IMAGING TECHNIQUES AND SAFETY

The evaluation of abdominal pain in pregnancy and puerperium requires a careful approach due to the safety of both mother and fetus.

Among the imaging modalities available are:

1.Ultrasound: this is the technique of choice due to its accessibility and absence of ionizing radiation, although its effectiveness may be reduced in the third trimester.

2.Magnetic Resonance Imaging (MRI): it is safe and adequate to evaluate complex conditions, but should be avoided in the first trimester as well as the use of gadolinium.

3.Computed Tomography (MDCT): reserved for critical situations, applying the ALARA (As Low As Reasonably Achievable) principle.

Effects of radiation during pregnancy

• Preimplantation phase (conception to 2nd week): risk of non-implantation or undetected embryo death.
• Organogenesis phase (3rd-8th week): risk of malformations with threshold doses of 100-200 mGy or more.
• Fetal development phase (9th-25th weeks): fetal doses > 100 mGy may reduce IQ.

According to RSNA, no deterministic effects are expected at doses below 100 mGy (10 rads) and no stochastic effects are expected at doses below 20-50 mGy (2-5 rads). table[1]

 Pregnancy CT Protocol

• Detailed information to the patient.
• Informed consent.
• Previous dose estimation: We consider that MDCT involves a radiation on the uterus of 20mGy.
• Subsequent calculation by the Radioprotection Service.

Use of contrast media

Iodinated contrast media can cross the placenta and affect the fetal thyroid, although no sequelae have been described for short exposures.

The FDA classifies them as level “B”, allowing their use in pregnant women if necessary for diagnostic purposes.

Gadolinium also crosses the placenta; its use is justified if the benefits outweigh the risks.

Iodinated contrasts should be used only when essential, with informed consent, and exposed infants should be screened for hypothyroidism in the first week.

FINDINGS AND PROCEDURE DETAILS

OBSTETRIC COMPLICATIONS

Obstetric complications include ectopic pregnancy, placental abruption, placenta accreta spectrum, uterine rupture and postpartum hemorrhage.

 1.Ectopic pregnancy

Implantation of the blastocyst outside the endometrium, being more common in the fallopian tubes (95%). Incidence of 2% of pregnancies and cause of maternal death in the first trimester. fig[2]

Risk factors include a history of ectopic pregnancy, pelvic inflammatory disease and infertility treatments.

The classic clinical triad (amenorrhea, abdominal pain and vaginal bleeding) occurs in less than 50% of cases, and abdominal pain in fertile women should be considered.

Transvaginal ultrasound (first line): most effective method for diagnosis.fig[3]

• Adnexal mass without intrauterine sac with βHCG levels >1000-1500 mIUI/mL (96% specificity).
• Extrauterine gestational sac visible in some cases.

Differential diagnosis:

     Corpus luteum: “Ring of fire” appearance on color Doppler.

     Hemoperitoneum can be associated with rupture ectopic pregnancy and hemorrhagic cyst.

MDCT

Useful in inconclusive diagnosis or incidental finding of ectopic pregnancy.fig[4], fig[5]

Characteristic findings:

• High density complex adnexal mass.
• Peri adnexal fat striation.
• Haemoperitoneum.

MRI

Indicated in indeterminate ultrasound.

Findings:

• Gestational sac

                 -Hyperintense on T2WI.

                 -Hypo-isointense in T1WI, with areas of acute haemorrhage.

• Adnexal haematomas and tubal dilatation due to haemosalpinx.
• Tubal wall enhancement.

Differential diagnosis: corpus luteum cyst.

Located in the ovary,thick wall.Thick wall,slightly hyperintense on T1WI and hypointense on T2WI.Homogeneous enhancement after contrast.

2.Uterine Rupture

Complete tearing of the uterine layers that can cause severe bleeding and fetal distress. Rare complication during the third trimester of pregnancy, delivery or immediately thereafter.

Uterine dehiscence is an incomplete separation of the myometrium in a previous scar.

Risk factors include previous surgeries (caesarean and myomectomies), abnormal placental implantation and abdominal trauma.

Ultrasound

• Intra- or extraperitoneal haematoma.
• Evidence of uterine dehiscence.
• Parts/whole fetus in extrauterine location, focal protrusion of membranes through dehiscence.

MDCT  fig[6]

Modality of choice for postpartum patients.

• Hypodense areas across the myometrium.
• Solutions of continuity.
• Haemoperitoneum.

MRI:

More accurate than ultrasound in differentiating between uterine rupture and other uterine wall defects, such as dehiscence. It allows identification of the intact serosal layer, which is key in this differentiation.

Treatment varies: rupture is managed with caesarean section, while dehiscence is treated conservatively.

3.Postpartum haemorrhage fig[7]

Classification; primary (within 24 hours) and secondary (between 24 hours and 6 weeks postpartum).

Severe complication and cause of maternal death in the puerperium.

Risk factors are uterine atony, retained products of conception, placenta accreta, endometritis and uterine vascular malformations.

Ultrasound: confirm or rule out retained products of conception

In complex cases, MDCT and MRI are used to assess lesions, infections and plan treatment.

Angiography is necessary for accurate diagnosis and invasive treatment.

