(Radiographics. 2001;21:943-955.)
© RSNA, 2001
A New View of the Neonatal Brain: Clinical Utility of Supplemental Neurologic US Imaging Windows1
Donald N. Di Salvo, MD
1 From the Department of Radiology, Brigham and Women’s Hospital, Harvard Medical School, 75 Francis St, Boston, MA 02115. Received October 26, 2000; revision requested January 5, 2001 and received February 21; accepted February 22. Address correspondence to the author (e-mail: ddisalvo@partners.org).
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Abstract
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Anterior fontanelle imaging has traditionally been the mainstay of neonatal cranial ultrasonography (US). However, this traditional approach has limited diagnostic accuracy, which has led to increasing use of two alternate neurologic US imaging techniques: posterior fontanelle imaging and mastoid fontanelle imaging. These alternate techniques can help detect pathologic conditions and structural malformations in the neonatal brain. Posterior fontanelle imaging allows improved detection of intraventricular hemorrhage. This technique better demonstrates subtle differences in echogenicity between clot and a choroid glomus and depicts clot extending into the occipital and temporal horns. Mastoid fontanelle imaging is particularly useful in detecting hemorrhage involving the brainstem, cerebellum, and subarachnoid cisterns. It greatly facilitates clot detection in the fourth ventricle and cisterna magna because the tissues surrounding these structures are normally echogenic. Mastoid fontanelle imaging can also help distinguish holoprosencephaly from aqueductal stenosis and identify small malformations of the posterior fossa. Color Doppler US may help identify normal variants such as calcar avis and lobular choroid plexus, and adjunct magnetic resonance imaging can help distinguish normal structures from a true Dandy-Walker variant. Use of posterior fontanelle imaging and mastoid fontanelle imaging can significantly augment the diagnostic power of neurologic US.
Index Terms: Brain, abnormalities, 10.14 • Brain, anatomy, 10.92 • Brain, hemorrhage, 10.367 • Brain, hydrocephalus, 10.145 • Brain, US, 10.1298 Skull, anatomy, 10.92 • Skull, US, 116.1298 • Ultrasound (US), in infants and children, 10.1298
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SUPPLEMENTAL MATERIAL
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Movie clips to supplement this article are available online at radiographics.rsnajnls.org/cgi/content/full/21/4/943/DC1.
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Introduction
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Neonatal cranial ultrasonography (US) has traditionally relied on the anterior fontanelle as the primary acoustic window. This approach displays supratentorial anatomy and disease processes very well; however, its accuracy in demonstrating posterior fossa abnormalities is limited, due primarily to the increased distance between the neonatal cerebellum and the transducer. The resulting poor delineation of the infratentorium means that potentially significant brainstem or cerebellar hemorrhage, subarachnoid hemorrhage, and Dandy-Walker variants may be overlooked. Two alternate neurologic US imaging techniques have been emphasized in the recent literature in an attempt to redress this problem: imaging through the posterior fontanelle for improved detection of small intraventricular hemorrhage (1) and through the mastoid fontanelle for improved visualization of the brainstem, cerebellum, and subarachnoid cisterns (2,3).
In this article, we discuss and illustrate imaging technique in US of the neonatal brain as well as normal brain anatomy, normal variants (calcar avis, lobular choroid plexus, foramen of Magendie), and pathologic conditions (intraventricular, subarachnoid, and brainstem hemorrhage; congenital anomalies [aqueductal stenosis, Dandy-Walker malformation]). Throughout the article, we emphasize the clinical utility of posterior and mastoid fontanelle imaging in achieving greater diagnostic accuracy.
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Imaging Technique
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Optimal imaging of the neonatal brain requires a good US window, which in most preterm and many full-term infants includes the unfused cranial sutures and fontanelles. The anterior, posterior, and mastoid (posterolateral) fontanelles are particularly useful (Fig 1). We generally use phased-array, multiple-foci 5.0- or 7.0-MHz sector transducers (XP; Acuson, Mountain View, Calif). The proximity to the brainstem and posterior fossa afforded by the alternate windows (ie, the posterior and mastoid fontanelles) allows the use of higher-frequency probes in these areas, thereby increasing resolution.
