with e l e c t r o n i c s and we have u s e d t h i s technique to p r o d u c e t h r e e - d i m e n s i o n a l displays from diasonograph recordings. I will not go into any g r e a t detail about the diasonograph. It has been d e s c r i b e d at p r e v i o u s c o n f e r e n c e s in t h i s s e r i e s and
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in m a n y places in the published literature. I will give only the briefest review of some of its features.
THE DIASONOGRAPH The diasonograph, which is produced by Nuclear Enterprises Ltd, Edinburgh, employs the pulse-echo technique familiar in radar and sonar systems. The transducer generates a short pulse (about 2~s) of ultrasound (in the frequency range I. 55MHz) which i s p r o p a g a t e d into the patient. E c h o e s p r o d u c e d by the v a r i o u s i n t e r f a c e s between t i s s u e s of d i f f e r e n t a c o u s t i c i m p e d a n c e in the patient r e t u r n e n e r g y to the t r a n s d u c e r at t i m e s p r o p o r t i o n a l to t h e i r r a n g e . The r e s u l t i n g output f r o m the t r a n s d u c e r is a m p l i f i e d and u s e d to modulate the intensity of the t r a c e on a c a t h o d e - r a y tube. The t r a c e i s a r r a n g e d to r e p r e s e n t , at all t i m e s , the position and d i r e c t i o n of the sound b e a m within the patient. This i s a c h i e v e d by c o n t r o l l i n g the t i m e b a s e g e n e r a t o r f r o m p o t e n t i o m e t e r s m e a s u r i n g the X, Y and angular c o - o r d i n a t e s . A s the t r a n s d u c e r i s m o v e d a c r o s s the patient a p o l a r o i d c a m e r a r e c o r d s all the e c h o - s p o t s to f o r m a c r o s s - s e c t i o n a l image of the r e g i o n scanned. At the Queen M o t h e r ' s Hospital, Glasgow, this technique i s c u r r e n t l y u s e d for the d i a g n o s i s of p r e g n a n c y f r o m 5 w e e k s gestation, a b n o r m a l i t i e s of pregnancy, d e t e r m i n a t i o n of multiple pregnancy, the lie of the foetus, l o c a l i s a t i o n of pla~ centa, detection of abdominal c y s t s and t u m o u r s . At o t h e r c e n t r e s , the s a m e technique i s u s e d to investigate the liver, kidneys and s p l e e n . E f f o r t s a r e a l s o being m a d e to obtain c r o s s - s e c t i o n a l p i c t u r e s of the head. The i n t e r p r e t a t i o n of the p i c t u r e s is dependent on e x p e r i e n c e and knowledge of the r e l e v a n t anatomy and pathology. It is often n e c e s s a r y to m e n tally c o r r e l a t e a n u m b e r of p i c t u r e s , e a c h of which may show only a f r a g m e n t of the o b j e c t s of i n t e r e s t . This i s b e c a u s e the p i c t u r e s show only t h o s e p a r t s of the object in the plane of scan and b e c a u s e e c h o e s a r e r e t u r n e d only f r o m s u r f a c e s which a r e n e a r l y n o r m a l to the sound b e a m . A method of displaying the i n f o r m a t i o n t h r e e - d i m e n s i o n a l l y i s t h e r e f o r e very attractive. Fig 3 show e x a m p l e s of r e c o r d i n g s obtained using the d i a s o n o graph. Fig 3a shows a t r a n s v e r s e s e c t i o n a c r o s s a patient who was expecting twins. The head of one of them is c l e a r l y outlined. Fig 3b i s a n o t h e r t r a n s v e r s e s e c t i o n through the s a m e patient--it s h o w s the head of the o t h e r twin.
