Ultrasound in obstetrics and gynaecology

Ultrasound in obstetrics and gynaecology

REVIEW  Focus e this should be set to the depth of interest  Gain e reducing the gain can help to minimize artefact  Noise reduction e using harmo...

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REVIEW

 Focus e this should be set to the depth of interest  Gain e reducing the gain can help to minimize artefact  Noise reduction e using harmonics may reduce the amount of background sound in the image  Frame rate e adjust the sector width as relevant to the image  Colour and power Doppler e this can be used to assess vascularity

Ultrasound in obstetrics and gynaecology Shreelata Datta

Abstract

Preparation

Ultrasound (US) is a key investigation in Obstetrics and Gynaecology and is commonly used in screening, diagnosing, treatment and follow-up. It is quick, cheap, widely available, portable and does not involve ionizing radiation. US can also be used to guide aspiration cytology or biopsy, as well as being a screening tool. This review discusses the role of ultrasound in both benign and malignant gynaecology, early pregnancy and obstetrics. The ultrasound findings of common female pelvic and obstetric pathology are considered and related to current evidence-based practice.

Keywords cancer; Doppler; ectopic pregnancy; endometrioma; fibroids;

Knowing the patient’s presenting complaint and medical history is essential. As well as obtaining consent to perform an ultrasound, preparatory steps include checking the patient’s identity and asking her to empty her bladder before performing a TV ultrasound. If performing a TA ultrasound, the bladder should be full. It is a good practice in Gynaecological US scanning, to begin with transabdominal US mapping followed by TV scanning as abdominal US provides an overall wide view of the abdominopelvic organs. The practical steps needed before performing US are summarized in Box 2.

gestational trophoblastic disease; ovarian cysts; transabdominal; transvaginal; ultrasound

Gynaecological ultrasound Uterine pathology The size and shape of the uterus together with the thickness of the uterine walls varies according to parity and age. The appearance of the endometrium also varies, depending on the timing of scan in relation to the menstrual cycle and the effect of any drugs.

Introduction Ultrasound is one of the most common investigations used in Obstetrics and Gynaecology. It is quick and inexpensive, easy to perform yet specific. It is particularly useful in Obstetrics as it does not expose the unborn fetus to ionizing radiation. US in pregnancy appears to be safe, although fetal exposure times should be minimized, using the lowest possible power output needed to obtain the necessary information. Its uses in clinical practice include screening, diagnosis and treatment of disorders. However, views depend on patient BMI, operator experience and optimal equipment settings.

Leiomyoma: leiomyoma (fibroids) are the most common benign uterine tumours and are found in upto 40% of women in their reproductive years. They are composed of smooth muscle and a variable amount of fibrous tissue and are located within the uterine wall. US is used initially to identify whether they are submucosal, intramural or subserosal in location. They appear as well circumscribed, hypoechoic, rounded or oval masses within the uterine body, and may cause acoustic shadowing. Cystic areas within them can indicate degeneration whilst calcification can be seen after the menopause. High velocity blood flow may be seen, particularly within large fibroids. A subserous or pedunculated fibroid may have a similar appearance to a solid ovarian tumour. MRI may be needed to characterize fibroids, particularly where multiple large fibroids are difficult to see in detail on ultrasound, or where UAE is being considered.

How does ultrasound work? Ultrasound uses the pulse-echo principle of sound above 20 kHz, i.e. a frequency above the human audible range. Each pulse of sound transmitted into the patient generates echoes through reflection and scatter, at different depths within the tissue (Venables 2011). Using modern equipment ensures that reliable images are produced, together with the appropriate technique (transabdominal versus transvaginal). Transabdominal (TA) US is ideal in women who are not yet sexually active, but requires a full bladder to provide a “window” for views. Transvaginal (TV) US provides a more detailed view of the endometrium and ovaries. Box 1 considers the differences between TV and TA ultrasound. To optimize equipment settings, being familiar with the following is key:  Frequency e this ensures adequate penetration

