MR Imaging of the Fetal Cerebellar Vermis in Utero: Description of some Useful Anatomical Criteria for Normal Development

A Robinson, S Blaser, A Toi, D Chitayat, M Gundogan, S Laughlin, S Pantazi, G Ryan The Hospital for Sick Children Departments of Diagnostic Imaging, Clinical & Metabolic Genetics and Neuropathology Mount Sinai Hospital Departments of Medical Imaging, Prenatal Diagnosis & Medical Genetics and Obstetrics & Gynaecology The University of Toronto, Ontario, Canada

Purpose Atlas of sagittal views of fetal cerebellar vermis: normal To define easily identifiable and reproducible measurements and markers of normal anatomic development of the fetal cerebellar vermis in vivo. 17.5 weeks 19.3 weeks 19.7 weeks 20.4 weeks Embryology (what is already known) Actual appearances of ex-utero fetal specimens on MRI (adapted from Hansen 1993, Babcock 1996, Chong 1997) developing

inferior vermis smaller 4th ventricle covered 1° fissure inferior vermis fully formed & fastigial point visible inferior vermis still smaller 2° fissure appears IV ventricle (roof IV ventricle (roof IV ventricle vermis not yet visible) only just visible) roof thickens complete 21.0 weeks 21.7 weeks 22.4 weeks 23.9 weeks 11-12 weeks 13-14 weeks 16 weeks 18 weeks

Materials & Methods Retrospective analysis of midline sagittal view of 189 consecutive fetal MRI examinations was performed for CNS and non-CNS indications. Analysis included identification of the fastigium and vermian fissures , pre-pyramidal fissure pre-culmenate fissure degree of coverage (closure) of the 4th ventricle, cranio-caudal appears appears diameter, measurement & ratio of cerebellar tissue above and below the fastigium-declive line - a line drawn from the fastigium to the dorsal most point of the vermis (the declive ), and the tegmento-vermian 24.2 weeks 26.7 weeks 27.0 weeks 27.8 weeks angle. Fetuses imaged for assessment of abnormalities affecting the posterior fossa structures (n=42) were evaluated separately.

Anatomy and measurements (image adapted from Duvernoy 1995) pre-culmenate primary fissure fissure other lobules becoming visible lobules fully formed

declive 30.8 weeks 32.9 weeks 34.6 weeks 37.0 weeks

lingula folium iter tuber fastigium pre-pyramidal obex fissure

pyramid

nodule uvula secondary or post-pyramidal fissure

cranio-caudal diameter fastigium-declive line tegmento-vermian angle Results Gestational age ranged from 14.0 to 38.6 weeks with a mean of 26.7 weeks. Useful midline sagittal views were obtained in 152 studies for a total of 230 measurements. Craniocaudal diameter of the cerebellar vermis follows a first order polynomial equation with an R2 value of 0.89: diameter (mm) = 0.74 x gestational age (weeks) – 6.11 Average height above and below the fastigial point also increased linearly, with percentages above and below between 39.8% and 59.3% (average 47.9% & 52.1% respectively), and no significant change in this ratio with gestational age. The tegmento-vermian angle was almost always 0° in normal fetuses, only one normal fetus had an angle of between 6-9°.

There was linear growth of the cerebellar There was linear growth of the vermis both Conclusions vermis throughout gestation above and below the fastigium 1. The fastigium should be clearly visualized at any gestational age in

30 14.0 the normal fetuses.

12.0 th 25 2. The vermis should cover the 4 ventricle by 17-18 weeks gestation, 10.0 2 = 0.84 but initially may be slightly smaller inferiorly. 20 2 = 0.89 8.0 y = 0.39x - 3.32 R 3. There should be linear and symmetrical growth of the vermis 15 y = 0.74x - 6.11 R 6.0 2 = 0.81 throughout gestation.

Height (mm) 4.0 10 4. The ratio of vermian tissue above and below the fastigium-declive line Diameter (mm) y = 0.35x - 2.79 R 2.0 5 should be slightly below unity and not change significantly. 0.0 15 20 25 30 35 40 0 5. A slightly increased tegmento-vermian angle may be a normal variant 15 20 25 30 35 40 Gestational age (weeks) and does not necessarily predict an adverse outcome. Gestational age (weeks) Height above fastigium Height below fastigium Normals Abnormals Trendline (Normals only) Trendline (Height above fastigium) Trendline (Height below fastigium) 6. The declive and primary fissure should always visible in normal fetuses from 17.5 weeks. The other lobules (besides declive) become visible from 24 weeks and most were visible by 27 weeks. 7. The other fissures should be visible as follows: the secondary (post- The ratio of vermian tissue above:below the A slightly increased tegmento-vermian angle pyramidal) from 20 weeks, the pre-pyramidal from 21 weeks, and fastigium was, on average, slightly below may be a normal variant pre-culmenate from 22 weeks. There is therefore a delay of between 3-5 weeks between identification of these features in live fetuses unity and did not change with gestational age 60 versus fetal specimens (Chong 1997, Nakayama 1999). 1.6 50 1.4 References 1.2 40

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Angle(degrees) •Chong BW. Babcook CJ. Pang D. Ellis WG. A magnetic resonance template for normal 0.4 10 cerebellar development in the human fetus. Neurosurgery. 41(4):924-8; discussion 928- 0.2 9, 1997 Oct. 0.0 0 15 20 25 30 35 40 15 20 25 30 35 40 •Duvernoy H. The Human and Cerebellum. Springer Verlag 1995. •Hansen PE. Ballesteros MC. Soila K. Garcia L. Howard JM. MR imaging of the Gestational age (weeks) Gestational age (weeks) developing . Part 1. . Radiographics. 13(1):21-36, Normals Trendline (Normals only) Normals Abnormals 1993 Jan.