AJR 2001; 176:995-1001
© American Roentgen Ray Society
High-Frequency Linear Array Transducers for Neonatal Cerebral Sonography
Glen D. Thomson1,2 and
Rita L. Teele1
1
Department of Radiology, National Women's Hospital & Starship Children's
Hospital, Private Bag 92024, Auckland, New Zealand.
2
Present address: Department of Radiology, Children's and Women's Health Center
of British Columbia, 4480 Oak St., Vancouver, BC, V6H 3V4 Canada.
Received June 19, 2000;
accepted after revision August 30, 2000.
Address correspondence to R. L. Teele.
Introduction
Transfontanelle sonography is an imaging technique that is well established
in its use in the neonatal nursery. It is the accepted initial investigation
for the diagnosis of germinal matrix, intraventricular hemorrhage, or both in
the premature infant. Hydrocephalus complicating intraventricular hemorrhage
or shunt blockage can be readily diagnosed and monitored. Similarly,
sonography can be used to diagnose ventricular enlargement or other causes of
macrocephaly and to diagnose congenital intracranial anomalies. For these
applications, a curved or phased array transducer of medium frequency (5-7.5
MHz) provides sufficient penetration and resolution of the neonatal brain.
For a number of other clinical indications—for example, seizures or
perinatal asphyxia in full-term infants—MR imaging and CT are
established as the imaging modalities of choice. However, CT and MR imaging
equipment may not be universally available, and neonates may be too unstable
to leave the intensive care unit. Sonography is frequently an interim
investigation in such situations until more definitive imaging can be arranged
and performed.
Using a transducer of medium frequency, it is possible to identify gross
cerebral edema, hemorrhage, or large structural abnormalities. The shape of a
sector image limits information regarding the near field. Using a linear array
transducer of high frequency, it is possible to assess, with much greater
clarity, the superior sagittal sinus, the parafalcine meningeal spaces, and
certain portions of the cerebral cortex.
Technique
Evaluation of the neonatal brain with high-frequency linear transducers
should be performed as an adjunct to standard sector scanning. The sector
scans provide most of the diagnostic information and direct attention to the
areas of the brain needing more thorough evaluation. As with sector scans, the
anterior and posterior fontanelles, the sagittal suture, and the pterion may
be used as acoustic windows.
If firm transducer pressure is applied to the anterior fontanelle, the
superior sagittal sinus may be compressed and the subarachnoid space distorted
(Fig.
1A,1B,1C,1D).
Large amounts of coupling gel are advisable to maintain adequate contact,
particularly if the infant has a lot of hair.
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Fig. 1A. —This male neonate had normal vaginal delivery at term.
Because head circumference was above 95th percentile, sonography was performed
at 1 day of age. Imaging revealed normal findings, and initial and follow-up
neurologic examinations also revealed normal findings. Coronal sonogram
obtained via anterior fontanelle shows subarachnoid space with vessels
(arrows) and normal homogeneous brain parenchyma.
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Fig. 1B. —This male neonate had normal vaginal delivery at term.
Because head circumference was above 95th percentile, sonography was performed
at 1 day of age. Imaging revealed normal findings, and initial and follow-up
neurologic examinations also revealed normal findings. Color Doppler sonogram
obtained at same level as A reveals triangular superior sagittal sinus
(blue and open arrow) and vein (red and solid
arrow) within subarachnoid space.
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Fig. 1C. —This male neonate had normal vaginal delivery at term.
Because head circumference was above 95th percentile, sonography was performed
at 1 day of age. Imaging revealed normal findings, and initial and follow-up
neurologic examinations also revealed normal findings. Magnified coronal
sonogram reveals subarachnoid space (calipers) that extends from
superior sagittal sinus to cerebral cortex.
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Fig. 1D. —This male neonate had normal vaginal delivery at term.
Because head circumference was above 95th percentile, sonography was performed
at 1 day of age. Imaging revealed normal findings, and initial and follow-up
neurologic examinations also revealed normal findings. Magnified coronal
sonogram shows obliteration of superior sagittal sinus (arrow) and
reduction in size of subarachnoid space when compression is applied.
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Because the flow in the superior sagittal sinus may normally be of low
velocity, another important technical consideration is to ensure equipment
settings are appropriate to maximize the Doppler sensitivity. Beam steering
may be necessary because flow in the sinus is perpendicular to the
transducer.
We routinely use a multihertz (10-5 MHz) small-footprint linear transducer
mated to the HDI 3000 platform (Advanced Technology Laboratories, Bothell,
WA). Five focal zones are used, and output is set at the lowest amplitude
possible. For a typical study, the thermal index for soft tissues and the
mechanical index are low. In gray-scale imaging, the usual values are 0.2 for
the thermal index and 0.6 for the mechanical index. Use of color Doppler
sonography increases these indexes, but not dramatically (thermal index = 0.3
and mechanical index = 0.7). Fontanelles and sutures are only small acoustic
windows, so potential exists for insonation of the adjacent calvarium. Because
the absorption coefficient of sound for bone is an order of magnitude greater
than that for soft tissues, care should be taken to exclude bone from the
image whenever possible.
