Transient interruption of contrast
Transient interruption of contrast (TIC) is a common flow artifact seen in CT pulmonary angiography (CTPA) studies. The contrast opacificiation of the pulmonary arteries is suboptimal due to an increase in the flow of unopacified blood from the inferior vena cava (IVC) to the right side of the heart, often during deep inspiration .
Transient interruption of contrast incidence is more common in pregnant women due to increased intra-abdominal pressure and becomes less common as patient age increases .
This artifact is commonly attributed to deep inspiration, although there is debate in the literature on the significance of breathing and breath-holding on image quality . Although CT pulmonary angiography protocols vary between radiology departments, a common protocol involves injecting a contrast bolus in the veins of the upper extremity, followed by a timed or bolus-triggered breathing instruction to suspend breathing. The instruction is intended to occur during a normal tidal volume. If the patient instead takes in a deep breath before holding it (i.e., inspiratory breath-hold, which is often employed in routine chest CTs), the intrathoracic pressure suddenly decreases. The decreased intrathoracic pressure increases systemic venous return, including of unopacified blood from the inferior vena cava, to the right heart and subsequently pulmonary arteries.
Transient interruption of contrast during CT pulmonary angiography is identified as suboptimal contrast enhancement in the pulmonary artery despite optimal contrast administration . Adequately timed contrast administration can be verified visually as contrast opacification in both the superior vena cava and, usually to a lesser degree, the left heart or aorta. A short interruption in vascular opacification can lead to an indeterminate study or be misinterpreted as a pulmonary embolus .
Imaging that was timed too late so as to miss the contrast bolus may be seen as contrast opacification in the systemic arterial system but not the venous system and pulmonary arteries.