European Journal of N u c l e a r
Eur. J. Nucl. Med. 2, 201-203 (1977)
Medicine © by Springer-Verlag 1977
Ventilation-Perfusion Lung Scans for Pulmonary Emboli Accuracy of Reporting N i g e l T. B a t e m a n , A n t h o n y J. C o a k l e y , D e s m o n d N. C r o f t , a n d J a m e s R. W. L y a l l Departments of Nuclear Medicine and Medicine, St.Thomas' Hospital, London, SE1 7EH, England
Abstract. R e p o r t s w e r e m a d e o n c o m b i n e d v e n t i l a t i o n - p e r f u s i o n l u n g scans by t h r e e o b s e r v e r s o n three o c c a s i o n s a n d by a n o t h e r o b s e r v e r once. R e p r o d u c ibility for e a c h o b s e r v e r v a r i e d b e t w e e n 80 a n d 8 8 % . There was complete agreement about the areas of scans r e p o r t e d as a b n o r m a l . A g r e e m e n t b e t w e e n observers o n w h e t h e r o r n o t the a b n o r m a l i t y r e p r e s e n t e d a pulmonary embolus averaged 77%. There was 86% a g r e e m e n t w i t h the final clinical d i a g n o s i s . O u r results s h o w t h a t r e p o r t i n g o f v e n t i l a t i o n p e r f u s i o n lung scans by eye is r e p r o d u c i b l e . T h e y s u p p o r t t h e c l a i m s t h a t , u n d e r r o u t i n e clinical c o n d i t i o n s , the t e c h n i q u e is 9 1 % to 9 5 % a c c u r a t e f o r p u l m o n a r y e m b o l i .
Introduction T h e c o m b i n e d v e n t i l a t i o n - p e r f u s i o n l u n g s c a n has b e e n c l a i m e d to be h i g h l y sensitive a n d 9 4 % - 9 5 % a c c u r a t e in t h e d i a g n o s i s o f p u h n o n a r y e m b o l i s m ( W i l l i a m s et al., 1974; B a t e m a n et al., 1976; M c N e i l , 1976; A l d e r s o n et al., 1976). T h e s e figures suggest t h a t w h e r e a g a m m a c a m e r a is a v a i l a b l e this w o u l d b e the i n v e s t i g a t i o n o f c h o i c e , as s c a n n i n g is safe and requires only one intravenous injection. However the s c a n r e p o r t o n w h i c h t h e s e c l a i m s a r e b a s e d were o b t a i n e d by eye a n d w e r e s u b j e c t to o b s e r v e r variation. W e h a v e t h e r e f o r e m e a s u r e d t h e r e p r o d u c i b i l i t y o f r e p e a t e d r e p o r t s b y i n d i v i d u a l o b s e r v e r s , a n d the v a r i a b i l i t y b e t w e e n o b s e r v e r s in t h e i r r e p o r t i n g o f c o m b i n e d v e n t i l a t i o n p e r f u s i o n l u n g scans. T h e s e rep o r t s w e r e c o m p a r e d w i t h f i n a l clinical diagnosis.
Methods The 25 combined ventilation-perfusion lung scans performed during May 1976 were used for the study. These were routine requests For offprints contact." Dr. D.N. Croft
from clinicians in St. Thomas' and were performed as a diagnostic service in the Department of Nuclear Medicine and were therefore unselected. Details of the method of ventilation scanning using ~33Xe and perfusion scanning using 99mTc labelled microspheres have been previously described by Williams et al. (1974). Scans are recorded on polaroid film using an Ohio Nuclear Series 100 gamma camera fitted with a special diverging collimator. When the original routine service report was made by Observer 1 the clinical details provided on the request form, and usually the chest radiograph, were available. This report is included in the analysis. During July 1976 the 25 scans were reported by three trained observers (II, III and IV). Each observer reported each scan three times. The position of an abnormal area was indicated and he was asked to word his report so that the scan could be placed on one of three categories: normal, pulmonary embolism (mismatch with perfusion impaired more than ventilation) or parenchyreal lung disease (matching impairment of perfusion and ventilation). No other instruction was given and no training or discussion allowed. The scans were reported by number, with other identification obscured, and they were randomly mixed before each reporting session by someone not concerned with the reporting. The chest radiograph was available for 17 of the scans but no other clinical information was given. There was at least five days between reporting sessions. The three reports on each scan were assessed for the area of the scan considered abnormal and for the diagnosis. To allow comparison a simple scoring system was devised (Armitage et al., 1966). Three identical reports scored 2 points, two identical reports 1 point, and if all three were different no points. The maximum score was 50 points. The scores expressed as percentages are shown in Table 1. For each observer a consensus conclusion was prepared for each scan, this being the majority verdict of each of his three reports for the scan. Where all three reports were at variance his secondary comments on the reports were taken into account. Using the consensus diagnoses inter observer reproducibility was assessed (Table 2). A score of 1 was given for each concurring scan report, identity of reports in all cases thus scoring 25 (I00%). In order to prevent the high incidence of normals from influencing our conclusions (Armitage et al., 1966), the results have been analysed for the whole sample (25), and for the group in which there was one or more than one positive report [17]. Comparison was also made with the original report (Observer 1). The consensus diagnoses for Observers II, III and IV and the report for Observer I were compared with the final clinical diagnosis taken from the medical records. It took account of the history, clinical examination, and the results of laboratory tests (blood tesfs, ECG, chest radiograph, lung scan in all patients and bilateral peripheral phlebograms in two and pulmonary angiograph~) in one), and the response to treatment.
