Background and Purpose

Treatment with flow-diverter (FD) stent is provided for giant unruptured aneurysm of the internal carotid artery (ICA).¹ Nevertheless, due to the characteristics of this stent, which divert the blood flow and promote thrombosis, a careful follow-up on the status of the aneurysmal embolization and patency of the parent artery is necessary.² Follow-up of ICA aneurysms treated with FD stent using three-dimensional time of flight MR Angiography (3D TOF-MRA) is difficult due to the stent related susceptibility artifacts and thrombus T1 shortening in the aneurysm. A few studies have reported the possibility of depicting the flow in the stents using ultrashort TE (UTE) MRA with arterial spin labeling (ASL).^2-5 However, this technique can obtain only 3D data of one specific post labeling delay. The newly developed UTE 4D-MRA using the ASL method might be used to visualize the hemodynamic vascular flow. As for this MRA sequence, the multi-phase acquisition is possible with a temporal resolution of 165 milliseconds. Therefore, UTE 4D-MRA may visualize not only the depiction of flow in the stent but also hemodynamic vascular flow in aneurysms treated with FD stents. The purpose of this study was to assess the usefulness of non-contrast enhanced UTE 4D-MRA for giant internal carotid artery aneurysms treated with FD stents.

Methods

Eleven aneurysms treated with FD stents underwent UTE 4D-MRA. Six aneurysms were treated with single FD stent, two were treated with two FD stents, two were treated with single FD stent and coils, and one was treated with three FD stents and coils. UTE 4D-MRA was performed on the next day of intravascular treatment. All studies were performed with a 3-T MR imaging system (Vantage Galan 3T ZGO, Canon Medical Systems, Otawara, Japan). The parameters for the UTE 4D-MRA were as follows: TR / TE, 3 / 0.1 msec ; flip angle, 6°; field of view, 224 × 224 mm, matrix 224 × 224 ; section thickness, 1.0 mm ; number of section, 150 ; NAQ, 2 ; bandwidth, 244.1 Hz ; number of segments, 200 ; number of trajectories, 9800 ; TI, ΔTI, and final TI, 200 msec, 165 msec, and 860 msec, respectively ; number of acquisition phases, five ; and acquisition time, 9 minutes 33 seconds. Moreover, delay phase scan parameters were as follows: number of trajectories, 10000 ; TI, ΔTI, and final TI, 1100 msec, 318 msec, and 1418 msec, respectively ; number of acquisition phases, two ; and acquisition time, 8 minutes 4 seconds. X-ray catheter-based intraarterial cerebral digital subtraction angiography (DSA) was performed using the following biplane angiographic systems: Artis Q. BA Twin (Siemens AG, Erlangen, Germany). The image matrix and field of view were 1024 × 1024 and 170 mm × 170 mm, respectively; the temporal resolution was 4 frames per second. Selective manual internal carotid artery injections were administered according to the aneurysm location. Two neuroradiologists independently reviewed the UTE 4D-MRA images, and subjectively scored the flow in the stents on a scale of 1 to 4 : 1, not visible ; 2, poor (slightly visible, but not adequate for diagnosis) ; 3, acceptable (roughly visible, diagnosable images) ; 4, excellent (images almost equal to X-ray DSA). We scored the vascular flow in aneurysms status on a scale of 1 to 3 : 1, poor ; 2, roughly visible ; 3, excellent (images almost equal to X-ray DSA). Moreover, we evaluated the visualization of eclipse sign. The X-ray DSA images during the treatment were used as a reference standard. The scores of MRA images from the two observers were averaged, and the scores were compared between observers. Weighted κ statistics was used to evaluate interobserver agreement.

Results

The average score ± standard deviation for flow in the stents and vascular flow in aneurysms were 2.59 ± 0.67 and 2.86 ± 0.56, respectively. Interobserver agreement was good agreement (κ = 0.82) for flow in the stents and good agreement (κ = 0.81) for the vascular flow in aneurysms, respectively. Eclipse sign was demonstrated in eight cases using both X-ray DSA and UTE 4D-MRA.

Discussion

The newly developed UTE 4D-MRA uses a UTE combined with ASL. ASL technique is used as a preparation pulse for the visualization of the blood flow and data acquisition is based on 3D radial sampling.³ UTE minimizes phase dispersion of signal of the labeled blood flow and decreases the susceptibility artifacts of FD stent. Therefore, UTE 4D-MRA could visualize flow in the stents at the acceptable image quality for diagnosis. In UTE 4D-MRA, hemodynamic vascular flow in aneurysms was comparable to those of X-ray DSA. The temporal resolution of UTE 4D-MRA and X-ray DSA are 165msec and four frames per second (250 msec), respectively. Thus, UTE 4D-MRA could visualize precisely. Moreover, FD stent with coiling case, UTE 4D-MRA showed intraaneurysmal flow within the coil which was difficult to visualize even with X-ray DSA (Fig.2).

Conclusion

Non-contrast enhanced UTE 4D-MRA is a useful technique for the follow-up tool of giant internal carotid artery aneurysms treated with flow-diverter stents.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Number JP16H06280. Grant-in-Aid for Scientific Research on Innovative Areas- Resource and technical support platforms for promoting research 'Advanced Bioimaging Support’.