Ectopic cerebellar tissue of the posterior cranial fossa

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(Fig. 1a). Postnatal transcranial ultrasound (US) imaging al- so revealed a large posterior fossa cyst and a solid mass isoechoic with cerebellary tissue (Fig. 1b).
Childs Nerv Syst DOI 10.1007/s00381-015-2826-z

CASE REPORT

Ectopic cerebellar tissue of the posterior cranial fossa: diffusion tensor tractography and MR spectroscopy findings Hediye Pınar Gunbey 1 & Meltem Ceyhan Bilgici 2 & Kerim Aslan 1 & Canan Aygün 3 & Handan Celik 4

Received: 13 May 2015 / Accepted: 9 July 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract Purpose Well-differentiated ectopic cerebellar tissue is extremely rare, with only 12 cases in the literature. Here, we describe a unique case of radiologically proven ectopic cerebellar tissue, using diffusion tensor tractography (DTT) and MR spectroscopy (MRS) findings, in a 6-day-old newborn. Case A 6-day-old newborn who had previously a fetal MRI referred to our department with the suspicion of an arachnoid cyst of the posterior fossa. Including the central nervous system, all of his physical examination tests were normal. Postnatal transcranial ultrasound (US) imaging and brain MRI also revealed a large posterior fossa cyst and a solid mass nearby the cerebellar tissue. The tissue showed a small connection and isointense signal with the cerebellum. Upon DTT, both the cerebellum and nearby solid tissue represented the same FA values. Tractographic studies showed a connection with fibers extending along the left cerebellar hemisphere from this tissue. The single voxel MRS of this solid tissue also revealed high choline (Cho) and a smaller N-acetylaspartate (NAA) concentration similar to that of the normal newborn cerebellum. Conclusion Ectopic cerebellar tissue can be characterized by advanced neuroimaging tools, like DTT and MRS, which

* Hediye Pınar Gunbey [email protected] 1

Department of Radiology, Neuroradiology Section, Ondokuz Mayıs University Faculty of Medicine, Samsun, Turkey

2

Department of Radiology, Pediatric Radiology Section, Ondokuz Mayis University Faculty of Medicine, Samsun, Turkey

3

Department of Pediatrics, Neonatalogy Division, Ondokuz Mayıs University Faculty of Medicine, Samsun, Turkey

4

Department of Obstetrics and Gynecology, Ondokuz Mayıs University, Kurupelit, 55139 Samsun, Turkey

provide information about brain metabolite concentrations and the microstructural integrity. In this way, unnecessary surgery can be avoided in order to obtain a histopathological diagnosis. Keywords Ectopic cerebellum . DTI . MRS

Introduction Ectopic cerebellar tissue is an extremely rare condition, with twelve previously reported cases in the literature. It is believed to be the result of embryogenesis and cell migration failure. With the exception of one case [1], the diagnosis of these cases was made via histopathology after surgery. As the quality of brain imaging improves, with advanced neuroimaging techniques like diffusion tensor tractography (DTT) and MR spectroscopy (MRS), the requirements of surgery decrease for tissue identification. Here, we describe a unique case of radiologically proven ectopic cerebellar tissue using DTT and MRS findings.

Case report A 6-day-old newborn with the suspicion of an arachnoid cyst of the posterior fossa was referred to our radiology department. He was delivered with a cesarean section at 38 weeks and 3 days of gestation. Including the central nervous system, all of his physical examination tests were normal. He previously had a fetal MRI, with findings of a large posterior fossa arachnoid cyst and a nearby solid mass (Fig. 1a). Postnatal transcranial ultrasound (US) imaging also revealed a large posterior fossa cyst and a solid mass isoechoic with cerebellary tissue (Fig. 1b). To explain this solid mass and to exclude associated abnormalities, a brain MRI was performed.

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Fig. 1 a Previously taken fetal MRI shows a large posterior fossa arachnoid cyst and nearby a solid mass. b Postnatal transcranial US imaging revealed a large posterior fossa cyst and a solid mass isoechoic with cerebellary tissue

MRI studies were performed with 1.5-T (Archieva Philips, Netherlands). Axial T2W TSE (TR/TE, 5000/100 ms; matrix, 256 × 256; slice thickness, 5 mm), coronal FLAIR (TR/TE, 6000/120 ms; matrix, 256 × 256; slice thickness, 2 mm), and T1-weighted 3D gradient-echo sequence (TR/TE, 7.2/33 ms; matrix, 256 × 256; slice thickness, 5 mm; NSA, 1; FOV, 256 mm; slice thickness, 1 mm; gap, 0 mm; flip angle, 8°) for the entire brain. To investigate the solid mass nearby cerebellum, diffusion tensor imaging (DTI) and single voxel MRS were performed. The DTI data were obtained using a single-shot spinecho planar image (SE-EPI) sequence (matrix, 128 × 128; field of view, 256 mm with a measured voxel size of 2.69 × 2.69 × 2.7 mm and a reconstructed voxel size of 2.00 × 2.00 × 2.7 mm; TE/TR, 90/10 ms; SENSE factor, 2; EPI factor, 67; b, 1000 mm2 s−1; NSA, 3; and slice thickness, 2.3 mm, gap, 0 mm). The diffusion sensitizing gradients were applied simultaneously along 16 non-collinear directions (b = 1000 s/mm2) as well as an acquisition without diffusion