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Fig 3 (a) Diasonograph r e c o r d taken f r o m a patient expecting twins showing the head of one of t h e m (b) Diasonograph r e c o r d of the s a m e patient a s in Fig 3a showing the head of the o t h e r twin
Fig 2 Photograph of a holographic r e c o n s t r u c t i o n
The d i a s o n o g r a p h r e c o r d i n g s a r e n o r m a l l y made on p o l a r o i d film so the next step i s to c o n v e r t t h e m into d i a p o s i t i v e s on photographic p l a t e s . T h e s e t r a n s p a r e n c i e s a r e then c o m b i n e d holographically so that when r e c o n s t r u c t e d they appear in t h e i r c o r r e c t r e l a t i o n s h i p to one a n o t h e r . This i s done by diffusely illuminating each t r a n s p a r e n c y in turn and allowing the t r a n s m i t t e d light to fall on a high r e s o l u t i o n photographic plate a s shown in Fig 4. Simultaneously a c o h e r e n t r e f e r e n c e b e a m f a l l s on the plate. The r e s u l t i s a multiple e x p o s u r e h o l o g r a m f r o m which each t r a n s p a r e n c y can be r e c o n s t r u c t e d in i t s c o r r e c t spatial r e l a t i o n s h i p to the o t h e r s . This i s done by illuminating the developed plate with a r e p l i c a of the r e f e r e n c e wave or by a wave which b e a r s s o m e s i m p l e r e l a t i o n ship to it. The effect i s r a t h e r like looking through a row of t r a n s p a r e n t d o m i n o e s . However, the r e c o n s t r u c t i o n s a p p e a r to be s e l f - l u m i n o u s so that the r e s u l t i s s u p e r i o r to the one which would be obtained if the t r a n s p a r e n c i e s t h e m s e l v e s w e r e viewed. ULTRASONICS J a n u a r y 1969
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It i s difficult to i l l u s t r a t e the t h r e e - d i m e n s i o n a l quality of the u t e r u s r e c o n s t r u c t i o n s in a t w o - d i m e n s i o n a l f i g u r e . I n s t e a d the F i g s 5 and 6 show p h o t o g r a p h s of s o m e s i m u l a t e d r e c o r d s . F o r s i m p l i c i t y , each t r a n s p a r e n c y contained enly a single v e r t i c a l bar and five of t h e s e w e r e m u l t i p l e x e d to p r o d u c e a t h r e e - d i m e n s i o n a l d i s t r i b u t i o n of b a r s . Fig 5 shows p h o t o g r a p h s taken f r o m two d i f f e r e n t viewpoints and Fig 6 shows the r e s u l t of f o c u s i n g the c a m e r a on d i f f e r e n t p a r t s of the r e c o n s t r u c t i o n . So f a r s e v e n t e e n s e c t i o n s , taken at 1/2cm i n t e r v a l s , have b e e n s u c c e s s f u l l y multiplexed, though many m o r e could be a c c o m m o d a t e d b e f o r e the n o i s e i n h e r e n t in the r e c o n s t r u c t i o n p r o c e s s would be o b t r u s i v e . At the m o m e n t , the l i m i t on the n u m b e r which can be usefully s t o r e d is set by the nature of the s u b j e c t . F o r e x a m p l e , if many b r i g h t a r e a s appear behind one a n o t h e r it i s difficult to d i s t i n g u i s h t h e m . However, it i s e x p e c t e d that t h i s difficulty will be o v e r c o m e by the use of a s a m p l i n g technique, in which a g r i d is i n s e r t e d i m m e d i a t e l y a f t e r the t r a n s p a r e n c y at the multiplexing stage. If n e c e s s a r y , g r i d s could be c h o s e n to suit p a r t i c u l a r s u b j e c t s . F o r e x a m p l e , a l a r g e r p r o p o r t i o n might be r e m o v e d f r o m w h e r e ~he i n f o r mation d e n s i t y is high than f r o m w h e r e it i s low.
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Having c o n v e r t e d the s e q u e n c e of d i a s o n o g r a p h r e c o r d i n g s into a t h r e e - d i m e n s i o n a l holographic display, one can think about making use of s o m e of the f a c i l i t i e s , such a s a c c e s s i b i lity, which a holographic r e c o r d i n g a f f o r d s . F o r e x a m p l e , one can m e r g e a holographic r e c o n s t r u c t i o n of the u t e r u s with a t h r e e - d i m e n s i o n a l g r a t i c u l e to allow m e a s u r e m e n t s to be m a d e d i r e c t l y on the r e c o n s t r u c t i o n . The g r a t i c u l e i t s e l f could of c o u r s e be g e n e r a t e d h o l o g r a p h i c a l l y using the multiplexing p r o c e d u r e and could even be s t o r e d in the s a m e h o l o g r a m plate a s the u t e r u s r e c o r d i n g s t h e m s e l v e s . Recently, we have d e s c r i b e d how t h r e e - d i m e n s i o n a l x - r a y i m a g e s can be p r o d u c e d holographically. T h e r e s e e m s no r e a s o n in p r i n c i p l e why a 3-D x - r a y h o l o g r a m of a patient should not be s u p e r i m p o s e d on the 3-D u l t r a s o n i c visual d i s play. Indeed at a pinch one could c o m p l e t e the p r o c e d u r e by s u p e r i m p o s i n g a 3-D r e c o n s t r u c t i o n of the patient a s viewed by o r d i n a r y light. R e t u r n i n g to the s u b j e c t of the s i m p l e u l t r a s o n i c t h r e e - d i m e n sional display t h e r e i s the p o s s i b i l i t y of using t i m e - l a p s e holography to follow the d e v e l o p m e n t of a p r e g n a n c y . By v a r y ing the r e f e r e n c e - b e a m angle one can s t o r e s u c c e s s i v e s t a g e s of a p r e g n a n c y on one h o l o g r a m plate a l m o s t independently of one a n o t h e r . An a n i m a t e d r e c o n s t r u c t i o n of the d e v e l o p m e n t of the p r e g n a n c y m i g h t then be p o s s i b l e m e r e l y by r o t a t i n g the r e c o n s t r u c t i n g b e a m about the h o l o g r a m plate
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(a) The multiplexing p r o c e d u r e and (b) the m u l t i p l e x ing a p p a r a t u s
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O n e of the d i s a d v a n t a g e s of the 3-D r e c o n s t r u c t i o n of the
u t e r u s , which we have p r o d u c e d so far, is the l i m i t e d f i e l d of view. The p a t i e n t i s r e c o n s t r u c t e d at full s c a l e a s though she w e r e s i t u a t e d at a d i s t a n c e of about 6 i n c h e s behind the h o l o g r a m plate which a c t s a s though it w e r e a window ( d i m e n s i o n s : five i n c h e s by four inches). Ideally, of c o u r s e , one would like to view the patient f r o m all d i r e c t i o n s , and one m e t h o d of doing this i s what I would like to d e s c r i b e now. T h i s m e t h o d m a k e s use of the fact that the d i a s o n o g r a p h allows one not only to get t r a n s v e r s e s e c t i o n s through a patient but also to obtain longitudinal ones. The end product of the s c h e m e which I am going to d e s c r i b e will be a view of the patient a p p a r e n t l y s u s p e n d e d in an octagonal g l a s s tube.
