Keywords
direct surgery - tentorial dAVF - Onyx embolization - smart metal artifact reduction
Introduction
Although Onyx (Micro Therapeutics, Inc., Irvine, CA, USA) is approved as an embolic
material for arteriovenous malformation (AVM) and dural arteriovenous fistula (dAVF)
with good outcomes,[1]
[2] metal artifacts due to Onyx on CT remain problematic after embolization.[3]
[4]
We report the feasibility of CT angiography (CTA) using a metal artifact reduction
(MAR) algorithm in the preoperative evaluation for direct surgery after transarterial
Onyx embolization of tentorial dAVF.
Case Report
A 45-year-old male patient had right pulsatile tinnitus and visited a nearby hospital.
By cranial magnetic resonance imaging, right tentorial dural arteriovenous fistula
(dAVF) was suspected and he was referred to our department.
Cerebral angiography demonstrated arteriovenous shunts on the tentorial dura of the
right petrous bone. The marginal tentorial artery (MTA), the posterior convexity branch
of the right middle meningeal artery (MMA), the right accessory meningeal artery (AMA),
and the mastoid branch of the right occipital artery (OA) were the main feeders, and
the right petrous veins were draining veins. The shunt flow was refluxed backward
to the veins of the cerebellum and basal vein (Borden type III, Cognard type III,
Lawton type V)[5]
[6]
[7] ([Fig. 1A–C]).
Fig. 1 Angiograms of the right internal maxillary artery (A), right OA (B), right internal carotid artery (ICA) (C) demonstrated right tentorial dAVF fed by the posterior convexity branch of the right
MMA (A: arrow), the right AMA (A: arrow head), the mastoid branch of the right OA (B: arrow head), and the right MTA (C: arrow head). The dAVF drained into the right petrous veins, and the shunt flow was refluxed
backward to the veins of the cerebellum and basal veins. After transarterial embolization,
the angiogram of the right ECA (D) and right ICA (E) showed the disappearance of feeding arteries from the right ECA and a surviving
feeder from the right MTA.
As the first-line treatment, we performed transarterial embolization. During the first
session, we selected the branch of the right MMA and injected 0.27 mL of Onyx-18 into
the dAVF, followed by 0.47 mL of Onyx-18 into the branch of the right AMA. During
the second session, the branch of the right MMA and the branch of the right OA were
selected for embolization, into which 0.36 and 0.57 mL of Onyx-18, respectively, was
injected. After two sessions of transarterial embolization, feeding arteries from
the right external carotid artery (ECA) disappeared, but the dAVF was still fed by
the right MTA, and the petrous veins still refluxed backward to the veins of the cerebellum
and basal vein ([Fig. 1D, E]).
Aiming at a complete cure of dAVF, we planned direct surgery. We performed CTA as
a preoperative evaluation of direct surgery. CTA was performed by high-definition
CT (Revolution EVO; General Electric Healthcare, Chicago, IL, USA). The patient was
in the supine position. A bolus of contrast material (Iopamidol, Oypalomin 370 mgI/mL;
Fuji Pharma Co., Ltd. Tokyo, Japan) was injected into the median cubital vein at a
flow rate of 3 mL/s for 25 seconds. The raw data were reconstructed conventionally
or with a commercial MAR algorithm (Smart MAR; General Electric Healthcare, Chicago,
IL, USA) and transferred to the Aquarius NET Thin Client Viewer (TeraRecon, Foster
City, CA, USA). Although evaluation of the lesion was difficult on normal CTA because
of artifacts by Onyx ([Fig. 2A]), CTA using MAR enabled a detailed planning of the direct surgery of the lesion
([Fig. 2B, C]). On CTA images using MAR, we were able to recognize the three petrosal veins as
the draining veins originating from the main drainer at the cerebellopontine angle,
one of which was filled with Onyx ([Fig. 2B, C]).
Fig. 2 On standard CTA (A), evaluation of the lesion was difficult because of strong artifacts by Onyx. However,
on CTA using MAR (B), we were able to assess the lesion in detail and recognize the three petrosal veins
as the draining veins (C) (arrowheads).
Direct surgery was performed through right retrosigmoid craniotomy. The main drainer
of the dAVF originated at the petrosal surface ([Fig. 3A]). We coagulated it at the site that penetrated the dura to reduce its volume ([Fig. 3B]). Referencing CTA images using MAR, we identified the three petrosal veins originating
from the main drainer. They were coagulated and cut ([Fig. 3C–E]), leading to complete occlusion of the dAVF ([Fig. 3F]).
