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Basic radiography 9-fluoroscopy
BASIC RADIOGRAPHY 9-Fluoroscopy J. C. Rockley
Conventional r a d i o g r a p h i c t e c h n i q u e s have many outstanding advantages f o r ndt, not l e a s t of which i s the ability to ' s e e ' the i n t e r n a l s t r u c t u r e of an object. However, the lack of a method for the ' r e a l - t i m e ' p r e s e n t a t i o n of i n f o r m a t i o n can often be u n s a t i s f a c t o r y . It is h e r e that f l u o r o s c o p y can be ext r e m e l y useful. In this a r t i c l e , the author d i s c u s s e s the b a s i c
p r i n c i p l e s of f l u o r o s c o p y and d i s t i n g u i s h e s between ' c o n v e n tional' f l u o r o s c o p y and f l u o r o s c o p y aided by i m a g e i n t e n s i f i e r s . D e t a i l s c o n c e r n i n g the choice of phosphor, the c h a r a c t e r i s t i c s of f l u o r e s c e n t s c r e e n s , and the a p p a r a t u s and t e c h niques u s e d a r e given. Finally, the author d i s c u s s e s the m e c h a n i s m s and use of i m a g e i n t e n s i f i e r s and a m p l i f i e r s .
F l u o r o s c o p y ( ' s c r e e n i n g ' ) is a radiological method of ins p e c t i o n in which an i m a g e of the s p e c i m e n undergoing examination is f o r m e d on a f l u o r e s c e n t (light-emitting) s c r e e n . Two s y s t e m s of f l u o r o s c o p y a r e available. T h e s e a r e (a) conventional f l u o r o s c o p y in which the i m a g e on the s c r e e n i s viewed d i r e c t l y by the o p e r a t o r without any additional aid, and (b) image intensification or amplification in which the screen image is processed electronically before viewing and, if desired, presented on a television monitor screen. Both of these systems are characterised by certain features that distinguish them from radiography. They offer the immediate presentation of information and the ability to study moving objects, but do not produce any permanent record unless the basic system is supplemented by an additional recording stage that may be either photographic or electronic in nature.
conditions; in addition, it m u s t also s a t i s f y the following requirements:
BASIC PRINCIPLES All f l u o r o s c o p i c s y s t e m s a r e b a s e d on the l u m i n e s c e n t p r o p e r t y of x - r a y s and g a m m a - r a y s , ie that p r o p e r t y which c a u s e s the e m i s s i o n of light by c e r t a i n s u b s t a n c e s (phosphors) when they a r e i r r a d i a t e d by t h e s e r a d i a t i o n s . Many s u b s t a n c e s exhibit this effect, but not n e c e s s a r i l y in a m a n n e r that m a k e s them suitable f o r i n c o r p o r a t i o n in a f l u o r o scopic s y s t e m . The suitability of a phosphor for this p u r p o s e is d e t e r m i n e d by the m a n n e r in which the eye r e s p o n d s to the s t i m u l u s of light. The r e s p o n s e of the eye i s m o s t efficient in t e r m s of both c o n t r a s t p e r c e p t i o n (ability to p e r c e i v e c h a n g e s of b r i g h t n e s s ) and visual acuity (ability to p e r c e i v e fine detail) when it i s functioning at r e l a t i v e l y high b r i g h t n e s s l e v e l s such as might be e n c o u n t e r e d in b r i g h t day-light. At t h e s e l e v e l s the eye is capable of detecting v a r i a t i o n s in b r i g h t n e s s of about 1°~ above or below the ambient b r i g h t n e s s . At l o w e r l e v e l s this capability is s e v e r e l y r e d u c e d , and at s o m e e x t r e m e conditions a s m e t with in conventional f l u o r o s c o p y it may be no b e t t e r than 20% to 40%. The e f f i c i e n c y of the eye i s also influenced by the colour of light. This i s known a s the P u r k i n j e effect. At low b r i g h t n e s s l e v e l s the e y e ' s s p e c t r a l s e n s i t i v i t y i s at a m a x i m u m in the y e l l o w - g r e e n (5200~--10-1Om) r e g i o n of the v i s i b l e s p e c t r u m . It follows f r o m t h e s e c o n s i d e r a t i o n s that a p h o s p h o r intended f o r use in f l u o r o s c o p y m u s t have p r o p e r t i e s that m e e t t h e s e The author is with the M i n i s t r y of Technology, AQD L a b o r a t o r i e s , Harefield, Uxbridge, Middlesex, UK 184
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It m u s t be stable both c h e m i c a l l y and in r e s p e c t to its l i g h t - e m i t t i n g c h a r a c t e r i s t i c s under i n t e n s e and p r o l o n g e d p e r i o d s of i r r a d i a t i o n
2
It m u s t be unaffected by a t m o s p h e r i c conditions
3
The s p e c t r a l e m i s s i o n must be independent of the e n e r g y of the radiation e m p l o y e d
4
T h e r e must be an a b s e n c e of ' a f t e r - g l o w ' , ie r e t e n tion of i m a g e a f t e r the exciting radiation has been cut off
F r e e d o m f r o m a f t e r - g l o w is an e s s e n t i a l in any p h o s p h o r u s e d for the p u r p o s e s of fluoroscopy. P e r s i s t e n c e of the image r e s u l t i n g f r o m this effect would r e s u l t in p r a c t i c e in a confusion of s u p e r i m p o s e d i m a g e s , o r a b l u r r e d , s m e a r e d a p p e a r a n c e in the c a s e of a moving s p e c i m e n , thus making interpretation impossible. Most m o d e r n f l u o r o s c o p i c s c r e e n s a r e b a s e d on the type o r i g i n a l l y developed by Levy and West 1, 2. The p h o s p h o r in t h i s type of s c r e e n c o n s i s t s of a m i x t u r e of zinc sulphide and c a d m i u m sulphide, the r e l a t i v e p r o p o r t i o n s of which a r e c a r e f u l l y c o n t r o l l e d to g o v e r n the colour of the e m i t t e d light. A f u r t h e r d e g r e e of c o n t r o l i s e f f e c t e d through the addition of s m a l l a m o u n t s of s i l v e r to the p h o s p h o r to obtain a peak f l u o r e s c e n t e m i s s i o n in the region of 5, 200A (10-1Om). The a f t e r - g l o w effect that is p a r t i c u l a r l y c h a r a c t e r i s t i c of zinc sulphide i s inhibited by the addition of nickel in the p r o p o r t i o n of 1 p a r t to 2 million. It follows that the m a n u f a c t u r e of this type of phosphor, which i s c l a s s i f i e d a s being s i l v e r - a c t i v a t e d , n i c k e l - i n h i b i t e d , i s a m a t t e r of v e r y c l o s e and a c c u r a t e c h e m i c a l control.
