A prevailing theory of tear drainage suggests that the lacrimal sac fills when the eyelids are closed and empties into the nasolacrimal duct when the eyelids are open. This is aided by the contraction of the orbicularis muscle during each blink. The Eye Institute at the National University Hospital in Singapore undertook a study to ascertain the possibility of seeing the dynamic movement of tears in the nasolacrimal system during blinks, using magnetic resonance dacryocystography (MR-DCG).

Researchers initially localized the lacrimal sac with a three-plane gradient echo sequence using a 1.5-T MRI platform. They then carried out fast, dynamic MR-DCG after instilling topically balanced salt solution (BSS) in five subjects and 0.5% gadolinium in seven subjects. The volunteers were asked to close and open their eyes during the fast imaging. The images were digitized to enable investigators to see the actual movement of fluid in the system.

The tear movement was clearly seen as a bolus in the volunteers where BSS was used. The fluid passed into the nasolacrimal duct after several blinks when patent; the sac was never seen to empty completely. Though researchers saw differential filling between the upper and lower part of the sac, they had difficulty seeing the actual fluid movement in the volunteers when topical gadolinium was used. Their findings support the notion of fluid travel in the form of a bolus through the sac. Once a threshold volume is reached in the lower end of the sac, the fluid is seen to pass through the nasolacrimal duct. This happens after several blinks.