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Vascular and Interventional Radiology |
1 From the Departments of Radiology (A.M.R., M.P., A.T.R., M.P.L., Z.J.R.) and Surgery (T.O., F.J.V.), Albert Einstein College of Medicine and Montefiore Medical Center, 111 E 210th St, Bronx, NY 10467. Received May 14, 2002; revision requested July 3; revision received August 5; accepted September 25. Address correspondence to A.M.R. (e-mail: allaroz@earthlink.net).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: CT scans were retrospectively evaluated for the presence of endoleaks after endovascular treatment of AAAs in 33 patients with endoleak (positive group) and 40 patients without evidence of endoleak or aneurysm enlargement (negative group). All patients underwent unenhanced and biphasic contrast material–enhanced CT. The CT scans were reviewed in the following combinations: (a) arterial phase and unenhanced scans (uniphasic/unenhanced set), (b) arterial and delayed phase scans only (biphasic set), and (c) arterial and delayed phase scans with unenhanced scans (complete set). Each set was reviewed by two radiologists blinded to the diagnosis of endoleak. Findings were recorded as positive, negative, or indeterminate for endoleak.
RESULTS: Within the positive group, endoleaks were diagnosed with the uniphasic/unenhanced, biphasic, and complete image sets in 30 (91%), 32 (97%), and 33 (100%) patients, respectively. With the uniphasic/unenhanced set, three (9%) endoleaks (seen only on delayed phase images) were missed. With the biphasic set, one (3%) endoleak was interpreted as indeterminate. Within the negative group, uniphasic/unenhanced, biphasic, and complete image sets were negative for endoleaks in 100%, 80%, and 100% of patients, respectively. With the biphasic set, results were indeterminate in 20% of cases.
CONCLUSION: A delayed CT acquisition enables detection of additional endoleaks, while an unenhanced acquisition helps eliminate indeterminate results. Thus, both acquisitions contribute to accurate diagnosis of endoleaks when combined with an arterial phase acquisition.
© RSNA, 2003
Index terms: Aneurysm, aortic, 981.73 • Aorta, CT, 981.12914 • Computed tomography (CT), comparative studies • Computed tomography (CT), phase imaging, 981.12914 • Grafts, interventional procedures
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Contrast material–enhanced helical computed tomography (CT) is recognized as the modality of choice for follow-up of patients with EVGs. CT angiographic techniques have been shown to be highly sensitive for the detection of endoleaks (4–6). Arterial phase CT imaging is performed with fairly standard technical parameters, with a few manufacturer-related variations (1,6–9). However, additional CT acquisitions, including acquisition of unenhanced and delayed phase images, are not performed routinely in all institutions, and their role in detection of endoleaks is uncertain. Different authors advocate various combinations of the three CT acquisitions for achieving optimal results. Some suggest a uniphasic arterial acquisition combined with an unenhanced acquisition (6,7), others recommend a biphasic (arterial phase and delayed phase) acquisition without an unenhanced acquistion (5), and still others have suggested the use of all three acquisitions (1).
Because all three acquisitions are used routinely in our practice, the purpose of our study was to assess unenhanced and delayed phase CT acquisitions combined with arterial phase CT aquisitions in the detection of endoleaks in patients who have undergone endovascular treatment for abdominal aortic aneurysm. For this purpose, we retrospectively compared the diagnostic utility of different combinations of CT acquisitions in two groups of patients: one group with a certain diagnosis of endoleak (positive group) versus another group with a definite absence of endoleak (negative group).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Our positive group consisted of 33 patients in whom perigraft flow was confirmed at follow-up CT (n = 27), angiography (n = 13), or both (n = 7). There were 30 men and three women between 60 and 89 years of age (mean, 76 years); their aneurysms measured 4.6–9.0 cm (mean, 6 cm) in diameter. Endoleaks were classified at angiography (n = 21), surgery (n = 2), or CT and clinical follow-up (n = 10) as type I, II, III, and IV in 12, 14, one, and no patients, respectively; the remaining six endoleaks were categorized as undefined—that is, either angiographically negative (n = 4) or of uncertain type (n = 2). The EVGs used in these patients included tube grafts (n = 4), aortounifemoral grafts (n = 8), and bifurcated grafts (n = 21). The follow-up period ranged from 0.2 to 36 months (mean, 10.6 months) after the detection of endoleak. For patients with type I or type III endoleak, the mean follow-up period was 2.3 months (range, 0.2–5 months), while for patients with type II or undefined endoleak, the mean follow-up period was 16 months (range, 6–36 months).