CHARACTERISTICS OF FLUOROSCOPIC SCREENS A f l u o r o s c o p i c s c r e e n c o n s i s t s of a r i g i d s u p p o r t of thin c a r d o r p l a s t i c s h e e t on which i s coated a p h o s p h o r layer. The p h o s p h o r i s finely ground and i s held in p o s i t i o n by a suitable b i n d e r . The p a r t i c l e s i z e of the p h o s p h o r and the t h i c k n e s s of the p h o s p h o r l a y e r e x e r t s o m e influence on the b r i g h t n e s s of the i r r a d i a t e d s c r e e n , and up to a point an inc r e a s e in e i t h e r will give an i n c r e a s e in b r i g h t n e s s . However, an i n c r e a s e in the s i z e of the p h o s p h o r p a r t i c l e s and the s i z e of t h e i r c o n g l o m e r a t e s has an a d v e r s e effect on the ability of the s c r e e n to r e s o l v e fine detail. Similarly, an inc r e a s e in l a y e r t h i c k n e s s r e s u l t s in a g r e a t e r d e g r e e of
light s c a t t e r i n g within the l a y e r with a c o n s e q u e n t r e d u c t i o n in the r e s o l v i n g p o w e r of the s c r e e n . I n v e s t i g a t i o n s m a d e into the influence of t h e s e and o t h e r f a c t o r s have shown 3, 4 that a typical f l u o r o s c o p i c s c r e e n may have an i n h e r e n t uns h a r p n e s s of 0 . 5 to 1 . 3 m m . T h i s i s c o n s i d e r a b l y g r e a t e r than that of a f i n e - g r a i n x - r a y film (about 0 . 0 1 m m ) and s i g n i f i c a n t l y g r e a t e r than that of a s c r e e n - t y p e x - r a y s c r e e n e x p o s e d with a p a i r of a v e r a g e - s p e e d s a l t intensifying s c r e e n s (about 0 . 3 r a m ) . The i n t e n s i t y of the light e m i s s i o n f r o m a f l u o r o s c o p i c s c r e e n i s p r o p o r t i o n a l to the i n t e n s i t y of the x - o r g a m m a r a d i a t i o n r e a c h i n g the s c r e e n . An a r e a of g r e a t e r x - r a y int e n s i t y will give r i s e to a c o r r e s p o n d i n g a r e a of i n c r e a s e d b r i g h t n e s s in the s c r e e n i m a g e . In the i n s t a n c e of a s p e c i m e n containing a flaw in the f o r m of a void, eg a g a s - h o l e in a casting, the flaw will be r e v e a l e d a s a l i g h t e r a r e a a g a i n s t the d a r k e r b a c k g r o u n d of the s u r r o u n d i n g i m a g e of the sound m e t a l . The s c r e e n p i c t u r e in conventional f l u o r o s c o p y can thus be r e g a r d e d as a p o s i t i v e in r e l a t i o n to the negative a p p e a r a n c e of a r a d i o g r a p h . This c o m p l e t e r e v e r s a l of the d e n s i t y - b r i g h t n e s s r e l a t i o n s h i p s m u s t constantly be b o r n e in mind when i n t e r p r e t i n g f l u o r o s c o p i c evidence. This effect, which can be o v e r c o m e in s o m e i m a g e - a m p l i f i e r s y s t e m s , is the only difficulty likely to be e n c o u n t e r e d in i n t e r p r e t a t i o n , s i n c e the p r o j e c t e d i m a g e p a t t e r n s of c a s t i n g and weld flaws r e m a i n the s a m e in f l u o r o s c o p y as in r a d i o g r a p h y . R e f e r e n c e h a s b e e n made to both x - r a y s and g a m m a - r a y s when d e s c r i b i n g the p r o p e r t i e s of p h o s p h o r s . In p r a c t i c e , v e r y little u s e i s made of g a m m a - r a y s and the technique of f l u o r o s c o p y in its r o u t i n e a p p l i c a t i o n s is l a r g e l y one a s s o c i a ted with x - r a y s . T h i s a r i s e s f o r two r e a s o n s . F i r s t l y , the production of a f l u o r e s c e n t s c r e e n i m a g e of adequate b r i g h t n e s s r e q u i r e s that a r e l a t i v e l y high i n t e n s i t y of r a d i a t i o n r e a c h e s the s c r e e n . An x - r a y set i s g e n e r a l l y capable of supplying the i n t e n s i t y r e q u i r e d , and in any event can b e p o s i t i o n e d r e a s o n a b l y c l o s e to the s c r e e n without any s i g n i ficant i m p a i r m e n t of i m a g e u n s h a r p n e s s b e c a u s e of i t s r e latively s m a l l s i z e of focal spot. G a m m a - r a y s o u r c e s a r e poor e m i t t e r s of r a d i a t i o n and to p r o d u c e the i n t e n s i t i e s r e quired they would n e e d to have p h y s i c a l d i m e n s i o n s that would s e r i o u s l y i n c r e a s e the g e o m e t r i c u n s h a r p n e s s of the s c r e e n image. The s e c o n d r e a s o n r e l a t e s to the l u m i n e s c e n t e f f i c i e n c y of p h o s p h o r s at h i g h e r e n e r g i e s of radiation. As the e n e r g y of the r a d i a t i o n i s i n c r e a s e d , t h e r e i s p r o p o r t i o n ally l e s s a b s o r p t i o n of the r a d i a t i o n in the p h o s p h o r layer, with a c o n s e q u e n t r e d u c t i o n in the i n t e n s i t y of the f l u o r e s c e n t e m i s s i o n . The l u m i n e s c e n t e f f i c i e n c y of the p h o s p h o r thus f a l l s off as the e n e r g y of the r a d i a t i o n is i n c r e a s e d . Since,
f8
/ 1
2 ¢
3
/.
Observer 1 x - r a y tube; 2 baffle to r e s t r i c t i r r a d i a t e d field; 3 i n n e r lead shielding; 4 p h o s p h o r - c o a t e d s c r e e n ; 5 plane m i r r o r ; 6 l e a d g l a s s window; 7 s p e c i m e n ; 8 o u t e r casing. B r o k e n l i n e s r e p r e s e n t path of x - r a y s , solid l i n e s r e p r e s e n t light path to viewer F i g 1 S c h e m a t i c d i a g r a m ¢~ i n d i r e c t v i s i o n screening c a b i n e t
with one exception, all the c o m m o n l y u s e d g a m m a - r a y s o u r c e s e m i t r e l a t i v e l y high e n e r g y radiation, t h e r e i s little s c o p e f o r t h e i r u s e in routine f l u o r o s c o p y . However, s p e c i a l i n s t a n c e s have b e e n r e p o r t e d 5 w h e r e the use of c o m p l e x s c r e e n s c o n s i s t i n g of a lead f i l t e r in conjunction with a p h o s p h o r s c r e e n has enabled high e n e r g y f l u o r o s c o p y to be p e r f o r m e d . In t h i s c a s e the s c r e e n r e a c t s p r i m a r i l y to the p h o t o - e l e c t r o n s e j e c t e d f r o m the lead and has led to the s u c c e s s f u l u s e of f l u o r o s c o p y with 31MeV (50 x 1 0 - 1 9 j ) x-radition.
APPARATUS
FOR CONVENTIONAL
FLUOROSCOPY
In conventional f l u o r o s c o p y the x - r a y tube, s p e c i m e n and f l u o r e s c e n t s c r e e n s a r e housed in a s h i e l d e d cabinet called a ' f l u o r o s c o p e ' o r ' s c r e e n i n g c a b i n e t ' . The b a s i c g e n e r a l a r r a n g e m e n t of such a cabinet i s shown in Fig 1. S c r e e n i n g c a b i n e t s may be d e s i g n e d for g e n e r a l use o r for a s p e c i f i c p u r p o s e , in which c a s e it would be capable of handling only the type of s p e c i m e n for which it was designed. In e i t h e r ins t a n c e the d e s i g n m u s t take into account the r e q u i r e m e n t s of the c u r r e n t s t a t u t o r y r e g u l a t i o n s 6 governing the i n d u s t r i a l u s e of ionising r a d i a t i o n s . The d e s i g n m u s t o b s e r v e , t h e r e f o r e , the following b a s i c p r i n c i p l e s :
1
2
3
4
The cabinet m u s t be s h i e l d e d to e n s u r e that the leakage of s c a t t e r e d and p r i m a r y radiation d o e s not e x c e e d 2 . 5 m r / h if c l a s s i f i e d w o r k e r s only a r e affected by i t s use and p r e s e n c e When the cabinet i s opened f o r any p u r p o s e , eg the i n s e r t i o n o r r e m o v a l of the s p e c i m e n , the x - r a y b e a m m u s t be cut off by i n t e r r u p t i o n of the supply to the ht g e n e r a t o r Any manipulation of the s p e c i m e n in the b e a m of x - r a y s m u s t be e f f e c t e d by m e a n s of c o n t r o l s ext e r n a l to the cabinet W h e r e p r a c t i c a b l e the s c r e e n i m a g e must be i n s p e c ted i n d i r e c t l y through a m i r r o r
The l e a d - g l a s s window through which the s c r e e n is viewed m u s t have shielding p r o p e r t i e s equal to t h o s e of the m e t a l l i c shielding of the walls of the cabinet. Lead g l a s s is n o r m a l l y g r a d e d in t e r m s of its lead equivalent. A l a y e r of lead g l a s s having a 2mm lead equivalent would have the s a m e shielding effect a s a 2mm l a y e r of m e t a l l i c lead. H, t h e r e f o r e , it i s n e c e s s a r y to provide 4mm lead shielding on a cabinet, it would be n e c e s s a r y to glaze the window with two l a y e r s of 2mm lead g l a s s . It i s e s s e n t i a l to e n s u r e that a good o v e r lap e x i s t s b e t w e e n the g l a s s of the window and the adjacent m e t a l l i c lead shielding by wrapping the lead shielding around the s q u a r e e d g e s of the g l a s s s h e e t s . I n d i r e c t viewing i s called for in o r d e r to o v e r c o m e the s o m e what r e m o t e p o s s i b i l i t y that the l e a d - g l a s s window may c r a c k o r s h a t t e r while the f l u o r o s c o p e i s in use. H t h i s o c c u r r e d the o b s e r v e r would undoubtedly be e x p o s e d to the p r i m a r y b e a m of r a d i a t i o n for as long as he c h o s e to r e m a i n in the viewing position, and for this r e a s o n the use of a m i r r o r should be r e c o m m e n d e d . N e v e r t h e l e s s , its introduction without doubt c r e a t e s a n o t h e r f r u s t r a t i o n in an a l r e a d y arduous p r o c e s s , and r o b s the v i e w e r of the s a t i s f a c t i o n of a good c l e a r look at the s c r e e n itself. When a m i r r o r i s employed it is d e s i r a b l e that it should be of the s u r f a c e - s i l v e r e d type. T h i s will p r e v e n t the f o r m a t i o n of double i m a g e s which, in the i n s t a n c e of s p e c i m e n s giving l o c a l i s e d b r i g h t a r e a s in an o t h e r w i s e d a r k image, can lead to confusion in i n t e r p r e t a t i o n . This effect i s i l l u s t r a t e d d i a g r a m m a t i c a l l y in Fig 2. Care m u s t be taken to clean r e g u l a r l y the s u r f a c e s of the s c r e e n , m i r r o r , and viewing window. A significant light l o s s can o c c u r through the p r e s e n c e of a c c u m u l a t e d l a y e r s of dust in the optical path of a f l u o r o s c o p e . An i m p o r t a n t f e a t u r e in the design of a f l u o r o s c o p e is the p r o v i s i o n of movable m a s k s at the x - r a y s o u r c e to enable the i r r a d i a t e d a r e a of the s c r e e n to be r e d u c e d in s i z e to that r e q u i r e d to a c c o m m o d a t e the s p e c i m e n image. This will r e m o v e the b r i g h t g l a r e that would o t h e r w i s e r e s u l t f r o m the unshielded a r e a s of the screen.
