OB-OB IMAGING

• Obstetrical ultrasound


  • It is fundamental to prenatal care
  • It is used to confirm gestational age and viability
  • It is used to detect and characterize abnormalities of the fetus, amniotic fluid, and placenta
  • It is used to assist with diagnostic and therapeutic procedures

  
• Ultrasound practice continues to evolve
• The number of components included in the second-trimester standard and detailed fetal anatomical surveys has expanded
• With improved image resolution, fetal abnormalities are increasingly identified in the late first trimester
• This has prompted the requirement for a limited anatomical survey during the standard first-trimester examination and has led to the development of a new detailed first-trimester examination
• Detailed placental evaluation is a new specialized examination to aid detection and characterization of placenta accreta spectrum
• Ultrasound


  Sound waves traveling at a frequency above 20,000 hertz (Hz)
• Ultrasound transducers


  • They contain groups of piezoelectric crystals that convert electrical energy into sound waves and convert returning sound waves back into electrical energy
  • The sound waves are emitted in synchronized pulses
• Gray-scale imaging (B-mode)


  Reflected waves are displayed as bright echoes on the screen
• Fluid


  Generates few reflected waves and appears dark
• Higher-frequency transducers


  Yield better image resolution
• Lower-frequency transducers


  Penetrate tissue more effectively
• Transducer frequency range


  • 5-12 MHz in early pregnancy
  • 4-6 MHz in first and second trimesters
  • 2-5 MHz in third trimester (particularly for obese patients)
• ALARA principle


  • Ultrasound should be performed only for a valid medical indication and use the lowest possible exposure setting to gain necessary information
• No causal relationship has been demonstrated between diagnostic ultrasound and any recognized adverse effect in human pregnancy
• Thermal index


  Measures the relative probability that the examination may raise the temperature enough to induce injury
• Mechanical index


  Measure of the likelihood of adverse effects related to rarefactional pressure, such as cavitation
• The use of ultrasound for any non-medical purpose, such as "keepsake fetal imaging", is considered contrary to responsible medical practice and is not condoned
• Operator safety


  • The reported prevalence of work-related musculoskeletal discomfort or injury among sonographers approximates 70 percent
  • The most common injuries are capsulitis and tendonitis of the shoulder, epicondylitis of the elbow, carpal and cubital tunnel syndrome, and neck or back strain
• Guidelines to help avert injury


  1. Position the patient close to you on the examination table

  2. Adjust the table or chair height so that your forearm is parallel to the floor

  3. Use a chair with back support if seated

  4. Face the monitor squarely and position it so that it is viewed at a neutral angle from the horizon

  5. Avoid reaching, bending, or twisting while scanning

  6. Take frequent breaks to help prevent muscle strain
• Gestational age


  Based on the certainty of the woman's last menstrual period (LMP) date and measurements of the embryo or fetus at the initial ultrasound examination
• Crown-rump length (CRL)


  The most accurate method to establish or confirm gestational age
• Biometric parameters


  Biparietal diameter, head circumference, abdominal circumference, and femur length
• Before 22 weeks, gestational age assessment using these four biometric parameters is accurate to within 7 to 10 days
• If the initial ultrasound examination is performed at or beyond 22 weeks' gestation, the pregnancy is suboptimally dated

  
• Fertilization is presumed to occur 2 weeks after a confident LMP
• Gestational age assessment using biometric parameters


  Accurate to within 7 to 10 days
• (American College of Obstetricians and Gynecologists, 2019c)
• Various nomograms available for other structural measurements


  • Transverse cerebellar diameter
  • Orbital distances
  • Thoracic circumference
  • Length of the ear
  • Length of the kidney
  • Length of long bones
  • Length of feet
• These may be used to address specific questions regarding organ system abnormalities or syndromes (Appendix, pp. 1238–1241)
• If the initial ultrasound examination is performed at or beyond 22 weeks' gestation


  The pregnancy is suboptimally dated

  
• In such cases


  Subsequent ultrasound evaluation in 3 to 4 weeks may be considered

  
• Fertilization is presumed to occur 2 weeks after a confident LMP


  1. For pregnancies achieved with in vitro fertilization and fresh transfer, 266 days are added to the egg-retrieval/fertilization date to calculate the EDD

  2. For pregnancies conceived with a day-3 frozen embryo, adding 263 days accounts for the days of embryo culture

  3. For pregnancies conceived with intrauterine insemination, LMP is used
• Three types of first-trimester examinations


  • Standard ultrasound
  • Nuchal translucency evaluation—between 11 and 14 weeks' gestation
  • Detailed first-trimester anatomy evaluation between 12 and 14 weeks' gestation
• Indications for the standard first-trimester examination


  • Confirm an intrauterine pregnancy
  • Estimate gestational age
  • Confirm cardiac activity
  • Diagnose/evaluate a multifetal gestation, including chorionicity and amnionicity
  • Assess for certain fetal anomalies, such as anencephaly
  • Measure fetal nuchal translucency, when part of an aneuploidy screening program
  • Evaluate for uterine abnormalities or pelvic masses
  • Evaluate for suspected ectopic pregnancy
  • Evaluate for suspected gestational trophoblastic disease
  • Evaluate for the cause of vaginal bleeding
  • Evaluate for the cause of pelvic pain
  • Serve as adjunct to embryo transfer, chorionic villus sampling, and intrauterine device localization and removal

  
• Components of the standard first-trimester examination


  • Gestational sac size, location, and number
  • Embryo and/or yolk sac identification
  • Crown-rump length
  • Gestational age assessment
  • Fetal number, including amnionicity and chorionicity of multifetal gestations
  • Embryonic/fetal cardiac activity, documented with M-mode or 2-dimensional video clip
  • Fetal anatomy assessment including calvarium, nuchal region, ventral wall cord insertion, and presence of limbs (depending on gestational age and fetal size)
  • Fetal nuchal translucency assessment
  • Maternal uterus, adnexa, and cul-de-sac evaluation

  
• Nuchal translucency


  The maximum thickness of the subcutaneous translucent area between the skin and soft tissue overlying the fetal spine at the back of the neck
• The nuchal translucency (NT) is measured in the sagittal plane between 11 and 14 weeks' gestation using precise criteria (Table 14-4)
• (American Institute of Ultrasound in Medicine, 2018a; International Society of Ultrasound in Obstetrics and Gynecology, 2016)

  
• When the NT measurement is increased


  The risk for fetal aneuploidy and various structural anomalies—in particular heart defects—is significantly elevated