From among the remaining 210 patients without evidence of endoleaks at completion angiography and initial CT, we selected a "negative" group. The following inclusion criteria were used: no CT evidence of endoleaks for at least 12 months after the procedure, and either stable or decreasing size of the aneurysm sac. Similar but less strict criteria are used to define the primary clinical success of the procedure, according to the Society for Vascular Surgery/International Society for Cardiovascular Surgery reporting standards (3). To meet the previously mentioned criteria, we excluded 54 patients who underwent only unenhanced follow-up CT, 91 patients who were followed up for less than 12 months, seven patients with enlarging aneurysm sacs, and 18 patients who were lost to follow-up. The remaining 40 patients constituted the negative group. There were 39 men and one woman who were aged between 66 and 89 years (mean, 75 years); their aneurysms measured 4.8–7.8 cm (mean, 5.9 cm). The follow-up period ranged from 12 to 65 months (mean, 27 months) after the endovascular procedure. EVGs used in these patients included tube grafts (n = 3), aortounifemoral grafts (n = 10), and bifurcated grafts (n = 27). Table 1 lists the specific endografts used in the positive (n = 33) and negative (n = 40) groups.
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CT Scanning
All patients underwent CT scanning of the abdomen and pelvis performed with a single–detector row HiSpeed Advantage CT scanner (GE Medical Systems, Milwaukee, Wis). Unenhanced helical CT images were obtained through the entire endovascular device with either 5–7-mm collimation and a pitch of 1.0–1.5 and reconstruction to 5-mm intervals or in an incremental fashion, with 10-mm collimation and 10-mm intervals. Iohexol 300 (Omnipaque 300; Sanofi-Winthrop, New York, NY) was then administered intravenously in a single uniphasic power injection at a rate of 3–4 mm/sec to a total of 120–150 mL, with a scan delay of 25–45 seconds.
Between 1993 and 1996, an injection-to-scan delay was calculated by using a time-density curve that was derived from a test scan obtained after intravenous administration of a 20-mL bolus of contrast material. Later, this technique was reserved for use in patients with cardiomegaly and known congestive heart failure. In the majority of patients, an empiric estimate of the scan delay of 25–30 seconds was used.
Helical CT scans were acquired by using 280–310 mA and 120 kVp. Two helical image sets were acquired to cover the entire abdomen and pelvis. A 30-second breath hold was used for the abdominal acquisition. A 15–20-second pelvic acquisition was obtained after a 7–10-second interval, with or without a breath hold, depending on the patient’s tolerance. Scanning began 1 cm above the celiac artery and ended at the femoral arteries. A collimation of 3 mm and a pitch of 2.0 were used to image the entire anatomic region. The images were retrospectively or prospectively reconstructed at 1.5-mm intervals. In addition, delayed helical CT scanning of the abdomen, including the entire EVG, was performed 100–130 seconds after the injection of contrast material with either a 10-mm section thickness and 10-mm intervals or 5–7-mm collimation and a pitch of 1.0–1.5, reconstructed to 5–7 mm intervals.
The protocol for delayed CT at our institution has undergone changes over the years. The incremental technique was used early in our experience, when x-ray tube cooling time was lengthy. With advances in technology, the protocol was changed to 7-mm helical scanning with a pitch of 1.0. Two years ago, we changed the protocol to 5-mm collimation with a pitch of 1.5. However, 7-mm collimation is still used if adequate technical parameters cannot be achieved with thinner collimation. No oral contrast material was administered.