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they attain m a x i m u m s e n s i t i v i t y under t h e s e conditi(m:~ This effect will have been e x p e r i e n c e d by anyone who, t o t example, has e n t e r e d a d a r k e n e d c i n e m a f r o m a brightly lii s t r e e t . The p r o c e s s of dark adaptation may extend over a period of 60rain b e f o r e it has been completed, but the maioc i m p r o v e m e n t in s e n s i t i v i t y is usually achieved ~Lfter about 20min. The d a r k - a d a p t e d condition is lost once the e y e s art-, again e x p o s e d to bright white light and a f u r t h e r period of adaptation would then be r e q u i r e d to r e s t o r e the f o r m e r condition. L o s s of dark adaptation does not o c c u r with red light, and it is thus p o s s i b l e to illuminate the raom in which the f l u o r o s c o p e is i n s t a l l e d with this colour, at a level that p e r m i t s the writing of n o t e s a~ld the inspection of s p e c i m e n s .
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Fig 2 Formation of multiple images by a back-silvered mirror. The main beam is shown by a solid line and and secondary reflections by dotted l i n e s
ASSESSMENT OF CONVENTIONAL FLUOROSCOPY
TECHNIQUES OF CONVENTIONAL FLUOROSCOPY The p r o c e d u r e to be adopted when c a r r y i n g out s c r e e n i n g examination will v a r y with the type of s p e c i m e n that is involved and the type of i n f o r m a t i o n that is sought. In an application of f l u o r o s c o p y w h e r e the s p e c i m e n is e a s i l y p e n e t r a t ed and the i n f o r m a t i o n sought i s in the f o r m of l a r g e detail, t h e r e is no g r e a t need f o r s p e c i a l l y c o n t r o l l e d viewing conditions or r e g u l a t e d working d r i l l s . It is usually sufficient in such i n s t a n c e s to c a r r y out the examination in a n o r m a l l y lit r o o m p r o v i d e d that the s c r e e n is shaded f r o m the d i r e c t r o o m lighting by s o m e f o r m of viewing port through which the o p e r a t o r can study the s c r e e n . Screening under such conditions can be c a r r i e d on continuously with no m o r e fatigue than would r e s u l t f r o m the d e g r e e of c o n c e n t r a t i o n involved in studying d e t a i l s under n o r m a l conditions of lighting. However, if the i n f o r m a t i o n sought g i v e s r i s e only to low c o n t r a s t or fine detail in the s c r e e n image it i s e s s e n t i a l that p a r t i c u l a r c a r e should be taken to e n s u r e that suitable viewing conditions a r e employed. Such conditions include the installation of the f l u o r o s c o p e in a d a r k e n e d r o o m , o r in one that is lit with r e d light, and a p e r i o d of dark adaptation b e f o r e s c r e e n i n g is c o m m e n c e d . It i s also d e s i r a b l e that the study of the s c r e e n should be l i m i t e d to continuous p e r i o d s not exceeding 30rain. The e y e s a r e not i m m e d i a t e l y s e n s i t i v e to low l e v e l s of illumination. A p e r i o d of dark adaptation i s r e q u i r e d b e f o r e
The main advantages of conventional f l u o r o s c o p y are low o p e r a t i o n a l cost, i m m e d i a t e p r e s e n t a t i o n of information. ability to study s p e c i m e n s in motion, and lack of a need for a high d e g r e e of o p e r a t i v e skill. In r e g a r d to the latter, howe v e r , it m u s t be u n d e r s t o o d that the o p e r a t o r must be capable of c a r r y i n g out on his own initiative the i n t e r p r e t a t i o n and a s s e s s m e n t of the f l u o r o s c o p i c evidence. The d i s a d v a n t a g e s include a v e r y r e s t r i c t e d application, eg l i m i t e d to s p e c i m e n s capable of being adequately p e n e t r a t e d at a m a x i m u m of 150 to 200kV. This r e p r e s e n t s a p p r o x i m a t e l y 115in (63ram) of aluminium alloy and about 1/sin (3mm) of s t e e l . The quality of r e s u l t obtainable in t e r m s of s e n s i t i v i t y to d e f e c t s and r e s o l u t i o n of detail i s given in Fig 3 and Table I. The r e s u l t s given in Table 1 w e r e obtained by s c r e e n i n g tlat p l a t e s of d u r a l u m i n to the tube side of which w e r e attached copper w i r e s of v a r i o u s d i a m e t e r s . The kilovoltage was v a r i e d to suit the s p e c i m e n t h i c k n e s s and did not e x c e e d 150kV (0. l m m focal spot) and 200kV (5mm focal spot). The image e n l a r g e m e n t in the f o r m e r c a s e was obtained by p r o jective magnification, ie by r e m o v i n g the s p e c i m e n f r o m the s c r e e n while retaining a fixed t u b e - s c r e e n distance.
IMAGE INTENSIFIERS AND AMPLIFIERS One of the main c a u s e s of the i n f e r i o r quality of the image obtained in conventional f l u o r o s c o p y is the low level of b r i g h t n e s s of the s c r e e n image. This disadvantage has given r i s e to the d e v e l o p m e n t of e l e c t r o n i c m e t h o d s for significantly i n c r e a s i n g the b r i g h t n e s s of the s c r e e n .
