Search

In this paper, investigation of position sensitive avalanche photodiodes (PSAPDs) as optical detectors for reading out segmented scintillation arrays of LSO in high resolution PET modules is reported. PSAPDs with 8 x 8 mm(2) and 14 x 14 Turn 2 area have been characterized with single LSO crystals and arrays. Energy resolution of 19% (FWHM) for 511 keV gamma-rays and coincidence timing resolution of similar to3 ns (FWHM) have been recorded with PSAPD coupled to 1 x I x 20 mm(3) LSO detectors. Flood histogram studies have been successfully conducted by coupling multi-element element LSO arrays (1 mm pixels, 20 mm tall) to the PSAPDs. Finally, depth of interaction (DOI) resolution of

We are developing PET detectors with depth of interaction (DOI) capability based on lutetium oxyorthosilicate (LSO) scintillator arrays coupled at both ends to position-sensitive avalanche photodiodes (PSAPDs). The detector module consists of a 5x5 block of LSO crystals (1.5 X 1.5 X 20 [mm.sup.3]) coupled to 8 X 8 [mm.sup.2] PSAPDs at both ends. We present the intrinsic spatial resolution for two complete modules and the radial resolution component obtained with and without parallax correction using DOI information. DOI resolution was measured to be ~3 mm. Intrinsic spatial resolution (FWHM) averaged 1.15 mm for opposing crystal pairs. The detectors were then rotated and moved closer together consistent with a 16 cm diameter cylindrical geometry scanner. Resolution was measured at positions corresponding to a radial offset of 4.7 cm and 7 cm, with the annihilation photons incident on the detector surface at 30[degrees] and 52[degrees] respectively. At an angle of 52[degrees] the intrinsic spatial resolution was degraded to 7.7 mm. By incorporating DOI information, the measured spatial resolution (FWHM) was improved to 1.8 mm. This demonstrates that approximately 90% of the resolution degradation due to the parallax error can be removed using these depth-encoding detectors for a field of view with a diameter that is 87.5 % of the detector separation. Index Terms--APD, depth-of-interaction, LSO, positron emission tomography (PET), spatial resolution.

We report here on a novel design of a digital, intraoperative imaging probe intended for use in radio-guided surgical procedures in conjunction with radiolabels such as [sup.131]I and [sup.18]F. The probe allows the user to rapidly localize tumors by detecting the highly penetrating gamma rays, and then image the tumor with the short-range beta rays. The system provides a rapid, high-resolution, image of the interrogated area, fulfilling the need for clear delineation of tumors during radio-guided surgical procedures. The beta imaging sensor consists of a micro-columnar CsI(TI) scintillator screen capable of providing very high detection efficiency, high light output and excellent spatial resolution coupled to a CCD via a flexible, coherent fiberoptic bundle. The gamma sensor is a shielded piece of crystalline CsI(TI) coupled to a photodiode located behind the image sensitive front end. The feasibility of this design was studied by separately testing the beta imaging and gamma detection components. The operation of the components was characterized with intrinsic performance measurements of count rates, signal-to-noise ratios, spatial resolution, as well as time for acquiring useful images using radionuclide and anthropomorphic phantoms. Index Terms--Cesium iodide, charge coupled devices, nuclear imaging, radioguided surgery, surgical probes.

We are currently developing high-resolution high-efficiency microcolumnar LiI films for thermal neutron imaging. The films are produced by the vapor deposition of LiI on a fiber-optic substrate and hermetically sealed in a specially designed aluminum package. Our work has produced up to 1.2-mm-thick films with column diameters of approximately 30 [micro]m. We have also performed imaging studies by optically coupling some of these films to a fiber-optic taper-based charge-coupled device. The imaging performance of the system was experimentally evaluated at Radiation Monitoring Devices, Inc., as well as at the thermal neutron port of the University of Massachusetts Lowell Research Reactor. The LiI films exhibited excellent scintillation characteristics with a spatial resolution as high as 4.5 lp/mm (line pairs per millimeter). This paper outlines the film characterization and performance evaluation conducted during the course of the study. The new sensor described here is expected to usher in the development of large-area high-resolution digital thermal neutron detectors with improved detection efficiency and dynamic range and faster readout times than the current sensors. Index Terms--Digital neutron imaging, neutron radiography, structured scintillator, thermal neutron imaging.

