Solid state gamma camera

Details Solid state gamma camera Solid state gamma camera

1999-05-24

2001-06-05

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Semiconductor system

C250S370060, C250S370090, C250S370010

Reexamination Certificate

active

06242745

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to methods for reading out a matrix of elements in a Solid State Gamma Camera.
BACKGROUND OF THE INVENTION
The use of solid state detectors for the detection of ionizing radiation is well known. Furthermore, the use of a mosaic of groups of detector electrodes on a single substrate of material such as CdZnTe has been mooted.
However, the application of such a matrix in a practical gamma camera is nearly obviated by the lack of a suitable fast readout system capable of reading out individual counts from the very large array of detector electrodes desirable for such a camera.
U.S. Pat. No. 4,672,207 describes a readout system for a mosaic of NXM scintillator/photodetector elements. In this system the photodetectors feed row and column amplifiers which indicate, for signals having the proper pulse height, that an event has occurred in the nth row and the mth column of the mosaic. However, this system requires a large number of scintillator crystals and, if applied to the solid state CdZnTe camera, as postulated above, would be unable to discriminate events which occur near or at the boundary between elements or to discriminate events which result in Compton scattering events.
In published PCT Application WO 95/33332 a method of reading out a matrix is described in which charge, generated as a result of events at points in the matrix, is stored at those points and the entire matrix is read out seriatim. This method, although mooted as being useful for a gamma camera utilizing CdZnTe, CdTe or a number of other materials at pages 45-48, is not capable of distinguishing individual events which would be necessary for the energy discrimination of events, used, for example, to eliminate events caused by Compton scattering.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a solid state gamma camera system having an improved readout system.
It is an object of some aspects of the present invention to provide a solid gamma camera system in which the outputs of individual pixels are recorded without the need to individually address the pixels.
It is an object of some aspects of the present invention to provide a solid state gamma camera in which events which occur near the boundaries of pixels and to some extent near the boundaries of crystals are properly detected.
It is an object of some aspects of the present invention to provide a solid state gamma camera in which the cells are all in a “talk-only” mode, in which no noise producing interrogating signals are necessary and in which each pixel transmits its data immediately after it detects an event.
It is an object of some aspects of the invention to provide a system which detects events which produce signals in more than one pixel, without collision of the data which is generated on these adjoining pixels.
A solid state gamma camera, in accordance with a preferred embodiment of the invention, is made up of a mosaic of crystals of CdTe (or alternatively of CdZeTe, HgI
2
, InSb, Ge, GaAs, Si, PbCs, PbS or GaAlAs). One side of each crystal is preferably covered by a single, common, electrode and the other side of the crystal is preferably covered by a rectangular (preferably square) matrix of closely spaced electrodes. This matrix of electrodes defines the cells or pixels of the gamma camera image. In a preferred embodiment of the invention, the matrix comprises 16×16 elements having a size of 2×2 mm. However, the size of the elements and the matrix size may vary over a relatively wide range depending on the desired spatial resolution and count rate. In particular, crystal sizes of 1×1 to 4×4 mm appear to be reasonable in the practice of the invention.
Generally, a rectangular mosaic of crystals each with its associated matrix of elements is used to provide a camera of the required size. This mosaic may have a dimension of 20×20 crystals or greater.
When a gamma ray impinges on the crystal, energy which is transferred to the crystal creates charge carriers within the normally insulating crystal such that it becomes temporarily conducting. When a high voltage is applied between the electrodes in the matrix and the common electrode, this charge generation results in current flow between them. This current generally lasts between 50 and 600 nanoseconds, depending on the depth of penetration of the gamma ray prior to its interaction with the crystal, and the crystal quality. The total charge collected by the matrix of elements is substantially proportional to the energy of the absorbed gamma ray. In this regard, each element can be considered as a signal source which produces a signal when a gamma ray absorption event occurs at or sufficiently close to its associated pixel.
In principle, the current resulting from a particular event (i.e., an absorbed gamma ray) should be limited to a single element of the matrix. However, a number mechanisms act to cause current to be measured at, generally, adjoining matrix elements.
One type of mechanism which induces current in more than one electrode is when an event occurs at or near a boundary between two or four matrix elements. Clearly, an event which occurs precisely at the boundary will cause an equal division of current between the adjacent two or four electrodes. Furthermore, events which occur near a boundary will also cause current to flow in adjoining elements since the gamma ray creates a small, but finite cloud of charge carriers which may overlap more than one cell and which diffuses and widens during its travel toward the electrodes. Thus, part of the current associated with an event near the boundary will be detected in an adjacent pixel element.
For each of the above effects, the energy of the gamma ray is deposited at substantially one point in the crystal and its effects are measured at more than one pixel element. Some events do not deposit their energy at only one point in the crystal. Rather they may undergo Compton scattering so that a portion of their energy is deposited at various points in the crystal. Each of these energy deposits causes currents to flow in corresponding pixel elements.
The above effects are dependent on both the energy of the gamma ray photons and the depth of penetration of the photon when it interacts with the crystal. Higher energy photons produce a larger electron cloud and have a higher probability of Compton scattering, such that, for 500 KeV photons, less than half will deposit their energy at a single point. The depth of penetration of the photon will determine the amount of spreading of the electron cloud prior to its being collected by one or more of the matrix elements.
While there is a relatively large probability that current will be collected in neighboring electrodes, the probability that current will be collected by non
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neigboring electrodes is small, for the energies used in Nuclear Medicine.
The determination of the position and energy of an event, especially for the situation where more than one matrix element receives current from the event, requires that (i) current generated by each event be separately received for each event and (ii) that the response at each matrix element be separately received, or at least that all currents for a particular event be added to give a proper measure of the energy of the event. This would appear to require that each pixel be connected, separately or in a multiplex fashion, to the main data processing computer. Such a connection would be impractical.
In accordance with a preferred embodiment of the present invention a pre-processing and multiplexing unit is attached to each crystal. This unit, referred to herein as an “ASIC” unit, determines the distribution of charge (i.e., energy) associated with each event and the position of the event. For events whose charge is associated with more than one pixel, the ASIC unit determines the amount of charge associated with each of the pixels. It is this reduced amount of information, namely, the energy associated with each pixel which is involved in an ev

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