Fetal Head

Skull - Cranium - Face - Neck

I - Skull and cranium:

Views from higher to lower level: 

Trans-ventricular, Trans-thalamic, Trans-cerebellar view

 

Standard Landmarks of fetal cranium

 

Check the following:

·Skull bones are seen all around with normal shape of the skull

·Biparietal diameter (BPD) and Head Circumference (HC) correlates with the gestational age and other fetal measures.

The above 2 points exclude most skull anomalies and hydrocephalus.

Cranium is checked for structures from anterior to posterior:

- Line of falx cerebri is seen and not shifted to one side and structures on both sides of the falx are similar almost mirror image this excludes many brain anomalies.

- Size and shape of cerebral ventricles

- Cavum septum pellucidum

- Thalami

- Cerebellum and vermis

- Cisternal Magna < 10mm

- Nuchal fold < 6mm

The standard views used:

·Trans-ventricular view which is just above the transthalamic view the lateral cerebral ventricles are visualized with the echogenic choroid plexus seen filling the bodies of the lateral ventricles, as gestation progresses from 18 to 24 weeks the ventricles and choroid plexus appear less pronounced with the growth of the cerebral hemispheres.

Measurement of the cerebral ventricle in an axial plane should not exceed 8mm before 25 weeks gestation.

·Trans-thalamic view try to document visualization of the thalami (1) and between them is the third ventricle (2) and anteriorly a midline fluid filled cavity (larger than the third ventricle) is the cavum septum pellucidum (3) lying between the anterior horns of the lateral ventricle (4).

The BPD is taken at this level.

·Transcerebellar view is below the transthalamic view and is obtained when the cerebellum and cisterna magna are visualized. At this plane the cerebellum, cisterna magna (anteroposterior diameter from vermis to inner aspect of occipital bone should not exceed 10mm) and nuchal fold (should not exceed 6mm) are evaluated.

·Facial structures are revised orbit, nose and upper lip, with special attention to upper lip view to exclude hare lip.

 

Illustrations of Intracranial structures

Algorithm of some common skull and brain anomalies

It is important to appreciate that the DD of lesions is extremely difficult and thus it is better to describe a lesion as detailed as you can rather than suggest a definite diagnosis.

II- Fetal Face:

Usually I start in axial view from the frontal bones of the skull going down the fetal face, you will be able to spot the following structures in order from cranial to caudal:

The 2 orbits

The lenses

Nasal structures

Cheeks and zygomatic arches

Upper Alveolar ridge and upper lip

The tongue

Pharynx (epiglottis appears here as an equal sign =)

The 2 rami of the mandible

The lower alveolar ridge

The tip of the mandible

The resolution of the ultrasound is extremely important to be able to view these structures.

Then In the midsagittal view you should examine:

Frontal bone (Sloping forehead in cases of Microcephaly)

Nasal bridge (Suggested marker for Down Syndrome is prefrontal thickening)

Nasal bone (Absent or hypoplastic in Down Syndrome)

Palate in lateral view (Protruding upper lip in cases of bilateral cleft palate)

Tongue (Microglossia / Macroglossia)

Mandible (micrognathia / retrognathia)

III - Fetal Neck:

- Assure no cysts or masses

- Not routine but in the midsagittal view sometimes the fetus swallows, during swallowing movement you will be able to trace the tongue movement just assure that the tongue during swallowing never touches the posterior pharynx, because if the back of the tongue reaches the posterior pharynx this fetus has absent or cleft palate.

Some Normal Variations encountered during scan:

* Choroid plexus cyst: cystic like space seen in the choroid plexus due to entrapment of CSF in neuroepithelial fold, mostly detected during second trimester and would resolve by the third trimester without consequences. However, 10% of feti with trisomy 18 show Choroid plexus cyst (s).

* Mega cisterna Magna: This term describes an enlarged cisterna magna (>10mm) but with intact vermis, absence of hydrocephalus and normal fourth ventricle, it is considered a normal variation.

*Cavum Veil Interpositi: Small midline intrhemispheric cystic dialtation (<10mm) commonly seen in second trimester scans. It is considered a normal variation and has no genetic association.

Definitions:

*Ventriculomegaly: Enlarged ventricles (>10mm) but BPD and HC normal for gestational age

*Hydrocephalus:: Enlarged ventricles leading to fetal head enlargement (BPD and HC) for the gestational age.

*Acrania: Absence of cranial vault bones

*Anencephaly: Absence of cranial vault bones and cerebral hemispheres

*Cephalocele: Herniation of intracranial structures through a defect in the cranium.

*Meningocele: Cephalocele that contains only meninges and CSF

*Encephalocele: Cephalocele with meninges, CSF and brain tissue

*Holoprosencephaly: Group of defective formation of the forebrain due to incomplete devision of the cerebral hemispheres. It is classified to 4 types (Alobar,  Semilobar,  Lobar and Midline interhemispheric) all characteriseed by different degrees of absence of falx cerebri division of the cerebral hemispheres.

*Agenesis of the corpus callosum: There will absent cavum septum pellucidum with mild ventriculomegaly espacially affecting the posterior horn of the lateral ventricle leading to "tear drop" appearance of the lateral ventricle, there will also be 3 lines separating the cerebral hemispheres (Falx + medial surfeace of each hemisphere)

*Dandy Walker malformation: Cystic enlargment of the fourth ventricle with partial or complete agenesis of the cerebellar vemis.

*Absent Vermis: Cerbellar vermis absence will appear as a cleft separating the inferior part of the cerebellar hemispheres.

*Arachnoid cyst: Simply it is a cyst that contains CSF. It does not communicate with the ventricles, subarachnoid space and does not show any activity with doppler mode (unlike vein of gallen aneurysm)

*Chiari malformation: Involves 4 types of displacement of the cerebellum into the foramen magnum different signs are mentioned (Lemon shape of the occipital bone, Banana shape of the cerebellum) .

*Microcephaly: Head size (BPD, HC) below 3 standard deviations from the mean, other fetal measures correlate with the gestational age (FL, AC, HL..)

*Macrocephaly: Head Larger than 98th percentile without cranial anomalies.

*Hemimegalencephaly: Assymetry of the cerebral hemispheres with shift of the midline and dialtation of the lateral ventricle on the affected side.

*Lissencephaly: Smooth cortex with no sulci and gyri due to abnormal neuronal migration.

*Schizencephaly: CSF filled clefts of the cerebral hemispheres extending from the subarachnoid space to the lateral ventricle splitting the brain.

*Hydranencephaly: Replacement of the normal brain tissue with a large fluid collection covered by leptomeninges and dura, the falx cerebri is still seen..

*Porencephaly: Fluid filled area that results in destruction of previously formed brain tissue with subsequent cavitary formation. It commonly communicates with the ventricles, subarachnoid spance or both.

*Aneurysm of the vein of Gallen: Central cavitary lesion with positive doppler activity

*Other lesions as tumours (e.g. teratomas)or infection (e.g CMV) may occur