Suspected pulmonary embolism at 26 weeks' gestation: V/Q versus CTPA
A 29-year-old woman at 26 weeks' gestation presents with sudden dyspnea, pleuritic chest pain and a heart rate of 112 bpm; oxygen saturation is 94% on room air. D-dimer is elevated and the clinical probability of pulmonary embolism is moderate. Chest radiograph is normal, with no effusion or consolidation. She has no history of asthma and normal renal function. The nuclear medicine physician is asked to perform a ventilation-perfusion study. The patient also mentions that she still breastfeeds her 22-month-old daughter twice a day, and asks whether that will be a problem. The intensive care team is pushing for CT pulmonary angiography instead because it is faster, and asks why a nuclear study would be preferable in her case.
1. Which study would you recommend, and which specific protocol would you use? Explain the dosimetric reasoning.
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Recommend a perfusion study with 99mTc-MAA, and explain why in terms the ICU team will accept.
Choosing the modality. With a normal chest radiograph, ventilation-perfusion imaging is the preferred test in pregnancy. Current guidance favors V/Q over CTPA when the radiograph is normal; CTPA is preferred when the radiograph is abnormal, because a matched defect behind an opacity cannot be interpreted. A normal perfusion scan with a normal radiograph effectively excludes pulmonary embolism and is the useful outcome here.
Choosing the protocol. Because the radiograph is normal, a perfusion-only study is sufficient and avoids the ventilation component and its added dose. Use a reduced particle number (on the order of 100,000 to 200,000 particles) with about 1 to 2 mCi of 99mTc-MAA; clumping must be avoided because hot spots create false defects.
Dosimetry. The fetal dose from a low-activity MAA perfusion study is very small, well under 1 mGy, and lower than the maternal breast dose associated with CTPA. Shielding the abdomen offers little benefit because the concern is internal deposition, not external exposure; do not let shielding rituals substitute for using the lowest adequate activity. If the perfusion study is non-diagnostic, proceed to ventilation imaging or CTPA rather than repeat blindly.
2. What do you tell the patient about breastfeeding?
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Tell her that she can continue breastfeeding without interruption. 99mTc-MAA is a particulate agent that is trapped in the pulmonary capillary bed; the amount excreted into milk is negligible, and current professional guidance does not require interruption of breastfeeding after a technetium-99m MAA perfusion study.
If she prefers reassurance, she may pump and discard one feed and resume afterward, but she does not need to wean, and she does not need to stop for days. This matters practically: a mother who is wrongly told to stop may stop permanently, and unnecessary interruption is a real harm.
Contrast this with the agents where interruption is mandatory: radioiodine (131I) requires cessation of breastfeeding before administration, and iodine-123 studies require interruption as well. If a pertechnetate study were ever needed, the usual advice is to interrupt for roughly 12 to 24 hours. Making that distinction explicitly helps her understand why this particular test is safe and prevents confusion later if she has thyroid imaging.
Document the counseling, including who gave it, so the advice is traceable.
3. What technical pitfalls specific to pregnancy and to perfusion imaging could make this study falsely abnormal or non-diagnostic?
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Several, and each has a fix:
- Too many particles or clumping. Keep the particle number low but adequate; clumped MAA produces hot spots and apparent defects. Inspect the syringe and use a fresh preparation.
- Free pertechnetate. Check for thyroid, salivary or gastric uptake, which indicates poor labeling and degrades image quality.
- Patient positioning and breathing. Supine imaging in a dyspneic gravid patient with a large uterus is uncomfortable and motion-prone; use a portable camera at the bedside if needed and coach quiet breathing. Motion artifact mimics mismatched defects.
- Correlation with the radiograph. A defect underlying a radiographic opacity is matched and not evidence of embolism; always read the two together.
- Use SPECT. Tomographic perfusion imaging improves detection of small or subsegmental defects that planar images miss.
- Do not skip the ventilation study reflexively. If perfusion shows an unmatched defect, a ventilation study (or CTPA) is needed to confirm mismatch.
A technically poor scan in this patient risks either a missed embolism or unnecessary anticoagulation for the rest of pregnancy, so quality control deserves explicit attention.