4.Placental pathology:

4.1. Placental abruption; premature separation of the normally implanted placenta.

Risk factors: chronic hypertension, pre-eclampsia, cocaine and tobacco use, multiple gestations, advanced age, multiparity and abdominal trauma.

Classification according to the location of the haematoma; retroplacental, marginal or subchorionic, and preplacental or subamniotic.

Clinical signs include vaginal bleeding, pain and uterine contractions, mainly in the second and third trimester of pregnancy. In some cases, bleeding may be ‘hidden’, as in the case of retroplacental haemorrhage.

Ultrasound is the first and often the only diagnostic modality used.

Ultrasound signs include:

• Presence of a retroplacental haematoma.
• Intraplacental anechoic áreas.
• Separation and rounding of the placental margin.
• Thickening of the placenta and retroplacental myometrium.
• Alterations of the retroplacental circulation.
• Intra-amniotic echoes due to haemorrhage.

MRI

• In highly suspicious cases with negative ultrasound findings.
• The signal intensity of the haemorrhage on T1 and diffusion vary over time determined by the presence of paramagnetic substances derived from haemoglobin.

4.2. Placental implantation anomalies; 

Implantation of the placenta in the lower uterine segment, which may cover the cervical os.

Transvaginal ultrasound: Sensitivity 100%, specificity 98.8%.

MRI: Useful for identifying other abnormalities such as placenta accreta or percreta.

Placenta accreta, increta and percreta result from failure of the normal process of placental decidual formation and implantation, with placental villi adhering to, invading or passing through the myometrium.

Placental implantation abnormalities

• Placenta accreta: villi attached directly to the myometrium.
• Placenta increta: villi invade the myometrium.
• Placenta percreta: Villi invade the myometrium and may invade adjacent organs.

Risk factors are previous caesarean sections, abnormal implantation sites, multiparity, advanced age.

Ultrasound and MRI are key diagnostic tools to identify these placental abnormalities, allowing a detailed assessment of the placenta and surrounding tissues.

Ultrasound:

• Variable sensitivity and specificity (50-100% and 50-98%).
• Findings: vascular lacunae (‘Swiss cheese’), disruption of retroplacental flow, myometrial thinning, exophytic masses.

MRI: fig [8],fig [9]

• Sensitivity 80-85%, specificity 65-100%.
• Indicated in inconclusive ultrasound scans or suspected placenta percreta.
• Findings: hypointense T2 bands, loss of normal uterine morphology, placental protrusion with invasion of adjacent structures (bladder, rectum, etc.).

Complications: Massive haemorrhage, uterine rupture, damage to neighbouring structures due to placental invasion.

NON-OBSTETRIC COMPLICATIONS IN PREGNANCY

1.Gastrointestinal

1.1Acute appendicitis:

 Main non-obstetric surgical cause.

• Ultrasound (technique of choice): non-compressible appendix >6mm.
• MRI: Hyperintense in T2. fig[10]
• If MRI is not available, CT is an alternative.The risk of misdiagnosis without accurate imaging outweigh the small potential risk of ionising radiation. fig[11]

1.2 Bowel obstruction: More frequent in the third trimester due to uterine compression. fig[12]

• Ultrasound limited value.
• MRI T2 weighted images.
• Diagnosis confirmed by CT scan, useful to identify ischaemia.

1.3 Inflammatory bowel disease: Activity decreases, but outbreaks may occur. fig[13]

• MRI: mural thickening, strictures, fistulas.

2.Hepatic-biliary

2.1 HELLP: Associated with pre-eclampsia. fig[14]

HELLP syndrome is a pregnancy-related condition and is an abbreviation for:

• haemolysis
• elevated liver enzymes and 
• low platelets

General features predominantly involve hepatic sequelae:

• hepatomegaly: especially the right lobe.
• hemorrhage,subcapsular hematoma, rupture.
• hepatic infarction.

Ultrasound is usually preferred over CT to avoid ionizing radiation.

The place of CT is mainly to assess for complications.

2.2 Cholecystitis: Second surgical cause in pregnant women.

• Ultrasound (technique of choice): gallbladder distension, pericholecystic fluid.
• MRI: complications such as perforation or abscesses.

2.3 Acute pancreatitis: common causes include cholelithiasis and hyperlipidaemia.

• Ultrasound diagnosis: lithiasis.
• MRI: peripancreatic oedema.

3.Urogenital

3.1 Urolithiasis/pyelonephritis: most common cause of non-obstetric hospitalisation.fig[15]

• Initial ultrasound diagnosis
• MRI for complications.

3.2 Ovarian torsion: Rare, mainly in the first trimester.fig[16]

• Ultrasound: abnormal ovarian position, whirlpool sign.
• MRI: torsioned pedicle.

3.3 Uterine leiomyomas: Red degeneration associated with pregnancy.

Hemorrhagic infarction secondary  to venous thrombosis.fig [17]

• Ultrasound
• MRI: haemorrhagic necrosis.

4.Vascular

Ovarian vein thrombosis: rare postpartum complication. fig[18]

• Diagnosis by ultrasound (no Doppler flow).
• CT scan extension to inferior vena cava.