Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1998-11-12
2001-09-11
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S367000, C250S363030
Reexamination Certificate
active
06288399
ABSTRACT:
TECHNICAL FIELD
This invention relates to the field of positron emission tomography devices. More particularly, the present invention is related to a detector block for measuring depth of interaction activity in high performance positron emission tomography.
BACKGROUND ART
In the field of positron emission tomography, or PET, it is well known that in order to improve the capability for investigating the living human brain, for example, with regard to blood flow, metabolism and receptor characteristics for small structures such as cortical sublayers and nuclei, the spatial resolution has to be improved relative to what is available today, as disclosed by K. Herholz, et al., “Preoperative activation and intraoperative stimulation of language-related areas in glioma patients,”
Neurosurgery,
1997; and L. Farde, et al., “A PET study of [
11
C]FLB 457 binding to extrastriatal D2-dopamine receptors in health subjects and antipsychotic drug treated patients,”
Psychopharmacology
, vol. 133, no. 4, 1997, spatial resolution of 2 mm or less may be necessary to reach these research goals. Such a resolution approaches the physical limits of the annihilation process itself; the range of positron in tissue and the non-collinearity of the annihilation photons.
The highest spatial resolution positron camera system commercially available for human investigations presently is the ECAT EXACT HR, as discussed by M. E. Casey, et al., “A multi-crystal two dimensional BGO detector system for positron emission tomography,”
IEEE Trans. Nucl Sci.,
vol. 33, pp. 460-463, 1986, with a spatial resolution in the reconstructed image planes of less than 4 mm. The ECAT EXACT HR system uses cost effective block technology and is based on BGO scintillators with 7×8 crystals per block with individual crystal sizes of approximately 2.9×5.9×30 mm
3
. By reducing the geometry of the system, the non-collinearity of the annihilation gamma rays are reduced, resulting in a spatial resolution of around 3 mm for a 40 cm diameter system. However, to maintain this resolution over a 20 cm FOV, the DOI in the 30 mm deep crystals must be obtained, which is a challenge with low light yield scintillators, such as BGO or GSO. Also the time response of the used scintillator has to accommodate an excellent timing resolution to suppress random coincidences with a short coincidence window of ~5 ns. In addition, with the low light from BGO, the 3 mm crystal dimension is probably a practical lower limit which can be resolved with the present BGO based block technology. A cost effective BGO positron camera system based on the block concept with a 2 mm spatial resolution has not been shown to be possible to construct.
Recently a new scintillator has become available, lutetium-oxyorthosilicate (LSO) with a scintillation light yield between four to five times that of BGO and a scintillation decay time of around 40 ns, as disclosed by C. L. Melcher, et al., “Cerium-doped lutetium oxyorthosilicate: A fast, efficient new scintillator,”
IEEE Trans. Nucl. Sci.,
vol. 39, no. 4, pp. 502-505, 1992. The high light yield implies that small crystals now can be identified with the block technology with only a small identification degradation due to photon statistics. The short scintillation decay time implies low dead time losses in the detectors. In addition, detectors based on LSO have a good time resolution which offers the possibility of using a short coincidence time window thus reducing the random coincidence contribution. Positron camera systems with a spatial resolution of 2 mm or less are now feasible for investigations of the living human brain as well as for animal studies.
Other related articles include: W. J. Jagust, et al., “The cortical topography of temporal lobe hypometabolism in early Alzheimer's disease”,
Brain Research
, vol. 629, no. 2, pp. 189-198, 1993; C. Carrier, et al., “Design of a high resolution positron emission tomograph using solid state scintillation detectors,”
IEEE Trans. Nucl. Sci.,
vol. 35, no. 1, pp. 685-690, 1988; and M. E. Casey, et al., Investigation of LSO crystals for high resolution positron emission tomography,”
IEEE Trans. Nucl. Sci.,
vol. 44, no. 3, pp. 1109-1113, 1997. Carrier, et al., disclose a phoswich combination of scintillator detectors for use in a high resolution positron emission tomograph.
An ECAT HRRT, as discussed by Casey, et al., is an octagonal design with 8 detector heads and an axial dimension of 25.2 cm. The distance between two opposing heads is 46.9 cm. Because of the small ring diameter and the large axial and transaxial width, corrections for depth-of-interaction is required to meet the ambitious goal of ~2 mm spatial resolution. The depth-of-interaction information is extracted from differences in scintillation decay time between the two crystal layers. Casey also disclosed the possibilities to use a LSO-LSO combination of the same dimensions using the differences in scintillation decay time.
An object of the present invention is to provide a new block detector for a new brain camera design, the ECAT HRRT, with a phoswich combination of LSO and GSO crystals with an individual crystal sizes, for example, of around 2.1×2.1×7.5 mm
3
in each crystal layer.
DISCLOSURE OF THE INVENTION
Other objects and advantages will be accomplished by the present invention which is provided for improving the spatial resolution and uniformity in modem high resolution brain Positron Emission Tomography (PET) systems over the entire field of view (FOV). An LSO crystal layer, a GSO crystal layer, and a light guide are stacked on each other and mounted on a 2×2 photomultiplier tube (PMT) set, so that the corners of the phoswich are positioned over the PMT centers. The crystal phoswich is cut into a matrix of discrete crystals. The separation of pulses from the LSO and the GSO layers by pulse shape discrimination allows discrete DOI information to be obtained. The block design provides an external light guide used to share the scintillation light in four PMTs.
The 4 PMT signals S
i
are connected to an amplifier box which offers a 4 pole semi-Gaussian shaping for each of the four PMT signals, a sample clock for triggering the ADC cards and a fast sum signal &Sgr;
i
S
i
of the four PMT signals S
i
for pulse shape discrimination. A constant fraction discriminator (CFD) provides a START signal for the time to pulse height converter. The fist sun signal is in addition differentiated and integrated with a fast filter amplifier and connected to a CFD, which provides a STOP signal for the TAC. The outputs of the shaped PMT signals and the TAC are connected to two ADC cards running under computer control.
For the flood source measurements, both cards are triggered with the sample clock signal of the amplifier box, initialized by the sum signal of the four PMTs. For the line spread function measurements, the CFD signal initialized by the sum signal and the CFD signal initialized by the reference detector are connected to a coincidence board. The reference detector is a single GSO crystal mounted on a single PMT. The PMT signal is amplified and connected to a CFD. The CFD signal in coincidence with the block CFD signal gives the trigger for the ADC cards. The data are acquired on the computer in list mode or directly as histogram data and are then transferred for analysis. A flood source is placed a selected distance from the crystal face to uniformly irradiate the block. The ADC card trigger line is connected to the sample clock. Software is provided for analyzing the data for identification of each single LSO and GSO crystal and for calculation of the events, energy centroid and energy resolution per crystal. The data are processed in two separate runs. The first run is to determine the thresholds in the zero cross time spectrum of the pulse shape discrimination data. The acquired time entries in the list mode data are sorted into a histogram. A double Gaussian is fit to the data to define the time boundaries, identifying the layer of the event intera
Andreaco Mark S.
Casey Michael E.
Nutt Ronald
Williams Charles W.
CTI Pet Systems, Inc.
Epps Georgia
Hanig Richard
Pitts & Brittian P.C.
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