202
N.T. Bateman et al. : Ventilation@effusionLung Scans for Emboli: Accuracyof Reporting
Table 1. Intra observer reproducibility. Percentage agreement Observer
All scans
Abnormal scans
II III IV
88% 86% 80%
82% 79% 71%
Mean
85%
77%
Discussion
For all scans 100% =score of 50 for abnormal scans 100% =score of 34 Table 2. Inter observer reproducibility. Percentage agreement
Observer
I
II
III
IV
All Abnormal All Abnormal All Abnormal scans scans scans scans scans scans 72%
II III
59%
72% 72%
The consensus diagnosis agreed with the final clinical diagnosis in 86 out of the 100 attempts. The individual scores were Observer I, 20, Observer II, 21, Observer III, 21, Observer IV, 24 (Maximum Score=25).
59% 59%
76% 88% 84%
65% 82% 76%
Mean for all s c a n s = 7 7 % Mean for abnormal s c a n s = 6 7 % For all scans 100% = score of 25 and abnormal scans 100% = Score of 17
The four observers who took part in this study have two (I), three (II), four (IV) and more than five (III) years experience at reporting ventilation-perfusion lung scans.
Results
Intra-observer reproducibility of the three reports on each scan was between 80% and 88% when all scans are considered, and between 71% and 82% for abnormal scans (Table 1). Only one scan was reported as showing three different diagnoses on the three attempts. This was the scan of a patient who had received radiotherapy to the mediastinum and regional nodes for the treatment of lymphoma, and was not reported consistently by any observer. When comparing the consensus diagnosis of Observers II, III and IV with each other and with the original report (Observer I), there was inter observer reproducibility of between 72% and 88% (average 77%) for all scans and 59% and 82% (average 67%) for abnormal scans (Table 2). All observers agreed which part of the scan was abnormal. Nineteen scans (76%) were reported identically on 8 or more of the ten attempts, and eleven scans achieved ten out of ten identical reports. Six scans were inconsistently reported and of these, three had five reports in one category and five in the others. By majority view t6 of the 25 scans were normal, three showed mismatch indicating pulmonary embolism and six showed matching defects due to parenchymal lung disease or, in one case, a grossly dilated heart.
