Lymphatic tracing is generally more often done with dyes in the case of removal of sentinal nodes due to e.g. cancers.
However, there been work in injecting radioimmunoagent tracers into lymphatics especially in the case of melanomas. The likely reason why these haven't before detected these meningeal lymphatic vessels is likely because: (1) We don't bother imaging the brain because "the brain doesn't have lymphatics" and (2) tracers generally flow with the direction of fluid and the direction of lympth from the CNS is likely efferent rather than afferent.
Furthermore it is difficult these days to do such experiments especially on live subjects "just because".
Indeed! Though mouse anatomy doesn't entirely transfer over to human anatomy. I interpreted your comment to refer to experimentation in humans, I apologise if I misunderstood.
You seem to be surprised that our current knowledge of basic human anatomy is not yet exhaustive. Two recent notable discoveries are:
* Anterolateral ligament (Ligament of the knee, discovered 2013 after macroscopic examination of cadavers)
* Dua's layer (Newly discovered layer of the cornea, discovered in 2013 using electron microscopy)
You may also be surprised about the amount of variation in anatomy found between people. Some structures (commonly muscles) do not exist in some people, or may exist on only one side of the body. Similarly vessels and nerves can sometimes also take alternative or strange courses. This has clinical implications, especially wrt surgery.
However, there been work in injecting radioimmunoagent tracers into lymphatics especially in the case of melanomas. The likely reason why these haven't before detected these meningeal lymphatic vessels is likely because: (1) We don't bother imaging the brain because "the brain doesn't have lymphatics" and (2) tracers generally flow with the direction of fluid and the direction of lympth from the CNS is likely efferent rather than afferent.
Furthermore it is difficult these days to do such experiments especially on live subjects "just because".
Hindsight is 20/20.