Obstetric ultrasound: A guide for medical students
At some point within your medical training, whether it’s during your first Obstetrics attachment or if you venture to the Radiology department, you are likely to encounter the use of Obstetric ultrasound scanning. However, it can be a complex subject to master and is not covered extensively in the undergraduate core curriculum or textbooks. This article discusses the key essentials that will help you make the most of your learning experience in these situations.
What is ultrasound?
Ultrasound is comprised of sound waves oscillating at frequencies greater than the human hearing range (above approximately 20 kHz). At present, the most widely used medical application of ultrasound is for diagnostic imaging, which was pioneered by Karl Dussik in 1942. Whilst in-depth knowledge of physics is by no means essential to perform (or observe!) ultrasound scanning, it is useful to understand the basic principles.
Ultrasound waves are generated and received by ultrasound transducers. According to the application of the ultrasound scan, the operator can select the frequency of ultrasound wave emitted (ranging from approximately 2 – 10 MHz). Higher frequency transducers generally have reduced penetration (i.e. ultrasound waves are not transmitted very deep) but better resolution (i.e. there is better ability to distinguish two objects in close proximity) than lower frequency transducers. The ultrasound waves are emitted from the transducer and then reflected from the target structures. The resulting reflected “echo” waves are detected and are processed to generate an on-screen image.
Ultrasound scanning has advantages over other imaging techniques because it is non-invasive and low–risk; ultrasound does not involve maternal-fetal exposure to ionising radiation (which can be harmful to both mother and baby). From a practical perspective, ultrasound machines provide a very rapid imaging technique and are relatively inexpensive (costing typically between £10-60K depending on the model, whereas MRI scanners can cost approximately £900K) 1. The key disadvantage of US scanning is that its usefulness can be greatly limited by the skill of the operator.
Whilst arguably the most well-known medical application of ultrasound (and the focus of this article) is obstetric imaging, ultrasound imaging has applications in many diverse areas of medicine, examples of which are given in Box 1. Therapeutic ultrasound is having an increasing role in the clinic and is an interesting area of medical research, with applications including lithotripsy, tumour ablation, acoustic targeted drug delivery and even ultrasound-assisted lipectomy.
As shown in Figure 1, the machines are usually comprised of a large processing unit, printer, monitor, keyboard, interface and probes. Before starting, the machine should be attached to the power supply, the probe wiring should be tidy and the probes to be used should be connected to the machine and clean.
User interfaces will vary between machine models. Most have a large trackball in the centre, with a select button that provides the basis for navigation through the system. Patient data can be input through the keyboard so that any future printouts are labelled with patient information (which typically includes patient ID, last menstrual period, age, estimated due date and estimated fetal weight.
Depending on the purpose of the US scan, the user will need to choose from a selection of parameter pre-sets (this saves the operator having to manually select parameters such as acoustic power, frequency, etc.). Examples of pre-sets used in an obstetric setting might include "first trimester”, “second trimester”, “fetal echocardiogram”, etc.). The probe chosen for use also needs to be selected on screen.
Most machines have a choice of probes for different purposes. The operator can choose from transabdominal or transvaginal probes; linear or curvilinear probes. It is important to decide which frequency of the probe (e.g. 9MHz, 6MHz, 4MHz etc.) depending on the subject of the scan. Higher frequency probes should be used when superficial structures need to be visualised and lower frequency probes should be used for visualising deep structures and visualising a larger field. Generally, higher frequency probes are used in early pregnancy, and lower frequency probes in later pregnancy.
Once this is set up, the operating mode of ultrasound should be selected. The operating modes can also be alternated between during the scan using the user interface, examples are described in Box 2.
Sometimes for early pregnancy scans when taking a transabdominal approach, it is preferred that the woman is scanned with a full bladder so the uterus can be identified more easily. A water-based coupling gel is applied to the skin (to reduce acoustic impedance from the surrounding air) and then the probe can be positioned and moved around over the abdomen according to the area of interest. For interpretation of the findings, knowledge of anatomy and technical skill is required. Transvaginal scanning is indicated if the uterus cannot be identified with a transabdominal approach, in cases of maternal obesity, or in very early gestation (less than 8 weeks).
During the scan, at any point the operator can take a print screen image, or record video footage. An example of a typical screen layout is demonstrated in Figure 2. The "freeze" function is particularly useful - this temporarily pauses the ultrasound emitted and freezes the image on screen. The operator can then use the on-screen tools to make measurements of structures visualised on screen, e.g. biparietal diameter, femoral length, head or abdominal circumference measurements.
