|ECR 2019 / C-0802|
|Knowing the art of brain ultrasound|
Findings and procedure details
In this poster we will detail how to carry out an adequate systematic technique of brain ultrasound, its limitations and its main indications in the pathology of the newborn.
-Systematic technique for the study of brain ultrasound
Brain ultrasound starts with basic grey scale imaging using a linear-array transducer through anterior fontanelle in the coronal and sagittal planes. The most relevant anatomical structures should be clearly represented in two planes. Typically, six coronal images are obtained beginning at the frontal lobes just anterior to the frontal horns and extending to the occipital lobe posterior to the lateral ventricle trigones Fig. 2. After that, the transducer is rotated 90°, and approximately five images are obtained, including a midline sagittal view of the corpus callosum and cerebellar vermis in addition to bilateral parasagittal images beginning in the midline and progressing laterally through the peripheral cortex Fig. 3. Secondly, colour Doppler images may be obtained for screening vascular structures. The arterial system is assessed for patency and resistance to flow by obtaining a colour Doppler image of the circle of Willis or pericallosal artery Fig. 4 . A spectral tracing with peak systolic velocity (PSV), end-diastolic velocity (EDV), and resistive index (RI) should be also performed. The venous system is evaluated for patency by obtaining a colour Doppler image of the sagittal sinus.The objective will be to analyze the morphology of the curve and the resistance index (RI). Premature neonates present high resistance curves with RI between 0.8 and 0.9. However, the healthy infants usually have lower resistance index (RI of 0.6-0.8). A RI between 0.6 and 0.9 can be used as an approximate value to encompass normal values for both term and preterm infants. The RI becomes lower with increases in diastolic flow as it occurs when there is cerebral vascular dilatation resulting from acute hypoxia or ischemia of any cause. The RI becomes higher with decreases in diastolic flow as in cerebral oedema.
Power-doppler is useful in the evaluation of areas of hyper or hypovascularity due to vascular occlusion, ischemia or infarction.
It is recommended to use curved transducers with a frequency between 7 and 9 MHz and in a small size, since they fit better to the fontanelle. Although in infants, a larger sectoral probe with a lower frequency may be necessary (5-8 MHz). Linear transducers with high frequency (10-18 MHz) are better in the evaluation of the extracerebral space and the sulcation brain and cerebellar pattern.
-Main indications in pathology of neonatal brain
1. Intracranial haemorrhage
- Germinal matrix haemorrhage (GMH)
Periventricular haemorrhage, preterm caudothalamic haemorrhage or intracranial haemorrhage grade 1 are also names for GMH. These haemorrhages occur in the highly vascular but also stress sensitive germinal matrix, which is located in the caudothalamic groove. This is the subependymal region between the caudate nucleus and thalamus.
The germinal matrix is present as a region of thin-walled vessels, migrating neuronal components and vessel precursors. It has matured by 34 weeks gestation, such that haemorrhage becomes very unlikely after this age.
Most GMHs occur in the first week of life. GMHs start in the caudothalamic groove and may extend into the lateral ventricle and periventricular brain parenchyma. These haemorrhages are subsequently found on follow-up ultrasounds and most of the patients are asymptomatic or demonstrate subtle signs that are easily overlooked Fig. 5
-Intracranial haemorrhage grade 2
It consists in a haemorrhage placed in the caudothalamic groove that extends into the ventricular system without ventriculomegaly Fig. 6
-Intracranial haemorrhage grade 3
Haemorrhage extends into the ventricular system with acute dilatation of one or both ventricles Fig. 7
-Intracranial haemorrhage grade 4
These grade 4 haemorrhages are venous haemorrhagic infarctions, which are the result of compression of the outflow of the veins by the subependymal haemorrhage. Previously, they were thought to be the result from subependymal bleeding into the adjacent brain Fig. 8 Venous infarctions resolve with cyst formation that can merge with the lateral ventricle and results into a porencephalic cyst. Fig. 9
In general, grade 1 and 2 haemorrhage have a good prognosis whereas grade 3 and 4 have variable long-term deficits. Outcome in grade 3 haemorrhages is often good if no parenchymal injury has occurred.
2. Cerebellar hemorrhage
It is relatively frequent in the most premature patients and its appearance worsens the neurological prognosis. They are diagnosed through the anterior fontanel when they have a considerable volume, but it is more appropriate to use the mastoid window because of their greater sensitivity.
3. Subarachnoid haemorrhage
It presents as an increased echogenity on the sulci and widening of the horizontal portion of the Sylvian fissure. However, it can only be detected if the quantity of blood is large, so CT is the preferred modality of choice Fig. 10
4. Subdural and epidural haemorrhage
Both of them are more common in term infants due to trauma. The imaging findings include an elliptical of lineal fluid collection between the brain and the skull and sometimes mass effect over the anatomical structures. Differentiation between subdural an epidural haemorrhage is often difficult through brain ultrasound so CT is the key for diagnosis Fig. 10
Post-hemorrhagic hydrocephalus is the most frequent type of hemorrhage in preterm infants as a complication of intraventricular hemorrhage. Brain ultrasound is essential in its diagnosis and management as well as in the follow-up while the fontanelle is still open. The role of the radiologist will be:
1.To identify dilated ventricles and intraventricular haematic deposits as well as its quantity and distribution.
2. To measure the size of the ventricular system and quantify the degree of dilation.
The measurement of the ventricular system should be done in an easy and reproducible sonographic plane. A coronal section through the lateral ventricles slightly posterior to the foramen of Monro where 3 echogenic dots representing the choroid plexus of the lateral ventricles and the roof of the third ventricle should be obtained. Furthermore, a symmetrical image of the Sylvian fissure on both sides and the hippocampus should be also shown Fig. 11.
