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Accurate screening for abdominal aortic aneurysm
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1992 Clin. Phys. Physiol. Meas. 13 135 (http://iopscience.iop.org/0143-0815/13/2/005) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 150.135.239.97 This content was downloaded on 02/10/2015 at 08:01
Please note that terms and conditions apply.
Clin. Phys. Physiol. Meas., 1992, Vol. 13, No. 2, 135.138. Printed in the UK
Accurate screening for abdominal aortic aneurysm G M Grimshawt and M F Dockerg iDeparment of Vascular Surgery, Queen Elizabeth Hospita1,Queen Elizabeth Medical Centre, Edgbasron, Birmingham B15 2TH, UK $ Department of Medical Physics, Birmingham Maternity Hospital, Queen Elizabeth Medical Centre, Edgbasron, Birmingham B15 ZTG,UK Received 12 June 1991, in final form 25 October 1991
Abstract. In Birmingham a screening programme has been set up to detect abdominal aortic aneurysm in the male population between h e ages of 65 and 75 years. T h e men to be screened are invited t o their o m general practitioners surgely and offered an abdominal uluasound.The accuracy with which abnormalities can be detected is of crucial importance to a screening programme. Repearability has been assessed for the single operator involved and is found to be i 1.3 mm. The acccu~~cy of a single scan has been calculated by comparison with the measured diameter from a CT scan of the patient, taken subsequently. This comparison indicates that 95% of the differences between the two measurements can be expected to be less than 3.5 mm.
1. Introduction
Surgeons are becoming increasingly concerned at the number of patients presenting as emergency admissions with rupture of the abdominal aorta. T h e loss of elasticity in the aortic wall leading to aneurysm formation is a progressive condition with few symptoms. Nevertheless, deaths due to rupture of the abdominal aorta are predicted to rise to 10 000 this year in the UK. T h e mortality of aortic rupture is very high. Most aneurysms rupture at home and 50% of all cases will die before reaching hospital. Of those reaching hospital all will need surgical replacement of the aorta but despite this a further 38% will die within a 30 day period. I n contrast to this poor prognosis, if an aneurysm is detected before rupture and operated upon electively the surgical mortality is less than 5% in most centres. A strong case can be made for early detection of aortic aneurysm in terms of both lives saved and surgical costs. Until such time as better predictors of rupture are available the maximum diameter of the aorta is taken as a measure of rupture risk. Several trials have been conducted in the U K to test the feasibility of general population screening for early detection of aortic aneurysm. T h e most successful of these trials have taken portable ultrasound B scanners to general practitioners’ surgeries and invited patients to attend for a simple measurement of the diameter of the aorta. These trials have demonstrated that a very high rate of acceptance of a single letter of invitation can be achieved and that the rate of abnormalities detected in males aged 65 to 75 years is around 10%. The majority of these trials were set up to establish the prevalence of the condition (Collin et al 1988, O’Kelly and Heather 1989). In Birmingham we have conducted our own trial. One of the key elements of this trial was to assess the feasibility of ultrasound scanning in the community with 0143-08151921020135 + 04 53.50 0 1992 Institute ofphysical Sciences in Medicine
135
136
G M Grimshaw and M F Docker
cheap portable equipment in the far from ideal conditions found in ordinary consulting rooms. Of central importance to the assessment of feasibility is the ability of a screening technique to detect abnormality accurately. As aortic diameter is used as a measure of the degree of abnormality, the accuracy with which it can be measured is of crucial importance when deciding thresholds for action. This issue bas been addressed in this trial. 