Volume 11 Issue 1 (February) - Australasian Society for Ultrasound ...

Jaqueline Cartmill, Ann Quinton, Michael Peek
Fig. 18: Prenatal MRI demonstrating mass occupying almost the entire
cranium.
Fig. 20: Prenatal MRI. Note size of head relative to body.
are eliminated 6 . Difficulties experienced with USS include
poor visualisation of the non-dependent cerebral hemisphere
due to reverberation from the calvarial wall. Sonography of
the posterior fossa anatomy may be impaired by shadowing
from the fetal skull, particularly in the third trimester 1 , and
fetal positioning can pose problems with obtaining coronal
and sagittal views on USS 6 .
Compared to ultrasound, MRI has been widely reported
to be most useful in evaluating ventriculomegaly (in particular,
anomalies occurring in conjunction with ventricular dilatation,
which account for most morbidity and mortality) 7,8 ,
abnormalities of the posterior fossa or corpus callosum, and
in the detection of parenchymal or migrational disorders
which are too subtle to be visualised sonographically 1,9,10 .
MRI also permits diagnosis of encephalomalacia prenatally,
with increased sensitivity compared with USS, which has
important implications in counselling and management in
Fig. 19: Prenatal MRI demonstrating large intra-cranial haemorrhage.
cases of intra-uterine death of a monozygotic twin 1 .
One study by Malinger, et al. has challenged the concept
of MRI as a useful adjunct to ultrasound in the assessment
of fetal CNS abnormalities. They found that in 42 fetuses
referred with suspected intracranial anomalies, detailed neurosonography
performed by a dedicated neurosonographer
in the setting of a Fetal Neurology Clinic was equal to MRI
in the diagnosis of fetal brain anomalies 11 .
In all three of our cases, the fetal intracranial abnormality
was not detected until the third trimester (in two of the cases
this was as a result of women presenting late for antenatal
care), which made sonographic imaging of CNS anatomy
difficult. In all cases, the ultrasound findings were available
to the MRI personnel, and neither maternal sedation nor
muscular blockade of the fetus were performed.
In the case of hydranencephaly, unfavourable fetal
position contributed significantly to the problems we
encountered with visualisation of intracranial structures
and was mainly responsible for the referral for MRI. The
accuracy of diagnosis achieved with MRI had a major
impact on counselling and the subsequent decision to
terminate the pregnancy.
In the case of schizencephaly, additional information
from the MRI enabled us to obtain multidisciplinary input
from a paediatric neurologist, thus providing the patient with
more accurate counselling regarding likely outcome and
prognosis. Although the postnatal MRI diagnosis of schizencephaly
differed from the antenatal report of porencephalic
cyst, these conditions are thought to have a similar underlying
pathophysiology, occurring as a result of ischaemic insult
in the territory of the middle cerebral artery. Schizencephaly
is thought to occur as a result of early brain injury during the
second trimester and is differentiated from porencephaly by
the absence of scar tissue around the defect. Porencephaly
is caused by local ischaemia after 26 weeks of gestational
age. The injured brain tissue subsequently dissolves,
leaving a fluid filled cavity or cyst, which can resemble
the cleft seen in schizencephaly. These two conditions
may be distinguished by the fact that in schizencephaly, the
cyst cavity is lined by grey matter, whereas in porencephaly
4 ASUM Ultrasound Bulletin 2008 February 11 (1)