The anterior fontanelle, located at the junction of the coronal and sagittal sutures, is the largest fontanelle and typically fuses by 12 months of age. Although anterior fontanelle imaging can be performed with the neonate in any position, imaging through the posterior and mastoid fontanelles is facilitated by placing the infant in the lateral decubitus position. In many premature infants, however, satisfactory images of the occipital horns may be obtained through the posterior aspect of the sagittal suture with the infant in the supine position.
The midline posterior fontanelle, located at the junction of the lambdoid and sagittal sutures, closes somewhat earlier (by about 3 months of age). It can be located with palpation along the midline cranium several fingerbreadths above the external occipital protrusion (inion). Correct placement of the transducer at this location with subsequent tilt of the imaging plane slightly off midline will demonstrate the lateral ventricular trigone with its occipital horn in the near field (Fig 2; see also Movie 1 at radiographics.rsnajnls.org/cgi/content/full/21/4/943/DC1). A choroid glomus is visible, as are its two anterior extensions into the ventricular body and temporal horn. The occipital horn, being devoid of choroid plexus, should be completely anechoic. Rocking the transducer into left and right angled parasagittal planes will display each occipital horn. Because the occipital horns are more lateral than the frontal horns, a slight rotation of the transducer with its caudal end slightly further away from the midline may be needed for optimal imaging. Slight elevation of the infant’s head (with either a folded cloth or the "nonimaging" hand) is frequently needed to visualize the "upper" ventricle (eg, the left lateral ventricle when the infant is in the right lateral decubitus position). Next, transverse images of both occipital horns are obtained by rotating the transducer into the axial plane. A sweep of the entire trigone from cephalic (ventricular bodies) to caudal (occipital horn) can then be performed to look for evidence of intraventricular clot. This plane is especially useful for detecting dependently layering clot.
The mastoid (posterolateral) fontanelle is located at the junction of the squamosal, lambdoidal, and occipital sutures. This fontanelle may not fuse until 2 years of age. This broad fontanelle blends rostrally into the squamous portion of the temporal bone, which is itself a good US window. The goal of mastoid fontanelle imaging is clear depiction of the brainstem and posterior fossa (Fig 3; see also Movie 2 at radiographics.rsnajnls.org/cgi/content/full/21/4/943/DC1). The brainstem (thalami, midbrain, third ventricle, aqueduct of Sylvius, basilar [perimesencephalic] cistern) is best imaged with the US trans-ducer angled semicoronally and placed just anterior to the external auricle. Imaging in a similar plane with the probe placed just behind the pinna of the ear demonstrates the posterior fossa structures (cerebellar hemispheres and vermis, fourth ventricle, tentorium, cisterna magna). The normal US appearance of these structures has been described (4,5) and will be briefly summarized and illustrated in the following section. Two in utero US images of the relevant anatomy in a third-trimester fetus are included to emphasize the similarity between mastoid fontanelle imaging and obstetric imaging (Fig 4).
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Normal Anatomy
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Brainstem
The major anatomic structures that can be seen in the brainstem area are the thalami, midbrain, third ventricle, aqueduct of Sylvius, and perimesencephalic cistern. At US, the thalami appear as a hypoechoic, inverted heart-shaped structure. The midbrain, consisting of cerebral peduncles and corpora quadrigemina, are paired hypoechoic lenticular structures contiguous with and immediately caudal to the thalami. The third ventricle appears as an anechoic, slitlike cleft between the thalami, whereas the aqueduct in its normal state manifests as an echogenic line or occasionally as a thin, anechoic slit in the midbrain. The perimesencephalic basilar cistern appears as a thin, echogenic curvilinear structure surrounding the midbrain.