Intensifiers 20
The e a r l i e s t work in this field was undertaken by HolsU and his c o - w o r k e r s . If light is allowed to i r r a d i a t e a photos e n s i t i v e l a y e r such a s s e l e n i u m , p h o t o - e l e c t r o n s will be e j e c t e d f r o m its s u r f a c e . If the p h o t o - s e n s i t i v e l a y e r f o r m s p a r t of a negatively c h a r g e d e l e c t r o d e (a photo-cathode), the e l e c t r o n s can be a c c e l e r a t e d t o w a r d s a positively c h a r g e d anode. The p h o t o - e l e c t r o n s will a c q u i r e an additional amount of e n e r g y that will be d e t e r m i n e d by the a c c e l e r a t i n g potential
A lS
10 b0
Table 1 Discernibility of detail with c(mventional fluoroscopy Specimen t h i c k n e s s (in-duralumin) I
0
I
I
I
1 2 Specimen thickness [ inches of aluminium ]
3
Fig 3 Sensitivity of conventional fluoroscopy to simulated casting defects in alumininm, in comparison with steptype IQI sensitivity (Rockley, J. C., Introduction to industrial radiology, Butterwort~ London, 1969) n o n - d e s t r u c t i v e t e s t i n g June 1970
(in--copper)
I
Curve A s i m u l a t e d sand and oxide inclusions; Curve B s i mulated gas holes; Curve C s t e p - t y p e IQI. To c o n v e r t in to ram. u s e the f a c t o r x 2 5 . 4
186
D i a m e t e r of s m a l l e s t v i s i b l e w i r e
5mm focal spot (xl)
0. l m m focal spot (:'<3)*
0. 5(12. 7ram)
0.01(0. 254mm)
0. 005(0. 127ram)
1.0(25.4ram)
0.02(0. 508mm)
0. 008(0. 203mm)
1.5(38.1ram)
0.03(0. 762mm)
0. 008(0. 203ram)
2.0(50.8mm)
0.03(0. 762mm)
0. 011(0. 279mm)
* D e g r e e of image e n l a r g e m e n t (Rockley, J. C., Introduction to i n d u s t r i a l radiology, B u t t e r worth, London, 1964)
between the photo-cathode and the anode. In an image intensifier (Fig 4) the photo-cathode consists of a curved fluorescent screen having its fluorescent layer coated with a thin photo-sensitive film. The anode is basically a thin aluminium foil coated on the side remote from the photo-cathode with a fluorescent layer. These two electrodes are mounted inside an evacuated glass container. A potential difference of about 15kV is applied to these electrodes. X-rays enter the intensifier from the end adjacent to the photocathode. These react with the fluorescent layer to give rise to an emission of light. This is immediately absorbed by the photo-sensitive layer which, in turn, emits photo-electrons in an intensity that is proportional to the light intensity generated in the fluorescent layer of the photo-cathode. Since the photoelectrons are emitted in a direction normal to the surface of the selenium layer, they are directed by the curvature of the photo-cathode towards the anode. The accelerating potential provides enough energy for the photo-electronsto penetrate the thin aluminium foil of the anode. They are thus able to pass through the anode and are absorbed in the fluorescent layer on its far side. The energy of the photo-electrons is re-emitted in the form of light. This process is in essence a system in which the light from the fluorescent layer of the photo-cathode is delivered to, and concentrated in, the small screen on the anode. A gain in brightness is thus brought about and is equal to the ratio of the area of the photo-cathode to that of the anode. There is, however, an additional gain due to the extra energy acquired by the electrons while under acceleration. This will be dependent upon the accelerating potential. The final gain is the product of the two factors, and in practice gains of x 500 to × i000 can be achieved. In an apparatus of this type the final image is necessarily of small diameter. Provision is made, therefore, for it to be examined by means of a low-power microscope. A number of advantages are gained through the use of an image intensifier. The need for dark-adaptation is no longer of great importance, and viewing can be carried out in a room that is not blacked-out but dimly lit. The gain in image brightness in itself increases the contrast of the image. It also enables lower kilo-voltages to be employed for a given specimen compared with conventional fluoroscopy, thus producing an additional increase in image contrast. The IQI sensitivity is improved, and in terms of step-type IQI sensitivity is almost equal to that of a radiograph. The thickness that can be penetrated is increased to approximately 4in (100ram) of aluminium and i/2in (12mm) of steel at 150kV. Since the viewing screen of the image intensifier is studied by means of an optical enlarging system, it follows that the grain of the screen phosphor will also be enlarged. It is for this reason that the image intensifier is unable to resolve fine detail unless some degree of projective magnification of the specimen image is introduced. This is normally accomplished by positioning the specimen at a point between the x-ray tube and the intensifier that gives the desired degree of magnification. The use of this method requires a fine-focus x-ray tube to ensure that the penumbral width of the image is kept within acceptable limits. Areas of the screen that are not covered by the specimen will be the source of considerable glare and should be eliminated by masking around the specimen and by means of opaque putty plugs in holes in the specimen that are axially oriented in the x-ray beam. Increasing use is now made of fluoroscopic systems in which the image intensifier is linked optically to a closed circuit television system so that the specimen image is presented on a monitor screen. Such equipment is expensive to install and for this reason is used only when there is a requirement to examine large numbers of a standard type of specimen.
Amplifiers The image i n t e n s i f i e r i s to s o m e extent e n e r g y dependent and shows a c o n s i d e r a b l y r e d u c e d r e s p o n s e to x - r a y s g e n e r a t e d at l e s s than about 35kV. In addition, at kilovoltages in e x c e s s of about 150kV t h e r e i s an i n c r e a s i n g t e n d e n c y for the viewing s c r e e n to be activated d i r e c t l y by the b e a m of r a d i a tion f r o m the x - r a y tube with a consequent l o s s of i m a g e
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F i g 4 Diagrammatic representation of an image intensifier
quality. T h e s e c h a r a c t e r i s t i c s a r e not a p p a r e n t in the i m a g e a m p l i f i e r . In this s y s t e m , an i m a g e is f o r m e d on a f l u o r e s c e n t s c r e e n . This i m a g e is r e f l e c t e d through 90 ° by a plane m i r r o r and i s r e c e i v e d by a w i d e - a p e r t u r e lens s y s t e m that f o c u s e s it on to a t e l e v i s i o n c a m e r a tube. This s y s t e m is r e l a t i v e l y independent of r a d i a t i o n e n e r g y and by the u s e of suitable t y p e s of f l u o r e s c e n t s c r e e n s can o p e r a t e s a t i s f a c t o r i l y f r o m about 20kV to 31MV. The use of the i m a g e a m p l i f i e r at 31MV f o r the f l u o r o s c o p i c i n s p e c t i o n of hot s t e e l b i l l e t s up to 12in (0.3m) thick has been r e p o r t e d S . Image a m p l i f i e r s y s t e m s usually i n c o r p o r a t e s o m e f o r m of r e c o r d ing s y s t e m . This may be in the f o r m of a 16mm movie c a m e r a s e t up to photograph a second m o n i t o r working in p a r a l l e l with the viewing m o n i t o r , or may be a v i d e o - t a p e r e c o r d i n g unit. Vidicon tubes that a r e d i r e c t l y s e n s i t i v e to x - r a y s have been developed for i n c o r p o r a t i o n into c l o s e d circuit television systems. The use of c l o s e d c i r c u i t t e l e v i s i o n s y s t e m s g i v e s a f u r t h e r c o n t r o l o v e r i m a g e b r i g h t n e s s and c o n t r a s t . In addition, it i s p o s s i b l e to c o n v e r t e l e c t r o n i c a l l y the n o r m a l ' p o s i t i v e ' s c r e e n i m a g e into one having the ' n e g a t i v e ' a p p e a r a n c e of a r a d i o g r a p h . It has b e e n found in p r a c t i c e that a c e r t a i n advantage i s to be gained f r o m this facility when s m a l l detail h a s to be d e t e c t e d against a b a c k g r o u n d that would n o r m a l l y be b r i g h t . It is also p o s s i b l e to e n l a r g e the i m a g e e l e c t r o n i cally. The t e l e v i s i o n s y s t e m e m p l o y e d f o r f l u o r o s c o p y is usually of a high definition type, eg 1,000 line, t r i p l e i n t e r lace. N e v e r t h e l e s s , it will be found that the b e s t r e s u l t s a r e obtained when p r o j e c t i v e e n l a r g e m e n t of the i m a g e is i n t r o duced by the use of a f i n e - f o c u s x - r a y tube in the m a n n e r previously described.