Follow-up CT was performed in all patients during the first month after the endovascular procedure and then every 6–12 months if the results of the first CT examination were negative. If an endoleak was detected, patients underwent follow-up CT every 3–6 months. Patients with endoleak and enlarging aneurysms who had negative results at completion angiography underwent follow-up angiography or surgery.
Image Analysis
For the purpose of this retrospective study, the images obtained at each CT examination were divided into three different sets to be analyzed independently. A uniphasic/unenhanced set contained the arterial phase and unenhanced images; a biphasic set included the combination of arterial and delayed phase images, without unenhanced images; and a complete set included the arterial and delayed phase images combined with the unenhanced CT images. Each set was reviewed separately at a computer workstation by two of three radiologists (A.M.R. and M.P. or A.M.R. and A.T.R.) who were blinded to the diagnosis of endoleak. During the review, all patient identifiers were removed from the screen. The radiologists evaluated images simultaneously, using the criteria indicated below, and all conclusions were made in consensus.
The results were recorded as positive, negative, or indeterminate for endoleak. The criteria for a positive result were based on the detection of contrast medium within the aneurysm sac—that is, interval change between the components of each image set—as perceived by means of visual assessment, without measurements. A uniphasic/unenhanced image set was considered positive when at least one area of high attenuation was present within the aneurysm sac on the contrast-enhanced image but absent on the unenhanced image. A biphasic study was considered positive when there was a perceptible difference in the appearance of attenuation and/or the size of at least one area of high attenuation within the aneurysm sac between images from the two phases. Examination results were interpreted as indeterminate for endoleak when a change between the two components of each image set could not be ascertained—that is, the areas of high attenuation could not be definitively characterized as perigraft contrast medium versus calcification. Linear and curvilinear zones within the aneurysm sac with attenuation higher than that of intravascular contrast material were considered to represent calcification. We did not attempt to classify endoleaks by their etiology or grade them by size at this time.
In addition, the conspicuity of endoleaks was compared between the arterial and delayed phase CT images. Endoleaks were considered equally conspicuous when their size was unchanged and their attenuation relative to that of endoluminal contrast material was similar on images obtained during both phases. A higher degree of conspicuity on images from one of the phases was assigned to endoleaks that had either a larger size or greater attenuation relative to that of endoluminal contrast material on images from one phase when the images were compared with those from the other phase. Conspicuity was not graded.
The retrospective reviewing procedure was performed in nine sessions separated by 2-week intervals to limit learning bias. The observers interpreted uniphasic/unenhanced image sets in the first three sessions, biphasic image sets in the next three sessions, and complete image sets in the subsequent three sessions. The specific image sets were randomly selected by another author (Z.J.R.) who did not participate in the review. During these sessions, the reviewers evaluated images from a single CT examination per patient for the presence of endoleak and recorded aneurysm size.
For the positive group, images from a CT examination of the earliest prospectively diagnosed endoleak were selected. In the negative group, images from a CT examination performed within 1 month after the endovascular procedure were used. To evaluate for interval change, the reviewers measured the size of the aneurysm sac on images from the latest follow-up CT examination and compared the measurements with those obtained from preoperative CT images during two additional review sessions. The aneurysm sac was measured perpendicular to the aortic axis at its largest dimension by using an electronic cursor. A change in the size of the sac (either an increase or decrease) was recorded if it was 2 mm or greater. The remaining aneurysms were considered stable. For the positive group, the reviewers correlated a change in sac size with the type of endoleak.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Contrast-enhanced helical CT is an effective method for detecting endoleaks (4–8); it is routinely used to follow up patients treated with EVGs. At CT, perigraft flow is seen as a collection of contrast material that is located anywhere between the aortic wall and an EVG. A relatively large perigraft collection that is located away from the metallic component of an EVG or calcified aortic wall is easily demonstrated at CT. However, the detection of a small collection in the proximity of a material, such as metal or calcification, that is of high attenuation may be difficult and requires a thorough analysis of CT scans (8).
