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COMPARISON OF DOPPLER ULTRASOUND, MAGNETIC RESONANCE ANGIOGRAPHIC TECHNIQUES AND CATHETER ANGIOGRAPHY IN EVALUATION OF CAROTID STENOSIS
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CLINICAL RADIOLOGY
REFERENCES 1 Hollingworth W, Todd CJ, Bell MI, et al. The diagnostic and therapeutic impact of MRI: an observational multi-centre study. Br J Radiology 2000;55:825±831.
doi:10.1053/crad.2000.0748, available online at http://www.idealibrary.com on
COMPARISON OF DOPPLER ULTRASOUND, MAGNETIC RESONANCE ANGIOGRAPHIC TECHNIQUES AND CATHETER ANGIOGRAPHY IN EVALUATION OF CAROTID STENOSIS SIR ± I read the paper by Johnson et al. [1] with interest. I am concerned that the methodology may show ultrasound in an unnecessarily disadvantageous light. The authors made no mention of the time interval between the various investigations. Ultrasound was used as the screening method for entry into the study and must, therefore, have been the earliest investigation for each patient. Fig. 1a of their paper shows that six out of 12 carotid arteries shown by catheter angiography to be occluded were found by ultrasound to be patent. This is a most unusual error for ultrasound. Ultrasound does not detect ¯ow when none is present. The more expected error is that ultrasound will diagnose an occlusion when there is in fact still trickle ¯ow present. Consequently, it seems highly likely that the tight stenoses doi:10.1053/crad.2000.0749, available online at http://www.idealibrary.com on
COMPARISON OF DOPPLER ULTRASOUND, MAGNETIC RESONANCE ANGIOGRAPHIC TECHNIQUES AND CATHETER ANGIOGRAPHY IN EVALUATION OF CAROTID STENOSIS SIR ± I read with interest the paper by Johnson et al. [1]. The authors have concluded from the study that the sensitivity of ultrasound for the detection of signi®cant internal carotid artery stenosis was as low as 65% compared with sensitivities varying from 82% to 100% for magnetic resonance angiographic techniques. There are a number of weaknesses which warrant closer scrutiny. The Doppler ultrasound examinations were performed by a number of examiners and the degree of stenosis calculated using standard criteria. However, there is no indication of what the standard criteria measurements were, nor is there any indication of the strati®cation of calculated stenoses. The description of the methodology for carotid Doppler ultrasound is sparse in comparison to the detailed description given to both the magnetic resonance angiographic techniques and conventional (catheter) angiography. In this regard, carotid angiography was performed on arch aortogram studies and not using selective catheterization of the common carotid artery. The degree of stenosis was then calculated based on NASCET criteria [2]. Based on the given data, it is inappropriate to make conclusions about the sensitivity of ultrasound in the assessment of carotid artery stenosis when the methodology is as uncontrolled as in the present study. At best, the study demonstrates the superiority of a tightly controlled magnetic resonance angiographic technique in comparison with a routine clinical service (although I suspect the vascular laboratory performing the Doppler ultrasound examinations would be alarmed at the low sensitivity ®gures for ultrasound), against a sub-optimal gold standard of arch aortography. If catheter angiography is to be used as the gold standard for evaluating other imaging modalities and the NASCET criteria are to be used, selective common carotid angiography is mandatory. The NASCET study [2] insisted on selective examinations
2 Sunshine JH, McNeil BJ. Rapid method for rigorous assessment of radiologic imaging technologies. Radiology 1997;202:549±557. 3 Miles KA. An approach to demonstrating cost-eectiveness of diagnostic imaging modalities in Australia illustrated by positron emission tomography. Australasian Radiology, in press. diagnosed by the initial ultrasound progressed to occlusion by the time of angiography. If this is true, then it throws into doubt the supposed ultrasound underestimation of the severe stenosis category also. The study methodology would have been better if the patients had had their ultrasound examinations repeated at the time of the MRA. This would have produced a more realistic assessment of ultrasound's accuracy. DR M. WESTON
Ultrasound Department, St James's University Hospital, Beckett Street, Leeds, LS9 7TF U.K.
