Ultrasound Surveillance of Endovascular Aneurysm Repair: A Safe Modality versus Computed Tomography
Article Outline
Routine ultrasound surveillance is adequate and safe for monitoring endovascular aneurysm repairs (EVARs). A retrospective chart review including 160 endograft patients was performed from August 2000 to September 2005. All ultrasound examinations (n = 359) were performed by a board-certified vascular surgery group's accredited laboratory. Registered vascular technologists utilized the same equipment consisting of Siemens Antares high-definition ultrasonography with tissue harmonics and color flow Doppler. An identical protocol was followed by each technologist: scan body and both limbs of the endograft and distal iliac vessels, measure anterior-posterior aneurysm sac size, and detect intrasac pulsatility and color flow. Statistical analysis utilized Pearson's correlation coefficient and the paired t-test. Forty-one endoleaks were discovered out of the 359 exams (11.4%). There were type I (7, 17%), type II (26, 63%), and combined type I with type II (8, 20%) endoleaks. Correlation with computed tomography (CT) was obtained in 35 of these cases. CT discovered three endoleaks that were not seen with ultrasound. However, these particular ultrasound exams were inadequate due to additional factors (bowel gas, body habitus, hernia), which prompted CT investigation and, hence, endoleak discovery. Of the 41 endoleaks found on ultrasound, only 14 were seen on CT. Specifically, 26 type II endoleaks were seen with ultrasound versus only nine during CT. Additional factors addressed included comparison between ultrasound and CT of residual aneurysm sac measurements and conditions limiting ultrasound examination. Although criticized in the past, color flow ultrasonography is a safe and effective modality for surveillance of aortic endografts. Utilizing ultrasound to analyze abdominal aortic aneurysm (AAA) sac dimensions and endoleak detection is statistically sound for screening AAA status post-EVAR.
Introduction
Ultrasound surveillance of abdominal aortic aneurysms has been well documented and utilized for numerous years.1 Endovascular aneurysm repair (EVAR) currently utilizes computed tomography (CT) as the gold standard for graft surveillance. However, with new technology and training, ultrasonography may indeed be the new standard of care. The goal of this study was to focus on how ultrasound is adequate and safe for EVAR surveillance.
A number of studies have dealt with this exact question of CT versus ultrasound with no definitive answer.1, 2, 3, 4, 5, 6, 7, 8 Due to this debate, this study encompasses 5 years' experience of a community-based vascular surgery group, adding to evidence for using ultrasound as surveillance after EVAR. This study provided a unique situation where early in the post-EVAR follow-up it was decided to utilize the group's dedicated Intersocietal Commission for the Accreditation of Vascular Laboratories (ICAVL) facility and personnel to track postoperative endovascular repairs. Multiple factors including the side effects of CT, which will be discussed, prompted this group to follow EVAR patients with ultrasound as the primary modality. Reviewing these data retrospectively, the aim was to determine if ultrasonography is a safe and effective modality for surveillance of aortic endografts.
Methods
A retrospective chart review of 160 endograft patients from August 2000 to September 2005 was performed. All procedures were completed by one community-based, six-member, board-certified vascular surgery group. Endografts were from various manufacturers: Ancure 51%, AneuRx 39% (Medtronic, Sunnyvale, CA), other 9% (Table I).
Table I. Breakdown of types of endografts utilized
| Ancure | AneuRx | Other | Total |
|---|---|---|---|
| 82 | 63 | 15 | 160 |
All ultrasound examinations (n = 359) were performed by this group's certified vascular laboratory. Within this laboratory, all studies were performed by registered vascular technologists utilizing the same equipment consisting of Siemens (Malvern, PA) Antares high-definition ultrasonography with tissue harmonics and color flow Doppler. An identical protocol2, 3 was followed by each technologist:
Interpretation of the ultrasonographic images included pulsation and/or blood flow within the aneurysm sac, evidence and types of endoleaks, and limitations to adequacy of the study. These interpretations were made by both the vascular surgeon and certified technologist. All ultrasounds were performed every 6 months, with the first study performed within 1 month of EVAR. Type II endoleaks were identified utilizing all aspects of ultrasound including color flow. Specifically with color flow, the leaks could be better delineated to be inflow versus outflow as well as single versus multiple vessels.