Other uterine pathology:  Congenital uterine anomalies may be seen using ultrasound. A septate uterus can be seen particularly well in the transverse plane.  Uterine polyps are seen as distinct hyperechoic areas within the endometrium protruding into the endometrial cavity. They are best seen in the proliferative part of the menstrual cycle, although their appearance remains constant throughout the menstrual cycle. Their appearance can be highlighted by hysterosonography, where saline is instilled into the endometrial cavity to outline lesions within the cavity.  Although ultrasound features may be subtle, adenomyosis can be seen as hypoechoic heterogeneous areas with anechoic lakes within the myometrium, an enlarged

Shreelata Datta BSc (Hons) MBBS MRCOG LLM is a Locum Consultant Obstetrician and Gynaecologist at St Helier Hospital, Carshalton, UK. Conflicts of interest: none declared.

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The differences between transabdominal and transvaginal ultrasound C C

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Transabdominal ultrasound provides a wider field of view Transvaginal ultrasound provides better resolution of smaller fibroids Transvaginal ultrasound is invasive and should be avoided in virgo intacta

Box 1

“globular” appearance and an indistinct endometrial/ myometrial border. An asymmetrical thickening of the anterior and posterior myometrium may also be seen.  Intrauterine contraceptive devices are seen as highly reflective echoes in the uterine cavity and are best seen in the sagittal view (Figure 1).

Figure 1 An anteverted uterus, with normal endometrial thickness, showing the characteristic acoustic shadowing from an intrauterine device, correctly located in the upper part of the uterine cavity.

cystadenoma is the second most common benign tumour of the ovary after dermoid cysts. They are typically unilocular, thinwalled cysts which can measure upto 20 cm. Mucinous cystadenoma present as large multilocular cystic lesions, which contain multiple fine septations. Endometriomas appear as discrete well-defined thick-walled cysts, with a characteristic “ground glass” appearance. This represents fluid with a high density of solid particles within it (Figure 2). Complex adnexal masses include teratomas, tubo-ovarian abscesses, dysgerminomas and haemorrhagic cysts. Dermoid cysts are seen on US as a distinctive complex mass with highly echogenic components. In patients presenting acutely with ovarian cysts, ovarian torsion must be excluded. The risk of ovarian torsion increases with ovarian volume and requires a comprehensive scan with Doppler flow to confirm the diagnosis. However, it is important to remember that ovarian torsion may be present without a compromised blood flow in the early stages, due to the dual blood supply by the ovarian and uterine artery.

Ovarian pathology The ovaries are usually seen inferior to the pelvic vessels on the lateral pelvic walls. They are usually elliptical and slightly hypoechoic. Ovarian cysts may be physiological (usually smooth, <10 cm in size, unilateral, hypoechoic and simple) or pathological (usually larger, asymmetrical and complex). Ultrasound is the first imaging modality of choice to detect and characterize adnexal masses. Whilst it can distinguish between benign and malignant adnexal masses, it has no role for staging. Physiological cysts are common and include follicular cysts in the first half of the menstrual cycle and corpus luteum cysts in the latter half of the cycle. Many resolve spontaneously and therefore should be re-evaluated in 6e8 weeks time. Cystic adnexal masses include simple ovarian cysts, cystadenomas, cystadenofibromas and hydrosalpinges. Serous

Practical tips for performing ultrasound scans C

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Familiarize yourself with the preset settings on your ultrasound machine as these are suited to specific types of examination Check the orientation of the images you obtain and find a way to identify orientation without having to look repeatedly at the probe Use the highest frequency that provides adequate penetration to improve spatial resolution Adjust the focal depth to improve lateral resolution and contrast to delineate small structures clearly throughout the examination Always check the patient’s name, date of birth and address. Obtain consent, check whether they have a latex allergy and have a chaperone available Figure 2 An endometrioma, demonstrating the classic “ground glass” appearance, with normal ovarian tissue seen peripherally.