2. Method The group of patients most at risk of abdominal aortic aneurysm, males aged 65-75 years, were invited to attend their own GFs’ surgeries for screening. The aorta was examined with a Pie Data 150s scanner using a 3.5 MHz mechanical sector transducer. All patients were scanned anteriorly in the abdominal midline below the sternum. T h e aorta was identified and surveyed longitudinally to establish tortuosity and general shape. The aorta was then scanned transversely and the diameter measured at several points down the length of the aorta as far as the bifurcation. The diameter was measured from anterior to posterior to utilise the maximum inherent resolution of the ultrasound beam. The diameter was taken as the internal diameter of the apparent lumen. Where thrombus was present the callipers were placed on the interior of the wall echoes. Significant variation in estimates of aortic size can be expected if the front of the anterior wall echo and the back of the posterior wall echo are used. These errors arise from variation during the cardiac cycle and artefacts caused by echoiinterface behaviour. The maximum diameter of the aorta was quoted rather than a ratio of infrarenal tu suprarenal diameter. It is our experience that the suprarenal portion of the aorta is too difficult to image to make it worthwhile for general screening. Twcnty paticnts who had been scanned at surgery and whose aortas were aneurysmal subsequently had CT scans. This enabled us to use the CT diameter measurement as a ‘gold standard’ to facilitate comparison between methods. The accuracy of the scanner itself was established using the Cardiff Test Object and reproducibility was measured by performing 10 serial scans on each of five patients with the calliper reading concealed from the operator and read by an observer. 3. Results
The scanner used for screening had an axial resolution of 1 mm, a lateral resolution of 2.5 mm and a calliper accuracy of 1 mm as assessed on the Cardiff Test Object at a depth of 8 cm, the depth at which we can expect to see the aorta. Ten measurements of the aortic diameter of each of five patients with mean diameters in the range 16 to 44 mm were taken, repositioning the transducer after each measurement. The total sample standard deviation was 1.3 mm and the sample fulfilled the BSI standard (BS5497 part I) for reproducibility in that 95% of all the measurements taken were within two standard deviations of the mean of the sets of measurements. We can therefore expect a single operator to be able to repeat measurements with an accuracy of 1.3 mm. During the feasibility study 2668 patients were scanned at their own surgery. Of the patients scanned, 219 (8.2%) had dilatation of the aorta. T h e distribution of
*
Accurate screeningfor abdominal aortic aneurysm
137
aortic size in the normal population is within the range 13 mm to 30 mm, with a mode of 20 mm. Twenty patients had ultrasound scans followed b y CT scans within four weeks. The range of aortic diameters was between 31 and 86mm (no patients with normal aortas were offered CT scans). T o derive a figure for accuracy of the ultrasound technique from comparison with CT scanning, correlation coefficients have been quoted. However, a very high degree of correlation would be expected between these two measurement techniques and quotation of a conventional correlation coefficient may still hide any underlying trend. Although giving the strength of a relationship between two variables, a correlation coeffient does not necessarily indicate agreement. The analysis technique chosen was one first proposed by Bland and Alunan (1986). They propose a plot of the difference between the two measurements being compared, plotted against the mean of the two measurements. T h e resulting distribution gives much more information 'about the scatter of the points. Figure 1 shows this plot for our data. 4
3 2
-
1
-
. .
. . .
0 -1
-
-2 -
. . . .
. .
-3 -4
It can be seen that there is no obvious relationship between the points, and certainly no skew across the range of diameters, but we can summarise this by calculating the bias of the distributed points as the mean (x) and standard deviation (s) of the distribution shown in figure 1 . The mean is 0.1 mm and the standard deviation between C T and ultrasound is 1.8 mm. T h e mean of 0.1 mm demonstrates that there is no consistent bias between the measurements. Assuming that the differences are normally distributed we can use x ? 1.96 s to express our limits of agreement. T h e other essential requirement, that 95% of all points should fall within two standard deviations, is fulfilled. We can state, therefore, that 95% of our ultrasound measurements are expected to lie within 3.5 mm of an aortic diameter measured by CT scan. As the value of aortic diameter as measured by CT scan is taken as an absolute for clinical decisions, no discussion of the accuracy of CT scanning is necessary here. The magnification factor noted by other authors (Ellis ez al 1990) using ultrasound does not seem to be present in this series.
