An image tube scintillation camera for use with radioactive isotopes emitting low-energy photons.

Abstract

Many radioactive isotopes decay with the emission of low-energy photons, which are either (a) gamma rays or (b) x rays emitted subsequent to electron capture or internal conversion. It is impossible to define on an absolute scale the term “low-energy photon”; and, arbitrarily, in this context the term will be applied to electromagnetic radiation emitted with an energy lower than 150 kiloelectron volts. Several authors, among them Harper, Myers, Sodee, and Wagner, have investigated the usefulness of several such radioactive isotopes in the visualization of organs and anatomical structures by scintiscanning (1–6). For these procedures, radioactive isotopes emitting low-energy photons exhibit both advantages and disadvantages as compared to isotopes emitting higher energy electromagnetic radiation. The high absorbability of low-energy photons in matter provides two main advantages in scintiscanning: 1. Low-energy photons can be efficiently detected by means of detectors with a relatively low cross-sectional density. Such detectors are easy to shield, they exhibit a relatively low background due to their low cross-section for the absorption of higher energy photons, and they facilitate considerably the design of certain types of scintillation cameras. 2. The second advantage derived from the high absorbability of low-energy photons consists of their efficient collimation by means of multi-hole collimators with relatively thin septa. In such collimators, the thickness of the septa is determined by the energy of the electromagnetic radiation which is to be collimated, and one of the limiting factors in both the resolution and efficiency of a scintillation camera is contributed by the thickness of the septa used. Thus, higher collimation resolution and efficiency can be achieved with lower energy photons than with higher energy radiation. In general, the design and construction of efficient collimators for low-energy photons is simple because of the near absence of septal penetration of the radiation. Collimators for low-energy radiation are light and easy to handle. The high absorbability of low-energy photons exhibits the disadvantage in organ visualization of requiring more activity in observing deep-seated structures because of the high attenuation of the radiation in the tissues interposed between the organ and the detector. This limitation, however, was found to be of minor importance in clinical work (7, 8). The major disadvantage in the use of low-energy photon emitters for organ visualization results from the fact that scattering of lower energy, photons takes place with little loss of energy; and consequently scattered radiation cannot be as efficiently eliminated from the examination as can higher energy radiation.