Cerebral arterial variations may develop from failure of fusion/regression or abnormal fusion/regression during early embryonic stages. Traditionally, cerebral arterial variations have been reported in cadaver dissections, surgeries, and catheter angiographies. Recently, MR angiography has been widely used for the evaluation of cerebral arterial lesions, and its image quality has improved, especially in images obtained with a 3-Tesla scanner. Thus, the incidence of cerebral arterial variations incidentally diagnosed by MR angiography is increasing [6].
AChAs have a low incidence of hyperplasia and their prevalence on catheter angiography is reported to be 2.3% [5]. In contrast, the prevalence on MR angiography was reported to be only 0.55% [8]. This discrepancy between the prevalence on catheter and MR angiographic studies may be due to the low spatial resolution of MR angiography. Usually, one of the branches of the PCA arises from this hyperplastic AChA. The PCA usually has 4 major cortical branches: the anterior temporal, posterior temporal, calcarine, and parieto-occipital arteries. The hyperplastic uncal branch, which supplies part of the anterior temporal artery of the PCA, is relatively small in caliber. Thus, this type of variation may be overlooked on MR angiography. The majority of PCA branches arising from the AChA are temporal arteries [5, 8]. Rarely, the parieto-occipital and calcarine arteries arise from the AChA [4, 5]. In 2016, the term “hyperplastic AChA” was suggested to replace “accessory PCA” [8].
It is extremely rare for all branches of the PCA arise from an AChA. This type of variation is termed “replaced PCA” [2, 7, 8]. This variation may be misdiagnosed as transposition of the AChA and PCoA. The proximal segment of the replaced PCA takes a similar course to the cisternal segment of the AChA [7]. Catheter angiography can reveal the ventricular segment of the AChA arising from this large artery [2]. If a tiny artery arising from the supraclinoid segment of the ICA just proximal to the origin of the large anomalous artery is detected, it indicates a hypoplastic PCoA. Therefore, if a small PCoA is identified, as in our patient, a correct diagnosis can be obtained. Replaced PCAs have been reported to be associated with aneurysms [2] or infundibular dilatation [7] at the origin of the hypoplastic PCoA.
The first large cortical branch of the PCA is the temporal artery, which rarely arises from the proximal segment of the PCA, forming an early bifurcated PCA. The prevalence of MR angiography was reported to be 0.34% [8]. Very rarely, the temporal artery arises from the PCoA of a fetal-type PCA [9]. Our patient had an extremely early branching temporal artery in the replaced PCA. To our knowledge, no similar cases have been reported in the relevant English-language literature.
Accessory PCAs, including replaced PCAs, can be associated with other cerebral arterial variations. A case of accessory PCA associated with an accessory middle cerebral artery was recently reported [6]. PCA-accessory PCA anastomoses have been reported using CT or MR angiography [1, 10]. There are several locations of anastomosis between the PCA and accessory PCA [3].
Two types of MR angiography reconstruction exist: maximum-intensity-projection (MIP) and VR images. MIP images are used more frequently than VR images in daily clinical practice. However, VR images are suitable for demonstrating arterial variations because they are 3-dimensional, which overcomes the superimposition of arterial branches (Fig. 1). Tiny arteries may not be identified on either MIP or VR images because of the limitation of the spatial resolution. Thus, careful observation of the source images and reformatted images is important and necessary for identifying tiny arteries (Supplemental files, Fig. 2)
