Advanced Fetal Ultrasonography and Therapy

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What is a “level II” obstetric ultrasound examination, and what elements of fetal anatomy are included?

A level II obstetric ultrasound examination is a comprehensive real-time ultrasonography (US) evaluation of fetal anatomy to detect fetal anomalies, whereas a level I obstetric ultrasound is a more general evaluation to detect obstetric problems.

For the elements of fetal anatomy that are included in a level II obstetric ultrasound examination, see Table 60-1 .

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What is a “detailed extended” obstetric ultrasound examination?

The detailed extended obstetric ultrasound examination involves real-time US maternal evaluation plus detailed fetal anatomic examination. This is used when there is an increased risk of fetal abnormalities. The elements included in the detailed extended obstetric ultrasound examination are more detailed themselves in comparison to those of the level II examination. For the elements of fetal anatomy that are included in a detailed extended obstetric ultrasound examination, see Table 60-2 .

In addition to the fetal anatomic survey elements of a level II obstetric ultrasound examination, a detailed extended obstetric ultrasound examination usually includes:

• Doppler US evaluation of the umbilical artery, umbilical vein, and ductus venosus.

• Measurement of the fetal nasal bone length.

• Documentation of the fetal extremities (includes imaging the forearms and forelegs to show the radius, ulna, tibia, and fibula and detailed imaging of the hands and feet to show five digits).

• Cardiac outflow tracts.

• Fetal gender.

Other anatomy that is evaluated depends on the suspected or detected abnormalities, including:

• Doppler US evaluation of one or both middle cerebral arteries.

• Demonstration of the corpus callosum (and pericallosal artery on color flow Doppler US).

• Measurement of fetal external ear length.

• Measurement of the cerebellar width and cisterna magna anteroposterior diameter.

• Demonstration of the anterior maxilla and mandible on transverse views.

• Aortic and ductal arch views.

• Cardiothoracic circumference (or area) ratio.

• Pulmonary vein drainage to the left atrium.

• Fetal gallbladder.

• Fetal spleen.

• Fetal adrenal glands.

• Sagittal and transverse kidney images.

• Fetal perineum (showing anal orifice).

• Fetal genitalia.

• Measurement of long bones (all four extremities) as well as foot length.

• Measurement of other skeletal structures such as clavicles, scapulae, and finger lengths.

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What are the common nonemergent indications to perform a detailed extended obstetric ultrasound study?

Nonemergent indicators of abnormal pregnancy that may prompt use of a detailed extended examination include:

• Abnormal maternal screening. This includes the sequential screen which includes measurement of some combination of maternal serum alpha fetoprotein (AFP), pregnancy-associated plasma protein A (PAPP-A), estriol, human chorionic gonadotropin (hCG), and dimeric inhibin A (DIA). More recent screening involves the cell-free DNA screening test, also referred to as noninvasive prenatal testing (NIPT), which examines the fetal DNA found in maternal blood and can detect trisomy 13, 18, and 21 and Turner syndrome. A patient who has an abnormal result of chorionic villus sampling or amniocentesis would also be a candidate for a detailed extended ultrasound study.

• Size-date discrepancy (includes polyhydramnios and oligohydramnios as causes).

• Changes in fetal movement. This can be an emergency if fetal activity changes from normal to markedly decreased or absent, as it indicates fetal distress at a minimum and fetal demise in the worst case scenario.

• History of previous pregnancy (or pregnancies) resulting in abnormal fetuses or with spontaneous abortions that have shown chromosomal abnormalities.

• Advanced maternal age (i.e., the patient is, or will turn, ≥ 35 years of age during the pregnancy).

• Family history of congenital anomalies.

• Maternal infections exposure (TORCH-toxoplasmosis, other, rubella, cytomegalovirus, herpes; other includes such known teratogenic virus as parvovirus B19).

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What is the significance of an elevated maternal serum AFP level?

