Nt scan images

“First Things First”: Images for a Proper Nuchal Translucency in the First Trimester 11–13+6 Weeks Scan

“First Things First”: Images for a Proper Nuchal Translucency in the First Trimester 11–13⁺⁶ Weeks Scan

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• Practice Guideline
• Published: 01 November 2014

• Chinmayee Ratha¹ &
• Ashok Khurana² 

Journal of Fetal Medicine volume 1, pages 55–57 (2014)Cite this article

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Abstract

This section of articles will concentrate on the technical aspects of imaging in fetal medicine. The “First Things First” series will deal with all aspects of the nuchal translucency (NT) scan, one at a time, in the current and forthcoming issues of the Journal. The present article aims at summarizing the ideal protocol for the measurement of the fetal NT at the 11–13⁺⁶ weeks scan based on the Fetal Medicine Foundation (FMF), UK guidelines and some practical tips for beginners.

Introduction

The first trimester nuchal translucency (NT) scan is being recognized now as the first important fetal scan in a pregnancy which can establish many facts that help in stratifying risk at various levels regarding both the fetus and mother. This series of articles entitled “First Things First”, will elaborate on each important aspect of the imaging and interpretation of the first trimester, 11–13⁺⁶ weeks scan.

The NT measurement is so important in this scan that this extensive imaging procedure has been allotted the popular nomenclature of an “NT scan”. NT is the sonographic appearance of a collection of fluid under the skin behind the fetal neck in the first-trimester of pregnancy [1].

Protocol for Measuring NT

The protocol for measuring the NT has been given in detail on the Fetal Medicine Foundation website and is accessible for reference to all interested readers. The important points of this protocol are enumerated as follows [2]:

1. 1.

   The gestational period must be 11–13 weeks and six days.

2. 2.

   The fetal crown-rump length (CRL) should be between 45 and 84 mm.

3. 3.

   The magnification of the image should be such that the fetal head and thorax occupy the whole screen.

4. 4.

   A mid-sagittal view of the face should be obtained. This is defined by the presence of the echogenic tip of the nose and rectangular shape of the palate anteriorly, the translucent diencephalon in the center and the nuchal membrane posteriorly. Minor deviations from the exact midline plane would cause non-visualization of the tip of the nose and visibility of the zygomatic process of the maxilla.

5. 5.

   The fetus should be in a neutral position, with the head in line with the spine. When the fetal neck is hyperextended the measurement can be falsely increased and when the neck is flexed, the measurement can be falsely decreased.

6. 6.

   Care must be taken to distinguish between fetal skin and amnion.

7. 7.

   The widest part of translucency must always be measured.

8. 8.

   Measurements should be taken with the inner border of the horizontal line of the callipers placed on the line that defines the NT thickness—the crossbar of the calliper should be such that it is hardly visible as it merges with the white line of the border, not in the nuchal fluid (Fig. 1).

   Fig. 1

   Nuchal translucency (NT) measurement

   Full size image

9. 9.

   In magnifying the image (pre or post freeze zoom) it is important to turn the gain down. This avoids the mistake of placing the calliper on the fuzzy edge of the line which causes an underestimate of the nuchal measurement.

10. 10.

    During the scan more than one measurement must be taken and the maximum one that meets all the above criteria should be recorded in the database.

11. 11.

    A new approach for the measurement of NT which improves the accuracy of measurements, is with the use of a semi-automated technique. The operator places a box in the nuchal area and the machine automatically selects the best measurement, which uses an algorithm that 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 measurement obtained by this method is similar to that obtained manually and it is therefore applicable to the software of the Fetal Medicine Foundation.

12. 12.

    The umbilical cord may be round the fetal neck in about 5 % of cases and this finding may produce a falsely increased NT. In such cases, the measurements of NT above and below the cord are different and, in the calculation of risk, it is more appropriate to use the average of the two measurements.

Practical Tips for Taking a Good NT Image

The sonographers attempting to take NT measurements at the 11–13⁺⁶ weeks scan should familiarize themselves with the above protocol. The scan can usually be done transabdominally and although a transvaginal approach is becoming increasingly popular to delineate fetal anatomy better at this gestation, the measurement of NT remains easier transabdominally as the range of probe movement is more. Magnification of the image is of utmost importance and it is useful to use the “box zoom” at the beginning of the scan to get the fetus into the range of the box (Fig.  2). Then the “pan zoom” knob can be used to enhance the area of interest (in this case, fetal head, neck, and upper thorax) so that the final image is satisfactory.

Use of “box zoom” for optimizing image

Full size image

A perfect mid-sagittal section is required and it is rather easy to obtain this if you can obtain an axial view of the head (Fig. 3) such that the midline falxcerebri is perpendicular with the orbits oriented superiorly on the screen.

Axial view of fetal head in first trimester

Full size image

With the falxcerebri at the center of the probe, if the probe is rotated 90°, a mid-sagittal view of the fetal face can be obtained. Minor adjustments to perfect the image will include reducing the gain to clearly demarcate the lines in the neck region and eliminate the fuzziness causing confusion about where exactly to place the callipers for measuring the NT (Fig.  4). At this stage, revision of the FMF protocol will help optimize the image to measure a perfect NT.

