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Medical Image Perception
Theoretical and Applied Aspects of Eye Movement Research A.G. Gale and F. Johnson (Editors) 0 Elsevier Science Publishers B.V. (North-Holland), 1984
295
MEDICAL IMAGE PERCEPTION INTRODUCTION A l a s t a i r G. Gale, D i v i s i o n o r Radiology, Queen's Medical Centre, N o t t i ngham, England. Diagnostic r a d i o l o g y u t i l i s e s various techniques f o r d i s p l a y i n g the i n t e r n a l morphology o f t h e body e i t h e r i n a s t a t i c o r a dynamic manner. The problem o f medical imaging i s t h a t d e s p i t e the t e c h n o l o g i c a l advances medical diagnosis i s s t i l l s u b j e c t t o human f r a i l t i e s . Nowhere was t h i s more w r y l y commented upon than by Llewellyn-Thomas (1976) i n d e s c r i b i n g one o f t h e techniques: "It i s an i n c r e d i b l e machine, a triumph o f science and engineering, and y e t i n t h e end some j e r k can be l o o k i n g a t i t and miss something" (p. 352). Various mass surveys and experimental s t u d i e s have found e r r o r r a t e s as high as 30%. This type o f f i n d i n g has spurned t h e experimental i n v e s t i g a t i o n o f t h e d i a g n o s t i c process. Most research has concentrated on chest radiograpty i f f o r no o t h e r reason than the vast number o f these radiographs which are examined each year. The f o l l o w i n g papers document the most r e c e n t work i n t h i s area. Kundel, Nodine and Toto r e p o r t a lung nodule search task where the d i s p l a y was d i r e c t l y r e l a t e d t o the observer's l i n e o f s i g h t i n a manner s i m i l a r t o McConkie and colleagues ( t h i s volume). A s i n g l e chest radiograph was presented t o observers so t h a t on some occasions a t a r g e t nodule was present. However t h i s could o n l y be seen v i a a v a r i a b l e s i z e d 'window' which was yoked t o t h e observer's gaze l o c a t i o n . The observers were unaware o f t h i s window and t h e arrangement meant t h a t w h i l s t p e r i p h e r a l v i s i o n o f the r a d i o graph was unaffected i t was o f no use i n a i d i n g nodule d e t e c t i o n . The authors r e p o r t t h a t as window s i z e increased so t h e number o f nodules scanned increased. Performance a l s o increased b u t l e v e l l e d o f f w i t h window sizes l a r g e r than 3.5 degrees. With i n c r e a s i n g window s i z e t h e time t o f i x a t e t h e nodule decreased. This type o f study, where t h e r e l a t i v e c o n t r i b u t i o n s o f f o v e a l and p e r i p h e r a l v i s i o n are teased apart, represents t h e c u r r e n t s t a t e o f t h e a r t and these e a r l y r e s u l t s promise f u r t h e r e x c i t i n g developments. Carmody a l s o i n v e s t i g a t e s the e f f e c t o f a t t e n d i n g t o s e l e c t e d port i o n s o f t h e chest radiograph. Manipulation o f t h e d i s p l a y such as by tachi s t o s c o p i c p r e s e n t a t i o n o r segmented viewing a l t e r s the observer's d e c i s i o n making a b i l i t y . Such s t u d i e s e s s e n t i a l l y prevent the comparison o f a suspected abnormality s i t e w i t h o t h e r d i s p l a y areas. The author r e p o r t s a stuQ which d i r e c t l y i n v e s t i g a t e s the e f f e c t o f p e r m i t t i n g o r p r e v e n t i n g such comparative scanning. I n a ' c o n s t r a i n e d ' experimental c o n d i t i o n observers had t o determine whether a nodule was present w i t h i n a given area o f a chest radiograph w i t h the d i s p l a y being terminated i f t h e i r gaze s t r a y e d beyond t h e d e f i n e d l i m i t s . Thus p e r i p h e r a l v i s i o n o f the r e s t o f t h e d i s p l a y was p o s s i b l e b u t foveal examination o f such areas was prevented. I n a second 'comparison' c o n d i t i o n t h e observers were allowed t o compare t h i s area w i t h o t h e r p a r t s o f the radiograph. Results demonstrated t h a t the comparison viewing c o n d i t i o n produced s u p e r i o r performance. The a b i l i t y t o adequately compare one p a r t o f a radiograph w i t h another t h e r e f o r e seems t o be an important f a c t o r . As Carmody p o i n t s o u t how then can r a d i o l o g i s t s be encouraged t o use t h i s technique? The author goes on t o address the compli-
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296
A.G. Gale