When warranted, CT was performed. Helical CT was done using an Aquilion 64 (Toshiba, Tokyo, Japan) multislice scanner. Precontrast studies were completed. Three phase-contrast images were obtained. Nonionic iodinated contrast medium (125 cc) was injected at 4 cc/sec. A slice thickness of 2.5 mm was used throughout the scanning process. Scans were performed at least from above the celiac axis to the femoral bifurcation. All CT scans were read by board-certified radiologists.
Ultrasound imaging was compared to CT scanning when endoleaks were discovered on ultrasound (n = 41). Aneurysm size was analyzed between the two modalities utilizing the Pearson correlation coefficient and the paired t-test. p < 0.05 was considered significant.
Statistical analysis utilized Pearson's correlation coefficient and the paired t-test to analyze data comparing CT and ultrasound measurements of abdominal aortic aneurysm (AAA) sacs.
There was no outside funding received for this study. All procedures and postoperative visits were performed by one group of surgeons and their vascular technologists in an accredited vascular laboratory.
Results
Of the 160 EVAR patients, 359 ultrasound examinations were performed (Table II). These scans were performed every 6 months after the procedure for surveillance until the AAA sac resolved. CT scanning was done on a selective basis from the ultrasound result: enlargement of the AAA sac and evidence of an endoleak. Thirty-five CT scans were completed (Table II).
Table II. Identified endoleaks4 (combined = type I with type II)
| Ultrasound | CT | |
|---|---|---|
| Total | 359 | 35 |
| Type I | 7 | 1 |
| Type II | 26 | 9 |
| Combined | 8 | 4 |
| No leak | 318 | 21 |
Aneurysm diameter as well as identification of endoleak were the parameters compared between the two scanning modalities (Fig. 1).1, 2, 4 There was a statistically significant correlation coefficient established, 0.969 (p < 0.0001). Moreover, the dimensions obtained via ultrasound were not statistically different when compared to CT (Table III).
Fig. 1
Diameter of aneurysm after EVAR. Correlation coefficient significantly different from 0 (0.969, p < 0.0001).1, 2, 4
Table III. Diameter of aneurysm after EVAR1, 2, 4
| n | Mean | Standard deviation | Minimum | Maximum | |
|---|---|---|---|---|---|
| Ultrasound | 33 | 5.4333 | 1.1185 | 4.00 | 8.20 |
| CT size | 33 | 5.4182 | 1.0333 | 4.10 | 8.00 |
Type I and type II endoleaks were discovered (Table II). Ultrasound identified a number of leaks that were not visualized on CT. Type I endoleaks (Fig. 2) were treated with endovascular and open techniques. Type II endoleaks (Fig. 3) were observed or treated via radiological techniques. There were patients who received CT studies for other reasons (not related to EVAR), which allowed additional comparisons (Table IV). Of these seven scans, four showed no endoleak, which confirmed ultrasound findings. The remaining three displayed type II endoleaks that were not discovered on ultrasound; however, no increase in AAA sac size was detected on CT or ultrasound. No intervention has been required in these patients, and all continue to be monitored with ultrasound per established protocol.
Fig. 2
Ultrasound image of type I(b) endoleak (solid arrow, end of graft; dashed arrow, endoleak).
Fig. 3
Ultrasound image of type II endoleak (solid arrows, graft; dashed arrow, endoleak).
Table IV. Available CT results comparable to ultrasound studies2
| All endoleaks | Ultrasound-negative | Ultrasound-positive |
|---|---|---|
| CT-negative | 4 | 14 |
| CT-positive | 3 | 10 |
| No CT | 311 | 17 |
Subjectivity is a common complaint among ultrasound disadvantages. Thus, a subjective comparison of CT and ultrasound was compiled (Table V). Comparing CT and ultrasound characteristics helped distinguish some variables between these two modalities. In addition, barriers to examining the aorta with ultrasound were recorded based on ultrasound operator reports (Table VI).
