: a . , • i LOFT ORNLP 1545 i.. 2 l ' . ' 136 ST ALS 125 1.4 1.6 MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF STANDARDS - 1963 ORNr-P-1545 Corf-650313-4 MASTER USEP 20 1965 COLLIMATORS FOR LOW-ENERGY GAMMA SAYS C. Craig Harris RELEASED POZ. AMMOSHCEMENT IN NÜLLRAR SCIENCE ABITricis Oak Ridge National Laboratory Oak Ridge, Tennessee The so-called focused collimator for scanning was designed to obtain increased counting rate without a loss in spatial resolution. With the focused collimator this is achieved only near the focal plane due to geometri- cal effects. Penetration, by gamma rays, of the walls between the collimat- ing channels causes additional degradation of the spatial resolving properties of the focused collimator. The walls between the holes, therefore, must be kept thick enough to keep gamma ray penetration within acceptable limits.. This sets an upper limit on the total area of all the holes and hence on the counting rate obtainable at a given resolution. A decade ago, the one or two collimator types we had were designed as compromises for the range 280 to 420 keV. At the upper end of this range we just had to live with the fact that the septa were not thick enough. There was too much leakage but most people felt that it would cost too much to do any better. At the lower end of the energy range the walls were perfectly opaque, so the open detector area --- and hence, counting rate --- was un- necessarily low. There has been considerable interest of late in radionuclides emitting only game rays below 160 keV. When older collimators, designed for higher energies, are used for scanning these low-energy emitters counting rates are about a factor of two lower than they should be. To take best advantage Research sponsored by U. S. Atomic Energy Commissiou vnder contract with Union Carbide Corporation. -2- of the low-energy emitters, we should use collimators specifically designed for this energy range. This would allow twice the counting rate for the same spatial resolution, or better resolution for the same counting rate. The difference between collimators of the same length but designed for the two energy ranges we have discussed may be seen in Fig. 1. The frac- tion, expressed in per cent, of the area of the detector not covered by the walls of the holes is called the transmission. Note that, since the walls can be very thin for low energies, transmission can be about doubled for the same hole dimensions. There are some problems, however, anà even some pitfalls when it comes to making such collimators. The first problem arises because of the thin walls between holes. Ary misalignment in casting or slips in machining can result in the disappearance of septa, since their thickness is of the order of customary tolerances. It therefore becomes practical to make collimators of thin-walled lead tubes formed on el mandrel (1) as shown in Fig. 2. This technique, used by Bell and rancis in their early work, seems to be satisfactory since such colli- . mators are now commercially available and some are even being homemade in certain hospitals. The thing that makes these collimators good is their length. They are generally made the same length as the medium-energy collimator they are intended to replace. The collimator shown in Fig. 2 is 3 inches long and is made of about 109 individual holes almost the same as in the ORNL 61-hole 3-inch collimator. It therefore has about the same spatial reso- lution (R, radius of optical circle of resolution, is about 0.25 inch). The septa are 20 thousandths of an inch thick at the large end and 10 thousandths thick at the small end of the collimator. The transmission is about 84 per cent. Since the collimator is 3 inches long, the septum thickness is ade- quate up to 160 keV. It is immediately obvious that little lead is needed for protection against leakage at low energies, and this fact has tempted people to make very short coilimators. Short collimators provide a definite advantage in that they allow better detection geometry; the focal point can be closer to the detector. There is a drawback, however. To make a collimator half as long as the one of Fig. 2, but with the same transmission, resolution : and collimator to focus distance would require four times as many holes (about 440) and septa half as thick as in the longer one. Such a collimator can be made by the foil tube technique but the work becomes tedious. When such a collimator is made by casting, the transmission is necessarily reduced. This reduces counting-rate advantage we were seeking in the first place. Collimators (for: 3-inch diameter crystals) shorter than 12 inches long are exceedingly difficult to make without a sacrifice in performance. Some as short as I inch have been made and provide excellent scans with Tc (140 keV), but transmission of these collimators appear to be about 30 to 35 per cent. Thus the counting rate, though large due to the large allow- able number of millicuries of Tc99m, and improved by better geometry, is no greater than that obtained with a collimator with somewhat poorer geom- etry but longer and with higher transmission. The large number of holes in the short collimator (over 700 in this collimator) adds to the manufac- turing difficulty and cost. In summary, it would appear to be a most wholesome idea to optimize the design of collimators for a specific range of gamma ray energies. In particular, it appears to be rewarding for energies below 160 keV. If, how- ever, one gets carried away with the idea of making a super-short colli- mator just because little lead is required, results will necessarily be less than optimum. ACKNOWLEDGEMENTS The author gratefully acknowledges the support of his colleagues . . M. M. Satteriield and D. A. Ross: the former for constructing the colli- . . . . . mator of Fig. 2 and the latter for preparation of the two illustrations. . . . . . REFERENCES . - - - ... - - - ..... 1. C. C. Harris, J. C. Jordan, M. M. Satterfield, Jack K. Goodrich, f. L. Stone and Rebecca Hill, "A Collimator for Scanning with Low-energy Photons," J. Nucl. Med. 5:653-656, 1964. . . . ..... . contra dimicama...............ve in.r . . . . . - - - - matrimoni - ... - te - - - - s -- - - - . . .. .. .. , Laat MEDIUM ENERGIES LOW ENERGIES TRANSMISSION 85% TRANSMISSION 45% . L + HE v . 1 I I MENRIC . ....... .. . T 1 - . . Y . KH 1 nu : SW . .. . * " , . . 1. .' . END kthim DATE FILMED 1 / 19 /66 . 2