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Comparison of cardiothoracic ratio by miniature and standard chest radiography
linica ommun
Comparison miniature
of and
cardiothoracic standard
chest
rati radiography
W. L. A&on, M.B., M.R.C.Y. G. Boss, M.B., B.S., D.C.H. London, England
T
he use of chest radiography in the diagnosis of cardiovascular disease is generally accepted, but little critical attention has been paid to the quantitative aspects of the subject. The evident success of experienced judgment of heart size is based on radiological examination of large numbers of patients with relatively severe forms of heart disease in advanced stages. It might be clinically and epidemiologitally useful to establish whether any relationship exists between the cardiothoracic ratio (CTR) as estimated on miniature chest skiagrams and standard chest skiagrams of the same subject. If this revealed a statistically valid relationship, an extended investigation of CTR could be undertaken, based on the large population samples subjected to miniature mass radiography (MMR). There is disagreement among physicians experienced in this field as to whether or not useful inferences about CTR may be made from !JIMR films. Resolution of this disagreement would lead, at least, to consistent diagnostic interpretation. If a consistent relationship were demonstrable,
MMR could become a powerful epidemiological tool in cardiovascular study. It might also attain clinical usefulness similar to that so well established in the diagnosis of pulmonary tuberculosis. Methods Subjects. These were selected from inpatient and outpatient practice on the basis of each patient’s ability to undergo routine radiological examination. No attempt was made to avoid age or sex bias, and no patient was included who had thoracic skeletal deformity, evident either on clinical examination or x-ray. R!Iajor chest surgery also precluded a patient from study. Patients examined had a variety of cardiovascular and respiratory diseases, and included some clinically normal persons. In addition, some patients had other disorders, such as evidence of gastrointestinal, locomotor, and metabolic disease. The patients examined were in 3 groups. 1. A special group of 12 patients was examined on one occasion by three 100 mm. films and three standard 5 ft. films taken
From
Grove Park Hospital, Lewisham Chest Clinic, and The South East London Mass Kadiography Service, London, Ewland. Received for publication Xov. 12, 1969. Reprint requests to: Dr. W. L. Ashton, Grave Park Hospital, Lewisbarn Group Hospital Management Committee, Marvels Lane. Lee, London. S.E. 12. England. 588
American
Heart Journal
NovezbeT, 1970
Vol.
80, No.
5, pp. 588-596
589
Chest radiogra#hy
Cross and Lewisham Chest Clinic being equipped with identical models. 2. New Cross films were taken with a Watson Roentgen IV equipment, at 6 ft. tube film distance, using 65 kv. with 0.08 X 300 Ma. = sec. exposure. 3. Lewisham films were taken with a Dean Matchlett equipment at 5 ft. tube distance, using 60 kv. with 0.04 X 300 Ma. = sec. exposure.
by one radiographer. Each group of three films was used to provide a mean estimate of CTR. Each mean estimate was used in the analysis. This procedure was considered to minimize any variability attributable to such uncontrollable factors as day-to-day changes in cardiovascular and respiratory activity, exposure at different times in the cardiac cycle, technical differences between radiographers, and alterations of x-ray tube function. This special group included inpatients and outpatients, thought to have large, normal, and small hearts and selected because of this supposed variation. 2. An unselected group of 31 patients, who were outpatients or inpatients, was examined purely on the basis of having 100 mm. and a standard 5 ft. film taken within a 10 day period, both available for measurement. These patients were from Grove Park Hospital or Lewisham Chest Clinic. Both these groups were examined by observer W. L. A. 3. .An unselected group of 13 subjects was examined following recall for large film after examination by South East London Mass Radiography Service. They thus resembled the preceding group in that they had films of both types taken within a few d,ays. The observations were made by a different observer (G. B.) and the x-rays were from different departments. In every subject examined, the films were measured by the method of Appleton, Hamilton, and Simon’ and the CTR was computed. Procedures. RADIOLOGICAL
STATISTICAL
ANALYSIS.
1. Every subject examined gave 2 estimates of CTR based on the 2 sizes of film. The product moment correlation coefficient (r) was computed for the bivariate array thus obtained. The values of r based on “special films” only, and “nonspecial” films taken at Lewisham and New Cross, were treated separately, and the values of r tested for significant variation by the z transformation. 2. A further test of consistency was based on the hypothesis that the 2 techniques were measurements of the same variable (CTR), and that there should, therefore, be no significant difference between the mean CTR measured by the 2 techniques if no distortion occurred. This hypothesis was tested by parametric analysis of variance, Gosset’s t ratio, and Wilcoxon’s test. 3. The ranking of CTR obtained by the 2 methods should be statistically consistent to support the hypothesis of statistical identity. This was examined by Spearman’s rank coefficient. Results Tables I, II, and III show the observations of CTR obtained by the 2 methods in each of the 56 subjects, with their correlation coefficients (r).
TECHNIQUES.
1. All 100 mm. films were taken with an 0delc.a camera, both the South East Londlon Mass Radiography Service at New Table I. Special series CTR’s -__ x (5 ft. 2.04 2.20 2.14 2.49 r = $
CTR)
1
’ ?%?‘)
1 x (5 ft.
2.02 2.04 2.09 2.44
CI.9854; n = 12; z = 2.27291;
CTR)
1.68 1.66 1.78 1.83 standard
error
= 0.3333;
1
’
(%~‘)
/
x (5ft.
