Summary

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Jamieson CW. Positron emission tomography in venous ulceration and liposderosis: Study of regional tissue function. Br Med J 1983; 286:333-6. 28. Siegel ME, Stewart CA. Thallium-201 peripheral perfusion scans: Feasibility of single dose, single day rest and stress study. AJR 1981; 136:1179.

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29. Siegel ME, Williams GM, Giargiana FA. A useful objective criterion for determining the healing potential of an ischemic ulcer. J Nucl Med 1975; 16:993. 30. Evens RG, Siegel BA, Welch MJ, Ter-Pogossian MM. Cost analysis of positron emission tomography for clinical use. AJR 1983; 141:1073-6.

Summary Robert W. Barnes, M.D., Little Rock, Ark. The four previous articles have reviewed the current status and future application of duplex scanning, digital subtraction angiography (DSA), nuclear magnetic resonance (NMR) imaging and spectroscopy, and positron emission tomography (PET). Despite the innovative technologic advances inherent in each of these modalities, certain problems of cost, complexity, and sophistication of data acquisition and interpretation require comment to permit clinicians to make appropriate use of these techniques in the detection and management of patients with vascular disease. I will summarize my views about the relative value of these various methods for both clinical practice and research compared with more Conventional diagnostic techniques. Duplex scanning brings together the combined advantages of Doppler ukrasonic flow detection and analysis and real-time B-mode ultrasonic imaging for noninvasive interrogation of arterial and venous disease. Recent advances in this technology have permitted more precise sampling of blood flow velocity, greater sophistication in Doppler signal analysis, and morphologic verification of the site of Doppler sampiing from the real-time B-mode image. In addition, high-resolution B-mode imaging permits detection of relatively minor vascular wall defects including, in certain circumstances, arterial ulceration, which may be important in patients with carotid artery disease. The combination of imaging and Doppler analysis is particularly important in determining the presence From the Department of Surgery, Universityof Arkansas for Medical Sciences, and the Surgical Service, John L. McClellan Memorial Veterans Administration Hospital. Presented at the combined breakfast program of the Society for Vascular Surgery and the International Society for Cardiovascular Surgery, North American Chapter, Atlanta, Ga., June 9, 1984. Reprint requests: Robert W. Barnes, M.D., Mail Slot 520, UAMS, 4301 West Markham, Little Rock, AR 72205.

of blood flow in vessels with advanced atherosclerotic plaques that, with imaging alone, may appear occluded. Such definition of lumen patency in severely stenotic vessels is particularly important in carotid disease, since such lesions are amenable to endarterectomy. Finally, duplex imaging is particularly helpful to identify and interrogate deep-lying visceral vessels such as renal arteries, mesenteric arteries, and the portal venous circulation. Despite these advantages, there are several limitations to the routine use of duplex scanning at the present time. One major problem relates to the variability in image resolution or Doppler fidelity in many of the commercial devices currently available. Some manufacturers have emphasized high resolution in the real-time B-mode image with relatively tittle emphasis on the quality of the Doppler signal. Other instruments have excellent Doppler frequency characteristics with relatively poor image detail and resolutiorl.~Until these problems are resolved, many clinicians will fail to take fifll advantage of both image quality and Doppler spectral characteristics from a single instrument. A second major limitation to routine use of duplex scanning is the need for extensive education and experience on the part of the technologist using the instrument. Although this device permits the greatest accuracy in screening for arterial and venous disease when employed by an experienced and knowledgeable technologist, many errors are possible in the hands of less-qualified individuals. Misinterpretation of both morphologic images and Doppler spectra is frequently made by the inexperienced observer. I would agree with the statement by Dr. Strandness that both the performance and interpretation of duplex scanning is best left to the experienced technologist. However, appropriate integration of such information with the clinical data base requires con-

