Materials & MethodsRetrospective 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 , degree of coverage (closure) of the 4th ventricle, cranio-caudal 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 angle. Fetuses imaged for assessment of abnormalities affecting the posterior fossa structures (n=42) were evaluated separately.

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

PurposeTo define easily identifiable and reproducible measurements and markers of normal anatomic development of the fetal cerebellar vermis in vivo.

Conclusions1. The fastigium should be clearly visualized at any gestational age in

the normal fetuses.

2. The vermis should cover the 4th ventricle by 17-18 weeks gestation, but initially may be slightly smaller inferiorly.

3. There should be linear and symmetrical growth of the vermis throughout gestation.

4. The ratio of vermian tissue above and below the fastigium-declive line should be slightly below unity and not change significantly.

5. A slightly increased tegmento-vermian angle may be a normal variant and does not necessarily predict an adverse outcome.

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-pyramidal) from 20 weeks, the pre-pyramidal from 21 weeks, and pre-culmenate from 22 weeks. There is therefore a delay of between 3-5 weeks between identification of these features in live fetuses versus fetal specimens (Chong 1997, Nakayama 1999).

References•Babcook CJ. Chong BW. Salamat MS. Ellis WG. Goldstein RB. Sonographic anatomy of the developing cerebellum: normal embryology can resemble pathology. American Journal of Roentgenology. 166(2):427-33, 1996 Feb.•Chong BW. Babcook CJ. Pang D. Ellis WG. A magnetic resonance template for normal cerebellar development in the human fetus. Neurosurgery. 41(4):924-8; discussion 928-9, 1997 Oct.•Duvernoy H. The Human Brainstem and Cerebellum. Springer Verlag 1995.•Hansen PE. Ballesteros MC. Soila K. Garcia L. Howard JM. MR imaging of the developing human brain. Part 1. Prenatal development. Radiographics. 13(1):21-36, 1993 Jan.

ResultsGestational age ranged from 14.0 to 38.6 weeks with a mean of 26.7 weeks. Useful midline sagittal views were obtained in 152studies 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°.

Atlas of sagittal views of fetal cerebellar vermis: normal

inferior vermis smaller 4th

ventricle covered 1° fissure & fastigial point visible inferior vermis still smaller

inferior vermis fully formed2° fissure appears

pre-pyramidal fissure appears

pre-culmenate fissure appears

other lobules becoming visible lobules fully formed

17.5 weeks 19.3 weeks 20.4 weeks

21.0 weeks

24.2 weeks 26.7 weeks 27.0 weeks 27.8 weeks

30.8 weeks 32.9 weeks 34.6 weeks 37.0 weeks

19.7 weeks

21.7 weeks 22.4 weeks 23.9 weeks

Anatomy and measurements (image adapted from Duvernoy 1995)

fastigium-declive line

lingula

central lobule

culmen

declive

folium

tuber

pyramid

uvulanodule

primary fissure

secondary or post-pyramidal fissure

pre-pyramidal fissure

pre-culmenate fissure

iter

obex

fastigium

11-12 weeks 13-14 weeks 16 weeks 18 weeks

developing cerebellum

IV ventricle (roof not yet visible)

IV ventricle (roof only just visible)

IV ventricle roof thickens

vermis complete

Embryology (what is already known)Actual appearances of ex-utero fetal specimens on MRI (adapted from Hansen 1993, Babcock 1996, Chong 1997)

tegmento-vermian anglecranio-caudal diameter

A Robinson, S Blaser, A Toi, D Chitayat, M Gundogan, S Laughlin, S Pantazi, G RyanThe 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 & GynaecologyThe University of Toronto, Ontario, Canada

A slightly increased tegmento-vermian angle may be a normal variant

0

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15 20 25 30 35 40

Gestational age (weeks)

Angl

e(de

gree

s)

Normals Abnormals

The ratio of vermian tissue above:below the fastigium was, on average, slightly below

unity and did not change with gestational age

0.0

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0.8

1.0

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Gestational age (weeks)

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Normals Trendline (Normals only)

There was linear growth of the cerebellar vermis throughout gestation

y = 0.74x - 6.11 R2 = 0.89

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Gestational age (weeks)

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met

er (m

m)

Normals Abnormals Trendline (Normals only)

There was linear growth of the vermis both above and below the fastigium

y = 0.35x - 2.79 R2 = 0.81

y = 0.39x - 3.32 R2 = 0.84

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Gestational age (weeks)

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ght (

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Height above fastigium Height below fastigiumTrendline (Height above fastigium) Trendline (Height below fastigium)