Box 2

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Bilateral ovarian enlargement may be seen in PCOS, or in ovarian hyperstimulation syndrome in patients undergoing assisted conception. A polycystic ovary is defined as an ovary which contains 10 or more cysts, each measuring 2e8 mm in diameter with an increase in ovarian stroma. In patients undergoing fertility treatment, ovarian hyperstimulation syndrome is seen on US as massively enlarged ovaries bilaterally, multicystic with multiple follicles. Ascites may also be present. TV ultrasound is used to retrieve oocytes from each ovary after hCG injection using a specialized needle guide. It is important to remember when scanning that complications of early pregnancy (e.g. ectopic pregnancy) are more common after fertility treatment.

IOTA rules to distinguish benign and malignant tumours (NB. If both benign and malignant features are present, the simple rules are inconclusive) Malignant tumour features

Unilocular cyst Presence of solid components Acoustic shadows

Irregular solid tumours Ascites Minimum four papillary projections Irregular multilocular solid tumour (diameter >10 cm) High Doppler blood flow

Smooth multilocular tumour No Doppler blood flow

Fallopian tube pathology Normal fallopian tubes may be difficult to visualize on ultrasound, but are easier to see when distended by fluid. Causes include tubo-ovarian abscess, tubal pregnancy or pelvic inflammatory disease. Hydrosalpinges may be mistaken for ovarian cysts, but are usually avascular. A tubo-ovarian abscess is typically seen as a multilocular mass with thick septations with areas of differing echogenicity.

Table 1

 Shape/outline of gestational sac  Presence of yolk sac  Presence of embryo  CRL  Presence/absence of fetal heart movement  Uterine/adnexal pathology  Nuchal measurement The presence of a gestational sac within the uterus is the earliest ultrasonic confirmation of an intrauterine pregnancy. It is usually seen from 4 þ 3 weeks gestation (31 days) as a circular transonic area surrounded by a thick bright ring, eccentrically placed at the uterine fundus. The “double decidual sign” is demonstrated by the inner and outer layer of the decidual reaction surrounding the gestational sac. The earliest accurate sign of pregnancy is the yolk sac, which is a circular mass within the gestation sac which can first be seen at about 35 days, when it measures 3e4 mm in diameter. It reaches a maximum diameter of 6 mm at 10 weeks. An embryo can usually be seen from approximately 37 days on transvaginal ultrasound and initially appears as a bright echo, adjacent to the yolk sac. Once a fetal pole can be seen, the crownrump length (CRL) and gestational sac measurements must be recorded. Fetal cardiac activity can be detected from 6 weeks gestation. All embryos with a CRL >6 mm should have visible cardiac activity.

Diagnosing malignancy on ultrasound Endometrial cancer: in post-menopausal women, the endometrial thickness should measure less than 5 mm; if it is more than this, uterine malignancy must be excluded. It is important to note that a thickened endometrium is a sensitive but non-specific indicator of endometrial disease. As such, benign endometrial polyps cannot be distinguished ultrasonographically from endometrial hyperplasia or endometrial carcinoma. Endometrial hyperplasia has a characteristic appearance: the endometrium is usually thickened with an increased echogenicity and occasional small cystic areas. Other causes of increased endometrial thickness include HRT or tamoxifen therapy, or endometrial hyperplasia. Ovarian cancer: suspicious US features include thick septations (>3 mm), wall irregularity, papillary projections, solid components, size greater than 4 cm and ascites. The presence of colour Doppler flow can also suggest neovascularisation, associated with malignancy. Table 1 summarizes the five rules based on the IOTA study to distinguish benign and malignant ovarian tumours.