The simple perfusion lung scan for the diagnosis of pulmonary embolism is highly sensitive but is subject to many false positive results (McNeil, 1976; Tow and Simon, 1975). The combined ventilation-perfusion technique using a gamma camera and 133Xenon has increased accuracy by reducing false positive perfusion scans (Williams et al., 1974; McNeil, 1976). Alderson et al. (1976) have shown a correlation of 91% between combined ventilation-perfusion scans and pulmonary angiography, and of 95% between the same ventilation-perfusion scans and the final clinical diagnosis. We therefore used the final clinical diagnosis and found an 86% correlation. The final clinical diagnosis had some advantages compared with pulmonary angiographie, which is known to miss small peripheral pulmonary emboli, since it includes clinical data, response to treatment and results of investigations such as peripheral phlebograms. These high correlations with angiographic and clinical evidence of pulmonary embolism have implied that the ventilation-perfusion lung scan could replace pulmonary angiography in the diagnosis of pulmonary embolism. They have all relied upon a qualitative visual interpretation which has been untested until now. Other studies from Our own and other units have shown that, when scan and x-rays were reported out of clinical context, it was hard to achieve reproducibility in excess of 80% (Davies et al., 1972. ; C o n n and Spencer, 1972.; Koran, 1975) and our figures compare favourably. For this study the complex qualitative report used routinely was reduced to one of three firm conclusions. Although this did not reflect the scope of conventional reporting, clinical action based on the interpretation of the routing report entails a simplification similar to that we adopted and we believe our study measured the scan's clinical usefulness. It is known that tuberculosis (Lopez-Majono et al., 1965), radiotherapy (Bateman and Croft, 1976) and other causes of pulmonary vasculitis (McNeil et al., 1974) will produce false positive combined lung scans. This is illustrated by the difficulty experienced in reliably reporting the scans of 3 patients. One had inactive apical tuberculosis and the reports on his scan were widely divergent (5 normal; 5 pulmonary embolus). The second had received radiotherapy (5 normal; 4 pulmonary embolus; 1 parenchymal disease). The third patient had polyarteritis nodosa (5
N. T. Bateman et al. : Ventilation-Perfusion Lung Scans for Emboli: Accuracy of Reporting
parenchymal disease; 4 pulmonary embolus; 1 normal) and the final diagnosis, made after a long and complicated illness, was spontaneous thrombosis of a pulmonary artery secondary to active arteritis. These results emphasise that care should be taken when reporting the scans of patients with diseases causing obliterative arteritis. Our results reveal a consistency of reporting which is comparable to those of other qualitative reporting techniques. They confirm the negligible false negative rate, and the high correlation of the combined scan with the final clinical diagnosis. The difficulties of reporting have been exaggerated in this study since clinical information, which resolves some of the problem scans, was not available to three of the four observers. However present reporting techniques, though qualitative, provide information which is 91% to 95% specific and suggest that the combined ventilation perfusion lung scan is the method of choice for diagnosing pulmonary embolism.
203
References
Armitage, P., Blendis, L.M., Smyllie, H.C.: The measurement of observer disagreement in the recording of signs. J. R. statist. Soc. 129, 98-109 (1966) Bateman, N.T., Croft, D.N.: False-positive lung scans and radiotherapy. B.M.J. 1, 807 808 ('1976) Bateman, N.T., Croft, D.N., Lyall, J.R.W. : Is the combined ventilation perfusion lung scan an advance in the diagnosis of pulmonary embolism? Br. J. Dis, Chest. 70, 216-217 (1976) Conn, H.O., Spencer, R.P. : Observer error in liver scans. Gastroenterology, 62, 1085-1090 (1972) Davies, R.J., Cotton, P.B., Sweetland, C.A., Vernon, M., Croft, D.N.: Comparison of liver scans and liver snaps. Lancet 1, 927-929 (1972) Koran, L.M. : The reliability of clinical methods, data and judgements. New Engl. J. Med. 293, 695-701 (1975) Lopez-Majano, V., Wagner, H.N,, Tow, D.E., Chernick, V.: Radioisotope scanning of the lungs in pulmonary tuberculosis. J. Am. med. Ass. 194, 1053-1058 (1965) McNeil, B.J.: A diagnostic strategy using ventilation-perfusion studies in patients suspect for pulmonary embolism. J. nucl. Med. 17, 613-616 (1976) McNeil, B.J., Holman, L., Adelstein, S.J. : The scintigraphic definition of pulmonary embolism. J. Am. reed. Ass. 227, 753-756 (1974) Tow, D.E., Simon, A.L.: Comparison of lung scanning and pulmonary angiography in the detection and follow-up of pulmonary embolism: Progress in cardiovascular disease 17, 239-245 (1975) Williams, O., Lyall, J.R.W., Vernon, M, Croft, D.N.: Ventilationperfusion lung scanning for pulmonary emboli. B.M.J. 1, 600 602 (1974)
Alderson, P.O., Rujanavech, N., Secker-Walker, R.H., McKnight, R.C. : The role of 133Xe ventilation studies in the scintigraphic detection of pulmonary embolism. Radiology. 120, 633-640 (1976)
Received January 28, 1977
Acknowledgements. We wish to thank the staff of the Department of Nuclear Medical for their help with the study. We thank Dr. A.V. Swan for statistical advice.
