Variations of the cerebellar arteries at CT angiography

Transcription

1 DOI /s z Original Article Variations of the cerebellar arteries at CT angiography Yeliz Pekcevik Ridvan Pekcevik Received: 28 July 2013 / Accepted: 14 September 2013 / Published online: 24 September 2013 Springer-Verlag France 2013 Abstract Purpose There are few studies that investigated variations of the cerebellar arteries, and the prevalence of some variations has not been reported. We aim to identify and evaluate variations of the cerebellar arteries using CTA. Methods The findings in 341 patients who underwent 64-slice CTA were reviewed for anatomical variations of the cerebellar arteries. We assessed variations and classified our findings to describe the results of our analysis. We attempted to create an anatomical model for CTA that is somewhat different from previous studies and described our findings according to vascular anatomy. Results Only 11.7 % of the 341 patients had all the cerebellar arteries without anatomical variations. Vertebral artery continuation as posterior inferior cerebellar artery (PICA) was found in 4.4 % of the patients. Absence of the anterior inferior cerebellar artery (AICA) was observed in 36.1 %, and absence of the PICA was observed in 6.7 % of the patients. The origin of the PICA was extradural in 71 (20.8 %) patients. Double origin of the PICA, PICA duplication, and fenestration were rarely found. Duplication of the AICA (7.9 %), and superior cerebellar artery (SCA) (20.5 %), and early bifurcation of SCA (9.4 %) were frequently observed. Superior cerebellar artery and posterior cerebral artery (PCA) common trunk and SCA originating from PCA were found 9.4 and 8.2 %, respectively. Y. Pekcevik (*) Department of Radiology, Izmir Tepecik Training and Research Hospital, Gaziler Cd No: 468, Yenişehir, Izmir, Turkey yelizpekcevik@yahoo.com R. Pekcevik Department of Radiology, Izmir Bozyaka Training and Research Hospital, Izmir, Turkey Conclusion Variations of the cerebral arteries can be easily evaluated by CTA. Recognizing and reporting them at cerebral CT angiography may be clinically important. Keywords Cerebellar artery Multidetector computed tomography Angiography Anatomical variations Abbreviations ct computed tomography DSA Digital subtraction angiography SCA Superior cerebellar artery AICA Anterior inferior cerebellar artery PICA Posterior inferior cerebellar artery MIP Maximum intensity projection MPR Multiplanar reformatting Introduction Computed tomography angiography (CTA) has become a valuable diagnostic technique for evaluation of the cerebral vasculature. Owing to recent advances in CT technology it allows for better visualization of small vessels and has partially replaced digital subtraction angiography (DSA) [3]. The typical vertebrobasilar system is an arterial network and consists of two vertebral arteries that give rise to the posterior inferior cerebellar arteries (PICA), a basilar artery that gives rise to anterior inferior cerebellar arteries (AICA), superior cerebellar arteries (SCA) and other small branches, and finally two posterior cerebral arteries (PCA). The complete occurrence of this well-known anatomy is not frequent, and it is important to know anatomical variations while interpreting imaging studies.

