State of the Art Cranial Ultrasound Imaging in Neonates

Summary

Cranial ultrasound (CUS) is a valuable tool for brain imaging in critically ill neonates. This video shows a comprehensive approach for neonatal (Doppler) CUS for both clinical and research purposes, including a bedside demonstration of the technique.

Abstract

Cranial ultrasound (CUS) is a reputable tool for brain imaging in critically ill neonates. It is safe, relatively cheap and easy to use, even when a patient is unstable. In addition it is radiation-free and allows serial imaging. CUS possibilities have steadily expanded. However, in many neonatal intensive care units, these possibilities are not optimally used. We present a comprehensive approach for neonatal CUS, focusing on optimal settings, different probes, multiple acoustic windows and Doppler techniques. This approach is suited for both routine clinical practice and research purposes. In a live demonstration, we show how this technique is performed in the neonatal intensive care unit. Using optimal settings and probes allows for better imaging quality and improves the diagnostic value of CUS in experienced hands. Traditionally, images are obtained through the anterior fontanel. Use of supplemental acoustic windows (lambdoid, mastoid, and lateral fontanels) improves detection of brain injury. Adding Doppler studies allows screening of patency of large intracranial arteries and veins. Flow velocities and indices can be obtained. Doppler CUS offers the possibility of detecting cerebral sinovenous thrombosis at an early stage, creating a window for therapeutic intervention prior to thrombosis-induced tissue damage. Equipment, data storage and safety aspects are also addressed.

Introduction

Since its clinical introduction in the late 1970’s cranial ultrasound (CUS) has been widely used for detecting congenital anomalies and acquired brain lesions during the neonatal period. In many neonatal intensive care units (NICUs), CUS has become indispensable in the care for critically ill neonates. Major advantages are its relatively low cost and the fact that it can be performed at bedside, even when a patient is unstable. In addition it is radiation-free and allows for serial imaging. Another technique often used for neuroimaging in critically ill neonates is magnetic resonance imaging (MRI). MRI provides excellent image quality, but its clinical use in NICU’s is currently limited because of logistic and safety issues¹.

Over time, quality of CUS has drastically improved, with advancing technique leading to higher resolution, faster image processing and digital display and back-up. Important brain structures can be adequately visualized using optimal settings. Traditionally, images are obtained through the anterior fontanel. This approach is less suitable for evaluation of infratentorial structures because they are located far away from the transducer and the highly echoic tentorium impedes their assessment. Use of high-frequency linear transducers through alternative acoustic windows and adapted settings also provides access to these brain regions. Examples of these supplemental acoustic windows are the lambdoid (posterior), mastoid and lateral (temporal) fontanels. So far, however, only few NICUs use these additional acoustic windows routinely^2-5. Doppler techniques can be used for screening patency of intracranial vessels. Flow velocities and indices in cerebral arteries can also be obtained. Some manufacturers now provide hardware to visualize flow around 2 cm/sec (Raets, et al., unpublished data). Small vessels are well displayed: medullary trunks and channels, subependymal veins tributing to the thalamostriate veins, and perforator arteries.

We present our approach of neonatal CUS, focusing on the use of different transducers, multiple acoustic windows and Doppler techniques. Neonatologists and radiologists use this approach in daily clinical practice but is also suitable for research purposes. In the practical part of the video we demonstrate bedside use in the NICU.

Protocol

NOTE: This protocol follows the guidelines of the local human research ethics committee.

1. General Considerations

NOTE: General considerations regarding equipment, data storage and safety are addressed in the Discussion.

1. Obtain images using a high-resolution, real-time, mobile 2D ultrasound machine with multiple transducers with a band of frequencies (see Discussion). Typically, obtain images of good quality using a probe with a frequency of 7.5 to 8.5 MHz.

2. Preparation of the CUS Exam

1. Schedule the CUS examination so that it does not coincide with other procedures such as blood sampling.
2. Ensure that a health care worker or a parent is available to support and/or comfort the neonate during the examination, using strategies such as those according to the principles of Newborn Individualized Developmental Care and Assessment Program⁶.