An elevated maternal serum AFP level may prompt an amniocentesis because an elevated level in the amniotic fluid is strong evidence of a fetal anomaly, usually a neural tube or abdominal wall defect. Maternal serum AFP is usually high in multiple gestations and is normally higher than the overall population average in some ethnic groups.

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What is the significance of size-date discrepancies?

Size-date discrepancies are frequently the result of errors in dating, usually because of an unknown last menstrual date or irregular menstrual cycles. Implantation bleeding can be mistaken by the patient as a menstrual period and cause a dating error. US is used to check on the fetal size compared with that predicted from dates. This may require sequential US studies to demonstrate a normal growth rate or to show an abnormal one.

Large for dates fetuses can result from maternal diabetes mellitus, either gestational or type II. Assuming that an error in dating has been excluded, a large for dates pregnancy (i.e., when the fundal height measured by the obstetrician is greater than expected) can also mean multiple gestations, high amniotic fluid level (polyhydramnios), or fetal macrosomia. Small for dates fetuses can result from too low of an amniotic fluid level (oligohydramnios), intrauterine growth restriction, or constitutionally small fetus (a diagnosis of exclusion). US is useful for both large and small for gestational age pregnancies to determine the etiology and plan follow-up.

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A detailed extended obstetric ultrasound examination can take considerably longer than a level II study. What are the safety elements of obstetric US?

Though ultrasound is not ionizing radiation and is generally regarded as safe, use of ultrasound does mean exposing tissues to mechanical energy. The mechanical energy of ultrasound has two basic effects in tissue: heating, and mechanical compressive and shear forces. Heating, or thermal, effects result from the sound energy causing tissues to vibrate. This vibration results in a rise in temperature. Compressive and shear forces can disrupt some tissues and have been shown to cause hemorrhages in bowel and lung (in experimental animal studies). The mechanical forces are much intensified if gas is present (which should not be so in a fetus) or near the edges or surfaces of bone because the reflection from bone can be so strong. The shear forces can also cause cavitation, resulting in the formation and collapse of small bubbles. This exerts tremendous local forces and can disrupt normal tissues. In nonmedical situations, cavitation exerts a powerful enough force to erode metal.

The United States Food and Drug Administration (FDA) has imposed output limits on ultrasound machines because of these thermal and mechanical effects. The maximum output is 720 milliwatts (mW) per square centimeter (cm ² ). However, for fetal US, the FDA limits output to 94 mW/cm ² . This power limit is the reason that an obstetric ultrasound study is performed using the ultrasound machine's “OB” setting. Machines set up the output power limits based on examination type. In agreeing to higher output limits for US machines overall, the FDA required manufacturers to display indices of the output power being used. This is known as the output display standard (ODS) and is the reason for the display on each image of two indices: the thermal index (TI) and the mechanical index (MI) ( Figure 60-1 ).

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Why does the TI have a letter after it?

The thermal index has three variants depending on the tissue types that will be encountered: TIs for soft tissue, TIb for bone, and TIc for the cranium. The thermal index is the ratio of the power being produced by the machine to the power needed to raise tissue temperature by 1° centigrade. The mechanical index can be thought of as the likelihood of causing cavitation. Use of color flow Doppler and spectral Doppler US increases the output power. For documentation of embryonic and fetal cardiac activity, M-mode may be used because it does not require output power greater than regular gray scale B-mode imaging.

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What values of TI and MI are recommended for fetal US studies?

TI and MI recommendations for fetal studies include:

• TIs values are used for first trimester studies.

• TIb values are used for second and third trimester studies.

• TI <0.5 should be used particularly for first trimester studies.

• TI <0.5 may likely be used for extended scan times.

• TI >0.5 and <1.0 should be limited to scan times of less than 30 minutes.

• TI >2.5 should be limited to scan times of less than one minute.

• MI ≤1.9 (the current FDA limit) can be used as needed in the absence of gas bodies.

In general, as for ionizing radiation, the principle of “as low as reasonably achievable (ALARA)” should be employed for ultrasound, particularly for obstetric studies.

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