Although not fully documented in the literature, most vascular surgeons believe that saccular aortic aneurysms have more ominous natural history than the typical fusiform aneurysm.¹ The purpose of our study is to characterize the flow abnormalities within the saccular aneurysm of the abdominal aorta as compared to the ordinary fusiform aneurysm, which is more common.

Patients

Study population consisted of three groups of patients who underwent 3D cine PC MR imaging (4D-Flow). 1) Four patients with abdominal saccular aneurysm (3 male and 1 female, 62-85 years), 2) fourteen patients with abdominal fusiform aneurysm (14 male, 52-84 years) and 3) fifteen patients with non-dilated abdominal aorta (11 male and 4 female, 41-78 years). The diagnosis of saccular, fusiform or non-dilated were determined by preceding CT study.

MR imaging

The MR imagings were conducted on a 1.5T (Signa Twinspeed 1.5T, GEHC) or a 3.0T MR scanner (Discovery 750, GEHC), with torso-array coil. Prior to 4D-Flow, 2D cine PC with velocity encoding values (VENC) of 200 cm/s was performed on the transverse section within aorta. The maximum flow velocity was calculated within the thoracic aorta, and the VENCs were optimized. Time resolved contrast enhanced 3 dimensional MR angiography (Gd3DMRA) was performed for determination of aortic boundary. ECG-gated respiratory compensated coronal based 4D-Flow was then performed. The parameters used were the following; TR/TE/FA/NEX of 4.5-5.5/2.0-3.0/15/1, FOV of 32 cm, Matrix of 224x224, 2 mm thickness, 60 partitions, 12-20 /cardiac cycle. The imaging time for the 4D-Flow was about 10 min with reduction factor of 2 by using parallel imaging.

Flow Data Analysis

The acquired data was post-processed with flow analysis software (flova II, R’Tech, Japan). The velocity data derived from 4D-Flow and the geometric data of the boundary of the aortic wall determined by Gd3DMRA were interpolated. The wall shear stress (WSS) of whole cardiac phase within the aortic wall were measured. Stream lines of the abdominal aorta were depicted for streamline analysis.

Statistical Analysis

The peak systolic and diastolic WSS within saccular and fusiform aneurysm was compared using Mann-Whitney test. The peak systolic and diastolic WSS of non-dilated portion of aorta within patients with aneurysm were also compared with that of patients with non-dilated aorta. P<0.05 was considered to be significant.

The peak systolic WSS of saccular aneurysm was significantly lower than that of fusiform aneurysm (p=0.025) (figure 1). The peak systolic WSS of non-dilated portion of aorta within patients with aneurysm was also significantly lower than the peak systolic WSS of aorta within patients with non-dilated aorta (saccular; p=0.028, fusiform; p=0.045) (figure 5).The streamline analysis showed vortex flow within the saccular aneurysm during whole cardiac cycle, which is isolated from the mainstream of the aorta. This trend was not observed within the fusiform aneurysm in which turbulent but relatively laminar flow was seen (figure 2).Majorities of physiological evidences indicate that the decline of WSS produces an expression of pro-atherogenic genes.² Spatially averaged WSS of saccular aneurysm was significantly lower than that of fusiform aneurysm during systole, which may be reflecting that saccular aneurysms are more severely affected by continuous atherogenic state. The flow within the saccular aneurysm was separated from that within the parent artery, which was not observed within fusiform aneurysm. The separated turbulent flow within the saccular aneurysm may be the reason for even lower WSS compared to the fusiform aneurysm.The peak systolic WSS of undilated portion of aorta within aneurysmal patients was significantly lower than that of patients with non-dilated aorta. The lower systolic WSS of the aortic wall might be the triggering factor of atherosclerotic change that might proceed to formation of aneurysm.Vortex flow within the saccular aneurysm of the abdominal aorta, which is separated from the mainstream of the aortic flow, and consequent low WSS of its wall may be reflecting the continuing risk of atherogenic changes of the saccular aortic aneurysm compared to fusiform aneurysm.