MR Imaging of the Fetal Cerebellar Vermis in Utero: Description of

MR Imaging of the Fetal Cerebellar Vermis in Utero: Description of

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
cerebellum
IV ventricle (roof
not yet visible)
11-12 weeks
IV ventricle (roof
only just visible)
13-14 weeks
IV ventricle
roof thickens
16 weeks
inferior vermis smaller 4th
ventricle covered 1° fissure
& fastigial point visible
inferior vermis still smaller
21.0 weeks
21.7 weeks
pre-pyramidal fissure
appears
pre-culmenate fissure
appears
24.2 weeks
26.7 weeks
vermis
complete
inferior vermis fully formed
2° fissure appears
22.4 weeks
23.9 weeks
27.0 weeks
27.8 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 ,
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.
Anatomy and measurements (image adapted from Duvernoy 1995)
culmen
pre-culmenate
fissure
primary
fissure
central
lobule
other lobules becoming
visible
lobules fully formed
30.8 weeks
declive
32.9 weeks
34.6 weeks
37.0 weeks
lingula
folium
iter
tuber
fastigium
pre-pyramidal
fissure
obex
pyramid
nodule uvula
cranio-caudal diameter
secondary or post-pyramidal fissure
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
vermis throughout gestation
Conclusions
There was linear growth of the vermis both
above and below the fastigium
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 (postpyramidal) 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).
14.0
30
12.0
25
2
20
.74
y=0
15
.11 R
6
x
9
= 0.8
Height (mm)
Diameter (mm)
1.
10
0.84
=
.32 R
3
.39x
0
=
y
10.0
2
8.0
6.0
0.81
=
.79 R
2
x
5
= 0.3
2
4.0
y
2.0
5
0.0
15
0
15
20
25
30
35
Abnormals
25
30
35
40
Gestational age (weeks)
40
Gestational age (weeks)
Normals
20
Height above fastigium
Trendline (Height above fastigium)
Trendline (Normals only)
Height below fastigium
Trendline (Height below fastigium)
A slightly increased tegmento-vermian angle
may be a normal variant
The ratio of vermian tissue above:below the
fastigium was, on average, slightly below
unity and did not change with gestational age
60
1.6
50
Angle(degrees)
1.4
1.2
Ratio
1.0
0.8
0.6
References
40
30
20
0.4
10
0.2
0
0.0
15
20
25
30
Gestational age (weeks)
Normals
Trendline (Normals only)
35
40
15
20
25
30
Gestational age (weeks)
Normals
Abnormals
35
40
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