Pelvic ultrasound (transabdominal)

Background

Pelvic anatomy.

• Ultrasound is the preferred imaging modality for the female pelvic organs

Indications

• To evaluate female reproductive organs in pediatric patients or those that are not sexually active or refuse transvaginal exam
• For evaluation during later pregnancy
• Start with transabdominal ultrasound when evaluating to rule out ectopic (less invasive than transvaginal), particularly if bHCG >6,000

Technique

• 3.5 MHz curvilinear probe
• Start above pubic symphysis in longitudinal axis
• Locate endometrial stripe (echogenic line) which represents the center of the uterus
• Scan through longitudinal axis of uterus and sweep laterally to try to visualize ovaries (though often difficult to visualize on transabdominal ultrasound)
• Rotate probe to transverse plane above pubic symphysis and scan through uterus from cervix to fundus
• Scan hepatorenal recess to assess for free fluid

Normal Findings

Uterus

• Anteverted: Fundus pointing to anterior abdominal wall
• Retroverted: Fundus pointing to posterior wall
• Endometrial stripe: Measured from thickest echogenic area (from one basal endometrial interface to the other, including canal)
  • Pre-menopausal:
    • During menstruation: 2-4mm
    • Early proliferative phase: 5-7mm
    • Late proliferative phase: 11mm
    • Secretory phase: 7-16mm
    • After D&C or SAB: >5mm
      • Consider retained POC if thicker
  • Post-menopausal:
    • With vaginal bleeding: <5mm
    • Without vaginal bleeding: <11mm

Ovaries

• Physiologic cysts present
  • <3 cm diameter
  • Ovarian follicle or corpus luteum
  • Typically don't cause complications

Pregnancy (1st trimester)

• 4-5 weeks
  • Gestational sac
    • First sign of early pregnancy, usually seen between 3-5 weeks gestation
  • "Double decidual sac" sign
    • Two concentric rings (uterine lining and gestation lining) around anechoic gestational sac)
    • Highly suggestive of intrauterine pregnancy (before yolk sac or embryo embryo visualized
  • "Intradecidual sign"
    • Thickened decidua on one side of uterine cavity surrounding anechoeic sac

• 5-6 weeks:
  • Yolk Sac
    • Circular (or sometimes as two parallel lines) echogenic structure with thick walls within gestational sac
    • Definitive evidence of intrauterine pregnancy
    • Multiple yolk sacs is earliest sign in multiple gestational pregnancy
  • "Double bleb" sign
    • Yolk sac and amniotic cavity that look like two bubbles within gestational sac

• 6-7 weeks
  • Fetal Pole
    • First visual manifestation of fetus (or "embryo")
    • Thickening of margin on yolk sac
  • Cardiac activity present (FHR ~100-115 bpm)
  • Crown rump length: ~5mm
• 8-9 weeks
  • Limb buds appear
  • Head identifiable
• 9-10 weeks
  • FHR ~170-180 bpm
  • Fetal movement visible
  • End of embryonic period

Abnormal Findings

Abnormal Pregnancy

• Gestational sac >10mm without visible yolk sac
• Gestational sac >18mm without fetal pole
• Collapsed gestational sac
• Pseudogestational sac
  • Endometrial breakdown during ectopic pregnancy
  • May be erroneously interpreted as true gestational sac in ectopic pregnancy
  • Irregularly shaped
  • Located in endometrial cavity, instead of eccentrically within endometrium
  • No yolk sac present
  • May not have double decidual sac sign
• Absence of fetal heart beat in embryo with CRL >5mm
• FHR <90 bpm
• Molar pregnancy
  • Many small, irregular cystic structures in endometrium
  • "Snowstorm appearance"

Ectopic pregnancy

• Implantation of blastocyst outside of endometrium
• Occur in fallopian tubes, cervix, ovaries, peritoneal cavity, or scar of prior uterine surgery
• Most definitive sonographic sign is gestational sac with yolk sac, embryo, or fetal heart beat outside of the endometrium
• Tubal pregnancy
  • "Tubal ring" sign: echogenic thick-walled anechoic sac visualized between uterus and ovary (separate from ovary when pressure applied to fallopian tube with probe)
• Interstitial Pregnancy
  • Implantation of gestational sac in muscular wall of uterus at proximal regional of fallopian tubes
  • "Interstitial line" sign: hyperechoic line from endometrial stripe to cornual region adjacent to interstitial gestational sac
• Cervical Pregnancy
  • Implantation of gestational sac below internal cervical os
  • Can cause life-threatening hemorrhage
  • Must distinguish between cervical ectopic and spontaneous abortion at level of cervix

Ovarian Torsion

• Bimodal age distribution: young women (15-30yo) and post-menopausal women
• Typically occur in enlarged ovaries (>5cm)
  • Benign mature cystic teratomas
  • Hemorrhagic/large ovarian cysts
  • Cystic neoplasms
  • Cystadenomas
  • Polycystic ovarian syndrome
• Sonographic signs associated with torsion
  • Enlarged ovary from edema, engorgement, hemorrhage
  • Midline ovary
  • Free pelvic fluid
  • Underlying ovarian lesions
  • Peripherally displaced follicular cysts
  • Doppler findings:
    • Decreased/absent venous or arterial flow
    • Absent/reversed diastolic flow
    • Can have normal flow (from dual supply from uterine and ovarian arteries)

Pearls and Pitfalls

• Best performed with full bladder to provide good window to view posterior structures and to move bowel gas out of view, however make sure to not confuse bladder with other cystic structures that may be present (ie. large ovarian cyst)
• Free pelvic fluid with positive hCG is ectopic pregnancy until proven otherwise
• Presence of doppler flow cannot rule out ovarian torsion