ULTRASOUND
The use of ultrasound in medicine was developed from SONAR (Sound Navigation and Ranging) systems used to detect submarines. Sound is the orderly transmission of mechanical vibrations through a medium. The number of vibrations that occur per second is known as the frequency and is measured in hertz (Hz), 1 Hz being 1 cycle/second. The human ear can detect frequencies in the range of 20 Hz to 20,000 Hz (20 kilohertz). Sound above this range is known as ultrasound. Most medical diagnostic equipments have a range of 1-10 megahertz (MHz), 1 MHz is 1,000,000 cycles/second.
When an interface between structures of different tissue densities is encountered some of the energy is reflected back. The reflected energy is displayed on the screen in shades of grey. A dense structure like bone appears white on the screen (echogenic) while fluid appears black (anechoic).
Ultrasound has proved very valuable in monitoring the pregnancy. It is safe and is devoid of any fetal risk or biological hazard.
Early Pregnancy
DIAGNOSIS OF INTRAUTERINE PREGNANCY Demonstration of a gestation sac within the uterus is the earliest ultrasonic confirmation of an intrauterine pregnancy. The gestation sac may be visualised from the time of the missed period by using the transvaginal method and from 5 weeks using the transabdominal method. Gestation sac volume is used to estimate gestational age with the help of reference tables.
VIABILITY OF PREGNANCY The pregnancy is said to be viable when fetal heart pulsations are demonstrated within the gestation sac. These are visualised by how many week using the vaginal method and by 6 weeks using the abdominal method.
MULTIPLE PREGNANCY Multiple gestation sacs can be identified as early as singleton sacs. The diagnosis of an ongoing multiple pregnancy is made when the fetal viability is confirmed in all the sacs.
Complications of Early Pregnancy
Missed abortion: This is diagnosed when a fetal pole is visible but no fetal heartbeat is heard. When the fetal pole is not seen in the gestation sac, blighted ovum is diagnosed.
Ectopic pregnancy: The main role of ultrasound in this condition is to confirm or exclude an intrauterine pregnancy. In the absence of intrauterine pregnancy, other signs of an ectopic gestation should be sought. An adnexal mass, especially if associated with free fluid in pouch of Douglas, is very suggestive of an ectopic pregnancy.
Hydatidiform mole: The ultrasound diagnosis of hydatidiform mole is usually easy. It has a classic snowstorm appearance.
Incompetent cervix: The normal cervix has a length of 2.5 cm or more and the width of the cervical canal at the level of internal os is less then 4 mm. If the length of the cervix is less than 2 cm or the interna os is more than 8 mm, an incompetent cervix is diagnosed.
Estimation of Gestational Age
The parameters used to estimate gestational age are:
1.Gestational sac volume (GS) used at 4-7 weeks.
2.Crown-rump length (CRL) used at 6-12 weeks. It predicts gestational age with an error of 3-5 days Between 15 and 24 weeks, a combination of measurements is used to accurately estimate gestational age.
3.Biparietal diameter (BPD). It predicts gestational age with an error of 7-10 days.
4.Femur length (FL) and humerus length correlate strongly with gestational age with an error of 7-11 days.
5.Head circumference (HC)
6.Abdominal circumference (AC)
After 26 weeks, the estimation of gestational age is not very accurate. The error in the measurement of BPD and FL is 14-21 days.
A combination of measurements is better than a single measurement for assessment of gestational age.
Estimation of Fetal Growth
Ultrasound is used to detect the fetus whose growth deviates from the normal pattern. In order to assess fetal growth, the age of the fetus must be accurately established before 24 weeks of gestation.
The growth rates of the fetal head and the abdomen are studied to assess the fetal growth. The measuring of serial biparietal diameter alone is associated with high false negative rates. This is due to a brain spacing effect where the brain is less affected by growth retardation than organs such as liver and spleen. Therefore, a transverse measurement of the fetal trunk at the level of the liver (HC) is a better parameter for the detection of intrauterine growth retardation.
In women at risk of small for gestational age (SGA) babies, both HC and AC are measured and their ratio (H:A) is calculated. This ratio falls with increasing gestational age and is approximately equal to one at 36 weeks gestation. The ratio is mainly used for differentiating between symmetrical and asymmetrical SGA. A large head-trunk ratio is seen in an asymmetrically growth retarded fetus. A symmetrically small fetus shows normal head- trunk ratio, suggesting either fetal abnormality or normal but a genetically small fetus. An estimate of amniotic fluid volume is made by measuring the largest volume of amniotic fluid in a vertical plane. Normal values are from 2-8 cm. Screening for [small-for-date fetuses is needed in high-risk pregnancies for earlier and better diagnosis of growth retardation.
