The cumulative results of the ratings of occlusion rate are shown in Table 1. All residual aneurysms were consistently detected with every technique except in one patient, where TOF-MRA was impaired due to clip-related signal extinction (Fig. 1a–d). That patient was excluded from the TOF-MRA rating.
Correlation analysis (Pearson) of occlusion rate revealed the following results: DSA vs. TOF-MRA: r = 0.889 (P < 0.001); DSA vs. ACT: r = 0.893 (P < 0.001); ACT vs. TOF-MRA: r = 0.913 (P < 0.001). TOF-MRA achieved the highest score by depicting 16 residual necks, followed by ACT (14) and DSA (12).
The case presented in Fig. 2 illustrates a residual neck of an Acom aneurysm, which is equally well detectable in ACT as well as in TOF-MRA (Fig. 2c,d), but not in DSA due to the unfavorable orientation (Fig. 2a,b). The residual aneurysm presented in Fig. 3 is reliably detected in all three techniques (Fig. 3a,b: DSA; c: TOF-MRA; d: ACT). The stretched and partially endothelialized coil remnant in the right ICA and A1 is well visualized by ACT (Fig. 3d).
Table 2 presents the results of the κ statistics. Interobserver agreement was very good for all groups (κw > 0.8) [16, 17].
All data are presented cumulatively. Firstly, a subjective assessment of the diagnostic performance of each technique compared to the other was made. Both ACT and TOF-MRA received inferior ratings when compared to DSA, being the gold standard (MRA vs. DSA: favors MRA: n = 14, no preference: n = 16, favors DSA: n = 33; ACT vs. DSA: favors ACT: n = 14, no preference: n = 15; favors DSA: n = 37).
Comparing MRA to ACT presents a slight trend in favor of TOF-MRA (favors MRA: n = 29; no prevalence: n = 14; favors ACT: n = 20). Dividing these data into two subgroups (aneurysms that have been treated with stent remodeling, n = 21 vs. those that have been treated without stent remodeling, n = 42) reveals a trend towards TOF-MRA being rated superior to ACT when no stent had been implanted, while the methods seem to be equivalent in the group of patients with stent remodeling.
Secondly, a subjective assessment of the impairments due to artifact load was asked in comparison between TOF-MRA and ACT. We were aware of not defining the character of artifacts we asked for because many ACT artifacts have not been described yet and are therefore hard to anticipate. Comparable to the ratings of the diagnostic value, TOF-MRA is superior to ACT when no stent was implanted (favors MRA: n = 23, no prevalence: n = 7, favors ACT: n = 12). A slight superiority of ACT is observed for patients with stent remodeling (favors MRA: n = 7, no prevalence: n = 3, favors ACT: n = 11). A second subgroup analysis was performed regarding the aneurysm size. A small aneurysm was defined as having a maximal diameter below 10 mm (n = 13 patients), a large aneurysm having a maximal diameter of 10 mm or more (n = 8 patients). Here equivalence results when comparing the artifact load in TOF-MRA and ACT regarding small aneurysms (favors MRA: n = 14, no prevalence: n = 8, favors ACT: n = 17), but a clear superiority of TOF-MRA over ACT regarding large aneurysms (favors MRA: n = 16, no prevalence: n = 2, favors ACT: n = 6).
One crucial artifact of ACT is demonstrated in Fig. 4. A large ophthalmoplegic ICA aneurysm was treated by stent-protected coil embolization. The follow-up DSA showed a slight central residual filling. ACT provides good visualization of the large coil package using 3D MIP. Concerning the interior of the coil package, ACT suffers from hardening artifacts presenting as an amorphous attenuation comparable to the contrast media filling of the parent vessel. Performing a subtraction only shows some movement artifacts on the edge of the coil package, but no residual filling. The residual filling is confirmed by TOF-MRA.