Neuroimaging 1: cranial ultrasound

1. Basic Ultrasound Physics

Ultrasound uses ultra high frequency sound pulses (3-10MHz). At these frequencies ultrasound has unusual properties:

• It travels and reflects in straight lines (like radar).
• It does not transmit through air or air-containing structures.
• It transmits very easily through liquids, and partially through solids.
• It is reflected at air-liquid interfaces, cell membranes and by subcellular particles.

The most common form of cranial ultrasound is “2-dimensional real-time imaging” using a “sector scanning” probe. A 2-dimensional image is constructed from echoes received as the probe scans rapidly to and forth. The pattern of delay in echoes received is converted into a black and white image that changes in real-time as the probe scans. The actual appearance of the image (the grey scale/contrast etc.) is constructed artificially by the software to represent something that radiologists and clinicians can recognise. Reflective areas and interfaces appear light (echogenic) whereas echolucent (i.e. fluid-filled) areas appear dark. Solid tissue produces a “grey” appearance due to an interference effect from multiple echoes.
The higher the frequency, the less tissue penetration that is possible but the higher the potential spatial resolution that can be achieved. As ultrasound transmits very poorly through bone, cranial ultrasound is only feasible through an ultrasonic window i.e. a patent fontanelle or a widely separated skull suture. In practice virtually all cranial ultrasound is performed using the anterior fontanelle. As ultrasound will not pass through air, it is essential to use a liquid or liquid gel to obtain good ultrasonic contact with the skin.

 

2. How to scan

Before placing the probe on the patient there are 6 things you need to consider outlined below.
i) Probe
Ensure the correct (smaller) probe is attached to the scanner for cranial ultrasound examination. The larger probe is used for scanning the heart. Change the probes by lifting the latch on the side of the scanner and gently removing the probe; ensure that the probe is securely latched in before starting. Take extreme care when handling the probe head and its cable – it is an incredibly delicate and expensive part of the scanner.
ii) Orientation
A side identifying mark should be recorded on each image. For consistency with standard radiological practice, coronal images should be viewed with the right side of the baby on the left hand side of the screen, and sagittal images with the nose towards the left hand side of the screen (when looking at the screen).
iii) Frequency
The scanner uses a variable frequency linear array probe. You can adjust the frequency by pressing multifunction key B. By adjusting the frequency the depth of imaging can be altered – the lower the frequency the deeper the ultrasound will penetrate but the lower the resolution; the higher the frequency ultrasound will penetrate less far but will result in a higher resolution image. For term infants use frequencies between 5-7MHz and for preterm infants 8-10MHz.

iv) Gain
Appropriate gain (sometimes known as Time Gain Compensation) settings should be used to produce a uniform echo pattern in the near and far fields. This can be achieved by adjusting the overall gain control (G) or gain sliders.
v) Depth
The depth control should be adjusted for each infant to ensure that the whole of the brain is included with optimum magnification, so that the image fills the screen.
vi) Focus
The narrowest part of the ultrasound beam will give the greatest resolution, known as the focal point. It is possible to have more than one focal point. Adjust the number of focal points and their position by pressing multifunction button E. The focal point(s) should be adjusted to the area around the ventricles and periventricular region.

 


3. Who to scan

 

4. Basic Anatomy

You should be able to recognise the following structures:

Skull bones. Highly echogenic. In the parasagittal view the anterior, middle and posterior cerebral compartments can be seen.
Lateral ventricles. Echolucent. Seen in cross-section inn coronal scans and as C-shaped structure in parasagittal scans. Each lateral ventricle has a frontal and temporal horn and an occipital pole (trigone). The germinal layer/germinal matrix lines the lateral ventricle but is not normally visible by ultrasound.
Choroid plexus. Highly echogenic because of its complex vascular structure. Lines the inner aspect of the lateral ventricle and passes down into the thalamo-caudate notch. Very variable in size and shape.
Third ventricle. Present in the midline below the level of the frontal horns and communicates with each lateral ventricle by the intraventricular foramen (foramen of Munro).
Thalamus. Sits in the bend of the C-shaped lateral ventricle.
Head of the caudate nucleus. Anterior to the thalamus below the frontal horn of the lateral ventricle. Creates the thalamo-caudate notch which is the commonest site of germinal layer haemorrhages.
Periventricular white matter. Slightly echodense regions adjacent to the lateral ventricles peripherally. Site of periventricular leukomalacia.
Sylvian fissure (lateral sulcus). Seen in the coronal section as an echodense horizontal Y shape with the pulsatile middle cerebral artery within it.
Fourth ventricle, aqueduct and cerebellum. Seen on a sagittal midline scan. The vermis of the cerebellum appears very echogenic.
Cavum septum pellucidum. Echolucent fluid-filled structure which lies in the midline between the two frontal horns. It is only seen in preterm infants when it may be very large and cause confusion. By term the fluid space obliterates forming the septum pellucidum. It is of no pathological significance.
Corpus callosum. Seen immediately above the lateral ventricles and the cavum septum pellucidum on a sagittal scan.

