What is the indication for a Ventilation-Perfusion (VQ) scan in a pregnant patient?

From the Guidelines

Pregnant patients with suspected pulmonary embolism should undergo a ventilation-perfusion (VQ) scan as the initial diagnostic test, rather than a CT pulmonary angiography (CTPA), due to the lower radiation exposure to the fetus and maternal breast tissue. The recommended approach is to start with a perfusion-only scan using a reduced dose of technetium-99m labeled macroaggregated albumin (typically 40-50 MBq instead of the standard 100-150 MBq) 1. This delivers approximately 0.1-0.2 mGy to the fetus, well below the threshold for fetal harm. If the perfusion scan is normal, no ventilation scan is needed. If abnormalities are seen, a ventilation scan using Tc-99m DTPA aerosol or Xenon-133 can be added to complete the study. Some key points to consider when performing a VQ scan in pregnant patients include:

• Using a reduced dose of technetium-99m labeled macroaggregated albumin to minimize radiation exposure to the fetus 1
• Performing a chest X-ray before the VQ scan to rule out other pathologies that might mimic pulmonary embolism 1
• Informing pregnant patients about the minimal risks and the importance of diagnosis, as untreated pulmonary embolism poses a greater risk to both mother and fetus than the radiation exposure from diagnostic imaging 1
• Considering the use of CTPA as an alternative diagnostic test if the VQ scan is nondiagnostic or if the patient has an abnormal chest X-ray 1 It is essential to weigh the benefits and risks of each diagnostic test and to consider the individual patient's circumstances and preferences when making a decision. However, based on the current evidence, VQ scanning is the preferred initial diagnostic test for suspected pulmonary embolism in pregnant patients due to its lower radiation exposure and comparable diagnostic accuracy to CTPA 1.

From the Research

Ventilation-Perfusion Scan in Pregnant Patients

• The ventilation-perfusion (V/Q) scan is a valuable technique in the management of pregnant women suspected of having pulmonary embolism (PE) 2.
• A study found that V/Q imaging is a useful tool in diagnosing PE in pregnant women, with 31 patients showing normal scans, 19 with low probability scans, 14 with intermediate probability scans, and 18 with high probability scans for PE 2.
• The study also found that anticoagulation was continued or started in 31 patients, and no complications of anticoagulation were observed, with no adverse outcomes of pregnancy reported 2.

Fetal Absorbed Dose in V/Q Scan

• A study aimed to calculate the fetal absorbed dose in V/Q scan in three trimesters of pregnancy using Monte Carlo simulation, and found that the fetus received the highest absorbed dose in the third trimester of pregnancy 3.
• The study recommended using lower administration activity in the third trimester of pregnancy to minimize the fetal absorbed dose 3.
• The fetal absorbed dose for 200 MBq 99mTc-MAA was found to be 1.01-1.97 mGy, which is higher than the values recommended by the International Commission on Radiological Protection (ICRP) 3.

Alternative Diagnostic Methods

• Lung ultrasound is a promising alternative diagnostic method for PE, with a bivariate weighted mean sensitivity of 87.0% and specificity of 81.8% 4.
• However, the diagnostic accuracy of lung ultrasound for PE diagnosis is still limited by methodological drawbacks of primary studies, and further well-designed accuracy studies are necessary 4.
• Ventilation/perfusion SPECT has largely replaced conventional planar V/Q scan in nuclear medicine departments for PE diagnosis, but its diagnostic performance and role in diagnostic management are still debated 5.

Clinical Outcomes and Management

• A systematic review found that V/Q SPECT has been widely implemented in daily clinical practice, but the exact diagnostic performance of V/Q SPECT for PE is still unknown 5.
• The review identified knowledge gaps and set the agenda for future research on the diagnostic accuracy and clinical outcomes of V/Q SPECT for PE diagnosis 5.
• A study found that the goal of V/Q scanning is not detection of pulmonary emboli per se, but rather the identification of patients at a high or low risk for future embolic events if they are not anticoagulated 6.