cating i s s u e s raised by the findings t h a t t r a i n i n g observers t o use such a comparative technique does not d r a s t i c a l l y improve performance and f u r t h e r more t h a t eye movement s t u d i e s of radiologistsshow l i t t l e evidence of such comparison viewing in c l i n i c a l p r a c t i c e . One of the problems of radiological reporting i s t h a t i t i s c a r r i e d out a t d i f f e r e n t times of t h e day. In o t h e r tasks involving visual search,performance has been found t o be a f f e c t e d by circadian v a r i a t i o n . Gale e t a l . i n v e s t i g a t e t h i s f a c t o r in a pulmonary nodule detection task a t three times of the day. Using medical students no v a r i a t i o n i n p e r f o n a n c e was i n i t i a l l y found b u t when gaze position upon nodule detection was incorporated i n t o the analysis t o give a location response then a drop in s e n s i t i v i t y between morning and lunch session was found. In c o n t r a s t no v a r i a b i l i t y in t h e observer's self-assessed a b i l i t y was reported. Some,mainly minor, v a r i a b i l i t y in eye movement parameters was a l s o reported, The authors argue fromacognitive standpoint t h a t such r e s u l t s may i n d i c a t e a change in s t r a t e g y by the observers a t d i f f e r e n t times of the day. Whilst e x t o l l ing care in extrapolation these findings do i n d i c a t e the need t o i n v e s t i gate circadian v a r i a t i o n f u r t h e r . The f i n a l paper in t h i s section i s by Papin, Metges and Amalberti and rep o r t s three s t u d i e s . The f i r s t i s again concerned with examining c h e s t radiographs and complements the others. The authors then extend the recording technique t o the dynamic s i t u a t i o n of the ultrasound examination. Finally the usefulness of recording visual behaviour as a t r a i n i n g a i d i s considered. These papers demonstrate the variety of research c u r r e n t l y being undertaken t o examine t h e problems of medical image perception. Considering the great financial outlay involved in purchasing any p a r t i c u l a r p a t i e n t imaging system and t h e amount of technical e f f o r t exerted i n producing good q u a l i t y images i t i s always s u r p r i s i n g t h a t r e l a t i v e l y l i t t l e a t t e n t i o n i s paid t o the psycho1 ogi cal f a c t o r s involved. Re f e r e nce Llewellyn-Thomas, E. Advice t o the searcher, i n : Monty, R.A. and Senders, J.W. (Eds.) Eye movements and psychological processes (Erlbaum, H i l l s d a l e 1976).
295
MEDICAL IMAGE PERCEPTION INTRODUCTION A l a s t a i r G. Gale, D i v i s i o n o r Radiology, Queen's Medical Centre, N o t t i ngham, England. Diagnostic r a d i o l o g y u t i l i s e s various techniques f o r d i s p l a y i n g the i n t e r n a l morphology o f t h e body e i t h e r i n a s t a t i c o r a dynamic manner. The problem o f medical imaging i s t h a t d e s p i t e the t e c h n o l o g i c a l advances medical diagnosis i s s t i l l s u b j e c t t o human f r a i l t i e s . Nowhere was t h i s more w r y l y commented upon than by Llewellyn-Thomas (1976) i n d e s c r i b i n g one o f t h e techniques: "It i s an i n c r e d i b l e machine, a triumph o f science and engineering, and y e t i n t h e end some j e r k can be l o o k i n g a t i t and miss something" (p. 352). Various mass surveys and experimental s t u d i e s have found e r r o r r a t e s as high as 30%. This type o f f i n d i n g has spurned t h e experimental i n v e s t i g a t i o n o f t h e d i a g n o s t i c process. Most research has concentrated on chest radiograpty i f f o r no o t h e r reason than the vast number o f these radiographs which are examined each year. The f o l l o w i n g papers document the most r e c e n t work i n t h i s area. Kundel, Nodine and Toto r e p o r t a lung nodule search task where the d i s p l a y was d i r e c t l y r e l a t e d t o the observer's l i n e o f s i g h t i n a manner s i m i l a r t o McConkie and colleagues ( t h i s volume). A s i n g l e chest radiograph was presented t o observers so t h a t on some occasions a t a r g e t nodule was present. However t h i s could o n l y be seen v i a a v a r i a b l e s i z e d 'window' which was yoked t o t h e observer's gaze l o c a t i o n . The observers were unaware o f t h i s window and t h e arrangement meant t h a t w h i l s t p e r i p h e r a l v i s i o n o f the r a d i o graph was unaffected i t was o f no use i n a i d i n g nodule d e t e c t i o n . The authors r e p o r t t h a t as window s i z e increased so t h e number o f nodules scanned increased. Performance a l s o increased b u t l e v e l l e d o f f w i t h window sizes