1.69 1.69 1.80 1.74 p << 0.001 (very
1.83 2.12 1.48 2.74 highly
significant).
CTR)
1
’ (‘%? 1.83 2.09 1.49 2.81
590
Ashton and Boss
Table II. Unselected Lewisham and Grove Padz CTR’s x (5”fL
y (100 mm.) CTR
CTR)
1.73 2.28 1.66 1.78 2.02 2.44 1.92 2.15 2.38 2.00
1.97 2.16 1.65 1.86 1.77 2.42 2.44 2.16 2.30 2.14
r = -b 0.8652;
n = 31; z = 1.31287;
Table III.
n (6ft.
x (4 ft.
y (100 mm.) CTR
CTR)
2.52 2.21 2.04 2.28 2.40 2.29 1.68 1.97 1.97 2.12 2.1s
standard
error
x (5).
2.34 2.26 2.13 2.37 2.42 2.52 1.65 2.03 2.07 2.11 2.09
= 0.1890;
p << 0.001 (very
y (100 aznz.) CTR
CTR)
1.80 2.12 2.54 1.75 1.86 2.26 1.96 1.65 2.17 1.68
highly
1.67 2.11 2.58 1.54 1.73 2.22 2.06 1.57 2.18 1.70
significant).
Unselected New Cross CTR’s
CTR)
/
’ (l%;m’)
2.00 2.10 1.95 1.76
1.75 1.91 1.62 1.72
r = C 0.9159;
n = 13; z = 1.56359;
!
x (6 ff.
CTR)
1
’ (li%?)
2.11 2.14 1.73 1.95 2.12
standard
error
= 0.3162;
i,,,,.
1.83 2.00 1.62 1.71 1.92
p < 0.001
The results are shown graphically in Figs. 1, 2, and 3 (separately for each group) and 4 (all observations pooled). The graphs are individually and collectively suggestive of a close functional relationship. The value of r computed for each confirms a very high significance probability for the relationship for each separate array. The r values are compared in Table IV using the z transformation. The z values are designated zi, ZZ, and ~3, respectively, for the three correlation coefficients. This finding is interpreted as indicating that r for the special series is significantly higher than for either of the other series. It also indicates that these data provide no evidence for assuming a difference to exist between r’s for either unselected series. The factors outlined in the description as being likely to produce technically based
(very
CTR) 2.24 1.67 1.98 2.94
highly
1
’ (l%?) 2.42 1.72 2,oo 2.95
significant).
changes in CTR--variation between individual radiographers, day-to-day fluctuations in x-ray tube performance, and changes in the physiology of the subjectwould have been virtually eliminated by the technique used in the special series. This technique would also have greatly minimized variation attributable to exposure in systole and diastole, with either type of film. It seems a very plausible inference that these factors are responsible for the highly significant increase of r in the special series. Analysis of variance and Gusset’s f test (Student t test). Parametric analysis of variance showed no evidence of significant differences attributable to the origin of observations, to the 2 radiological techniques, or to the 2 observers. The t test was, therefore, applied to the testing of two related features of the data.
Chest radiography
IN =
121
'R =
0.9854
Z =
247741
P e 0.001
1.5
1.6
I.7
1.8
1.9
2.0
2.1
2.2
X [STANDARD
Fig.
1. Scatter
diagram,
special
2.3
24
2.5
2.6
2.7
2.8
2.9
3.0
C.T.R.)
series.
.3& 2.9
IN =
311
2.8
R =
0.8652
2.7
Z =
1.31287
2.6'
P <( 0.001
2.5 % G 2.4 ti s 2'3 8 2.2 Z > 2.1 2.0 i.9 10 1’0 x
I.7
*
1.6 1.5 1.4Y 1.4
Fig.
2. Scatter
diagram,
1.5
1.6
unselected
1.7
1.8
Lewisham
1.9
2.0
2.1 2.2 2.3 2.4 X [STANDARD C.T.R.1
and
Grove
Park
series.
2.5
2.6
2.7
2.8
2.9
3.0
591
592
Amw.Xelw 3.
Ashton and Boss
Nowmbor,
3.1 IN =
131
2.1
R =
0.9159
2.;
z =
I.56359
2,:
P c 0.001
2.t 2.i a 5
2.1
$
2.:
8 5
2.2
F
2.1 2.c 1.9 1.8 1.7
1.5 1.4 1.5
1.6
1,7
1.8
1.9
2.0
2.1
2.2
X (STANDARD
Fig.
3. Scatter
diagram,
unselected
mass
2.9
IN =
56)
2.8
R =
0.9109
2.7
z =
radiography
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
C.T.R.)
series.
153367
P (( 0.001
2.6 2.5 2 i-’ ti
2.4
5 2.3 I 0 2.2 s1 >
2.1 2.0 I.9 1.8
x”
1.7
Y OF SYMBOLS e
X
1.6
LEWISHAM
-
1.5
SPECIAL SERIES
MASS 1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
X ISTANDARD
Fig.
4. Scatter
diagram,
pooled
data.