Eur. J. Nucl. Med. 2, 201-203 (1977)
Medicine © by Springer-Verlag 1977
Ventilation-Perfusion Lung Scans for Pulmonary Emboli Accuracy of Reporting N i g e l T. B a t e m a n , A n t h o n y J. C o a k l e y , D e s m o n d N. C r o f t , a n d J a m e s R. W. L y a l l Departments of Nuclear Medicine and Medicine, St.Thomas' Hospital, London, SE1 7EH, England
Abstract. R e p o r t s w e r e m a d e o n c o m b i n e d v e n t i l a t i o n - p e r f u s i o n l u n g scans by t h r e e o b s e r v e r s o n three o c c a s i o n s a n d by a n o t h e r o b s e r v e r once. R e p r o d u c ibility for e a c h o b s e r v e r v a r i e d b e t w e e n 80 a n d 8 8 % . There was complete agreement about the areas of scans r e p o r t e d as a b n o r m a l . A g r e e m e n t b e t w e e n observers o n w h e t h e r o r n o t the a b n o r m a l i t y r e p r e s e n t e d a pulmonary embolus averaged 77%. There was 86% a g r e e m e n t w i t h the final clinical d i a g n o s i s . O u r results s h o w t h a t r e p o r t i n g o f v e n t i l a t i o n p e r f u s i o n lung scans by eye is r e p r o d u c i b l e . T h e y s u p p o r t t h e c l a i m s t h a t , u n d e r r o u t i n e clinical c o n d i t i o n s , the t e c h n i q u e is 9 1 % to 9 5 % a c c u r a t e f o r p u l m o n a r y e m b o l i .
Introduction T h e c o m b i n e d v e n t i l a t i o n - p e r f u s i o n l u n g s c a n has b e e n c l a i m e d to be h i g h l y sensitive a n d 9 4 % - 9 5 % a c c u r a t e in t h e d i a g n o s i s o f p u h n o n a r y e m b o l i s m ( W i l l i a m s et al., 1974; B a t e m a n et al., 1976; M c N e i l , 1976; A l d e r s o n et al., 1976). T h e s e figures suggest t h a t w h e r e a g a m m a c a m e r a is a v a i l a b l e this w o u l d b e the i n v e s t i g a t i o n o f c h o i c e , as s c a n n i n g is safe and requires only one intravenous injection. However the s c a n r e p o r t o n w h i c h t h e s e c l a i m s a r e b a s e d were o b t a i n e d by eye a n d w e r e s u b j e c t to o b s e r v e r variation. W e h a v e t h e r e f o r e m e a s u r e d t h e r e p r o d u c i b i l i t y o f r e p e a t e d r e p o r t s b y i n d i v i d u a l o b s e r v e r s , a n d the v a r i a b i l i t y b e t w e e n o b s e r v e r s in t h e i r r e p o r t i n g o f c o m b i n e d v e n t i l a t i o n p e r f u s i o n l u n g scans. T h e s e rep o r t s w e r e c o m p a r e d w i t h f i n a l clinical diagnosis.
Methods The 25 combined ventilation-perfusion lung scans performed during May 1976 were used for the study. These were routine requests For offprints contact." Dr. D.N. Croft
from clinicians in St. Thomas' and were performed as a diagnostic service in the Department of Nuclear Medicine and were therefore unselected. Details of the method of ventilation scanning using ~33Xe and perfusion scanning using 99mTc labelled microspheres have been previously described by Williams et al. (1974). Scans are recorded on polaroid film using an Ohio Nuclear Series 100 gamma camera fitted with a special diverging collimator. When the original routine service report was made by Observer 1 the clinical details provided on the request form, and usually the chest radiograph, were available. This report is included in the analysis. During July 1976 the 25 scans were reported by three trained observers (II, III and IV). Each observer reported each scan three times. The position of an abnormal area was indicated and he was asked to word his report so that the scan could be placed on one of three categories: normal, pulmonary embolism (mismatch with perfusion impaired more than ventilation) or parenchyreal lung disease (matching impairment of perfusion and ventilation). No other instruction was given and no training or discussion allowed. The scans were reported by number, with other identification obscured, and they were randomly mixed before each reporting session by someone not concerned with the reporting. The chest radiograph was available for 17 of the scans but no other clinical information was given. There was at least five days between reporting sessions. The three reports on each scan were assessed for the area of the scan considered abnormal and for the diagnosis. To allow comparison a simple scoring system was devised (Armitage et al., 1966). Three identical reports scored 2 points, two identical reports 1 point, and if all three were different no points. The maximum score was 50 points. The scores expressed as percentages are shown in Table 1. For each observer a consensus conclusion was prepared for each scan, this being the majority verdict of each of his three reports for the scan. Where all three reports were at variance his secondary comments on the reports were taken into account. Using the consensus diagnoses inter observer reproducibility was assessed (Table 2). A score of 1 was given for each concurring scan report, identity of reports in all cases thus scoring 25 (I00%). In order to prevent the high incidence of normals from influencing our conclusions (Armitage et al., 1966), the results have been analysed for the whole sample (25), and for the group in which there was one or more than one positive report [17]. Comparison was also made with the original report (Observer 1). The consensus diagnoses for Observers II, III and IV and the report for Observer I were compared with the final clinical diagnosis taken from the medical records. It took account of the history, clinical examination, and the results of laboratory tests (blood tesfs, ECG, chest radiograph, lung scan in all patients and bilateral peripheral phlebograms in two and pulmonary angiograph~) in one), and the response to treatment.
