While Röntgen had demonstrated that dark and light areas on photographic film were caused by differential absorption of rays passing through various densities, it was obvious that additional contrast would be needed to distinguish kidneys, ureters or bladder.
But not until 1906, when German surgeons Alexander von Lichtenberg and Fritz Voelcker reported success instilling silver solution, Collargol, through the bladder up into the kidneys, would physicians have a way to evaluate specific urinary tract areas. Retrograde pyelography made previously undisclosed genitourinary disorders vulnerable to disclosure. Revolutionary? Yes. But the technique was hardly problem-free, which made the introduction of excretory urography in 1929, by American urologist Moses Swick, even sweeter. Working with von Lichtenberg in Berlin, he injected an organically-bound iodine compound—later named Uroselectan—into a vein, taking X-rays as the material cleared the body through the urinary tract. Swick's intravenous pyelogram (IVP) heralded a new era in urologic diagnosis.