2,3
& GROVE
RADIOGRAPHY 2.4
C.T.R.1
2.5
2.6
PARK SERIES SERIES 2.7
2.8
2.9
3.0
1970
Yolume
80
Namber
3
Chest radiogra$hy
593
Table IV. Values under test .ZI and z3 and .II and
Standard difference
z2 z2 z3
error of for pair
0.3832 0.3864 0.4594
Value difference
of of 3’s
1.16454 0.25072 0.91382
Ratio standard
of diference to error of differeme 3.0390t 0 6806 1.9892%
*Probably significant (p fi 0.05). tVery highly significant (p 2 0.001).
The hypothesis x = y was tested for each array separately and for all results pooled (x being CTR based on standard films and y being CTR based on miniature films). The hypothesis x - y = 0 was also tested, by subtracting all the individual 100 mm. CTR’s from the corresponding standard CTR and applying a one-tailed t test to the difference of the mean value from 0. For all arrays, there was no significant difference demonstrated between x and y when the first method was applied. For the second method, it was found that x - y z 0 (t = 2.79: p = 0.02) in the third series of films only. Since in this series, there were uncontrolled conditions of x-ray examination and also a different observer, this Iprovided no support for the view that distortion of CTR may be attributed to using 100 mm. film. It seems likely that this nonidentity may be due to a greater variance of observations under these conditions. This is not inconsistent with the result of analysis of variance, which merely shows that for this sample size, there is no convincing evidence of nonhomogeneity of variance. Ot.ber significance tests. To avoid the assumption of homogeneous variance implied by the parametric tests, the data were tested by Wilcoxon’s test, the sign test (applicable only to all 56 results pooled), and the Spearman rank coefficient. Wilcoxon’s test (applied to each group separately and to all groups pooled) showed no significant difference between CTl? estimates by either method. The sign test also supported this conclusion. Spear-man’s test gave consistent findings. The values of R were all highly significant
(p < 0.001 for all except the New Cross series, for which 0.001 < p < 0.01). It therefore seems that there is no convincing objective ground for the view that CTR is seriously distorted by 100 mm. as opposed to standard size chest films. Discussion
The radiological assessment of heart size by any physician is usually based on one chest skiagram taken at a finite time in the patient’s recent past. There is awareness that apparent changes in CTR may occur as a result of technical and physiological variations between different times of filming the same subject, quite apart from those changes, due to heart disease, whose presence and severity are under assessment. These results establish that there is a very highly significant correlation between the estimate of CTR made with 100 mm. and 5 or 6 ft. standard films. The fact that the correlation is significantly higher in the special series reflects, in all likelihood, the lessening of variation due to technical changes in the x-rays taken as between different radiographers on different days, and day-to-day physiological changes in the subjects under study. The point to consider in interpreting the findings is that the measurement obtained as an estimate of CTR appears statistically identical whether it is based on miniature or standard films. Thus, any fallacies inherent in evaluating CTR based on miniature films are those based on shortcomings of CTR as a useful measure and not to any distortion of the size of cardiac silhouette by either technique. Consideration of the geometry of the chest x-ray does not support any basis for expecting that CTR
594
Ashton
and Boss
would be altered in technically adequate miniature films. It is apparent that objections can be made to CTR as a measure of heart size. Not the least important is that where right ventricular enlargement is predominant, the transverse diameter of the heart is affected relatively little. Thoracic deformity, in addition, may add difficulties in assessment. Whatever the difficulties posed by either type of film, the evaluation of heart size is most frequently based on the posteroanterior chest skiagram. The quantitative aspects of assessment remain ill-defined at present; a ratio of 2.1 as suggested by Woo@ seems widely accepted. Work is at present in progress to try to define limits of CTR which are more clearly associated with the presence or absence of diagnosable cardiovascular abnormality. It seems from present findings that MMR can offer as much information as standard chest radiography in cardiovascular disease. Abnormal form of the heart and great vessels and evidence of rib notching or erosionthose radiological features that may be as readily called “nonquantitative’‘-are revealed by MMR as by standard films. R,Iost subjects with cardiovascular disease, !lowever, have hypertension, ischemic heart disease, or acquired valve disease (if pulmonary heart disease is excluded because of owing its presence to lung or skeletal disease). With the exception of some cases of mitral stenosis and ischemic heart disease, these conditions nearly always lead to significant cardiac enlargement; and there are seldom clearcut morphological changes, other than enlargement, to aid recognition. Indeed, very few patients with any type of heart disease fail to show cardiac enlargement detectable at necropsy, but the congenital lesions are more likely to lead to recognizable deformation of cardiovascular silhouette and they are relatively far commoner in children. It is thus difficult to evade the conclusion that MMR is potentially a powerful tool in epidemiology and also in selecting subjects likely to benefit clinically from full cardiological assessment. Large populations can be examined by MMR swiftly and cheaply. The method is already generally accepted by the public (always of basic
importance in any investigative technique of ma.jor epidemiological interest). The possible implications, immediate and remote, for cardiovascular epidemiology and clinical cardiology may be considerable. At present, cardiovascular assessment of population samples is based on the relatively cumbersome methods of routine clinical evaluation with electrocardiography and, in a few centers, ballistocardiography. These are time consuming, expensive, and thus of doubtful applicability to population samples outside various highly selected groups of relatively small size. It is also worth considering that chest radiography, giving as it does a cardiovascular assessment based on heart muscle mass and cardiac filling, may offer a more direct simple test of cardiac function than other Iaboratory methods, except for ballistocardiography. At present, MMR films are being reported by physicians who are divisible into those who believe in MMR as a useful measure of heart size, and those who do not consider it helpful. In the absence of evidence establishing which of these viewpoints is correct, it is difficult to evade the conclusion that MMR surveys are inevitably less effective than they might be. For either a considerable amount of detectable heart disease is being overlooked by the nihilistic school, or a considerable amount of unnecessary, anxiety-inducing, and occasionally dangerous investigation is being carried out-at considerable public expense-by referrals for spurious “cardiac enlargement.” Neither of these situations is desirable and both are avoidable. The hypothesis derived from the observations described here can be tested in several ways. The most obvious method applied at present is the comparison of clinical, ECG, and laboratory findings in persons recalled after MMR because of apparent cardiomegaly. (This is considered at present as indicated by a ratio below 1.80 on the basis of a pilot study now in progress.) Age- and sex-matched subjects with “normal” hearts are also being recalled. A highly significant excess of detectable heart disease in the “large” hearts would be expected if the MMR estimate is, as we consider, a valid estimate of CTR.