202
N.T. Bateman et al. : Ventilation@effusionLung Scans for Emboli: Accuracyof Reporting
Table 1. Intra observer reproducibility. Percentage agreement Observer
All scans
Abnormal scans
II III IV
88% 86% 80%
82% 79% 71%
Mean
85%
77%
Discussion
For all scans 100% =score of 50 for abnormal scans 100% =score of 34 Table 2. Inter observer reproducibility. Percentage agreement
Observer
I
II
III
IV
All Abnormal All Abnormal All Abnormal scans scans scans scans scans scans 72%
II III
59%
72% 72%
The consensus diagnosis agreed with the final clinical diagnosis in 86 out of the 100 attempts. The individual scores were Observer I, 20, Observer II, 21, Observer III, 21, Observer IV, 24 (Maximum Score=25).
59% 59%
76% 88% 84%
65% 82% 76%
Mean for all s c a n s = 7 7 % Mean for abnormal s c a n s = 6 7 % For all scans 100% = score of 25 and abnormal scans 100% = Score of 17
The four observers who took part in this study have two (I), three (II), four (IV) and more than five (III) years experience at reporting ventilation-perfusion lung scans.
Results
Intra-observer reproducibility of the three reports on each scan was between 80% and 88% when all scans are considered, and between 71% and 82% for abnormal scans (Table 1). Only one scan was reported as showing three different diagnoses on the three attempts. This was the scan of a patient who had received radiotherapy to the mediastinum and regional nodes for the treatment of lymphoma, and was not reported consistently by any observer. When comparing the consensus diagnosis of Observers II, III and IV with each other and with the original report (Observer I), there was inter observer reproducibility of between 72% and 88% (average 77%) for all scans and 59% and 82% (average 67%) for abnormal scans (Table 2). All observers agreed which part of the scan was abnormal. Nineteen scans (76%) were reported identically on 8 or more of the ten attempts, and eleven scans achieved ten out of ten identical reports. Six scans were inconsistently reported and of these, three had five reports in one category and five in the others. By majority view t6 of the 25 scans were normal, three showed mismatch indicating pulmonary embolism and six showed matching defects due to parenchymal lung disease or, in one case, a grossly dilated heart.
