Pelvic ultrasound

Pelvic ultrasound is usually the initial modality for imaging gynecologic pathology, including acute pelvic pain and chronic pelvic pain. The exam normally involves two components: a transabdominal (TA) evaluation and a transvaginal (TV) / endovaginal (EV) evaluation.

Normal ultrasound anatomy

Uterus

(see: uterus)

• consists of endometrium, junctional zone, and myometrium
• the appearance of the endometrium depends on which part of a woman's menstrual cycle she is in, and can vary from 2-15 mm
• the endometrium normally has a three-layer appearance at midcycle but is usually more homogeneous later
• the junctional zone may be difficult to detect on ultrasound
• the uterus is normally tilted toward the anterior abdominal wall (anteverted)
• if the uterus is enlarged, the transabdominal exam may be needed to evaluate the full size

Adnexa

• the ovaries also vary in size with the menstrual cycle
  • due to varying sizes of follicles
  • nulliparous: maximum volume of 9 mL
  • parous: maximum volume of 15 mL
• a dominant follicle in the ovaries reaches 20-25 mm diameter at maturity
  • do not call an anechoic ovarian structure a "cyst" in a premenopausal woman unless it is >30 mm
  • the dominant follicle becomes the corpus luteum
• fallopian tubes are not normally seen on ultrasound unless dilated or surrounded by free pelvic fluid

Technique

Pelvic ultrasound usually includes two components:

• transabdominal (TA) evaluation
• transvaginal (TV) evaluation
  • some prefer the term "endovaginal" (EV)

The transabdominal component is always performed first. The transvaginal component is performed second and, because of the higher resolution of the transvaginal probe, is usually very helpful for an evaluation of the pelvic structures.

Despite the usefulness of the transvaginal exam, it may not be performed if:

• the patient does not want it
• the patient has never had sexual intercourse (virgo intacta) 
• pediatric patient

Transabdominal exam

A mid-low frequency transducer (e.g. up to 5 MHz) is usually used. A full bladder is used as an acoustic window to achieve better imaging of the uterus and adnexa.

The transabdominal exam is used for a general overview of the pelvis and may be necessary in some situations in order to image the entirety of some pelvic pathologies.

Transvaginal/endovaginal exam

A mid-high frequency probe (e.g. >7 MHz) is usually used. The bladder is emptied before the exam. The higher frequency allows a better spatial resolution and Doppler evaluation of the pelvis.

Practical points

• mural nodularity can be missed in large cystic lesions, make sure to carefully evaluate the entire wall
• if there is trouble determining which organ a mass is arising from, a bimanual technique can be used in which the mass is imaged while the two organs are manually pushed in different directions
  • if the mass moves with an organ, it arises from the organ
  • if the mass slides past an organ, it arises from the adjacent organ
• if the uterus is retroverted, consider endometriosis

See also

• uterus
• differential for endometrial thickening
• ovaries
• cervix