Chest radiography
Absolute proof could be established in the long term by necropsy study but this would require a long and complex degree of organization of observations, and would depend on a high necropsy rate in both “normal-” and “large-heart” patients, as well as on frequent MMR films to ensure that a recent estimate of CTR was available at the time of death. Since modern cardiological diagnosis is technically so reliable, it is considered that the detection of a significant excess of cardiovascular disease in the “large-heart” patients would be adequate preliminary proof of the hypothesis, and it would be, at present, both ineffilzient and superfluous to attempt a necropsy survey, even if this could be organized. The technique of measurement and calculation required has been considered. Setsquare and slide rule were used in this study, for measurement and computing. Each. film took 2 to 3 minutes to examine in this way. It is considered that a device using 2 pairs of calipers, moving on each of 2 parallel horizontal scales attached to the lower margin of the viewing box, should make rapid measurement possible. The application of the findings of this study depends on the following factors: 1. A definition of a value of CTR produci:ng the maximum segregation of subjects into those with and those without significant cardiomegaly is necessary. 2. There must be an establishment of the reliability of MMR observers in selecting films for measurement. Some observers appear to be as capable of detecting cardiomepaly in 100 mm. films as competent cardiologists are in 5 ft. or 6 ft. films. Others seem less reliable, but error seems comistently to be in the direction of judging normal-sized hearts to be enlarged. The validity of these impressions, and the extent to which training improves the ability of observers to detect “large” heart shadows, requires testing by suitable operational research. 3. A suitable technique must be evolved for the rapid measurement of CTR in the cases selected as being suspicious. The caliper device suggested in outline would probably be simplest; it might be possible to d.evelop a photoelectric scanning device
59s
coupled to the necessary computing elements required to calculate the ratio. An attempt to define the CTR value giving maximum segregation into “normal” and “abnormal” patients is at present being made, using samples of persons examined by MMR. The other factors require further technical study. The problems are not directly relevant to the hypothesis being tested here, that CTR measured by 100 mm. film is identical with that measured by standard film. The assumption that MMR is measuring the same dimensions is being applied to samples of persons being examined by miniature radiography, in an attempt to reach a clearer definition of the range of “normal” CTR. Related to assessment of heart size by either standard or miniature radiography is the problem of how far evaluation is affected by physiological variations in heart size. Exposure during systole and diastole affects the film obtained. Variations in respiration may affect the result. Recent physical activity is important in affecting heart size in health and disease.2 Earlier work by Gorlin and Braunwald3v4 suggested that increased somatic muscle activity diminishes ventricular systolic volume in healthy subjects. Analysis of observations in transverse cardiac diameter or CTR of successive films under various standard conditions might enable more reliable criteria for judging significance of changes to be established. Although the 12 patients examined in the “special” series each had 6 films for analysis, the data are too restricted to examine this point precisely because of the fact that all these films were taken in rapid succession with the subject at rest, that is, in circumstances intended to minimize functional changes. The most important point illustrated by these findings is that the opinion frequently expressed by physicians-that miniature films give an impression of increased heart size-has no objective basis. Presumably this opinion arises from illusory perception associated with the smaller films. Be this as it may, it has evidently led to doubt about the application of MMR to any aspect of cardiovascular investigation. It
596
Ashton and Boss
underlines quantitative
the importance of applying methods wherever possible.
Miss M. Brown took the special x-rays, Mrs. V. Pike gave valuable secretarial assistance, and Mr. J. Hale, of Goldsmith’s College, kindly made calculating facilities available. Dr. J. M. Morgan gave helpful advice, and Mr. J. J. Lyons of the G. L. C. reviewed and supplemented the statistical analysis.
REFERENCES 1. Appleton, A. B., Hamilton, W. j., and Simon, G.: Surface and radiological anatomy, Cambridge, 1946, W. Heffer & Sons, Ltd. ?-. Boelling, G. X., Phillips, W. J., Frerking, H. W., et al.: Roentgenographic exercise test. A new test of myocardial state, J.A.M.A. 202:275, 1967. 3. Braunwald, E., et al.: Circ. Res. 13:448, 1963. 4. Gorlin, R.: Pathophysiology of cardiac pain, Circulation 32:361, 1965. r3. Wood, P. : Disorders of the heart and circulation, London, 1957, Eyre and Spottiswoode.
