High Yield Overview
Imaging in Pregnancy: Safety Considerations
—Teratogenic Threshold
100Greater than mGy
—Typical Pelvic X-ray
1mGy fetal dose
—MRI Safety
—Preferred over CT
—US Safety
—No radiation, safe any trimester
Imaging Modality Safety in Pregnancy
Ultrasound: Safe, no radiation
MRI (no Gd): Safe, preferred for many indications
X-ray: Low dose, generally safe with precautions
CT: Higher dose, use MRI alternative if possible
Nuclear: Avoid unless essential
Key: Never withhold clinically indicated imaging that will change management
Critical Must-Knows
- MRI and ultrasound have no ionising radiation - preferred in pregnancy
- X-ray/CT: Fetal dose typically well below teratogenic threshold
- Greatest fetal risk: 2-8 weeks gestation (organogenesis)
- Clinical benefit must be weighed against theoretical risk
- Do not delay essential imaging that affects management
Examiner's Pearls
- "MRI preferred over CT in pregnancy (no radiation)
- "Shield fetus if pelvis not being imaged
- "Gadolinium crosses placenta - avoid unless essential
- "Iodinated contrast: Limited data, use if truly needed
- "Most diagnostic X-rays: Fetal dose less than 10 mGy
Clinical Imaging
Imaging Gallery
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Exam Warning
Imaging in pregnancy is a common viva topic. Know that MRI is preferred over CT, most X-ray exposures are well below teratogenic thresholds, and clinical benefit usually outweighs theoretical risk. The key message: do not withhold essential imaging that will affect patient care.
No imaging study should be withheld if it is clinically necessary and will change patient management. The risk of missing a serious diagnosis usually outweighs the minimal radiation risk from diagnostic imaging.
Fetal Radiation Effects
Radiation Effects by Gestational Age
| Period | Gestation | Primary Risk | Threshold |
|---|---|---|---|
| Pre-implantation | 0-2 weeks | All-or-nothing (loss or no effect) | Greater than 100 mGy |
| Organogenesis | 2-8 weeks | Teratogenesis, major malformations | Greater than 100 mGy |
| Early fetal | 8-15 weeks | Mental retardation, microcephaly | Greater than 100 mGy |
| Mid-late fetal | 15-25 weeks | Reduced IQ (milder) | Greater than 100 mGy |
| Late fetal | Greater than 25 weeks | Minimal structural risk | Stochastic risk only |
Key Threshold Concept
Deterministic effects (malformations, mental retardation) have a threshold of approximately 100 mGy, well above any diagnostic imaging dose. Stochastic effects (cancer) have no threshold but very low probability at diagnostic doses. Most X-rays deliver less than 10 mGy to the fetus.
Fetal Doses from Common Examinations
Estimated Fetal Doses
| Examination | Fetal Dose (mGy) | Risk Assessment |
|---|---|---|
| Chest X-ray | Less than 0.01 | Negligible |
| Extremity X-ray | Less than 0.01 | Negligible |
| Lumbar spine X-ray | 1-5 | Low |
| Pelvic X-ray | 1-2 | Low |
| Hip X-ray | 0.2 | Negligible to low |
| CT head | Less than 0.01 | Negligible |
| CT chest | 0.1-0.3 | Low |
| CT abdomen/pelvis | 10-50 | Moderate (but still below threshold) |
Practical Interpretation
Almost all diagnostic X-ray procedures result in fetal doses well below 10 mGy - far below the 100 mGy threshold. Even CT abdomen/pelvis, while delivering higher doses, typically remains below teratogenic thresholds. The risk-benefit calculation almost always favours imaging when clinically indicated.
Modality Selection
Imaging Modality Choice in Pregnancy
| Modality | Safety Status | Key Considerations |
|---|---|---|
| Ultrasound | Safe (first choice for many) | No radiation, excellent for soft tissue |
| MRI (no contrast) | Safe (preferred over CT) | No radiation, excellent for many MSK indications |
| X-ray | Generally safe with precautions | Shield fetus, collimate, minimise exposures |
| CT | Use if MRI not suitable | Higher dose, but safe if indicated |
| Nuclear medicine | Avoid if possible | Consider risk-benefit carefully |
| MRI with Gd | Avoid unless essential | Gadolinium crosses placenta |
| CT with contrast | Use if needed | Limited human data, appears safe |
Mnemonic
US-MRI FirstPregnancy Imaging Priority
U
Ultrasound: First choice if it can answer the question
M
MRI: Preferred over CT (no radiation)
X
X-ray: Acceptable with precautions
C
CT: If MRI not available/suitable
A
Always: Clinical need outweighs theoretical risk
Memory Hook:Choose the modality that will answer the clinical question with the lowest radiation exposure
Contrast Agents in Pregnancy
Gadolinium in Pregnancy
Gadolinium crosses the placenta and is excreted into amniotic fluid where it has prolonged half-life. Animal studies show teratogenicity at high doses. Human data limited but no confirmed adverse effects at diagnostic doses. Recommendation: Avoid unless the information is essential and cannot be obtained otherwise. If used, use minimum dose of Group 3 (macrocyclic) agent.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
EXAMINER
"A 28-year-old woman at 12 weeks gestation presents with severe back pain and possible cauda equina syndrome. She requires urgent imaging."
