In symptomatic intracranial atherosclerotic disease refractive to medical treatment intracranial angioplasty and stenting (ICAS) is a therapeutic option increasingly used in specialized stroke centers. However, restenosis rates ranging from 8% to 30% have been reported, underlining the necessity for regular follow-up examinations to exclude significant restenosis [¹?⁴]. CT angiography (CTA) and MR angiography (MRA) are known to suffer from considerable artifacts that often prevent assessment of the stent lumen. Therefore, current follow-up techniques include transcranial Doppler sonography and conventional intraarterial digital subtraction angiography (ia-DSA). However, diagnostic ia-DSA is an invasive technique with a rate of examination-related neurological complications ranging from 0.5% to 0.8% [⁵, ⁶]. In addition, ia-DSA is not offered on an outpatient basis at every medical center so patients have to be admitted for at least one night, significantly increasing the treatment costs. Furthermore, patient compliance with follow-up examinations may be a problem because the ia-DSA procedure is not very comfortable.

The new angiographic systems equipped with flat-panel detectors which allow rotational acquisition of volume data provide the possibility of CT-like low-contrast imaging [⁷]. The administration of even highly diluted contrast medium intraarterially gives good delineation of the vessels indicating that intravenous administration of contrast medium could be sufficient. We sought to combine this property with the possibility of nearly artifact-free imaging of small-vessel stents described previously [⁸].

DSA and angiographic CT (ACT) examinations were performed on an AxiomArtis dBA biplane angiography system equipped with flat panel detectors (Siemens Medical Solutions, Forchheim, Germany). The acquisition of rotational datasets was performed with the following parameters (20s-1k protocol): 20?s rotation, 538 projections, 220? total angle, no zoom (detector size 30?40?cm), CTDI_weighted approximately 22?mGy (manufacturer?s information). Postprocessing of the image data to a volume dataset (ACT) was performed on a Leonardo workstation (DynaCT, InSpace 3D software, Siemens). The software includes the application of system-specific filter algorithms in order to correct for beam hardening, radiation scatter, truncated projections and ring artifacts. Postprocessing resulted in volume datasets each defined by a batch of about 400 slices in a 512?512 matrix. Voxel size was about 0.1?0.1?0.1?mm³. The ACT datasets were further processed to multiplanar reconstructed (MPR) slices with the thin-slice and thick-slice maximum intensity projection (MIP) technique parallel and perpendicular to the stent-supplied section of the vessel, respectively. The contrast medium used in the follow-up ACT examination was Imeron 350 (Bracco ALTANA Pharma, Konstanz, Germany). The contrast medium was injected to a volume of 100?ml into a cubital vein at a flow rate of 5?ml/s, and the start delay for rotational acquisition was 20?s. The patient was asked to close his or her eyes and to breathe calmly during the examination.

CTA was performed on a 16-slice CT scanner (Aquilion, Toshiba Medical Systems, Tokyo, Japan) with 0.5-mm slice collimation, intravenous injection of 60?ml Imeron 350 (Bracco ALTANA Pharma), and a CTDI_vol of approximately 50?mGy [⁹]. Postprocessing was performed on a Vitrea II workstation (Vital Systems, Minnetonka, Minn.).

In patients suffering from intracranial atherosclerosis, ICAS is a promising new therapeutic option [¹, ³, ¹¹]. To identify an in-stent restenosis a reliable imaging technique is needed. Here, DSA is the current gold standard, but it carries a 0.5% to 0.8% risk of permanent neurological impairment [⁵, ⁶]. Additionally, not every center is able to perform ia-DSA on an outpatient basis. The risk may be less with ia-ACT if the injection is performed in the aortic arch. However, this is still an invasive procedure.

In the study reported here we demonstrated that iv-ACT can reliably depict the lumen of small-vessel stents with high spatial resolution. As the low contrast resolution of ACT is comparable to that of conventional CT, a relatively low concentration of intravascular contrast medium is needed. All intracranial vessels can be viewed simultaneously in a high-quality image. CT-like images of the brain parenchyma are provided as well [¹⁰]. It therefore represents a new radiological imaging technique between CTA and ia-DSA.

Even though this new method needs to be evaluated in a larger number of patients, it possibly has many advantages for patients and physicians as a new option in the imaging of cerebrovascular disease: Examinations can be performed on an outpatient basis and the risk of neurological complications is lowered practically to zero. The examination can be performed quickly, in a time similar to that required for a CTA examination. The radiation exposure for the patient from a rotational acquisition is lower than that from a CTA examination (manufacturer?s information) and has been shown to be considerably lower than that from a single biplane DSA series [¹²].

Possible drawbacks of the iv-ACT technique first include movement artifacts. The patient has to lie nearly motionless for about 20?seconds, which requires a high level of compliance and is not always feasible in patients suffering from cerebrovascular disease.

The first stent we examined is a relatively new balloon-mounted stent designed for intracranial use (Pharos, Micrus Endovascular, Renens, Switzerland). All dedicated intracranial stents (Neuroform and Wingspan, Boston Scientific, Natick, Mass.; LEO, BALT, Montmorency, France; and Enterprise, Cordis, Miami Lakes, Fl.) have a similar strut size of about 0.06?mm [¹³]. In contrast, common coronary stents have thicker struts (e.g. Driver, Medtronic, Minn.: 0.09?mm) [¹⁴]. Our second patient demonstrates that the lumen of these stents can be visualized as well. This issue is not unimportant because until recently dedicated intracranial stents were not available. Therefore, the majority of stents implanted into intracranial vessels are still coronary stents.

Iv-ACT is a promising new technique for minimally invasive follow-up after intracranial stenting. Other possible applications can be envisaged, such as diagnostic and follow-up imaging before and after endovascular treatment of intracranial aneurysms or arteriovenous malformations.

Conflict of interest statement J.-H. Buhk and M. Knauth have received grants for travel expenses from Siemens Medical Solutions (Forchheim, Germany). P. Lingor declares no conflict of interest.

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