Accuracy of nuchal translucency measurement depends on equipment used and its calibration.
OBJECTIVE
To determine the effect of different ultrasound machine-probe combinations on nuchal translucency (NT) measurements and to assess how this impacts on the accuracy of the NT-derived component of first-trimester screening for trisomy 21.


METHODS
Sixteen different ultrasound machine-probe combinations were used for axial measurement of 2.0-, 3.0- and 4.0-mm spaced targets in an ultrasound phantom. Differences between the measured and known values were determined. The mean of the axial measurements was used to calculate adjusted risks for trisomy 21, given specific clinical scenarios.


RESULTS
Differences observed using different machine-probe combinations for the 2.0-mm target ranged from 1.8-2.2 mm; for the 3.0-mm target, 2.7-3.2 mm; and for the 4-mm target, 3.7-4.3 mm, and exceeded those due to intraobserver variability. For a fetal crown-rump length of 50.0 mm and NT measurement of 2.0 mm, the maximum/minimum measurements in the fetus of a 40-year-old woman led to derived risks ranging from 1 in 32 (NT, 2.2 mm) to 1 in 189 (NT, 1.8 mm) and in the fetus of a 20-year-old with an NT of 3.0 mm these ranged from 1 in 102 (NT, 3.2 mm) to 1 in 160 (NT, 2.7 mm).


CONCLUSIONS
We have described the effect of machine-probe combinations on small but very precise ultrasound measurements. Such machine-probe combinations led to greater variability than those ascribed to intraobserver differences, and significantly affected the screening risk for the same fixed measurement. This finding has implications for Down syndrome screening algorithms and audit of ultrasound operators. Furthermore, most ultrasound machines are neither calibrated nor specified for measurements of tenths of a mm.
Medical Physics and Clinical Engineering, Addenbrookes Hospital, Cambridge University Hospitals NHS Foundation Trust, Cambridge, UK; Postgraduate Medical Institute, Anglia Ruskin University, Chelmsford, UK.
R G
RG
Axell
A
A
Gillett
D
D
Pasupathy
T
T
Chudleigh
J
J
Brockelsby
P A
PA
White
C C
CC
Lees
D000328:Adult; D002138:Calibration; D004314:Down Syndrome / Q000000981:diagnostic imaging*; D005260:Female; D006801:Humans; D016014:Linear Models; D048208:Nuchal Translucency Measurement / Q000295:instrumentation* / Q000592:standards; D015588:Observer Variation; D011247:Pregnancy; D011261:Pregnancy Trimester, First
Down syndrome screening; NT; intraoperator; nuchal translucency; phantom; ultrasound equipment; variability