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Fig 5 P h o t o g r a p h s of the r e c o n s t r u c t i o n f r o m a synthetic h o l o g r a m showing p a r a l l a x
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Fig 7 is a s c h e m a t i c d i a g r a m intended to show how the method w o r k s . ~t shows four h o l o g r a m p l a t e s which a r e a r r a n g e d in the f o r m of half an octagon. A l s o shown a r e two diffusing s c r e e n s e a c h of which can be i l l u m i n a t e d by a b e a m of c o h e r e n t ( l a s e r ) light. In the middle of the figure a r e d e p i c t e d t r a n s p a r e n c i e s made f r o m six typical d i a s o n o g r a p h r e c o r d i n g s - - t h r e e longitudinal and t h r e e t r a n s v e r s e . In p r a c t i c e m o r e could be u s e d . Multiplexing t a k e s place in two s t a g e s . F i r s t of all the t r a n s v e r s e t r a n s p a r e n c i e s a r e i l l u m i n a t e d s u c c e s s i v e l y by m e a n s of light diffused by the u p p e r diffusing s c r e e n . The t r a n s m i t t e d light by the d i a s o n o g r a p h s i s allowed to fall on holog r a m p l a t e s 3 and 4. Then light f r o m the o t h e r d i f f u s e r i s
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Fig 6 P h o t o g r a p h s of the r e c o n s t r u c t i o n of a synthetic hologram (a) f o c u s s e d on the f r o n t (b) f o c u s s e d on the back
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allowed to i l l u m i n a t e the longitudinal t r a n s p a r e n c i e s s u c c e s sively. The light they t r a n s m i t is allowed to fall on h o l o g r a m p l a t e s 1 and 2. S i m u l t a n e o u s l y of c o u r s e at e a c h of the h o l o g r a p h i c r e c o r d ing s t a g e s a r e f e r e n c e b e a m i s allowed to fall on the a p p r o p r i a t e h o l o g r a m p l a t e . When the p l a t e s have b e e n developed they a r e r e p l a c e d in the s a m e p o s i t i o n and i l l u m i n a t e d by r e p l i c a s of the r e f e r e n c e w a v e s s i m u l t a n e o u s l y . When the eye m o v e s a r o u n d the octagonal a r r a y one then obtains the d e s i r e d v i e w s of the patient.
THREE POINTS IN CONCLUSION F i r s t l y what I have d e s c r i b e d i s just one application of one s o r t of h o l o g r a p h i c multiplexing. O t h e r multiplexing t e c h niques allow one to produce t h r e e - d i m e n s i o n a l r e c o n s t r u c t i o n s of l a r g e o b j e c t s and people and e v e n x - r a y i m a g e s . Secondly, the s o r t of multiplexing I have d e s c r i b e d is not l i m i t e d to m e d i c a l a p p l i c a t i o n s n o r n e e d the t r a n s p a r e n c i e s be r e p r e s e n t a t i o n s of p h y s i c a l o b j e c t s . They may, for example r e p r e s e n t contour l i n e s or a f a m i l y of g r a p h s . F i n a l l y with r e g a r d to d i a s o n o g r a p h v i s u a l d i s p l a y s . A l though the h o l o g r a p h i c technique is at p r e s e n t r a t h e r l a b o -
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Longitudinal diosonograph sections
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plates 1 2 3 and /~
Fig 7 Method of o p e r a t i o n of h o l o g r a p h i c multiplexing rious, it a p p e a r s to offer two p o s s i b i l i t i e s . F i r s t l y to allow the i n t e r p r e t a t i o n of i m a g e s which have h i t h e r t o been o b s c u r e d and secondly to a s s i s t in the t r a i n i n g of c l i n i c i a n s in the use and p o t e n t i a l i t i e s of the d i a s o n o g r a p h equipment.