Posterior Fossa
The major anatomic structures seen in the posterior fossa at US include the cerebellar hemispheres and vermis, fourth ventricle, cisterna magna, and tentorium. The cerebellar hemi-spheres are ovoid, hypoechoic lateral masses just caudal to the echogenic tentorium, whereas the vermis appears as an echogenic midline structure between the fourth ventricle and the anechoic cisterna magna. The fourth ventricle manifests as an anechoic, semilunar midline space located between the hemispheres and anterior to the vermis. Inferiorly, a thin outlet—the foramen of Magendie—may be seen on steeply angled scans. The tentorium manifests as echogenic, linear meningeal folds coursing obliquely superior to the cerebellar hemispheres.
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Normal Variants
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Calcar Avis
Calcar avis is a paramedian protrusion of the calcarine gyrus into the medial aspect of the lateral ventricle at the junction of the trigone with the occipital horn. On parasagittal oblique images, these normal parenchymal structures may simulate intraventricular clots. They can be recognized by their characteristic location, their contiguity with the calcarine gyri, and the presence of a central echogenic sulcus (Fig 5). If further confirmation is needed, color Doppler US may be performed to demonstrate normal perfusion in the calcar avis in contradistinction to absence of flow in a clot.
Lobular Choroid Plexus
Although the echogenic choroid plexus is usually smooth, lobulations may occasionally be seen and mistaken for adherent clot (Fig 6). Again, color or power Doppler US may be helpful in distinguishing this variant from small clots. Stability of appearance on repeat images also favors the impression of a lobular choroid plexus.
Foramen of Magendie
The foramen of Magendie is a normal posteroinferior midline outlet of the fourth ventricle that may be visualized on inferiorly angled images of the posterior fossa (Figs 7, 8; see also Movie 3 at radiographics.rsnajnls.org/cgi/content/full/21/4/943/DC1). In addition, a closely related structure, the cerebellar vallecula (a deep recess or valley between the inferior surfaces of the cerebellar hemispheres), may mimic the appearance of the fourth ventricle with an inferior vermian cleft, a feature of the Dandy-Walker variant. This troubling appearance has also been observed on angled prenatal scans of the posterior fossa (6). Distinguishing between these two normal structures (ie, cerebellar vallecula, foramen of Magendie) and a true Dandy-Walker variant may be difficult. However, the cerebellar vallecula is a variable-sized subarachnoid space not directly contiguous with the fourth ventricle, and the foramen of Magendie is thinner than the vermian cleft in a Dandy-Walker variant. In doubtful cases, correlation with magnetic resonance (MR) imaging may be needed for definitive evaluation until further studies have more clearly elucidated the anatomic relationships and US appearances of these normal structures.
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Pathologic Conditions
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Grade 2 Hemorrhage
Grade 2 hemorrhage is defined as the presence of blood within a nondilated ventricular system. These lesions typically result from transependymal extension of a germinal matrix hemorrhage, although less commonly they may originate from the choroid plexus itself. Small amounts of clot tend to settle in the dependent occipital horns in the supine infant and are difficult to detect with anterior fontanelle imaging because they are in the far field of the transducer. They are especially likely to be missed in the absence of germinal matrix hemorrhage (Fig 9) or when layering nonclotted blood is present (Fig 10).
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Figure 10a. Layering right occipital horn hemorrhage in a 30-week-old infant with prior grade 3 intraventricular hemorrhage. (a) Follow-up US image obtained through the anterior fontanelle shows mild ventricular dilatation without clots. (b) US image obtained through the posterior sagittal suture with the patient supine shows layering clot in the right occipital horn ( ). The left occipital horn appeared normal on the posterior fontanelle view (not shown).
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Figure 10b. Layering right occipital horn hemorrhage in a 30-week-old infant with prior grade 3 intraventricular hemorrhage. (a) Follow-up US image obtained through the anterior fontanelle shows mild ventricular dilatation without clots. (b) US image obtained through the posterior sagittal suture with the patient supine shows layering clot in the right occipital horn (). The left occipital horn appeared normal on the posterior fontanelle view (not shown).