Miscarriage Ultrasound features in early pregnancy which suggest a poor outcome include a fetal heart rate below 85 bpm at over 7 weeks gestation, a small sac size relative to the embryo, an enlarged or abnormally shaped yolk sac and sub-chorionic haematoma. These features do not offer definitive guidance and are not part of any current protocol on which to make a diagnosis of miscarriage. Miscarriage should be suspected in any woman with a positive pregnancy test and vaginal bleeding. The ultrasound diagnosis of miscarriage should only be considered with:  A mean gestational sac diameter 25 mm (with no visible yolk sac)  Fetal pole with no evidence of heart activity and a crownrump length of 7 mm

Early pregnancy ultrasound The current RCOG guidelines recommend transvaginal scanning in all women under 12 weeks gestation. A normal intrauterine pregnancy (IUP) should be visible transvaginally when the serum B-HCG levels are above 1000 m IU/mL, which occurs between 5 and 6 weeks gestation. TA ultrasound is less reliable in early pregnancy but can be done first where women are uncertain of their dates. In upto 30% of women, a diagnosis cannot be made by ultrasound on the initial visit. Most of these are failing pregnancies, or normal very early pregnancies, which cannot yet be seen on ultrasound. Observations during an early pregnancy ultrasound include:  Number of sacs  Mean sac diameter

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Benign tumour features

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Where there is any doubt about the diagnosis, or if a woman requests a repeat scan, this is repeated at least 1 week later. No growth in gestation sac size or CRL strongly suggests a nonviable pregnancy in the absence of embryonic structures. A pregnancy is classified as a “pregnancy of uncertain viability” when the mean gestational sac diameter is <25 mm with no yolk sac or embryo visible, or when there is an embryo with no visible heartbeat and a CRL of <7 mm.

and amnionicity), an accurate gestational age can be calculated. Any gross fetal abnormalities and pelvic pathology can also be picked up. Estimating the due date: an accurate CRL is the most accurate means of estimating the gestational age, but requires an unflexed, longitudinal section of the embryo. The CRL measures the maximum extended and unflexed length in which the endpoints of crown and rump are clearly defined (BMUS CRL criteria). The yolk sac should not be included as this overestimates the gestational age. Technical difficulties in measuring the CRL include patient obesity and a retroverted uterus, as well as excessive fetal movement. It becomes much easier to identify the end-points of the embryo after 7 weeks. Identifying the fetal spine (seen beyond 9 weeks) can be used as a guide to assess the true fetal length. Beyond the first trimester, gestational age can be estimated using the biparietal diameter, head circumference, abdominal circumference and femur length. Table 2 summarizes the parameters used to estimate gestational age. Assumptions made when using US to date a pregnancy include a normal embryo which is normal- sized, an accurate measurement technique and correctly calibrated equipment.

Ectopic pregnancy An ectopic pregnancy is defined as implantation of the fertilized ovum outside the uterine cavity and is most commonly located in the fallopian tubes, cornua or ovary, in conjunction with an empty uterus. In 10e30% of cases, a pseudogestational sac which is incompletely surrounded by myometrium and thickened endometrium may be seen in utero. Unlike a gestational sac, a pseudosac is typically avascular. Ectopic pregnancies should be suspected in patients who have a positive pregnancy test, pelvic pain and no intrauterine pregnancy visible. Typical characteristics include an adnexal “ring-like” structure, comprising of a hypoechoic centre surrounded by a thick echogenic ring may be seen. This must be distinguished from the corpus luteum, which is a physiological structure associated with pregnancy which produces progesterone and is predominantly seen in the first trimester. It is usually cystic with a thick circumferential wall with flow on pulsed Doppler. It should be noted however that 78% of ectopic pregnancies are ipsilateral to the corpus luteum. Fluid in the pouch of Douglas is seen in upto 25% of ectopic pregnancies. Blood and clots may appear as hyperechoic fluid on ultrasound, suggesting a tubal abortion or a ruptured ectopic pregnancy.

Screening: screening for trisomy 13, 18 and 21 can also be done at the dating (12 week) and anomaly (20 week) scan. First trimester screening should include nuchal translucency measurements, together with serum markers such as hCG and PAPPA. The nuchal translucency is defined as a collection of subcutaneous fluid behind the neck, seen between 11 and 14 weeks gestation. Increased nuchal translucency thickness (>3 mm) assesses the risk of trisomy 21 in combination with maternal age, serum B-HCG and PAPP-A at 11e14 weeks.

Gestational trophoblastic disease (molar pregnancy) Gestational trophoblastic disease is rare and US is unreliable in making the diagnosis, which requires histology. However, a “snowstorm” appearance is seen on ultrasound or “cluster of grapes” due to the presence of cystic placental tissue.

