1. CT Angiography in Stent-Graft Sizing: Impact of Using Inner vs Outer Wall Measurements of Aortic Neck Diameters J ENDOVASC THER 2011 Institute of Radiology and Vascular Surgery Rome, Italy

2. INTRODUCTON Endovascular aneurysm repair (EVAR) is playing an increasing role in the treatment of abdominal aortic aneurysm (AAA). Patients treated with EVAR require reintervention during mid-and long-term follow-up due to complications related to the procedure.

3. Incorrect graft sizing can lead to both attachment site endoleaks as well as migration. The instructions of current endografts recommend the reference diameter measurement method to use for sizing: outer-to-outer wall (adventitia-to- adventitia) or inner-to-inner wall (intima-to-intima), but there is no consensus with regard to the correct strategy for endograft sizing in EVAR planning.

4. In some clinical situations, the neck measurements could be made in advance of selecting the stent-graft type to implant. CTA of preoperative evaluation acquires static images of the aorta at any random moment during the cardiac cycle. However, dynamic imaging has documented significant changes in aortic diameters during the cardiac cycle.

5. The presence of significant pulsatile aortic neck variations may have serious implications for EVAR design( endograft size). The purpose of this study was to assess the potential impact of using inner vs. outer wall measurements based on static CTA to select stent-graft sizes.

6. METHODS Prospective single-center pilot study. 40 consecutive patients>75years old with infrarenal AAA and referred to our institution for routine preoperative CTA. 29 men; mean age 78.9+6 years, range 75–89. Excluded standard: Patients 1.5 mg/dL, or creatinine clearance rate <60 mg/mL.

7. Mean body mass index 26.266+4.16 kg/m2 (range 22–35.2). All enrolled patients underwent both standard static and dynamic ECG-gated CTA with double CT acquisition and double contrast injection. The ethical conduct of the study was approved by our departmental review board. All patients provided written informed consent for the protocol with specific acceptance of double CT acquisitions.

8. Scanning Protocols A double-phase (unenhanced and contrastenhanced arterial) static CT acquisition was first performed on a 64-detector row helical scanner. (Light Speed VCT XT; GE Medical System) using standard parameters (120 kV, 800 mAs). From suprarenal abdominal aorta to the common femoral artery. Trigger scanning after capture of 150 HU on the abdominal aorta at the level of the celiac trunk.

9. Dynamic scan mode acquires data in a nonstop, helical fashion while an independent ECG trace is generated. Dynamic ECG-gated datasets were acquired with a low-dose acquisition protocol (100 kV) Range extending from the origin of the celiac trunk to the aortic bifurcation using the same settings as above.

10. Standardized incremental 10% reconstructions were done by the technologist from 5% to 95% of the R-R cardiac cycle at 10 equidistant time points.

11. Image Evaluation All images were processed and evaluated using the cardiac review program on a dedicated 3- dimensional workstation. The specific image sets were randomly selected by a radiologist not involved in image analysis.

12. The coordinator of the study selected for evaluation 3 static and 3 dynamic images located ①1 cm above the highest renal artery (suprarenal level) ② Immediately below the lowest renal artery (juxtarenal level) ③1 cm below the lowest renal artery (infrarenal level).

13. Measurements were performed on a plane perpendicular to the largest portion of the aneurysm. First on sagittal multiplanar reconstructions (MPR), then on coronal MPR, creating a ‘‘modified axial’’ plane perpendicular to the long axis of the aortic neck in 2 orthogonal planes (Fig. 1).

14. Static imaging datasets were read in consensus by an experienced vascular interventional radiologist and vascular surgeon.

15. Measured maximum aortic neck diameters using an electronic cursor from intima-to-intima (inner wall) and from adventitia- to-adventitia (outer wall). On the basis of these measurements, they selected the size of the stent-graft according to the institution’s recommended oversizing (20%–30%).