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Grade 3 Hemorrhage and Subarachnoid Hemorrhage
Grade 3 hemorrhages are those with intraventricular blood as well as ventricular dilatation. Lateral ventricle clots are larger and thus more easily seen at anterior fontanelle imaging, although it may be difficult to distinguish acute clot from a normally echogenic choroid glomus. Posterior fontanelle imaging can improve diagnostic accuracy in many cases by better showing subtle differences in echogenicity between clot and a choroid glomus and by showing clot extending into the occipital and temporal horns. Clot in the remainder of the ventricular system may occasionally be seen in the third ventricle and foramina of Monro at anterior fontanelle imaging, but mastoid fontanelle imaging greatly facilitates clot detection in the fourth ventricle and aqueduct because the surrounding tissues are normally echogenic (Fig 11).
Subarachnoid blood is notoriously difficult to detect due to the normal variation in the US appearance of subarachnoid cisterns. Although some success has been reported in the detection of hemorrhage in the basilar cisterns with anterior fontanelle imaging, this technique has not enjoyed wide application (7). Mastoid fontanelle imaging, again through increased resolution achieved by relocating these normally distant cisterns in the near field of the US transducer, is a promising technique for the detection of subarachnoid hemorrhage around the brainstem as well as in the posterior fossa. Because the presence of subarachnoid blood is believed to be the proximate cause of posthemorrhagic hydrocephalus, earlier detection may facilitate prediction of this late complication, especially when intraventricular blood volume is low (Fig 12).
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Figure 12a. Grade 2 intraventricular hemorrhage with large subarachnoid hemorrhage in a 26-week-old infant. (a) Coronal US image obtained through the anterior fontanelle shows mild fullness of the choroid plexus without definite clots. The posterior fossa is not well seen. (b) Findings on a sagittal US image of the right lateral ventricle obtained through the anterior fontanelle suggest intraventricular hemorrhage, but the occipital horn is poorly visualized. An anterior fontanelle view of the left lateral ventricle demonstrated similar findings. (c) US image of the left lateral ventricle () obtained through the posterior fontanelle demonstrates an occipital horn clot. A posterior fontanelle view of the right lateral ventricle (not shown) also demonstrated an occipital horn clot. (d) On a sagittal midline US image obtained through the anterior fontanelle, the cerebellum appears indistinct, a finding that suggests posterior fossa hemorrhage. However, it is unclear whether this finding represents subarachnoid blood or a cerebellar hemorrhage. (e) On a US image obtained through the mastoid fontanelle with the patient facing to the right (same US window and patient orientation used for e-h), a large subarachnoid hemorrhage is seen filling the cisterna magna () and thickening the tentorium ( ) while sparing the cerebellar parenchyma. (f) US image shows the fourth ventricle filled with blood (calipers), which extends posteriorly through a dilated foramen of Magendie (). (g) US image shows the midbrain encircled by blood. The quadrigeminal plate cistern (Q [ , left]), perimesencephalic cistern (), and interpeduncular cistern (I [, right]) all show increased echogenicity, findings that are consistent with subarachnoid hemorrhage. (h) US image reveals blood in both frontal horns () and in the foramina of Monro (), third ventricle ( ), and upper aqueduct ( ). The patient developed severe posthemorrhagic hydrocephalus, requiring placement of a ventriculoperitoneal shunt (cf Fig 11).
