Stephen E. Derenzo

Emission Computer Assisted Tomography with Single-Photon and Positron Annihilation Photon Emitters

Computed transverse section emission tomography using 99mTc with the Anger camera is compared to positron annihilation coincident detection using a ring of crystals and 68Ga. The single-photon system has a line spread function (LSF) of 9 mm full width at half maximum (FWHM) at the collimator and gives a transverse section reconstruction LSF of 11 mm FWHM with 144 views. The positron ring has a LSF of 6 mm at the center with a transverse section reconstruction LSF of 7.5 mm FWHM. Correction for uniformity of detector response and accurate center of rotation determination is essential in both techniques. The signal-to-noise ratio in a reconstruction is diminished by a factor of 1.2 x (number of resolution elements)1/4 over that expected from the average number of events per resolution element. Attenuation compensation causes more noise to appear in the center than the edge for both modes and an average increase in uncertainty of 30%. The effects of attenuation result in more loss of data for positron coincidence imaging than for single-photon imaging even at energies of 80 keV. For a 20-cm cylinder imaged in transverse section, only 20% of the positron annihilation events are not scattered; however, at 140 keV, 40% of the photons are not scattered. The relative crystal efficiency gives single-photon imaging an advantage of 5. On the other hand, the solid angle advantage of positron photon coincidence imaging is about 100 for the comparisons of this paper. Taking these factors into account, we find positron-computed section imaging has a tenfold increase in sensitivity over multiple-view imaging with the scintillation camera, which gives multiple sections but requires camera or patient rotation.

A positron tomograph with 600 BGO crystals and 2.6 mm resolution

A description is given of the imaging performance of the Donner 600-Crystal Positron Tomograph, a single 60-cm-diameter ring of 3-mm-wide bismuth germanate (BGO) crystals coupled individually to 14-mm phototubes. With a pulse height threshold of 200-keV and a slice thickness of 5 mm, the sensitivity is 7024 events/s per mu Ci/Ml in a 20-cm cylinder of water. The measured rates for 18 mu Ci/ml are 95000 trues/s plus 20000 randoms/s. A 0.3-mm-diameter /sup 22/Na line source near the center of the tomograph has a circular point-spread function (PSF) with a full-width at half-maximum (FWHM) of 2.6 mm. At 5 cm from the center the PSF is elliptical with a FWHM of 2.7 mm tangential*3.2 mm radial. At 10 cm the PSF has a FWHM of 2.8 mm tangential*4.8 mm radial. Attenuation data have been accumulated with a 20 mCi /sup 68/Ge orbiting transmission source, and 100 million coincident events have been collected in 200 s. >

Imaging Properties of a Positron Tomograph with 280 Bgo Crystals

The basic imaging properties of the Donner 280-BGO-Crystal positron tomograph were measured and compared with the same system when it was equipped with 280 NaI(T1) crystals. The NaI(T1) crystals were 8 mm × 30 mm × 50 mm deep, sealed in 10 mm wide stainless steel cans. The BGO crystals are 9.5 mm wide × 32 mm × 32 mm deep and as they are not hygroscopic do not require sealed cans. With a shielding gap of 3 cm (section thickness 1.7 cm FWHM) the sensitivity of the BGO system is 55,000 events per sec for 1 ?Ci per cm3 in a 20 cm cylinder of water, which is 2.3 times higher than the NaI(T1) system. For a 200 ?Ci/cm line source on the ring axis in a 20 cm diameter water cylinder, the BGO system records 86% of the scatter fraction and 66% of the accidental fraction of the NaI(T1) system. The lower light yield and poorer time resolution of BGO requires a wider coincidence timing window than NaI(T1); however, the ability to use full-energy pulse height selection with a 2.3-fold improvement in sensitivity results in an overall reduction in the fraction of accidental events recorded. The in-plane resolution of the BGO system is 9-10 mm FWHM within the central 30 cm diameter field, and the radial elongation at the edge of the field in the NaI(T1) system has been nearly eliminated.

Temperature dependence of CsI(Tl) gamma-ray excited scintillation characteristics

The gamma-ray excited, temperature dependent scintillation characteristics of CsI(Tl) are reported over the temperature range of −100 to + 50°C. The modified Bollinger-Thomas and shaped square wave methods were used to measure the rise and decay times. Emission spectra were measured using a monochromator and corrected for monochromator and photocathode spectral efficiencies. The shaped square wave method was also used to determine the scintillation yield as was a current mode method. The thermoluminescence emissions of CsI(Tl) were measured using the same current mode method. At room temperature, CsI(Tl) was found to have two primary decay components with decay time constants of τ1 = 679±10 ns (63.7%) and τ2 = 3.34±0.14 μs (36.1%), and to have emission bands at about 400 and 560 nm. The τ1 luminescent state was observed to be populated by an exponential process with a resulting rise time constant of 19.6±1.9 ns at room temperature. An ultra-fast decay component with a < 0.5 ns decay time was found to emit about 0.2% (about 100 photons/MeV) of the total scintillation light. Except for the ultra-fast decay time, the rise and decay time constants were observed to increase exponentially with inverse temperature. At −80°C τ1 and τ2 were determined to be 2.22±0.33 μs and 18.0±2.59 μs, respectively, while the 400 nm emission band was not observed below −50°C. At +50°C the decay constants were found to be 628 ns (70.5%) and 2.63 μs (29.3%) and both emission bands were present. The scintillation yield of CsI(Tl) was observed to be only slightly temperature dependent between −30 and +50°C, peaking at about −30°C (about 6% above the room temperature yield) and monotonically decreasing above and below this temperature. Four different commercially available CsI(Tl) crystals were used. Minimal variations in the measured scintillation characteristics were observed among these four crystals. Thermoluminescence emissions were observed to have peak yields at −90, −65, −40, +20, and possibly −55°C. The relative magnitudes and number of thermoluminescence peaks were found to vary from crystal to crystal.