REFERENCES 1
Levy, L. and West, D. W., B r i t i s h J o u r n a l of Radiology, Vol 6 (1935)
2
Levy, L. and West, D. W., T r a n s a c t i o n s of the F a r a d a y Society, Vol 35 (1939)
3
Halmshaw, R. and Hunt, C., Unpublished M i n i s t r y of Supply R e p o r t (1957)
4
O'Connor, D. T. D. and Polansky, D., Navord R e p o r t 2168 (1951)
5
Luckerath, W., Fink, K. and Flossman, R., Iron and Steel, Vol 33, Nos I0 and 11 (1960)
6
Statutory Instruments No 808, The Ionising Radiations (Sealed Sources) Regulations (1969)
7
Holst, G., de Boer, J. H., Teves, M. C. and Veenemans, C. F., Physica, Vol 1 (1937) p 297
8
Oosterkamp, W. J., Proper, J. and Teves, M. C., Metallurgia, Vol 62 (1960) p 347 n o n - d e s t r u c t i v e t e s t i n g June 1970
187
Conventional r a d i o g r a p h i c t e c h n i q u e s have many outstanding advantages f o r ndt, not l e a s t of which i s the ability to ' s e e ' the i n t e r n a l s t r u c t u r e of an object. However, the lack of a method for the ' r e a l - t i m e ' p r e s e n t a t i o n of i n f o r m a t i o n can often be u n s a t i s f a c t o r y . It is h e r e that f l u o r o s c o p y can be ext r e m e l y useful. In this a r t i c l e , the author d i s c u s s e s the b a s i c
p r i n c i p l e s of f l u o r o s c o p y and d i s t i n g u i s h e s between ' c o n v e n tional' f l u o r o s c o p y and f l u o r o s c o p y aided by i m a g e i n t e n s i f i e r s . D e t a i l s c o n c e r n i n g the choice of phosphor, the c h a r a c t e r i s t i c s of f l u o r e s c e n t s c r e e n s , and the a p p a r a t u s and t e c h niques u s e d a r e given. Finally, the author d i s c u s s e s the m e c h a n i s m s and use of i m a g e i n t e n s i f i e r s and a m p l i f i e r s .
F l u o r o s c o p y ( ' s c r e e n i n g ' ) is a radiological method of ins p e c t i o n in which an i m a g e of the s p e c i m e n undergoing examination is f o r m e d on a f l u o r e s c e n t (light-emitting) s c r e e n . Two s y s t e m s of f l u o r o s c o p y a r e available. T h e s e a r e (a) conventional f l u o r o s c o p y in which the i m a g e on the s c r e e n i s viewed d i r e c t l y by the o p e r a t o r without any additional aid, and (b) image intensification or amplification in which the screen image is processed electronically before viewing and, if desired, presented on a television monitor screen. Both of these systems are characterised by certain features that distinguish them from radiography. They offer the immediate presentation of information and the ability to study moving objects, but do not produce any permanent record unless the basic system is supplemented by an additional recording stage that may be either photographic or electronic in nature.
conditions; in addition, it m u s t also s a t i s f y the following requirements:
BASIC PRINCIPLES All f l u o r o s c o p i c s y s t e m s a r e b a s e d on the l u m i n e s c e n t p r o p e r t y of x - r a y s and g a m m a - r a y s , ie that p r o p e r t y which c a u s e s the e m i s s i o n of light by c e r t a i n s u b s t a n c e s (phosphors) when they a r e i r r a d i a t e d by t h e s e r a d i a t i o n s . Many s u b s t a n c e s exhibit this effect, but not n e c e s s a r i l y in a m a n n e r that m a k e s them suitable f o r i n c o r p o r a t i o n in a f l u o r o scopic s y s t e m . The suitability of a phosphor for this p u r p o s e is d e t e r m i n e d by the m a n n e r in which the eye r e s p o n d s to the s t i m u l u s of light. The r e s p o n s e of the eye i s m o s t efficient in t e r m s of both c o n t r a s t p e r c e p t i o n (ability to p e r c e i v e c h a n g e s of b r i g h t n e s s ) and visual acuity (ability to p e r c e i v e fine detail) when it i s functioning at r e l a t i v e l y high b r i g h t n e s s l e v e l s such as might be e n c o u n t e r e d in b r i g h t day-light. At t h e s e l e v e l s the eye is capable of detecting v a r i a t i o n s in b r i g h t n e s s of about 1°~ above or below the ambient b r i g h t n e s s . At l o w e r l e v e l s this capability is s e v e r e l y r e d u c e d , and at s o m e e x t r e m e conditions a s m e t with in conventional f l u o r o s c o p y it may be no b e t t e r than 20% to 40%. The e f f i c i e n c y of the eye i s also influenced by the colour of light. This i s known a s the P u r k i n j e effect. At low b r i g h t n e s s l e v e l s the e y e ' s s p e c t r a l s e n s i t i v i t y i s at a m a x i m u m in the y e l l o w - g r e e n (5200~--10-1Om) r e g i o n of the v i s i b l e s p e c t r u m . It follows f r o m t h e s e c o n s i d e r a t i o n s that a p h o s p h o r intended f o r use in f l u o r o s c o p y m u s t have p r o p e r t i e s that m e e t t h e s e The author is with the M i n i s t r y of Technology, AQD L a b o r a t o r i e s , Harefield, Uxbridge, Middlesex, UK 184
n o n - d e s t r u c t i v e t e s t i n g June 1970
1
It m u s t be stable both c h e m i c a l l y and in r e s p e c t to its l i g h t - e m i t t i n g c h a r a c t e r i s t i c s under i n t e n s e and p r o l o n g e d p e r i o d s of i r r a d i a t i o n
2
It m u s t be unaffected by a t m o s p h e r i c conditions
3
The s p e c t r a l e m i s s i o n must be independent of the e n e r g y of the radiation e m p l o y e d
4
T h e r e must be an a b s e n c e of ' a f t e r - g l o w ' , ie r e t e n tion of i m a g e a f t e r the exciting radiation has been cut off
F r e e d o m f r o m a f t e r - g l o w is an e s s e n t i a l in any p h o s p h o r u s e d for the p u r p o s e s of fluoroscopy. P e r s i s t e n c e of the image r e s u l t i n g f r o m this effect would r e s u l t in p r a c t i c e in a confusion of s u p e r i m p o s e d i m a g e s , o r a b l u r r e d , s m e a r e d a p p e a r a n c e in the c a s e of a moving s p e c i m e n , thus making interpretation impossible. Most m o d e r n f l u o r o s c o p i c s c r e e n s a r e b a s e d on the type o r i g i n a l l y developed by Levy and West 1, 2. The p h o s p h o r in t h i s type of s c r e e n c o n s i s t s of a m i x t u r e of zinc sulphide and c a d m i u m sulphide, the r e l a t i v e p r o p o r t i o n s of which a r e c a r e f u l l y c o n t r o l l e d to g o v e r n the colour of the e m i t t e d light. A f u r t h e r d e g r e e of c o n t r o l i s e f f e c t e d through the addition of s m a l l a m o u n t s of s i l v e r to the p h o s p h o r to obtain a peak f l u o r e s c e n t e m i s s i o n in the region of 5, 200A (10-1Om). The a f t e r - g l o w effect that is p a r t i c u l a r l y c h a r a c t e r i s t i c of zinc sulphide i s inhibited by the addition of nickel in the p r o p o r t i o n of 1 p a r t to 2 million. It follows that the m a n u f a c t u r e of this type of phosphor, which i s c l a s s i f i e d a s being s i l v e r - a c t i v a t e d , n i c k e l - i n h i b i t e d , i s a m a t t e r of v e r y c l o s e and a c c u r a t e c h e m i c a l control.