The finding of a large abdominal circumference (on or above the 90th centile) or of a ferus with accelerated growth should suggest the possibility of maternal diabetes mellitus. Excessive fetal growth associated with polyhydramnios (amniotic column of more than 8 cm) and a large placenta (more than 4 cm thick) suggests poor control of maternal diabetes.
Estimation of Fetal Weight
The measurement of fetal weight is usually done by measuring BPD and AC. In late pregnancy if fetal head is engaged, or in breech presentation if the shape of the head is dolicocephalic, then the measurement of BPD becomes unreliable. In these situations, AC and FL are used to estimate fetal weight.
Estimation of Fetal Well-being
Fetal well-being is assessed by biophysical profile.
Estimation of Amniotic Fluid Volume
The amniotic fluid volume can be assessed either subjectively or by measurement of maximum vertical pocket or amniotic fluid index. A maximum vertical pocket of 2-8 cm is considered normal. Amniotic fluid index is calculated by adding vertical depths of the largest pocket in each of the four uterine quadrants. Normal value is between 5 and 24 cm.
CAUSES OF POLYHYDRAMNIOS
Multiple pregnancy
Fetal abnormality, e.g., anencephaly, duodenal atresia, hydrops fetalis
Maternal diabetes mellitus
CAUSES OF OLIGOHYDRAMNIOS
■Rupture of membranes
■Intrauterine growth retardation
■ Anomalies of the female urinary tract like renal agenesis, urethral stenosis.
Detection of Fetal Anomalies
Ultrasound allows the assessment of fetal anatomy. The earliest time at which the fetus can be studied for structural anomalies is 16-18 weeks. However, some anomalies like anencephaly and encephalocele may be diagnosed as early as 9-10 weeks.
SOME SPECIFIC ANOMALIES
Anencephaly: Absence of a cranial vault gives a 'frog's eye' appearance to the fetus. This anomaly is readily detected from 14 weeks
Hydrocephalus: It is defined as the presence of excessive cerebrospinal fluid and is diagnosed by the enlargement of the ventricles, i.e. ventriculomegaly
Duodenal atresia: This gives rise to the 'double bubble appearance on ultrasound
Jejunal atresia: It produces a 'triple bubble' sign Diaphragmatic hernia: This is suspected by the finding of cystic structures within the chest. Parts of bowel may be seen passing through the diaphragm.
Clubfoot: This is diagnosed when the whole length of the foot can be seen in the same section as the tibia and fibula.
Single umbilical artery: The umbilical cord should normally contain two arteries and one vein. Eighty per cent of the fetuses with single umbilical artery are normal but it may be associated with abnormalities of urinary tract, heart and gastrointestinal tract.
Hydrops fetalis: In this condition, there is either fetal ascites or ascites, plural and pericardial effusion with skin edema. It may be due to Rh isoimmunisation or other nonmmune causes.
Markers of chromosomal abnormalities: Certain sonographic abnormalities may be indicators of an underlying chromosomal abnormality, e.g., cystic hygroma may be associated with Turner syndrome or trisomy 21; duodenal atresia may be associated with trisomy 21; omphalocele may be associated with trisomy 13 or 18.
Hyperechogenic bowel: It has been associated with fetal aneuploidy and cystic fibrosis.
The Placenta
The placenta is easily recognised by a stronger echo pattern compared to that of the underlying myometrium. By 8 weeks the placental site is easily recognised. Placental localisation is an integral part of an obstetrics examination and is reasonably accurate in skilled hands (Fig. 54.10). Texture and pattern of the placenta have been graded by ultrasound according to Grannum's grading system.In pregnancies complicated by hypertensive disease and intrauterine growth retardation, a mature placenta may be seen in earlier gestational periods.
Safety of Ultrasound
Ultrasound waves can cause human tissue damage from heat and cavitation at very high intensities. However, the low intensity at which real time imaging is done is devoid of any fetal risks.
CONCLUSION
The ionising irradiation (x-rays) causes fetal damage, whereas the biological effects of diagnostic ultrasound have been shown to be harmless. Ultrasound has made a huge contribution in obstetric management, especially in early pregnancy failures, detection of multiple pregnancy, locating placental position and detection of fetal anomalies.