Experimental Assessment of the Gain Achieved by the Utilization of Time-of-Flight Information in a Positron Emission Tomograph (Super PETT I)

The gain achieved in image quality by utilizing, in the image forming process, the time-of-flight information (TOF) of positron annihilation photons between their inception and detection was measured experimentally by means of a positron emission tomograph (PET)-Super PETT I. The measurements were carried out by imaging a 35 cm cylindrical uniform phantom containing different positron activity concentrations. The gain achieved through the incorporation of TOF information, defined as the ratio of variances in images reconstructed with and without TOF information, was found to be approximately 3 at the lowest activity concentration and 5-8 in the activity concentration range typically encountered in clinical studies especially in fast or dynamic studies. This increase in gain with activity was interpreted as resulting from the reduction of random coincidences when TOF information is used. Further image improvement is yielded by incorporating TOF information into the PET attenuation correction provided by the measurement of transmission of annihilation photons in the object imaged.     

Molten poly(methyl methacrylate) under electron irradiation: Studies of depropagation and chain scission

The degradation rates of free radical polymerized and syn-diotactic poly(methyl methacrylate) (PMMA) under electron radiation were explored at temperatures ranging from 225 to 325°C and at electron dose rates from 10,000 rads/min to 100,000 rads/min. Scission of bonds between main-chain atoms resulted in pairs of radical chain ends, of which some disportionated, thereby reducing polymer molecular weight without loss of volatile, while others proceeded to depropagate via the usual radical transfer, causing complete volatilization of the chain without significant change of molecular weight of the residual polymer. Electron irradiation at a dose rate of 100,000 rads/min produced a significant enhancement in the reaction, rate, and reduced the overall activation energy from 140 KJ/g-mole observed for thermally induced depropagation to a value of 75 KJ/g-mole observed during simultaneous irradiation and/thermal depropagation.     

Performance of a Borrelia burgdorferi bacterin in borreliosis-endemic areas

The safety and efficacy of a commercially available Borrelia burgdorferi bacterin were examined under field conditions in 1,969 dogs that received a total of 4,033 doses of bacterin during a 20-month period. For comparison, 4,498 dogs that were not vaccinated also were evaluated. All dogs were examined at 1 of 3 veterinary practices in borreliosis-endemic areas. Owners of all dogs were educated about the benefits of and methods for avoiding infected ticks. Vaccination was found to be safe regardless of previous exposure to B burgdorferi or history of borreliosis. Thirty-eight (1.9%) of the 1,969 dogs had minor reactions; 1 of these dogs had a reaction after the first and second doses of B burgdorferi bacterin. Reactions resolved without complications immediately or within 72 hours after vaccination. Cumulative incidence of borreliosis was 1.0% (20/1,969) in vaccinated dogs and 4.7% (211/4,498) in nonvaccinated dogs. Preventable fraction, a measure of the reduction of incidence of disease in vaccinated dogs vs nonvaccinated dogs, was 78%. Preventable fraction in dogs that were seropositive prior to vaccination (ie, previously exposed to B burgdorferi) was 58% and in dogs that were seronegative prior to vaccination (ie, without evidence of prior exposure to B burgdorferi) was 86%. This suggested that, in borreliosis-endemic areas, dogs may be most effectively protected by vaccinating them when they are young and before they are exposed to infected ticks, and that in areas where borreliosis is not endemic, it may be useful to initiate vaccination programs so that dogs will be protected prior to exposure to infected ticks.     

System Design of Fast Pet Scanners Utilizing Time-of-Flight

Recent advances in PET designs have shown that a gain in signal-to-noise ratio can be expected by incorporating time-of-flight data in positron emission tomography over the conventional PET mode. It has also been shown that cesium fluoride (CsF) offers the potential of faster timing and high detection efficiency which would be required for a clinical scanner utilizing time-of-flight information. Our research with CsF and the results of a feasibility study of time-off-light positron emission tomography reconstruction have shown that, indeed, a significant improvement in image quality results from such an approach and that coincidence resolving times of less than 500 psec FWHM are easily achievable with CsF detectors. However, the design of fast tomographic systems with multiple detectors which maintain this fast coincidence timing poses a challenging technical problem. The solution to this problem requires a departure from the conventional mode of PET designs to a fast on-line microprocessor based system which is capable of compressing and correcting the data for timing differences, normalization and image function. Such a system is described in this paper and its advantages and disadvantages are discussed.     

The Effect of Accidental Coincidences in Time-of-Flight Positron Emission Tomography

An accidental coincidence is defined as the erroneous registration of two photons, originating from separate positron annihilations, as having originated from the same positron annihilation. Previous analyses which did not consider accidental coincidences indicated that for a certain radioactivity distribution a gain in image signal-to-noise ratio of about 5 dB is achieved by the time-of-flight method over the conventional method. Subsequent experiments have validated this prediction in low counting rate situations. For higher, typical counting rates these experiments showed a significantly larger gain of about 9 dB, which was attributed to the way in which the time-of-flight method suppresses the degrading effects of accidental coincidences. We present an analytical model, extended from a previous model, which considers accidental coincidences. Calculations of signal-to-noise ratio, using this model, compare well with the experiments and show that the additional gain is indeed due to the treatment of accidental coincidences. An understanding of the model leads to an intuitive explanation of the gain mechanism and a determination of an effective coincidence-timing window that is achieved by the time-of-flight method.     

The bright future of organic LEDs

Electro-luminescence is light emission from a solid through which an electric current is passed. Electro-luminescence from organic compounds was discovered in the early '60s, but the subject did not receive much attention until the discovery and development of conductive polymers. Compared to other electro-luminescent technologies (such as inorganic compound semiconductors, porous silicon and liquid crystals), polymer/organic light-emitting diodes (PLEDs/OLEDs) are very attractive. The reasons are their very low operating voltage, high brightness and their tunability to produce the three fundamental colors (red, blue and green). Furthermore, they are lightweight and can be grown on flexible substrates. They are fairly easy and inexpensive to fabricate. Today, PLEDs/OLEDs are suitable for applications such as automotive displays. In the future, they will probably conquer a large portion of the micro display market. Their higher efficiency and lower weight will make them quite competitive with LCD displays, the currently favorite technology. The article concludes that the future of organic light-emitting diodes seems very bright indeed.     

Super PETT I: A Positron Emission Tomograph Utilizing Photon Time-of-Flight Information

The physical characteristics and some imaging capabilities of Super PETT I, a positron emission tomograph utilizing time-of-flight (TOF) in its image reconstruction process were assessed experimentally by means of measurements carried out in phantoms and clinical imaging studies. The performance characteristics assessed included sensitivity, spatial resolution, image improvements resulting from time-of-flight information utilization, system dead time, and linearity. The clinical examples included imaging of the brain, the heart, the liver, and a demonstration of Super PETT I's capability of achieving cardiac gating.