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Editorial |
1 From the Department of Radiology, Winthrop-University Hospital, 259 First St, Mineola, NY 11501, and the School of Medicine, State University of New York at Stony Brook. Received June 20, 2001; accepted June 21. Address correspondence to D.S.K. (e-mail: dsk2928@pol.net).
Index terms: Arteries, CT, 92.12912, 92.12915, 92.12916, 92.12917 • Arteries, extremities, 92.72, 92.73 • Computed tomography (CT), angiography, 92.12916
You on the current edge of technology have already made yesterday’s impossibilities the commonplace realities of today. Ronald Reagan, Former United States President
Berland and Smith, in a 1998 editorial in Radiology (1), commented that the introduction of multi–detector row helical computed tomography (CT) represented more than an evolutionary advance in technology and that it once again created new opportunities for imaging patients and disease processes. Since that editorial, a variety of articles have documented the advantages of and the advances with multi–detector row helical CT, compared with single-section helical CT (1–4), such as markedly increased volumetric coverage and/or improved z-axis resolution with thinner sections because of an as much as eight times increase in scanner speed and isotropic or nearly isotropic voxels if very thin images are acquired. Other advantages of multi–detector row helical CT include better separation of different vascular phases of enhancement, more efficient use of contrast material administered intravenously, and at least comparable image quality. For example, the liver can now be routinely imaged in its entirety with thin sections in a single short breath hold; repeatedly scanned during early arterial, late arterial, and venous phases; and examined with high-quality multiplanar reconstructions, which are generated immediately and easily (5,6).
The application that has already most greatly benefited from the introduction of multi–detector row helical CT is clearly CT angiography, which exploits all of the advantages of this relatively new technologic innovation. For instance, recently Ghaye et al (7) demonstrated the adequate depiction of almost 90% of subsegmental pulmonary arteries by using 1.25-mm thick images generated with four-channel multi–detector row helical CT, and Qanadli et al (8) reported a sensitivity of 90% and a specificity of 94% for dual-channel multi–detector row helical CT pulmonary angiography in the depiction of pulmonary embolism. In an innovative preliminary report, Ohnesorge et al (9) described a technique that was used to generate images with high spatial resolution and without artifacts for electrocardiographically gated multi–detector row helical CT angiography of the coronary arteries.
Since its introduction in the early 1990s, helical CT has revolutionized computerized tomography, particularly with respect to the creation of CT angiography, which has been increasingly used in the past decade for noninvasive imaging of the vascular system in an increasing number of applications. Following the initial description of CT angiography in 1993 by Dr Rubin and colleagues (10,11), it is now a given that imaging of the aorta and its branches can be performed with helical CT scanners routinely on a daily basis, in lieu of catheter angiography for a wide variety of emergent and elective indications, throughout the world. Compared with conventional angiography, CT angiography is less costly and faster, does not require assembly of an angiographic team to perform the study, permits a wider variety of manipulations of the volumetric data set for image display and analysis in contrast to the limited projections routinely obtained during conventional angiography, and has fewer potential complications (3,12).
In the first report of its kind, to our knowledge as of the time that this editorial was prepared, Dr Rubin and his co-investigators (13) further push the CT imaging envelope with their initial published description of multi–detector row helical CT angiography of the entire lower extremity arterial circulation and of the aortoiliac system, which they performed in 24 symptomatic patients with a single CT acquisition each. They report, in the current issue of Radiology (13), 100% concordance for the absence or extent of disease in 351 arterial segments at direct comparison with conventional angiography in 18 patients and excellent opacification of the arterial system overall. Their technique uses only a simple timing bolus and a uniphasic injection of approximately 180 mL of iodinated contrast material, requires no special equipment or software other than the multi–detector row helical CT scanner, and uses approximately the same amount of contrast material as does their conventional angiographic protocol (13). A variety of image reconstructions are performed from the data set of 2.5-mm images, particularly maximum intensity projections (13), all of which can be done with relative ease (perhaps with the exception of the initial removal of the bones from the data set) with current CT workstations, which are now standard components of multi–detector row helical CT systems.
The results of the current preliminary study are impressive for several reasons. For the first time, it is now possible to routinely and noninvasively image the entire abdominal aorta, the iliac arteries and their branches, and both lower extremity arterial systems by using a single injection of contrast medium and a single imaging acquisition. Although the study was not blinded, there were 27 arterial segments that could be evaluated at CT but not at conventional angiography; 17 of these segments were not seen at conventional angiography in one patient with aortic and common iliac arterial occlusion probably because the contrast material bolus on the conventional angiogram did not fill the collateral vessels supplied by the inferior epigastric or inferior mesenteric arteries. Another explanation for the nondepiction of the other 10 arterial segments at conventional angiography as opposed to their identification at CT angiography is the high contrast resolution of CT combined with cross-sectional and multi-projectional imaging display (14).
