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Invited Commentary |
1 Department of Radiology, University of Pennsylvania Medical Center, Philadelphia, Pennsylvania
Two articles in this issue describe state-of-the-art vascular imaging techniques. In the first, Gates and Hartnell (1) review techniques for performing and optimizing conventional angiography. In the second, Watanabe et al 135 (2) describe their experience with dynamic contrast-enhanced MR angiography and suggest various clinical situations to which this technique can be applied. At first glance, the articles appear similar in their description of techniques used to evaluate the vascular system, but on further scrutiny they are very different. The first represents the culmination of over 100 years of experience with an established technique; the other represents an initial experience with a new technique.
The history of vascular imaging is a long one. In January 1896, one month after Roentgen's discovery, Hascheck and Lindenthal obtained the first in vitro angiogram by injecting a mixture of bismuth, lead, and barium salts into the blood vessels of an amputated hand. Since that time, the growth of vascular imaging has kept up with the development of contrast materials and paralleled the changes and advances made in imaging.
In 1924, Brooks obtained the first clinical femoral and pulmonary arteriograms in humans using sodium iodide. Because of the toxic effects associated with this contrast agent, clinically useful arteriography was put on hold until safer organic iodine–containing compounds were developed in the mid-1930s.
The next major advance occurred after the introduction of the percutaneous transfemoral catheterization method developed by Seldinger in 1953. This technique allowed percutaneous access to the aorta and all its branches. Angiography was further stimulated by the introduction of rapid film changers, automatic injectors, image-amplified fluoroscopy, and cine angiography in the 1960s. It was the safety and sophistication of percutaneous coronary arteriography that allowed surgeons to first attempt coronary artery bypass surgery over 30 years ago.
The article by Gates and Hartnell (1) is an excellent, concise summary of basic methods and strategies used to optimize conventional angiography. These techniques are standard practice for interventional radiologists today and reflect the long history of diagnostic angiography.
Conversely, MR angiography has a relatively short history. In the late 1980s, flowing blood was merely a nuisance producing artifacts on MR images. Pulse sequences were developed that suppressed these "flow artifacts." It was only a few years later that these same sequences were used to visualize (rather than suppress) flowing blood. It is truly remarkable how far MR angiography has advanced during the past decade.
The article by Watanabe et al (2) describes subtraction MR angiography. This is a relatively new MR imaging technique; as a result, this article represents more of a feasibility study or a work in progress.
Dozens of MR imaging techniques have been proposed during the past decade, reflecting advances in hardware and software. In the near future, it is likely that MR angiography will be performed in a dynamic way, with visualization of blood entering and leaving a vascular bed. It will probably also be possible to perform organ perfusion studies, and MR images of the heart will be obtained at rates of 30–40 per second.
However, at present, there is no single MR angiographic technique that images all the vessels of the body with the same accuracy as conventional angiography. When MR angiography is performed, different techniques are used to optimize signal from each area being examined. For example, one technique may be useful for imaging the aorta and its branch vessels, whereas another is better for imaging tibial and pedal arteries. Dynamic subtraction contrast-enhanced MR angiography is an example of the former. Watanabe et al (2) present both fine images of large proximal vessels such as the aorta and disappointing images of smaller peripheral vessels. As a result, the authors' suggestion that this technique be used to image patients with peripheral vascular occlusive disease is unrealistic. Imaging of small vessels requires small fields of view to achieve the resolution needed to quantitate severity of disease and make revascularization decisions. The images of the peripheral vessels shown in this article could not possibly accomplish either of these goals. Nevertheless, the subtraction technique used by these authors allows rapid, high-quality dynamic imaging of the large vessels of the abdomen and thorax. With modifications (including use of surface coils), there is no reason why the peripheral vessels could not be demonstrated satisfactorily.
Modern conventional angiography has benefited from over a century's worth of experience. During the past 100 years, we have learned much about the technique and the vessels we are studying. Today, modern image intensifiers, digital subtraction angiography, pulsed fluoroscopy, road mapping, and nonionic contrast agents have made examinations remarkably accurate and yet safe for both the patient and the physician. However, the invasive nature of having to insert a catheter into the vascular system is an inherent drawback of this technique. This drawback is the reason why noninvasive vascular imaging modalities have proliferated during the past 15 years. Although it is hazardous to predict the future, it appears all but certain that one of the cross-sectional imaging techniques (computed tomographic angiography or MR angiography) will replace conventional angiography in the near future. One can envision a time when a patient with vascular disease will be screened noninvasively with one of the cross-sectional modalities and, if necessary, referred for a vascular intervention. This imaging algorithm is already commonplace in many hospitals (3).
Vascular imaging in the 21st century promises to be as exciting and challenging as it was in the past 100 years.
References
Related Articles
This article has been cited by other articles:
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  T. Leiner, K. Y. J.A.M. Ho, J. M.A. van Engelshoven, and Y. Watanabe Techniques of Dynamic Subtraction Contrast-enhanced MR Angiography RadioGraphics, July 1, 2000; 20(4): 1113 - 1114. [Full Text] [PDF]
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