Table V. Cost and complication comparison of ultrasonography versus CT
| Ultrasound | CT | |
|---|---|---|
| Cost | + | +++ |
| MD fee | + | ++ |
| Operator-dependent | +++ | + |
| Nephrotoxicity | − | ++ |
| Radiation exposure | − | +++ |
| Time | ++ | + |
| Technologist fee | ++ | + |
Table VI. Identifiers of poor ultrasound candidates3
| Bowel gas | 36 |
| Body habitus | 21 |
| Hernia | 8 |
| Tortuosity | 7 |
| Ascites | 1 |
| Sac not visualized | 4 |
| Intolerance | 1 |
There were a total of seven deaths among the 160 patients. However, none of these deaths was a direct result of complications from EVAR.
Discussion
Numerous studies have been performed both confirming and rejecting the utilization of ultrasonography for EVAR surveillance. This community-based practice provided the unique opportunity to retrospectively study this exact question. Early EVAR surveillance using periodic CT scanning became problematic for a number of patients for various reasons (kidney failure, contrast allergy, etc.). Therefore, practice guidelines switched to periodic monitoring with ultrasound (every 6 months until AAA sac resolved) versus CT scanning. Of note, no patients have expired due to complications directly related to EVAR.
Currently, surveillance of AAA, in general, is with ultrasonography. With advancing technology and more long-term follow-up of EVAR patients, ultrasound does seem to be an excellent choice for periodic monitoring. First, with measurements of post-EVAR AAA sac size, CT and ultrasound are comparable (Table III). No statistical difference was detected when analyzing this set of data between the two modalities. Moreover, the two sets of data statistically correlated well (Fig. 1) with a calculated coefficient of 0.969 (p < 0.0001). This supports that ultrasound measurements are as good as CT measurements in post-EVAR evaluations. Next, endoleak detection was again obtainable on ultrasound (Fig. 2, Fig. 3). The data supported that more endoleaks were discovered on ultrasound when compared to CT (Table II). Statistically, ultrasound would be termed more “sensitive,” which for screening and surveillance purposes is ideal. Of the leaks detected on ultrasound that were not seen on CT, none was clinically significant enough to warrant intervention. Also, none of the endoleaks detected only on CT (Table IV) needed corrective procedures and all continue to be followed by ultrasonography.
Neither CT nor ultrasonography is without complications. Both modalities have advantages and disadvantages (Table 5).9 Various communities must weigh which modality is best both economically and technically. Previous studies2 have expressed concern about not being able to keep up with technology and obtaining up-to-date equipment. However, with these community-based practice methodologies and data, ultrasound proves to be an effective modality in studying post-EVAR patients. However, this is not to eliminate CT scanning and its advantages. There are a number of patient characteristics that hinder ultrasonography (Table VI).3 To avoid such complications, preparatory instructions are given to each patient. Within this prep, the following are requested of the patient: no food or fluids after midnight (except water), avoid gas-producing foods 2-3 days prior, avoid smoking and gum chewing the morning of exam, and take morning medications if possible on an empty stomach. When confronted with these criteria, CT imaging may be superior to ultrasonography.
Conclusion
EVAR surveillance with the “gold standard” of CT is questionable. The goal of this study was to focus on how ultrasound is adequate and safe for EVAR surveillance. Both the measurement of AAA sac dimensions and evidence of endoleak were analyzed and found to be adequate and, more importantly, safe when using ultrasound as the primary modality of EVAR monitoring.
With advancing technology of both ultrasound and CT, more data can and should always be obtainable. Additional long-term follow-up data on EVAR will aid in determining the best method of surveillance. This was a retrospective review. A randomized or even prospective study design may solicit different statistically significant findings. Specifically, ultrasound operator dependence and patient characteristics continue to limit its use; however, with continuously evolving technology, these issues may no longer hinder such a useful surveillance tool.
Overall, ultrasonography is a safe and effective modality for surveillance of aortic endografts.
References
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- Hiatt MD, Rubin GD. Surveillance for endoleaks: how to detect all of them. Semin Vasc Surg 17:268-278.
Presented at the Seventeenth Annual Winter Meeting of the Peripheral Vascular Surgery Society, Steamboat Springs, CO, January 26-28, 2007.
PII: S0890-5096(07)00280-4
doi:10.1016/j.avsg.2007.07.009
© 2007 Annals of Vascular Surgery Inc. Published by Elsevier Inc All rights reserved.
