[Commentary]

History Corner The Barium Enigma When you look over the history of the radiological sci 7) Following the discovery of nuclear fission, many fission ences (and I am putting nuclear medicine in this cat products found application in medicine. One of them, egory), a curious pattern emerges: the element 30-year cesium-13 7 (discovered in 1941), found appli barium has played a key role in an astonishing number of cation as a long-lived radiation source for teletherapy discoveries, each one using a different property of the element. and for industrial applications, and as a calibration stan 1) When Roentgen discovered x-rays ( 1895), his first dard. (Cesium-137 is actually a pure beta emitter; the clue that his cathode-ray tubes had produced a new type useful 660-keV gamma actually comes from its daugh of radiation was the faint glow of a barium platinocyanide ter, 2.5-minutebarium-137m.) screen lying on a nearby table more than a meter away. 8) Ultrasound, initially developed to detect underwater Roentgen knew that cathode rays traveled only about 8 objects (icebergs, submarines), was slow in finding med cm in air; the black cardboard covering the tube elimi ical applications until the first practical medical trans nated ultraviolet and visible light; he knew that he had ducer material (barium titanate, 1950) was developed, discovered something new. (Barium platinocyanide is which could be switched on and off very rapidly,enabling intensely fluorescent to x-rays.) detection of echoes at close range. (Barium titanate has 2) When the Curies discovered radium ( 1896), their last a high electromechanical coupling coefficient, enabling step in proving that it was a new element was separating rapid dissipation of acoustic energy.) it from chemically similar barium. (Radium and barium 9) There was very poor standardization of I-131 thyroid are sequential Group II elements.) uptake measurements until Marshall Brucer introduced 3) An early and widely accepted measure of x-ray dose was "mock iodine" (1950), a radionuclide mixture which the color change in barium platinocyanide pastilles, intro gave off gamma rays of approximately the same energy duced by Holzknecht (1902). (Barium platinocyanide, as I-131, but which had a half-life of about 10years. For when exposed to x-rays, undergoes a slowly reversible the first time, RAIU measurement became a reproducible color change from pale yellow to deep brick red, the procedure. ( "Mock iodine " consists of a mixture of 10- color depending on the total dose.) year Ba-133, gamma energy 356 keV, and 30-year Cs- 4) When Rutherford and Soddy discovered nuclear trans 137, whose 662-keV gamma comes from its daughter mutation (1910), they proved that "thorium emanation" Ba-137m.) (radium-228) grew with time in a thorium solution, that 10)The coincidence principle of PET allows us to assign a it could be separated from thorium by precipitating it line in space which contains the source but not to assign with barium sulfate, and that it was therefore a differ an exact position along that line. Time-of-flight (TOF) ent element. (Barium sulfate precipitates are exceed measurements refine the position estimation but require inglyfine; their high surface-to-volume ratio makes them faster crystal resolving times than are available with ideal adsorbers for small amounts of substances.) sodium iodide or bismuth germanate. The breakthrough 5) Also in 1910 a Bonn gastroenterologist, Paul Krause, came with fast barium fluoride BaF2 crystals. Newer found by accident that barium sulfate was surprisingly materials have taken the place of BaF2, but it was the nontoxic, and an almost ideal gastrointestinal contrast first crystal fast enough for TOF-PET. (Barium fluo agent. It quickly became the standard, replacing the ride scintillators have very fast rise and decay times.) expensive, toxic bismuth salts. (Barium sulfate is non- That's ten, folks—and the amazing thing is, no two of toxic,fine-grained, and inexpensive.) these properties of the element barium are linked. That is the 6) When Fermi tried to produce transuranic elements by Barium Enigma. bombarding uranium with neutrons, he obtained an activ Do you have any barium stories? Send them to me for full ity which neither he nor his best chemists could sepa attribution. Thanks to Charles Sondhaus for the title. rate from barium. Otto Hahn and his group in Berlin showed (1939) that the reason Fermi couldn't separate —Dennis D. Patton MD.is the SNM historian and professor of it from barium was that it was barium, and that the ura radiology and optical sciences at University Medical Center, nium had undergone fission. (Barium, element 56, has Tucson Arizona a high fission yield from uranium, about 6%.)

24N THE JOURNALOFNUCLEARMEDICINE•Vol. 40 •No. 7 •July 1999