The first radiological imaging of the ureters first occurred via X-rays, although this was made more difficult by the thick abdomen, which the low power of the original X-rays was not able to penetrate enough to produce clear images. More useful images were able to be produced when Edwin Hurry Fenwick in 1908 pioneered the use of tubes covered in material visible to X-rays inserted into the ureters, and in the early 20th century when contrasts were injected externally into the urinary tract (retrograde pyelograms). Unfortunately, much of the earlier retrograde pyelograms were complicated by significant damage to the kidneys as a result of contrast based on silver or sodium iodide. Hryntshalk in 1929 pioneered the development of the intravenous urogram, in which contrast is injected into a vein and highlights the kidney and, when excreted, the urinary tract. Things improved with the development by Moses Swick and Leopold Lichtwitz in the late 1920s of relatively nontoxic contrast media, with controversy surrounding publication as to who was the primary discoverer. Side-effects associated with imaging improved even more when Tosten Almen published a ground-breaking thesis in 1969 based on the less toxic low-osmolar contrast media, developed based on swimming experiences in lakes with different salinity.