REFERENCE 1 Johnson MB, Wilkinson ID, Wattam J, Venables GS, Griths PD. Comparison of Doppler ultrasound, magnetic resonance angio-
of the carotid artery as opposed to the European Carotid Surgery Trial [3] which accepted any form of angiographic imaging. Although selective examinations of carotid artery carries added risk [4], it remains the gold standard imaging examination of the carotid bifurcation. Undoubtedly ultrasound is the most subjective examination of the carotid bifurcation, and is limited by a wide range of patient factors; it remains the most robust screening method of the carotid bifurcation. Due to technical advances, magnetic resonance angiography will become more sophisticated and accurate, but the same will hold true for ultrasound. However, just as there are patient factors which in¯uence an ultrasound examination, patient factors such as claustrophobia, cardiac pacemakers and other metallic foreign bodies will limit access to magnetic resonance angiography. The published results of Johnson et al. [1] do injustice to ultrasound, not because the technique is unreliable but due to ¯awed methodology. DR PAUL S. SIDHU
Department of Clinical Radiology, King's College Hospital, Denmark Hill, London, SE5 9RS, U.K.
REFERENCES 1 Johnson MB, Wilkinson ID, Wattam J, Venables GS, Griths PD. Comparison of Doppler ultrasound, magnetic resonance angiographic techniques and catheter-angiography in evaluation of carotid stenosis. Clin Radiol 2000;55:912±920. 2 North American Symptomatic Carotid Endarterectomy Trial. Bene®cial eect of carotid endarterectomy of symptomatic patients with a high grade stenosis. New Engl J Med 1991;325:445±453. 3 European Carotid Surgery Trial. MRC European carotid surgery trial. Lancet 1991;337:1235±1243. 4 Hankey GJ, Warlow CP, Sellar RJ. Cerebral angiographic risk in mild cerebro-vascular disease. Stroke 1990;21:209±222.
CLINICAL RADIOLOGY
REFERENCES 1 Hollingworth W, Todd CJ, Bell MI, et al. The diagnostic and therapeutic impact of MRI: an observational multi-centre study. Br J Radiology 2000;55:825±831.
doi:10.1053/crad.2000.0748, available online at http://www.idealibrary.com on
COMPARISON OF DOPPLER ULTRASOUND, MAGNETIC RESONANCE ANGIOGRAPHIC TECHNIQUES AND CATHETER ANGIOGRAPHY IN EVALUATION OF CAROTID STENOSIS SIR ± I read the paper by Johnson et al. [1] with interest. I am concerned that the methodology may show ultrasound in an unnecessarily disadvantageous light. The authors made no mention of the time interval between the various investigations. Ultrasound was used as the screening method for entry into the study and must, therefore, have been the earliest investigation for each patient. Fig. 1a of their paper shows that six out of 12 carotid arteries shown by catheter angiography to be occluded were found by ultrasound to be patent. This is a most unusual error for ultrasound. Ultrasound does not detect ¯ow when none is present. The more expected error is that ultrasound will diagnose an occlusion when there is in fact still trickle ¯ow present. Consequently, it seems highly likely that the tight stenoses doi:10.1053/crad.2000.0749, available online at http://www.idealibrary.com on
COMPARISON OF DOPPLER ULTRASOUND, MAGNETIC RESONANCE ANGIOGRAPHIC TECHNIQUES AND CATHETER ANGIOGRAPHY IN EVALUATION OF CAROTID STENOSIS SIR ± I read with interest the paper by Johnson et al. [1]. The authors have concluded from the study that the sensitivity of ultrasound for the detection of signi®cant internal carotid artery stenosis was as low as 65% compared with sensitivities varying from 82% to 100% for magnetic resonance angiographic techniques. There are a number of weaknesses which warrant closer scrutiny. The Doppler ultrasound examinations were performed by a number of examiners and the degree of stenosis calculated using standard criteria. However, there is no indication of what the standard criteria measurements were, nor is there any indication of the strati®cation of calculated