Superior Sagittal Sinus
The superior sagittal sinus lies just deep in relation to the anterior
fontanelle. It is also possible to visualize the sinus via the posterior
fontanelle. If the parietal bones are not overlapping, the sinus may be seen
throughout its entire length.
Thrombosis of the superior sagittal sinus is believed to be more common
than once thought. Likely, it is underdiagnosed because of its nonspecific
clinical presentation [1].
Thrombosis has been associated with dehydration, sepsis, and ischemia.
Thrombosis due to extracorporeal membrane oxygenation is a recognized
complication [2]. There have
also been concerns that thrombosis may be a complication of head cooling for
asphyxia.
The color Doppler evidence of thrombosis of the superior sagittal sinus is
as one would expect—that is, an absence of color flow (Figs.
2A,2B
and
3A,3B).
The gray-scale findings are of the normally concave lateral margins of the
sinus becoming convex and bulging toward the cerebral hemispheres. A thrombus
within the sinus tends to have low-level echoes within it
(Fig. 4).
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Fig. 2A. —1-day-old female neonate, born at term, who sustained severe
hypoxic ischemic insult. Sagittal power Doppler sonogram obtained via anterior
fontanelle shows no appreciable flow in anterior portion of superior sagittal
sinus (arrow). Flow is present only in tissues superficial to
sinus.
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Fig. 3A. —3-month-old male infant with hypoxic ischemic injury to brain
caused by severe shaking. Sagittal sonogram of anterior superior sagittal
sinus shows normal flow in region of color box. There is echogenic material
seen within sinus more posteriorly (arrow).
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Fig. 4. —Female neonate, in first day of life, born by forceps
delivery at term who was initially well but went into shock a few hours
postpartum and died shortly thereafter. Postmortem examination revealed
massive intraperitoneal hemorrhage from unidentified source. This close-up
coronal sonogram, obtained a few minutes before death, shows bulging lateral
walls (arrows) of superior sagittal sinus, which is filled with
clot.
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Meningeal Spaces
For normally grown term infants, the width of the subarachnoid space has
been measured (range, 0-3.3 mm; mean, 1.6 mm)
[3,
4]. The extraaxial space may
become enlarged when there is communicating hydrocephalus or necrosis and
shrinkage of the underlying brain. There is also a population of infants,
developmentally normal, who have large heads and capacious extraaxial
spaces—so-called benign macrocrania. The extraaxial space is effaced if
there is cerebral edema such as in asphyxia.
The subarachnoid space may become echogenic and the pia mater thickened in
cases of meningitis [5]
(Fig. 5).
For many years it has been known that sonography can depict a subdural
collection if it is large enough and in a examinable area
[6]. More recently, sonography
has been matched to findings on MR imaging and CT. A subdural collection
displaces the veins within the subarachnoid space away from the skull toward
the brain (Figs.
6A,6B,6C
and
7A,7B).
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Fig. 6A. —5-month-old male infant who presented with acute increase in
head circumference. Coronal sonogram shows hypoechoic right-sided subdural
collection displacing subarachnoid space away from calvarium
(arrows).
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Fig. 6B. —5-month-old male infant who presented with acute increase in
head circumference. Axial CT scan shows differential density between right
subdural collection (arrow) and subarachnoid space.
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Fig. 7A. —5-day-old female infant with factor VII deficiency who
presented with seizures and falling hemoglobin level. Axial sonogram obtained
through right pterion shows anterior pole of right temporal lobe (white
arrow), displaced from calvarium by subdural hematoma of mixed
echogenicity (arrowhead), and farther echogenic component medial to
temporal lobe (black arrow).
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Fig. 7B. —5-day-old female infant with factor VII deficiency who
presented with seizures and falling hemoglobin level. Axial CT scan obtained
at equivalent level as A shows large right subdural hematoma
(arrows). Configuration of collection correlates well with that seen
on sonographic image.
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Cerebral Cortex
Sonographic features of cerebral edema include effacement of sulci,
slitlike ventricles, and a generalized increase in cerebral echogenicity (Fig.
8A,8B).
When cerebral echogenicity is markedly increased or when changes are focal or
unilateral, scanning with a transducer of medium frequency is generally
sufficient for diagnosis. Subtle, generalized cerebral involvement is more
apparent with a linear array transducer of high frequency. Gray—white
differentiation in the healthy infant is minimal; both components of the
cerebrum are hypoechoic and homogeneous in appearance
[7]. Enhancement of
gray—white differentiation—a band of echolucent gray matter
surrounding white matter that is slightly echogenic—is the early sign of
cerebral edema. As edema increases, gray matter may also become blotchy or
diffusely blurred [7]
(Fig. 9). Hemorrhage in areas
of ischemic damage is a secondary phenomenon. Even small petechial hemorrhages
are identifiable if they are in the field of view of the transducer (Fig.
10A,10B,10C).
