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 Australia.