Obstetric ultrasonography applications
During the course of their pregnancy, women are offered at least two ultrasound scans. The first is commonly known as the “dating scan”, which is performed at approximately 8-14 weeks. The dating scan serves to assess viability of pregnancy (confirmed upon visualisation of fetal heart pulsation), estimate the gestational age (by measuring the crown-rump length), as well as diagnosing multiple pregnancies, ectopic and molar pregnancies. At this stage, nuchal translucency can be assessed (see Figure 3) if a sagittal image of the fetus is obtained. This is a measurement of the collection of lymphatic fluid posterior to the fetal neck under the skin. An increased distance (>3mm) can be a predictor for fetal chromosomal abnormalities (e.g. Down’s syndrome), congenital heart or neuromuscular defects.
The second scan is known as the “anomaly scan”, and is performed between 18 – 21 weeks, during the second trimester. The anomaly scan is performed no earlier than 18 weeks as this would be prior to adequate organ development. Scans later than 21 weeks gestation are not ideal due to shadows created from bone ossification. Features that are assessed in the anomaly scan are head shape; spinal appearance; abdominal and thoracic appearance and content; appearance of arms and legs2. Further scans for investigation may be undertaken for further investigation in a specialist Fetal Medicine clinic if necessary.
Ultrasound scanning is also be used to provide visual guidance when performing invasive procedures such as amniocentesis, chorion biopsy, fetal blood sampling or intracardiac injection. It can also be used for visual guidance for antenatal in utero surgical intervention.
Interpreting obstetric scans
Application of some basic principles and familiarity with the most common ultrasound scanning modes can help in interpreting results. Maternal-fetal anatomy is recommended reading but beyond the scope of this short article.
B Mode ultrasound
B (“brightness”) Mode ultrasound will provide a two-dimensional image. It should be remembered that, generally, the scan will show the most superficial structures (closest to the probe) at the top of the image, and the deepest structures at the bottom. A key principle is that different tissue types have different acoustic properties – some absorb ultrasound waves strongly whilst others reflect strongly. Therefore fluids, which strongly transmit ultrasound waves, will appear black; bone, which is denser and strongly reflects ultrasound, will appear white; and soft tissue will appear in numerous shades of grey. It is also worthwhile being aware of enhancement and acoustic shadowing artefacts: tissues deep to a highly conductive region will appear enhanced, whereas tissues deep to a very reflective region (e.g. bone) will appear attenuated.
Findings from B Mode scans can be supplemented by 4D scans, which are particularly useful in assessing anatomical abnormalities. For example, in diagnosis of exomphalos, a congenital disorder that results in herniation of the fetal abdominal contents into the umbilical cord (Figure 4).
Doppler ultrasound can be used to investigate the maternal-fetal circulation. Doppler assessment of the umbilical artery, uterine artery and fetal middle cerebral artery waveforms can be performed. An example of a normal umbilical artery Doppler sonogram is shown in Figure 5A. Abnormal waveforms that are seen in placental failure include absent end-diastolic flow (Figure 5B) and, in severe cases, reversed end-diastolic flow (Figure 5C).
Is obstetric ultrasound safe?
Ultrasound safety is an interesting field of debate and an area of present research. Antenatal ultrasound examination exposes the developing fetus to potentially harmful mechanical and thermal stress but, as yet, epidemiological data is inconclusive as to the potential for subtle long-term adverse effects. The risk of harm can be evaluated quantitatively using the output display indices which are displayed on screen which include the mechanical index (MI) and thermal index (TI). In the context of obstetric imaging, the heating effect is of greatest concern3 and therefore guidance advises to use restrict ultrasound use to when clinically indicated. Moreover, when using modes such as pulsed wave Doppler, which generate the greatest TI, the operator should limit the time of the scan to minimise risk to the fetus.
How do I learn more?
There are some specialist books available on the subject and ultrasound practical courses. As a medical student, clinic attendance and Student Selected Components are good ways of gaining exposure to the field.
1. Department of Health. Managing high value capital equipment in the NHS in England. National Audit Office. March 2011. Available online at http://www.official-documents.gov.uk/document/hc1011/hc08/0822/0822.pdf [Accessed 15 May 2013]
2. Collins S, Arulkumaran S, Hayes K, Jackson S, Impey L. Oxford Handbook of Obstetrics and gynaecology, 2nd Ed. Oxford University Press, 2008. pp114.
3. ter Haar G. The new British Medical Ultrasound Society Guidelines for the safe use of diagnostic ultrasound equipment. Ultrasound May 2010 vol. 18 no. 2 50-51 doi: 10.1258/ult.2010.100007