Intra- and interoperator repeatability of the nuchal translucency measurement

Invasive Prenatal Diagnostic Testing for Aneuploidies in Singleton Pregnancies: A Comparative Review of Major Guidelines

Sophisticated screening protocols for genetic abnormalities constitute an important component of current prenatal care, aiming to identify high-risk pregnancies and offer appropriate counseling to parents regarding their options. Definite prenatal diagnosis is only possible by invasive prenatal diagnostic testing (IPDT), mainly including amniocentesis and chorionic villous sampling (CVS). The aim of this comparative review was to summarize and compare the existing recommendations on IPDT from the most influential guidelines. All the reviewed guidelines highlight that IPDT is indicated based on a positive screening test rather than maternal age alone. Other indications arise from medical history and sonography, with significant variations identified between the guidelines. The earlier time for amniocentesis is unequivocally set at ≥15 gestational weeks, whereas for CVS, the earlier limit varies from ≥10 to ≥11 weeks. Certain technical aspects and the overall approach demonstrate significant differences. Periprocedural management regarding Rhesus alloimmunization, virologic status and use of anesthesia or antibiotics are either inconsistent or insufficiently addressed. The synthesis of an evidence-based algorithm for IPDT is of crucial importance to healthcare professionals implicated in prenatal care to avoid unnecessary interventions without compromising optimal prenatal care.

United States' experience in nuchal translucency measurement: variation according to provider characteristics in over five million ultrasound examinations

OBJECTIVE

The Nuchal Translucency Quality Review (NTQR) program has provided standardized education, credentialing and epidemiological monitoring of nuchal translucency (NT) measurements since 2005. Our aim was to review the effect on NT measurement of provider characteristics since the program's inception.

METHODS

We evaluated the distribution of NT measurements performed between January 2005 and December 2019, for each of the three primary performance indicators of NT measurement (NT median multiples of the median (MoM), SD of log₁₀ NT MoM and slope of NT with respect to crown-rump length (CRL)) for all providers within the NTQR program with more than 30 paired NT/CRL results. Provider characteristics explored as potential sources of variability included: number of NT ultrasound examinations performed annually (annual scan volume of the provider), duration of participation in the NTQR program, initial credentialing by an alternative pathway, provider type (physician vs sonographer) and number of NT-credentialed providers within the practice (size of practice). Each of these provider characteristics was evaluated for its effect on NT median MoM and geometric mean of the NT median MoM weighted for the number of ultrasound scans, and multiple regression was performed across all variables to control for potential confounders.

RESULTS

Of 5 216 663 NT measurements from 9340 providers at 3319 sites, the majority (75%) of providers had an NT median MoM within the acceptable range of 0.9-1.1 and 85.5% had NT median MoM not statistically significantly outside this range. Provider characteristics associated with measurement within the expected range of performance included higher volume of NT scans performed annually, practice at a site with larger numbers of other NT-credentialed providers, longer duration of participation in the NTQR program and alternative initial credentialing pathway.

CONCLUSIONS

Annual scan volume, duration of participation in the NTQR program, alternative initial credentialing pathway and number of other NT-credentialed providers within the practice are all associated with outcome metrics indicating quality of performance. It is critical that providers participate in ongoing quality assessment of NT measurement to maintain consistency and precision. Ongoing assessment programs with continuous feedback and education are necessary to maintain quality care. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.

A Systematic Review and Meta-analysis of the Use of High-Fidelity Simulation in Obstetric Ultrasound

SUMMARY STATEMENT

There is little global consensus on how to train, assess, and evaluate skills in obstetric ultrasound. The outcomes of curricula, where present, are often based on the number of clinical cases completed, rather than objective outcomes. The central question in this review is whether simulation enhances training and prepares trainees for clinical practice. A systematic review was conducted of the currently available literature in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Studies considering the use of simulators in training or assessment of sonographers were eligible for inclusion. We conclude that simulation is best used for acquisition of technical skills and image optimization. Best outcomes are observed when simulation augments traditional learning, with a strong focus on specific, objective, and measurable skills. Integrating simulation into training curricula could allow trainees to contribute to clinical service while learning. How skills learned in a simulated environment translate to the clinic is poorly addressed by the literature.

Dimensionless squared jerk: An objective differential to assess experienced and novice probe movement in obstetric ultrasound

OBJECTIVE

Widely accepted, validated and objective measures of ultrasound competency have not been established for clinical practice. Outcomes of training curricula are often based on arbitrary thresholds, such as the number of clinical cases completed. We aimed to define metrics against which competency could be measured.

