1. Field of the Invention
This invention pertains to gamma imaging devices and more particularly to that class of devices known as scintillation cameras.
In the diagnosis of certain illnesses, radioactive agents are administered to patients. These administered agents have the characteristic of localizing in certain tissues and either not localizing, or localizing to a lesser degree, in other tissues. For example, iodine 131 will localize in thyroid glands. A representation of the spatial distribution and concentration of administered iodine 131 in a thyroid gland provides an image of the gland itself which is useful in diagnosing the condition of the gland.
2. Description of the Prior Art
Generally speaking, two classes of devices known as scanners and cameras have been used to detect and represent the spatial distribution and localization of radioactive isotopes. Typically, a scanner has a scintillation probe which is moved along a plurality of spaced parallel paths. Gamma energy detected by the probe results in a display through either a photographic or a dot image representative of the spatial distribution and localization of an isotope. A clinically successful scanner is described in greater detail in the above-referenced patent Re 26,014.
The devices known as cameras remain stationary with respect to the patient as a representation of the spatial distribution of radioactivity is developed. With many of these cameras, a relatively large disc-shaped scintillation crystal is positioned to be stimulated by radiation emitted from the patient. In most cameras, a collimator is interposed between the patient and the crystal so that, for example, with a parallel hole collimator the rays striking the crystal are all generally perpendicular to it.
The crystal scintillates as it converts gamma energy impinging on it to light energy. The light is transmitted through a suitable light pipe, to an array of phototubes. When a phototube is stimulated by light generated in a crystal by a scintillation, an electrical signal is emitted which is proportional to the intensity of light energy received by that tube. When a scintillation causes all or substantially all of the phototubes to emit signals, these signals are emitted concurrently and are then summed to provide a signal known as the Z signal. This Z signal is conducted to a pulse-height analyzer to determine whether the signal reflects the occurrence of a so-called photopeak event to the isotope which has been administered to the patient. That is, the Z-signal is of appropriate strength to reflect the full conversion of the energy of a gamma ray emitted from the administered isotope to light energy by the crystal.
Summing and ratio circuits are also provided which develop what are known as X and Y signals. These X and Y signals cause a dot to be produced on the screen of the oscilloscope at a location corresponding to the location of the detected scintillation. Thus, the oscilloscope dots are displaced relatively, each at a location corresponding to the location of the corresponding scintillation in the crystal and the oscilloscope dots are integrated to produce an image. Suitable circuits for producing an oscilloscope image of spatial distribution of a radioactive isotope are described in greater detail in the HINDEL application.
The phototubes, the circuits and the oscilloscope function as a unit to provide a light amplifier such that each dot produced on the oscilloscope is a brightened representation of a scintillation. Through the use of a persistence screen on the scope, or a photographic camera, these dots are integrated to produce an image.
With cameras of the type using an array of phototubes, the literature has described a spacing of the phototubes a sufficient distance from the crystal so that the tubes "view coextensive areas". Typically, there will be a spacing of the order of two inches between a crystal and a phototube. More specifically, since the typical crystal is thallium activated sodium iodide, it is hygroscopic and must be hermetically enclosed. At the output side of the crystal, the typical hermetic enclosure includes a glass window which has a thickness of about one-half inch. A light pipe, such as the light pipe described in greater detail in the MARTONE application, is optically coupled to both the window and the phototubes. Typically, the light pipe will have a thickness of the order of one and one-half inches.
Thus, in a camera of these typical dimensions, any scintillation occurring to the crystal must, as a minimum, be at least two inches from the nearest phototube. Proposals for somewhat thinner light pipes are known, but the thinnest of these has been one and three-eighths inches, which, if coupled with a one-half inch glass, results in a minimum scintillation-to-phototube distance of one and seven-eighths inches.
It may generally be said that the further a scintillation is from the phototube, the weaker will be the light signal received by the phototube and accordingly, the weaker the electrical output of the phototube. Accordingly, the closer the phototubes are to the scintillation, the stronger will be the signals. Both theory and experiment indicate that this will better the spatial resolving power of the instrument.
As noted above, it has been taught that the tubes must be sufficiently spaced to view overlapping, coextensive areas in the scintillator. In addition, which known light pipe constructions, if the spacing between the crystal and the phototubes is too small, there is a loss of uniformity and linearity. That is, the response of the system to a uniform source of activity will exhibit bright and dark areas not related to the isotope concentration and furthermore, light signals produced on the oscilloscope will be displaced from the desired position and the result is a distorted image. Additionally, it is known there will be a loss of uniformity of system sensitivity. That is, the pictures formed by integrating light dots will exhibit light or dark areas not related to isotope concentration, this indicating a preferential ability to detect scintillations in certain parts of the crystal.