Increased nuchal translucency is seen in several conditions, including cardiac defects, diaphragmatic hernia, exomphalos, achondroplasia and Noonan syndrome. The first trimester fetal heart rate in chromosomally abnormal fetuses may be tachycardic in trisomy 13 and Turners syndrome, whereas in trisomy 18 and triploidy there is a tendency for bradycardia. In trisomy 21, there is a mild increase in fetal heart rate. In spina bifida, the neural tube fails to close, which normally occurs by the 6th week of gestation. US may show an abnormal “lemon shaped” head, associated with an abnormal cranial

Obstetric ultrasound The NICE guidelines for Antenatal Care recommend routine US at 12 and 20 weeks gestation for all pregnant women. Ultrasound in pregnancy allows the following:  Establishing gestational age and estimate due date  Identifying the location of pregnancy  Confirming fetal viability  Identifying fetal abnormalities (including CVS/ amniocentesis)  Screening for pre-eclampsia  Evaluating placental location  Uterine fibroids and adnexal masses which may interfere with labour The first ultrasound scan is between 11 and 13 þ 6 weeks and achieve many of the aims outlined above.

Ultrasound parameters used to estimate gestational age (NB gestational age cannot be accurately determined by ultrasound beyond 24 weeks) Gestational age (weeks) Transvaginal scan 4e6 weeks 6e13 weeks Transabdominal scan 13þ to 24 weeks

11e14 week dating scan Routine first trimester ultrasounds confirms the presence of an intrauterine pregnancy and its viability. As well as identifying the number of fetuses (and in multiple pregnancies the chorionicity

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Parameter

Mean gestational sac diameter/volume Crown-rump length Biparietal diameter, femur length

Table 2

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 Head circumference  Fetal weight is based on AC, or in combination with HC, FL and BPD. AC is the most sensitive predictor of fetal weight  Doppler US can be used to measure the blood flow velocity in fetal vessels. The changes seen in the umbilical arteries precede those of the fetal middle cerebral and thoracic aorta. Absent end diastolic blood flow may be seen in IUGR, prematurity, oligohydramnios or SLE The amniotic fluid (AF) index is measured by dividing the uterus into four quadrants and the deepest vertical pocket of AF is measured in the four quadrants. An AF index of 10e17 cm is normal.

shape and the “banana” sign, which relates to a curved cerebellum. As well as early diagnosis of major fetal defects, other benefits of the 11e14 week scan include the detection of multiple pregnancies, with reliable identification of chorionicity, the main determinant of outcome in multiple pregnancies. Twins account for approximately 1% of pregnancies, with 2/3 being dizygotic and 1/3 being monozygotic. To determine chorionicity by ultrasound, the number of placentas and characteristics of the membrane between the two amniotic sacs must be seen. When diagnosing multiple pregnancy, the “lambda” sign identifies dichorionic diamniotic twins, which occurs as a result of the amniotic and chorionic membranes surrounding each fetus separated slightly at the insertion site by a tongue of placental tissue. All dyzygotic twins have separate placentas and therefore separate chorionic and amniotic sacs. By comparison, the “T sign” is seen at the placental insertion of the amniotic membranes in monochorionic diamniotic twins. Chorionicity is best evaluated before 14e15 weeks.

Biophysical profile This is a tool for antepartum surveillance to identify a compromised fetus. It is a non-stress test which evaluates fetal movement, tone, breathing and the amniotic fluid index over 20e40 minutes. A composite score is allocated, although management depends on the gestational age. A 2007 Cochrane systematic review supported the use of the biophysical profile as a test of fetal well-being.

The anomaly scan The anomaly US is performed between 18 and 22 weeks. It involves examining the brain, face, spine, heart, stomach, bowel, kidneys and limbs. It also reviews placental morphology and amniotic fluid volume.

