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Ultrasound in obstetrics. By Dr. Khattab KAEO Assis. Prof. of Obstetrics and Gynaecology Faculty of Medicine, Al-Azhar University, Damietta. Introduction. Ultrasound has a frequency >20 000 Hz (20kHz). However, ultrasound machines have frequencies of 2-10 mega Hz (MHz).
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Ultrasound in obstetrics By Dr. Khattab KAEO Assis. Prof. of Obstetrics and Gynaecology Faculty of Medicine, Al-Azhar University, Damietta
Ultrasound has a frequency >20 000 Hz (20kHz). However, ultrasound machines have frequencies of 2-10 mega Hz (MHz). • Higher frequencies give better resolution, but decreased tissue penetration; thus, used to examine near structures. Conversely, lower frequency probes are used to examine deep structures. For instance, abdominal probes give 3-5 MHz, while vaginal probes give 5-7.5 MHz. • It is best to reduce 'depth' as much as possible. • Increasing the 'gain' increases the echoes, and thus, may improve the image where obesity is csausing attenuation.
Early pregnancy scanning Aim:To determine location of pregnancy (intra- or extra-uterine), viability, gestational age and fetal number, in addition to adnexal pathology (an ovarian cyst mostly). Uterine abnormalities may be seen as an empty cavity adjoining the pregnancy sac.
Gestational sac: It could be detected by TAS from 6 weeks’ amenorrhoea, while TVS may detect it from 4.5 weeks (2-4mm). Normal sac growth is 0.7-1 mm/day. It is considered abnormal if its tro-phoblastic reaction is <2 mm. Shape of the sac may be affected by uterine contraction or bladder fullness.
The embryo: • Yolk sac (10mm) is the first structure to be seen within the sac. Itshould be detec-ted within an intrauterine gestational sac of a 20 mm diameter using TAS, or 8 mm using TVS. It is first seen at 5 weeks’ gestation on TVS and at 6 weeks on TAS. It has no predictive value but confirms that the pregnancy is intrauterine. • Embryo is first visible with heart pulsa-tion on TVS at 5 weeks (2-4mm embryonic length). • Heart tone is first visible on TVS at 6.5 weeks (sac diameter of 15-20 mm).
Early pregnancy assessment clinic (EPAC) Aim:avoidance of admission or reduced hospital stay (& cost).
Ultrasonography results: # Viable intrauterine pregnancy: Most women are suitable for immediate discharge and GP follow-up.
# Fetal pole, no cardiac activity: Some are viable, while others represent delayed miscarriage. Early embryos typically appear adjacent to the yolk sac in the periphery of the gesta-tional sac. CRL is the key for management. CRL 6mm = home & re-scan in 7-10d CRL >6 mm = termination.
A dead embryo of CRL >6mm and no cardiac activity as seen by M mode ultrasonography.
# Empty gestational sac: Some are viable, while others represent blighted ovum. The mean sac diameter (MSD = the mean of 3 perpendicular measurements), rather than volume, is the key for management. MSD 20mm = home & re-scan in 7-10 d. MSD >20 mm = termination of pregnancy. You should look for a second opinion.
Empty gestational sacs (absent embryo, even if amniotic sac is seen [arrow]). Arrow heads point to thin decidual reaction. The lowermost sonogram shows an abnormally large yolk sac, presented for comparison with the first one sonogram.
# Retained products of conception: Mixed echogenicity with irregular echo-bright areas (it is difficult to differentiate blood clots from retained tissues). Mostly the tissue is of <30 mm maximum diameter with light blood loss and no signs of infection, and so, management is conservative. Large volume of tissue or heavy blood loss = evacuation.
# Empty uterus: Differential diagnosis: 1- very early pregnancy; 2- complete miscarriage; or 3- ectopic pre gnancy. Consider: 1- history for risk factors for ectopic pregnancy, 2- examination findings and 3- -hCG level as well as its rate of disappearance. If tissues have been passed, this should be examined microscopically for chorionic villi. When -hCG level exceeds 1000 iu/l, intrauterine pregnancy would be visible by TVS (5000 iu/l for TAS). -hCG level is the key for management. A- -hCG <1000 iu/l: All the 3 possibilities are probable. If there are no risk factors for ectopic pregnancy and no peritonism, review after 48 hours by TVS and -hCG. The absolute level of -hCG (1000IU/L) should be relied upon rather than the rate of rise in -hCG level. Some ectopic pregnancies (13%) show normal rate of -hCG rise. Some normal pregnancies (15%) show slow rate of -hCG rise. (Between the 2nd & 4th post-ovulation weeks the level of -hCG doubles every 48 hours; ectopic pregnancy and abortion shows <66% rise). B- -hCG 1000 IU/L, only ectopic pregnancy or complete abortion are possible. Laparoscopy can be considered or selectively with review in 48 hours. Complete abortion can be confirmed by a -hCG fall to 20% by 48 hours. History may assist decision-making. Regarding the disappearance rate of hCG: if it is less than 1.4 days, the most likely diagnosis is miscarriage. If it is greater than 7 days, the case is almost always ectopic pregnancy.
