Vascular imaging is changing dramatically. It is no longer enough simply to find that a patient has an arterial stenosis. Now, physicians want to see the circulation to the region, evaluate the vessel wall, and determine the composition of plaques so they can better select and monitor treatment. There also is a growing push to screen for vascular disease beyond the heart in order to prevent myocardial infarction, stroke, and amputation.

Increasingly, vascular diagnosis is being done by CT and MR rather than traditional catheter techniques. High spatial and temporal resolution, less or no radiation and iodinated contrast medium, and the ability to reconstruct the volumetric data in three dimensions are the appealing features of CTA and MRA. 1 More and more, catheter angiography is restricted to interventional therapeutic procedures.

CT ANGIOGRAPHY

MR arterial image of the lower leg and foot, using time-resolved imaging of contrast kinetics. Image courtesy of Steven D. Wolff, MD, PhD, Lenox Hill Hospital, New York.

As multidetector-array CT (MDCT) scanners have moved into more hospitals, there has been an “exponential increase” 2 in CTA use. With this technology, it is no longer necessary to sacrifice high spatial resolution along the z axis to obtain extensive coverage. Procedures are completed with greater dispatch, and peripheral vessels unseen with classic helical CT become visible. Dedicated workstations and software have reduced the complexities of data editing and processing. The importance of the latter feature of the new equipment must not be underestimated, as an angiographic study on an MDCT scanner generates more than 1,000 axial images. 2 Image volumes can be reconstructed in a variety of ways, such as by maximum intensity projection, volume rendering, and multiplanar and curved planar reconstruction, depending on the user’s needs.

Of course, these benefits have a price. “The faster that scan technology gets, as with 16-slice MDCT scanners, the less forgiving it is,” 2 making parameter selection and contrast timing especially demanding.

A particular concern on MDCT scanners is the higher radiation dose. 2 Among the recommended measures are scanning only the area of direct interest and being selective about performing precontrast scans. If such a run is necessary, a low-mA protocol should be used. Dose modulation software is suggested for runoff studies. 2

Several clear indications for CTA have emerged, 2 such as elective and emergency imaging of aortic aneurysms, stenoses, or occlusions; critical ischemia or claudication in peripheral vessels; and several types of visceral angiography. For example, the renal vasculature may be examined in potential kidney donors or to select the best type of ureteropelvic junction repair for a particular patient. Other indications for CTA are many types of mesenteric, hepatic, coronary, cerebral, and pulmonary disease and venography in patients being considered for reconstruction.

A team at Stanford University described the use of CTA in 14 patients scheduled for upper-extremity reconstruction, praising the study for the excellent depictions of the bones and soft tissues as well as the blood vessels. 3 In two patients, the CTA findings “significantly altered” the surgical plan. Moreover, the average charge for CTA was $1,140, whereas the charge for the same study done by catheter angiography was $3,900. 3

Peripheral CTA studies can triage patients to various treatments with a scan time of less than 1 minute, but “[t]his is without a doubt the most demanding study on the MDCT scanner and post-processing software,” 2 and accurate reconstruction of the smaller leg and pedal vessels must be done manually from individual slices. 4 Still other accepted CTA applications are traumatic vascular emergencies, where it obviates conventional angiography, 5 and small-bowel enteroclysis for identifying tumors, including lymphoma, Crohn’s disease, and tuberculosis. 6 Localization of gastrointestinal bleeding by CTA is being evaluated. 2