2 456 Recognizing and reporting variations of the cerebellar arteries at CTA may be clinically important. Arterial blood supply regions may show differences due to variations [15]. These differences should be considered in evaluating ischemic lesions and clinical symptoms in stroke patients [4]. Deviation and prominence of one vessel may cause compression of the cranial nerves. Trigeminal neuralgia and hemifacial spasm were reported to be associated with the arterial variations [7, 15]. Fenestrations may increase the incidence of aneurysm [11, 16]. They may be misinterpreted as an aneurysm or a dissection at magnetic resonance imaging. Evaluating the cerebellar arteries variations may give valuable information prior to surgical and endovascular treatments for planning the surgical and interventional approach. There are few studies that investigated anatomical variations of the cerebellar arteries [14, 15, 17], and prevalence of some variations on CTA has not been reported before. In the era of CTA, these variations should be reclassified for anatomical and systematic evaluation. The purpose of this study was to identify and evaluate the anatomical variations of the cerebellar arteries by using CTA. We also classified the observed variations according to their anatomy for summarizing our findings. Materials and methods Patients Between January 2012 and May 2013 a total of 341 consecutive patients (193 male, 148 female, mean age 52.8 years, range years), who underwent 64-slice CTA in our department, were reviewed for variations of the cerebellar arteries. Patients were referred for cerebral CTA because of known or suspected cerebral aneurysm, intracerebral hemorrhage and suspected arteriovenous malformation. We excluded all patients with a history of surgical or interventional treatment of vertebrobasilar system, with previous posterior fossa surgery and with occlusive disease in the vertebrobasilar system. The institutional review board approved the study protocol and the requirement for informed patient consent was waived. CT scan All CT examinations were performed by a 64-slice CT scanner (Aquilion 64, Toshiba Medical Systems, Tochigi, Japan). For venous access, an upper extremity vein (antecubital vein of the right arm) and a 20-gauge IV cannula was used. After precontrast images, a total of ml of contrast media with high iodine concentration ( mg/ml) was injected with a flow rate of 5 ml/s, followed by a 20 ml saline chaser. The scanning parameters included 120 kv, 225 ma, section thickness of 0.5 mm and reconstruction interval of 0.3 mm. The scan revolution time was 0.4 s. Data for CTA were obtained in a caudocranial direction. Image analysis All multidetector CTA data were transferred from the archive to a workstation (Aquarius workstation, TeraRecon, San Mateo, CA, USA), via internal network connections, providing 3D postprocessing options, multiplanar image reformatting and maximum intensity projections (MIP). All examinations were reviewed by two radiologists independently with respect to the anatomy of the cerebellar arteries. In cases of discrepancy, the CTA images were reviewed again, to reach a consensus on all variations. We assessed the anatomy and variations of the cerebellar arteries and classified our findings to describe the results of our analyses. We attempted to create an anatomical model for CT angiography that is somewhat different from previous studies. The classification criteria and terms used in our assessment are explained below: Normal anatomy: two PICAs originated from intracranial vertebral arteries, two AICAs and SCAs originated from basilar artery. Absence of AICA and PICA: when they were not seen in their normal and variant localization. Vertebral artery continued as PICA: if the vertebral artery size was equal to that of the PICA size and vertebral artery had hypoplasia just after its PICA branch (Fig. 1). Duplication: two distinct arteries with separate origins and no distal arterial convergence [2] (Fig. 2). Fenestration: division of an arterial lumen into distinctly separate lumens that had convergence on their both ends [9]. Double origin of the PICA: two distinct PICAs with separate origins and with distal arterial convergence [8] (Fig. 3). SCA originating from PCA: SCA took its origin from PCA rather than basilar artery [18] (Fig. 4). Early bifurcation of the SCA: the bifurcation of the SCA occurred in the proximal part of the anterior mesencephalic segment of the SCA [19] (Fig. 5). SCA PCA common trunk: SCA and P1 segment of the PCA had common origin [2] (as shown in Fig. 1). Extradural origin of the PICA: PICA took its origin proximal to the intracranial segment of the vertebral artery (Fig. 6).

3 457 Fig. 1 Vertebral artery continued as PICA. CT angiogram shows equal-sized vertebral artery and PICA (double arrows). Vertebral artery has hypoplasia just after its PICA branch. There is also a unilateral common trunk of the superior cerebellar artery and P1 segment of the posterior cerebral artery (arrow) Arrowhead indicates basilar artery fenestration Results Anatomical variations of the PICA, AICA and SCA were listed in Tables 1, 2 and 3 A total of 40 (11.7 %) of the 341 patients had all the cerebellar arteries without anatomical variations. Absence of the AICA was more frequent on the left side whereas absence of the PICA was more frequent on the right side. Absence of the AICA was observed in 123 (36.1 %) patients and absence of the PICA was observed in 131 (38.4 %) patients, either bilateral or unilateral. Of the 26 patients with AICA duplication, 18 were identified on the right side. The origin of the PICA was outside of the cranium in 71 (20.8 %) patients. Extradural origin was more commonly observed on the left side. The most common anomalies of the SCA were duplication and early bifurcation. Duplication was associated with SCA PCA common trunk and SCA originating from PCA in six and one patients, respectively. Fig. 2 Duplication of the SCA (arrow). There is also hypoplasia of P1 segment of the PCA (double arrows) Discussion Our study results show that, the prevalence of anatomical variations in the cerebellar arteries is high. Only 11.7 % of the patients had a complete anatomy without variations. Performing a systematic anatomical analysis of cerebral CTA can help for accurate interpretation. Recognizing and reporting of these variations may be clinically important. The terms anomalous are used to define any variant form observed in less than 1 % of the general population. Some of the variability in the reported incidence likely reflects referral bias and even variability in definitions of anomalous and normal variant. We preferred to use anatomical variations term for describing our findings. The reported prevalence of cerebellar arteries variations is mostly from anatomic and angiographic studies. Given that CTA has now partially replaced DSA, knowing the prevalence on CTA is important. Vertebral artery continuation as PICA was observed in 15 (4.4 %) patients in our study. This was described in Osborn s textbook [8] but its incidence has not been reported. It might be partly due to identification of the variations in different ways. We tried to use anatomical terms and vertebral artery continuation as PICA seemed to

