Optimized Angiographic CT Using Intravenous Contrast Injection: A Noninvasive Imaging Option for the Follow-Up of Coiled Aneurysms?

Discussion

To the best of our knowledge, ivACT has not yet been evaluated for follow-up of coiled aneurysms. In our study, ivACT demonstrated high sensitivity and specificity in detecting remnants down to a minimal size of 1.0 × 0.9 mm. Assessment of remnant size by ivACT showed a significant correlation with DSA. On the basis of differentiating any remnant and no remnant, agreement of ivACT with DSA was identical in assessing the aneurysmal occlusion rate.

ivACT was performed as a dual rotational 10-second acquisition run by using the bolus-watching technique to obtain the best arterial vessel opacification. Using a dual-volume technique (iDentify) with newly implemented reconstruction modes, one can generate freely rotatable 3D images. Fusing 2 separately reconstructed datasets, 1 optimized for vessel visualization and 1 for coil delineation, allows an improvement in image quality compared with MPR images that are obtained from only 1 reconstruction step.

An advantage compared with DSA is that ivACT allows evaluation of several coiled aneurysms in only 1 examination step (comparison shown in Fig 2). Implanted stents after stent-assisted coiling induced no substantial artifacts (comparison shown in Fig 4). Moreover, the stent lumen can be assessed precisely in ivACT,¹⁷ which is not achievable in MRA. In the case of volume bare platinum coils, ivACT might be less suitable than MRA because these coils seem to induce compromising beam-hardening artifacts. However, our results concerning these coils are limited due to the small sample size, but that might be a subject for further investigations.

In angiographic CT using intra-arterial contrast material injection, substantial “eggshell” artifacts are reported for coil packages larger than 10 mm in diameter, supposedly the result of insufficient reconstruction algorithms.¹² In contrast, in our study by using newly implemented reconstruction algorithms and iDentify instead of MPR reconstructions, we did not observe such artifacts in larger aneurysms. Additionally, image evaluation might be facilitated by injecting the contrast material intravenously, leading to a homogeneous contrast mixing without high-flow artifacts, which can occur with intra-arterial injection.

TOF-MRA also demonstrated high sensitivity and specificity in remnant detection on almost the same level in ivACT. Assessment of remnant size was comparable with that of ivACT and demonstrated a significant correlation with DSA. Stents induced no artifacts substantially compromising aneurysm evaluation, but a reliable assessment of the stent lumen is not viable in MRA.

Our results are in line with those in other MR imaging studies that found substantial agreement between DSA and MRA in evaluating coiled aneurysms. The reported sensitivity and specificity of remnant detection are comparable with our results.^8,18,19 One study found MRA sufficient for therapeutic decision-making in the retreatment of recanalization.¹⁸ On the other hand, evaluation of MRA performed with 3T MR imaging revealed a greater discrepancy with DSA in aneurysms with a small (1–3 mm) residual lumen than in those with a larger lumen because for a small residual lumen, the precise differentiation between subtotal and incomplete occlusion does not seem possible.⁸ Similarly, sufficient accuracy of MRA is reported, in general, for the screening of residual flow after coil embolization, but in particular in small aneurysms (<6 mm), the detection of residual flow was limited.¹⁷ Therefore, ivACT might be considered more favorable than MRA in small aneurysms because we found no restrictions with ivACT in evaluating small aneurysms. On the contrary, because ivACT might be limited in evaluating large aneurysms with a maximum diameter of >10 mm, ivACT and MRA could be considered complementary imaging techniques, which might be selected individually, depending on aneurysm size or coil material. Here, further investigations with an increased patient population are necessary.

In our study, observer agreement with DSA was almost perfect for both ivACT and MRA. Interobserver variability was not assessed because our study aimed first to demonstrate the feasibility of ivACT and to assess ivACT and MRA as close as possible to the usual clinical setting with only 1 reader. Because MRA and ivACT reached nearly identical accuracy in remnant detection and in the assessment of remnant size, they seem equally capable of a noninvasive follow-up; both may be reliable alternatives to DSA. However, because MRA can be performed without radiation exposure to the patient, it may serve as the preferred technique in routine cases. Nevertheless, in patients with contraindications to MR imaging (heart pacemaker and so forth) or in the case of compromising susceptibility artifacts due to additional metal implants (eg, aneurysm clips, comparison shown in Fig 5), ivACT can be considered a reliable alternative. ivACT could also be an option in patients treated with ventriculopertioneal shunts to avoid potential maladjustment or damage. The advantages of ivACT are that it has a short acquisition time, it is less time-consuming than DSA and MRA, and it can be performed on an outpatient basis, of interest from an economic point of view.

Concerning radiation exposure, evaluations by using an Alderson-Rando phantom revealed an effective dose of 2.9 mSv for the 10-second run of an ivACT if the entire head was scanned. However, by collimating the FOV on the coil package, the radiation dose can be substantially reduced by up to 0.5 mSv.²⁰ For comparison, the effective dose of a biplane 2D-DSA series was measured as 1 mSv. Thus, especially in the follow-up of multiple coiled cerebral aneurysms, the use of ivACT could significantly reduce radiation exposure to the patient compared with DSA. Advances in the rapidly developing flat panel detector technology might further contribute to increasing dose effectiveness and improving image quality.

Our findings are limited because mainly 1 type of coil was used for aneurysm treatment in our study population. Moreover, the mean aneurysm size and the mean remnant size were relatively low in our population; thus, an objective for further investigations could be to add more large aneurysms to our series. Head motion and the resulting artifacts lead to a compromised image quality in both modalities, making restless or noncompliant patients less suitable for both imaging techniques. The development of special cups for better head fixation might be helpful for reducing motion artifacts in ivACT. As an objective for further investigations, the implementation of reconstruction modes with automated metal artifact reduction might further contribute to improved ivACT image quality.