J. L. Cahoon

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.

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. >

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.

Initial Results from the Donner 600 Crystal Positron Tomograph

We describe a positron tomograph using a single ring of 600 close-packed 3 mm wide bismuth germanate (BGO) crystals coupled to 14 mm phototubes. The phototube preamplifier circuit derives a timing pulse from the first photoelectron, and sends it to address and coincidence circuits only if the integrated pulse height is within a pre-set window. The timing delays and pulse height windows for all 600 detectors and the coincidence timing windows are computer adjustable. An orbiting positron source is used for transmission measurements and a look-up table is used to reject scattered and random coincidences that do not pass through the source. Data can be acquired using a stationary mode for 1.57 mm lateral sampling or the two-position clam sampling mode for 0.79 mm lateral sampling. High maximum data rates are provided by 45 parallel coincidence circuits and 4 parallel histogram memory units. With two-position sampling and 1.57 mm bins, the reconstructed point spread function (PSF) of a 0.35 mm diam 22Na wire source at the center of the tomograph is circular with 2.9 mm full-width at half-maximum (fwhm) and the PSF at a distance of 8 cm from the center is elliptical with a radial fwhm of 4.0 mm and tangential fwhm of 3.0 mm.

The Donner 280-Crystal High Resolution Positron Tomograph

We describe a stationary positron tomooraph desiqned for the rapid, accurate, three-dimensional imaging of positron-labeled compounds anywhere in the human body. The system consists of a large gantry containing lead shieldinq and a 92 cm diam ring of 280 closely-packed rectanqular NaI(Tl) crystals, lightpipes and phototubes; coincidence and address circuits; semiconductor histogram memory; a hard-wired reconstructor; and a gray-level CRT. Event rates, backgrounds and resolution test images are presented.

Three dimensional imaging of the myocardium with radionuclides.

Transverse sections of the distribution of 129Cs and 201Ti in the human myocardium were obtained using 36 and 72 views of the thorax with a large field of view Anger camera. The cardiac cycle was divided into 100-msec intervals to obtain motion images of an average cycle of the beating heart. At least 300,000 events must be detected for each cardiac phase of each section for quantitative work. Over 8 million events from the upper thorax must be accumulated in gated studies if three or more sections are obtained for 8 intervals of 100 msec to 150 msec.

Preparation of 11C-methyl iodide and l-[S-methyl-11C]methionine by an automated continuous flow process

A continuous flow procedure has been developed for the synthesis of 11C-methyl iodide and L-[S-methyl-11C]methionine from 11CO2. The preparation can be completed in 20 min after the end of bombardment. This procedure results in the production of approximately 44 mCi 11C-methyl methionine with a specific activity of 3.3 Ci/mmol. Radiochemical purity is greater than 96%. The procedure has been automated so that the preparation can be carried out remotely, thus minimizing exposure of personnel to radiation.

The Electronics for the Donner 600-Crystal Positron Tomograph

The data acquisition system, designed for the Donner 600-Crystal Positron Tomograph, is described. Coincidence timing resolution of less than five nanoseconds full width at half maximum and data rates in excess of one million events per second are achieved by using high-speed emitter coupled logic circuits, first-in first-out memory to derandomize data flow, and parallel architecture to increase throughput. These data rates allow the acquisition of adequate transmission data in a reasonable amount of time. Good timing resolution minimizes accidental coincidences and permits data rates greater than 100,000 image-forming events per second for high-speed dynamic emission tomography. Additional scatter and accidental rejection are accomplished for transmission data by using an orbiting source and a look-up table for valid events. Calibration of this complex electronic system is performed automatically under computer control.

Data Acquisition, Reconstruction and Display for the Donner 280-Crystal Positron Tomograph

The imaging system for the Donner 280-Crystal Positron Tomograph is described. The architecture of the hardwired data acquisition system (histogrammer) allows a data rate in excess of two million events/second while simultaneously correcting for accidental events. The histogrammer has eight memory cards to allow separate accumulation of time-slices of the cardiac cycle, or multiple-buffering for rapid sequential studies. Transfer time from the double-bussed histogrammer to a disk of the host computer system is less than one second. A hardwired device is able to reconstruct a 256×256 image from 140 angles in less than three seconds, but since calibration and correction for attenuation are presently performed in software, this time is increased to about 10 seconds. Images are displayed on a 256×256 raster scan display system with image manipulation capabilities.