3. Examination Through Anterior Fontanel

1. Install the ultrasound machine along the incubator or cot.
2. Apply transducer gel to the probe to ensure good contact between the probe and the skin. Consider warming the gel before use.
3. Start imaging through the anterior fontanel with a convex probe in B-mode. Place the probe in the middle of the fontanel with the marker on the probe turned to the right side of the neonate. The left side of the brain will then be displayed on the right side of the monitor.
   NOTE: Imaging through the anterior fontanel can be performed with the neonate in any position³. For research purposes it may be necessary to strive for a standard head position.
   1. Record images in at least five coronal and five sagittal planes. In the first image adjust depth, gain and time gain compensation settings to produce an image filling the sector, containing the cranial contours, avoiding too bright or dark images and aiming for an equilibrium between reflections from nearby and deeper structures.
   2. Coronal planes
      NOTE: Try to obtain perfectly symmetrical images. When lesions near the frontal lobe convexity are suspected, consider recording specific oblique coronal sections, so that one hemisphere is displayed in better detail (Figure 1).
      1. For the coronal prefrontal image, angle the probe forwards to visualize the frontal lobes, anterior to the frontal horns of the lateral ventricles at the level of the olfactory sulci.
      2. For the coronal image at the level of Monro, angle the probe to visualize the coronal section anterior to the tela choroidea to depict the frontal horns of the lateral ventricles, cavum septi pellucidi, corpus callosum, sulcus cinguli. Note the echogenicity of parts of the basal ganglia.
      3. For the coronal image at the level of thalamus, angle the probe backwards to identify the lateral fissures, tela choroidea in the roof of the third ventricle and temporal lobes. Note the echogenicity of the thalamus (especially ventrolateral nuclei) in relation to the basal ganglia. Note that network injury in pulvinar may be visualized in an extra coronal section just in front of the atria.
      4. For the coronal image at the level of atria, visualize the lateral ventricles at the level of the choroid plexus. Identify the temporal lobes and cerebellar hemispheres. Note the echogenicity of periventricular white matter compared to choroid plexus. Compare optic radiation with the normal hyperechoic areas above and lateral to the atria in preterm neonates.
      5. For the coronal parieto-occipital image, angle the probe backwards to the level of the parieto-occipital sulcus to identify the parietal and occipital lobes.
   3. Sagittal planes
      1. Rotate the probe 90° with the marker on the probe facing towards the neonate’s face. The anterior part of the brain will be displayed on the left side of the monitor. Record images at the level of the following structures (Figure 2).
      2. For the midsagittal image, visualize the corpus callosum, cavum septi pellucidi (CSP), third and fourth ventricle, vermis, cisterna magna, pons and mesencephalon. Note the presence of cavum Vergae and cavum veli interpositi⁷.
      3. For the parasagittal image through one gangliothalamic ovoid (e.g., the right), angle the probe sideways for a parasagittal view through the lateral ventricle. Identify the choroid plexus and note the echogenicity of thalamus and basal ganglia. The scanned side for parasagittal planes should be appropriately indicated with text tools.
      4. For the parasagittal insular image, angle the probe further lateral through the insula. Identify the lateral fissure and frontal-, temporal-, parietal- and occipital lobes.
      5. Repeat parasagittal images for the contralateral side (i.e., the left).
   4. Color Doppler
      1. Continue imaging through the anterior fontanel with a convex probe using color Doppler. Consider evaluating flow velocities in cerebral arteries and veins and obtaining derived indices.
         NOTE: Resistive index (RI) is defined as peak systolic velocity – end diastolic velocity / peak systolic velocity. RI is angle-independent, absolute velocity values are not^8-10. RI is not similar in arteries of different caliber. Serial measurements are only useful if performed in the exact same location of the same vessel.
   5. Record images in coronal planes of the following vessels (Figure 3):
      1. Visualize the transverse sinuses at the level of the cerebellum. If only one or none the transverse sinuses is visualized, try lowering the pulse repetition frequency (PRF). If then still only one or none of the transverse sinuses can be identified through the anterior fontanel, use a high frequency linear probe for visualization through the mastoid fontanel (see section 4.4.2).
      2. Visualize the circle of Willis with internal carotid arteries, middle cerebral arteries and anterior cerebral arteries at the level of the frontal horns of the lateral ventricles. Distinguishing the left and right anterior cerebral arteries is often challenging, but is usually unnecessary. Identify the striatal candelabra of arteries.
      3. Angle the probe backwards to visualize the basilar artery with adjacent jugular veins.
      4. Angle even more backwards to visualize the internal cerebral and thalamostriate veins.
   6. Record an image in the sagittal plane of one anterior cerebral artery (Figure 4). Assess flow velocity and RI at a specific part of this vessel (usually below the genu of the corpus callosum). Near the midline velocities in the internal cerebral vein can be readily measured.
   7. Using a high frequency linear probe in a coronal plane in the anterior fontanel, identify the superior sagittal sinus. If this should fail, reduce the amount of pressure applied with the probe to the fontanel.
      NOTE: The linear probe can be used for detailed visualization of superficial structures (meninges, arachnoid and subdural space, cortex). Tangential vessels are in the subarachnoid space. Ideally, Doppler imaging as described in the previous steps will be performed during the first CUS examination of the neonate. During follow-up examination some of the steps may be skipped. In case of suspected cerebral sinovenous thrombosis Doppler imaging as described in steps 3.3.5.1, 3.3.7 and 4.4.2 should be performed.