In order to quantify the degree of dilatation Levene index can be used up 40 weeks of gestational age. It measures the distance between the interhemispheric lens and the external edge of the lateral ventricle in a coronal view through the anterior fontanelle, at the level of the foramina of Monro. When the distance is 4 mm above the 97th percentile, the treatment of hydrocephalus is considered. The Levene index is performed for the left and right side and can be compared to the reference curve which is quite useful for further follow-up.
The ventricular index or frontal horn ratio should be used after 40 weeks. It is calculated by a ratio of the distance between the lateral sides of the ventricles and the biparietal diameter.
Another way of quantify the degree of dilatation of the ventricular system is measuring the width of the frontal horn Fig. 12
-Normal: less than 3 mm.
-Mild: between 6-9mm
-Severe: more than 10mm.
A ventricular dilatation is also considered when thalamo-occipital distance is greater than 24 mm Fig. 12
3. To evaluate the progressive increase of the extracerebral space and monitor the ventricular size in cases treated with derivations.
4. To detect complications such as entrapment of the fourth ventricle or associated parenchymal involvement.
5. To assess permeability of the aqueduct of Silvio by Doppler.
6. To detect children with severe alteration of intracranial compliance and at risk of developing intracranial hypertension.
6. Hypoxic-Ischemic Encephalopathy
In the preterm, child hypoxic-ischemic encephalopathy is also known as periventricular leukomalacia (PVL) and affects the periventricular zones. It is considered a white matter disease.
Ischemia was thought to be the single cause of PVL, but nowadays, it is known that other causes such as infection or vasculitis play an additional role.
PVL presents as areas of increased periventricular echogenicity. Normally, the echogenicity of the periventricular white matter should be less than the echogenicity of the choroid plexus. Physiologically, preterm infants have a periventricular echogenicity discreetly increased in the first 7-10 days of life. However, if the hyperechogenicity is persistent, superior to the echogenicity of the plexuses, heterogeneous or markedly diffuse, it is considered pathological.
Detection of PVL is essential because a significant percentage of surviving premature infants with PVL develop cerebral palsy, intellectual impairment or visual disturbances.
More than 50% of infants with PVL or grade III haemorrhage develop cerebral palsy.
It can be classified in 4 grades Fig. 13
-Grade I: persistent periventricular hyperechogenicity.
-Grade II: cysts located next to the lateral ventricles.
-Grade III: large cysts in white frontoparietal and occipital white matter.
-Grade IV: extensive cysts with diffuse subcortical extension.
Patients with a greater degree of PVL will have worse long-term neurological prognosis.
In term infants, hypoxic-ischemic encephalopathy may happen as a consequence of asphyxia. When hypoxic-ischemic encephalopathy is suspected the first neuroimaging test should be a brain ultrasound. If the patient is treated with hypothermia, a brain ultrasound will be performed every 24 hours during the 3 days of treatment.
The brain ultrasound will detect signs of prenatal brain damage or pathology unnoticed in fetal ultrasounds and will assess which pattern of hypoxic-ischemic encephalopathy predominates:
· Peripheral: focal or diffuse periventricular hyperechogenicity Fig. 13
· Central: gangliothalamic involvement which implies worse prognosis.
In mild ischemic hypoxic encephalopathy, a focal or diffusely echogenic cortex, moderate-severe oedema, poor definition of the corticosubcortical bundle, and ventricles with a slit form are seen.
Late imaging findings of hypoxic ischemic encephalopathy include parenchymal atrophy, increased of the ventricular size, and cystic encephalomalacia.
The basal ganglia and thalamus are very vulnerable to ischemia and are shown as focal or diffusely echogenic.
An important association has been demonstrated between the increase of the echogenicity in parenchymal and the development of motor or intellectual deficits. Neurological sequelae occur in approximately 90% as opposed to 10% who have a normal brain parenchyma and associate neurological involvement.
7. Congenital brain malformation
Protocolized fetal ultrasound performed during pregnancy is able to detect cerebral malformations in the prenatal period, so many times the postnatal brain ultrasound has the function of confirming the prenatal findings.
The agenesis of corpus callosum (ACC) is one of the most frequent malformations and, therefore, its imaging findings must be known by radiologists Fig. 14 . ACC is characterized by:
1. A morphology on bull or tear horns of the lateral ventricles.
2. An absence of cavum septi pellucidum.
3. Bilateral colpocephaly with parallel disposition of both lateral ventricles.
4. An absence of corpus callosum and cingulate groove in the sagittal plane.
The presence of cortical malformations can also be suspected by brain ultrasound.
In cases of polymicrogyria, the Silvio fissures are thickened and heterotopias may be seen as subependymal nodules in the lateral ventricles.
In the evaluation of post-anterior fossa malformations brain ultrasound is used as the first diagnostic approach for evaluation of ventriculomegaly or other associated anomalies.
8. Normal variants
Cavum septi pellucidi, cavum vergae and cavum of the velum interpositum are well known variants. The more premature the baby, the more frequently these cavities are present. They can persist until adulthood Fig. 15
The cavum septi pellucidi is a normal variant of cerebral fluid space seen in between the lateral ventricles.
The cavum of the velum interpositum presents as a cyst-like structure in the region of the tectum with a helmet shape. It can easily be confused with a subarachnoid cyst or a cyst of the pineal gland, so take into consideration this entity is important in order to avoid diagnostic errors.
The cavum vergae is the posterior extension of the cavum septum pellucidum. It is posterior to the anterior columns of the fornix and lies anterior to the splenium of the corpus callosum.
Another common variant is the choroid plexus cyst which is an incidental finding without clinical consequences.
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