4 458 Fig. 4 SCA originating from PCA. CT angiogram shows bilateral SCA (arrows) that take their origin from the posterior cerebral arteries Fig. 3 Double origin of the PICA. CT angiogram shows two distinct posterior inferior cerebellar arteries with separate origins and distal convergence (arrow) us correct explanation of equal-sized vertebral artery and PICA with hypoplasia of the ipsilateral vertebral artery. Knowing this variation may help explaining the extent of ischemic lesions due to stenosis, embolism or thrombosis in posterior circulation strokes. Fenestrations of the PICA, AICA, and SCA are extremely rare. The incidences 0.3, 0.3, and 0.9 % in our study that were found for PICA, AICA, and PCA fenestrations, respectively, have not been reported in the literature. Cerebellar arteries are very small vessels and fenestration of these vessels may facilitate occlusion. Fenestrations may be associated with aneurysms [11, 16]. A post-mortem study of 291 brains demonstrated that the PICA was absent bilaterally in 2.3 % and absent unilaterally in 19 % of the cases [19]. Higher prevalence was found in our study when compared with cadaveric studies. Although thin-slab MIP images were performed for evaluating images, considering such small vessels, we might have overestimated the results. Absence of the PICA was more frequent on the right side as opposed to AICA. An extradural origin of the PICA occurs in 5 18 % of cases [8, 10]. In our study 20.8 % of the patients had extradural Fig. 5 Bilateral early bifurcation of the SCA. CT angiogram MIP image shows early bifurcation of the bilateral superior cerebellar arteries (arrows) that is proximal to the expected localization origin and it was found to be higher on the left side. Knowing and reporting this variation is clinically important because it might be the origin of the subarachnoid hemorrhage due to associated aneurysm. It is also important in planning surgical or endovascular treatment approach for avoiding complications during surgery [10]. Double origin of the PICA is a rare variation. The reported prevalence of this anomaly in a previous study

5 459 Table 2 Anterior inferior cerebellar artery (AICA) anatomical variations Type of the variation No. of patients (%) (n = 341) Absence 123 (36.1) Right 19 (5.6) Left 55 (16.1) Bilateral 49 (14.4) Duplication a 27 (7.9) Right 18 (5.3) Left 8 (2.3) Fenestration b 1 (0.3) a Patients with duplication of the AICA were associated with ipsilateral absence of the PICA. Only two patients had PICA that originated from V3 segment of the vertebral artery (extradural origin of the PICA) b It was right-sided Table 3 Superior cerebellar artery (SCA) anatomical variations Fig. 6 Extradural origin of the PICA. CT angiogram demonstrates that the PICA takes its origin proximal to the intracranial segment of the vertebral artery (arrow). Double arrows indicate origin of the normal contralateral PICA Table 1 Posterior inferior cerebellar artery (PICA) anatomical variations Type of the variation No. of patients (%) (n = 341) Absence 131 (38.4) Right 65 (19.1) Left 43 (12.6) Bilateral 23 (6.7) Extradural origin of the PICA 71 (20.8) Right 19 (5.6) Left 33 (9.7) Bilateral 19 (5.6) Vertebral artery continued as PICA 15 (4.4) + ipsilateral hypoplasia Right 8 (2.3) Left 7 (2.1) Double origin of the PICA 3 (0.9) Right 2 (0.6) Left 1 (0.3) Duplication a 2 (0.6) Fenestration b 1 (0.3) Type of the variation No. of patients (%) (n = 341) 1 (0.3) Duplication 60 (17.6) Right 32 (9.4) Left 28 (8.2) Bilateral 10 (2.9) SCA PCA common trunk 17 (4.9) Right 8 (2.3) Left 6 (1.8) Bilateral 3 (0.9) SCA originating from PCA 16 (4.7) Right 8 (2.3) Left 4 (1.2) Bilateral 4 (1.2) Duplication + common trunk 6 (1.8) Right 4 (1.2) Left 1 (0.3) Bilateral 1 (0.3) Duplication + SCA originating from PCA a Early bifurcation 32 (9.4) Right 16 (4.7) Left 13 (3.8) Bilateral 3 (0.9) a It was bilateral PCA posterior cerebral artery a/b Right-sided on catheter angiography was 1.45 % [6]. The frequency of double origin of the PICA on CTA has not been reported before. In our study, it was only 0.9 %. The double origin of the PICA was described as being potentially on the left side [6]; but it has also been reported as being on the right side [5]. We found a 2:1 ratio right-sided preponderance in our study. No matter on which side, it is important to