stenoses. The description of the methodology for carotid Doppler ultrasound is sparse in comparison to the detailed description given to both the magnetic resonance angiographic techniques and conventional (catheter) angiography. In this regard, carotid angiography was performed on arch aortogram studies and not using selective catheterization of the common carotid artery. The degree of stenosis was then calculated based on NASCET criteria [2]. Based on the given data, it is inappropriate to make conclusions about the sensitivity of ultrasound in the assessment of carotid artery stenosis when the methodology is as uncontrolled as in the present study. At best, the study demonstrates the superiority of a tightly controlled magnetic resonance angiographic technique in comparison with a routine clinical service (although I suspect the vascular laboratory performing the Doppler ultrasound examinations would be alarmed at the low sensitivity ®gures for ultrasound), against a sub-optimal gold standard of arch aortography. If catheter angiography is to be used as the gold standard for evaluating other imaging modalities and the NASCET criteria are to be used, selective common carotid angiography is mandatory. The NASCET study [2] insisted on selective examinations
2 Sunshine JH, McNeil BJ. Rapid method for rigorous assessment of radiologic imaging technologies. Radiology 1997;202:549±557. 3 Miles KA. An approach to demonstrating cost-eectiveness of diagnostic imaging modalities in Australia illustrated by positron emission tomography. Australasian Radiology, in press. diagnosed by the initial ultrasound progressed to occlusion by the time of angiography. If this is true, then it throws into doubt the supposed ultrasound underestimation of the severe stenosis category also. The study methodology would have been better if the patients had had their ultrasound examinations repeated at the time of the MRA. This would have produced a more realistic assessment of ultrasound's accuracy. DR M. WESTON
Ultrasound Department, St James's University Hospital, Beckett Street, Leeds, LS9 7TF U.K.
REFERENCE 1 Johnson MB, Wilkinson ID, Wattam J, Venables GS, Griths PD. Comparison of Doppler ultrasound, magnetic resonance angio-
of the carotid artery as opposed to the European Carotid Surgery Trial [3] which accepted any form of angiographic imaging. Although selective examinations of carotid artery carries added risk [4], it remains the gold standard imaging examination of the carotid bifurcation. Undoubtedly ultrasound is the most subjective examination of the carotid bifurcation, and is limited by a wide range of patient factors; it remains the most robust screening method of the carotid bifurcation. Due to technical advances, magnetic resonance angiography will become more sophisticated and accurate, but the same will hold true for ultrasound. However, just as there are patient factors which in¯uence an ultrasound examination, patient factors such as claustrophobia, cardiac pacemakers and other metallic foreign bodies will limit access to magnetic resonance angiography. The published results of Johnson et al. [1] do injustice to ultrasound, not because the technique is unreliable but due to ¯awed methodology. DR PAUL S. SIDHU
Department of Clinical Radiology, King's College Hospital, Denmark Hill, London, SE5 9RS, U.K.
REFERENCES 1 Johnson MB, Wilkinson ID, Wattam J, Venables GS, Griths PD. Comparison of Doppler ultrasound, magnetic resonance angiographic techniques and catheter-angiography in evaluation of carotid stenosis. Clin Radiol 2000;55:912±920. 2 North American Symptomatic Carotid Endarterectomy Trial. Bene®cial eect of carotid endarterectomy of symptomatic patients with a high grade stenosis. New Engl J Med 1991;325:445±453. 3 European Carotid Surgery Trial. MRC European carotid surgery trial. Lancet 1991;337:1235±1243. 4 Hankey GJ, Warlow CP, Sellar RJ. Cerebral angiographic risk in mild cerebro-vascular disease. Stroke 1990;21:209±222.