CT protocols that have been suggested for optimizing the detection of endoleaks vary between authors. Most authors agree on the optimal timing of the arterial acquisition, the injection rate, the section thickness, and the reconstruction intervals, although there are some manufacturer-related variations. There is, however, disagreement regarding the use of unenhanced CT, as well as disagreement regarding the value of uniphasic versus biphasic contrast-enhanced acquisitions. Unenhanced CT scanning is often used as a localizing procedure for arterial phase scanning. More importantly, unenhanced CT images are acquired so that they can be compared with arterial phase images to enable identification of small perigraft leaks and differentiation of such leaks from a calcified luminal thrombus or the metallic portion of an EVG (4,8). Golzarian et al (5) did not use an unenhanced CT phase in their study and suggested that calcification can be differentiated from endoleak on a delayed CT image, which they obtained 50–80 seconds after contrast material injection. These authors concluded that a biphasic acquisition (arterial and delayed phases) is superior to the uniphasic arterial acquisition alone and that delayed CT is valuable for detecting low-flow leakage (5). In their series, five (11%) additional endoleaks that were either not visible or missed on the arterial phase images were diagnosed at analysis of the delayed phase images. These authors used relatively large collimation (5 mm) and reconstruction intervals (4 mm), which may limit detection of small leaks with arterial phase images (5). Similarly large collimation (5.5 mm) and slightly smaller reconstruction intervals (2.5 mm) were used by Gorich et al (1), who also performed a biphasic contrast-enhanced CT acquisition but combined it with an unenhanced acquisition. Gorich et al (1) found that no additional information was provided by the delayed phase (at 100 seconds) compared with the arterial phase. Dorffner et al (20) used the combination of an unenhanced acquisition and a single arterial phase acquisition with 3-mm collimation and 1-mm reconstruction intervals. Their technique was shown to be superior to angiography in detecting endoleaks (20).
We routinely use 3-mm collimation and 1.5-mm reconstruction intervals to maximize spatial resolution and enable three-dimensional rendering when necessary. In our study, biphasic CT images were more sensitive than uniphasic/unenhanced images for the detection of endoleaks: Three additional endoleaks were demonstrated on the delayed phase images. Two of these were not seen on arterial phase images, even in retrospect. Our results are in concordance with those of Golzarian et al (5). The proportion of delayed leakage was slightly lower in our study (9% versus 11% in the study of Golzarian et al [5]). This may be related to the higher-spatial-resolution technique we used for arterial acquisition. Delayed endoleaks are termed low-flow leaks (21) and have uncertain clinical importance. However, they may require further investigation and treatment, because endoleaks of various natures are associated with aneurysm expansion and rupture (5,20,21). Two of the three patients with delayed endoleaks in our study did not require additional intervention: The aneurysm sac decreased in size in one patient and was stable in the other during follow-up. However, in the third patient (who had a type II endoleak), the size of the aneurysm sac increased, and this patient required additional treatment.
In general, the type of endoleak influences the method of treatment (2). Most type I and type III endoleaks require reintervention or, less frequently, open conversion, while type II endoleaks are considered benign because they may heal spontaneously and thus can be safely observed without treatment (17,22,23). For clinically important type II endoleaks, transarterial or translumbar embolization or laparoscopic clipping of the feeding vessels can be performed (2,23). A detailed discussion of various treatment options is beyond the scope of this work. Any type of endoleak associated with an enlarging aneurysm necessitates additional treatment. It is becoming recognized that not only type I and type III but also type II endoleaks can produce marked intrasac pressure and result in negative outcomes (23,24). In our series, 92% of all type I and III endoleaks were associated with enlargement of the aneurysm sac, while sac enlargement was observed in 57% of type II endoleaks during follow-up. Type II endoleaks may be low-flow leaks and may therefore escape detection at arterial phase CT. Subsequent progression of such an endoleak, resulting in aneurysm enlargement, has been documented in the literature (5) and was observed in one patient in our series.
The optimal timing for detection of low-flow endoleaks is not known. In our study, we performed delayed CT between 90 and 120 seconds after contrast material injection. We originally chose this timing because it was determined to be optimal for the renal imaging performed in conjunction with CT angiography at our institution. Additionally, intravascular contrast material, both arterial and venous, could still be discerned up to 2 minutes after the injection.