Volume 2 Number 2 March 1985

siderable sophistication on the part of the physicians relating to or supervising the noninvasive laboratory. Thus it is incumbent on both physician and technologist to devote the time necessary for acquiring the knowledge and experience to achieve the skill in the use and interpretation of this modality. This final problem with duplex instrumentation now is the cost of this diagnostic modality. Unfortunately, the cost for equipment outlay as well as physician and technologist training and equipment maintenance preclude its acquisition by many diagnostic laboratories. However, the clinician must realize that much of the diagnostic information that may be generated ideally by duplex scanning can be achieved by more simple and less costly devices, particularly continuous-wave Doppler ultrasound. In my own experience much of the diagnostic accuracy possible with duplex scanning may be achieved by an experienced technologist using continuous-wave Doppler and sound spectrum analysis. However, such information in the absence of imaging capability places much of the burden for the diagnostic examination on the technologist. Nevertheless, such data acquisition requires an experienced technologist regardless of the equipment used. However, the high cost of duplex scanning often leads to abuses of the instrument in screening large numbers of patients who might otherwise be evaluated by more simple and less costly means. I am particularly concerned about the increasing tendency of many laboratories to acquire more sophisticated diagnostic equipment; the financing of these devices is recovered by more extensive screening of patients who might best be served by more simple clinical follow-up or patient education. In particular, the current practice of using noninvasive laboratories for routine screening of asymptomatic patients as a prelude to possible angiography and prophylactic carotid endarterectomy deserves critical attention. It is my belief that these noninvasive techniques might best be used to evaluate the natural history of such patients in certain academic centers to document the validity of the hypothesis that prophylactic carotid endarterectomy is warranted. Until such studies are forthcoming, however, many physicians will continue to use these newer diagnostic techniques to screen patients for a potential prophylactic operation, the efficacy of which is questionable. DSA represents a technologic advance that was perhaps prematurely exploited by clinicians to evaluate patients for vascular disease. Although newer generations of digital equipment will unquestionably change the practice of radiology in this country, the

Symposium: New VascularDiagnostic Techniques 367

use of intravenous DSA was uncritically accepted by many physicians before the accuracy of this method was clarified. Unfortunately, the initial enthusiasm was followed by a negative attitude toward this procedure, which itself may not be completely justified. Intravenous DSA provides the virtues of increased patient safety and reduced costs but has limitations of reduced image resolution and frequent ambiguity of vascular diagnosis. Although newer generations of equipment have led to constant improvement in sensitivity and clarity of image detail, many problems remain. These include overlapping of vascular images, motion artifacts, and reduced image resolution, particularly of intracranial vessels. Despite these limitations many physicians continue to employ intravenous DSA as a routine diagnostic screening test, reserving intra-arterial imaging for patients with ambiguous or technically inadequate studies. Many radiologists are now using DSA technology for processing intra-arterial angiograms. The advantage of this technique is that higher image resolution is possible with the lower volumes of contrast material, which may be injected through a smaller intra-arterial catheter at less risk to the patient. There remains a definite role for intravenous DSA. I believe that those centers with current-generation equipment may employ this modality for many patients with vascular disease providing that initial noninvasive screening is used to select the appropriate angiographic modality. In my own practice, I follow an algorithm very similar to that outlined by Dr. Sumner. Patients with symptoms of transient ischemic attack or recovered stroke are initially screened by direct carotid noninvasive studies. Ifa significant stenosis is suggested, intravenous DSA is recommended, and if confirmatory of carotid stenosis, I perform endarterectomy based on the intravenous image. If the noninvasive study suggests that the carotid artery is normal or minimally diseased, I recommend intra-arterial angiography to define the presence or absence of an operable carotid plaque. If the noninvasive study suggests internal carotid artery occlusion, an intravenous DSA may be performed. With this approach, few patients have required intraarterial angiography after ambiguous or unsuccessful intravenous DSA. For patients with atypical symptoms or asymptomatic carotid bruits, noninvasive direct carotid screening is carried out to define the presence or absence of significant internal carotid stenosis. If severe bilateral disease is documented, intravenous DSA may be carried out and endarterectomy performed for the appropriate lesions. If unilateral ste-

