True coincidence count (TCC) and noise equivalent count (NEC) curves were measured with a standardized 20-cm-diameter nylon cylinder for five different CTI/Siemens PET (positron emission tomography) scanners with several scanner-collimator combinations: (1) 831/08-12 with 1-mm collimator septa; (2) 933/08-12 and 933/08-16 with 3 to 1-mm tapered collimator septa; and (3) 931/08-12 with 3 to 1-mm tapered and a 1-mm collimator septa and the 931/08-16 with 3 to 1-mm tapered collimator septa In addition, TCC and NEC curves on the 933/08-12 were compared with those from an Alderson brain phantom In general, it is found that the TCC curves indicated peak count rates and activity levels that were as much as 50% higher than the corresponding values from NEC curves The primary factor causing this difference is the noise effect of the randoms component It is demonstrated that, compared to the Alderson brain phantom, the standard 20-cm cylinder is a poor predictor of count rate performance for PET brain imaging >

Measuring PET scanner sensitivity: relating countrates to image signal-to-noise ratios using noise equivalents counts

Coded aperture imaging in high energy astronomy represents an important technical advance in instrumentation over the full energy range from X- to γ-rays and is playing a unique role in those spectral ranges where other techniques become ineffective or impracticable due to limitations connected to the physics of interactions of photons with matter. The theory underlying this method of indirect imaging is of strong relevance both in design optimization of new instruments and in the data analysis process. The coded aperture imaging method is herein reviewed with emphasis on topics of mainly practical interest along with a description of already developed and forthcoming implementations.

Coded aperture imaging in X- and gamma-ray astronomy

Cerium‐activated phosphors are characterized by their fast luminescence decay. Gadolinium orthosilicate (Gd2SiO5) is a material possessing a high atomic number, and can also play host to the cerium activator. Cerium‐doped Gd2SiO5 single crystals were grown by the Czochralski technique, and their luminescence properties were examined. The light output was 1.3 times larger than that of the best Bi4Ge3O12, and the decay constant was 60 ns at room temperature.

Cerium‐activated Gd2SiO5 single crystal scintillator

Non-invasive method of forming prostheses of skeletal structures internal to a body for use in reconstructive surgery. The selected internal skeletal structure is measured by subjecting the body to radiant energy to produce radiant energy responses that are detected to obtain representations delineating the skeletal structure. Three dimensional coordinate data defining the skeletal structure is generated from the obtained representations. The coordinate data is employed to control a sculpting tool to form the prosthesis.

Method of forming implantable prostheses for reconstructive surgery

The trend in the design of scanners for positron emission computed tomography has traditionally been to improve the transverse spatial resolution to several millimeters while maintaining relatively coarse axial resolution (1-2 cm). Several scanners are being built with fine sampling in the axial as well as transverse directions, leading to the possibility of the true volume imaging. The number of possible coincidence pairs in these scanners is quite large. The usual methods of image reconstruction cannot handle these data without making approximations. It is computationally most efficient to reduce the size of this large, sparsely populated array by back-projecting the coincidence data prior to reconstruction. While analytic reconstruction techniques exist for back-projected data, an iterative algorithm may be necessary for those cases where the point spread function is spatially variant. A modification of the maximum likelihood algorithm is proposed to reconstruct these back-projected data. The method, the iterative image space reconstruction algorithm (ISRA), is able to reconstruct data from a scanner with a spatially variant point spread function in less time than other proposed algorithms. Results are presented for single-slice data, simulated and actual, from the PENN-PET scanner.

An Iterative Image Space Reconstruction Algorthm Suitable for Volume ECT

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 BGO's (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.

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

A scintillator for a gamma ray camera constituted by a crystal of NaI (Tl) having a surface roughness of less than 400 grit and a thickness of less than approximately 9 mm.

Scintillator for a gamma ray camera

Evaluation of slice shield collimators is carried out using theoretical event rate formulas. This approach permits calculation of system performance in terms of sensitivity, scatter fraction, and high-counting rate characteristics. Electronics counting efficiency is also taken into account. A rational approach to optimal design is presented along with design examples. Effectiveness of collimator replacement is suggested. A system using sub-collimator with relatively small detector ring radius is proposed. It achieves a level of performance similar to that of a conventional collimator system with large ring radius.

Evaluation of Slice Shield Collimators for Multi-Layer Positron Emission Computed Tomographs

A scintillator for a gamma ray camera includes a pair of crystals mechanically pressed together The surfaces of the crystals are roughened so as to form a photo-diffuser for the light generated as a result of scintillation

Scintillator for gamma ray camera

An advanced coded imaging system is described, and some results of phantom experiments are presented. The advanced method uses a pair of coherent codes (+1 and −1 codes) and has many advantages compared with conventional ones. One of the greatest advantages is that there are no sidelobes in the focal plane and only a few in other planes. Therefore, when an object can be regarded as two-dimensional, it is perfectly reconstructed with high detecting efficiency, and this is successfully simulated by a thyroid phantom with 99mTc. Moreover, this system has an ability to reconstruct tomograms, which is also shown by using ring phantoms piled on one another with some cold spots in their shells. From these experimental results it may be concluded that the new system is useful for practical applications, for example, to nuclear medicine.

Kenji Ishimatsu

Papers

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

Scintillator for a gamma ray camera

Evaluation of Slice Shield Collimators for Multi-Layer Positron Emission Computed Tomographs

Scintillator for gamma ray camera

Advanced coded-aperture imaging system for nuclear medicine.