Head ultrasound

Head ultrasound (HUS), also called cranial ultrasound (CUS), is obtained for the diagnosis and follow-up of premature and sick neonates.

Advantages

Head ultrasound has the advantages of:

• accessibility

• mobility, i.e. bedside scanning at the NICU and neonatal ward

• requiring no sedation

• enabling serial scans, e.g. for assessing brain maturation and/or lesion evolution

• no ionizing radiation

Doppler ultrasound may also be used to visualize anterior and middle cerebral arteries ^5,6.

Disadvantages

As with all ultrasound studies, head ultrasound is highly operator-dependent. This is not a true disadvantage as such but does necessitate that the radiographer performing the examination be well-trained in the acquisition technique, know how to utilize the available ultrasound machine(s), and be knowledgeable in intracranial anatomy and pathology, so as not to miss any significant finding.

An inherent limitation of head ultrasound is that the structures that comprise the acoustic windows (see Approach below, under Technique) all eventually close.

Indications

Indications for a neonatal head ultrasound include:

• routine head ultrasound for all premature neonates

• suspicion of brain anomalies on antenatal ultrasound

• any sick neonate in whom brain pathology is implicated

• a neonate that had not been screened prenatally

Technique

Approach

For a routine scan, the anterior (bregmatic) fontanelle serves as the acoustic window. Additional acoustic windows, used for visualizing specific intracranial structures: the posterior (lambdoid) fontanelle, mastoid fontanelle, squamosal part of the temporal bone (i.e. temporal window), foramen magnum, coronal suture, and squamosal sutures.

Transducers

• a 7.5-8 MHz micro convex transducer is best suited for viewing the neonatal brain through the fontanelles while affording good depth

• a linear transducer, usually 11-12 MHz, can be used in addition, for a more detailed depiction of superficial structures and lesions

• a phased array transducer can be used if the fontanelle is small, e.g. at a later age in infancy, but has a narrower field of view and is generally less favored

Protocol

A basic routine scan is performed through the anterior fontanelle. To be sure, the brain of an extremely premature neonate (i.e. 28 weeks or less) will appear less developed than that of a term neonate, including less convoluted gyri and shallower sulci; the following should only serve as a rough guide.
On a coronal (transverse) scan, the transducer is first angulated anteriorly, then gradually rotated posteriorly. The following structures should be sought and assessed ¹:

• level of frontal lobes: frontal lobes, interhemispheric fissure, orbits

• level of frontal horns of lateral ventricles: frontal lobes, interhemispheric fissure, corpus callosum, frontal horns, cavum septi pellucidi, caudate nucleus, basal ganglia, temporal poles, Sylvian fissures

• level of the foramen of Monro: frontal lobes, interhemispheric fissure, cingulate sulcus, corpus callosum, frontal horns and cavum septi pellucidi, choroid plexus in ventricles, caudate nucleus, basal ganglia, temporal lobes, Sylvian fissures

• level of the bodies of the lateral ventricles: interhemispheric fissure, corpus callosum, bodies of lateral ventricles with choroid plexus, third ventricle, caudate nucleus, basal ganglia, temporal horns of lateral ventricles, parahippocampal gyrus, parietal lobes, temporal lobes, Sylvian fissures, midbrain, tentorium cerebelli, cerebellar hemispheres and vermis

• level of the trigones of the lateral ventricles: interhemispheric fissure, cavum vergae (in preterm neonates or as an anatomical variant), corpus callosum, trigones of the lateral ventricles with choroid plexus, parietal lobes, occipital lobes, calcarine fissure

• through the parieto-occipital lobes: cingulate sulcus, parieto-occipital fissure (well-formed in term neonates), calcarine fissure, parietal lobes, occipital lobes

• naturally, the skull should appear on all acquisitions

On a sagittal (longitudinal) scan, the transducer is positioned at the midline, then angulated all the way to the extreme right and from there gradually back to the midline. After having arrived back at the midline (i.e. the midsagittal structures are visible again), the same scan should be repeated on the left. The following structures should be assessed:

• midsagittal: cingulate sulcus, corpus callosum, cavum septi pellucidi, cavum vergae (premature neonates or variant), third ventricle, fornix, midbrain, pons, medulla, cerebellar vermis, calcarine fissure, parieto-occipital fissure, quadrigeminal plate, fourth ventricle, cisterna magna, interpeduncular cistern, Sylvian aqueduct

• extreme parasagittal plane (right/left): Sylvian fissure, insular cortex, precentral, central, and postcentral sulci

• parasagittal plane though the insula: frontal lobe, temporal lobe, Sylvian fissure, parietal lobe, occipital lobe, insular cortex and sulci (in premature neonates, the latter gradually become visible as the infant matures), precentral, central, and postcentral sulci

• parasagittal plane though the (right/left) lateral ventricle: frontal lobe, caudate nucleus, basal ganglia, thalamus, temporal lobe, cingulate sulcus, lateral ventricle - frontal horn, body, occipital horn, and temporal horn, choroid plexus; parahippocampal gyrus, cerebellar hemisphere, parietal lobe, parieto-occipital fissure, occipital lobe

Interpretation

The grey matter (cerebral corex and basal ganglia) is hypoechoic while white matter is hyperechoic on ultrasound ⁷. In preterm babies born before 24 weeks, cerebral cortex are usually smooth with only Sylvian fissures exhibited. Smooth cerebral cortex at this age should not be confused with lissencephaly. Parietooccipital fissure and cingulate sulci started to appear between 24 to 28 weeks of gestation. Branching of sulci appears at 30 weeks of gestation and continues until the baby is term ⁸.