CHARACTERISTICS OF FLUOROSCOPIC SCREENS A f l u o r o s c o p i c s c r e e n c o n s i s t s of a r i g i d s u p p o r t of thin c a r d o r p l a s t i c s h e e t on which i s coated a p h o s p h o r layer. The p h o s p h o r i s finely ground and i s held in p o s i t i o n by a suitable b i n d e r . The p a r t i c l e s i z e of the p h o s p h o r and the t h i c k n e s s of the p h o s p h o r l a y e r e x e r t s o m e influence on the b r i g h t n e s s of the i r r a d i a t e d s c r e e n , and up to a point an inc r e a s e in e i t h e r will give an i n c r e a s e in b r i g h t n e s s . However, an i n c r e a s e in the s i z e of the p h o s p h o r p a r t i c l e s and the s i z e of t h e i r c o n g l o m e r a t e s has an a d v e r s e effect on the ability of the s c r e e n to r e s o l v e fine detail. Similarly, an inc r e a s e in l a y e r t h i c k n e s s r e s u l t s in a g r e a t e r d e g r e e of
light s c a t t e r i n g within the l a y e r with a c o n s e q u e n t r e d u c t i o n in the r e s o l v i n g p o w e r of the s c r e e n . I n v e s t i g a t i o n s m a d e into the influence of t h e s e and o t h e r f a c t o r s have shown 3, 4 that a typical f l u o r o s c o p i c s c r e e n may have an i n h e r e n t uns h a r p n e s s of 0 . 5 to 1 . 3 m m . T h i s i s c o n s i d e r a b l y g r e a t e r than that of a f i n e - g r a i n x - r a y film (about 0 . 0 1 m m ) and s i g n i f i c a n t l y g r e a t e r than that of a s c r e e n - t y p e x - r a y s c r e e n e x p o s e d with a p a i r of a v e r a g e - s p e e d s a l t intensifying s c r e e n s (about 0 . 3 r a m ) . The i n t e n s i t y of the light e m i s s i o n f r o m a f l u o r o s c o p i c s c r e e n i s p r o p o r t i o n a l to the i n t e n s i t y of the x - o r g a m m a r a d i a t i o n r e a c h i n g the s c r e e n . An a r e a of g r e a t e r x - r a y int e n s i t y will give r i s e to a c o r r e s p o n d i n g a r e a of i n c r e a s e d b r i g h t n e s s in the s c r e e n i m a g e . In the i n s t a n c e of a s p e c i m e n containing a flaw in the f o r m of a void, eg a g a s - h o l e in a casting, the flaw will be r e v e a l e d a s a l i g h t e r a r e a a g a i n s t the d a r k e r b a c k g r o u n d of the s u r r o u n d i n g i m a g e of the sound m e t a l . The s c r e e n p i c t u r e in conventional f l u o r o s c o p y can thus be r e g a r d e d as a p o s i t i v e in r e l a t i o n to the negative a p p e a r a n c e of a r a d i o g r a p h . This c o m p l e t e r e v e r s a l of the d e n s i t y - b r i g h t n e s s r e l a t i o n s h i p s m u s t constantly be b o r n e in mind when i n t e r p r e t i n g f l u o r o s c o p i c evidence. This effect, which can be o v e r c o m e in s o m e i m a g e - a m p l i f i e r s y s t e m s , is the only difficulty likely to be e n c o u n t e r e d in i n t e r p r e t a t i o n , s i n c e the p r o j e c t e d i m a g e p a t t e r n s of c a s t i n g and weld flaws r e m a i n the s a m e in f l u o r o s c o p y as in r a d i o g r a p h y . R e f e r e n c e h a s b e e n made to both x - r a y s and g a m m a - r a y s when d e s c r i b i n g the p r o p e r t i e s of p h o s p h o r s . In p r a c t i c e , v e r y little u s e i s made of g a m m a - r a y s and the technique of f l u o r o s c o p y in its r o u t i n e a p p l i c a t i o n s is l a r g e l y one a s s o c i a ted with x - r a y s . T h i s a r i s e s f o r two r e a s o n s . F i r s t l y , the production of a f l u o r e s c e n t s c r e e n i m a g e of adequate b r i g h t n e s s r e q u i r e s that a r e l a t i v e l y high i n t e n s i t y of r a d i a t i o n r e a c h e s the s c r e e n . An x - r a y set i s g e n e r a l l y capable of supplying the i n t e n s i t y r e q u i r e d , and in any event can b e p o s i t i o n e d r e a s o n a b l y c l o s e to the s c r e e n without any s i g n i ficant i m p a i r m e n t of i m a g e u n s h a r p n e s s b e c a u s e of i t s r e latively s m a l l s i z e of focal spot. G a m m a - r a y s o u r c e s a r e poor e m i t t e r s of r a d i a t i o n and to p r o d u c e the i n t e n s i t i e s r e quired they would n e e d to have p h y s i c a l d i m e n s i o n s that would s e r i o u s l y i n c r e a s e the g e o m e t r i c u n s h a r p n e s s of the s c r e e n image. The s e c o n d r e a s o n r e l a t e s to the l u m i n e s c e n t e f f i c i e n c y of p h o s p h o r s at h i g h e r e n e r g i e s of radiation. As the e n e r g y of the r a d i a t i o n i s i n c r e a s e d , t h e r e i s p r o p o r t i o n ally l e s s a b s o r p t i o n of the r a d i a t i o n in the p h o s p h o r layer, with a c o n s e q u e n t r e d u c t i o n in the i n t e n s i t y of the f l u o r e s c e n t e m i s s i o n . The l u m i n e s c e n t e f f i c i e n c y of the p h o s p h o r thus f a l l s off as the e n e r g y of the r a d i a t i o n is i n c r e a s e d . Since,
f8
/ 1
2 ¢
3
/.
Observer 1 x - r a y tube; 2 baffle to r e s t r i c t i r r a d i a t e d field; 3 i n n e r lead shielding; 4 p h o s p h o r - c o a t e d s c r e e n ; 5 plane m i r r o r ; 6 l e a d g l a s s window; 7 s p e c i m e n ; 8 o u t e r casing. B r o k e n l i n e s r e p r e s e n t path of x - r a y s , solid l i n e s r e p r e s e n t light path to viewer F i g 1 S c h e m a t i c d i a g r a m ¢~ i n d i r e c t v i s i o n screening c a b i n e t
with one exception, all the c o m m o n l y u s e d g a m m a - r a y s o u r c e s e m i t r e l a t i v e l y high e n e r g y radiation, t h e r e i s little s c o p e f o r t h e i r u s e in routine f l u o r o s c o p y . However, s p e c i a l i n s t a n c e s have b e e n r e p o r t e d 5 w h e r e the use of c o m p l e x s c r e e n s c o n s i s t i n g of a lead f i l t e r in conjunction with a p h o s p h o r s c r e e n has enabled high e n e r g y f l u o r o s c o p y to be p e r f o r m e d . In t h i s c a s e the s c r e e n r e a c t s p r i m a r i l y to the p h o t o - e l e c t r o n s e j e c t e d f r o m the lead and has led to the s u c c e s s f u l u s e of f l u o r o s c o p y with 31MeV (50 x 1 0 - 1 9 j ) x-radition.
APPARATUS
FOR CONVENTIONAL
FLUOROSCOPY
In conventional f l u o r o s c o p y the x - r a y tube, s p e c i m e n and f l u o r e s c e n t s c r e e n s a r e housed in a s h i e l d e d cabinet called a ' f l u o r o s c o p e ' o r ' s c r e e n i n g c a b i n e t ' . The b a s i c g e n e r a l a r r a n g e m e n t of such a cabinet i s shown in Fig 1. S c r e e n i n g c a b i n e t s may be d e s i g n e d for g e n e r a l use o r for a s p e c i f i c p u r p o s e , in which c a s e it would be capable of handling only the type of s p e c i m e n for which it was designed. In e i t h e r ins t a n c e the d e s i g n m u s t take into account the r e q u i r e m e n t s of the c u r r e n t s t a t u t o r y r e g u l a t i o n s 6 governing the i n d u s t r i a l u s e of ionising r a d i a t i o n s . The d e s i g n m u s t o b s e r v e , t h e r e f o r e , the following b a s i c p r i n c i p l e s :
1
2
3
4
The cabinet m u s t be s h i e l d e d to e n s u r e that the leakage of s c a t t e r e d and p r i m a r y radiation d o e s not e x c e e d 2 . 5 m r / h if c l a s s i f i e d w o r k e r s only a r e affected by i t s use and p r e s e n c e When the cabinet i s opened f o r any p u r p o s e , eg the i n s e r t i o n o r r e m o v a l of the s p e c i m e n , the x - r a y b e a m m u s t be cut off by i n t e r r u p t i o n of the supply to the ht g e n e r a t o r Any manipulation of the s p e c i m e n in the b e a m of x - r a y s m u s t be e f f e c t e d by m e a n s of c o n t r o l s ext e r n a l to the cabinet W h e r e p r a c t i c a b l e the s c r e e n i m a g e must be i n s p e c ted i n d i r e c t l y through a m i r r o r
The l e a d - g l a s s window through which the s c r e e n is viewed m u s t have shielding p r o p e r t i e s equal to t h o s e of the m e t a l l i c shielding of the walls of the cabinet. Lead g l a s s is n o r m a l l y g r a d e d in t e r m s of its lead equivalent. A l a y e r of lead g l a s s having a 2mm lead equivalent would have the s a m e shielding effect a s a 2mm l a y e r of m e t a l l i c lead. H, t h e r e f o r e , it i s n e c e s s a r y to provide 4mm lead shielding on a cabinet, it would be n e c e s s a r y to glaze the window with two l a y e r s of 2mm lead g l a s s . It i s e s s e n t i a l to e n s u r e that a good o v e r lap e x i s t s b e t w e e n the g l a s s of the window and the adjacent m e t a l l i c lead shielding by wrapping the lead shielding around the s q u a r e e d g e s of the g l a s s s h e e t s . I n d i r e c t viewing i s called for in o r d e r to o v e r c o m e the s o m e what r e m o t e p o s s i b i l i t y that the l e a d - g l a s s window may c r a c k o r s h a t t e r while the f l u o r o s c o p e i s in use. H t h i s o c c u r r e d the o b s e r v e r would undoubtedly be e x p o s e d to the p r i m a r y b e a m of r a d i a t i o n for as long as he c h o s e to r e m a i n in the viewing position, and for this r e a s o n the use of a m i r r o r should be r e c o m m e n d e d . N e v e r t h e l e s s , its introduction without doubt c r e a t e s a n o t h e r f r u s t r a t i o n in an a l r e a d y arduous p r o c e s s , and r o b s the v i e w e r of the s a t i s f a c t i o n of a good c l e a r look at the s c r e e n itself. When a m i r r o r i s employed it is d e s i r a b l e that it should be of the s u r f a c e - s i l v e r e d type. T h i s will p r e v e n t the f o r m a t i o n of double i m a g e s which, in the i n s t a n c e of s p e c i m e n s giving l o c a l i s e d b r i g h t a r e a s in an o t h e r w i s e d a r k image, can lead to confusion in i n t e r p r e t a t i o n . This effect i s i l l u s t r a t e d d i a g r a m m a t i c a l l y in Fig 2. Care m u s t be taken to clean r e g u l a r l y the s u r f a c e s of the s c r e e n , m i r r o r , and viewing window. A significant light l o s s can o c c u r through the p r e s e n c e of a c c u m u l a t e d l a y e r s of dust in the optical path of a f l u o r o s c o p e . An i m p o r t a n t f e a t u r e in the design of a f l u o r o s c o p e is the p r o v i s i o n of movable m a s k s at the x - r a y s o u r c e to enable the i r r a d i a t e d a r e a of the s c r e e n to be r e d u c e d in s i z e to that r e q u i r e d to a c c o m m o d a t e the s p e c i m e n image. This will r e m o v e the b r i g h t g l a r e that would o t h e r w i s e r e s u l t f r o m the unshielded a r e a s of the screen.