ULTRASONICS J a n u a r y 1969
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Holographic display of data from ultrasonic scanning J. D. Redman*, W. P. Walton*, J. E. F l e m i n g * * and A. M. Hall**
A m e d i c a l a p p l i c a t i o n of a h o l o g r a p h i c m u l t i p l e x i n g t e c h n i que i s d e s c r i b e d . T h e t e c h n i q u e i s not l i m i t e d to m e d i c a l a p p l i c a t i o n s and can be e x t e n d e d to r e p r e s e n t c o n t o u r l i n e s o r a f a m i l y of g r a p h s . In i t s u s e with d i a s o n o g r a p h e q u i p m e n t , the h o l o g r a p h i c t e c h n i q u e a l l o w s i n t e r p r e t a t i o n of i m a g e s p r e v i o u s l y o b s c u r e d and a s s i s t s in t r a i n i n g of c l i n i c i a n s in the u s e of the e q u i p m e n t .
H o l o g r a p h i c m u l t i p l e x i n g can be u s e d to d i s p l a y d a t a in a way which m a k e s i n t e r p r e t a t i o n e a s i e r . T h i s p a p e r i s about an a p p l i c a t i o n of one type of m u l t i p l e x i n g . U s i n g it, we have b e e n able to p r o d u c e t h r e e - d i m e n s i o n a l u l t r a s o n i c r e c o n s t r u c t i o n s of the u t e r u s .
What the h o l o g r a m plate h a s done i s to r e c o r d both the a m p l i tude and the p h a s e of the s c a t t e r e d w a v e - f r o n t . What you s e e when you view the r e c o n s t r u c t e d w a v e s i s the object a s though v i e w e d t h r o u g h a window f o r m e d by the p h o t o g r a p h i c p l a t e . You hold the p h o t o g r a p h i c p l a t e up and you look t h r o u g h it. T h r o u g h the plate you s e e the o r i g i n a l object.
HOLOGRAPHIC VISUAL DISPLAYS At the l a s t U l t r a s o n i c s f o r I n d u s t r y C o n f e r e n c e , J. H a l s t e a d of I m p e r i a l C o l l e g e gave an e x c e l l e n t p a p e r on u l t r a s o u n d h o l o g r a p h y and the w r i t t e n v e r s i o n which a p p e a r e d in U l t r a s o n i c s , A p r i l 1968, g i v e s a d e t a i l e d r e v i e w of the t h e o r y of h o l o g r a p h y , in g e n e r a l . I will not a t t e m p t to d u p l i c a t e H a l s t e a d ' s p a p e r but I will r e v i e w , in q u a l i t a t i v e t e r m s , t h o s e a s p e c t s of h o l o g r a p h y which a r e r e l e v a n t to v i s u a l d i s p l a y s . F i r s t l y , optical h o l o g r a p h y is a m e t h o d of m a k i n g t h r e e d i m e n s i o n a l p i c t u r e s . The r e s u l t s a r e so r e a l i s t i c that it i s difficult to d i s t i n g u i s h b e t w e e n a h o l o g r a p h i c r e c o n s t r u c t i o n of an object and the object i t s e l f . To m a k e an optical h o l o g r a m an object or s c e n e i s u s u a l l y i l l u m i n a t e d c o h e r e n t l y by m e a n s of a l a s e r b e a m . The s c a t t e r e d light i s allowed to fall on a p h o t o g r a p h i c p l a t e . S i m u l t a n e o u s l y , a c o h e r e n t r e f e r e n c e b e a m of light f a l l s on the p h o t o g r a p h i c p l a t e . T h i s r e f e r e n c e b e a m is a s i m p l e wave which can e a s i l y be r e p r o d u c e d - - s u c h a s a p l a n e wave or diverging beam. At the p h o t o g r a p h i c p l a t e the light s c a t t e r e d by the object and the light in the r e f e r e n c e b e a m p r o d u c e an i n t e r f e r e n c e p a t t e r n . W h e n the p l a t e i s developed, the r e s u l t i s a v e r y c o m p l i c a t e d type of d i f f r a c t i o n g r a t i n g . When t h i s g r a t i n g i s i l l u m i n a t e d by a r e p l i c a of the r e f e r e n c e wave, a r e p l i c a of the s c a t t e r e d w a v e - f r o n t s f r o m the object i s r e c o n s t r u c t e d and t r a v e l s o u t w a r d s f r o m t h e p h o t o g r a p h i c plate-a l m o s t a s if it had n e v e r b e e n i n t e r r u p t e d . When t h e s e w a v e s fall on the eye, the eye l e n s f o c u s e s and one s e e s the o r i g i n a l object in t r u l y t h r e e - d i m e n s i o n a l f o r m i n c l u d i n g m o n o c u l a r a s well a s b i n o c u l a r p a r a l l a x . By c h a n g i n g o n e ' s viewpoint one c a n s e e d i f f e r e n t a s p e c t s of the object j u s t a s one would if the object w e r e r e a l l y t h e r e .
