Diagnostic ultrasound ( PDFDrive )

1092 PART IV Obstetric and Fetal Sonography
supported by two organizations: the Nuchal Translucency
Quality Review program based in the United States and the
Fetal Medicine Foundation based in the United Kingdom. he
criteria and caliper placement for measuring the NT are illustrated
in Fig. 31.2 (see also Video 31.1 for real time scanning
without measurement). he speciic details for credentialing
can be found at www.ntqr.org or www.fetalmedicine.org.
he fetal CRL must be between 45 and 84 mm although
variations exist among laboratories that perform the biochemical
component. he accuracy of the NT measurement and the CRL
is critical because the NT measurement is converted into multiples
of the median (MoM) based on the CRL. An adequate sonographic
image to measure the CRL requires the fetus to occupy a majority
of the image space and be in a neutral position. he longest
straight line between the fetal crown and rump is measured at
least three times and the average of three good measurements
is used.
Haddow et al. 38 showed that accuracy is critical when
obtaining an NT measurement. hey studied 4412 women who
underwent irst-trimester screening with biochemistry and NT
in which no speciic training in NT measurement was required
although the method for measuring NT was a standard protocol.
Measurement of the NT varied considerably between
centers and could not be reliably incorporated into risk calculations.
Furthermore, the center with the highest success rate
in obtaining an NT measurement (100%) had the lowest sensitivity
(0%) for identifying trisomy 21. Results from the BUN
trial revealed that ater training, measurements were initially
smaller than expected compared with normative values developed
by Fetal Medicine Foundation. With increasing experience,
the measurements of the BUN trial were in concordance
with published norms. 23,39
Criteria for an Accurate Crown-Rump Length
Fetal Nuchal Translucency (NT) Measurement
Technique
ALARA: Thermal Index BONE < 0.7 40
CLEAR NT MARGINS
Thin NT line
Angle of insonation perpendicular to NT space
Fetus horizontal on image
TIPS for optimal imaging:
1. Optimize your focal zone
2. Reduce your dynamic range
3. Reduce the gain
4. Review harmonics. Possible edge enhancement
optimized with harmonics off
5. Avoid post freeze zoom
6. Narrow your sector
FETUS IN MIDSAGITTAL PLANE
Fetal spine midsagittal in thoracic and cervical region
Tip of nose in proile
Third and fourth ventricles in brain demonstrated
FETUS OCCUPIES MAJORITY OF IMAGE
Head, neck, and upper thorax ill image
Fetus occupies more than 50% of image space; a second
fetus of the same size would not it in the image space
HEAD IN A NEUTRAL POSITION
Head in line with the spine, angle of neck and chest is less
than 90 degrees
Pocket of luid should be visible between chin and neck
AMNION SEEN SEPARATELY FROM NT LINE
Fetus is seen away from uterine wall and separate from
the amnion
MEASUREMENT
Calipers (cursors) must be + (plus/positive)
Crossbar of caliper is on the NT line at the inner border
adjacent to the lucency
Measurement is perpendicular to long axis of fetus
Measurement is taken at the WIDEST part of the lucency
(If nuchal cord, measure above and below the cord and
average)
ALARA: Thermal Index BONE < 0.7 40
MAGNIFICATION
The fetus ills the majority of the image space available
MIDSAGITTAL VIEW
Fetal spine midsagittal
Proile, spine, and rump are visible
NEUTRAL POSITION
The spine is in line with the head
Fluid is visible between the fetal chin and chest
MEASUREMENT
Angle of insonation perpendicular to fetus
Fetus horizontal on image
Calipers are placed on the outer border of the skin at
crown and rump
A poorly done NT measurement has a negative impact on
detection of aneuploidy, and inaccuracy of 0.5 mm decreases
sensitivity by 18%. 41 Training an inexperienced examiner to obtain
reliable reproducible NT measurements takes 80 to 100 scans. 42
Screening using an NT measurement is not always possible
because of limited availability of this specialized ultrasound in
certain areas and a variety of maternal conditions, including
large myomas and high maternal body mass index, which may
hamper the ability to obtain a reliable measurement. Initial studies
suggested that an NT value was obtainable in 99% of cases, but
clinical studies report an 80% initial success rate. 43,44
If an accurate NT measurement is not possible, a serum-only
version of the integrated test should be ofered. he performance
of the serum integrated test with no ultrasound component has