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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 first positron computed tomography system with cesium fluoride scintillation detectors, PETT VI, has been developed. The system provides 7 slice images with 4 detector rings (57 cm). Performances of the system are discussed and clarified based on experimental data. Adoption of CsF detectors decrease the random coincidence rate by achieving a short coincidence timing resolution. The timing resolution of a pair of detector heads is 1.5 nsec FWHM and 3.0 nsec FWTM. After a preliminary timing alignment of 288 detectors, the coincidence window width (2?) of 11 nsec or wiider has yielded maximum coincidence sensitivity, and operation at 5.9 nsec has given 91% of maximum. The full sensitivity for a cyclindrical uniform phantom (20 cm dia × 13 cm) is 354 kcps/microCi/cc/7 slices in a low resolution mode (intrinsic resolution at center is 11.7 mm FWHM). In a high resolution mode (7.1 mm), it is 31% of the full sensitivity. Coincidence rates ratio, [random/(true + scattered)], in the low resolution mode, is 0.16 ? with 5.9 nsec window width inside the phantom images reconstructed without a random correction, where ? is activity density (microCi/cc). The ratio is 1/3 of that obtained when operating at 20 nsec. Scattered coincidence fraction at the center of the phantom images, without the random correction process, is 9% of the [true + scattered].

As part of the Positron-Emission Tomography (PET) [1] reconstruction process, annihilation counts are normalized for photon absorption, detector efficiency and detector-pair duty-cycle. Several normalization methods of time-of-flight and conventional systems are analyzed mathematically for count bias and variance. The results of the study have some implications on hardware and software complexity and on image noise and distortion.

Various high-resolution scintillation detectors are analyzed for resolution and sensitivity. In particular, the effects of using a wedge face and inter-crystal tungsten-septa are studied for both BaF2 and BGO crystals. The results indicate that usage of septa alone causes an improvement in resolution in areas away from the center of the image, but also causes a degradation in sensitivity in those areas, and that usage of a wedge face alone causes no improvement in resolution, while usage of septa and a wedge face in combination causes a similar resolution improvement, but with a lesser degradation in sensitivity.

Incorporation of differential time-of-flight (TOF) information in positron emission tomography offers significant performance improvement at the cost of increasing the quantity of data to be processed. A tomographic unit utilizing TOF data, called Super-PETT, is currently nearing the final stage of construction. Design and implementation aspects of the data acquisition system are described with an emphasis on instrument operation in an on-line preprocessing mode.

A new image-reconstruction algorithm for time-of-flight (TOF) assisted positron-emission tomography is presented. This linear algorithm differs from previously published linear algorithms in the way the angular information of the measurements is used. The needed computations are reduced by coalescing data having angles within a range into a smaller number of "group" angles. This angle-reduction is made possible by the additional TOF measurement. It is shown that in the limit as the numer of groups is very large this reduced-angle weighting algorithm is the same as the confidence-weighting algorithm [1]. Similarly, if the data are grouped into a single angular range, the new algorithm is the same as the most-likely-position algorithm [1].

One recent development in image reconstruction utilizing time-of-flight information yields a compression of the three dimensional array used in forming the unfiltered "preimage." A reduction from 96 angles to 16 angles is shown to produce equivalent quality images and may form a basis for future architectural strategies. A second development in image reconstruction can strongly affect the quality of results when constraints such as cardiac gating are imposed, while not inherently sacrificing spatial resolution due to inadequate sampling. A correction for sampling density obtained by analyzing list mode data is shown to greatly reduce circular artifact production. This technique is extendable to systems without time-of-flight measurements and to scanning motions other than wobbling.

The ideal scintillation detector for positron tomography would be a very high Z material, nonhygroscopic, have a fast decay time, and copious light output. NaI(Tl) has good light output, and even though it is hygroscopic has found acceptance as a good scintillator. Recently BGO has been shown to satisfy the first two requirements for tomography, and has replaced NaI(Tl) as the detector of choice in tomography. However, its low light yield and poor coincidence timing make it unsuitable for fast scanners. Cesium fluoride (CsF) has been investigated by us as a possible scintillation detector for positron tomography, and shows great promise even though it is extremely hygroscopic and exhibits low scintillation efficiency as compared to NaI(Tl). Its very fast decay time, good detection efficiency, and fast coincidence timing make it an ideal detector for tomographs designed for fast dynamic studies. Moreover, it permits the incorporation of time of flight information with conventional tomography for improved signal to noise ratio in the image.