Standard image for NT measurement

Full size image

References

1. Nicolaides KH, Azar G, Byrne D, et al. Fetal nuchal translucency: ultrasound screening for chromosomal defects in first trimester of pregnancy. BMJ. 1992;304(6831):867–9.

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2. FMF website http://fetalmedicine.org/nuchal-translucency-scan.

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Conflict of interest

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Authors and Affiliations

1. Department of Fetal Medicine, Rainbow Hospitals, Road No. 10, Banjara Hills, Hyderabad, 500034, Telangana, India

   Chinmayee Ratha

2. The Ultrasound Lab, New Delhi, India

   Ashok Khurana

Authors

1. Chinmayee Ratha

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2. Ashok Khurana

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Correspondence to Chinmayee Ratha.

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About this article

12-14 Week Scan - Women's Imaging

This scan should be ideally performed between 12 weeks 5 days and 13 weeks 6 days of your pregnancy. Women’s Imaging conducts a detailed risk assessment for your baby in accordance with the Fetal Medicine Foundation. As well as checking that your baby is growing well and confirming your due date the main aim of the scan includes:

• To see if there are any structural abnormalities. Some can already be identified or suspected at this stage so a normal appearing scan is very reassuring for you.
• To check your uterus and ovaries to ensure they won’t be an issue during the pregnancy.
• To confirm the growth of the pregnancy and the due date.
• To check where the placenta is lying, where the umbilical cord is in relation to the placenta and if there is sufficient fluid around the baby.

Risks

Ultrasound is safe to use throughout your pregnancy.
Sometimes we need to do a vaginal scan. If you are allergic to latex prior to the vaginal scan or you don’t know then a latex-free cover will be used on the probe.

Occasionally there is some discomfort from probe pressure on a full bladder or from the vaginal probe manipulation. If this is extremely painful please let us know.

Benefits

You will be able to see all of your developing baby

We are able to take some important measurements which allows us to give you an accurate risk assessment for your baby.

You have the option of getting the results of the scan on the day by our consultant. Please let reception know at the time of booking.

What is an ultrasound?

An ultrasound scan uses high-frequency sound waves to create images of the inside of your body and baby. Sound waves are used instead of radiation which makes them safe.

What do I need to do to have a risk assessment for my baby?

You will need to have a blood test done before coming to Women’s Imaging for your ultrasound scan. Your doctor will organise this for you. Ideally you should have your 1st Trimester bloods/ Maternal Serum done at 11 weeks gestation. Your doctor will organise a form for you.

We like to scan you between 12.5 and 13.6 weeks of pregnancy.

Why is a 12-14 weeks scan different at Women’s Imaging?

We offer the most sensitive and comprehensive risk assessment for your baby between 12.5-13.6 weeks which includes pre-eclampsia and fetal growth restriction. We are currently the only practice in Tasmania that does this. Traditional chromosomal abnormalities such as Down’s Syndrome continue to be assessed. Our sonographers are trained to the highest standards to perform these specialist measurements of you and your baby. They hold Certificates in Competence from the Fetal Medicine Foundation which is recognised by the Royal Australian and New Zealand College of Obstetricians and Gynaecologists.

We measure:

• Your baby’s nasal bone
• The thin layer of fluid at the back of the baby’s neck (the nuchal translucency)
• Your blood pressure (we take 2 measurements from both arms)
• The blood flow between you and your baby

We also examine:

• Your blood results
• Your age
• Your weight
• Whether you have had any previous problems in pregnancies (if relevant).

When you choose to have your First Trimester scan with us, you can be confident that the risk assessment results you receive are the best available in obstetric scanning today.

Women’s Imaging provides a comforting and safe environment with the latest state of the art equipment to ensure that you have the best possible care and experience.

What happens on the day?

When you arrive for your scan you will be asked to fill out a form about your pregnancy to date.

You will then have your blood pressure taken on both arms and your height and weight will be measured if you are not sure. Sometimes your measurements are taken before you have your scan and sometimes after.

When the sonographer takes you through to the scanning room you will be asked to lie on the table and expose your tummy. A towel will be tucked into your pants to limit spread of the gel onto your clothes.

Clear gel is applied to your tummy and the sonographer moves the probe over your tummy recording images. The clear gel is water-soluble so it won’t stain your clothes. It’s just a little sticky!

Please come with a full bladder which will make it easier to obtain images of your baby. A full bladder pushes the uterus away from your lower pelvis thereby making it  easier for us to see and scan your baby.

We will take measurements of your baby and check your placenta and ovaries.

Occasionally a vaginal scan is also performed to give us a better view of your baby. You will be able to watch the entire scan on our wall-mounted plasma screen monitors!

Once the scan is completed the sonographer will leave the room. All of your measurements including your blood pressure and blood results will be reviewed by our radiologist or on-site obstetrician. Results will be sent to your referring doctor and they will give them to you at your next appointment.

You will be given your expected due date on the day and you will be given a online access card to view your images through our website.

How long will it take?

The 12-14 week scan takes approximately 45 minutes.

The 12-14 week scan and a consultation with our doctor on the day takes approximately 60 minutes.

(Please allow some extra time in your schedule when you come to see us for a 12-14 week scan. Occasionally we have to recalculate your measurements and this can take a few extra minutes)

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