Norimasa Nohara

Analytical study of the performance of a multilayer positron computed tomography scanner.

The image-forming performance of multilayer positron tomographs for extended sources is evaluated analytically. The analysis is simplified by “rotation transform,” by which three-dimensional photon detection problems are solved by two-dimensional treatment. Event rates of singles, unscattered true coincidence, and single- and double-scattered coincidence are formulated for a uniform cylinder phantom as functions of various design parameters. Angle factors for Compton scattering and other parameters used in the evaluation are presented. Scatter components in projections and their effect on the reconstructed images are also evaluated. The scatter component in the reconstructed image depends critically on the detector ring radius, phantom radius, method of attenuation correction, etc. When the detector ring radius is relatively small (40 ± 45 cm in diameter), the scatter/true ratio at the image center of a 20 cm diameter phantom may be larger than the scatter/true ratio in the event rates. Comparison with experimental data obtained with a head positron tomograph, positologica, showed reasonable agreement both in the total coincidence rates and in the scatter components in the images for a cylindrical phantom of 20 cm in diameter.

A quad BGO detector and its timing and positioning discrimination for positron computed tomography

Abstract Quad BGO detectors and the timing and positioning discriminators have been developed for high sensitivity multilayer positron computed tomographs. Each detector consists for rectangular BGO crystals and two cylindrical photomultiplier tubes. The design allows good optical coupling between the crystals and the photomultiplier tubes, which is essential in order to obtain good time resolution with reasonable spatial resolution of the system. The discriminator consists of a time pick-off circuit based on the first photoelectron detection method and a positioning circuit. The positioning circuit identifies the crystal absorbing an annihilation photon. The design criteria of the electronic system and the performance are described. With a suitable optical configuration of the detector, the erroneous positioning due to statistical noise is negligibly small and the coincidence time resolution for annihilation photon pairs is about 3.6 ns fwhm. The unit works satisfactorily at a count rate up to at least 360 kcps.

Positologica: A Positron ECT Device with a Continuously Rotating Detector Ring

A rotary positron emission computed tomography device is developed for human brain and animal studies. The device utilizes 64 rectangular BGO detectors arranged at unequal spacing on a circular ring. The detector ring is continuously rotated at a constant speed of 60 rpm or less. This single continuous motion of the ring with the detector array provides excellent sampling characteristics and high detector redundancy. The device has a field of view 24 cm in diameter with a slice thickness adjustable from 1 cm to 2 cm. Measured width of system response to a 2 mm diameter line source is 5.8 mm FWHM at the center and less than 9 mm FWHM within a circle 16 cm in diameter. Measured sensitivity including scattered coincidence events is about 17 kcps/pCi/ml for a 20 cm diameter water phantom and 2 cm thick slice.

Development of a High Resolution Positron CT for Animal Studies

A single ring high resolution positron CT for animal studies was developed which utilizes 64 gridded rectangular photomultipliers (l3mm square) and 128 rectangular Bismuth Germanate crystal (4mm wide, 10mm high and 20mm long). A photomultiplier is incorporated with a grid near the photocathode which controls photoelectron transport of half part of a tube. This enalbes us the identification of the fired crystal out of two coupled to a photomultiplier. Detector ring, animal port and field of view diameters are 265, 135 and 128mm, respectively. Detector arrangement was determined by POSITOLOGY. The spatial resolutions in fwhm after unfolding source spread (2mm dia.) were 2.2mm at center and 3.3mm (tangential) and 3.6mm(radial) at 60mm from the center. Hardware out-of-FOV event rejector was implemented which rejects events outside the field of view and reduces event rate by a factor of 3 in the case of calibration measurements.

Variable sampling-time technique for improving count rate performance of scintillation detectors

A new technique is presented to improve the count rate capability of a scintillation spectrometer or a position sensitive detector with minimum loss of resolution. The technique is based on the combination of pulse shortening and selective integration in which the integration period is not fixed but shortened by the arrival of the following pulse. Theoretical analysis of the degradation of the statistical component of resolution is made for the proposed system with delay line pulse shortening, and the factor of resolution loss is formulated as a function of the input pulse rate. A new method is also presented for determining the statistical component of resolution separately from the non-statistical system resolution. Preliminary experiments with a NaI(Tl) detector have been carried out, the results of which are consistent with the theoretical prediction. However, due to the non-exponential scintillation decay of the NaI(Tl) crystal, a simple delay line clipping is not satisfactory, and an RC high-pass filter has been added, which results in further degradation of the statistical resolution.