368 Symposium:New VascularDiagnostic Techniques

nosis is present, I recommend evaluation of the integrity of the circle of Willis using carotid compression ocular pneumoplethysmography of Gee. If good collateral circulation is present, I follow asymptomatic stenoses without surgical intervention unless the patient has transient ischemic attacks. If the circle of Willis is incomplete, I would recommend prophylactic carotid endarterectomy. I am particularly interested in the recent data by Strandness suggesting that patients with carotid stenoses >80% diameter reduction are at increased risk of symptomatic neurologic events. Such patients may deserve prophylactic carotid endarterectomy, although the validity of this hypothesis remains to be proved. Recent data suggest that combinations of noninvasive carotid screening with appropriate angiography in select cases is cost effective. More studies of such cost effectiveness are necessary to define the relative role of noninvasive techniques with angiography. Both NMR and PET have become important new modalities to increase our understanding of the metabolism, in vivo biochemistry, and pathophysiology of vascular disease. To date, NMR imaging has been the most extensively exploited method. I am concerned that the explosive interest in this modality reflects an enthusiasm for image generation similar to that experienced by intravenous DSA imaging. Without doubt NMR imaging provides new morphologic detail that is complementary to, and in certain cases, superior to that of CT imaging. However, the cost of this new modality should make the clinician pause before considering routine use of this technology for the evaluation of vascular disease. Certainly its routine use for imaging the aortoiliac system for aneurysmal or occlusive disease may not be cost effective. The mere achievement of excellent imaging should not justify routine use of such technology. Unfortunately, image resolution remains unsatisfactory for smaller vessels. Furthermore, the frequent presence of metallic implants precludes the use of this technology in such patients. I believe that one of the most important future applications of NMR technology is in spectroscopic evaluation of the basic metabolism of atherosclerotic disease and the enhancement of our understanding of the ischemic process and the methods of adaptation to ischemia, including the effects of exercise or newer drug therapies. Unfortunately, current limitations of magnet size and strength preclude the use of NMR spectroscopy in man except for evaluation of brain or limb physiology. However, future improvements in instrumentation may facilitate human

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applications. I would predict that much of our future understanding of the basic mechanisms of disease may come from both NMR and PET technologies. However, these applications should remain in academic centers for investigative purposes. I would hesitate to recommend enthusiastic embracing of these technologies by the clinician for routine diagnosis and management of vascular diseases. Nowhere do the virtues of data acquisition conflict more with the cost of health care delivery than with PET technology, which is more expensive than any of the aforementioned methods. The potential impact of this modality on increasing our understanding of the biochemistry and pathophysiology of vascular and other diseases is difficult to comprehend. Many of our recent advances in understanding the metabolism and effects ofischemia on brain function have been outgrowths of this technology. Increased applications in the evaluation of myocardial and peripheral circulatory disorders will greatly increase our understanding of the nature of, and adaptation to, ischemia. Unfortunately, the cost of the equipment, including detectors, cyclotron, and personnel necessary for the acquisition of this modality, restricts its use to select academic centers. Nevertheless, PET, as with NMR spectroscopy, should greatly illuminate our understanding of the mechanisms of vascular disease. However, the clinician will only indirectly reap the rewards of these technologies in the near future. In conclusion, both duplex scanning and DSA are becoming more prevalent in the detection and management of patients with vascular disease. Rather than being competitive, I feel that these two techniques are appropriately complementary to each other and can be used to construct cost-effective diagnostic algorithms in the management of patients with arterial or venous disease. It should be emphasized that duplex technology should not replace less costly and more simple noninvasive screening techniques. Furthermore, the recent trend toward use of DSA for intra-arterial injections should not detract from the virtues of intravenous DSA in select patients. Finally, NMR and PET are costly but innovative new technologies that will greatly enhance our understanding of the mechanisms of vascular disease and their treatment. While NMR imaging is assuming increasing importance, the greatest virtue of this modality will be the spectroscopic information permitting biochemical interrogation of intact man. This, along with PET, will provide indirect scientific validation of the indication for, and the efficacy of, therapeutic interventions for patients with vascular disease.