Studies in the velocity of blood flow.

"In 1925, Hermann Blumgart performed the first diagnostic procedure using radioactive indicators on humans; this first is well recognized. Less well recognized is the fact that Blumgart and his coworker Otto C. Yens, then a medical student, developed the first instrumentation used in a diagnostic procedure involving radioactive indicators. The instrumentation, a modified Wilson cloud chamber, turned out to be the detector most suitable for their purpose. Blumgart also showed remarkable foresight in outlining the requirements both for a satisfactory indicator (tracer) and for a satisfactory detector—requirements that still hold true today. The Blumgart–Yens modified cloud chamber was the birth of nuclear medicine instrumentation" (http://jnm.snmjournals.org/content/44/8/1362.long, accessed 03-2018).