G M Grimshaw and M F Docker
138
4. Discussion At present, rupture risk is assumed to increase linearly with aortic diameter and most vascular surgeons will use some arbitrary figure to define whether the aorta is aneurysmal or not. In an informal survey taken by the British Vascular Surgical Society most surgeons (95%) would replace aortas with a diameter in excess of 60 mm but only 50% would replace aortas with a diameter of 40 mm. Once a population is screened, a full range (13mm to 96" in the Birmingham Study) of aortic diameters are demonstrated. Further, although it is accepted that most aortas with a dismeter greater &an 60" will xpture, very little evidence is available to predict the behaviour of aneurysmal aortas of smaller diameter. Anecdotal evidence is available to suggest aneurysms of diameter 40 mm can rupture. This study has shown that accurate estimation of aneurysm size is both possible and repeatable in the far from ideal circumstances found when scanning in general practice. T h e accuracy of the community measurements compared with CT scanning is sufficiently good to allow suigical decisions to be made without further tests unless involvement of the renal arteries is suspected. Of equal importance in a screening programme is the management of the patient with small, but possibly growing, aortic diameter. These patients are offered monitoring scans at fixed intervals at their own surgery. Over 100 patients have been monitored for growth rate and the results from this series will be published after a suitable time. Here the accuracy can be estimated from the repeatability of the ultrasound scan rather than I-. .-..*I-..A" TL;" A~c..;.;,... ^F " *I.^I-" C"I..^t^-l y U a L L u io minimum detectable change in size. Only if more than one operator is likely to see the patient routinely should the inter-observer quality control methods used in x-ray departments be necessary. To ensure repeatability in screening programmes it may be preferable to arrange that each patient is monitored by a single operator. In this case only single operator quality control methods, such as frequent machine checks and annual performance tests, need to be applied. "J
-".....""".:.."
LY"Ap"0LL'"c
..'."."" I...., I.
"C 1 . 1 1 .
&.".I
^^-
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References Bland M J and Altman D G 1986 Statistical measurement for assessing agreement between two methods of clinical measurement Lancer 1307-10 Collin J, Walton 1, Aravjo L 1988 Oxford screening programme for abdominal aortic aneulysm in men aged 65 to 74 Lancetii 613-5 Ellis .M, Powell !T, Place J I990 The Coum and .Mcilnnpmlmr n f A n e ! q w ! I( Gree~halghznd 1 .A Marileck (London: Saunders) O'Kelly T ] and Heather B P 1989 General practice based population screening for abdominal aortic aneurysm: a pilot study Br.3.Surg. 76 479-SO
Search
Collections
Journals
About
Contact us
My IOPscience
Accurate screening for abdominal aortic aneurysm
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1992 Clin. Phys. Physiol. Meas. 13 135 (http://iopscience.iop.org/0143-0815/13/2/005) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 150.135.239.97 This content was downloaded on 02/10/2015 at 08:01
Please note that terms and conditions apply.
Clin. Phys. Physiol. Meas., 1992, Vol. 13, No. 2, 135.138. Printed in the UK
Accurate screening for abdominal aortic aneurysm G M Grimshawt and M F Dockerg iDeparment of Vascular Surgery, Queen Elizabeth Hospita1,Queen Elizabeth Medical Centre, Edgbasron, Birmingham B15 2TH, UK $ Department of Medical Physics, Birmingham Maternity Hospital, Queen Elizabeth Medical Centre, Edgbasron, Birmingham B15 ZTG,UK Received 12 June 1991, in final form 25 October 1991
Abstract. In Birmingham a screening programme has been set up to detect abdominal aortic aneurysm in the male population between h e ages of 65 and 75 years. T h e men to be screened are invited t o their o m general practitioners surgely and offered an abdominal uluasound.The accuracy with which abnormalities can be detected is of crucial importance to a screening programme. Repearability has been assessed for the single operator involved and is found to be i 1.3 mm. The acccu~~cy of a single scan has been calculated by comparison with the measured diameter from a CT scan of the patient, taken subsequently. This comparison indicates that 95% of the differences between the two measurements can be expected to be less than 3.5 mm.
