High-yield overview
Radiation, MRI, Ultrasound, and Contrast in Orthopaedic Practice
UltrasoundNo ionising radiation
MRIPreferred cross-sectional option
100 mGyDeterministic-risk threshold order of magnitude
CTStill justified when needed
Modality Hierarchy in Pregnancy
Ultrasound: first if it answers the question
MRI without gadolinium: preferred cross-sectional study
Radiographs: acceptable when indicated
CT: use when faster or more diagnostic than alternatives
Gadolinium: reserve for essential cases
Key: Choose the test that answers the question with the lowest reasonable fetal exposure, not the lowest exposure regardless of diagnostic value.
Critical Must-Knows
- Most orthopaedic radiographs expose the fetus to doses far below deterministic-risk thresholds.
- Ultrasound and non-contrast MRI are preferred when diagnostically adequate.
- Emergency imaging should not be delayed if maternal management depends on it.
- Gadolinium should be avoided unless the added diagnostic value is essential.
- Protocol optimisation matters more than reflexive fear of imaging.
Clinical Pearls
- “The best first answer in pregnancy imaging stems is often MRI without gadolinium if it answers the question.
- “A needed CT is safer than a missed diagnosis in major trauma or neurological emergency.
- “Do not overstate fetal risk from single diagnostic studies.
- “The exam usually rewards balanced risk-benefit reasoning, not blanket avoidance.
The Maternal Emergency Still Comes First
In trauma, suspected cauda equina syndrome, septic arthritis, unstable fracture, or suspected visceral injury, the correct principle is to image decisively. A missed maternal diagnosis is usually a bigger threat to mother and fetus than the radiation from a justified diagnostic study.
Mnemonic
SAFESAFE Imaging Hierarchy
S
Sonography first
Use ultrasound when it can answer the question
A
Avoid delay
Do not postpone needed imaging in emergency care
F
Favour MRI
Non-contrast MRI is the preferred cross-sectional option
E
Explain dose
Counsel using realistic thresholds and benefits
Hook:SAFE means choose the lowest-risk adequate test, not no test.
Mnemonic
DOSEDOSE Counselling
D
Diagnostic doses are usually low
Most single studies are far below deterministic thresholds
O
Optimise protocol
Collimate, minimise phases, and avoid unnecessary repeats
S
Substitute when appropriate
Prefer ultrasound or MRI if equally diagnostic
E
Essential imaging proceeds
Clinical need overrides theoretical risk
Hook:DOSE is how you counsel rather than frighten the patient.
Overview
Pregnancy imaging decisions are built around two facts. First, ultrasound and MRI avoid ionising radiation and therefore become preferred whenever they can answer the clinical question. Second, most diagnostic radiographs and many justified CT studies still deliver fetal doses well below the level associated with deterministic effects such as malformation or neurodevelopmental injury.
That means the real task is not to avoid imaging at all cost. The real task is to match modality to question, optimise dose when ionising radiation is used, and counsel patients honestly. Orthopaedic practice most often encounters this in trauma, suspected pelvic or spinal injury, infection, and severe back pain with neurological compromise.
Clinical Imaging
Imaging Atlas
Systematic Approach
| Step | Question | Preferred action |
|---|---|---|
| 1. Define the emergency | Is the question time-critical or limb/life threatening? | Image without delay |
| 2. Choose the safest adequate test | Will ultrasound or MRI answer it? | Use them first when equivalent |
| 3. Estimate exposure | If radiographs or CT are needed, what body region and protocol are involved? | Optimise collimation and avoid extra phases |
| 4. Consider contrast | Is contrast essential to answer the question? | Avoid gadolinium unless the extra information is necessary |
| 5. Counsel and document | Does the patient understand the rationale and expected risk? | Explain benefits, dose context, and alternatives |
High-Yield Exam Language
A strong viva answer explicitly states that the maternal indication drives imaging choice, MRI without gadolinium is preferred when suitable, and a justified CT is acceptable because diagnostic fetal doses are usually far below deterministic thresholds.
Fetal Radiation and Contrast
| Study | Typical fetal exposure pattern | Exam takeaway |
|---|---|---|
| Extremity radiograph | Negligible to extremely low | Usually safe when indicated |
| Pelvic radiograph | Low | Often far below deterministic thresholds |
| CT outside pelvis | Usually low fetal scatter | May still be justified in trauma |
| CT abdomen or pelvis | Higher than radiographs but often still below deterministic thresholds | Optimise protocol and justify clearly |
Gestational Windows and Deterministic Risk
The reason gestational age matters is that the fetus passes through phases of differing radiosensitivity. Counselling and protocol decisions should reflect this, while remembering that diagnostic doses rarely reach the relevant thresholds.