Comparison miniature
of and
cardiothoracic standard
chest
rati radiography
W. L. A&on, M.B., M.R.C.Y. G. Boss, M.B., B.S., D.C.H. London, England
T
he use of chest radiography in the diagnosis of cardiovascular disease is generally accepted, but little critical attention has been paid to the quantitative aspects of the subject. The evident success of experienced judgment of heart size is based on radiological examination of large numbers of patients with relatively severe forms of heart disease in advanced stages. It might be clinically and epidemiologitally useful to establish whether any relationship exists between the cardiothoracic ratio (CTR) as estimated on miniature chest skiagrams and standard chest skiagrams of the same subject. If this revealed a statistically valid relationship, an extended investigation of CTR could be undertaken, based on the large population samples subjected to miniature mass radiography (MMR). There is disagreement among physicians experienced in this field as to whether or not useful inferences about CTR may be made from !JIMR films. Resolution of this disagreement would lead, at least, to consistent diagnostic interpretation. If a consistent relationship were demonstrable,
MMR could become a powerful epidemiological tool in cardiovascular study. It might also attain clinical usefulness similar to that so well established in the diagnosis of pulmonary tuberculosis. Methods Subjects. These were selected from inpatient and outpatient practice on the basis of each patient’s ability to undergo routine radiological examination. No attempt was made to avoid age or sex bias, and no patient was included who had thoracic skeletal deformity, evident either on clinical examination or x-ray. R!Iajor chest surgery also precluded a patient from study. Patients examined had a variety of cardiovascular and respiratory diseases, and included some clinically normal persons. In addition, some patients had other disorders, such as evidence of gastrointestinal, locomotor, and metabolic disease. The patients examined were in 3 groups. 1. A special group of 12 patients was examined on one occasion by three 100 mm. films and three standard 5 ft. films taken
From
Grove Park Hospital, Lewisham Chest Clinic, and The South East London Mass Kadiography Service, London, Ewland. Received for publication Xov. 12, 1969. Reprint requests to: Dr. W. L. Ashton, Grave Park Hospital, Lewisbarn Group Hospital Management Committee, Marvels Lane. Lee, London. S.E. 12. England. 588
American
Heart Journal
NovezbeT, 1970
Vol.
80, No.
5, pp. 588-596
589
Chest radiogra#hy
Cross and Lewisham Chest Clinic being equipped with identical models. 2. New Cross films were taken with a Watson Roentgen IV equipment, at 6 ft. tube film distance, using 65 kv. with 0.08 X 300 Ma. = sec. exposure. 3. Lewisham films were taken with a Dean Matchlett equipment at 5 ft. tube distance, using 60 kv. with 0.04 X 300 Ma. = sec. exposure.
by one radiographer. Each group of three films was used to provide a mean estimate of CTR. Each mean estimate was used in the analysis. This procedure was considered to minimize any variability attributable to such uncontrollable factors as day-to-day changes in cardiovascular and respiratory activity, exposure at different times in the cardiac cycle, technical differences between radiographers, and alterations of x-ray tube function. This special group included inpatients and outpatients, thought to have large, normal, and small hearts and selected because of this supposed variation. 2. An unselected group of 31 patients, who were outpatients or inpatients, was examined purely on the basis of having 100 mm. and a standard 5 ft. film taken within a 10 day period, both available for measurement. These patients were from Grove Park Hospital or Lewisham Chest Clinic. Both these groups were examined by observer W. L. A. 3. .An unselected group of 13 subjects was examined following recall for large film after examination by South East London Mass Radiography Service. They thus resembled the preceding group in that they had films of both types taken within a few d,ays. The observations were made by a different observer (G. B.) and the x-rays were from different departments. In every subject examined, the films were measured by the method of Appleton, Hamilton, and Simon’ and the CTR was computed. Procedures. RADIOLOGICAL
STATISTICAL
ANALYSIS.
1. Every subject examined gave 2 estimates of CTR based on the 2 sizes of film. The product moment correlation coefficient (r) was computed for the bivariate array thus obtained. The values of r based on “special films” only, and “nonspecial” films taken at Lewisham and New Cross, were treated separately, and the values of r tested for significant variation by the z transformation. 2. A further test of consistency was based on the hypothesis that the 2 techniques were measurements of the same variable (CTR), and that there should, therefore, be no significant difference between the mean CTR measured by the 2 techniques if no distortion occurred. This hypothesis was tested by parametric analysis of variance, Gosset’s t ratio, and Wilcoxon’s test. 3. The ranking of CTR obtained by the 2 methods should be statistically consistent to support the hypothesis of statistical identity. This was examined by Spearman’s rank coefficient. Results Tables I, II, and III show the observations of CTR obtained by the 2 methods in each of the 56 subjects, with their correlation coefficients (r).
TECHNIQUES.
1. All 100 mm. films were taken with an 0delc.a camera, both the South East Londlon Mass Radiography Service at New Table I. Special series CTR’s -__ x (5 ft. 2.04 2.20 2.14 2.49 r = $
CTR)
1
’ ?%?‘)
1 x (5 ft.
2.02 2.04 2.09 2.44
CI.9854; n = 12; z = 2.27291;
CTR)
1.68 1.66 1.78 1.83 standard
error
= 0.3333;
1
’
(%~‘)
/
x (5ft.
