Imaging description

CT imaging for suspected pulmonary embolism is among the most challenging in terms of reproducible high image quality. Even careful timing with bolus tracking does not always guarantee that contrast enhancement levels will be sufficient to image the peripheral pulmonary arteries, as discussed in case 42. In patients with corrected congenital heart disease, the presence of conduits from arteries and veins to cardiac chambers can result in dramatically altered timing of contrast delivery to the right heart, further increasing the complexity of protocol design for imaging of the pulmonary arteries. In the case shown here, the standard timing method of bolus tracking the main pulmonary artery resulted in a non-diagnostic scan because the patient has a Fontan shunt between the inferior vena cava and right pulmonary artery. Markedly dense contrast is seen in the left pulmonary arterial system, with mixing artifact, and complete absence of contrast in the right pulmonary arterial system (Figure 44.1). A longer delay is required to opacify the conduit and pulmonary arterial system (Figure 44.2). Park et al. have reported that the optimal delay to enhance the Fontan conduit and pulmonary arteries is 3 minutes.

An additional pitfall shown in this case is a thrombosed Blalock–Taussig (BT) shunt mimicking acute pulmonary embolism (Figure 44.3). Patients with congenital heart disease may undergo BT shunting in early childhood, followed by other definitive surgical correction, such as a Fontan procedure. Knowledge of their surgical history is critical in recognizing that the thrombosed vessels coursing cranially from the pulmonary arteries are old BT shunts rather than lobar or segmental pulmonary emboli.

Importance

Following surgical repair for congenital heart disease, the standard CT timing for pulmonary artery imaging cannot be used. Timing must be tailored to the altered hemodynamics resulting from indwelling conduits.

Typical clinical scenario

In patients with congenital heart disease, multidetector CT (MDCT) may be requested to image the pulmonary arteries for suspected vascular stenoses or atresia as part of the spectrum of their cardiac anomalies or as a secondary complication that develops over time.