* Atomic Weapons Research Establishment, Aldermaston, Berkshire, England ** Q u e e n M o t h e r ' s H o s p i t a l , G l a s g o w , Scotland
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F i g 1 P h o t o g r a p h s of the h o l o g r a p h i c r e c o n s t r u c t i o n of a toy b u s (a) with c a m e r a f o c u s s e d on the f r o n t of the reconstruction (b) with c a m e r a f o c u s s e d on the back of the reconstruction F i g 1 s h o w s two p h o t o g r a p h s of the h o l o g r a p h i c r e c o n s t r u c t i o n of a toy b u s . T h e s e p h o t o g r a p h s i l l u s t r a t e the t h r e e - d i m e n s i o n a l i t y of the r e c o n s t r u c t i o n b e c a u s e in o n e of t h e m t h e c a m e r a w a s f o c u s e d at the f r o n t of the r e c o n s t r u c t i o n and in the o t h e r the c a m e r a w a s f o c u s e d on the back. I l l u s t r a t i o n s of t h i s type abound in the p u b l i s h e d l i t e r a t u r e on h o l o g r a p h y . F o r a t i m e , h o l o g r a p h y s e e m e d to be e x c l u s i v e l y c o n c e r n e d with a w o r l d of dinky t o y s , c h e s s m e n and the like. It w a s r e l a t i v e l y e a s y to m a k e h o l o g r a m s of s m a l l o b j e c t s which could be h e l d s t a t i o n a r y to within a f r a c t i o n of a w a v e l e n g t h of light duririg the e x p o s u r e . S m a l l o b j e c t s w e r e u s e d so t h a t the light f r o m the l a s e r could be c o n c e n t r a t e d into a r e l a t i v e l y h i g h i n t e n s i t y b e a m which w a s s t a t i o n a r y to within a f r a c t i o n of a w a v e l e n g t h . T h i s c o n t r o l m e a n t t h a t the i n t e r f e r e n c e p a t t e r n at the h o l o g r a m plate would not be s m e a r e d out d u r i n g the e x p o s u r e . One could not m a k e h o l o g r a m s of people and outdoor s c e n e s . N e v e r t h e l e s s , s p e a k e r s on h o l o g r a p h y f r e q u e n t l y i l l u s t r a t e d
Ul=~l~i=l~Sl~llC8 F O R ~ their lectures using illustrations such as the one in Fig 2 which is a photograph of a holographic reconstruction. In order to m a k e a hologram of this object, one simply takes a photograph of the person in ordinary light with an ordinary camera. This photograph is then m a d e into a transparency. This transparency is then illuminated diffusely from behind and the light it transmits is allowed to fall on a hologram plate. Simultaneously a reference b e a m falls on the plate. W h e n the plate is developed and illuminated by a replica of the reference b e a m a replica of the light transmitted by the transparency is reconstructed. And when you look through the hologram plate you see 'the diffusely illuminated transparency. Of course no matter how m u c h you change your viewpoint in looking at the holographic reconstruction you cannot see round people in the transparency. However, b e c a u s e the t r a n s p a r e n c y was p l a c e d at a c e r t a i n d i s t a n c e behind the h o l o g r a m plate when the h o l o g r a m was being made, the r e c o n s t r u c t i o n of the t r a n s p a r e n c y a p p e a r s to be at that d i s t a n c e . So a s you move your head about, a s you a r e looking at the t r a n s p a r e n c y , you see p a r a l l a x between it and the h o l o g r a m plate. It s e e m s to move about r e l a t i v e to the window you a r e looking at it through. It i s a s if you a r e looking through a window at a t r a n s p a r e n c y which i s being held up s o m e d i s t a n c e behind it. J u s t a s in o r d i n a r y photography one can e x p o s e s e v e r a l v i e w s on the s a m e f r a m e so one can e x p o s e s e v e r a l holographic v i e w s on the s a m e hologram. In the c a s e of o r d i n a r y photography, h o w e v e r , t h e e x p o s u r e of s e v e r a l v i e w s on the s a m e f r a m e often l e a d s to confusion and it is u s e d only in v e r y s p e c i a l i s e d applications. In holography, one can use the t h r e e - d i m e n s i o n a l i t y of the r e c o n s t r u c t i o n to avoid t h i s confusion. We can s t o r e many t r a n s p a r e n c i e s in one h o l o g r a m plate in such a way that they r e c o n s t r u c t at d i f f e r e n t d i s t a n c e s behind the h o l o g r a m plate. The s t o r a g e of many w a v e - f r o n t s in the s a m e h o l o g r a m plate i s known a s multiplexing--by analogy with e l e c t r o n i c s and we have u s e d t h i s technique to p r o d u c e t h r e e - d i m e n s i o n a l displays from diasonograph recordings. I will not go into any g r e a t detail about the diasonograph. It has been d e s c r i b e d at p r e v i o u s c o n f e r e n c e s in t h i s s e r i e s and
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in m a n y places in the published literature. I will give only the briefest review of some of its features.