METHOD

We undertook a prospective, observational study of obstetric sonographers at a UK University Teaching Hospital. Participants were either experienced in fetal ultrasound (n = 10, >200 ultrasound examinations) or novice operators (n = 10, <25 ultrasound examinations). We recorded probe motion data during the performance of biometry on a commercially available mid-trimester phantom.

RESULTS

We report that Dimensionless squared jerk, an assessment of deliberate hand movements, independent of movement duration, extent, spurious peaks and dimension differed significantly different between groups, 19.26 (SD 3.02) for experienced and 22.08 (SD 1.05, p = 0.01) for novice operators, respectively. Experienced operator performance, was associated with a shorter time to task completion of 176.46 s (SD 47.31) compared to 666.94 s (SD 490.36, p = 0.0004) for novice operators. Probe travel was also shorter for experienced operators 521.23 mm (SD 27.41) versus 2234.82 mm (SD 188.50, p = 0.007) when compared to novice operators.

CONCLUSION

Our results represent progress toward an objective assessment of technical skill in obstetric ultrasound. Repeating this methodology in a clinical environment may develop insight into the generalisability of these findings into ultrasound education.

Automatic measurement of fetal Nuchal translucency from three-dimensional ultrasound data

The Nuchal translucency (NT), which is the collection of fluid at the back of the fetal neck, is related to chromosomal defects and early cardiac failure in first trimester of pregnancy. In clinic, the thickness of NT is used as an important marker in prenatal screening, and is manually measured by sonographers in the mid-sagittal plane. In this paper, an automatic method based on dynamic programming is proposed to detect the thickness and area of NT in the mid-sagittal plane. Furthermore, the volume of NT in the whole three-dimensional ultrasound data is also measured. A novel cost function for dynamic programming is proposed and results in higher accuracy of NT border detection. As the nuchal translucency is the collection fluid part, higher dimensional markers of NT possess more potential to represent the amount of the fluid.

A prospective study to assess the clinical impact of interobserver reliability of sonographic measurements of fetal nuchal translucency and crown-rump length on combined first-trimester screening

OBJECTIVE:

To examine the interobserver variability for fetal biometric parameters and to focus on how this variability affects the combined first-trimester screening test results.

METHODS:

Sixty-four singleton pregnancies who were scheduled for combined first-trimester screening were investigated. Two experienced sonographers performed all examinations at the same visit. The risk calculations of screening were performed according to the each operator’s individual ultrasonographic fetal measurements. Interobserver variability in measurements of fetal nuchal translucency (NT) and crown-rump length (CRL) and the effect of this interobserver variability on the screening results were assessed.

RESULTS:

Interobserver reliabilities for NT and CRL were 0.687 and 0.945 (p<0.001), respectively. Interobserver reliability coefficients for trisomy 21 and trisomy 13/18 were 0.945 and 0.963 (p<0.001), respectively.

CONCLUSION:

Our study demonstrated quite high interobserver reliability for CRL measurement, while interobserver agreement for NT was lower. Despite this lower reliability and agreement for NT measurement, interobserver reliability for chromosomal abnormalities was quite high.

Accuracy of nuchal translucency measurement depends on equipment used and its calibration

OBJECTIVES

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.

Repeat measurements of nuchal translucency at 11-14 weeks of gestation: when do we need them?

OBJECTIVE

Previous studies have recommended that several repeat measurements of nuchal translucency (NT) be obtained to optimize sensitivity of the screening process. However, truncation is applied within the risk calculation for small NT measurements; therefore, repeating NT measurements in the lower range may be unnecessary. The aim of this study was to determine the optimal number of NT measurements and whether this is dependent on the initial value obtained.

METHODS

We simulated the expected distribution of sets of five repeat NT measurements and their corresponding likelihood ratios (LR) for a variety of crown-rump length (CRL) values at 11-14 weeks' gestation, based on the published mixture model for first-trimester NT measurements and on published data regarding NT measurement variability. The ratio between LR obtained from the highest and the initial measurement in each set were computed, with a ratio of 1 indicating that repeat measurements would have no effect on risk estimation. We calculated NT cut-offs below which a change in estimated LR would occur in fewer than 10% of cases if repeat measurements were obtained, and we tested this approach on a real first-trimester screening dataset.