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Figure 12b. Grade 2 intraventricular hemorrhage with large subarachnoid hemorrhage in a 26-week-old infant. (a) Coronal US image obtained through the anterior fontanelle shows mild fullness of the choroid plexus without definite clots. The posterior fossa is not well seen. (b) Findings on a sagittal US image of the right lateral ventricle obtained through the anterior fontanelle suggest intraventricular hemorrhage, but the occipital horn is poorly visualized. An anterior fontanelle view of the left lateral ventricle demonstrated similar findings. (c) US image of the left lateral ventricle () obtained through the posterior fontanelle demonstrates an occipital horn clot. A posterior fontanelle view of the right lateral ventricle (not shown) also demonstrated an occipital horn clot. (d) On a sagittal midline US image obtained through the anterior fontanelle, the cerebellum appears indistinct, a finding that suggests posterior fossa hemorrhage. However, it is unclear whether this finding represents subarachnoid blood or a cerebellar hemorrhage. (e) On a US image obtained through the mastoid fontanelle with the patient facing to the right (same US window and patient orientation used for e-h), a large subarachnoid hemorrhage is seen filling the cisterna magna () and thickening the tentorium () while sparing the cerebellar parenchyma. (f) US image shows the fourth ventricle filled with blood (calipers), which extends posteriorly through a dilated foramen of Magendie (). (g) US image shows the midbrain encircled by blood. The quadrigeminal plate cistern (Q [, left]), perimesencephalic cistern (), and interpeduncular cistern (I [, right]) all show increased echogenicity, findings that are consistent with subarachnoid hemorrhage. (h) US image reveals blood in both frontal horns () and in the foramina of Monro (), third ventricle (), and upper aqueduct (). The patient developed severe posthemorrhagic hydrocephalus, requiring placement of a ventriculoperitoneal shunt (cf Fig 11).
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Figure 12c. Grade 2 intraventricular hemorrhage with large subarachnoid hemorrhage in a 26-week-old infant. (a) Coronal US image obtained through the anterior fontanelle shows mild fullness of the choroid plexus without definite clots. The posterior fossa is not well seen. (b) Findings on a sagittal US image of the right lateral ventricle obtained through the anterior fontanelle suggest intraventricular hemorrhage, but the occipital horn is poorly visualized. An anterior fontanelle view of the left lateral ventricle demonstrated similar findings. (c) US image of the left lateral ventricle () obtained through the posterior fontanelle demonstrates an occipital horn clot. A posterior fontanelle view of the right lateral ventricle (not shown) also demonstrated an occipital horn clot. (d) On a sagittal midline US image obtained through the anterior fontanelle, the cerebellum appears indistinct, a finding that suggests posterior fossa hemorrhage. However, it is unclear whether this finding represents subarachnoid blood or a cerebellar hemorrhage. (e) On a US image obtained through the mastoid fontanelle with the patient facing to the right (same US window and patient orientation used for e-h), a large subarachnoid hemorrhage is seen filling the cisterna magna () and thickening the tentorium () while sparing the cerebellar parenchyma. (f) US image shows the fourth ventricle filled with blood (calipers), which extends posteriorly through a dilated foramen of Magendie (). (g) US image shows the midbrain encircled by blood. The quadrigeminal plate cistern (Q [, left]), perimesencephalic cistern (), and interpeduncular cistern (I [, right]) all show increased echogenicity, findings that are consistent with subarachnoid hemorrhage. (h) US image reveals blood in both frontal horns () and in the foramina of Monro (), third ventricle (), and upper aqueduct (). The patient developed severe posthemorrhagic hydrocephalus, requiring placement of a ventriculoperitoneal shunt (cf Fig 11).
