Obstetrics Simplified - Diaa M. EI-Mowafi
Ultrasonography in Obstetrics
Physics
The highest audible sound waves ranges between 18.000-20.000 hertz (18-20 KHz) where the hertz is one cycle or wave per second. Ultrasound is sound waves above this audible range and for diagnostic purposes it ranges between 2.25 and 7.5 megahertz (usually 3.5 MHz is used).
The transducer of the ultrasound machine contains piezoelectric crystals previously were made of quartz but now these are synthetic crystals. These crystals have the property of changing the electric to sound waves and vice versa.
The electric current supplied to the machine generates ultrasound waves from these crystals to be reflected from tissue interfaces at various depths and by different echoes according to the nature of the medium i.e. fluid, tissue or bone.
The reflected sound waves is re-transformed again into electric waves presented as dotes or lines on the ultrasound screen.
The ultrasound waves can pass through fluid and solid media but not effectively through gases, so a gel should be applied between the transducer and skin.
In abdominal ultrasonography, the full bladder is essential for effective transmission of the waves and delineation of the pelvic organs in gynaecological and first trimester obstetric diagnosis. There is no need for full bladder if a vaginal probe is used.
Vaginal transducer of frequency 5.0-6.5 MHz is more convenient for detection of tubal and ovarian masses and early pregnancy at least one week earlier than the abdominal ultrasound.
Modes
A (Amplitude) mode
It is a unidimensional system used to measure the depth of structures not used in obstetrics.
B (Brightness) mode
It is two-dimensional system. Returning echoes are displayed as bright spots on the screen.
Static: rarely used.
Real-time: 40 images or more are obtained per second so that the movement of the structures is shown. This can be achieved by 3 different techniques:
- Linear: giving a rectangular image ( more convenient for obstetric examination).
- Sector: giving a wedge-shaped image (more convenient for gynaecological examination).
- Convex: giving an image midway between linear and sector.
T-M (Time -Motion) mode
The static structure appears as a straight line and movement is represented by a wave giving a pattern similar to ECG of the foetal heart.
Three-dimensional mode
The most recent one giving an image of 3 dimensions.
Indications
Diagnosis of:
- Intrauterine pregnancy.
- Ectopic pregnancy.
- Gestational age as follow:
- from 5-6 weeks by measurement of gestational sac,
- from 6-12 weeks by measurement of the crown- rump length (CRL),
- from 12-26 weeks by measurement of the biparietal diameter (BPD- most convenient at this time).
- > 26 weeks - BPD and/ or femur length (FL) measurement.
- Foetal viability: by foetal heart movement and limb movement from 6 weeks.
- Multiple pregnancy: may be detected in the early weeks but diagnosed reliably by 16 weeks.
- Congenital major structural anomalies: can be detected by 16 weeks.
- Placental site, size, maturity and separation in abruptio placentae.
- Foetal wellbeing: (see before).
- Different types of abortion including blighted ovum, threatened, inevitable, incomplete, complete and missed abortions.
- Vesicular mole.
- Intrauterine growth retardation
- Intrauterine foetal death.
- Amount of liquor amnii.
- Foetal presentation and position.
- Retained products in case of postabortive or postpartum haemorrhage.
- An aid for invasive procedures as:
- Amniocentesis.
- Chorionic villus biopsy.
- Culdocentesis.
- Intrauterine foetal therapy.
Hazards of Ultrasound
Up till now, there is no data about any deleterious effects on the foetus or the mother.
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