THE DIASONOGRAPH The diasonograph, which is produced by Nuclear Enterprises Ltd, Edinburgh, employs the pulse-echo technique familiar in radar and sonar systems. The transducer generates a short pulse (about 2~s) of ultrasound (in the frequency range I. 55MHz) which i s p r o p a g a t e d into the patient. E c h o e s p r o d u c e d by the v a r i o u s i n t e r f a c e s between t i s s u e s of d i f f e r e n t a c o u s t i c i m p e d a n c e in the patient r e t u r n e n e r g y to the t r a n s d u c e r at t i m e s p r o p o r t i o n a l to t h e i r r a n g e . The r e s u l t i n g output f r o m the t r a n s d u c e r is a m p l i f i e d and u s e d to modulate the intensity of the t r a c e on a c a t h o d e - r a y tube. The t r a c e i s a r r a n g e d to r e p r e s e n t , at all t i m e s , the position and d i r e c t i o n of the sound b e a m within the patient. This i s a c h i e v e d by c o n t r o l l i n g the t i m e b a s e g e n e r a t o r f r o m p o t e n t i o m e t e r s m e a s u r i n g the X, Y and angular c o - o r d i n a t e s . A s the t r a n s d u c e r i s m o v e d a c r o s s the patient a p o l a r o i d c a m e r a r e c o r d s all the e c h o - s p o t s to f o r m a c r o s s - s e c t i o n a l image of the r e g i o n scanned. At the Queen M o t h e r ' s Hospital, Glasgow, this technique i s c u r r e n t l y u s e d for the d i a g n o s i s of p r e g n a n c y f r o m 5 w e e k s gestation, a b n o r m a l i t i e s of pregnancy, d e t e r m i n a t i o n of multiple pregnancy, the lie of the foetus, l o c a l i s a t i o n of pla~ centa, detection of abdominal c y s t s and t u m o u r s . At o t h e r c e n t r e s , the s a m e technique i s u s e d to investigate the liver, kidneys and s p l e e n . E f f o r t s a r e a l s o being m a d e to obtain c r o s s - s e c t i o n a l p i c t u r e s of the head. The i n t e r p r e t a t i o n of the p i c t u r e s is dependent on e x p e r i e n c e and knowledge of the r e l e v a n t anatomy and pathology. It is often n e c e s s a r y to m e n tally c o r r e l a t e a n u m b e r of p i c t u r e s , e a c h of which may show only a f r a g m e n t of the o b j e c t s of i n t e r e s t . This i s b e c a u s e the p i c t u r e s show only t h o s e p a r t s of the object in the plane of scan and b e c a u s e e c h o e s a r e r e t u r n e d only f r o m s u r f a c e s which a r e n e a r l y n o r m a l to the sound b e a m . A method of displaying the i n f o r m a t i o n t h r e e - d i m e n s i o n a l l y i s t h e r e f o r e very attractive. Fig 3 show e x a m p l e s of r e c o r d i n g s obtained using the d i a s o n o graph. Fig 3a shows a t r a n s v e r s e s e c t i o n a c r o s s a patient who was expecting twins. The head of one of them is c l e a r l y outlined. Fig 3b i s a n o t h e r t r a n s v e r s e s e c t i o n through the s a m e patient--it s h o w s the head of the o t h e r twin.
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Fig 3 (a) Diasonograph r e c o r d taken f r o m a patient expecting twins showing the head of one of t h e m (b) Diasonograph r e c o r d of the s a m e patient a s in Fig 3a showing the head of the o t h e r twin
Fig 2 Photograph of a holographic r e c o n s t r u c t i o n
The d i a s o n o g r a p h r e c o r d i n g s a r e n o r m a l l y made on p o l a r o i d film so the next step i s to c o n v e r t t h e m into d i a p o s i t i v e s on photographic p l a t e s . T h e s e t r a n s p a r e n c i e s a r e then c o m b i n e d holographically so that when r e c o n s t r u c t e d they appear in t h e i r c o r r e c t r e l a t i o n s h i p to one a n o t h e r . This i s done by diffusely illuminating each t r a n s p a r e n c y in turn and allowing the t r a n s m i t t e d light to fall on a high r e s o l u t i o n photographic plate a s shown in Fig 4. Simultaneously a c o h e r e n t r e f e r e n c e b e a m f a l l s on the plate. The r e s u l t i s a multiple e x p o s u r e h o l o g r a m f r o m which each t r a n s p a r e n c y can be r e c o n s t r u c t e d in i t s c o r r e c t spatial r e l a t i o n s h i p to the o t h e r s . This i s done by illuminating the developed plate with a r e p l i c a of the r e f e r e n c e wave or by a wave which b e a r s s o m e s i m p l e r e l a t i o n ship to it. The effect i s r a t h e r like looking through a row of t r a n s p a r e n t d o m i n o e s . However, the r e c o n s t r u c t i o n s a p p e a r to be s e l f - l u m i n o u s so that the r e s u l t i s s u p e r i o r to the one which would be obtained if the t r a n s p a r e n c i e s t h e m s e l v e s w e r e viewed. ULTRASONICS J a n u a r y 1969