The simple perfusion lung scan for the diagnosis of pulmonary embolism is highly sensitive but is subject to many false positive results (McNeil, 1976; Tow and Simon, 1975). The combined ventilation-perfusion technique using a gamma camera and 133Xenon has increased accuracy by reducing false positive perfusion scans (Williams et al., 1974; McNeil, 1976). Alderson et al. (1976) have shown a correlation of 91% between combined ventilation-perfusion scans and pulmonary angiography, and of 95% between the same ventilation-perfusion scans and the final clinical diagnosis. We therefore used the final clinical diagnosis and found an 86% correlation. The final clinical diagnosis had some advantages compared with pulmonary angiographie, which is known to miss small peripheral pulmonary emboli, since it includes clinical data, response to treatment and results of investigations such as peripheral phlebograms. These high correlations with angiographic and clinical evidence of pulmonary embolism have implied that the ventilation-perfusion lung scan could replace pulmonary angiography in the diagnosis of pulmonary embolism. They have all relied upon a qualitative visual interpretation which has been untested until now. Other studies from Our own and other units have shown that, when scan and x-rays were reported out of clinical context, it was hard to achieve reproducibility in excess of 80% (Davies et al., 1972. ; C o n n and Spencer, 1972.; Koran, 1975) and our figures compare favourably. For this study the complex qualitative report used routinely was reduced to one of three firm conclusions. Although this did not reflect the scope of conventional reporting, clinical action based on the interpretation of the routing report entails a simplification similar to that we adopted and we believe our study measured the scan's clinical usefulness. It is known that tuberculosis (Lopez-Majono et al., 1965), radiotherapy (Bateman and Croft, 1976) and other causes of pulmonary vasculitis (McNeil et al., 1974) will produce false positive combined lung scans. This is illustrated by the difficulty experienced in reliably reporting the scans of 3 patients. One had inactive apical tuberculosis and the reports on his scan were widely divergent (5 normal; 5 pulmonary embolus). The second had received radiotherapy (5 normal; 4 pulmonary embolus; 1 parenchymal disease). The third patient had polyarteritis nodosa (5
N. T. Bateman et al. : Ventilation-Perfusion Lung Scans for Emboli: Accuracy of Reporting
parenchymal disease; 4 pulmonary embolus; 1 normal) and the final diagnosis, made after a long and complicated illness, was spontaneous thrombosis of a pulmonary artery secondary to active arteritis. These results emphasise that care should be taken when reporting the scans of patients with diseases causing obliterative arteritis. Our results reveal a consistency of reporting which is comparable to those of other qualitative reporting techniques. They confirm the negligible false negative rate, and the high correlation of the combined scan with the final clinical diagnosis. The difficulties of reporting have been exaggerated in this study since clinical information, which resolves some of the problem scans, was not available to three of the four observers. However present reporting techniques, though qualitative, provide information which is 91% to 95% specific and suggest that the combined ventilation perfusion lung scan is the method of choice for diagnosing pulmonary embolism.
203
References
Armitage, P., Blendis, L.M., Smyllie, H.C.: The measurement of observer disagreement in the recording of signs. J. R. statist. Soc. 129, 98-109 (1966) Bateman, N.T., Croft, D.N.: False-positive lung scans and radiotherapy. B.M.J. 1, 807 808 ('1976) Bateman, N.T., Croft, D.N., Lyall, J.R.W. : Is the combined ventilation perfusion lung scan an advance in the diagnosis of pulmonary embolism? Br. J. Dis, Chest. 70, 216-217 (1976) Conn, H.O., Spencer, R.P. : Observer error in liver scans. Gastroenterology, 62, 1085-1090 (1972) Davies, R.J., Cotton, P.B., Sweetland, C.A., Vernon, M., Croft, D.N.: Comparison of liver scans and liver snaps. Lancet 1, 927-929 (1972) Koran, L.M. : The reliability of clinical methods, data and judgements. New Engl. J. Med. 293, 695-701 (1975) Lopez-Majano, V., Wagner, H.N,, Tow, D.E., Chernick, V.: Radioisotope scanning of the lungs in pulmonary tuberculosis. J. Am. med. Ass. 194, 1053-1058 (1965) McNeil, B.J.: A diagnostic strategy using ventilation-perfusion studies in patients suspect for pulmonary embolism. J. nucl. Med. 17, 613-616 (1976) McNeil, B.J., Holman, L., Adelstein, S.J. : The scintigraphic definition of pulmonary embolism. J. Am. reed. Ass. 227, 753-756 (1974) Tow, D.E., Simon, A.L.: Comparison of lung scanning and pulmonary angiography in the detection and follow-up of pulmonary embolism: Progress in cardiovascular disease 17, 239-245 (1975) Williams, O., Lyall, J.R.W., Vernon, M, Croft, D.N.: Ventilationperfusion lung scanning for pulmonary emboli. B.M.J. 1, 600 602 (1974)
Alderson, P.O., Rujanavech, N., Secker-Walker, R.H., McKnight, R.C. : The role of 133Xe ventilation studies in the scintigraphic detection of pulmonary embolism. Radiology. 120, 633-640 (1976)
Received January 28, 1977
Acknowledgements. We wish to thank the staff of the Department of Nuclear Medical for their help with the study. We thank Dr. A.V. Swan for statistical advice.