1.69 1.69 1.80 1.74 p << 0.001 (very
1.83 2.12 1.48 2.74 highly
significant).
CTR)
1
’ (‘%? 1.83 2.09 1.49 2.81
590
Ashton and Boss
Table II. Unselected Lewisham and Grove Padz CTR’s x (5”fL
y (100 mm.) CTR
CTR)
1.73 2.28 1.66 1.78 2.02 2.44 1.92 2.15 2.38 2.00
1.97 2.16 1.65 1.86 1.77 2.42 2.44 2.16 2.30 2.14
r = -b 0.8652;
n = 31; z = 1.31287;
Table III.
n (6ft.
x (4 ft.
y (100 mm.) CTR
CTR)
2.52 2.21 2.04 2.28 2.40 2.29 1.68 1.97 1.97 2.12 2.1s
standard
error
x (5).
2.34 2.26 2.13 2.37 2.42 2.52 1.65 2.03 2.07 2.11 2.09
= 0.1890;
p << 0.001 (very
y (100 aznz.) CTR
CTR)
1.80 2.12 2.54 1.75 1.86 2.26 1.96 1.65 2.17 1.68
highly
1.67 2.11 2.58 1.54 1.73 2.22 2.06 1.57 2.18 1.70
significant).
Unselected New Cross CTR’s
CTR)
/
’ (l%;m’)
2.00 2.10 1.95 1.76
1.75 1.91 1.62 1.72
r = C 0.9159;
n = 13; z = 1.56359;
!
x (6 ff.
CTR)
1
’ (li%?)
2.11 2.14 1.73 1.95 2.12
standard
error
= 0.3162;
i,,,,.
1.83 2.00 1.62 1.71 1.92
p < 0.001
The results are shown graphically in Figs. 1, 2, and 3 (separately for each group) and 4 (all observations pooled). The graphs are individually and collectively suggestive of a close functional relationship. The value of r computed for each confirms a very high significance probability for the relationship for each separate array. The r values are compared in Table IV using the z transformation. The z values are designated zi, ZZ, and ~3, respectively, for the three correlation coefficients. This finding is interpreted as indicating that r for the special series is significantly higher than for either of the other series. It also indicates that these data provide no evidence for assuming a difference to exist between r’s for either unselected series. The factors outlined in the description as being likely to produce technically based
(very
CTR) 2.24 1.67 1.98 2.94
highly
1
’ (l%?) 2.42 1.72 2,oo 2.95
significant).
changes in CTR--variation between individual radiographers, day-to-day fluctuations in x-ray tube performance, and changes in the physiology of the subjectwould have been virtually eliminated by the technique used in the special series. This technique would also have greatly minimized variation attributable to exposure in systole and diastole, with either type of film. It seems a very plausible inference that these factors are responsible for the highly significant increase of r in the special series. Analysis of variance and Gusset’s f test (Student t test). Parametric analysis of variance showed no evidence of significant differences attributable to the origin of observations, to the 2 radiological techniques, or to the 2 observers. The t test was, therefore, applied to the testing of two related features of the data.
Chest radiography
IN =
121
'R =
0.9854
Z =
247741
P e 0.001
1.5
1.6
I.7
1.8
1.9
2.0
2.1
2.2
X [STANDARD
Fig.
1. Scatter
diagram,
special
2.3
24
2.5
2.6
2.7
2.8
2.9
3.0
C.T.R.)
series.
.3& 2.9
IN =
311
2.8
R =
0.8652
2.7
Z =
1.31287
2.6'
P <( 0.001
2.5 % G 2.4 ti s 2'3 8 2.2 Z > 2.1 2.0 i.9 10 1’0 x
I.7
*
1.6 1.5 1.4Y 1.4
Fig.
2. Scatter
diagram,
1.5
1.6
unselected
1.7
1.8
Lewisham
1.9
2.0
2.1 2.2 2.3 2.4 X [STANDARD C.T.R.1
and
Grove
Park
series.
2.5
2.6
2.7
2.8
2.9
3.0
591
592
Amw.Xelw 3.
Ashton and Boss
Nowmbor,
3.1 IN =
131
2.1
R =
0.9159
2.;
z =
I.56359
2,:
P c 0.001
2.t 2.i a 5
2.1
$
2.:
8 5
2.2
F
2.1 2.c 1.9 1.8 1.7
1.5 1.4 1.5
1.6
1,7
1.8
1.9
2.0
2.1
2.2
X (STANDARD
Fig.
3. Scatter
diagram,
unselected
mass
2.9
IN =
56)
2.8
R =
0.9109
2.7
z =
radiography
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
C.T.R.)
series.
153367
P (( 0.001
2.6 2.5 2 i-’ ti
2.4
5 2.3 I 0 2.2 s1 >
2.1 2.0 I.9 1.8
x”
1.7
Y OF SYMBOLS e
X
1.6
LEWISHAM
-
1.5
SPECIAL SERIES
MASS 1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
X ISTANDARD
Fig.
4. Scatter
diagram,
pooled
data.