New third generation x-ray sources such as the Advanced Photon Source have created a need for a detector that can provide multiple frames of detailed x-ray images on the millisecond time scale. Such detectors will prove invaluable in applications such as time-resolved x-ray diffraction, x-ray microtomography, as well as materials science applications like polymer processing. Currently, detectors capable of acquiring high resolution x-ray images at such high speed do not exist, thus limiting progress in many of these important areas of research. To address these needs we have developed a prototype fast x-ray imaging system, using a structured CsI(Tl) scintillator coupled to a fast-frame 1K x 1K CCD. The system has been successfully employed to capture 1024 x 64 pixel x-ray images at a rate of 1000 frames per second (fps) with a 12 bit dynamic range. The system exceeds the capabilities of the current high speed x-ray imaging systems which typically operate at the rate of 30 fps. Fabrication of a large area detector is currently underway, using a microstructured CsI(Tl) scintillator coupled to a fast-frame CCD with a 3:1 fiberoptic taper. The camera will operate in a burst mode, acquiring 8 1K x 1K images at rates up to 1000 frames per second with 12 bit dynamic range. Higher image capture speeds can be accomplished by reducing the image area. This paper will discuss the specific characteristics of the CsI(Tl) screens, experimental details of the prototype and the new design for the large area detector being developed specifically for time-resolved x-ray diffraction experiments in structural biology.

We are developing large-area, thick, structured CsI(T1) imaging sensors for a wide variety of X-ray imaging applications. Recently we have fabricated structured CsI(T1) scintillators ranging from 30 [[micro]meter] (16 mg/[cm.sup.2]) to 2000 [[micro]meter] (900 mg/[cm.sup.2]) in thickness and up to 15 x 15 [cm.sup.2] in area. Even 2000-[[micro]meter]-thick film showed well-controlled columnar growth throughout the film. Material characterization confirmed that the film is crystalline in nature and that the stoichiometry is preserved. To improve the spatial resolution of thick films, post-deposition treatments were performed. The effect of these treatments on film characteristics was quantitatively evaluated by measuring signal output, modulation transfer function [MTF(f)], noise power spectrum [NPS(f)], and detective quantum efficiency [DQE(f)]. The data show that by proper film treatments, the film DQE(f) can be improved.

Film radiography has long served the aerospace industry as a principal method for hardware flaw detection. Although excellent in performance, this method is extremely time consuming, labor intensive, costly, and is unsuitable for real time inspections. Modern digital radiographic systems overcome some of these difficulties but are also limited in terms of speed of operation due to persistence of the sensor and a problematic tradeoff between the X-ray detection efficiency and spatial resolution. We are developing an innovative X-ray imaging detector consisting of a novel microstructured CsI scintillator coupled to a fiber-optic taper-based CCD. Thin-film deposition techniques, previously developed to produce thin, structured CsI screens have been extended to fabricate CsI screens, up to 450 mg/[cm.sup.2] (1,000 [[micro]meter]) in thickness. These sensors are suitable to provide high detection efficiency with high image quality for NDE applications. A prototype high energy imaging system was constructed by integrating these screens into a fiber-optic-based CCD camera. The performance was compared to that of the same system using a standard polycrystalline phosphor for NDE imaging applications. The experimental evaluations were carded out at Scientific Measurement Systems, Inc., Austin, TX.