Invasive procedures In obstetrics, US is used in:  Amniocentesis e this is usually performed at about 16 weeks gestation and is predominantly used to find out the fetal karyotype. It can confirm or exclude fetal anomalies associated with chromosomal abnormalities. It involves sampling the amniotic fluid under direct ultrasound vision and is associated with a 1% risk of miscarriage/IUD.  CVS e this is the first trimester alternative to amniocentesis which is performed between 10 and 12 weeks. However, there is a slightly higher rate of miscarriage and mosaicism from the placenta may necessitate a fetal blood sample. In gynaecology, US can be used to aspirate ascites or insert an abdominal drain. It also enables ERPCs to be performed under direct vision.

Placental location: it is checked in every second and third trimester US and is best identified by scanning the uterus longitudinally. It is documented as anterior, posterior, fundal, low lying or praevia in relation to the internal cervical os. Placenta praevia is defined as implantation of the placenta over the cervical os. In the second trimester, placenta praevia can be seen in upto 15% of patients. Ultrasound can also be used to diagnose placental implantation abnormalities. These include haemorrhage, multiple cysts (suggestive of triploidy) and placental masses (chorioangioma). Findings such as the loss of a normal hypoechoic rim of myometrial tissue under the placental surface or the presence of placental echo suggest these abnormalities. Placenta accreta is seen as the loss of normal retroplacental hypoechoic zone, with increased vascularity.

Conclusion As the resolution of US improves, we are able to identify a greater range of abnormalities, but this comes at a cost. In addition, we can now perform ultrasound not only to diagnose or screen patients, but also in a therapeutic manner. Obtaining the maximum amount of information from an ultrasound examination however, relies on up-to-date equipment, appropriate patient preparation and operator competence. An accurate diagnosis and effective management plan cannot be made by ultrasound alone, but requires an assessment of the patient’s presenting complaint, clinical examination and blood investigations, together with effective counselling on the findings of US. A

Cervical length assessment: whilst routine cervical length measurements are not cost-effective, in selected populations, cervical length measurements (distance between the internal and external os) are performed from 14 to 16 weeks gestation. Ultrasound has advantages over digital examination as the internal os can be visualized by sonography, detecting early changes of cervical incompetence. Most studies use a cut off length of 25 mm to predict the risk of preterm labour or indicate the need for the insertion of a cervical cerclage in high-risk women. It should be noted that in patients with multiple gestation, cervical length is significantly shorter.

FURTHER READING Chudleigh T, Thilaganathan B. Obstetric ultrasound: how, why and when. Elsevier, 2004. Datta S, Kunde K, Bourne T. Managing early pregnancy loss: diagnosing miscarriage is not as easy as you think. OGRM Sept 2012; 22: 269e70. Levitov A, Dallas A, Slonim A. Bedside ultrasonography in clinical medicine. McGrawHill, 2010.

Fetal biometry and growth scans Growth US rely on an accurately established gestational age. Fetal measurements include:  Abdominal circumference e this can be used to assess fetal growth and to evaluate any disproportion between head and body size

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Naidoo K. Transvaginal ultrasound in gynaecology e the normal and abnormal pelvis BMUS. Bulletin May 2000; 8: 12e5. Nicolaides K, Sebire N, Snijders JM. The 11e14 week scan: the diagnosis of fetal abnormalities. Parthenon Publishing, 1999. Venables H. How does ultrasound work? USS 2011; 19: 44e9.

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Always check the patient’s details prior to performing an ultrasound, take a basic history and obtain verbal consent Be prepared to perform a transabdominal ultrasound scan if limited views are obtained by transvaginal ultrasound, and vice versa

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Have a systematic approach to performing the scan, and go back to areas which are difficult to visualize in the first instance. Check your machine settings are optimized to help you obtain a clear image When speaking to the patient, be aware of the language you use and the implications of your findings If you are unable to view the ovaries clearly, press firmly with the ultrasound probe and identify the internal iliac vessels. If performing a transvaginal scan, ask the patient to apply pressure in the iliac fossa to displace bowel and to improve the definition of the ovaries Review the reports written by your consultants and sonographers as this will help you develop your own report-writing skills

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