Anatomy of the Neonatal Brain

 

5. Standard Views

In order to standardise images it is recommended that a minimum set of images are obtained.

The following image set is recommended:

CORONAL (5 images)

 

SAGITTAL (5 images)

 

 

Measurements
Measurements of the ventricular size are very important as they may be dilated. It is important to remember that the ventricular system is a complex three dimensional organ and dilatation can occur in many directions. The standard measurement is the ventricular width (sometimes called the ventricular index). This is taken from the coronal view with both foramens of Munro and the third ventricle visualised. Make sure that the view is as symmetrical as possible. The cursors are then placed on the outer limits of the lateral ventricle and the midline. The value in millimetres should be plotted on the standard chart. More recently normalised data has been obtained from the anterior horn width and the thalamo-occipital width. This is particularly useful in the assessment of hydrocephalus. The third ventricle can be measured either from a standard coronal section or from a transverse section: the probe is placed at the level just above a line from the outer canthus of the eye to the upper point of insertion of the ear.

 

It is important to get a good symmetrical midline view. If the baby has a parenchymal haemorrhagic infarct or an evolving porencephalic cyst, the measurement should be taken from the opposite ventricle as shown below.

 

The rate of increase in ventricular size is more important than the absolute measurement.

 

 

6. Prognostic implications of ultrasound lesions

One of the first things parents want to know when they have a newborn infant on NICU is “what is going to happen later?” – will their child survive and if so, will they be disabled. Clearly the road through neonatal intensive care is a rocky one and the baby remains at risk of complications capable of affecting neurodevelopmental outcome for many weeks. The introduction of routine neonatal cranial ultrasound scanning has provided one means by which clinicians can make some predictions regarding neurodevelopmental prognosis. The condition of the baby as a whole should always be considered when addressing outcome, but it is possible to use the ultrasound findings to give more informed advice about prognosis to the parents.

Many groups worldwide have published data relating cranial ultrasound lesions in preterm infants to neurodevelopmental outcome. The studies are of differing sizes and populations but they all tend to show similar findings.

In 1979 a large prospective follow-up study of very pre-term infants (<33 weeks) was set up at University College London with the aim of defining the relationship between ultrasound lesions detected in the neonatal period and long term neurodevelopmental outcome. This next section outlines the findings. For more detailed information about outcome studies see the chapter “Neurodevelopmental Outcome” in Rennie & Roberton’s Textbook of Neonatology, editor JM Rennie, Elsevier, 2012.

Definitions

Neurodevelopmental outcome measures

“Impairment”: Any abnormality of development or neurology with or without functional implications.
“Disabling impairment”: impairment with functional disability (eg. inability to walk, sensorineural hearing loss requiring aiding, cognitive scores >2SD below the mean).
“Impairment without disability”: impairment without a detectable functional disability (eg neurological signs without functional consequences, high tone hearing loss not needing aiding, cognitive scores of 70-79 ie 1.5-2SD below the mean).
“Total impairment”: impairments both with and without disability.

Scan definitions
“Low risk”: normal scan or any ONE of the following lesions:-
• Uncomplicated GMH or IVH (unilateral or bilateral)
• Parenchymal echodensity (not proceeding to cyst formation) or flare
• Ventricular dilatation

“Intermediate risk”: TWO or more of:-
• GMH-IVH
• Parenchymal echodensity (not proceeding to cyst formation) or flares
• Ventricular dilatation
• Hydrocephalus without atrophy

“High risk”: Loss of brain tissue from any cause including:
• HPI
• Cystic PVL
• Hydrocephalus plus focal brain atrophy ie porencephalic cyst
• Generalised atrophy

Distribution of ultrasound risk categories in survivors:
Within the cohort of infants born at <33weeks gestation, of those that survived to discharge from NICU, 74% had cranial ultrasound scans classed as low risk, 19% had intermediate risk and only 7% had high risk scans.

Overall neurodevelopmental status at 8 years in survivors:
Relating the scan findings to overall neurodevelopmental status produced the following results:

 

Ultrasound risk category    
  Disabling Total
Low 6% 27%
Intermediate 24% 46%
High 74% 90%

 

Thus there is a close correlation between the risk category of the scan and the percentage of children who develop a disabling impairment. However there are a small number of infants with low risk scans who will develop disabling neuroimpairment (6%). Within this group will be some infants with apparently normal scans in the neonatal period. At the other extreme although the majorioty of infants with high-risk lesions will develop disability, about 10% of infants with high risk lesions will be normal at follow-up with no significant impairment or disability. While this is only a small percentage of the total number, it does suggest that we can give some hope to those parents whose children have severe lesions.