n o n - d e s t r u c t i v e t e s t i n g June 1970
185
#
they attain m a x i m u m s e n s i t i v i t y under t h e s e conditi(m:~ This effect will have been e x p e r i e n c e d by anyone who, t o t example, has e n t e r e d a d a r k e n e d c i n e m a f r o m a brightly lii s t r e e t . The p r o c e s s of dark adaptation may extend over a period of 60rain b e f o r e it has been completed, but the maioc i m p r o v e m e n t in s e n s i t i v i t y is usually achieved ~Lfter about 20min. The d a r k - a d a p t e d condition is lost once the e y e s art-, again e x p o s e d to bright white light and a f u r t h e r period of adaptation would then be r e q u i r e d to r e s t o r e the f o r m e r condition. L o s s of dark adaptation does not o c c u r with red light, and it is thus p o s s i b l e to illuminate the raom in which the f l u o r o s c o p e is i n s t a l l e d with this colour, at a level that p e r m i t s the writing of n o t e s a~ld the inspection of s p e c i m e n s .
//
Fig 2 Formation of multiple images by a back-silvered mirror. The main beam is shown by a solid line and and secondary reflections by dotted l i n e s
ASSESSMENT OF CONVENTIONAL FLUOROSCOPY
TECHNIQUES OF CONVENTIONAL FLUOROSCOPY The p r o c e d u r e to be adopted when c a r r y i n g out s c r e e n i n g examination will v a r y with the type of s p e c i m e n that is involved and the type of i n f o r m a t i o n that is sought. In an application of f l u o r o s c o p y w h e r e the s p e c i m e n is e a s i l y p e n e t r a t ed and the i n f o r m a t i o n sought i s in the f o r m of l a r g e detail, t h e r e is no g r e a t need f o r s p e c i a l l y c o n t r o l l e d viewing conditions or r e g u l a t e d working d r i l l s . It is usually sufficient in such i n s t a n c e s to c a r r y out the examination in a n o r m a l l y lit r o o m p r o v i d e d that the s c r e e n is shaded f r o m the d i r e c t r o o m lighting by s o m e f o r m of viewing port through which the o p e r a t o r can study the s c r e e n . Screening under such conditions can be c a r r i e d on continuously with no m o r e fatigue than would r e s u l t f r o m the d e g r e e of c o n c e n t r a t i o n involved in studying d e t a i l s under n o r m a l conditions of lighting. However, if the i n f o r m a t i o n sought g i v e s r i s e only to low c o n t r a s t or fine detail in the s c r e e n image it i s e s s e n t i a l that p a r t i c u l a r c a r e should be taken to e n s u r e that suitable viewing conditions a r e employed. Such conditions include the installation of the f l u o r o s c o p e in a d a r k e n e d r o o m , o r in one that is lit with r e d light, and a p e r i o d of dark adaptation b e f o r e s c r e e n i n g is c o m m e n c e d . It i s also d e s i r a b l e that the study of the s c r e e n should be l i m i t e d to continuous p e r i o d s not exceeding 30rain. The e y e s a r e not i m m e d i a t e l y s e n s i t i v e to low l e v e l s of illumination. A p e r i o d of dark adaptation i s r e q u i r e d b e f o r e
The main advantages of conventional f l u o r o s c o p y are low o p e r a t i o n a l cost, i m m e d i a t e p r e s e n t a t i o n of information. ability to study s p e c i m e n s in motion, and lack of a need for a high d e g r e e of o p e r a t i v e skill. In r e g a r d to the latter, howe v e r , it m u s t be u n d e r s t o o d that the o p e r a t o r must be capable of c a r r y i n g out on his own initiative the i n t e r p r e t a t i o n and a s s e s s m e n t of the f l u o r o s c o p i c evidence. The d i s a d v a n t a g e s include a v e r y r e s t r i c t e d application, eg l i m i t e d to s p e c i m e n s capable of being adequately p e n e t r a t e d at a m a x i m u m of 150 to 200kV. This r e p r e s e n t s a p p r o x i m a t e l y 115in (63ram) of aluminium alloy and about 1/sin (3mm) of s t e e l . The quality of r e s u l t obtainable in t e r m s of s e n s i t i v i t y to d e f e c t s and r e s o l u t i o n of detail i s given in Fig 3 and Table I. The r e s u l t s given in Table 1 w e r e obtained by s c r e e n i n g tlat p l a t e s of d u r a l u m i n to the tube side of which w e r e attached copper w i r e s of v a r i o u s d i a m e t e r s . The kilovoltage was v a r i e d to suit the s p e c i m e n t h i c k n e s s and did not e x c e e d 150kV (0. l m m focal spot) and 200kV (5mm focal spot). The image e n l a r g e m e n t in the f o r m e r c a s e was obtained by p r o jective magnification, ie by r e m o v i n g the s p e c i m e n f r o m the s c r e e n while retaining a fixed t u b e - s c r e e n distance.
IMAGE INTENSIFIERS AND AMPLIFIERS One of the main c a u s e s of the i n f e r i o r quality of the image obtained in conventional f l u o r o s c o p y is the low level of b r i g h t n e s s of the s c r e e n image. This disadvantage has given r i s e to the d e v e l o p m e n t of e l e c t r o n i c m e t h o d s for significantly i n c r e a s i n g the b r i g h t n e s s of the s c r e e n .