EXCEPTIONAL ANSWER
This is a clinical emergency requiring urgent imaging regardless of pregnancy. I would recommend MRI lumbar spine without gadolinium as the first-line investigation. MRI has no ionising radiation and is safe in pregnancy. Counselling: I would explain that MRI is safe and preferred over CT in pregnancy. The risk of missing cauda equina syndrome (permanent neurological deficit) far outweighs any theoretical imaging risk. MRI provides excellent detail of the disc, neural structures, and spinal cord. If MRI is unavailable urgently and cauda equina is strongly suspected, CT myelography would be an alternative - the radiation dose to the fetus would be small and the clinical need justifies it.
KEY POINTS TO SCORE
MRI without Gd is safe in pregnancy
Clinical emergency - must image
Risk of missed CES far outweighs imaging risk
MRI preferred over CT (no radiation)
CT acceptable if MRI unavailable
COMMON TRAPS
✗Delaying imaging due to pregnancy concerns
✗Ordering CT when MRI would be better
✗Using gadolinium without strong indication
VIVA SCENARIOStandard
EXAMINER
"A pregnant woman at 20 weeks has fallen and requires imaging of her pelvis for suspected fracture. She is very concerned about radiation exposure."
EXCEPTIONAL ANSWER
I would counsel her that her concern is understandable but the risk from diagnostic X-rays is very low. Key points: (1) The fetal dose from a pelvic X-ray is approximately 1-2 mGy, far below the 100 mGy threshold for teratogenic effects. (2) X-rays are safe for diagnostic purposes when clinically indicated. (3) Not imaging a fracture could lead to worse outcomes. My imaging approach: Start with plain X-rays of the pelvis - these will diagnose most fractures with minimal radiation. If X-rays are inconclusive and further detail is needed, MRI can provide excellent fracture detail without radiation. CT would only be considered if MRI is unavailable and surgical planning requires it. The key message: a pelvic X-ray in pregnancy is safe when indicated.
KEY POINTS TO SCORE
Pelvic X-ray fetal dose: 1-2 mGy (well below threshold)
100 mGy threshold for teratogenic effects
X-ray first, MRI if more detail needed
CT only if MRI unavailable and necessary
Risk of missed fracture outweighs radiation risk
COMMON TRAPS
✗Refusing X-ray due to pregnancy
✗Over-stating radiation risks
✗Not explaining the threshold concept
VIVA SCENARIOStandard
EXAMINER
"A trauma patient at 8 weeks gestation requires CT of her abdomen and pelvis for suspected visceral injury after a motor vehicle accident."
EXCEPTIONAL ANSWER
At 8 weeks gestation, we are in the organogenesis period when the fetus is most sensitive to radiation. However, this is a trauma situation where CT may be life-saving. My approach: (1) Clinical benefit clearly outweighs risk - suspected visceral injury can be fatal. (2) CT abdomen/pelvis fetal dose is approximately 10-50 mGy - below the 100 mGy teratogenic threshold. (3) Proceed with CT if clinically indicated. (4) Optimise protocol: Use lowest dose protocol that provides diagnostic images, single phase if possible (not multiphasic). (5) Document the indication and the discussion. (6) After the acute situation, offer counselling about the exposure and reassure that the dose was below harmful thresholds. The principle: never withhold life-saving imaging because of pregnancy.
KEY POINTS TO SCORE
Trauma: Life-saving imaging must proceed
CT abd/pelvis: 10-50 mGy (below 100 mGy threshold)
Optimise protocol to minimise dose
Document indication and counselling
Reassure about threshold concept
COMMON TRAPS
✗Delaying life-saving imaging
✗Not knowing approximate fetal doses
✗Not counselling about exposure afterwards
Imaging in Pregnancy Quick Reference
High-Yield Exam Summary
Modality Priority
- •1. Ultrasound (safe, no radiation)
- •2. MRI without Gd (safe, preferred over CT)
- •3. X-ray (low dose, safe with precautions)
- •4. CT (if MRI unavailable)
Key Thresholds
- •Teratogenic threshold: Greater than 100 mGy
- •Most X-rays: Less than 10 mGy fetal dose
- •CT abd/pelvis: 10-50 mGy
- •Below threshold = no deterministic effects
Contrast Agents
- •Gadolinium: Avoid unless essential
- •Iodinated: Can use if indicated
- •Check neonatal thyroid if iodine near term
Key Message
- •Never withhold clinically indicated imaging
- •Risk of missed diagnosis outweighs radiation risk
- •Counsel and document
- •MRI preferred over CT when possible