An important application of MDCTA is guidance and follow-up of endograft placement for aortic aneurysms. Here, the superiority of CTA to MRA is clear, as MR cannot depict the calcification that affects the suitability of the iliac arteries for graft delivery and of the aorta for graft placement. 7 In patients with impaired renal function, CTA can be performed with gadolinium contrast. 7 Rapid acquisition, appropriate contrast doses, and thin slices are required. Although 2 mm has been the standard slice thickness, 1 mm may be better and is easily achievable with MDCT. 7 “Nowhere is there a more compelling demonstration of the increased accuracy of 3D imaging than in its application to determine [blood vessel] length” and measure angles for endograft placement. 7 Dedicated software facilitates accurate measurements of the aortic neck and the distal zone and also permits endoluminal views from the perspective of the catheter, endograft modeling, and simulation of placement of particular endograft designs. Postoperatively, according to the Society of Interventional Radiology, CTA is the “gold standard” for endograft follow-up. 7

NEW TECHNOLOGY FOR MRA

The exquisite soft-tissue detail from MRI has been available for some time, but until recently, the technology was too slow and the resolution too crude for most angiography applications. That has changed. New MRA technology has virtually erased the spatial resolution advantage formerly enjoyed by CTA, and manufacturers continue to press for greater speed. Although MR now needs 6 to 8 seconds to capture a single volume, whereas digital subtraction angiography acquires images at six frames per second, the goal is for MR to capture a three-dimensional volume every second. With improvements in speed and resolution, MRA is expected to supplant CT for some body sites, especially in diabetic patients, in whom the safety of x-ray contrast medium is of particular concern.

Steven D. Wolff, MD, PhD, director of advanced cardiovascular imaging and of cardiovascular MRI and CT at the Cardiovascular Research Foundation and chief of cardiovascular MRI at Lenox Hill Hospital in New York City, averages two MRA runoff studies in the legs every day.

“Historically, runoff studies have been difficult with MR, both because of the large field of viewfrom the belly button to the feetand because the scan must be carried out at the right speed to track the contrast, and the correct speed can be different in the two legs,” he notes. “In the past, we used a timing bolus. Now, we have time-resolved imaging of contrast kinetics. Appropriate images can be selected retrospectively to create three-dimensional depictions of the anatomy that can be viewed from any angle. We also have seen a 15-fold increase in the signal-to-noise ratio (SNR) over that of the traditional body coil with 0.5-mm resolution using new boot-shaped radiofrequency coils. In fact, the SNR is so high that we have reduced the amount of contrast we use in the lower legs from 20 mL to 8 mL. Not only does this save moneygadolinium contrast costs about $3 per milliliterbut we have less interfering contrast in the body when we do the scans of the upper legs. Because we do not need a timing bolus, and because we do not need to change coils in the middle of the procedure, patients can be moved in and out of the scanner faster. Also, there are almost no callbacks necessitated by poor images.”

Sohn and colleagues 8 have found that dynamic and time-resolved MRA is helpful in assessing intracerebral vascular lesions such as arteriovenous malformations. In patients with ischemic cerebrovascular accidents (CVAs), conventional and three-dimensional time-of-flight MRA improves diagnosis. Although their older scanner did not have optimal resolution, those investigators forecast growing substitution of MRA for conventional catheter angiography for the intracranial circulation. 8

DIGITAL FLUOROSCOPY

Digital flat-panel fluoroscopy units provide a much higher level of small-vessel detail without the need for more contrast and with lower patient and operator absorbed radiation doses. These systems also have a much wider dynamic range than the old image intensifiers and are free of the distortion created by the curved image intensifiers, with good image quality all the way to the edge of the screen. A study can be completed from the diaphragm almost to the pubic bone in one shot rather than two to four, as in the past. Less radiation is required for magnified views because magnification is created by displaying a block of pixels at full-screen size. Procedures go faster because the anatomy is depicted better. These detectors are capable of imaging the full length of the nitinol and other nonmagnetic stents that are becoming popular, whereas older systems show only the ends. Although three-dimensional reconstruction is more common with MR and CT than with fluoroscopy, it is valuable to reduce the number of contrast injections needed.