RESULTS

The simulations performed indicated that repeating NT measurements when the first NT obtained ranged between ≤ 0.9 and ≤ 1.6 mm at CRLs of 45 and 84 mm, respectively, is not useful. When applied to a real dataset, our approach allowed avoiding repetition of measurements in 47/165 cases (28%).

CONCLUSIONS

Although there is some variability in NT measurements, repeat assessment is not useful to optimize screening performance for the smallest NT measurements due to lower truncation limits that are applied in risk calculation. Our study provides NT cut-off values, in relation to CRL, below which it appears that there is no need to repeat measurements once a good quality image has been obtained.

Intra- and interoperator reliability of manual and semi-automated measurement of fetal nuchal translucency by sonographers with different levels of experience

OBJECTIVE

To compare the intra- and interoperator reliability of manual and semi-automated nuchal translucency thickness (NT) measurement by sonographers with different levels of experience.

METHODS

From our digital database we selected retrospectively 70 NT images without measurements and of highest quality according to the guidelines of The Fetal Medicine Foundation (FMF). Four operators (two experts who were consultants in fetal medicine, one general consultant in obstetrics and gynecology with average knowledge in obstetric ultrasound and one final-year medical student with no experience in ultrasound), having first attended the FMF's online NT measurement course, measured the fetal NT manually according to FMF guidelines and semi-automatically from the inner to inner borders of the two echogenic lines delineating the NT and from the inner border to the middle of the lower NT line. The semi-automatic measurement algorithm identifies the two NT lines within a manually chosen region of interest and standardizes the caliper placement process. Each operator was blinded to any pre-existing measurements. Each measurement was performed twice by each operator. Intraoperator repeatability was assessed by the within-operator SD and intraclass correlation coefficients (ICC). Interoperator reliability was assessed by comparison with the mean manual measurement of the experts, which was considered as the gold standard.

RESULTS

The mean NT measurement of the four sonographers was between 1.9 and 2.0 mm with both the manual and the semi-automated inner-inner NT measurement and it was between 2.2 and 2.3 mm with the semi-automated inner-middle method. The within-operator SDs of the manual measurement for both experts were similar and significantly lower than those of the non-experts, and all four sonographers performed similarly with the semi-automated measurement. Concerning the interoperator biases relative to the manual measurements of the experts taken as the gold standard, the manual measurements of the student and the general consultant were significantly lower, by 0.18 mm and 0.12 mm. The semi-automated inner-inner measurements of all four sonographers were much closer to the gold standard, although there was still a significant underestimation (mean differences from gold standard between 0.04 and 0.06 mm). With the semi-automated inner-middle measurement there was a significant and more pronounced overestimation of fetal NT compared with the gold standard, relative biases ranging from 0.08 to 0.23 mm above it.

CONCLUSION

Semi-automation of NT measurement leads to improved standardization of the NT assessment process. In particular, non-expert operators improve as the variability in their caliper placement can be reduced to the level of experts. However, the most important contributor to the inter- and intraoperator variability of NT assessment is that of image acquisition, which is not affected by semi-automation of NT measurement.

Semi-automated system for measurement of nuchal translucency thickness

OBJECTIVE

To estimate intersonographer and intrasonographer variance components of fetal nuchal translucency (NT) thickness measurement using the traditional manual approach and a new semi-automated system.

METHODS

A semi-automated method was developed for measurement of the NT. In this method, the operator places an adjustable box over the relevant area at the back of the fetal neck. The system draws a line through the center of the nuchal membrane and another line at the edge of the soft tissue overlying the cervical spine. The system then identifies the largest vertical distance between the two lines. The images of 12 fetuses at 11-13 weeks of gestation satisfying the guidelines of The Fetal Medicine Foundation for measurement of NT were selected. They were exported in DICOM format from the ultrasound system, and four versions of each image were stored under different names. The resulting 48 images were presented in random order for electronic assessment. A total of 20 sonographers measured the NT in each set of 48 pictures, twice using the semi-automated system and twice using the manual system, according to a randomized block design. Within- and between-operator variance components were estimated. Relative biases were assessed by comparing the means from the two methods.

RESULTS

The estimated between-operator SD using the semi-automated method was 0.0149 mm compared with 0.109 mm for the manual method. The respective within-operator SD values were 0.05 mm and 0.126 mm. The intraclass correlation coefficients for different sonographers measuring the same images were 0.98 and 0.85 for the semi-automated method and the manual method, respectively.