Schooling and cognitive development in survivors:
The need for extra provision at school matches closely with the neurodevelopmental status in the intermediate and high risk groups as shown here:

 

Ultrasound risk category Extra educational provision IQ (mean + SD)
Low 17% 100+16
Intermediate 30% 96+21
High 75% 73+25

How should ultrasound findings influence recommendations for follow-up?

a) Low risk:
• Check hearing and vision.
• Detailed follow-up is probably unnecessary and routine surveillance may be adequate. In extremely preterm infants neurodevelopmental assessment at 12 and/or 24 months may still be advised.

b) Intermediate risk:
• Check hearing and vision.
• Careful follow-up to 2 years including a structured neurological examination and developmental assessment.
• If no impairment or only subtle neurological signs at any stage, inform Community Child Development Team of increased risk of educational problems at school age.
• If disability becomes apparent, refer for specialist multidisciplinary follow-up.

c) High risk:
• Warn parents of risk of long-term neurological and developmental problems but stress the uncertainty of outcome and don’t destroy all hope.
• Check hearing and vision.
• Arrange physiotherapy assessment before discharge and referral for community physiotherapy.
• Refer to Community Child Development Team for on-going specialist multidisciplinary follow-up.
• Will probably require formal assessment of Special Educational Needs before entering school.

Summary
Neonatal cranial ultrasound scanning is a valuable tool in the management if the preterm infant. By categorising the lesions seen on ultrasound scan, useful information can be obtained about the likelihood of long-term neurodevelopmental impairment. However as always in medicine, diagnostic scan results should be interpreted with caution and in the light of the entire clinical picture. A normal or low risk scan, while reassuring, is not always associated with a normal neurodevelopmental outcome. At the other extreme a small proportion of infants with high risk scans will have little or no associated neurodisability. Knowing this we should be able to warn the parents of the increased risk of disability whilst not destroying all hope of a favourable outcome. Infants at significant risk of disability can be referred early for detailed surveillance thus allowing speciality therapy and educational input to be provided at an early stage.

 

7. Neonatal cerebral Doppler

 

The main role of Doppler cranial ultrasound in neonatal practice is for assessment of blood flow velocity in the major cerebral arteries.

The anterior cerebral artery is visualised from a sagittal scan, as it winds above the corpus callosum. The Doppler signal is obtained from the most anterior portion of the artery where the blood flow is aligned with the ultrasound beam from the anterior fontanelle.

The middle cerebral artery is insonated in the axial plane. The probe is placed on the squamous temporal bone just above and in front of the ear. With colour Doppler the circle of Willis can sometimes visualised and the middle cerebral artery runs almost directly in line with the insonating beam.

The RI is often used to obtain quantitative measurements from the cerebral vessels. Since it employs a ratio, the RI is largely independent of the insonating angle. An elevated RI tends to correlate with increased downstream vascular resistance in the cerebral circulation. Conversely a low RI correlates with a reduced downstream vascular resistance in the cerebral circulation. However it is important to realize that many other variables may influence the RI, such as the presence of a PDA causing reduced or reversed diastolic velocity, and changes in myocardial function. Hence it is important not to over interpret RI measurements.

 

It is important to remember that Doppler ultrasound is capable of measuring cerebral blood flow velocity but this is not the same as cerebral blood flow. The blood flow depends on the cross sectional area of the artery being insonated and in the brain this cannot be measured using conventional techniques. The neonatal arteries are known to be vasoactive and the diameter may change from second to second. Hence changes in blood flow velocity may not reflect changes in cerebral blood flow.

Assessment of Doppler velocity signals may be clinically useful in the following clinical situations:
Hypoxic-ischaemic encephalopathy – a reduced resistance index in the first days of life is associated with cerebrovascular vasodilatation and is associated with an increased risk of adverse outcome. Levene et al, Dev Med Child Neurol; 31: 427-434, 1989.
Patent ductus arteriosus – associated with a reduced, absent or reversed diastolic flow velocity due to “cerebral steal” through the ductus arteriosus in diastole. Reversed diastolic flow in this situation does not necessarily indicate a dangerously low cerebral blood flow.
Hydrocephalus – raised intracranial pressure may be associated with a reduced RI but it is dangerous to rely on this measure. Clinical assessment of intracranial pressure is more useful.
Focal arterial infarction – insonation of the supplying artery shows an absent Doppler signal.
Massive cerebral infarction or “brain death” – in this situation, cerebral blood flow ceases altogether because of massively raised intracranial pressure and vascular obstruction. Doppler ultrasound of the cerebral arteries shows a small “to and fro” signal with systolic and diastolic velocities opposite and equal. The prognosis is hopeless.
Aneurysm of the vein of Galen – colour Doppler shows a large multicoloured lesion in the centre of the brain. The appearances are due to high turbulent flow within the aneurysm.