Cerium fluoride, a new fast, heavy scintillator

The authors describe the scintillation properties of cerium fluoride CeF/sub 3/, a recently discovered, heavy (6.16 g/cm/sup 3/), inorganic scintillator. Its fluorescence decay lifetime, measured with the delayed coincidence method, is described by a single exponential with a time constant of 20+or-1 ns. The emission spectrum peaks at a wavelength of 340 nm, and drops to less than 10% of its peak value at 315 nm and 460 nm. When a 1-cm optical-quality cube of CeF/sub 3/ is excited with 511-keV photons, a photopeak with a 20% full width at half maximum is observed at approximately half the light output of a bismuth germanate (BGO) crystal with similar geometry. Also presented are measurements of the decay time and light output of CeF/sub 3/ doped with three rare-earth elements (Dy, Er, and Pr). The short fluorescence lifetime, high density, and reasonable light output of this scintillator suggest that it would be useful for applications where high counting rates, good stopping power, and nanosecond timing are important, such as medical imaging and nuclear science. >

Design of a high-resolution, high-sensitivity PET camera for human brains and small animals

The authors present design parameters for a 3-D PET camera with high sensitivity (35 cm detector ring diameter, 15 cm axial field of view) and isotropic high resolution provided by detector modules capable of depth of interaction (DOI) measurement. Detector modules are made of LSO crystals (3 mm square by 30 mm deep)-the small module size and short decay time of LSO reduce the detector dead time by a factor of 14 compared to conventional EGO detector modules and narrow the coincidence window width to 4 ns. This yields an expected peak noise equivalent count rate of 800 kcps and noise equivalent sensitivity of 1370 kcps//spl mu/Ci/cc with a 20 cm diameter phantom-three to five times higher than conventional scanners. With 5 mm fwhm DOI resolution, the expected reconstructed spatial resolution is <3.0 mm fwhm throughout the entire field of view. Depth of interaction measurement information is incorporated into the reconstruction algorithm by rebinning onto a regularly spaced grid. Attenuation correction is performed with an orbiting single transmission source.

Orbiting transmission source for positron tomography

Accidental suppression and effective data rates have been measured for the orbiting transmission source as implemented in the Donner 600-crystal positron-emission tomograph (PET). A mechanical description of the orbiting source and a description of the electronics used to discard scattered and accidental events are included. Since accidental coincidences were the rate-limiting factor in transmission data acquisition, this method allows sufficient transmission data to be acquired in a shorter time with a more-active transmission source. >

The scintillation properties of cerium-doped lanthanum fluoride

Abstract We report on the scintillation properties of cerium-doped lanthanum fluoride (LaF 3 ), a newly discovered dense (5.9 g/cm 3 ) heavy-atom scintillator. We have investigated four dopant concentrations (0.01%, 1%, 10% and 50% mole fraction of CeF 3 ) and measured the emission spectrum, light output and decay time distribution. The light output increases with increasing cerium concentration until a maximum is reached at 2200 photons/MeV at 10% CeF 3 , then decreases to 1900 photons/MeV at 50% CeF 3 . The emission spectrum depends on concentration, but tends to consist of a pair of peaks centered at approximately 350 and 290 nm, with the lower cerium concentrations having a greater fraction of shorter-wavelength emissions. While the decay time distribution also depends on concentration, all samples are well described by the sum of three exponential components (3.0 ns, 26.5 ns, and a long component that varies between 140 and 275 ns), with the fraction of the 3.0 ns fast component increasing from 10% at 50% cerium to 15% at 1% cerium, and the fraction of the long component increasing from 3% at 50% cerium to 21% at 1% cerium. These scintillating compounds may be useful for applications where high rates and good stopping power are required, such as medical imaging and nuclear instrumentation.

Design and Implementation of a Facility for Discovering New Scintillator Materials

We describe the design and operation of a high-throughput facility for synthesizing thousands of inorganic crystalline samples per year and evaluating them as potential scintillation detector materials. This facility includes a robotic dispenser, arrays of automated furnaces, a dual-beam X-ray generator for diffractometry and luminescence spectroscopy, a pulsed X-ray generator for time response measurements, computer-controlled sample changers, an optical spectrometer, and a network-accessible database management system that captures all synthesis and measurement data.

High Resolution Computed Tomography of Positron Emitters

High resolution computed transaxial tomography has been performed on phantoms containing positron-emitting isotopes. The imaging system consisted of two opposing groups of eight NaI(T1) crystals 8 mm × 30 mm × 50 mm deep and the phantoms were rotated to measure coincident events along 8960 projection integrals as they would be measured by a 280 crystal ring system now under construction. The spatial resolution in the reconstructed images is 7.5 mm FWHM at the center of the ring and approximately 11 mm FWHM at a radius of 10 cm. We present measurements of imaging and background rates under various operating conditions. Based on these measurements, the full 280 crystal system will image 10,000 events per sec with 400 ¿Ci in a section 1 cm thick and 20 cm in diameter. We show that 1.5 million events are sufficient to reliably image 3.5 mm hot spots with 14 mm center-to-center spacing and isolated 9 mm diameter cold spots in phantoms 15-20 cm in diameter.