It is important to note that some patients in this series had rest pain and not just intermittent claudication. Patients with rest pain typically have more severe disease that may be more difficult to image noninvasively, whereas most of the magnetic resonance (MR) angiography series reported to date have included only patients with intermittent claudication (15,16). The arterial anatomy could be evaluated in all the patients in this series without substantial venous contamination, despite the presence of severe asymmetric arterial disease. Also, as with other cross-sectional techniques and in contrast to conventional angiography, CT angiography can depict extravascular structures; in this series, soft-tissue inflammatory changes were noted in a minority of patients. In addition, the estimated radiation dose was almost four times greater with conventional angiography, as compared with the dose delivered with multi–detector row helical CT angiography (13). Finally, the representative maximum intensity projection images demonstrated in the current article are of excellent quality. They rival or exceed in their depiction of the vascular anatomy even the most state-of-the-art MR angiographic techniques, particularly those for imaging the calf and foot arteries and for the identification of collateral vessels throughout the imaging volume.
As Dr Rubin and colleagues point out in their article (13), this is not the first use of helical CT for imaging the lower extremity arterial system. In 1995, Lawrence et al (12) reported an accuracy of 95% for identifying occlusions and greater than 50% stenoses of the major lower extremity arteries with helical CT angiography, as compared with conventional angiography, in six patients with symptomatic peripheral vascular disease. Two helical acquisitions were required to complete the CT examinations, along with two separate boluses and a decrease in the tube current. In each patient, 5-mm images were acquired from the inguinal ligament to the proximal calves. In 1996, Rieker et al (14), in what is still the largest series published to date on CT angiography of the lower extremities for peripheral vascular disease, prospectively examined 50 patients with both CT angiography and conventional angiography. Because of improvements in CT technology, 5-mm images were obtained for this study from the groin to the lower calves in only one helical acquisition. Compared with conventional angiography, CT angiography had a sensitivity and specificity for the identification of occlusions and greater than 75% stenoses that varied from 100% to 73%, depending on the specific arterial level. It is important to note that nine stenoses distal to superficial femoral arterial occlusions, as well as six calf runoff arteries, were identified at CT angiography but not at conventional digital subtraction angiography (14).
Beregi et al (17) demonstrated additional advantages of cross-sectional imaging with CT angiography, compared with those of conventional angiography. In 26 patients with suspected popliteal arterial disease, seven stenoses were depicted to be due to six aneurysms only at CT, and in one case, cystic adventitial disease also was depicted only at CT. The etiology of two popliteal artery occlusions were also depicted only at CT—in one patient due to thrombosis of an aneurysm and in the other due to popliteal artery entrapment (17). Extraluminal anatomy and disease, which may be highly relevant to concurrent vascular disease in some patients, were therefore demonstrated only at cross-sectional imaging in this report.
Most recently, Soto et al (18) used CT angiography of the proximal legs and arms as the primary imaging method for evaluating arterial injuries in patients with trauma. CT angiography revealed partial or complete occlusions, arteriovenous fistulas, intimal flaps, and pseudoaneurysms in 61 of the 137 arterial segments, which were assessed in 134 patients. There was high accuracy, compared with the findings at surgery or angiography or based on clinical follow-up data (18). Despite these reports, to date, helical CT angiography of the lower extremities has not met with wide use or acceptance by radiologists and vascular surgeons because of substantial limitations in volumetric coverage.
However, with multi–detector row helical CT, there are no tradeoffs in coverage or z-axis resolution because the entire abdominal aorta through to the feet can be imaged with one contrast material bolus and one acquisition, ensuring that inflow aortoiliac disease, including stenoses, occlusions, collateral vessels, and aneurysms, is identified as is outflow disease in the distal popliteal and calf arteries. The potential of this technique in the near future is even greater, with imminent improvements in multi–detector row helical CT technology (4), as well as rapid dissemination of multi–detector row helical CT units into many radiology practice settings. With 8- and 16-plus channel detector systems promised in the near future by several CT manufacturers, there is the potential to routinely image the neck, thoracic, abdominal and pelvic, and lower extremity arteries in one acquisition.
Where does multi–detector row helical CT angiography of the lower extremities fit into current imaging algorithms, particularly when compared with gadolinium-enhanced MR angiography and conventional angiography? This is the million dollar question, and as correctly stated by Dr Rubin and his co-investigators, much additional research will be needed to answer it, especially with blinded prospective trials that include large numbers of patients and studies that further optimize image acquisition, reconstruction, and analysis. Given the recent track record of CT angiography in replacing many conventional diagnostic angiographic procedures, it would not be surprising if multi–detector row helical CT angiography of the lower extremity arterial system began to replace, for the common indication of peripheral vascular disease, conventional diagnostic angiography, which is the current, albeit imperfect, standard imaging study (12). Most vascular surgeons still insist on conventional angiography prior to surgery (19), and similarly, interventional radiologists routinely perform diagnostic angiography—that is, runoff studies—as part of or prior to angioplasty, stent placement, and other arterial interventional procedures.