weighting (b = 0). Re-alignment of the DTI images was performed using the diffusion registration software package provided by the manufacturer (Extended MR WorkSpace, version 2.5.3.0, Philips Medical Systems). DTT performed with the fiber assignment by continuous tracking (FACT) algorithm, which starts tracking by every single voxel and goes over all the voxels of the image volume, was used for reconstruction of these fiber tracts. Fiber tracking was terminated with a fractional anisotropy (FA) threshold of 0.20 and when the angle between two principal Eigen vectors was greater than 70° on colored FA maps. The T1weighted 3D MR images were used for anatomical reference, and regions of interest (ROIs) were drawn manually on the axial images. MRS was acquired with point-resolved spectroscopic (PRESS) sequence (TR/TE, 2000/144 ms; NSA, 128). Metabolic signals were measured at 2.0 ppm N-acetylaspartate (NAA), 3.0 ppm Creatin (Cr) and 3.2 ppm Choline (Cho). NAA/Cr, Cho/ Cr, and NAA/(Cho + Cr) ratios were also calculated for the cerebellum and for the nearby solid tissue.

Fig. 2 A large posterior fossa cyst compromising arachnoid cyst and a solid lesion nearby the left cerebellar hemisphere and arachnoid cyst which has isointense signals with cerebellum on T2 weighted (a), T1 weighted (b), and diffusion weighted (c) images

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A large posterior fossa cyst compromised of an arachnoid cyst, with isointense signals and cerebrospinal fluid on all sequences, and some mass effect adjacent to the cerebellar hemispheres were revealed in the MRI (Fig. 2). This solid lesion of the posterior fossa near the left cerebellar hemisphere and the arachnoid cyst were reported to be compatible with the previous fetal MRI and transcranial US findings. The tissue was isointense with the cerebellum. Additionally, the 3D T1W images showed a small connection with the cerebellum, suggesting that it may be related to the normal cerebellum (Fig. 2b). Upon DTI, both the cerebellum and nearby solid tissue represented the same FA values (FA, 0.331) (Fig. 3a). Tractographic studies showed a connection with fibers extending along the left cerebellar hemisphere from this tissue (Fig. 3b). The single voxel MRS of this solid tissue also revealed high Cho and a smaller NAA concentrations similar to the normal newborn cerebellum (Fig.3c). Thus, the suspicion of ectopic cerebellar tissue was confirmed with DTI and MRS findings.

In addition to these theories, the association of arachnoid cysts and CSF collection near ectopic cerebellary tissue, as in our case, emphasizes the importance of meningeal

Discussion An ectopic cerebellum is quite a rare condition and thought to be due to an error in the advent of embryogenesis and cell migration. With the exception of the common cerebellar ectopias associated with Chiari I and II malformations, ectopic cerebellar tissue may be found isolated in the middle and anterior cranial fossae, in an intraorbital location [2, 3]. Two cerebellar ectopias were previously described in frontal [4] and orbital encephaloceles [5]. Another description of cerebellar ectopia in a 21 month-old boy was in a hydromyelic dilatation of the spinal cord (myelocystocele), with a well-differentiated ectopic cerebellum, in connection with a cervicothoracic meningocele [6]. The suprasellar region has also been associated with an ectopic cerebellum, with a nasopharyngeal teratoma in a newborn [7]. Although today there are many theories, little is known about the etiology of an ectopic cerebellum. The disturbances of cell migration have been suggested to be the cause of these abnormalities, and an erroneous inductive stimulus on the undifferentiated cells of the fetal mantle layer is one possible explanation [6]. Some authors have postulated another theory about the development and maturation of ectopic cerebellar tissue which is located separately from its appropriate anatomical position, in that it arises from aberrant embryogenesis and results in the disturbed migration of primitive pluripotent stem cells. Chang et al. indicated that intrinsic intracellular signals seem sufficient to direct the development of the cerebellar cortex, independent of ascending and descending projections [7].

Fig. 3 a On DTI, both cerebellum and nearby solid tissue represented same FA values. b Tractography studies showed the connection with fibers extending along left cerebellar hemisphere from this tissue

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stimulus in the embryological process. Andrae et al. reported ectopic cerebellar tissue development with transgenic mice and suggested that the platelet-derived growth factor (PDGF) and meningeal structures can influence cerebellar development [8]. Therefore, it is possible that during the formation of the normal cerebellum, the germinal tissue migrates through the ependyma into the CSF. An ectopic cerebellum may occur in an obstruction of the normal flow of cerebrospinal fluid, with inflammation of the arachnoid membrane, resulting in a cyst. The improvements in brain imaging, with advanced neuroimaging tools, provide more information about white matter microstructures with DTI and metabolite concentrations with MRS imaging. MRS imaging has a potential to improve the diagnostic value from traditional anatomic imaging by providing a metabolic profile about the tissue. The DTT represents the white matter tracts and their extensions to the neighboring structures, and with the exception of one case, all of the ectopic cerebellum cases in the literature have gone to surgery to confirm the diagnosis. We believe that our case is an unusual aspect of the diagnosis of this malformation without surgery. In conclusion, the rare entity of an ectopic cerebellum should be considered in the differential diagnosis of discrete extra-axial masses with signal characteristics isointense to the cerebellum. In addition to conventional MR studies, DTI and MRS imaging provide useful information about the tissue organization and biochemical structure and may prevent unnecessary surgery.

Conflict of interest The authors declare that they have no conflict of interests. Financial disclosure None.

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