The last few years have seen dramatic changes in the preferred methods of imaging the vasculature. There are several reasons. First, the risks and discomfort of contrast angiography have driven searches for alternatives. Second, the deficiencies of that study in examining one of the most important diseases in Western societiesatherosclerosishave become all too clear. Acute coronary syndromes are most often caused by lesions that are angiographically insignificant or invisible. What is needed is a means of depicting the "vulnerable" plaque, rich in lipids and infiltrating macrophages and having a thin fibrous cap that is prone to rupture, leading to thrombosis and emboli. Third, the capabilities of CT and MR for vascular imaging have improved dramatically, and these modalities enable examination of the consequences of vascular disease or injury for the surrounding tissues.^1,2 Moreover, these modalities have capabilities such as three-dimensional reconstruction and manipulation of the images, eg, for background suppression. Also driving the implementation of angiography alternatives have been the invention of devices such as imaging guidewires and the demand for noninvasive techniques for monitoring clinical trials and screening for vascular disease. This article looks at some of the technology that is displacing contrast angiography.

IMAGING THE AORTA

Screening for abdominal aortic aneurysms by ultrasonography is a growing practice, with many areas of the country now served by traveling scanners. Monitoring of aneurysms also is done by sonography, with its triple advantage of speed, low cost, and noninvasiveness.

Traditionally, when an aneurysm was discovered and the decision was made to repair it or insert a stent-graft, aortography was the choice for preoperative planning. However, helical CT and CT angiography (CTA) are growing more popular, as they are faster and less invasive, depict the anatomy of surrounding organs,³ and permit accurate measurement of the dimensions of the aneurysmal sac. In patients with suspected aneurysm ruptures and acute dissections, CTA is the imaging method of choice.⁴

Patients who have received stent-grafts require lifelong follow-up to detect leaks and enlargement of the aneurysmal sac. A standard protocol is a plain film to examine the stent itself followed by color Doppler ultrasonography and CTA. Robert B. McLafferty, MD, and associates, of the Division of Vascular Surgery at Southern Illinois University, who studied 79 stent-graft patients, found that color-flow duplex ultrasonography detected all of the leaks found by CT with one false-positive examination.⁵ Those investigators suggested that the greater ease of ultrasonography might permit earlier identification and treatment of leaks. However, ultrasonography is not reliable for measurements of aneurysmal sacs, making it necessary to use another technique such as biphasic CTA to monitor changes.⁶ Magnetic resonance angiography (MRA) is employed for patients with impaired renal function or contrast allergy and may be superior in detecting certain types of leaks.⁶

EXAMINING THE BRAIN VASCULATURE

The concern about the morbidity of contrast angiography is particularly acute for studies of the brain: as many as 3% of patients undergoing intracranial angiography suffer neurologic complications.⁷ At the same time, the demand for such imaging has grown as intravascular thrombolysis has made occlusive stroke treatable. Here, MRA with perfusion and diffusion imaging has become dominant. Present-day ultrafast protocols provide three essential items of information: the site of the vascular lesion, the identity of the injured tissue, and the region of ischemic brain. A recent review from Massachusetts General Hospital⁸ suggests that future stroke treatment "will be tailored, not to a fixed time window, but to the physiological state of the ischemic tissue as defined by MRI."

EVALUATING THE CAROTID ARTERIES

With the confirmation that medical and surgical therapy can control symptoms and prevent strokes in patients with carotid atherosclerosis, screening by B-mode ultrasonography has become widespread.  Ultrasonographic measurement of intimalmedial thickness (IMT) is the only noninvasive imaging test recommended by the American Heart Association as a marker of cardiac risk, and it is a reliable means of monitoring progression and regression of early carotid atherosclerosis.^9,10  Changes in IMT have been used as a surrogate endpoint in several clinical trials, although, because the technique is not standardized, comparisons of the results of different trials can be difficult.¹¹

Ultrasonography also can characterize plaques. Echogenicity caused by lipid predicts rupture in symptomatic patients, although the results are observer dependent.⁹ Videodensitometry uses ultrasonography data to create a histogram of the frequency of different gray levels, which reflects the composition of tissues. It is observer independent and commercially available at a relatively low cost, although lack of standardization has inhibited its use. M. M. Ciulla, MD, and associates of the University of Milan compared the videodensitometric and histologic findings in 19 patients who underwent endarterectomy for stenoses of at least 70%, and found that the histograms enable correct classification of 90% of the plaques as lipidic, fibrolipidic, or fibrotic.¹²

A new technology with great potential is automated cardiac-gated three-dimensional ultrasonic imaging. In a recent preliminary trial,¹³ data acquisition could be completed in an average of 12 minutes, suggesting that the technique will be clinically useful.