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Figure 12d. Grade 2 intraventricular hemorrhage with large subarachnoid hemorrhage in a 26-week-old infant. (a) Coronal US image obtained through the anterior fontanelle shows mild fullness of the choroid plexus without definite clots. The posterior fossa is not well seen. (b) Findings on a sagittal US image of the right lateral ventricle obtained through the anterior fontanelle suggest intraventricular hemorrhage, but the occipital horn is poorly visualized. An anterior fontanelle view of the left lateral ventricle demonstrated similar findings. (c) US image of the left lateral ventricle () obtained through the posterior fontanelle demonstrates an occipital horn clot. A posterior fontanelle view of the right lateral ventricle (not shown) also demonstrated an occipital horn clot. (d) On a sagittal midline US image obtained through the anterior fontanelle, the cerebellum appears indistinct, a finding that suggests posterior fossa hemorrhage. However, it is unclear whether this finding represents subarachnoid blood or a cerebellar hemorrhage. (e) On a US image obtained through the mastoid fontanelle with the patient facing to the right (same US window and patient orientation used for e-h), a large subarachnoid hemorrhage is seen filling the cisterna magna () and thickening the tentorium () while sparing the cerebellar parenchyma. (f) US image shows the fourth ventricle filled with blood (calipers), which extends posteriorly through a dilated foramen of Magendie (). (g) US image shows the midbrain encircled by blood. The quadrigeminal plate cistern (Q [, left]), perimesencephalic cistern (), and interpeduncular cistern (I [, right]) all show increased echogenicity, findings that are consistent with subarachnoid hemorrhage. (h) US image reveals blood in both frontal horns () and in the foramina of Monro (), third ventricle (), and upper aqueduct (). The patient developed severe posthemorrhagic hydrocephalus, requiring placement of a ventriculoperitoneal shunt (cf Fig 11).
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Figure 12e. Grade 2 intraventricular hemorrhage with large subarachnoid hemorrhage in a 26-week-old infant. (a) Coronal US image obtained through the anterior fontanelle shows mild fullness of the choroid plexus without definite clots. The posterior fossa is not well seen. (b) Findings on a sagittal US image of the right lateral ventricle obtained through the anterior fontanelle suggest intraventricular hemorrhage, but the occipital horn is poorly visualized. An anterior fontanelle view of the left lateral ventricle demonstrated similar findings. (c) US image of the left lateral ventricle () obtained through the posterior fontanelle demonstrates an occipital horn clot. A posterior fontanelle view of the right lateral ventricle (not shown) also demonstrated an occipital horn clot. (d) On a sagittal midline US image obtained through the anterior fontanelle, the cerebellum appears indistinct, a finding that suggests posterior fossa hemorrhage. However, it is unclear whether this finding represents subarachnoid blood or a cerebellar hemorrhage. (e) On a US image obtained through the mastoid fontanelle with the patient facing to the right (same US window and patient orientation used for e-h), a large subarachnoid hemorrhage is seen filling the cisterna magna () and thickening the tentorium () while sparing the cerebellar parenchyma. (f) US image shows the fourth ventricle filled with blood (calipers), which extends posteriorly through a dilated foramen of Magendie (). (g) US image shows the midbrain encircled by blood. The quadrigeminal plate cistern (Q [, left]), perimesencephalic cistern (), and interpeduncular cistern (I [, right]) all show increased echogenicity, findings that are consistent with subarachnoid hemorrhage. (h) US image reveals blood in both frontal horns () and in the foramina of Monro (), third ventricle (), and upper aqueduct (). The patient developed severe posthemorrhagic hydrocephalus, requiring placement of a ventriculoperitoneal shunt (cf Fig 11).
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Figure 12f. Grade 2 intraventricular hemorrhage with large subarachnoid hemorrhage in a 26-week-old infant. (a) Coronal US image obtained through the anterior fontanelle shows mild fullness of the choroid plexus without definite clots. The posterior fossa is not well seen. (b) Findings on a sagittal US image of the right lateral ventricle obtained through the anterior fontanelle suggest intraventricular hemorrhage, but the occipital horn is poorly visualized. An anterior fontanelle view of the left lateral ventricle demonstrated similar findings. (c) US image of the left lateral ventricle () obtained through the posterior fontanelle demonstrates an occipital horn clot. A posterior fontanelle view of the right lateral ventricle (not shown) also demonstrated an occipital horn clot. (d) On a sagittal midline US image obtained through the anterior fontanelle, the cerebellum appears indistinct, a finding that suggests posterior fossa hemorrhage. However, it is unclear whether this finding represents subarachnoid blood or a cerebellar hemorrhage. (e) On a US image obtained through the mastoid fontanelle with the patient facing to the right (same US window and patient orientation used for e-h), a large subarachnoid hemorrhage is seen filling the cisterna magna () and thickening the tentorium () while sparing the cerebellar parenchyma. (f) US image shows the fourth ventricle filled with blood (calipers), which extends posteriorly through a dilated foramen of Magendie (). (g) US image shows the midbrain encircled by blood. The quadrigeminal plate cistern (Q [, left]), perimesencephalic cistern (), and interpeduncular cistern (I [, right]) all show increased echogenicity, findings that are consistent with subarachnoid hemorrhage. (h) US image reveals blood in both frontal horns () and in the foramina of Monro (), third ventricle (), and upper aqueduct (). The patient developed severe posthemorrhagic hydrocephalus, requiring placement of a ventriculoperitoneal shunt (cf Fig 11).