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Fig. 8A. —1-day-old female neonate, born at term after complicated
delivery, who sustained severe hypoxic ischemic insult. Coronal sonogram shows
brain parenchyma to be diffusely echogenic compared with normal parenchyma
(compare with Figure
1A,1B,1C,1D).
Subarachnoid space is almost completely obliterated by swollen gyri that are
squared off where they meet at interhemispheric fissure (arrow).
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Fig. 8B. —1-day-old female neonate, born at term after complicated
delivery, who sustained severe hypoxic ischemic insult. Magnified coronal
sonogram shows diameter of subarachnoid space marked by calipers.
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Fig. 9. —6-day-old female neonate delivered at term who sustained
severe hypoxic ischemic insult to brain as result of birth asphyxia. On this
sonogram, note blotchy echogenic pattern of subcortical white matter
(arrow) compared with that of gray matter.
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Fig. 10A. —Female neonate born at term who experienced birth asphyxia
and had poor initial Apgar scores but showed rapid clinical improvement during
first week of life. Coronal sonogram obtained via anterior fontanelle when
patient was 10 days old shows echogenic focus in subcortical white matter of
left parafalcine gyrus (arrow).
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Fig. 10B. —Female neonate born at term who experienced birth asphyxia
and had poor initial Apgar scores but showed rapid clinical improvement during
first week of life. Left parasagittal sonogram obtained when patient was 10
days old shows this area as diffuse band of echogenicity
(arrows).
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Fig. 10C. —Female neonate born at term who experienced birth asphyxia
and had poor initial Apgar scores but showed rapid clinical improvement during
first week of life. Axial CT scan obtained when patient was 11 days old shows
subtle hemorrhagic staining, which is more prominent on left, in frontal
cortex bilaterally (arrows).
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Severe cerebral necrosis and resultant cystic encephalomalacia are
characteristic of infection with Proteus, Enterococcus, Citrobacter,
and Serratia species. The area of cerebritis may be intensely
echogenic initially, then it rapidly liquefies and cavitates (Fig.
11A,11B,11C,11D).
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Fig. 11A. —Premature female neonate (29 weeks' gestational age) who had
rapid respiratory decompensation when 24 days old and was suspected of having
meningitis. Bacillus cereus was grown from blood cultures. Neonate
survived with major neurologic handicap and seizures. Initial coronal sonogram
shows intense echogenicity of white matter. This was widespread throughout
brain.
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Fig. 11B. —Premature female neonate (29 weeks' gestational age) who had
rapid respiratory decompensation when 24 days old and was suspected of having
meningitis. Bacillus cereus was grown from blood cultures. Neonate
survived with major neurologic handicap and seizures. Sonogram obtained 1 day
after A shows echogenicity is fading.
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Fig. 11C. —Premature female neonate (29 weeks' gestational age) who had
rapid respiratory decompensation when 24 days old and was suspected of having
meningitis. Bacillus cereus was grown from blood cultures. Neonate
survived with major neurologic handicap and seizures. Sonogram obtained 4 days
after onset of illness shows cystic encephalomalacia (arrows) is
developing.
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Fig. 11D. —Premature female neonate (29 weeks' gestational age) who had
rapid respiratory decompensation when 24 days old and was suspected of having
meningitis. Bacillus cereus was grown from blood cultures. Neonate
survived with major neurologic handicap and seizures. Sonogram obtained 2
weeks after onset of illness shows brain is shrinking away from calvarium,
resulting in large subarachnoid space that is devoid of vessels.
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Neuronal migrational disorders result in a disordered pattern of gray and
white matter. Such disorders include lissencephaly, pachygyria, and
polymicrogyria. All are better identified and characterized with the use of
linear array transducers of high frequency (Fig.
12A,12B,12C).
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Fig. 12A. —6-day-old male neonate who had seizure during first week of
life. Coronal sector sonogram obtained through anterior fontanelle shows
diffusely increased echogenicity in deep white matter tracts of both cerebral
hemispheres (arrows) and asymmetry of parafalcine gyri.
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Fig. 12B. —6-day-old male neonate who had seizure during first week of
life. Coronal sonogram obtained with linear array transducer shows small gyri
(polymicrogyria, arrows) and large left-sided gyrus (pachygyria,
arrowhead).
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Fig. 12C. —6-day-old male neonate who had seizure during first week of
life. Coronal T2-weighted MR image shows abnormal gray matter with pachygyria
affecting frontal lobes (arrows) and medial temporal lobes. Gray
matter of lateral aspects of temporal lobes has more normal appearance
(arrowhead).
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Conclusions
The use of high-frequency linear array transducers increases the diagnostic
capability of neonatal cranial sonography, particularly in cases in which the
meningeal spaces, superior sagittal sinus, or the cerebral cortex is abnormal.
As we have shown, these cases include thrombosis of the superior sagittal
sinus; abnormal meningeal spaces from meningitis, subdural hematoma, and
cerebral edema; and edema, hemorrhage, infection, or migrational anomalies
that involve the peripheral cortex.
The major limitations to the use of linear array transducers are the
limited access to the brain via fontanelles and sutures and the shallow depth
of view.
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