My name is Carol Bagnall and I am a sonographer
working in women's health.
This presentation gives guidance on how to
optimise the nuchal translucency examination.
Quality improvements to antenatal screening
were introduced in NZ in 2010 when maternal
serum was combined with the nuchal translucency
(NT) and crown rump length (CRL) measurements.
Previously maternal age and then NT were used
to screen for Down syndrome and other conditions.
A high quality 11–13 week scan and precise
NT and CRL measurements are essential in providing
women with an optimum risk result for antenatal
screening.
There is a tendency to under measure the nuchal
translucency.
It important the NT and CRL are measured correctly,
due to the amplification of errors that can
occur when they are not measured precisely.
This presentation provides guidance and a
refresher on best practice and includes tips
for optimising both the image and the measurements.
Note that different equipment will require
further tailoring of the examination.
As part of 12 week scan we should document
the early fetal anatomy and placental location.
This presentation is only about optimising
the NT and CRL measurements.
The results of the maternal serum test (known
as the MSS1) rely on an accurate CRL measurement.
The CRL is used to standardise the biochemistry.
If the CRL is under measured it may increase
the chance of a false negative screen rate
and if it is over measured it is more likely
to increase the false positive screen rate.
To obtain a precise CRL you need to take a
sagittal view of the fetus in a neutral position
horizontal on the screen.
Callipers are to be placed on the crown which
is the skin above the parietal bone and the
rump, which is inferior to the tip of the
sacrum.
For antenatal first trimester screening, or
MSS1, the CRL should be between 45 and 84
mm.
You need to use appropriate depth and magnification.
Ideally the scan is done around 12–13 weeks
when the CRL is 56 mm or greater.
The measured CRL axis should be at 90 degrees
to the ultrasound beam, measuring at angles
greater than 30 degrees will likely lead to
measurement error.
This slide demonstrates the fetus lying in
a horizontal position in a sagittal view with
the callipers placed correctly at the crown
and the rump of the fetus.
This slide show a fetus lying in the correct
position with the callipers placed at 30 degrees,
which is acceptable.
However, it would be more ideal to have the
callipers placed on the horizontal with respect
to the ultrasound beam.
This slide shows the fetus in correct sagittal
position but where magnification has not been
used making it harder to correctly place the
calipers.
The following three slides demonstrate how
much variation in measurment can occur depending
on fetal position, thus stressing the importance
of correct fetal position when measuring the
CRL.
This slide shows the fetus in the correct
position where appropriate magnification has
been used so a reliable CRL can be obtained.
This is the same fetus but with a poorly obtained
CRL demonstrating under measurement of the
CRL at 49.8 mm, which is a difference of 7.7
mm from the previous slide.
This slide shows the same fetus but where
the angle of the fetus with respect to the
ultrasound beam is greater than 30 degrees.
The fetus is also not in a sagittal position.
This demonstrates an under measurement of
the CRL at 53.2 mm which is a difference of
4.3 mm from the first slide.
To obtain a good quality NT measurement, the
fetus needs to be in a midline sagittal position
using the intra cranial landmarks of the diencephalon,
mid brain and 4th ventricle.
The maxilla should not be visible.
You should use the highest frequency transducer
available; this may be a linear probe a curve
linear or a trans-vaginal probe.
You need adjust the depth and magnification
so that you demonstrate only the head and
upper thorax in the field of view.
Adjust the gain settings.
Adjust the dynamic range and place the focus
on the correct location.
You need to measure on the line to on the
line that define the NT and measure the widest
part of the NT.
This slide shows NT measurement where appropriate
magnification has been used.
The fetus lies in a midline sagittal position
with the appropriate intra cranial landmarks
seen. You can see there is no maxilla present,
the intra cranial translucency at the back
of the fourth ventricle is present and the
amnion is visible at the back of the fetus.
To help guide you that the fetus is in the
correct position, you should note that the
fetal palate should be between 30 and 60 degrees
relative to the horizontal as shown in this
image.
The fetal head should not be too flexed onto
the chin, the nasal tip should be level or
above the anterior abdominal wall relative
to the horizontal and there should be a pocket
of fluid at least equivalent to the size of
the palate visible between the fetal chin
and chest.
Shown here is how the same fetus can be imaged
and differing NT measurements can be obtained.
The same frequency probe was used, but the
fetus is in a slightly different position,
and the only other adjustment was the gain.
The high quality image is on the left, demonstrating
an NT of 1.8 mm by adjusting the gain and
position you can see that the NT is 1.6 mm
which has resulted in an under measurement
of 0.2mm.
Here we show the difference in NT obtained
using different magnification settings.
This is the same image taken where in the
left hand side of the screen the appropriate
magnification is used and the NT measured
1.8 mm.
In the right hand image magnification was
not employed and the NT was 1.5 mm resulting
in a under measurement of 0.3 mm.
Using different frequency settings can change
the NT measurement.
On the left hand side a curve linear high
frequency probe was used and the NT was measured
at 1.8 mm.
In the right hand image a 1.5 MHz curved array
linear probe was used and the NT was measured
at 1.4 mm an under measurement of 0.4 mm.
In this slide the top left image was taken
using a high frequency 4–8 MHz curved array
probe and the NT is measured at 1.8 mm. In
the right sided image using the C1–5 MHZ
lower frequency probe with poor magnification
the NT is measured at 1.2 mm.
An under measurement of 0.6mm.
Note how the intracranial landmarks are not
well defined and caliper placement is poor.
This next slide demonstrates how a Transvaginal
Vaginal scan can help when the fetus is in
a difficult position.
The slide on the right shows that the fetus
was in a difficult position and the NT was
unable to be obtained, the slide on the left
is the same fetus using a TV scan where the
NT was obtained at 1.4 mm.
Operator and skill can affect the NT measurement
obtained.
Junior staff, practitioners who perform NT
less frequently and those needing to improve
the quality of their images should be provided
with support and mentoring by practitioners
with the skill to obtain high quality images.
The image on the left demonstrates the NT
obtained by someone who was less experienced
or requiring support in NT scanning.
The measurement taken here is 1 mm.
The image on the right is a high quality image
of the same fetus obtained by a more skilled
practitioner, the measurement obtained is
1.8 mm, a 0.8 mm difference.
This is the same fetus, demonstrating the
image on the left where the operator had not
taken a satisfactory CRL measurement and the
CRL is over measured.
The image on the right the appropriate CRL
is obtained and is measured at 62 mm.
There can be a degree of luck involved in
trying to manoeuvre a fetus into the optimal
position for obtaining an accurate NT.
In the top left image a fetus is shown in
a difficult position, the same fetus is shown
below this image and to the right where techniques
have been used to manoeuvre the fetus.
Techniques employed are at the sonographer's
discretion and depend on individual circumstances
on the day of examination and can include
getting the woman to fill or empty her bladder,
rolling the woman from side to side, or delaying
scanning by 30 minutes – 1 hour.
So, in conclusion, it is important to remember
it is the little things that count in obtaining
a high quality nuchal translucency and crown
rump length measurement.
To get the most reliable risk result for women
we need to optimise the ultrasound image every
time; use high frequency transducers when
possible; adjust settings: 1: gain, 2: magnification,
3: frequency, and 4: opt for TV scan when appropriate.
We may need to delay scanning. Remember, if
you cannot get a good NT image then second
trimester screening is available.