| Gestational window | Dominant concern | Practical implication |
|---|---|---|
| 0 to 2 weeks (pre-implantation) | All-or-nothing effect (loss vs unaffected survival) | Malformation risk not increased; pregnancy may not yet be recognised |
| 2 to 8 weeks (organogenesis) | Congenital malformation if dose is high | Most radiosensitive for structural anomaly, but threshold is around 100 to 200 mGy |
| 8 to 15 weeks | Highest risk of reduced IQ / severe intellectual disability | Most sensitive window for the developing CNS |
| 16 to 25 weeks | Lower CNS sensitivity | Effects only at substantially higher doses |
| All gestations | Small stochastic increase in childhood cancer | Relative risk small; absolute baseline childhood cancer risk is low |
The Numbers That Matter
Deterministic effects (malformation, growth restriction, neurological injury) have a practical threshold on the order of 100 to 200 mGy and are not expected below about 50 to 100 mGy. Essentially every single diagnostic orthopaedic radiograph or extremity CT delivers a fetal dose orders of magnitude below this. Even a CT of the abdomen and pelvis typically delivers a fetal dose well under the deterministic threshold. The stochastic risk (childhood cancer) has no threshold but is small in absolute terms.
Differential of the Pregnant Patient with Bone or Back Pain
Not every musculoskeletal complaint in pregnancy is mechanical. Several pregnancy-associated conditions can mimic trauma or degenerative disease, and recognising them changes both imaging choice and management.
| Condition | Distinguishing features | Preferred imaging |
|---|---|---|
| Pregnancy-associated osteoporosis / transient osteoporosis of the hip | Atraumatic hip or back pain, often third trimester or postpartum | Non-contrast MRI (marrow oedema); avoids radiation |
| Sacral or pubic insufficiency fracture | Low back / pelvic pain, often peripartum, normal radiographs | MRI most sensitive; radiograph often negative early |
| Pelvic girdle pain / symphyseal diastasis | Mechanical pain over symphysis or sacroiliac joints | Clinical; radiograph or MRI only if severe or refractory |
| Septic arthritis or osteomyelitis | Fever, raised inflammatory markers, joint effusion | Ultrasound-guided aspiration; MRI without gadolinium |
| Cauda equina syndrome | Saddle anaesthesia, bladder dysfunction, bilateral leg symptoms | Urgent MRI lumbar spine without gadolinium |
| Avascular necrosis of the hip | Groin pain, antalgic gait, risk factors (steroids) | Non-contrast MRI |
Do Not Anchor on Mechanical Back Pain
Back and pelvic pain are common in normal pregnancy, but red-flag features (neurological deficit, fever, atraumatic fracture, night pain) demand the same decisive imaging you would offer a non-pregnant patient. The default is non-contrast MRI, which carries no ionising-radiation risk.
Controversies and Areas of Uncertainty
How Strong Is the Gadolinium Signal?
The 2016 Ontario cohort (Ray et al, JAMA) found a statistically significant association between gadolinium MRI and stillbirth/neonatal death and a broad rheumatological/inflammatory skin outcome, but absolute numbers were small and confounding by indication is hard to exclude. Most authorities therefore counsel avoidance of gadolinium unless essential, while acknowledging the evidence is observational rather than causal.
Guidelines, Registries & Global Practice
Global Epidemiology
Trauma complicates roughly 6 to 8 percent of pregnancies and is a leading non-obstetric cause of maternal death, which is precisely why decisive maternal imaging is emphasised worldwide. Musculoskeletal complaints are near-universal in pregnancy: pelvic girdle pain and low back pain affect a large proportion of pregnancies, the majority mechanical and self-limiting, but a minority masking insufficiency fracture, transient osteoporosis, infection or neural compression.
Side-by-Side Guidance
| Body | Core position |
|---|---|
| ACOG (US) | Ultrasound and MRI are imaging of choice; do not withhold indicated radiography/CT; single diagnostic studies rarely reach deterministic thresholds |
| ACR / SPR (US) | Ionising studies appropriate when justified; emphasise protocol optimisation and realistic, region-specific dose communication |
| RANZCOG / RCR (UK) | Clinically justified imaging proceeds; non-contrast MRI preferred cross-sectional study; gadolinium avoided unless essential |
| ICRP (international) | Fetal doses under about 100 mGy are not a reason to terminate or withhold a justified examination |
| AO Foundation / trauma consensus | In major trauma, maternal stabilisation and decisive imaging take priority; fetal monitoring runs in parallel |
High- vs Limited-Resource Practice
In well-resourced settings, urgent MRI and dose-optimised multidetector CT are usually available around the clock, so the safest adequate test is rarely out of reach. In limited-resource settings, MRI may be unavailable or delayed; here the justified use of radiographs and CT becomes more important, and ultrasound (including FAST in trauma) carries even greater weight as a radiation-free, bedside, low-cost first-line tool. The underlying principle is universal: match the modality to the question, optimise dose, and never withhold imaging that will change maternal management.
