Investigation Of Transmolybdenum Fluorescent Anodes For Mammography

AbstractRecent work has suggested a new approach to film/screen mammography through the use of transmolybdenum (Z > 42) fluorescent anodes. An experimental rhodium (Z = 45) anode with K emission at 20.0 - 23.2 keV has been compared to a conventional "molybdenum anode. Previous spectral measurements have indicated a dose advantage for the Rh anode but no comparison of contrast has been made. A series of step wedges of different materials was imaged at several kVp and for several phantom thicknesses. The resulting data permits quantitative comparison of the radiographic contrast. At 40 kVp for an equivalent tissue thickness of 4.8 cm the radiographic contrast provided by the Rh anode is at least equal to and under some conditions actually superior to that provided by a Mo anode.IntroductionEarlier work has suggested that the Mo K radiation commonly used for film and film/screen mammography may be less than optimal from a dose standpoint. It is well accepted that a narrow band of radiation is desirable because if the energy is properly chosen, such radiation provides better contrast for a given surface exposure than that provided by a continuum of radiation. The key question concerning contrast then revolves about the optimum energy for mammography. Several authors have investigated the contrast provided by selectively filtered continua. 5 However it has been clearly demonstrated that such simulation of fluorescent sources is both inefficient from an output standpoint and ineffective from the standpoint of spectral purity. [The term spectral purity is defined here as the ratio of the exposure in the desired narrow energy band to the total exposure.]The question of optimal energy in terms of contrast is complicated further by the thickness and type of breast being imaged. Haus et al have demonstrated that a large percentage of the exit spectrum from a 5 cm breast lies in the high energy continum.-^ The percentage of the exit spectrum in this continuum is of course dependent on the filtration provided by the breast itself.The object of this effort is to investigate the possibility of a better fluorescent anode for film/screen mammography from a contrast standpoint.MethodsTwo fluorescent anodes were investigated - a conventional Mo (Z = 42 Koc = 17,5 keV) and an experimental Rh (Z = 45 Koc = 20.2 keV) . Details of the tubes are available elsewhere.1 The Mo anode included 0.5 mm Al equivalent inherent filtration. An additional 25 }.i of Mo was added. The Rh tube has a Be window so 0.5 mm Al was added to make a fair comparison of the Mo and Rh tubes. In addition 25 y of Rh was added to the Rh tube as selective filtration. The tubes were operated at 2 mA on a conventional single phase generator. Capacitance in the high tension cables kept the ripple below 25%. The tubes were operated at the same ripple. Initial contrast measurements were made with the step wedges in the Kodak ITO phantom. However these wedges did not provide the desired sensitivity or dynamic range. Thus a series of wedges of teflon, aluminum and polycarbonate (lexan) were constructed. Each wedge has 10 steps. The wedges were fixed to the front surface of a 0,95 cm thick 15 x 15 cm lucite sheet. Additional similar sheets were placed between the wedges and phantom to produce the total thickness of lucite.Images were made using Kodak TL film. This film is equivalent to RP/M which is no longer available under that designation. The sensitometric data are available from Kodak. The film is a fine grain high contrast nonscreen film. The H and D curve shows a very wide linear range which permitted contrast comparison over a broad range of exposures. While most mammography using film is done with film/screen combinations it was felt comparisons of radiographic contrast between the two anodes could be made under the condi­ tions imposed. The film was processed in a carefully monitored automatic processor using Kodak chemistry recommended for the TL film.*This work supported in part by NTH Grant # 1 R23 CA 25372-01, awarded by the National Cancer Institute, DHEW.