1. Introduction
Surgeons are becoming increasingly concerned at the number of patients presenting as emergency admissions with rupture of the abdominal aorta. T h e loss of elasticity in the aortic wall leading to aneurysm formation is a progressive condition with few symptoms. Nevertheless, deaths due to rupture of the abdominal aorta are predicted to rise to 10 000 this year in the UK. T h e mortality of aortic rupture is very high. Most aneurysms rupture at home and 50% of all cases will die before reaching hospital. Of those reaching hospital all will need surgical replacement of the aorta but despite this a further 38% will die within a 30 day period. I n contrast to this poor prognosis, if an aneurysm is detected before rupture and operated upon electively the surgical mortality is less than 5% in most centres. A strong case can be made for early detection of aortic aneurysm in terms of both lives saved and surgical costs. Until such time as better predictors of rupture are available the maximum diameter of the aorta is taken as a measure of rupture risk. Several trials have been conducted in the U K to test the feasibility of general population screening for early detection of aortic aneurysm. T h e most successful of these trials have taken portable ultrasound B scanners to general practitioners’ surgeries and invited patients to attend for a simple measurement of the diameter of the aorta. These trials have demonstrated that a very high rate of acceptance of a single letter of invitation can be achieved and that the rate of abnormalities detected in males aged 65 to 75 years is around 10%. The majority of these trials were set up to establish the prevalence of the condition (Collin et al 1988, O’Kelly and Heather 1989). In Birmingham we have conducted our own trial. One of the key elements of this trial was to assess the feasibility of ultrasound scanning in the community with 0143-08151921020135 + 04 53.50 0 1992 Institute ofphysical Sciences in Medicine
135
136
G M Grimshaw and M F Docker
cheap portable equipment in the far from ideal conditions found in ordinary consulting rooms. Of central importance to the assessment of feasibility is the ability of a screening technique to detect abnormality accurately. As aortic diameter is used as a measure of the degree of abnormality, the accuracy with which it can be measured is of crucial importance when deciding thresholds for action. This issue bas been addressed in this trial. 2. Method The group of patients most at risk of abdominal aortic aneurysm, males aged 65-75 years, were invited to attend their own GFs’ surgeries for screening. The aorta was examined with a Pie Data 150s scanner using a 3.5 MHz mechanical sector transducer. All patients were scanned anteriorly in the abdominal midline below the sternum. T h e aorta was identified and surveyed longitudinally to establish tortuosity and general shape. The aorta was then scanned transversely and the diameter measured at several points down the length of the aorta as far as the bifurcation. The diameter was measured from anterior to posterior to utilise the maximum inherent resolution of the ultrasound beam. The diameter was taken as the internal diameter of the apparent lumen. Where thrombus was present the callipers were placed on the interior of the wall echoes. Significant variation in estimates of aortic size can be expected if the front of the anterior wall echo and the back of the posterior wall echo are used. These errors arise from variation during the cardiac cycle and artefacts caused by echoiinterface behaviour. The maximum diameter of the aorta was quoted rather than a ratio of infrarenal tu suprarenal diameter. It is our experience that the suprarenal portion of the aorta is too difficult to image to make it worthwhile for general screening. Twcnty paticnts who had been scanned at surgery and whose aortas were aneurysmal subsequently had CT scans. This enabled us to use the CT diameter measurement as a ‘gold standard’ to facilitate comparison between methods. The accuracy of the scanner itself was established using the Cardiff Test Object and reproducibility was measured by performing 10 serial scans on each of five patients with the calliper reading concealed from the operator and read by an observer. 3. Results
The scanner used for screening had an axial resolution of 1 mm, a lateral resolution of 2.5 mm and a calliper accuracy of 1 mm as assessed on the Cardiff Test Object at a depth of 8 cm, the depth at which we can expect to see the aorta. Ten measurements of the aortic diameter of each of five patients with mean diameters in the range 16 to 44 mm were taken, repositioning the transducer after each measurement. The total sample standard deviation was 1.3 mm and the sample fulfilled the BSI standard (BS5497 part I) for reproducibility in that 95% of all the measurements taken were within two standard deviations of the mean of the sets of measurements. We can therefore expect a single operator to be able to repeat measurements with an accuracy of 1.3 mm. During the feasibility study 2668 patients were scanned at their own surgery. Of the patients scanned, 219 (8.2%) had dilatation of the aorta. T h e distribution of
*
Accurate screeningfor abdominal aortic aneurysm
137
aortic size in the normal population is within the range 13 mm to 30 mm, with a mode of 20 mm. Twenty patients had ultrasound scans followed b y CT scans within four weeks. The range of aortic diameters was between 31 and 86mm (no patients with normal aortas were offered CT scans). T o derive a figure for accuracy of the ultrasound technique from comparison with CT scanning, correlation coefficients have been quoted. However, a very high degree of correlation would be expected between these two measurement techniques and quotation of a conventional correlation coefficient may still hide any underlying trend. Although giving the strength of a relationship between two variables, a correlation coeffient does not necessarily indicate agreement. The analysis technique chosen was one first proposed by Bland and Alunan (1986). They propose a plot of the difference between the two measurements being compared, plotted against the mean of the two measurements. T h e resulting distribution gives much more information 'about the scatter of the points. Figure 1 shows this plot for our data. 4
3 2
-
1
-
. .