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It i s difficult to i l l u s t r a t e the t h r e e - d i m e n s i o n a l quality of the u t e r u s r e c o n s t r u c t i o n s in a t w o - d i m e n s i o n a l f i g u r e . I n s t e a d the F i g s 5 and 6 show p h o t o g r a p h s of s o m e s i m u l a t e d r e c o r d s . F o r s i m p l i c i t y , each t r a n s p a r e n c y contained enly a single v e r t i c a l bar and five of t h e s e w e r e m u l t i p l e x e d to p r o d u c e a t h r e e - d i m e n s i o n a l d i s t r i b u t i o n of b a r s . Fig 5 shows p h o t o g r a p h s taken f r o m two d i f f e r e n t viewpoints and Fig 6 shows the r e s u l t of f o c u s i n g the c a m e r a on d i f f e r e n t p a r t s of the r e c o n s t r u c t i o n . So f a r s e v e n t e e n s e c t i o n s , taken at 1/2cm i n t e r v a l s , have b e e n s u c c e s s f u l l y multiplexed, though many m o r e could be a c c o m m o d a t e d b e f o r e the n o i s e i n h e r e n t in the r e c o n s t r u c t i o n p r o c e s s would be o b t r u s i v e . At the m o m e n t , the l i m i t on the n u m b e r which can be usefully s t o r e d is set by the nature of the s u b j e c t . F o r e x a m p l e , if many b r i g h t a r e a s appear behind one a n o t h e r it i s difficult to d i s t i n g u i s h t h e m . However, it i s e x p e c t e d that t h i s difficulty will be o v e r c o m e by the use of a s a m p l i n g technique, in which a g r i d is i n s e r t e d i m m e d i a t e l y a f t e r the t r a n s p a r e n c y at the multiplexing stage. If n e c e s s a r y , g r i d s could be c h o s e n to suit p a r t i c u l a r s u b j e c t s . F o r e x a m p l e , a l a r g e r p r o p o r t i o n might be r e m o v e d f r o m w h e r e ~he i n f o r mation d e n s i t y is high than f r o m w h e r e it i s low.
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Reconstructing beam
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Having c o n v e r t e d the s e q u e n c e of d i a s o n o g r a p h r e c o r d i n g s into a t h r e e - d i m e n s i o n a l holographic display, one can think about making use of s o m e of the f a c i l i t i e s , such a s a c c e s s i b i lity, which a holographic r e c o r d i n g a f f o r d s . F o r e x a m p l e , one can m e r g e a holographic r e c o n s t r u c t i o n of the u t e r u s with a t h r e e - d i m e n s i o n a l g r a t i c u l e to allow m e a s u r e m e n t s to be m a d e d i r e c t l y on the r e c o n s t r u c t i o n . The g r a t i c u l e i t s e l f could of c o u r s e be g e n e r a t e d h o l o g r a p h i c a l l y using the multiplexing p r o c e d u r e and could even be s t o r e d in the s a m e h o l o g r a m plate a s the u t e r u s r e c o r d i n g s t h e m s e l v e s . Recently, we have d e s c r i b e d how t h r e e - d i m e n s i o n a l x - r a y i m a g e s can be p r o d u c e d holographically. T h e r e s e e m s no r e a s o n in p r i n c i p l e why a 3-D x - r a y h o l o g r a m of a patient should not be s u p e r i m p o s e d on the 3-D u l t r a s o n i c visual d i s play. Indeed at a pinch one could c o m p l e t e the p r o c e d u r e by s u p e r i m p o s i n g a 3-D r e c o n s t r u c t i o n of the patient a s viewed by o r d i n a r y light. R e t u r n i n g to the s u b j e c t of the s i m p l e u l t r a s o n i c t h r e e - d i m e n sional display t h e r e i s the p o s s i b i l i t y of using t i m e - l a p s e holography to follow the d e v e l o p m e n t of a p r e g n a n c y . By v a r y ing the r e f e r e n c e - b e a m angle one can s t o r e s u c c e s s i v e s t a g e s of a p r e g n a n c y on one h o l o g r a m plate a l m o s t independently of one a n o t h e r . An a n i m a t e d r e c o n s t r u c t i o n of the d e v e l o p m e n t of the p r e g n a n c y m i g h t then be p o s s i b l e m e r e l y by r o t a t i n g the r e c o n s t r u c t i n g b e a m about the h o l o g r a m plate
Fig 4
(a) The multiplexing p r o c e d u r e and (b) the m u l t i p l e x ing a p p a r a t u s
i
O n e of the d i s a d v a n t a g e s of the 3-D r e c o n s t r u c t i o n of the
u t e r u s , which we have p r o d u c e d so far, is the l i m i t e d f i e l d of view. The p a t i e n t i s r e c o n s t r u c t e d at full s c a l e a s though she w e r e s i t u a t e d at a d i s t a n c e of about 6 i n c h e s behind the h o l o g r a m plate which a c t s a s though it w e r e a window ( d i m e n s i o n s : five i n c h e s by four inches). Ideally, of c o u r s e , one would like to view the patient f r o m all d i r e c t i o n s , and one m e t h o d of doing this i s what I would like to d e s c r i b e now. T h i s m e t h o d m a k e s use of the fact that the d i a s o n o g r a p h allows one not only to get t r a n s v e r s e s e c t i o n s through a patient but also to obtain longitudinal ones. The end product of the s c h e m e which I am going to d e s c r i b e will be a view of the patient a p p a r e n t l y s u s p e n d e d in an octagonal g l a s s tube.