2,3
& GROVE
RADIOGRAPHY 2.4
C.T.R.1
2.5
2.6
PARK SERIES SERIES 2.7
2.8
2.9
3.0
1970
Yolume
80
Namber
3
Chest radiogra$hy
593
Table IV. Values under test .ZI and z3 and .II and
Standard difference
z2 z2 z3
error of for pair
0.3832 0.3864 0.4594
Value difference
of of 3’s
1.16454 0.25072 0.91382
Ratio standard
of diference to error of differeme 3.0390t 0 6806 1.9892%
*Probably significant (p fi 0.05). tVery highly significant (p 2 0.001).
The hypothesis x = y was tested for each array separately and for all results pooled (x being CTR based on standard films and y being CTR based on miniature films). The hypothesis x - y = 0 was also tested, by subtracting all the individual 100 mm. CTR’s from the corresponding standard CTR and applying a one-tailed t test to the difference of the mean value from 0. For all arrays, there was no significant difference demonstrated between x and y when the first method was applied. For the second method, it was found that x - y z 0 (t = 2.79: p = 0.02) in the third series of films only. Since in this series, there were uncontrolled conditions of x-ray examination and also a different observer, this Iprovided no support for the view that distortion of CTR may be attributed to using 100 mm. film. It seems likely that this nonidentity may be due to a greater variance of observations under these conditions. This is not inconsistent with the result of analysis of variance, which merely shows that for this sample size, there is no convincing evidence of nonhomogeneity of variance. Ot.ber significance tests. To avoid the assumption of homogeneous variance implied by the parametric tests, the data were tested by Wilcoxon’s test, the sign test (applicable only to all 56 results pooled), and the Spearman rank coefficient. Wilcoxon’s test (applied to each group separately and to all groups pooled) showed no significant difference between CTl? estimates by either method. The sign test also supported this conclusion. Spear-man’s test gave consistent findings. The values of R were all highly significant
(p < 0.001 for all except the New Cross series, for which 0.001 < p < 0.01). It therefore seems that there is no convincing objective ground for the view that CTR is seriously distorted by 100 mm. as opposed to standard size chest films. Discussion
The radiological assessment of heart size by any physician is usually based on one chest skiagram taken at a finite time in the patient’s recent past. There is awareness that apparent changes in CTR may occur as a result of technical and physiological variations between different times of filming the same subject, quite apart from those changes, due to heart disease, whose presence and severity are under assessment. These results establish that there is a very highly significant correlation between the estimate of CTR made with 100 mm. and 5 or 6 ft. standard films. The fact that the correlation is significantly higher in the special series reflects, in all likelihood, the lessening of variation due to technical changes in the x-rays taken as between different radiographers on different days, and day-to-day physiological changes in the subjects under study. The point to consider in interpreting the findings is that the measurement obtained as an estimate of CTR appears statistically identical whether it is based on miniature or standard films. Thus, any fallacies inherent in evaluating CTR based on miniature films are those based on shortcomings of CTR as a useful measure and not to any distortion of the size of cardiac silhouette by either technique. Consideration of the geometry of the chest x-ray does not support any basis for expecting that CTR
594
Ashton
and Boss
would be altered in technically adequate miniature films. It is apparent that objections can be made to CTR as a measure of heart size. Not the least important is that where right ventricular enlargement is predominant, the transverse diameter of the heart is affected relatively little. Thoracic deformity, in addition, may add difficulties in assessment. Whatever the difficulties posed by either type of film, the evaluation of heart size is most frequently based on the posteroanterior chest skiagram. The quantitative aspects of assessment remain ill-defined at present; a ratio of 2.1 as suggested by Woo@ seems widely accepted. Work is at present in progress to try to define limits of CTR which are more clearly associated with the presence or absence of diagnosable cardiovascular abnormality. It seems from present findings that MMR can offer as much information as standard chest radiography in cardiovascular disease. Abnormal form of the heart and great vessels and evidence of rib notching or erosionthose radiological features that may be as readily called “nonquantitative’‘-are revealed by MMR as by standard films. R,Iost subjects with cardiovascular disease, !lowever, have hypertension, ischemic heart disease, or acquired valve disease (if pulmonary heart disease is excluded because of owing its presence to lung or skeletal disease). With the exception of some cases of mitral stenosis and ischemic heart disease, these conditions nearly always lead to significant cardiac enlargement; and there are seldom clearcut morphological changes, other than enlargement, to aid recognition. Indeed, very few patients with any type of heart disease fail to show cardiac enlargement detectable at necropsy, but the congenital lesions are more likely to lead to recognizable deformation of cardiovascular silhouette and they are relatively far commoner in children. It is thus difficult to evade the conclusion that MMR is potentially a powerful tool in epidemiology and also in selecting subjects likely to benefit clinically from full cardiological assessment. Large populations can be examined by MMR swiftly and cheaply. The method is already generally accepted by the public (always of basic
importance in any investigative technique of ma.jor epidemiological interest). The possible implications, immediate and remote, for cardiovascular epidemiology and clinical cardiology may be considerable. At present, cardiovascular assessment of population samples is based on the relatively cumbersome methods of routine clinical evaluation with electrocardiography and, in a few centers, ballistocardiography. These are time consuming, expensive, and thus of doubtful applicability to population samples outside various highly selected groups of relatively small size. It is also worth considering that chest radiography, giving as it does a cardiovascular assessment based on heart muscle mass and cardiac filling, may offer a more direct simple test of cardiac function than other Iaboratory methods, except for ballistocardiography. At present, MMR films are being reported by physicians who are divisible into those who believe in MMR as a useful measure of heart size, and those who do not consider it helpful. In the absence of evidence establishing which of these viewpoints is correct, it is difficult to evade the conclusion that MMR surveys are inevitably less effective than they might be. For either a considerable amount of detectable heart disease is being overlooked by the nihilistic school, or a considerable amount of unnecessary, anxiety-inducing, and occasionally dangerous investigation is being carried out-at considerable public expense-by referrals for spurious “cardiac enlargement.” Neither of these situations is desirable and both are avoidable. The hypothesis derived from the observations described here can be tested in several ways. The most obvious method applied at present is the comparison of clinical, ECG, and laboratory findings in persons recalled after MMR because of apparent cardiomegaly. (This is considered at present as indicated by a ratio below 1.80 on the basis of a pilot study now in progress.) Age- and sex-matched subjects with “normal” hearts are also being recalled. A highly significant excess of detectable heart disease in the “large” hearts would be expected if the MMR estimate is, as we consider, a valid estimate of CTR.
