Pocket ObGyn – Imaging in ObGyn
See Abbreviations
Ultrasound (US)
- Transabdominal US: 4–5 mHz curvilinear transducer, better if pt’s bladder is full
- TVUS: 5–10 mHz transducer, better visualization of pelvic organs, pt’s bladder should be empty
- Doppler: To assess flow, change in frequency of reflected US shows bld flow
- SIS (aka SHG): Catheter passed through the cervix ® 10–20 cc sterile saline injected ® TVUS demonstrates uterine cavity contours. Contraindications: Preg, active pelvic infxn, obst of the cervix or vagina, Risk of infxn <1%.
• Nml measurements:
Uterus is 8 ´ 5 ´ 4 cm (smaller in prepubertal or postmenopausal women). Nml AP diameter 3–5 cm & length 6–10 cm.
EMS is <15 mm (premenopausal) & <8 mm (postmenopausal). In screening for postmenopausal vaginal bleeding, use nml <5 mm (PPV 9% & NPV 99% for endometrial cancer). EMS measured from echogenic interfaces of the anter & post basalis layers.
Ovary vol is 9.8 ± 5.8 cm3. Ovarian follicles up to 2.5 cm diameter. Avg nml ovary is 3.5 ´ 2.5 ´ 1.5 cm ® 2 ´ 1.5 ´ 1 cm postmenopausal.
Fallopian tubes are not normally visible; can see hydrosalpinx. Small amt of fluid in the post cul-de-sac may be nml physiologic.
Radiography (XR)
- Usual indications for fractures, trauma, other nonpregnant Abd shielding used.
- HSG: Inject radiopaque contrast via cervical canal ® visualize endocervical canal, endometrial cavity, lumen, & patency of fallopian tubes (via the spill of dye into the pelvis).
- DEXA: Assess bone density in the hip &
Computed Tomography (CT)
- In OBGYN, used most frequently to evaluate gyn malignancies or in the ER to evaluate the acute abd, postoperative sx, pelvic abscesses & masses, & rule out nongyn problems like appendicitis & IV contrast ok in Preg.
- Noncontrast CT: Nephrolithiasis, neuropathology (hemorrhagic stroke, head trauma, intracranial hemorrhage, intracranial lesions/masses, skull fracture)
- Contrast CT: Vascular pathology (aneurysm, dissection, ischemic stroke), trauma, bowel pathology (diverticulitis, appendicitis), abscesses, pulmonary embolism
Positron Emission Tomography (PET)
- Used mostly for malig. Radiochemical compounds measure specific metabolic processes. Eg, FDG identifies accelerated rates of glycolysis found in neoplastic
Magnetic Resonance Imaging (MRI)
- Used in w/u of uterine fibroids, adenomyosis, Müllerian duct anomalies (eg, to differentiate btw septate & bicornuate uteri), adnexal masses, fetal
Imaging During Pregnancy (Obstet Gynecol 2004;104:647; Am J Obstet Gynecol 2012;206:456)
- No reports of adverse fetal affects w/ US or MRI
- Ionizing radiation from CT or XR ® risks depending on exposure & GA
Extremely high-dose ionizing radiation ® “All or nothing” effect w/ early Preg loss. At <18 w, 500 rad is the estimated threshold for embryonic demise
| Fetal radiation exposure during imaging | |
| Procedure | Estimated fetal radiation exposure (mrad) |
| CXR (2 views) | 0.02–0.07 |
| Abdominal film (single view) | 100 |
| Hip film (single view) | 200 |
| CT scan of head or chest | <1000 |
| CT scan of abd & lumbar spine | 3500 |
| CT pelvimetry | 250 |
At term, 2000 rad is the threshold & fetal risks same as mat risks.
Risk of anomalies, growth restriction, or SAB not increased w/ radiation exposure of <5 rad. True threshold dose is likely >20 rad.
Risk of CNS effects (eg, microcephaly, mental retardation) highest at 8–15 w.There is no established risk at <8 w or >25 w.
The threshold dose of ionizing radiation ® mental retardation at <16 w is 35–50 rad. After 16 w, the threshold is 150 rad.
1–2 rad fetal exposure may leukemia risk by 1.5–2´, but baseline childhood can- cer risk is 0.2–0.3%; therefore, overall risk is still low.
No single diagnostic procedure provides radiation dose signif enough for adverse embryonic/fetal effect, esp mid to late Preg.
- Nuclear medicine: Radioactive iodine contraindicated in Preg. Tc-99m usually results in fetal exposure of <5 rad.
- Contrast agents: Iodine-based contrast safe for use in Gadolinium relative contraindicated in Preg – assess risks/benefits of contrast & obtain consent. Gadolinium crosses placenta ® excreted into amniotic fluid. Unk exposure duration & effect on fetus.
Ultrasound in Early Pregnancy
Ultrasound in Pregnancy (Obstet Gynecol 2009;113:451)
- 1st trimester US: TVUS best in early Preg to confirm IUP, evaluate ectopic Preg, determine GA, evaluate twin Preg chorionicity, confirm cardiac activity, evaluate adnexal Also obtain nuchal translucency, nasal bone for prenatal screening.
GS: Visible by ~4 w GA, eccentrically implanted in the midupper fundus w/ a bright decidual rxn (double-ring sign), visible in 2 planes. Not used to determine final GA. Mean sac diameter (the avg of 3 measurements in mm) + 30 = GA (days) ± 3 d.
Yolk sac: Visible at 5 w GA, should be seen when the mean GS diameter is
>13 mm.
Embryo: Visible at 6 w GA, or when mean GS diameter is ³20 mm.
1st trimester CRL is most accurate dating. If £9.5 w GA, CRL in mm + 42 = GA (days) ± 3 d.
FHM: Observed when the embryo is ³5 mm CRL. FHR = 100 bpm at 5–6 w GA, & ® peak 175 bpm at 9 w GA. If FHM is seen, SAB rate is 2–3% in asymptomatic low-risk women.Women <35 yo who p/w VB = 5% SAB rate if the US is nml & shows FHM.
To quickly estimate EDD from LMP, use Naegele’s rule: Add 1 y, subtract 3 mo, & add 7 d (= 280 d from LMP).
- 2nd & 3rd trimester US – see Chap.
- US for determination of GA: US dating takes preference over menstrual dating when the discrep is >7 d in the 1st trimester; >10 d in the 2nd In the 3rd trimester, accuracy of a US is w/i 3–4 w.