Twin BGO detectors for high resolution positron emission tomography

Abstract Twin BGO detectors and timing pulse shape discriminators for crystal identification have been developed for high spatial resolution positron emission tomography. Each detector consists of two BGO crystals and one single photomultiplier tube. The tube has a grid covering half the area of the photocathode window, and opposite to which stands one of the BGO crystals. The amplitude of the anode signals for the scintillation events in the grid side crystal is modulated by supplying negative pulses to the grid. Crystal identification is achieved by pulse shape discrimination for the detector signals. The design criteria of the electronic system and the performance are described. With suitable pulse shape discrimination, the erroneous positioning due to statistical noise is negligible and the coincidence time resolution for annihilation photon pairs is about 5.8 ns fwhm.

Utilization of Non-Negativity Constraints in Reconstruction of Emission Tomograms

Emission computed tomography (ECT) has gained recognition in the past decade as a valuable tool in nuclear medicine imaging. The ECT falls into one of two categories: positron ECT and single-photon ECT(SPECT)[1]. The ECT has the advantage of higher object contrast than planar imaging, but at the same time has often suffered from a lack of sufficient count densities to achieve statistically smooth images. The poor counting statistics is due to limited isotope dosage to patients, limited counting time and limited count rate capability of the imaging devices. Fast dynamic studies require reduction of total number of counts to be accumulated per image, and high resolution imaging further reduces count density per resolution cell. The magnitude of statistical noise of an image depends on various factors such as the radionuclide distribution, the total number of counts, the number of resolution cells in the object area, etc., but it also depends on the reconstruction algorithm, because the statistical noise is amplified in the stage of image reconstruction.

Time-of-flight positron imaging and the resolution improvement by an iterative method

A simple time-of-flight (TOF) positron imaging method is proposed in which the spatial resolution obtained with TOF information is improved by a novel iterative algorithm. The prototype has a pair of opposed detectors using BaF/sub 2/ (2*2*2 cm/sup 3/) and photomultiplier tubes (18.5-mm diameter). The measured TOF resolution full width at half maximum was 330 ps, which corresponds to 5.0 cm of spatial resolution for source localization between the detectors. Eight iterations with the algorithm improve the spatial resolution by a factor of about two for a point source. The algorithm is based on modifications of a Bayesian deconvolution algorithm. For more complex sources, it takes more iterations (e.g. 80); however the algorithm is about five times faster than the original Bayesian algorithm. >

Analytical Study of Performance of High Resolution Positron Emission Computed Tomographs for Animal Study

Performance of positron emission computed tomographs having spatial resolution as high as 3 mm FWHM was analytically evaluated. Increases in widths of line spread functions due to positron range and angular deviation were estimated. The slice thickness should be thin in accordance with the resolution in transverse axial plane. Singles rate, true coincidence rate, and scattered coincidence rate with high resolution tomographs of small diameter detector rings were estimated for three types of thin slice collimators. Feasibility of high resolution positron tomographs with small detector rings was discussed with applications to animal study.

Performance Study of Whole-Body, Multislice Positron Computed Tomograph POSITOLOGICA-II

A 3-detector ring, 5-slice whole-body positron CT has been developed and is being tested. The PCT, POSITOLOGICA-II, has a total of 480 BGOs (160/ring), and employs continuous rotation scan (0.5rps). By using a 15mm wide BGO, a 9.2mm FWHM spatial resolution for reconstructed image is obtained at the center of FOV. Measured phantom diameter dependence of the true count rate shows good agreement with theoretically anticipated characteristics, including maximum sensitivity at around 30cm dia. Sensitivities for 20cm dia. phantom are 28 and 38 kcps/¿Ci/ml for in-plane and cross-plane, respectively, including scattered coincidences. Relative system detection efficiency measured with a line source at FOV center is 96% at 15ns time window (90% at 12ns), basing on 100% efficiency in 20-24ns.