Preoperative evaluation of the carotid arteries is increasingly dependent on MRA, which is preferred by many surgeons because of its depiction of nearby anatomy.¹⁴ Also, gadolinium-enhanced MR has been used to measure the fractional blood volume at sites of carotid plaque as a means of determining the extent of neovascularization, which is associated with plaque inflammation and instability.¹⁵ Spiral CT is being tested as a means of identifying and characterizing carotid stenosis, but the techniques are not yet standardized. A potential drawback is CT's inability to depict lipid well.⁹ Use of MRA and CTA in carotid imaging is likely to increase as techniques are refined,¹⁶ although further data confirming the accuracy of these newer modalities will be required if they are to find a routine place in clinical practice.¹⁷

ABDOMINAL VASCULATURE

The introduction of protocols fast enough to avoid motion artifacts have made CTA and MRA of the abdomen so dominant that Vosshenrich and Fischer recently wondered "is there still a role for angiography?"⁴

For possible mesenteric ischemia, selective angiography is still the gold standard and has the advantage of permitting appropriate interventions such as infusion of a vasodilator.¹⁸ However, when, as is so often the case, the cause of abdominal pain is obscure, CT may be preferred,¹⁹ especially as multidetector array scanners become more readily available.²⁰ Contrast-enhanced MRA is often the choice for evaluating chronic mesenteric ischemia, examining the portal veins, or diagnosing and monitoring portal hypertension.^4,21

CTA has been used for some years to assess potential living kidney donors, especially when laparoscopy is being planned, as the smaller surgical field inhibits analysis of the anatomy. A more recent development is the comprehensive vascular, urographic, and parenchymal examination available with gadolinium-enhanced MRI. In one series,²² the sensitivity and positive predictive value of MRA with a torso phased-array coil at 1.5T were 75% and 95%, respectively. Nephrectomy was completed laparoscopically in 27 of the 28 donors on the basis of the MR study, the sole exception being a patient who proved to have complex venous anatomy too small to be seen by imaging.

ASSESSING PVD

The most common goals of imaging in the peripheral vessels are diagnosis of acute deep venous thrombosis and identification of critical stenoses amenable to relief. Here again, classic contrast studies are being replaced. For venous thrombosis, venography is yielding to duplex ultrasonography with color-flow Doppler analysis. A newer technique is administration of 99mtechnetium-labeled gpIIb/IIIa receptor antagonists or fibrin-binding compounds to identify recently formed clot (which is of greatest interest as a potential source of pulmonary emboli).²³ In preparation for revascularization, contrast-enhanced MRA is likely to become the dominant imaging method.²⁴

CORONARY CALCIUM SCANNING?

The most controversial use of vascular imaging is the application of electron beam tomography (EBT) or, more recently, multidetector-array CT^25-27 to screen asymptomatic persons for significant atherosclerotic coronary artery disease. Certainly, calcium scoring is accurate in predicting angiographic coronary artery disease in symptomatic patients,²⁸ in monitoring medical interventions such as statin therapy,²⁹ and perhaps in predicting the response of a coronary lesion to angioplasty.³⁰ However, its prognostic utility in asymptomatic subjects is less clear. Whereas some studies suggest that calcium scores are indeed predictive of cardiac events in this population,^31,32 other investigators have had contradictory results.³³ Moreover, calcium screening presents some ethical³⁴ and legal³⁵ issues of which radiologists must be aware before they decide to offer the study to asymptomatic persons.

SOME OTHER NEW DEVELOPMENTS

The foregoing does not exhaust the changes being seen in vascular imaging. MRA can depict the spinal vessels,³⁶ demonstrate the patency of coronary artery bypass grafts,³⁷ and contribute to a comprehensive evaluation of seriously injured joints.³⁸ Color Doppler ultrasonography and CTA are replacing angiography for diagnosing vascular injuries (although angiography is likely to remain the first choice if interventional treatment is contemplated).³⁹

A source of great excitement is the growing likelihood that plaques can be characterized by imaging (reviewed by Naghavi et al⁴⁰). One technique is elastography, an adjunct to intravascular ultrasonography (IVUS) that assesses the mechanical properties of tissues through measurement of strain. The differences in the degree of strain are color coded and plotted on the IVUS images. Soft plaques have a deformability of 1% to 2%, whereas hard (calcified) plaques register values of 0 to 0.2%.⁴¹ Thermography identifies the higher temperatures characteristic of plaques more heavily infiltrated with macrophages (ie, with greater inflammation).^42,43 Molecular imaging with site-targeted agents is in its infancy but in a position to benefit from recent discoveries about the chemical nature of vulnerable plaques. The ability to target vascular tissue has already been demonstrated.⁴⁴

One highly promising method does not examine the blood vessels at all. MR spectroscopic metabonomics provides a metabolic profile of a cell or tissue. In December, a team from the Imperial College of Science, Technology and Medicine in the United Kingdom demonstrated that application of pattern-recognition techniques to proton spectra of human serum could correctly demonstrate the presence and the severity of coronary artery disease.⁴⁵ The investigators noted that this was the first technique "capable of providing an accurate, noninvasive and rapid diagnosis of coronary heart disease [sic] that can be used clinically, either in population screening or to allow effective targeting of treatments such as statins." Confirmatory data are eagerly awaited.

CONCLUSION

These developments will have a profound effect on the equipment needs of hospitals. Barry T. Katzen, MD, of the Miami Vascular Institute, has written that vascular imaging is likely to become a specialty in its own right and that interventionalists should consider acquiring equipment for MR and CT.⁴⁶ He also pointed out that even though many applications of contrast angiography are being replaced by CTA and MRA, this does not mean that the need for angiography will decline. On the contrary. Because of the increasing age of the population and earlier diagnosis of disease, more interventional procedures are being performed, and a large number of these still require angiography.

Judith Gunn Bronson, MS, is a contributing writer for Decisions in Imaging Economics.

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