What are the risks and considerations for using Computed Tomography (CT) pelvimetry in patients, particularly pregnant women?

CT Pelvimetry in Pregnancy: Risks and Considerations

CT pelvimetry should be avoided in pregnant women unless absolutely necessary, with ultrasound and MRI being the preferred imaging modalities due to their lack of ionizing radiation.

Radiation Risks to the Fetus

CT pelvimetry exposes both the mother and fetus to ionizing radiation, which carries several important risks:

• Exposure >50 mGy is associated with doubled relative risk of childhood cancer 1
• Exposure >100 mGy may be associated with increased risk of mental retardation (estimated loss of 0.025 IQ points per mGy above 100 mGy) 1
• Risk of malformations increases with fetal exposure >150 mGy 1
• A threshold of 200-500 mGy has been suggested as a possible reason for considering pregnancy termination 1

While a single CT scan typically delivers radiation below these thresholds, the stochastic effects (potential DNA damage) are theoretically possible at any dose without a clear "threshold effect" 1.

Preferred Imaging Alternatives

First-line imaging options:

• Ultrasound: Should be the initial imaging modality of choice for pregnant women 1
• MRI without contrast: Safe alternative that does not expose the fetus to ionizing radiation 1

When CT might be considered:

• Only when benefits clearly outweigh risks
• When ultrasound and MRI are unavailable or inconclusive
• When urgent diagnosis is needed for serious pathology 1

Special Considerations for CT Pelvimetry

If CT pelvimetry is deemed absolutely necessary (which is rare in modern obstetric practice):

1. Use low-dose CT techniques that can reduce radiation exposure compared to conventional radiographic pelvimetry 2, 3
2. Digital radiography techniques (CT topogram/scout view) can reduce fetal radiation dose by a factor of 10-100 compared to conventional pelvimetry 4
3. A single CT slice through the low pelvis can provide accurate measurements without the need for compensation for magnification 2

Decision-Making Algorithm

1. Determine if pelvimetry is truly necessary:

   • Modern obstetric practice rarely requires pelvimetry for suspected cephalopelvic disproportion 3
   • Consider clinical assessment and trial of labor in most cases

2. If imaging is deemed necessary:

   • Start with ultrasound evaluation
   • If ultrasound is insufficient, consider MRI pelvimetry
   • Only consider CT pelvimetry if:
     • MRI is unavailable or contraindicated
     • Immediate information is needed for urgent delivery planning
     • The clinical benefit clearly outweighs radiation risk

3. If CT is performed:

   • Use low-dose protocols specifically designed for pelvimetry
   • Limit to scout views and minimal necessary slices
   • Document the clinical necessity and informed consent

Common Pitfalls to Avoid

• Unnecessary imaging: Pelvimetry is rarely indicated in modern obstetric practice
• Overestimating radiation risk: While caution is warranted, a single CT scan typically delivers <50 mGy, which carries a minimal increased cancer risk (estimated 1.1-3 per 1000) 1
• Underestimating alternatives: MRI provides excellent soft tissue contrast without radiation exposure
• Failing to obtain informed consent: Patients should understand both risks and benefits
• Using gadolinium contrast with MRI: Gadolinium-based contrast agents should be avoided during pregnancy as they cross the placenta 1

While the absolute risk from a single CT pelvimetry is relatively low, the availability of radiation-free alternatives (ultrasound and MRI) makes these the preferred options for evaluating the maternal pelvis during pregnancy.