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Figure 12g. Grade 2 intraventricular hemorrhage with large subarachnoid hemorrhage in a 26-week-old infant. (a) Coronal US image obtained through the anterior fontanelle shows mild fullness of the choroid plexus without definite clots. The posterior fossa is not well seen. (b) Findings on a sagittal US image of the right lateral ventricle obtained through the anterior fontanelle suggest intraventricular hemorrhage, but the occipital horn is poorly visualized. An anterior fontanelle view of the left lateral ventricle demonstrated similar findings. (c) US image of the left lateral ventricle () obtained through the posterior fontanelle demonstrates an occipital horn clot. A posterior fontanelle view of the right lateral ventricle (not shown) also demonstrated an occipital horn clot. (d) On a sagittal midline US image obtained through the anterior fontanelle, the cerebellum appears indistinct, a finding that suggests posterior fossa hemorrhage. However, it is unclear whether this finding represents subarachnoid blood or a cerebellar hemorrhage. (e) On a US image obtained through the mastoid fontanelle with the patient facing to the right (same US window and patient orientation used for e-h), a large subarachnoid hemorrhage is seen filling the cisterna magna () and thickening the tentorium () while sparing the cerebellar parenchyma. (f) US image shows the fourth ventricle filled with blood (calipers), which extends posteriorly through a dilated foramen of Magendie (). (g) US image shows the midbrain encircled by blood. The quadrigeminal plate cistern (Q [, left]), perimesencephalic cistern (), and interpeduncular cistern (I [, right]) all show increased echogenicity, findings that are consistent with subarachnoid hemorrhage. (h) US image reveals blood in both frontal horns () and in the foramina of Monro (), third ventricle (), and upper aqueduct (). The patient developed severe posthemorrhagic hydrocephalus, requiring placement of a ventriculoperitoneal shunt (cf Fig 11).
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Figure 12h. Grade 2 intraventricular hemorrhage with large subarachnoid hemorrhage in a 26-week-old infant. (a) Coronal US image obtained through the anterior fontanelle shows mild fullness of the choroid plexus without definite clots. The posterior fossa is not well seen. (b) Findings on a sagittal US image of the right lateral ventricle obtained through the anterior fontanelle suggest intraventricular hemorrhage, but the occipital horn is poorly visualized. An anterior fontanelle view of the left lateral ventricle demonstrated similar findings. (c) US image of the left lateral ventricle () obtained through the posterior fontanelle demonstrates an occipital horn clot. A posterior fontanelle view of the right lateral ventricle (not shown) also demonstrated an occipital horn clot. (d) On a sagittal midline US image obtained through the anterior fontanelle, the cerebellum appears indistinct, a finding that suggests posterior fossa hemorrhage. However, it is unclear whether this finding represents subarachnoid blood or a cerebellar hemorrhage. (e) On a US image obtained through the mastoid fontanelle with the patient facing to the right (same US window and patient orientation used for e-h), a large subarachnoid hemorrhage is seen filling the cisterna magna () and thickening the tentorium () while sparing the cerebellar parenchyma. (f) US image shows the fourth ventricle filled with blood (calipers), which extends posteriorly through a dilated foramen of Magendie (). (g) US image shows the midbrain encircled by blood. The quadrigeminal plate cistern (Q [, left]), perimesencephalic cistern (), and interpeduncular cistern (I [, right]) all show increased echogenicity, findings that are consistent with subarachnoid hemorrhage. (h) US image reveals blood in both frontal horns () and in the foramina of Monro (), third ventricle (), and upper aqueduct (). The patient developed severe posthemorrhagic hydrocephalus, requiring placement of a ventriculoperitoneal shunt (cf Fig 11).