l a r g e r than 3.5 degrees. With i n c r e a s i n g window s i z e t h e time t o f i x a t e t h e nodule decreased. This type o f study, where t h e r e l a t i v e c o n t r i b u t i o n s o f f o v e a l and p e r i p h e r a l v i s i o n are teased apart, represents t h e c u r r e n t s t a t e o f t h e a r t and these e a r l y r e s u l t s promise f u r t h e r e x c i t i n g developments. Carmody a l s o i n v e s t i g a t e s the e f f e c t o f a t t e n d i n g t o s e l e c t e d port i o n s o f t h e chest radiograph. Manipulation o f t h e d i s p l a y such as by tachi s t o s c o p i c p r e s e n t a t i o n o r segmented viewing a l t e r s the observer's d e c i s i o n making a b i l i t y . Such s t u d i e s e s s e n t i a l l y prevent the comparison o f a suspected abnormality s i t e w i t h o t h e r d i s p l a y areas. The author r e p o r t s a stuQ which d i r e c t l y i n v e s t i g a t e s the e f f e c t o f p e r m i t t i n g o r p r e v e n t i n g such comparative scanning. I n a ' c o n s t r a i n e d ' experimental c o n d i t i o n observers had t o determine whether a nodule was present w i t h i n a given area o f a chest radiograph w i t h the d i s p l a y being terminated i f t h e i r gaze s t r a y e d beyond t h e d e f i n e d l i m i t s . Thus p e r i p h e r a l v i s i o n o f the r e s t o f t h e d i s p l a y was p o s s i b l e b u t foveal examination o f such areas was prevented. I n a second 'comparison' c o n d i t i o n t h e observers were allowed t o compare t h i s area w i t h o t h e r p a r t s o f the radiograph. Results demonstrated t h a t the comparison viewing c o n d i t i o n produced s u p e r i o r performance. The a b i l i t y t o adequately compare one p a r t o f a radiograph w i t h another t h e r e f o r e seems t o be an important f a c t o r . As Carmody p o i n t s o u t how then can r a d i o l o g i s t s be encouraged t o use t h i s technique? The author goes on t o address the compli-
-
296
A.G. Gale
cating i s s u e s raised by the findings t h a t t r a i n i n g observers t o use such a comparative technique does not d r a s t i c a l l y improve performance and f u r t h e r more t h a t eye movement s t u d i e s of radiologistsshow l i t t l e evidence of such comparison viewing in c l i n i c a l p r a c t i c e . One of the problems of radiological reporting i s t h a t i t i s c a r r i e d out a t d i f f e r e n t times of t h e day. In o t h e r tasks involving visual search,performance has been found t o be a f f e c t e d by circadian v a r i a t i o n . Gale e t a l . i n v e s t i g a t e t h i s f a c t o r in a pulmonary nodule detection task a t three times of the day. Using medical students no v a r i a t i o n i n p e r f o n a n c e was i n i t i a l l y found b u t when gaze position upon nodule detection was incorporated i n t o the analysis t o give a location response then a drop in s e n s i t i v i t y between morning and lunch session was found. In c o n t r a s t no v a r i a b i l i t y in t h e observer's self-assessed a b i l i t y was reported. Some,mainly minor, v a r i a b i l i t y in eye movement parameters was a l s o reported, The authors argue fromacognitive standpoint t h a t such r e s u l t s may i n d i c a t e a change in s t r a t e g y by the observers a t d i f f e r e n t times of the day. Whilst e x t o l l ing care in extrapolation these findings do i n d i c a t e the need t o i n v e s t i gate circadian v a r i a t i o n f u r t h e r . The f i n a l paper in t h i s section i s by Papin, Metges and Amalberti and rep o r t s three s t u d i e s . The f i r s t i s again concerned with examining c h e s t radiographs and complements the others. The authors then extend the recording technique t o the dynamic s i t u a t i o n of the ultrasound examination. Finally the usefulness of recording visual behaviour as a t r a i n i n g a i d i s considered. These papers demonstrate the variety of research c u r r e n t l y being undertaken t o examine t h e problems of medical image perception. Considering the great financial outlay involved in purchasing any p a r t i c u l a r p a t i e n t imaging system and t h e amount of technical e f f o r t exerted i n producing good q u a l i t y images i t i s always s u r p r i s i n g t h a t r e l a t i v e l y l i t t l e a t t e n t i o n i s paid t o the psycho1 ogi cal f a c t o r s involved. Re f e r e nce Llewellyn-Thomas, E. Advice t o the searcher, i n : Monty, R.A. and Senders, J.W. (Eds.) Eye movements and psychological processes (Erlbaum, H i l l s d a l e 1976).