Our routine CT angiographic protocol includes an unenhanced CT acquisition. Comparison of images obtained before and during intravenous contrast material infusion allows unequivocal differentiation of endoleaks from calcifications (4,7,8). This is particularly important for the definitive exclusion of an endoleak. In the patients constituting our negative group, endoleaks were suspected to be present on the biphasic images in 20% of all true-negative CT studies. However, when the arterial phase images were compared with the unenhanced CT images, endoleaks were easily excluded. Moreover, with increasing experience, we noticed that thick unenhanced sections were often inadequate for detection of small or faint aortic calcifications, which can be confused with endoleaks on subsequent thin-collimation arterial phase images. Therefore, during the latter 2 years of this study, we routinely used 5-mm helical acquisitions reconstructed at 5-mm intervals for unenhanced CT instead of the thicker collimation and intervals that we used earlier.
In addition to depicting calcifications, unenhanced CT images may be helpful in the detection of some small endoleaks that are difficult to see on biphasic CT images alone, as was demonstrated in one of the patients in our study. The endoleak in this case was more conspicuous when the arterial phase image was compared with the unenhanced CT image than when the arterial and delayed phase images were compared with each other.
As previously discussed in the literature (5), an additional CT acquisition results in increased radiation exposure to the patient. Unfortunately, dosimetry is not routinely performed with our equipment, and data on patient radiation dose are unavailable. However, we believe that for diagnosis of endoleak, the benefits of an additional acquisition outweigh the risks of increased radiation in our patient population, in which the mean age was 75 or 76 years. Further studies are necessary to enable optimization of dose-efficient CT techniques.
Because of the lack of reference standards for the true-negative CT results in our study, we postulated that serial negative CT results for at least 12 months after the procedure combined with evidence of a stable or shrinking aneurysm sac enabled confident exclusion of the presence of an endoleak. This is in concordance with the definition of primary clinical success in aortic endograft placement according to the reporting standards of the Society for Vascular Surgery/International Society for Cardiovascular Surgery (3). In our negative group, the aneurysm decreased in size in 37 (92%) of 40 patients and remained unchanged in three (8%) patients; these results are consistent with clinical success. Several reports (2,24–27) indicate that 12-month stability is not a guarantee that an endoleak will not develop later. However, when combined with negative CT results, such stability is a good indicator of the absence of an endoleak 12 months after the procedure.
Because we used a minimal 12-month follow-up period, it was difficult to identify a large negative cohort among our patients, who were predominantly elderly and had multiple medical problems. A number of the patients treated for abdominal aortic aneurysm with EVG placement at our institution were followed up at outside institutions, so their CT results could not be used in our study. A relatively small cohort of patients with negative findings is one of the limitations of our study. Another limitation is that the study was retrospective rather than prospective and randomized. In addition, CT scans in our series were obtained with single–detector row equipment, and results may differ with multi–detector row helical CT equipment.
The results of our study indicate that biphasic CT is more sensitive than the combination of uniphasic and unenhanced CT for the diagnosis of endoleaks. However, results of biphasic CT can be indeterminate in 20% of negative cases. Unenhanced CT images allow confident exclusion of endoleaks in these cases. A low-flow endoleak can be missed on arterial phase images, even if thin collimation and reconstruction intervals are used. A delayed acquisition between 90 and 120 seconds in length may be helpful for detecting such a leak, although the clinical importance of a low-flow endoleak is not known. Thus, to maximize accurate detection of endoleaks, a combination of unenhanced and biphasic contrast-enhanced CT acquisitions seems to be appropriate.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, A.M.R., Z.J.R.; study concepts and design, A.M.R.; literature research, M.P., A.T.R., Z.J.R., A.M.R.; clinical studies, T.O., F.J.V.; data acquisition, T.O., F.J.V., A.T.R., M.P.; data analysis/interpretation, A.M.R., M.P., A.T.R., M.P.L.; manuscript preparation, A.M.R.; manuscript definition of intellectual content, A.M.R., M.P.L.; manuscript editing and revision/review, all authors; manuscript final version approval, A.M.R., M.P., A.T.R., Z.J.R.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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