Sonography, which is used by some clinicians to depict the lower extremity arterial system, is relatively inexpensive and noninvasive and uses no ionizing radiation or contrast material (at present). Sonography has some major disadvantages: It is time-consuming and operator-dependent and is particularly limited for the calf arteries and for detecting lesions distal to a high-grade stenosis (19). Although results vary among investigations, in one study (20) of 286 limbs in 150 patients, for example, the sensitivity of sonography for depicting greater than 50% stenosis in the popliteal artery was only 67%. In a recent meta-analysis, sonography was shown to have substantially lower sensitivity than MR for depicting peripheral vascular disease (21).
MR angiography of the lower extremities has substantially improved in the past several years with the routine use of gadolinium-based contrast material and fast three-dimensional gradient-recalled echo acquisitions, as well as MR power injectors, dedicated surface coils, and state-of-the-art MR hardware and software. MR angiography now has somewhat of an established role for imaging patients prior to potential surgical or interventional radiologic procedures (15,16). The role of MR angiography in patients, as with other noninvasive imaging studies, is not to determine whether disease is present—this is usually apparent on the basis of history and physical examination—but to precisely localize and quantify the extent of disease (15,19,22).
Aside from the radiation issue, which we agree is not that important in older patients who typically have lower extremity arterial and aortoiliac disease, a major advantage of MR angiography compared with any other imaging technique is that patients with preexisting renal disease can undergo imaging without risk. It is these patients—especially those with diabetes—who often have concurrent peripheral vascular disease (12,16,22). However, the biggest problem to date with MR angiography that is no longer a problem with CT angiography is the difficulty in covering the large volume that needs to be routinely imaged. This is particularly a problem with respect to the depiction of the relatively small outflow vessels in the calves, where there is the additional challenge of obtaining sufficient gadolinium enhancement (15,22). To circumvent this problem, most authorities have recommended using the less than ideal time-of-flight sequence to supplement gadolinium-enhanced sequences of the aorta, pelvis, and thighs (5–16).
In an attempt to solve the MR angiography coverage problem, multiple strategies, which vary in their ease of application, have been reported in the past few years with apparently successful results; however, we are not aware that they have disseminated into routine community practice. These have included the use of multiple acquisitions and multiple boluses, slower infusion rates combined with subtraction techniques, and automated stepping tables or manual movement of the patient between acquisitions to "chase" the gadolinium-based contrast material bolus (15,22–25).
For example, Ho et al (22), by using image subtraction and a moving MR bed reported 93% sensitivity and 98% specificity, compared with conventional angiography, for the depiction of hemodynamically significant stenoses in 28 patients with intermittent claudication. Huber et al (23) performed MR angiography by using three separate acquisitions, each with its own intravenous contrast material bolus, and achieved 100% sensitivity and around 94% specificity for 80 hemodynamically significant stenoses and 39 occlusions in 24 patients. Ruehm et al (24), by using a single-injection, two-station MR angiographic protocol with manual repositioning during a 10-second break along with a lower extremity vascular coil and an initial test bolus, reported 92% sensitivity and 97% specificity for hemodynamically significant disease. Reid et al (25) achieved 100% sensitivity and 92% specificity for identifying lower extremity arterial lesions that required intervention in 13 patients by using a moving table MR angiographic technique.
It is difficult to determine exactly what role multi–detector row helical CT angiography will play, compared with state-of-the-art gadolinium-enhanced MR angiography, for imaging the lower extremities in the near future, and to our knowledge, there are no data on the accuracy of one compared with the other at this time, although the current article notes that the spatial resolution of multi–detector row helical CT angiography is substantially superior to that of the current MR angiographic techniques (13) and will improve further with advances in multi–detector row helical CT technology. In addition, some of the newer MR angiographic protocols still do not permit coverage of most or all of the abdominal aorta (15,22–25).
There are many additional specific technical issues, as well as broader questions, that are raised as a result of the current work by Dr Rubin and his colleagues and that will hopefully be answered in future larger prospective investigations. Will asymmetric flow and venous contamination become a problem when a larger series of patients is studied? Is a timing bolus needed for all patients? In patients with clinically suspected severe disease, will it be worth performing an additional CT acquisition of the lower thighs to the feet following the initial "run" to further define the vascular anatomy? What is the optimal use of the three-dimensional data set? What is the best way to handle an imaging set with more than 2,000 transverse images? Should we even bother reviewing a 2,000-plus image set in the cine mode on a monitor, or should we dispense with this completely and rely purely on three-dimensional imaging reconstructions? Which reconstruction techniques will be easiest to perform in academic and in general and community radiologic practices? Will vascular surgeons operate on the basis of CT angiographic findings alone? Will multi–detector row helical CT angiography replace or substantially compete with conventional diagnostic angiography as it has in many other areas? What role will it play in patient follow-up after interventional procedures and surgery?
Although multi–detector row helical CT angiography of the aorta, iliac arteries, and lower extremity arterial system is in its infancy, it represents an exciting addition to the diagnostic armamentarium for treatment of lower extremity arterial disease. Multi–detector row helical CT, especially in the area of CT angiography, truly is fulfilling the promise of new opportunities that were presented by its introduction in 1998.
FOOTNOTES
See also the article by Rubin et al (pp 146–158 ) in this issue.
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