CONCLUSION

The measurement of fetal NT is more reliable when a semi-automatic approach is used rather than the traditional manual method.

Impact of nuchal cord on measurement of fetal nuchal translucency thickness

OBJECTIVE

To define the impact of nuchal cord on the measurement of fetal nuchal translucency thickness (NT).

METHODS

Between December 2004 and June 2006, we examined prospectively 53 fetuses that were observed on routine first-trimester ultrasound examination between 11 + 3 and 13 + 6 weeks of gestation to have nuchal cord causing an indentation in the skin in the nuchal region. The fetuses were re-examined after a median interval of 132 min, when the cord was no longer around the neck and indentation of the skin had resolved. Various NT measurements (highest, lowest, mean) with the cord located around the neck (nuchal cord) were compared with NT measurements in the absence of nuchal cord ('true' NT) in the same fetuses. Measurements were considered to be equal when they were within mean +/- 1.96 SD, defined by our own intraobserver repeatability according to the method of Bland and Altman.

RESULTS

The mean of the largest and the smallest of six measurements in the presence of nuchal cord fell within the mean +/- 1.96 SD of our own intraobserver repeatability, i.e. gave a correct estimate of the true NT, in 80% of fetuses, while the NT was overestimated in 10% and underestimated in 10% of fetuses. The largest and the smallest of six measurements in the presence of nuchal cord gave an underestimate of the true NT in 2% of fetuses and an overestimate in 4% of fetuses, respectively.

CONCLUSION

There is a wide scattering of measurements in fetuses with nuchal cord in comparison to the same fetuses in the absence of nuchal cord. This prevents accurate prediction of the true NT, although the largest and smallest of repeat measurements with nuchal cord can allow calculation of the highest and lowest possible risks, respectively. These facts must be taken into consideration in counseling patients.

[Down syndrome screening by ultrasound]

The nuchal translucency is the usual subcutaneous space, between the skin and the soft tissues covering the fetus'neck spine, observed by ultrasound during the first trimester of pregnancy. It increases with gestational age. The association between nuchal translucency thickness and risk of Down syndrome was firstly observed in high risk populations as an old maternal age or a history of a previous child affected by Down syndrome. However, these studies are retrospective and only report the issue among pregnancies associated with an abnormal nuchal translucency thickness. Furthermore, as the performance of a screening test is correlated to the prevalence of the disease in the studied population, such results can not be extrapolated to the general population. To date, eleven prospective studies have been published, assessing the performance of the nuchal translucency measurement in a general population, including about 125,000 patients. The global sensitivity of this screening was 70% for a false positive rate of 5%. For an adjusted risk on the maternal age, the detection rate increased up to 77%. Although nuchal translucency measurement is an early and performing screening tool, uncertainties remain nevertheless about its reproducibility in the general population. In order to correctly measure the nuchal translucency, a previous training course is necessary to guarantee adequate and reproducible measurements.

Variability of Nuchal Translucency Measurement

OBJECTIVES

To evaluate intra- and inter-observer variations of nuchal translucency (NT) measurements and study the duration of measurements.

SUBJECTS

One hundred and forty-seven singleton pregnant women with 10-14 weeks of gestation who had attended antenatal clinic during January 1st, 2000-August 31st, 2001 were included.

METHODS

Crown-rump length and NT were measured three times for each woman. Then, another examiner who was unaware of the previous results did the other three measurements. The duration of measurements was recorded. Intra-observer and inter-observer variations were analyzed by repeated ANOVA and paired t test, respectively. The correlation of NT measurements within each observer and between paired observers was evaluated by intraclass correlation coefficients (ICC). Kappa statistic was calculated for agreement.

RESULTS

Intra-observer variation of all 10 examiners at three institutes was varied from 0.20 +/- 0.27 to 0.33 +/- 0.41 mm. Inter-observer variation at Chiang Mai University was 0.40 +/- 0.37 mm which was the highest value compared with those at Prince of Songkla and Khon Kaen University (p < 0.05). The mean duration of measurements was in range of 8-12 min. The intra-observer repeatability of the first two measurements showed ICCs varying from 0.61 to 0.94. The inter-observer repeatability of paired examiners showed a variation in ICC from 0.28-0.90. The kappa value expressing the intra- and inter-observer repeatability as being >95th or < or = 95th percentile was 0.73 and 0.72, respectively.

