Olof Johnson

First results with high-resolution PET detector modules using wavelength-shifting fibers

The authors have constructed and tested prototype PET detector modules incorporating wavelength-shifting fiber ribbons alternating with thin CsI(Na) scintillator plates in multilayer stacks. Modules with cross-sectional area of 11 cm/spl times/11 cm and with 9 mm of total crystal thickness have been tested. A reconstructed spatial resolution of 3 mm FWHM has been obtained for a line source, with an energy resolution of <20% and a time resolution of 25 ns FWHM. Energy measurements and coincidence timing are provided by 4 photomultipliers in an Anger array on one 11 cm/spl times/11 cm module face. Multiplexed fiber readout has been implemented with multianode photomultipliers coupled to several fiber ribbons. The methods and resolution achieved are readily extended to larger systems in a straightforward and cost-effective manner. Results of tests with several phantoms are presented.

Scintillator crystal readout with wavelength-shifting optical fibers

Wavelength-shifting fiber readout of scintillator crystals permits efficient discrete event localization over large scintillator areas, by flexibly interfacing position-sensitive photosensors or photosensor arrays to scintillator crystal surfaces. We describe techniques for localizing gamma-ray interactions within large crystals, and for determining the crystal of interaction within multicrystal arrays. Measurements carried out with NaI(Tl) coupled through fibers to photomultipliers confirm the techniques expected light yield, and measurements with BGO coupled through fibers to avalanche photodiodes demonstrate the usefulness of this method for crystal-of-interaction determination. We use a block detector geometry, combining fiber localization with direct energy measurement by large photomultipliers. Potential applications for gamma cameras and for high resolution PET systems are discussed.<<ETX>>

Characterization of a new multianode PMT for optical fiber readout at low-level light intensity

We have characterized the new Hamamatsu R5900-L16 multianode photomultiplier with regard to its suitability for charge division position-sensitive readout with just two analog channels for each 16-anode PMT. Because of the low capacitance of the anode structure of the PMT, we obtain an output pulse width of less than 20 ns in response to narrow LED input pulses. We were able to obtain position-sensitive readout of light pulses corresponding to a single photoelectron with a spatial width of less than 1 mm FWHM for a collimated light source and of about 2 mm FWHM for a single fiber. Combined with the good single photoelectron peak for the R5900-L16, this provides excellent performance at very low-level light intensity. We discuss application of this device to the precise determination of gamma-ray interaction position within thin crystal scintillators.

Characterization of a new multianode PMT for low-level optical fiber readout

We have characterized the new Hamamatsu R5900-L16 multianode photomultiplier with regard to its suitability for position-sensitive readout with just two analog channels for each 16-anode PMT. Because of the low capacitance of the anode structure of the PMT, we obtain an output pulse width of less than 20 ns for narrow input pulses. We were able to obtain position-sensitive readout of single photoelectron pulses with a spatial width of less than 1 mm FWHM for a collimated light source and about 2 mm FWHM for single fiber. Combined with the good single photoelectron peak for R5900-L16, this provides excellent performance at very low light levels. We discuss application of this device to the precise determination of gamma-ray interaction position within thin crystal scintillators.

Development and testing of a large-area underwater survey device

A large-area underwater radioactive survey device was developed for a contamination survey of the Johnston Atoll. A towed device consisting of a single photomultiplier and a single scintillator crystal is described. Results from the development and deployment of the device are presented.

On the use of the PhotoDetection high-sensitivity 90cm bore PET scanner wavelength shifting fiber detector design to achieve high spatial resolution

One of the unique features of the PhotoDetection high-sensitivity 90 cm bore PET scanner is the use of wavelength shifting fibers (WLSF) integrated into the detector module design. The optical design includes WLSF embedded in the detector module in conjunction with block detector optics. A specialized light shaping element is employed in the optical readout of the block to improve spatial linearity and light yield uniformity. This design allows the PET scanner to achieve high trans-axial spatial resolution with large blocks. This resolution has been measured to be 4.2 mm FWHM at the center of the field of view. The design and implementation of the WLSF will be discussed, along with the crystal finding algorithm. This algorithm combines the WLSF and Anger co-ordinates using a position dependent weighting function. The results of the resolution measurements will be presented using a resolution phantom designed for better characterization of the PET scanner resolution. In addition, results are shown for spatial resolution as a function of block singles rate, across a range spanning clinically relevant system singles rates.

Performance study of a prototype PET scanner using CsI(Na) scintillator with wavelength-shifting fiber readout

We have designed and built a high resolution dual-head PET scanner using wavelength-shifting optical fibers coupled with thin plates of CsI(Na). Each detector head has the sensitive area of 114×114 mm2 and is being read out by 8 Hamamatsu multi-anode PMTs R5900-L16 using charge division method. Additional 4 single-anode 60 mm square PMTs per head provide trigger and total energy measurement. Being initially developed for Positron Emission Mammography the design of the system is also of general interest for gamma-imaging. The principles of operation are discussed and results of some measurements are presented.