Evidence Base
Evidence
ACOG Guidance on Diagnostic Imaging During Pregnancy and Lactation
American College of Obstetricians and Gynecologists • Committee Opinion 723 (2017)
Key Findings:
- Ultrasound and MRI are the imaging techniques of choice when they answer the clinical question.
- Withholding needed radiography or CT because of pregnancy is discouraged when the result will change management.
- Counselling should emphasise that necessary diagnostic imaging rarely approaches deterministic-risk thresholds.
Clinical implication: When speaking to patients or examiners, frame the decision around clinical necessity plus dose optimisation rather than blanket avoidance.
Limitation: Guideline-level evidence synthesises the literature rather than providing new prospective comparative trials.
Evidence
ACR Practice Guidance for Imaging Potentially Pregnant Patients
American College of Radiology • ACR-SPR Practice Parameter (2023)
Key Findings:
- Ionising studies remain appropriate when clinically justified and when the result will materially affect care.
- Protocol design and dose reduction are more important than avoiding every study regardless of diagnostic need.
- Risk communication should use realistic region-specific exposure rather than generic statements that overstate harm.
Clinical implication: A technically optimised, justified CT is often the right answer in major trauma or when MRI is unavailable or non-diagnostic.
Limitation: Practice parameters guide behaviour but do not replace local trauma workflow or urgent multidisciplinary judgement.
Evidence
MRI and Gadolinium Exposure in Pregnancy: Population Cohort
Ray JG, Vermeulen MJ, Bharatha A, Montanera WJ, Park AL • JAMA (2016)
Key Findings:
- Among 1,424,105 deliveries in Ontario, first-trimester MRI (n=1737) was NOT associated with increased stillbirth/neonatal death, congenital anomaly, neoplasm, or vision/hearing loss versus no MRI.
- Gadolinium MRI at any time in pregnancy (n=397) was associated with a higher risk of a broad set of rheumatological, inflammatory or infiltrative skin conditions (adjusted HR 1.36, 95% CI 1.09 to 1.69).
- Gadolinium exposure was also associated with increased stillbirth or neonatal death (adjusted RR 3.70, 95% CI 1.55 to 8.85).
Clinical implication: Non-contrast MRI, including in the first trimester, is reassuring; gadolinium should be avoided unless its diagnostic value is essential because it carries a measurable signal of fetal harm.
Limitation: Retrospective administrative-database design with small gadolinium-exposed numbers; cannot detect rare outcomes and residual confounding is possible.
Evidence
Fetal Dosimetry at CT: A Primer
Sensakovic WF, Royall I, Hough M, Potrebko P, Grekoski V, Vicenti R • RadioGraphics (2020)
Key Findings:
- No single diagnostic CT examination commonly used in pregnancy approaches the deterministic-effect dose thresholds for the fetus.
- Fetal dose cannot be measured directly in vivo and is estimated by medical physicists using validated methods; radiologists should understand the magnitude of these doses.
- Knowing dose thresholds for biologic effects allows confident, evidence-based counselling rather than reflexive avoidance.
Clinical implication: When CT is the correct study, proceed with a dose-optimised protocol and counsel using realistic estimated fetal dose figures.
Limitation: Educational review rather than primary comparative data; dose estimates depend on patient habitus, gestational age and scanner technique.
Evidence
Imaging Modalities for Cardiac/Thoracic Disease in Pregnancy
Ntusi NA, Samuels P, Moosa S, Mocumbi AO • Cardiovascular Journal of Africa (2016)
Key Findings:
- The commonly cited cautionary cumulative fetal threshold is 5 rad (50 mSv / 50 mGy), and no single diagnostic study approaches this dose.
- Echocardiography and cardiovascular MRI appear safe in pregnancy with no demonstrated adverse fetal effects, provided no general MRI contraindication exists.
- A single justified radiological study during pregnancy is considered safe and should be performed whenever clinically indicated.
Clinical implication: Reassure that cumulative diagnostic exposure rarely approaches the 50 mGy cautionary threshold, and that ultrasound and MRI carry no ionising-radiation risk.
Limitation: Narrative review of cardiothoracic imaging; thresholds are conservative estimates rather than trial-derived endpoints.
Evidence
Diagnostic Management of Suspected Pulmonary Embolism in Pregnancy
Robert-Ebadi H, Le Gal G, Righini M • Frontiers in Cardiovascular Medicine (2022)
Key Findings:
- Validated strategies combining clinical probability, D-dimer and lower-limb compression ultrasound can safely exclude PE without thoracic imaging in a significant proportion of pregnant women.
- When imaging is required, CT pulmonary angiography and ventilation-perfusion scanning both have very low diagnostic failure rates.