. . .
0 -1
-
-2 -
. . . .
. .
-3 -4
It can be seen that there is no obvious relationship between the points, and certainly no skew across the range of diameters, but we can summarise this by calculating the bias of the distributed points as the mean (x) and standard deviation (s) of the distribution shown in figure 1 . The mean is 0.1 mm and the standard deviation between C T and ultrasound is 1.8 mm. T h e mean of 0.1 mm demonstrates that there is no consistent bias between the measurements. Assuming that the differences are normally distributed we can use x ? 1.96 s to express our limits of agreement. T h e other essential requirement, that 95% of all points should fall within two standard deviations, is fulfilled. We can state, therefore, that 95% of our ultrasound measurements are expected to lie within 3.5 mm of an aortic diameter measured by CT scan. As the value of aortic diameter as measured by CT scan is taken as an absolute for clinical decisions, no discussion of the accuracy of CT scanning is necessary here. The magnification factor noted by other authors (Ellis ez al 1990) using ultrasound does not seem to be present in this series.
G M Grimshaw and M F Docker
138
4. Discussion At present, rupture risk is assumed to increase linearly with aortic diameter and most vascular surgeons will use some arbitrary figure to define whether the aorta is aneurysmal or not. In an informal survey taken by the British Vascular Surgical Society most surgeons (95%) would replace aortas with a diameter in excess of 60 mm but only 50% would replace aortas with a diameter of 40 mm. Once a population is screened, a full range (13mm to 96" in the Birmingham Study) of aortic diameters are demonstrated. Further, although it is accepted that most aortas with a dismeter greater &an 60" will xpture, very little evidence is available to predict the behaviour of aneurysmal aortas of smaller diameter. Anecdotal evidence is available to suggest aneurysms of diameter 40 mm can rupture. This study has shown that accurate estimation of aneurysm size is both possible and repeatable in the far from ideal circumstances found when scanning in general practice. T h e accuracy of the community measurements compared with CT scanning is sufficiently good to allow suigical decisions to be made without further tests unless involvement of the renal arteries is suspected. Of equal importance in a screening programme is the management of the patient with small, but possibly growing, aortic diameter. These patients are offered monitoring scans at fixed intervals at their own surgery. Over 100 patients have been monitored for growth rate and the results from this series will be published after a suitable time. Here the accuracy can be estimated from the repeatability of the ultrasound scan rather than I-. .-..*I-..A" TL;" A~c..;.;,... ^F " *I.^I-" C"I..^t^-l y U a L L u io minimum detectable change in size. Only if more than one operator is likely to see the patient routinely should the inter-observer quality control methods used in x-ray departments be necessary. To ensure repeatability in screening programmes it may be preferable to arrange that each patient is monitored by a single operator. In this case only single operator quality control methods, such as frequent machine checks and annual performance tests, need to be applied. "J
-".....""".:.."
LY"Ap"0LL'"c
..'."."" I...., I.
"C 1 . 1 1 .
&.".I
^^-
Y.
LLCCY'"CJ
CL.1.
Y.L..
"C
References Bland M J and Altman D G 1986 Statistical measurement for assessing agreement between two methods of clinical measurement Lancer 1307-10 Collin J, Walton 1, Aravjo L 1988 Oxford screening programme for abdominal aortic aneulysm in men aged 65 to 74 Lancetii 613-5 Ellis .M, Powell !T, Place J I990 The Coum and .Mcilnnpmlmr n f A n e ! q w ! I( Gree~halghznd 1 .A Marileck (London: Saunders) O'Kelly T ] and Heather B P 1989 General practice based population screening for abdominal aortic aneurysm: a pilot study Br.3.Surg. 76 479-SO