17. RESULTS An excellent interobserver repeatability coefficient was obtained (kappa 0.87). Aortic Pulsatility Changes in proximal neck diameters reflected the significant aortic pulsatility within the aneurysm neck during the cardiac cycle.

18. Mean variations for the inner and outer wall diameters of 9.75% +64.01% and 8.66%+6 3.71% The absolute changes in diameters were 1.826+0.63mm and 1.916+0.64 mm.

19. No statistically significant differences in aortic pulsatility at the 3 levels in the neck for the inner or outer wall diameters. Significant variability was seen between inner (mean 20.863.4 mm) vs outer (mean 23.764.3 mm; p<0.05) wall diameters. Stent-graft sizes significantly changed on the basis of the measurement method and device.

22. Selecting the stent-graft using the incorrect outer wall measurement changed the device size in 14 (35.5%) of 40 patients, with a consequent excessive oversizing in 8 (20%) patients. The oversizing was inadequate in 36 (90.3%) of 40 patients when the size of the stent-graft incorrectly selected on the basis of outer diameters. For a device that requires inner wall measurements as reference

23. When considering a stent-graft that requires outer wall diameter measurements as reference The stent-graft size changed in 15 (38.7%) of 40 patients when using the wrong inner wall diameters, with a consequent inadequate oversizing in 13 (32.5%) patients. The oversizing was inadequate in 17 (41.9%) of 40 patients when the size of the stent-graft selected incorrectly using inner diameters.

24. For example, Using the outer diameter to size a stent-graft that requires an inner diameter reference changed 36% of the selected stent-graft sizes, with,20% being excessively oversized. Conversely, using the inner diameter to size an outer- diameter–based stent-graft resulted in nearly 40% of the sizes being altered. Based on dynamic measurements, the changes were more dramatic: the oversizing was considered excessive in up to 90% of patients if the measurement method did not match the stent-graft’s stipulated reference.

25. DISCUSSION There are several factors that have been implicated in aortic neck dilatation and elongation: including aggressive stent-graft oversizing and the natural course of progressive aortic aneurysmal disease. Adequate oversizing is mandatory to obtain a seal between the stent-graft and the aortic wall to prevent subsequent perigraft.

26. As demonstrated by recent studies on dynamic CTA imaging, the aorta changes significantly during the cardiac cycle. Thus, both aortic pulsatility and the method of measuring the neck diameter play roles in successful EVAR planning.

27. Our data confirmed that significant aortic pulsatility exists within the aneurysm neck during the cardiac cycle. Furthermore, there is substantial variability in aortic neck inner wall and outer wall measurements on static CTA images. Thus, stent-graft sizing could be significantly impacted if one incorrectly used the inner or outer static diameters to select a stent-graft.

28. Using dynamic measurements is even more dramatic: in the first scenario above (outer diameter with an inner-diameter–based stent-graft), most of the devices would be excessively oversized. Inadequate oversizing due to incorrectly measured baseline diameters could explain postoperative stent-graft–related complications (such as migration, type I endoleaks, and subsequent poor patient outcomes).

29. In our opinion, to avoid incorrect (inadequate or excessive) oversizing, stent-graft sizing should be based on the mean value between diastolic and systolic diameters.

30. LIMITATIONS Main limitation relatively small number of patients examined. patients >75 years (to reduce the potential consequences of increased radiation exposure from the double CT acquisition). Aortic pulsatility may be age-dependent and correlate with cardiac status, so the results obtained in this patient cohort may differ from those seen in younger patients.

31. We have not suggested a ‘‘standard’’ over-sizing. It has to be selected individually on the basis of dynamic characteristics. If the oversizing performed on the basis of dynamic imaging impacted post-EVAR outcomes by reducing complications. This falls outside the scope of this pilot study and remains a goal for future studies.

32. CONCLUSIN Our study suggests that stent-graft sizing should follow the manufacturer’s recommendations for using inner or outer diameter references based on dynamic patterns (mean value between diastolic and systolic diameters).