- Optimised CT protocols can reduce radiation dose to both fetus and mother while preserving diagnostic accuracy.
Clinical implication: Apply stepwise non-imaging triage first, then choose the imaging test (CTPA or V/Q) by local availability and dose, rather than withholding imaging from a deteriorating patient.
Limitation: Review of management-outcome studies; pregnancy-specific D-dimer cut-offs are still being refined.
Evidence
Trauma in Pregnancy: A Narrative Review
April MD, Long B, Gottlieb M • The American Journal of Emergency Medicine (2024)
Key Findings:
- Trauma accounts for nearly half of all deaths among pregnant women, and the incidence of trauma in pregnancy is approximately 6 to 8 percent.
- Clinical assessment and resuscitation must focus on the mother first, beginning with the primary survey, because fetal survival depends on maternal survival.
- Ultrasonography (including FAST) is an attractive first imaging modality, but indicated CT should not be withheld in major trauma.
Clinical implication: In the pregnant trauma patient, prioritise the maternal primary survey and image decisively; do not delay CT when it answers a life-threatening question.
Limitation: Narrative review without pooled effect estimates; trauma evidence remains heterogeneous and scenario-specific.
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
Viva scenarioChallenging
Clinical prompt
“A 12-week pregnant patient presents with severe back pain and possible cauda equina syndrome.”
Practical approach
I would recommend urgent MRI of the lumbar spine without gadolinium. MRI avoids ionising radiation and is the best test for compressive neural pathology. The pregnancy does not justify delaying imaging because missing cauda equina syndrome risks permanent maternal neurological harm, which also threatens fetal wellbeing. If MRI were genuinely unavailable in a time-critical situation, CT-based alternatives could still be justified.
Key clinical points
MRI without gadolinium is the preferred cross-sectional study.
Do not delay imaging in neurological emergency.
Maternal diagnosis drives fetal safety.
Common pitfalls
Delaying imaging because of pregnancy alone.
Using gadolinium reflexively.
Failing to state why maternal emergency takes priority.
Further questions
“If MRI were unavailable overnight, how would you justify the next-best imaging pathway?”
Viva scenarioStandard
Clinical prompt
“A 20-week pregnant patient falls and may have a pelvic fracture.”
Practical approach
I would explain that clinically indicated pelvic radiographs are acceptable in pregnancy and that the expected fetal dose from diagnostic imaging is usually far below deterministic-risk thresholds. I would use the minimum projections necessary, optimise technique, and escalate to MRI or CT only if the radiographs do not answer the question. The key counselling point is that a missed unstable fracture is more dangerous than a justified diagnostic study.
Key clinical points
Pelvic radiographs are acceptable when indicated.
Use realistic threshold-based counselling.
Optimise the protocol and avoid unnecessary repeats.
Common pitfalls
Telling the patient radiographs are forbidden in pregnancy.
Overstating the fetal dose.
Forgetting that fracture diagnosis changes management.
Further questions
“If the radiographs are equivocal and the patient remains unstable, what would determine whether you choose MRI or CT next?”
Viva scenarioChallenging
Clinical prompt
“A major trauma patient at 8 weeks gestation may have intra-abdominal injury.”
Practical approach
Yes, if CT is the correct test for maternal management. At 8 weeks the embryo is radiosensitive, but a justified trauma CT is still appropriate because the immediate maternal threat is far greater than the theoretical risk of a missed diagnosis. I would use a dose-optimised protocol, avoid unnecessary multiphase acquisition, document the indication, and counsel afterwards using realistic exposure context.
Key clinical points
Maternal survival and diagnosis come first.
CT is justified when it answers the urgent trauma question best.
Dose optimisation and documentation are essential.
Common pitfalls
Delaying trauma CT because of pregnancy alone.
Ignoring protocol optimisation.
Providing vague reassurance without explaining the rationale.
Further questions
“What protocol choices can reduce fetal exposure without making the study non-diagnostic?”
Exam day cheat sheet
Pregnancy Imaging Quick Reference
Preferred Order
- Ultrasound first when adequate
- MRI without gadolinium for most cross-sectional questions
- Radiographs when clinically indicated
- CT when it best answers an urgent question
Counselling Principles
- Use realistic dose language
- Explain that most diagnostic studies are far below deterministic thresholds
- State that maternal diagnosis protects fetal outcome
- Document indication and discussion
Contrast Rules
- Avoid gadolinium unless essential
- Use iodinated contrast if the CT question requires it
- Do not sacrifice diagnosis simply to avoid contrast
- Follow local neonatal thyroid-check policy if relevant
Viva Bottom Line
- Do not withhold necessary imaging
- Optimise protocol rather than avoid diagnosis
- MRI is preferred, not mandatory
- Emergency imaging proceeds when management depends on it