Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2003-06-19
2004-05-25
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S397000
Reexamination Certificate
active
06740885
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to solid state photon detectors and in particular to the timing and energy resolution of pixelated photon detectors.
BACKGROUND OF THE INVENTION
Single photon emission computerized tomography (SPECT) and positron emission tomography (PET) depend upon the detection of gamma ray photons emitted by, or generated as a result of, radio-pharmacological compounds introduced into the body to image regions of the body. Photon detectors used in SPECT and PET imaging systems that provide data from the emitted photons in order to produce images provide a spatial location at which a photon is detected and generally a measure of the energy of the detected photon. In PET imaging two photons are detected in coincidence and in addition to providing a spatial location for a detected photon, photon detectors also provide an indication of the time at which they detect a photon.
Traditional photon detectors for PET, SPECT and other applications, generally use an NaI(Tl) or BGO scintillation crystal to detect photons. However, NaI(Tl) has a relatively low density and low stopping power per gram of material. In order to provide detectors having good detection efficiencies for photons, NaI(Tl) crystals used in these detectors must be made relatively thick. However, as the thickness of a detector crystal increases, generally, the spatial resolution of the detector decreases. BGO scintillation crystals on the other hand exhibit low light output for detected photons and therefore generally poor energy resolution Furthermore, scintillation light in both NaI(Tl) and BGO crystals caused by the passage of a photon through the crystal lasts for a relatively long time. This complicates accurate coincidence timing using these detectors.
Semiconductor materials with high atomic numbers and relatively high densities such as CdZnTe, CdTe, HgI
2
, InSb, Ge, GaAs, Si, PbCs, PbS, or GaAlAs, have a high stopping power for photons per centimeter path length in the semiconductor material. Therefore, for the same stopping power, crystals made from these materials are generally thinner than detector crystals made from other materials that are used for photon detectors. These materials if they are formed into crystals of sufficient thickness can therefore be used to provide detectors with good photon detection efficiencies and improved spatial resolution. However, it difficult to manufacture thick semiconductor crystals for photon detectors using present state of the art technology.
Gamma cameras for detecting photons for SPECT, PET and other applications requiring photon detection and location, have been fabricated from the semiconductor materials listed above. Often these gamma cameras comprise arrays of pixelated detector modules, hereinafter referred to as “pixelated detectors”. A pixelated detector is described in PCT publication WO 98/23974, the disclosure of which is incorporated herein by reference.
FIGS. 1A and 1B
schematically illustrate a pixelated detector and a gamma camera comprising pixelated detectors respectively.
FIG. 1A
shows a typical construction of a pixelated detector
20
comprising a crystal
22
formed from a semiconductor material such as one of those noted above. A face
24
of crystal
22
has a large single cathode electrode
26
. An opposite face
28
of crystal
22
has an anode
30
comprising a rectangular array of identical small square anode pixels
32
. Typically, sizes of anode pixels
32
vary between 1 and 4 mm
2
, and the thickness of crystal
22
, between anode
30
and cathode
26
is on the order of millimeters to a centimeter. In operation, a voltage difference is applied between anode and cathode so that an electric field, hereinafter referred to as a “detector field”, is generated in crystal
22
. This field is typically on the order of a few kilovolts per centimeter.
When a photon having an energy typical of the energies of photons used in SPECT or PET applications is incident on crystal
22
, it generally loses all its energy in crystal
22
by ionization and leaves pairs of mobile electrons and holes in a small localized region of crystal
22
. As a result of the detector field, the holes drift to cathode
26
and the electrons drift to anode
30
, thereby inducing charges on anode pixels
32
and cathode
26
. The induced charges on anode pixels
32
are sensed and generally partially processed by appropriate electronic circuits comprised in ASICs (not shown) located in a detector base
34
on which detector
20
is mounted. Detector base
34
comprises connection pins
36
for mounting to a motherboard (not shown) and transmitting signals from the ASICs to the motherboard. Signals from the induced charges on pixels
32
are used to determine the time at which a photon is detected, how much energy the detected photon deposited in the crystal and where in the crystal the photon interaction took place.
FIG. 1B
shows a rectangular gamma camera
38
comprising twenty pixelated detectors
20
arranged to form a rectangular array of five rows of four detectors
20
each. Detectors
20
are shown mounted on a motherboard
39
. In practice, gamma cameras comprising larger arrays of pixelated detectors are generally used.
Pixelated detectors provide reasonable spatial resolution and detection efficiency for incident photons. However, signals from anode pixels of a pixelated detector of this type are highly variable in shape and exhibit considerable jitter in signal timing with respect to the time that detected photons are incident on the detector. As a result, for applications such as PET, which require accurate coincidence measurements between different detectors, output signals from anode pixels are not readily useable. Furthermore, pixelated detectors do not provide highly accurate measurements of energies of detected photons such as are required in SPECT, PET and other applications.
The energy of a photon detected by a semiconductor detector is generally determined from an estimate of the total number of electron-hole pairs produced in the detector's crystal when the photon ionizes material of the crystal. This is generally determined from the number of electrons produced in the ionizing event, which is estimated from the charge collected on the anode of the detector. The energy resolution of the detector is a function of how accurately the number of electron-hole pairs produced in the detector by a detected photon can be estimated.
If all the electrons and holes produced by a photon detected in a semiconductor detector were collected by the detector electrodes, then the induced charge on either the anode or cathode of the detector would be an appropriate measure of the energy of the photon. However, the holes are generally subject to relatively severe charge trapping problems in the detector crystal and they drift in the crystal at approximately one tenth the velocity of the electrons. Charge trapping and the slow drift velocity of the holes degrade the accuracy with which the induced charges on either the cathode or anode of a semiconductor detector can be used to estimate the energy of a detected photon.
Semiconductor photon detectors with improved energy resolution that do not detect the location of detected photons, but only their energies, are described in “Unipolar Charge Sensing with Coplanar Electrodes—Application to Semiconductor Detectors,” by P. N. Luke, IEEE Transaction on Nuclear Science, vol. 42(4), 1995, p. 207, and in “Coplanar Grid Patterns and Their Effect on Energy Resolution of CdZnTe Detectors” by Z. He, et al, presented at the 1997 IEEE Nuclear Science Symposium and Medical Imaging Conference, Albuquerque, N.M., Nov. 9-15, 1997, which are incorporated herein by reference. For the described detectors, the effects of charge trapping and drift motion of holes on energy determinations of photons detected by the detectors are substantially reduced.
FIG. 2
shows a detector
40
of the type described in the referenced articles. Detector
40
comprises a semiconductor crystal
42
(CdZnTe is the semiconduct
Amrami Aharon
Hefetz Yaron
Pansky Amir
Wainer Naor
Elgems Ltd.
Fenster & Company
Gabor Otilia
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