4. Examination Through Alternative Acoustic Windows

1. Next, continue examination through alternative acoustic windows.
2. Consider recording images through the lambdoid (posterior) fontanel using a convex probe (Figure 5). The posterior fontanel is located at the junction of the sagittal and lambdoid sutures^3,11. Image through the posterior fontanel by placing the neonate in the lateral decubitus position.
   NOTE: In many premature infants satisfactory images may also be obtained through the posterior aspect of the sagittal suture with the infant in a supine position³.
   1. Position the probe in the middle of the posterior fontanel for a sagittal view. Angle the probe slightly off midline to identify the body of the lateral ventricle and its occipital horn. Turn the probe roughly 90° to obtain a coronal view. Identify the occipital horns of the lateral ventricles.
3. Consider recording images through the lateral (temporal) window using a convex or linear probe above the ear (Figure 6).
   1. If necessary, obtain images through the lateral window to allow a detailed view of the brainstem¹². Place the probe horizontally above and slightly in front of the ear. Move the probe until the cerebral peduncles are visualized.
      NOTE: Other structures that can be identified are the third ventricle, aqueduct and temporal lobes. Using color Doppler, the circle of Willis can be visualized.
4. Record images through the mastoid fontanel (Figure 7). The mastoid fontanel is located behind the ear, at the junction of the temporal, parietal and occipital bones⁴. Image through the mastoid fontanel by placing the neonate in a lateral decubitus position³.
   NOTE: In our experience, neonates often show signs of discomfort when images through the mastoid fontanel are obtained. Therefore, it would be best to do this after imaging through the anterior fontanel and other acoustic windows. We hypothesize that this discomfort could be caused by the mechanism of auditory response to pulses of radiofrequency energy¹³.
   1. Image through mastoid fontanel using a convex probe. Place the probe parallel to the ear to obtain a coronal view. Sweep the probe back and forth to identify the cerebellar hemispheres, vermis, third and fourth ventricle, pons and cisterna magna. In small preterms the contralateral cerebellar hemisphere may be well depicted.
   2. Image through mastoid fontanel using a linear probe. If (one of) the transverse sinuses cannot be identified through the anterior fontanel, use a high frequency linear probe for visualization through the mastoid fontanel. Place the probe parallel to the ear lobe to obtain a coronal view.
      1. Identify the cerebellar hemisphere and fourth ventricle. Using color Doppler, identify the transverse and sigmoid sinus, tentorial sinus and emissary veins.
5. Consider additional visualization of the posterior fossa through foramen magnum¹⁴.

Results

Examples of imaging made according to the described protocol are presented in Figures 1-7. Images should be carefully interpreted by an experienced observer. Symmetrical imaging is necessary for adequate interpretation of coronal images made through the anterior fontanel (Figure 1). Any suspected lesion should be visualized in both a coronal and (mid)sagittal plane or by visualization through an acoustic window other than the anterior fontanel. Use Color Doppler for visu...

Discussion

We describe and demonstrate a state-of-the-art approach for neonatal Doppler CUS. In experienced hands, this is an excellent tool for safe, serial bedside neonatal brain imaging. In many NICUs the described possibilities are not optimally used. Adding Doppler studies allows for screening of patency of intracranial arteries and veins. Flow velocities can be evaluated and indices obtained. Doppler CUS permits detection of cerebral sinovenous thrombosis at an early stage in the fragile transverse to sigmoid sinus angle, all...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
MyLab 70	Esaote (Genoa, Italy)		Ultrasound system