HEAD AND BRAIN (NORMAL ANATOMY)CHAPTER 1

In the late first trimester ultrasound the cranial vault, brain tissue and lateral ventricles occupied by the choroid plexuses can be visualized.

In the anomaly scan done between 18–22 weeks of gestation the ventricular system, subarachnoid space, the diencephalic and rhombencephalic structures can be evaluated in detail.

Visualization of the ventricular atrium, the cisterna magna and the cavum septum pellucidum on the standard imaging planes excludes most of the cranial anomalies.

Meticulous cranial biometry consisting of the biparietal diameter, head perimeter, occipito-frontal diameter, ventricular width, interhemispheric distance, cerebellar transverse diameter and depth of cisterna magna is essential as this can help in diagnosing many anomalies.

Section for cranial biometry consists of the thalamus, the third ventricle and the cavum septum pellucidum (Figs 1.1 and 1.2).

Biparietal diameter: Side to side measurement from the outer table of the proximal skull to the inner table of the distal skull.

Head perimeter: The total cranial circumference, which includes the maximum antero-posterior diameter.

Occipito-frontal diameter: Front to back measurement from the outer table on both sides.

The choroid plexus should occupy the whole of the body of the lateral ventricle (Figs 1.3 to 1.5).

Upper limit of normal being 10 mm.

The width of the body, anterior horn and posterior horn of the lateral ventricle are taken. (Normal value < 08 mm, borderline 08–10 mm and > 10 mm abnormal) (Fig. 1.6).

The lateral ventricle/interhemispheric width ratio is not very sensitive for early dilatation so is not used very often for defining hydrocephalus (Fig. 1.6).

When the choroid plexus does not occupy the whole of the body of the lateral ventricle see for the measurement of the medial separation of the choroid plexus from the wall of the lateral ventricle. (Normal value < 02 mm, Borderline 02–03 mm and > 03 mm is abnormal) (Fig. 1.7).

The cerebellum is seen as a ‘W’ turned 90 degrees (Fig. 1.8).

Measure cerebellar transverse diameter.

The cerebellar transverse diameter (CTD) is measured from the edges of both cerebellar hemispheres. The CTD in mm from 14–22 weeks is equal to the gestational age of the fetus in weeks (Fig. 1.9).

Look for any hypoplasia of the superior and inferior cerebellar vermis.

Whether there is any communication between fourth ventricle and cisterna magna.

The cisterna magna is seen posterior to the cerebellar vermis and anterior to the occipital bone (Fig. 1.10).

(Normal value < 08 mm, borderline 08–10 mm and > 10 mm abnormal).

Should not be more than 10 mm.

Few strands seen traversing the cisterna magna are normal.

Carefully check for any communication between the fourth ventricle and the cisterna magna through an abnormal cerebellar vermis. If there is any communication at gestational age less than 16 weeks revaluate the fetus after 2 weeks.

Check for any posterior fossa cyst.