A combined video and gamma ray imaging system has been developed which can rapidly determine the location, distribution, and intensity of gamma ray sources. This instrument includes both a conventional video camera and a gamma ray imaging system, which is based upon a position sensitive photomultiplier tube, a scintillator, and a pinhole collimator. The gamma camera records the position and energy of each interaction, determining the energy spectrum and count rate from each direction. We have used a prototype of such an instrument in preliminary field tests to image radioactive sources with gamma ray energies between 120 keV and 2.4 MeV. At 662 keV, this new system achieves an angular resolution of 4 [degrees] FWHM with an absolute efficiency of 1x[10.sup.-6] at 1 meter, with sensitivity such that a 0.5 mCi 137Cs source at 5 meters can be located in [less than]60 seconds. Higher efficiency and sensitivity can be achieved at lower spatial resolution.

A major impediment to the accurate measurement of neutron dose experienced by astronauts arises from the sensitivity of most neutron dosimeters to the large fluxes of protons which occur during a typical space mission. As a result, measurement of the neutron dose has remained largely an unsolved problem. Under NASA sponsorship, we have developed a special PIN diode that has good response to neutrons while being relatively insensitive to other forms of ionizing radiation. This new sensor has been incorporated into a very compact flight instrument to provide high quality real time measurement of neutron dose. The dosimeter is capable of measuring and periodically recording fast neutron dose ([E.sub.n] [is greater than or equal to] 140 keV) from 50 microgray to tens of milligrays (5 mRads to several Rads) over a flight of up to 14 days with an accuracy of [+ or -] 17.5 micrograys ([+ or -] 1.75 mRads) or better. The latest measurements of the PIN diode performance, with a detailed description of the overall instrument, are presented.

A compact, high resolution X-ray and gamma ray spectroscopy system has recently been developed which provides excellent energy resolution over a wide range of photon energies. Two detector types have been tested, a cooled silicon PIN photodiode for X-ray energies from 2 keV to 30 keV and a CdTe detector for energies from 20 keV to 1 MeV. The CdTe detector can be operated cooled or at room temperature. The heart of the system is a miniature preamplifier developed at AMPTEK which is used in combination with a small thermoelectric cooler to reduce the leakage current of the detectors. Using the cooled Si-PIN detector, 660 eV FWHM was achieved at 5.9 keV and using a CdTe detector 1.3 keV was achieved at 60 keV both with nearly 100% counting efficiency. Good resolution was also achieved at higher energies.

One of the most promising means for improving the quality of radiation therapy is the use of real-time imaging systems for routine portal position verification. We have constructed a prototype portal imager using a linear array of 256 CdTe photovoltaic detectors, each 2 x 2 x 2 mm. The array is attached to a compact linear scanner which is to be mounted in a cassette shaped package located below the patient table. The array of detectors is moved under the patient during image acquisition. The high stopping power of the CdTe allows a high contrast image to be made using a single linac pulse for each array position. In tests conducted with a 4 MV linac, this system produced 50 cm x 35 cm images with an open field signal-to-noise ratio of 143 and 2 mm spatial resolution in less than 3 seconds. This corresponds to a signal-to-noise ratio of 1 for 1% contrast objects. Ultimately, a signal-to-noise ratio greater than 5 at 1% contrast should be achievable with no loss of spatial resolution or increase in acquisition time.

One of the most promising new technologies for improving the quality of radiation therapy is the use of real-time systems to produce portal images. In our approach, we are constructing a linear array of 256 CdTe photovoltaic detectors attached to a very compact linear scanner, all of which will be mounted in a cassette shaped package to be located under the patient table. The high stopping power of the CdTe allows a high contrast image to be made using only a single Linac pulse per line, resulting in a high contrast image in under 5 seconds.

A neutron sensor which can be incorporated in a compact flight instrument was developed to aid astronauts in detecting high energy neutrons coming from primary and secondary sources of ionizing radiation. The new sensor utilizes a special high neutron sensitivity PIN diode which is insensitive to other forms of ionizing radiation. It can measure and record neutron doses over the range of 50 microgray to tens of milligrays and can also be applied for use in commercial facilities such as power plants, accelerator and industrial facilities.