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 andVascular Surgery Rome, Italy

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.

• 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.

• 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.

• 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.

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 <75 years old; with a

history of congestive heart failure; previous myocardial infarction; severe rhythm disturbances; moderate or severe renal impairment, serum creatinine levels >1.5 mg/dL, or creatinine clearance rate <60 mg/mL.

• 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.

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.

• 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 .

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

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.

• The coordinator of the study selected for evaluation 3 static and 3 dynamic images located

① 1 cm above the highest renal artery (suprarenal level)

② Immediately below the lowest renal artery (juxtarenal level)

③ 1 cm below the lowest renal artery (infrarenal level).

• 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).

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

• 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%).

RESULTSAn 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.

• 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.

• 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.

• 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

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.

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.

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 .

• 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.

• 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.

• 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).

• 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.

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.

• 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.

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).

Thanks for attention!