CONCLUSIONS

NT measurement was reproducible for small variation and good agreement. The duration of measurements was acceptable.

Audit on nuchal translucency thickness measurements in Flanders, Belgium: a plea for methodological standardization

OBJECTIVES

To audit nuchal translucency thickness (NT) measurements for fetal aneuploidy screening in Flanders, and to estimate the impact of small variations in NT measurement on the screening result of two first-trimester screening algorithms: maternal age + NT (Algorithm A), and maternal age + NT + pregnancy associated plasma protein-A + free beta-human chorionic gonadotropin (Algorithm B).

METHODS

We used the database of first-trimester combined screening, as collected by the General Medical Laboratory AML in Antwerp, Belgium, between 1 January 2001 and 1 April 2004. Audit was performed by establishing a delta-NT distribution curve for one trainee of The Fetal Medicine Foundation (FMF) and for a group of 263 other sonographers, in comparison with the FMF reference values. Risks for fetal aneuploidy were calculated at a cut-off value of 1 : 300 for Algorithm A and 1 : 150 for Algorithm B. These risks were recalculated in both algorithms after a modeled increase of all NT values by 0.1 or 0.2 mm.

RESULTS

In a total of 592 measurements performed by the FMF trainee, the 5th, 50th and 95th percentiles of delta-NT measurements were at -0.41, +0.03 and +0.68 mm, respectively. These values were close to the FMF reference values. The screen-positive rate for this set of data was 4.4% (26/592) in both algorithms. For the 12 555 measurements of the 263 other sonographers, the 5th, 50th and 95th percentiles of delta-NT were at -0.81, -0.14 and +0.73 mm, respectively, which clearly indicates underestimation of NT in the lower range. In this set of data the screen-positive rate was 3.5% for both algorithms (439/12 555 for Algorithm A and 436/12 555 for Algorithm B). Also in this group, 5% (59/1186) of negative screening results at maternal age > or = 35 years in Algorithm A became positive after a modeled 0.1-mm increase in NT, whereas this was only in 1.2% (134/11 369) of tests at maternal age < 35 years (P < 0.0001). The overall increase of screen-positive rate in Algorithm A after an NT modification of +0.1 mm was 1.2% (152/12 555), significantly more than in Algorithm B (86/12 555; 0.7%) (P < 0.0001).

CONCLUSION

In Flanders, there is a systematic underestimation of NT in comparison with the FMF reference range. Attempts to change these measurements according to the FMF criteria are crucial. This will mainly influence the screening results of women at advanced maternal age and of NT-based algorithms without the use of other parameters.

Ultrasound simulators: experience with the SonoTrainer and comparative review of other training systems

Ultrasound has become indispensable in prenatal diagnosis. Ultrasound training, however, still lacks proper quality assessment and control. Moreover, most fetal anomalies which must be diagnosed during pregnancy are extremely rare. Ultrasound simulators could provide an opportunity to overcome this dilemma. This review summarizes the potential benefits of simulator-based ultrasound training, briefly describes the properties of a variety of ultrasound simulators that have been developed for various applications including prenatal diagnosis, and presents the SonoTrainer sonography simulation system which makes it possible to run a real-time simulation of a complete prenatal ultrasound examination. We evaluated the system for the training of first- and second-trimester screening for both normal and pathological findings and found that physicians who received theoretical training and were additionally trained with the simulator (T + S) significantly improved their skills in measurements of nuchal translucency thickness (NT) and crown-rump length (CRL) as compared with colleagues who only underwent theoretical instruction (T) [mean +/- SD NT deviation: 0.31 +/- 0.1 mm (T + S) vs. 0.62 +/- 0.2 mm (T), P < 0.05; mean +/- SD CRL deviation: 1.48 +/- 2.0 mm (T + S) vs. 3.27 +/- 2.5 (T), P < 0.05]. Simulator-based training enabled physicians to diagnose rare fetal anomalies in the second trimester with a sensitivity of 86% and a specificity of 100%. In a study in which second-trimester scans including fetal anomalies were presented to physicians, 96% of the participants subjectively estimated their training effect as being good. We therefore conclude that simulator-based training would provide an ideal educational tool to test, improve and monitor a physician's or technician's ultrasound skills in detecting fetal anomalies.