Fig. 3 Intraoperative findings. The main drainer of the dAVF originated at the petrosal
surface (A). It was coagulated to reduce its volume (B). Referencing CTA images using MAR, three petrosal veins originating from the main
drainer were coagulated and cut (C–E), leading to complete occlusion of the dAVF (F).
After surgery, the right-sided pulsatile tinnitus disappeared with no postoperative
complications. Angiogram performed 1 week later confirmed no residual dAVF ([Fig. 4A, B]).
Fig. 4 Angiograms of the right ECA (A) and right ICA (B) demonstrated the obliteration of the fistula.
Discussion
Tentorial dAVF is a relatively rare type of dAVF, accounting for less than 4% of all
intracranial dAVF.[8] Compared with dAVF in other regions, tentorial dAVF has a higher risk of intracranial
hemorrhage, venous infraction, and aggressive neurological behavior. Furthermore,
those characterized by cortical venous drainage often have a high incidence of aggressive
behavior, hence the need for immediate and accurate treatment.[9]
[10] Although treatment options include surgical interruption of the draining vein,[7] stereotactic radiosurgery,[11] endovascular procedure,[12]
[13]
[14]
[15] and a combination of these options,[16]
[17] no optimal treatment has been established.
As endovascular procedures, we have strategies involving transvenous embolization
and transarterial embolization. The transvenous approach that sacrifices the dural
sinus has a high cure rate, but using this approach for tentorial dAVF is often difficult
because most directly drain into cortical veins without the main sinus.[15] Therefore, transarterial embolization is the main endovascular procedure. To achieve
complete cure by transarterial embolization, it is important that embolic material
is injected beyond the shunt point to penetrate the vein side. If endovascular therapy
is inappropriate or ineffective, surgical disconnection of the fistula is the most
efficacious treatment.[17]
In the present study, as the first-line treatment, we selected transarterial embolization.
After two sessions of transarterial embolization, we considered the complication risk
of transarterial embolization via MTA. Based on pathological analyses of tentorial
dAVF, the surgical approach is more likely to be curative and has a lower risk of
postoperative complications.[7]
[12]
[13]
[15]
Onyx was approved in Japan as “an embolic material for dAVF for which transvenous
embolization is difficult” in 2018. Onyx is liquid embolic matter of the separation
type and it is not adhesive. It is easy to control more embolic material, and one
of the advantages is that immediate judgment is not necessary.[15]
[18]
[19] Although transarterial Onyx embolization for tentorial dAVF has a higher complete
cure rate and is safer than conventional embolization, there have only been a few
cases of permanent cure by only transarterial Onyx embolization.[12]
[13]
[15]
Although the gold standard diagnostic method for dAVF is DSA, CTA has been reported
to be feasible as a less-invasive imaging technique.[20] CTA provides excellent visualization of small vascular details and their relationship
with the cranial bone in planning of craniotomy. Even in the planning of direct surgery
for dAVF, CTA is useful to understand the location of the shunt point and drainers
in relation to the cranial bone.[21]
However, for patients with a history of Onyx embolization, conventional CTA is not
useful for follow-up because it cannot precisely depict the status of dAVF due to
artifacts from Onyx. Evaluation of intracranial vessels is hampered by beam-hardening
and photon starvation artifacts caused by Onyx.[3] The underlying reason for the artifact production by Onyx on CT imaging is the high
absorption of photons by the admixed high-atomic number material tantalum (anatomic
number 73), resulting in beam hardening, scatter, and noise, which are visible as
dark and bright streaks in the reconstructed CT image. The higher degree of artifact
production by Onyx can accordingly be explained by the higher atomic number of tantalum.[3]
Several studies regarding CTA with commercially available metal artifact reduction
algorithms, including Smart MAR, have been reported.[22]
[23]
[24]
[25]
[26]
[27]
[28] With Smart MAR, platinum coil artifacts were greatly reduced, and we previously
reported that the status of treated aneurysms was able to be depicted for patients
with a history of clipping of recurrent aneurysms after coil embolization.[29] Our report may be the first that confirms the feasibility of MAR for the reduction
of metal artifacts on CTA after transarterial Onyx embolization for dAVF. Due to the
increase in the incidence of direct surgery after transarterial Onyx embolization
for AVM or dAVF, CTA using MAR may become more important.