!' i
!
Fig 5 P h o t o g r a p h s of the r e c o n s t r u c t i o n f r o m a synthetic h o l o g r a m showing p a r a l l a x
!
Fig 7 is a s c h e m a t i c d i a g r a m intended to show how the method w o r k s . ~t shows four h o l o g r a m p l a t e s which a r e a r r a n g e d in the f o r m of half an octagon. A l s o shown a r e two diffusing s c r e e n s e a c h of which can be i l l u m i n a t e d by a b e a m of c o h e r e n t ( l a s e r ) light. In the middle of the figure a r e d e p i c t e d t r a n s p a r e n c i e s made f r o m six typical d i a s o n o g r a p h r e c o r d i n g s - - t h r e e longitudinal and t h r e e t r a n s v e r s e . In p r a c t i c e m o r e could be u s e d . Multiplexing t a k e s place in two s t a g e s . F i r s t of all the t r a n s v e r s e t r a n s p a r e n c i e s a r e i l l u m i n a t e d s u c c e s s i v e l y by m e a n s of light diffused by the u p p e r diffusing s c r e e n . The t r a n s m i t t e d light by the d i a s o n o g r a p h s i s allowed to fall on holog r a m p l a t e s 3 and 4. Then light f r o m the o t h e r d i f f u s e r i s
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ULTRASONICS J a n u a r y 1969
a
b
Fig 6 P h o t o g r a p h s of the r e c o n s t r u c t i o n of a synthetic hologram (a) f o c u s s e d on the f r o n t (b) f o c u s s e d on the back
U
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allowed to i l l u m i n a t e the longitudinal t r a n s p a r e n c i e s s u c c e s sively. The light they t r a n s m i t is allowed to fall on h o l o g r a m p l a t e s 1 and 2. S i m u l t a n e o u s l y of c o u r s e at e a c h of the h o l o g r a p h i c r e c o r d ing s t a g e s a r e f e r e n c e b e a m i s allowed to fall on the a p p r o p r i a t e h o l o g r a m p l a t e . When the p l a t e s have b e e n developed they a r e r e p l a c e d in the s a m e p o s i t i o n and i l l u m i n a t e d by r e p l i c a s of the r e f e r e n c e w a v e s s i m u l t a n e o u s l y . When the eye m o v e s a r o u n d the octagonal a r r a y one then obtains the d e s i r e d v i e w s of the patient.
THREE POINTS IN CONCLUSION F i r s t l y what I have d e s c r i b e d i s just one application of one s o r t of h o l o g r a p h i c multiplexing. O t h e r multiplexing t e c h niques allow one to produce t h r e e - d i m e n s i o n a l r e c o n s t r u c t i o n s of l a r g e o b j e c t s and people and e v e n x - r a y i m a g e s . Secondly, the s o r t of multiplexing I have d e s c r i b e d is not l i m i t e d to m e d i c a l a p p l i c a t i o n s n o r n e e d the t r a n s p a r e n c i e s be r e p r e s e n t a t i o n s of p h y s i c a l o b j e c t s . They may, for example r e p r e s e n t contour l i n e s or a f a m i l y of g r a p h s . F i n a l l y with r e g a r d to d i a s o n o g r a p h v i s u a l d i s p l a y s . A l though the h o l o g r a p h i c technique is at p r e s e n t r a t h e r l a b o -
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q~m4Ferenoe Paper
Longitudinal diosonograph sections
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Diffusing
Transverse
~Holograrn
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plates 1 2 3 and /~
Fig 7 Method of o p e r a t i o n of h o l o g r a p h i c multiplexing rious, it a p p e a r s to offer two p o s s i b i l i t i e s . F i r s t l y to allow the i n t e r p r e t a t i o n of i m a g e s which have h i t h e r t o been o b s c u r e d and secondly to a s s i s t in the t r a i n i n g of c l i n i c i a n s in the use and p o t e n t i a l i t i e s of the d i a s o n o g r a p h equipment.
ULTRASONICS J a n u a r y 1969
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