Intra- and interoperator variability in fetal nasal bone assessment at 11-14 weeks of gestation

OBJECTIVES

Examination of the fetal nasal bones by ultrasound between 11 and 14 weeks of gestation has been proposed as an additional tool in the detection of trisomy 21 in a high-risk population. However the variability in the identification of fetal nasal bones by ultrasound has not yet been investigated. The aim of this study was to assess the intraobserver and interobserver reproducibility of fetal nasal bone identification by ultrasound between 11 and 14 weeks of gestation.

METHODS

A total of 1040 consecutive ultrasound examinations were performed at 11-14 weeks of gestation for nuchal translucency (NT) measurement and nasal bone identification by ultrasound. A total of 657 consecutive video-loops were assessed by three experienced operators. Each operator assigned cases to one of three categories, namely present, uncertain or absent nasal bones, and the results were compared between operators. To assess the intraoperator variability each operator reviewed 100 randomly selected videos out of the 657 loops and again used the same classification. Results were compared by pairwise unweighted and weighted Kappa index to evaluate the inter- and intraoperator variability.

RESULTS

Among the 1040 fetuses, there were 51 (4.9%) with an NT measurement above the 95th centile. Nasal bones were identified by ultrasound in 948, not seen in eight and impossible to assess in 84 fetuses. Four fetuses had trisomy 21 including three with absent nasal bones and increased NT and one with present nasal bones and normal NT. The Kappa and weighted Kappa values for interoperator variability between the three operators were between 0.26 and 0.37 and 0.33 and 0.44, respectively. The Kappa and weighted Kappa values for intraoperator variability were between 0.35 and 0.48 and 0.43 and 0.55, respectively.

CONCLUSION

The assessment of fetal nasal bones is only fairly reproducible. Although the performance of the test in fetuses at high risk for trisomy 21 has been reported to be good, its implementation as an additional screening technique in the general population must be accompanied by teaching and quality control programs.

Nuchal translucency thickness measurement: repeatability using a virtual ultrasound scanner

AIM

To use a PC-based virtual ultrasound scanner (VirUS) in the investigation of inter- and intraoperator nuchal translucency (NT) thickness measurement repeatability of experienced ultrasound operators.

METHODS

Realistic fetal ultrasound images of defined NT thickness were simulated with VirUS with emulation of scanner gain and time-gain compensation and gain-dependent echo size changes. A set of 50 images was generated with uniformly distributed NT thickness (range, 1-5 mm at 1-mm intervals) and translucency angle (mean +/- standard deviation of +/- 2.52 degrees +/- 1.85 degrees about the horizontal). Operators (n = 13) measured NT thickness in the image set on three occasions separated by at least 1 day, giving 150 measurements per operator (total measurements, 1950).

RESULTS

Inter- and intraoperator repeatabilities were +/- 0.41 mm and +/- 0.22 mm, respectively (at the 95% confidence level). There were significant correlations between repeatability and mean measured NT thickness (r = - 0.72, P = 0.005 at 4-mm interval), between gain and mean measured NT thickness (P </= 0.002, n = 8/13) and between gain and repeatability coefficient (P < 0.01, n = 6/13).

DISCUSSION

VirUS provides a consistent NT audit environment and demonstrates the need to both optimize repeatability vs. mean measured thickness and to set gain consistently. The technique has potential in operator training.

Screening for Down's syndrome: effects, safety, and cost effectiveness of first and second trimester strategies

OBJECTIVE

To compare the effects, safety, and cost effectiveness of antenatal screening strategies for Down's syndrome.

DESIGN

Analysis of incremental cost effectiveness.

SETTING

United Kingdom.

MAIN OUTCOME MEASURES

Number of liveborn babies with Down's syndrome, miscarriages due to chorionic villus sampling or amniocentesis, health care costs of screening programme, and additional costs and additional miscarriages per additional affected live birth prevented by adopting a more effective strategy.

RESULTS

Compared with no screening, the additional cost per additional liveborn baby with Down's syndrome prevented was 22 000 pound sterling for measurement of nuchal translucency. The cost of the integrated test was 51 000 pound sterling compared with measurement of nuchal translucency. All other strategies were more costly and less effective, or cost more per additional affected baby prevented. Depending on the cost of the screening test, the first trimester combined test and the quadruple test would also be cost effective options.

CONCLUSIONS

The choice of screening strategy should be between the integrated test, first trimester combined test, quadruple test, or nuchal translucency measurement depending on how much service providers are willing to pay, the total budget available, and values on safety. Screening based on maternal age, the second trimester double test, and the first trimester serum test was less effective, less safe, and more costly than these four options.