Intensifiers 20
The e a r l i e s t work in this field was undertaken by HolsU and his c o - w o r k e r s . If light is allowed to i r r a d i a t e a photos e n s i t i v e l a y e r such a s s e l e n i u m , p h o t o - e l e c t r o n s will be e j e c t e d f r o m its s u r f a c e . If the p h o t o - s e n s i t i v e l a y e r f o r m s p a r t of a negatively c h a r g e d e l e c t r o d e (a photo-cathode), the e l e c t r o n s can be a c c e l e r a t e d t o w a r d s a positively c h a r g e d anode. The p h o t o - e l e c t r o n s will a c q u i r e an additional amount of e n e r g y that will be d e t e r m i n e d by the a c c e l e r a t i n g potential
A lS
10 b0
Table 1 Discernibility of detail with c(mventional fluoroscopy Specimen t h i c k n e s s (in-duralumin) I
0
I
I
I
1 2 Specimen thickness [ inches of aluminium ]
3
Fig 3 Sensitivity of conventional fluoroscopy to simulated casting defects in alumininm, in comparison with steptype IQI sensitivity (Rockley, J. C., Introduction to industrial radiology, Butterwort~ London, 1969) n o n - d e s t r u c t i v e t e s t i n g June 1970
(in--copper)
I
Curve A s i m u l a t e d sand and oxide inclusions; Curve B s i mulated gas holes; Curve C s t e p - t y p e IQI. To c o n v e r t in to ram. u s e the f a c t o r x 2 5 . 4
186
D i a m e t e r of s m a l l e s t v i s i b l e w i r e
5mm focal spot (xl)
0. l m m focal spot (:'<3)*
0. 5(12. 7ram)
0.01(0. 254mm)
0. 005(0. 127ram)
1.0(25.4ram)
0.02(0. 508mm)
0. 008(0. 203mm)
1.5(38.1ram)
0.03(0. 762mm)
0. 008(0. 203ram)
2.0(50.8mm)
0.03(0. 762mm)
0. 011(0. 279mm)
* D e g r e e of image e n l a r g e m e n t (Rockley, J. C., Introduction to i n d u s t r i a l radiology, B u t t e r worth, London, 1964)
between the photo-cathode and the anode. In an image intensifier (Fig 4) the photo-cathode consists of a curved fluorescent screen having its fluorescent layer coated with a thin photo-sensitive film. The anode is basically a thin aluminium foil coated on the side remote from the photo-cathode with a fluorescent layer. These two electrodes are mounted inside an evacuated glass container. A potential difference of about 15kV is applied to these electrodes. X-rays enter the intensifier from the end adjacent to the photocathode. These react with the fluorescent layer to give rise to an emission of light. This is immediately absorbed by the photo-sensitive layer which, in turn, emits photo-electrons in an intensity that is proportional to the light intensity generated in the fluorescent layer of the photo-cathode. Since the photoelectrons are emitted in a direction normal to the surface of the selenium layer, they are directed by the curvature of the photo-cathode towards the anode. The accelerating potential provides enough energy for the photo-electronsto penetrate the thin aluminium foil of the anode. They are thus able to pass through the anode and are absorbed in the fluorescent layer on its far side. The energy of the photo-electrons is re-emitted in the form of light. This process is in essence a system in which the light from the fluorescent layer of the photo-cathode is delivered to, and concentrated in, the small screen on the anode. A gain in brightness is thus brought about and is equal to the ratio of the area of the photo-cathode to that of the anode. There is, however, an additional gain due to the extra energy acquired by the electrons while under acceleration. This will be dependent upon the accelerating potential. The final gain is the product of the two factors, and in practice gains of x 500 to × i000 can be achieved. In an apparatus of this type the final image is necessarily of small diameter. Provision is made, therefore, for it to be examined by means of a low-power microscope. A number of advantages are gained through the use of an image intensifier. The need for dark-adaptation is no longer of great importance, and viewing can be carried out in a room that is not blacked-out but dimly lit. The gain in image brightness in itself increases the contrast of the image. It also enables lower kilo-voltages to be employed for a given specimen compared with conventional fluoroscopy, thus producing an additional increase in image contrast. The IQI sensitivity is improved, and in terms of step-type IQI sensitivity is almost equal to that of a radiograph. The thickness that can be penetrated is increased to approximately 4in (100ram) of aluminium and i/2in (12mm) of steel at 150kV. Since the viewing screen of the image intensifier is studied by means of an optical enlarging system, it follows that the grain of the screen phosphor will also be enlarged. It is for this reason that the image intensifier is unable to resolve fine detail unless some degree of projective magnification of the specimen image is introduced. This is normally accomplished by positioning the specimen at a point between the x-ray tube and the intensifier that gives the desired degree of magnification. The use of this method requires a fine-focus x-ray tube to ensure that the penumbral width of the image is kept within acceptable limits. Areas of the screen that are not covered by the specimen will be the source of considerable glare and should be eliminated by masking around the specimen and by means of opaque putty plugs in holes in the specimen that are axially oriented in the x-ray beam. Increasing use is now made of fluoroscopic systems in which the image intensifier is linked optically to a closed circuit television system so that the specimen image is presented on a monitor screen. Such equipment is expensive to install and for this reason is used only when there is a requirement to examine large numbers of a standard type of specimen.
Amplifiers The image i n t e n s i f i e r i s to s o m e extent e n e r g y dependent and shows a c o n s i d e r a b l y r e d u c e d r e s p o n s e to x - r a y s g e n e r a t e d at l e s s than about 35kV. In addition, at kilovoltages in e x c e s s of about 150kV t h e r e i s an i n c r e a s i n g t e n d e n c y for the viewing s c r e e n to be activated d i r e c t l y by the b e a m of r a d i a tion f r o m the x - r a y tube with a consequent l o s s of i m a g e
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F i g 4 Diagrammatic representation of an image intensifier
quality. T h e s e c h a r a c t e r i s t i c s a r e not a p p a r e n t in the i m a g e a m p l i f i e r . In this s y s t e m , an i m a g e is f o r m e d on a f l u o r e s c e n t s c r e e n . This i m a g e is r e f l e c t e d through 90 ° by a plane m i r r o r and i s r e c e i v e d by a w i d e - a p e r t u r e lens s y s t e m that f o c u s e s it on to a t e l e v i s i o n c a m e r a tube. This s y s t e m is r e l a t i v e l y independent of r a d i a t i o n e n e r g y and by the u s e of suitable t y p e s of f l u o r e s c e n t s c r e e n s can o p e r a t e s a t i s f a c t o r i l y f r o m about 20kV to 31MV. The use of the i m a g e a m p l i f i e r at 31MV f o r the f l u o r o s c o p i c i n s p e c t i o n of hot s t e e l b i l l e t s up to 12in (0.3m) thick has been r e p o r t e d S . Image a m p l i f i e r s y s t e m s usually i n c o r p o r a t e s o m e f o r m of r e c o r d ing s y s t e m . This may be in the f o r m of a 16mm movie c a m e r a s e t up to photograph a second m o n i t o r working in p a r a l l e l with the viewing m o n i t o r , or may be a v i d e o - t a p e r e c o r d i n g unit. Vidicon tubes that a r e d i r e c t l y s e n s i t i v e to x - r a y s have been developed for i n c o r p o r a t i o n into c l o s e d circuit television systems. The use of c l o s e d c i r c u i t t e l e v i s i o n s y s t e m s g i v e s a f u r t h e r c o n t r o l o v e r i m a g e b r i g h t n e s s and c o n t r a s t . In addition, it i s p o s s i b l e to c o n v e r t e l e c t r o n i c a l l y the n o r m a l ' p o s i t i v e ' s c r e e n i m a g e into one having the ' n e g a t i v e ' a p p e a r a n c e of a r a d i o g r a p h . It has b e e n found in p r a c t i c e that a c e r t a i n advantage i s to be gained f r o m this facility when s m a l l detail h a s to be d e t e c t e d against a b a c k g r o u n d that would n o r m a l l y be b r i g h t . It is also p o s s i b l e to e n l a r g e the i m a g e e l e c t r o n i cally. The t e l e v i s i o n s y s t e m e m p l o y e d f o r f l u o r o s c o p y is usually of a high definition type, eg 1,000 line, t r i p l e i n t e r lace. N e v e r t h e l e s s , it will be found that the b e s t r e s u l t s a r e obtained when p r o j e c t i v e e n l a r g e m e n t of the i m a g e is i n t r o duced by the use of a f i n e - f o c u s x - r a y tube in the m a n n e r previously described.
REFERENCES 1
Levy, L. and West, D. W., B r i t i s h J o u r n a l of Radiology, Vol 6 (1935)
2
Levy, L. and West, D. W., T r a n s a c t i o n s of the F a r a d a y Society, Vol 35 (1939)
3
Halmshaw, R. and Hunt, C., Unpublished M i n i s t r y of Supply R e p o r t (1957)
4
O'Connor, D. T. D. and Polansky, D., Navord R e p o r t 2168 (1951)
5
Luckerath, W., Fink, K. and Flossman, R., Iron and Steel, Vol 33, Nos I0 and 11 (1960)
6
Statutory Instruments No 808, The Ionising Radiations (Sealed Sources) Regulations (1969)
7
Holst, G., de Boer, J. H., Teves, M. C. and Veenemans, C. F., Physica, Vol 1 (1937) p 297
8
Oosterkamp, W. J., Proper, J. and Teves, M. C., Metallurgia, Vol 62 (1960) p 347 n o n - d e s t r u c t i v e t e s t i n g June 1970
187