Chest radiography
Absolute proof could be established in the long term by necropsy study but this would require a long and complex degree of organization of observations, and would depend on a high necropsy rate in both “normal-” and “large-heart” patients, as well as on frequent MMR films to ensure that a recent estimate of CTR was available at the time of death. Since modern cardiological diagnosis is technically so reliable, it is considered that the detection of a significant excess of cardiovascular disease in the “large-heart” patients would be adequate preliminary proof of the hypothesis, and it would be, at present, both ineffilzient and superfluous to attempt a necropsy survey, even if this could be organized. The technique of measurement and calculation required has been considered. Setsquare and slide rule were used in this study, for measurement and computing. Each. film took 2 to 3 minutes to examine in this way. It is considered that a device using 2 pairs of calipers, moving on each of 2 parallel horizontal scales attached to the lower margin of the viewing box, should make rapid measurement possible. The application of the findings of this study depends on the following factors: 1. A definition of a value of CTR produci:ng the maximum segregation of subjects into those with and those without significant cardiomegaly is necessary. 2. There must be an establishment of the reliability of MMR observers in selecting films for measurement. Some observers appear to be as capable of detecting cardiomepaly in 100 mm. films as competent cardiologists are in 5 ft. or 6 ft. films. Others seem less reliable, but error seems comistently to be in the direction of judging normal-sized hearts to be enlarged. The validity of these impressions, and the extent to which training improves the ability of observers to detect “large” heart shadows, requires testing by suitable operational research. 3. A suitable technique must be evolved for the rapid measurement of CTR in the cases selected as being suspicious. The caliper device suggested in outline would probably be simplest; it might be possible to d.evelop a photoelectric scanning device
59s
coupled to the necessary computing elements required to calculate the ratio. An attempt to define the CTR value giving maximum segregation into “normal” and “abnormal” patients is at present being made, using samples of persons examined by MMR. The other factors require further technical study. The problems are not directly relevant to the hypothesis being tested here, that CTR measured by 100 mm. film is identical with that measured by standard film. The assumption that MMR is measuring the same dimensions is being applied to samples of persons being examined by miniature radiography, in an attempt to reach a clearer definition of the range of “normal” CTR. Related to assessment of heart size by either standard or miniature radiography is the problem of how far evaluation is affected by physiological variations in heart size. Exposure during systole and diastole affects the film obtained. Variations in respiration may affect the result. Recent physical activity is important in affecting heart size in health and disease.2 Earlier work by Gorlin and Braunwald3v4 suggested that increased somatic muscle activity diminishes ventricular systolic volume in healthy subjects. Analysis of observations in transverse cardiac diameter or CTR of successive films under various standard conditions might enable more reliable criteria for judging significance of changes to be established. Although the 12 patients examined in the “special” series each had 6 films for analysis, the data are too restricted to examine this point precisely because of the fact that all these films were taken in rapid succession with the subject at rest, that is, in circumstances intended to minimize functional changes. The most important point illustrated by these findings is that the opinion frequently expressed by physicians-that miniature films give an impression of increased heart size-has no objective basis. Presumably this opinion arises from illusory perception associated with the smaller films. Be this as it may, it has evidently led to doubt about the application of MMR to any aspect of cardiovascular investigation. It
596
Ashton and Boss
underlines quantitative
the importance of applying methods wherever possible.
Miss M. Brown took the special x-rays, Mrs. V. Pike gave valuable secretarial assistance, and Mr. J. Hale, of Goldsmith’s College, kindly made calculating facilities available. Dr. J. M. Morgan gave helpful advice, and Mr. J. J. Lyons of the G. L. C. reviewed and supplemented the statistical analysis.
REFERENCES 1. Appleton, A. B., Hamilton, W. j., and Simon, G.: Surface and radiological anatomy, Cambridge, 1946, W. Heffer & Sons, Ltd. ?-. Boelling, G. X., Phillips, W. J., Frerking, H. W., et al.: Roentgenographic exercise test. A new test of myocardial state, J.A.M.A. 202:275, 1967. 3. Braunwald, E., et al.: Circ. Res. 13:448, 1963. 4. Gorlin, R.: Pathophysiology of cardiac pain, Circulation 32:361, 1965. r3. Wood, P. : Disorders of the heart and circulation, London, 1957, Eyre and Spottiswoode.