“The way we treat arterial disease has changed within the last few years. Most diagnostic work now is done with no or only minimal invasiveness, such as CTA or MRA,” according to Michael Brunner, MD, vice chairmanradiology, director of the vascular laboratory, and chief of interventional radiology at Swedish Covenant Hospital in Chicago, and immediate past president of the Society of Interventional Radiology. “Angiographic procedures now are primarily therapeutic. Flat-panel detectors have increased what can be done. Previously, 3 mm was about the limit for seeing and doing; now, we can see 1 mm quite well, and with new balloons and stents, we can do things for those vessels that we could not do before. On the horizon are nanotechnology applications and drugs, such as those that stimulate angiogenesis, and we will need imaging both to deliver them and to confirm that they are effective.”

An example of the difference digital flat-panel technology has made can be seen in the patient admitted to the emergency department with a possible CVA. Previously, this patient would have required a wide view of the aortic arch, images of the carotid arteries, and studies of the head, perhaps six to eight contrast runs. If the patient does not have CTA or MRA first but rather is imaged with a flat-panel detector, the entire field of interest usually can be encompassed by a single study. Also, the greater detail of small vessels makes it easier to decide where to take a closer look.

ULTRASONOGRAPHY

The capabilities of vascular ultrasonography also are expanding, particularly with the availability of specific contrast agents. High-resolution real-time linear arrays have 94% specificity and 96% specificity in detecting carotid stenosis, and flow patterns can be examined with spectral and color-flow Doppler. 9 The echogenicity of a plaque suggests its composition and thus its risk of rupture: hypoechoic plaques generally have large lipid cores or hemorrhage, and therefore greater vulnerability to rupture, whereas hyperechoic plaques tend to be calcified and thus more stable. Echolucent plaques have been linked to an increase risk of vascular events; in one trial, subjects with such plaques in the carotid arteries had a relative risk of 13% for future cerebrovascular events. 10 Increased intimal-medial thickness, measured with precise tools, is an indicator of subclinical vascular disease and in several studies has been associated later with CVAs and myocardial infarction. 9 Contrast-enhanced high-definition ultrasonography can depict the vasa vasorum serving a plaque, a marker of atherosclerosis severity. 10 Potential future applications include specific identification of vulnerable plaques and monitoring of treatment such as with statin drugs. 11,12

HYBRID EQUIPMENT OPTIONS

Scanners combining CT and positron-emission tomography already have proven their value and seem to be only the beginning. Machines enabling both angiography and CT were adopted quickly in Japan for identification and embolization of hepatocellular carcinomas. Systems also are being installed in Europe and the United States. The demand in part reflects a growing demand for endovascular oncology, in which imaging is used to define the site or vascularity of a tumor and help the radiologist navigate instruments to it, perhaps via third- and fourth-order vessels, in order to deliver drugs, glue, or other occluding devices, and radioactive microspheres, whether for preoperative devascularization, definitive treatment, or palliation. The demand for imaging equipment in operating rooms also is increasing to permit combinations of open or laparoscopic surgery with percutaneous interventions.

SCREENING: BEYOND CALCIUM

There is a huge burden of undiagnosed vascular disease with a terrible cost in both human and economic terms. In 1997, the Society of Interventional Radiology, in collaboration with the American Heart Association and several other organizations, launched Legs for Life, a program to encourage ultrasound screening for peripheral vascular disease and aortic aneurysms. A growing number of practice groups are traveling to community and senior centers in smaller towns to offer such examinations with a preceding publicity campaign emphasizing the totally noninvasive nature of the test. On February 16, a bill was introduced in the US Senate that would mandate coverage of aneurysm screening of high-risk persons by Medicare. The prospects for the bill, which has bipartisan support, are rated good in view of the endorsement of screening by the US Preventive Services Task Force.

Additional Reading

Rowe VL, Tucker SW Jr. Advances in vascular imaging. Surg Clin North Am. 2004;84:11891202.

Sands MJ, Levitin A. Basics of magnetic resonance imaging. Semin Vasc Surg. 2004;17:6682.

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