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Brainstem Hemorrhage
Most neonatal intraparenchymal hemorrhage occurs as an extension of intraventricular hemorrhage in premature infants or as a complication of infarction or asphyxia in full-term infants. In both situations, the affected parenchyma is supratentorial, either periventricular in the so-called grade 4 hemorrhage of preterm infants or intrathalamic in the status marmoratus lesion of full-term infants. Brainstem hemorrhage (Fig 13) or cerebellarhemorrhage is detected at US much less frequently due to difficulty in adequately visualizing these deeper central structures. With the routine use of mastoid fontanelle imaging, a better understanding of the true frequency and prognosis of hemorrhages in these areas is possible (8).
Congenital Anomalies
With the increasing use of routine obstetric US, many congenital brain malformations are recognized prenatally, and the role of neurologic US has changed from allowing a primary diagnosis to confirming a known or suspected abnormality. Aqueductal stenosis, a common congenital cause of hydrocephalus, demonstrates severe dilatation of the third and lateral ventricles with no other structural abnormalities present. When dilatation of the lateral ventricles is severe and the cerebral mantle is thin, aqueductal stenosis may mimic holoprosencephaly. The crucial difference is the appearance of the thalami and third ventricle: Holoprosencephaly demonstrates fused thalami and an absent third ventricle, whereas aqueductal stenosis demonstrates splayed thalami and a dilated third ventricle (Fig 14). Mastoid fontanelle imaging affords excellent depiction of brainstem anatomy and can assist in distinguishing between these two anomalies, which have different prognoses and require different treatment.
The Dandy-Walker deformity represents a spectrum of posterior fossa malformations characterized by the absence of all or part of the cerebellar vermis in association with cystic expansion of the fourth ventricle and proximal hydrocephalus. Milder cases demonstrate absence of the inferior cerebellar vermis, mild hydrocephalus, and only a thin communication between the fourth ventricle and cisterna magna. Again, mastoid fontanelle imaging of the posterior fossa allows clearer depiction of the cerebellum and visualization of smaller clefts not readily visible through the anterior fontanelle. This is because, in mastoid fontanelle imaging, the cerebellar vermis is placed in the near field of the transducer and the US beam is more nearly perpendicular to the direction of the cleft. As a recent study has shown, caution must be exercised to distinguish between normal structures (eg, the foramen of Magendie) (Fig 8) and a true Dandy-Walker variant (Fig 15) (3).
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Conclusions
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Neonatal intracranial US has traditionally been performed through the anterior fontanelle. However, the use of two newer approaches—imaging through the posterior fontanelle and through the mastoid fontanelle—can significantly augment the diagnostic power of neurologic US in detecting small intraventricular hemorrhages, subarachnoid hemorrhage, and brainstem hemorrhage and in depicting structural abnormalities of the brainstem and cerebellum.
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References
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Anderson N, Allan R, Darlow B, Malpas T. Diagnosis of intraventricular hemorrhage in the newborn: value of sonography via the posterior fontanelle. AJR Am J Roentgenol 1994; 163:893-896.[Abstract/Free Full Text]
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Top
Abstract
SUPPLEMENTAL MATERIAL
) and anterior (
) are also seen. (b) US image of the posterior fossa obtained through the mastoid fontanelle depicts the fourth ventricle (
) with a central echogenic sulcus (
) as well as an occipital horn (