6 460 recognize and evaluate this anomaly because double origin of the PICA may have an increased association with intracranial aneurysms, and may represent a risk factor for subsequent development of intracranial aneurysm [5, 6]. Double origin of the PICA may be associated with fenestration [1]. Duplication of the PICA is different from double origin, and we used this term for two distinct arteries that originate from vertebral artery with no distal arterial convergence. It was found in 2 of 50 cerebellar specimens on the right side in a cadaveric study [12]. We had only two patients (0.6 %) with duplication of the PICA, which was also on the right side. We preferred the term duplication of AICA for two arteries that originate from the basilar artery. All the patients with PICA duplication have no AICA. All the patients with AICA duplication were associated with ipsilateral absence of the PICA, except two patients that had PICA originating from extradural segment of the vertebral artery. The prevalence of SCA variations has not been reported systematically on CT angiography. Variation of the SCA is a common finding and it usually has no clinical significance. But vascular compression due to their different sizes and course may rarely cause hemifacial spasm or trigeminal neuralgia [7, 15]. The prevalence of the duplication, early bifurcation, and SCA and PCA originating as a common trunk were reported as 29.2, 21 and 16.3 %, respectively, in a previous cadaveric study [15]. Although we used thin-slab MIP images for evaluating the cerebellar artery variations, prevalence rates observed in our study were lower than that of cadaveric studies. The superior cerebellar artery that originates from PCA is occasionally seen in the present study population and the prevalence was 4.7 %. Duplication was a frequent finding and it was rarely found to be associated with other SCA variations, SCA and PCA originating as a common trunk and SCA that originates from PCA. The present study had some limitations. First, it was performed retrospectively. Second, because it was based on the analysis of CTA images, prevalence of the associated anomalies might be different from anatomic studies. But CTA is now widely used, and this study may be used as an initial reference study for CTA images. Third, this study was conducted on the basis of image interpretations performed by means of consensus opinion. Fourth, variations were classified according to terms that are created for anatomic evaluation according to previous studies. But previous literature had some confusing data about cerebellar artery variations and we think that describing the detailed anatomy of all observed variations might be better. Last, although we evaluated thin-slab CTA images, we did not find some rare variations as revealed by angiography, such as aberrant origin of the cerebellar arteries from anterior cerebral circulation [13]. In conclusion, our study results show that, the prevalence of anatomical variations of the cerebellar arteries is high. Only 11.7 % of the patients had all the cerebellar arteries without anatomical variations. Performing a systematic anatomical analysis of cerebral CTA can help in accurate interpretation. Recognizing and reporting of these variations may be clinically important. Conflict of interest The authors have no conflict of interest to disclose. References 1. Cho YD, Han MH, Lee JY (2011) Double origin of the posterior inferior cerebellar artery with juxta-proximal fenestration of caudal component. Surg Radiol Anat 33: Dimmick SJ, Faulder KC (2009) Normal variants of the cerebral circulation at multidetector CT angiography. 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