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Chapter 5: “Technical”
Factors in the CPC Controversy
Several factors which may be loosely termed “technical” have
contributed to the rise of the CPC controversy by allowing a greater rate of
demonstration of abnormalities.
5.1 Improved Equipment
Equipment designers have continually pushed forward the
available resolution and image quality of ultrasound machines using phased,
linear, annular and curved array transducer technology which requires great
computing power and speed.
Piezo-electric crystal design and construction, microchip design and
computer control over an increasing number of the parameters of image acquisition
and display have dramatically improved the spatial and contrast resolution of
real-time transducers to the point where they well exceed the former “gold
standard” of static-scanning single-crystal image quality. Computerised beam-forming technology has
reduced many of the artifacts that once beset electronic imaging.
Equipment used in the early to mid1980’s, when most of the
data published in the literature was collected, was perhaps unable to display
detailed anatomy of the fetal brain and the anatomy elsewhere. High resolution views of the choroid plexus
were often not possible. Zoom functions
and variable focus settings were absent from many manufacturers’ equipment. Even in the more sophisticated machines
curved array probes, which are now probably the most frequently used for
obstetrical scanning, were not common.
Scanning performed with linear array probes, particularly lower quality
machines, was particularly prone to several types of artifact.
That equipment quality affected the
discovery of these cysts was highlighted in the 1989 paper by Ostlere146:
“The incidence of cyst detection was one in every 120 dating scans when using state-of-the-art equipment [Hitachi EUB 340] but only one in every 400 when using an older scanner [Hitachi EUB 25].”
Undoubtedly, as even better equipment is available from year
to year, new aspects of fetal structures will be become more evident, and other
controversies will arise.
During the Geelong Hospital study, we used an Aloka SSD 280
with 3.5 and 5.0 MHz sector (fixed focus) and linear array (variable focus)
probes, (purchased in 1984), an ATL Ultramark 9 with 3.5 and 5.0 MHz annular
array sector probes and a 3.5 MHz curved array probe (purchased in 1988), and a
Diasonics Spectra with 3.5 and 5.0 MHz curved array probes (purchased in
1992). Without doubt the ATL and
Diasonics machines produced better visualisation of fetal intracranial
structures, although the exact data are lacking as to the utilisation of each
machine. The ATL machine is capable of
pulse Doppler, the Diasonics of pulse and colour Doppler.
5.2 Concentration on the Fetal Brain
The fetal skull and its contents have always held centre
stage in obstetrical ultrasound. The biparietal
diameter measurement (BPD) was the first ultrasound parameter used for
estimating the fetal age. Landmarks in
the brain – the thalami, cerebral peduncles and septum cavum pellucidum – were
used to define the correct measurement plane for this measurement.
The AIUM and ASUM141 obstetric ultrasound
guidelines (reproduced in the Appendix) suggest views of the fetal ventricles
during second trimester ultrasound in order to assess the presence of
ventriculomegaly. Awareness has been
drawn by many researchers to the cranial and intra-cranial changes associated
with the Arnold-Chiari malformation of open neural tube defects such as spina
bifida cystica. In fetuses with open
neural tube defects the head is said to be “lemon-shaped,” the cisterna magna
is greatly reduced or absent, and the cerebellum is compressed against the
occiput in a characteristic “banana” shape139.
Recommendations were made in 1989 by Nyberg134
and also Filly77 for a “practical level of effort” in evaluating the
fetal cranium. They suggested three
views; a BPD, a posterior fossa view for the cerebellum, and a view of the
atrium of the ventricle (showing the glomus of the choroid plexus) with or
without a measurement to exclude ventriculomegaly. This measurement was based on the work of
Cardoza et al42, who further suggested that the morphology of the
choroid plexus within the ventricle can be used as a sign of ventriculomegaly
(the dangling choroid sign).
Sonographers seem to be following this suggested protocol,
with the concentration on the ventricles for the third view no doubt increasing
the rate of demonstration of choroid plexus abnormalities.
5.3 Sonographer Skill & Training
The educational opportunities for sonographers have improved
dramatically in the last two decades as this new profession finds its
feet. Because of the expanding role of
ultrasound in diagnostic medicine, the decision to specialize in sonography has
been taken by many medical imaging technologists, nurses and other allied
health professionals. Professional
advancement is generally dependant upon the legitimisation of knowledge by
formal qualifications. Most employers
require sonographers to have achieved, or be studying towards, a qualification,
and this forces exposure to the guidelines and recommendations mentioned
above. Inadequate sonographer training
and insufficient duration of the routine examination have been suggested as the
major cause of missed fetal abnormalities181.
At present, processes for voluntary accreditation of
sonography education and sonographers are being set-up in