Ken Meadors

Simultaneous PET and MR imaging

We have developed a prototype PET detector which is compatible with a clinical MRI system to provide simultaneous PET and MR imaging. This single-slice PET system consists of 48 2 x 2 x 10 mm3 LSO crystals in a 38 mm diameter ring configuration that can be placed inside the receiver coil of the MRI system, coupled to three multi-channel photomultipliers housed outside the main magnetic field via 4 m long and 2 mm diameter optical fibres. The PET system exhibits 2 mm spatial resolution, 41% energy resolution at 511 keV and 20 ns timing resolution. Simultaneous PET and MR phantom images were successfully acquired.

MicroPET II: design, development and initial performance of an improved microPET scanner for small-animal imaging

MicroPET II is a second-generation animal PET scanner designed for high-resolution imaging of small laboratory rodents. The system consists of 90 scintillation detector modules arranged in three contiguous axial rings with a ring diameter of 16.0 cm and an axial length of 4.9 cm. Each detector module consists of a 14 x 14 array of lutetium oxyorthosilicate (LSO) crystals coupled to a multi-channel photomultiplier tube (MC-PMT) through a coherent optical fibre bundle. Each LSO crystal element measures 0.975 mm x 0.975 mm in cross section by 12.5 mm in length. A barium sulphate reflector material was used between LSO elements leading to a detector pitch of 1.15 mm in both axial and transverse directions. Fused optical fibre bundles were made from 90 microm diameter glass fibres with a numerical aperture of 0.56. Interstitial extramural absorber was added between the fibres to reduce optical cross talk. A charge-division readout circuit was implemented on printed circuit boards to decode the 196 crystals in each array from the outputs of the 64 anode signals of the MC-PMT. Electronics from Concorde Microsystems Inc. (Knoxville, TN) were used for signal amplification, digitization, event qualification, coincidence processing and data capture. Coincidence data were passed to a host PC that recorded events in list mode. Following acquisition, data were sorted into sinograms and reconstructed using Fourier rebinning and filtered hackprojection algorithms. Basic evaluation of the system has been completed. The absolute sensitivity of the microPET II scanner was 2.26% at the centre of the field of view (CFOV) for an energy window of 250-750 keV and a timing window of 10 ns. The intrinsic spatial resolution of the detectors in the system averaged 1.21 mm full width at half maximum (FWHM) when measured with a 22Na point source 0.5 mm in diameter. Reconstructed image resolution ranged from 0.83 mm FWHM at the CFOV to 1.47 mm FWHM in the radial direction, 1.17 mm FWHM in the tangential direction and 1.42 mm FWHM in the axial direction at 1 cm offset from the CFOV. These values represent highly significant improvements over our earlier microPET scanner (approximately fourfold sensitivity increase and 25-35% improvement in linear spatial resolution under equivalent operating conditions) and are expected to be further improved when the system is fully optimized.

Development of a PET detector system compatible with MRI/NMR systems

We report the development of a prototype positron emission tomography (PET) scanner compatible with clinical magnetic resonance imaging (MRI) scanners and nuclear magnetic resonance (NMR) spectrometers. This single slice PET system consists of 72 2/spl times/2/spl times/5 mm lutetium oxyorthosilicate (LSO) crystals coupled by 2 mm diameter, 4 meter long double clad optical fibers to three multi-channel photomultiplier tubes (MC-PMTs) shielded inside an aluminum closure. The ring diameter is 54 mm and the slice thickness is /spl sim/1 mm FWHM. Measurements with a point source demonstrate that this PET system has a reconstructed resolution of 2.1 mm, a coincidence time resolution of 26 ns and a typical energy resolution of 45%. Simultaneously acquired PET and MR phantom images, show no significant artifacts or distortions. We also obtained simultaneous NMR spectra and PET images from an isolated, perfused rat heart, demonstrating the power of obtaining temporally correlated PET and NMR information in biological systems. Again, no artifacts in the PET or NMR data were apparent, despite the high field strength of 9.4 T. The challenge for the future is to scale up the design to develop a high resolution, high sensitivity device that can be used in simultaneous PET and MR studies of in vivo systems.

Optical fiber readout of scintillator arrays using a multi-channel PMT: a high resolution PET detector for animal imaging

The authors report the results from a new high resolution gamma ray imaging detector designed for use in a positron emission tomography (PET) system dedicated to small animal imaging. The detectors consist of an 8/spl times/8 array of 2/spl times/2/spl times/10 mm bismuth germanate (BGO) crystals coupled by 2 mm diameter double clad optical fibers to a 64 pixel multi-channel photomultiplier tube (MC-PMT). A charge division readout board is used to convert the 64 output channels into four position sensitive signals which determine the crystal of interaction. Measurements with a pair of these detectors demonstrate an intrinsic spatial resolution of 1.4 mm, a coincidence timing resolution of 15 ns and an energy resolution ranging between 35 and 60%. Based on these encouraging results, the design for a dedicated animal PET tomograph is proposed and simulations of this system project a reconstructed resolution of less than 2 mm within a 5 cm diameter transaxial field of view.

Techniques to improve the spatial sampling of MicroPET-a high resolution animal PET tomograph

A method is described to improve the spatial sampling of microPET, a high resolution PET scanner designed for imaging small laboratory animals. The high intrinsic resolution of the microPET detector (1.58 mm FWHM), in combination with the stationary ring geometry of the tomograph, generate an imaging system which is inherently spatially undersampled. As a result the imaging resolution measured with a three-dimensional (3-D) filtered backprojection (FBP) algorithm for a point source at the center of the field of view (CFOV) is only 1.8 mm FWHM and has large fluctuations at positions near the CFOV. A small wobble motion was introduced via a circular motion of the scanner bed in the transverse plane, with a wobble radius of 300 /spl mu/m. The data was acquired with a step-and-shoot method by dividing the wobble circle into a number of equidistantly sampled intervals. The separate sinograms were interpolated to a finely resampled sinogram, which was reconstructed with the 3-D filtered backprojection algorithm. The resulting images demonstrated full recovery of the intrinsic detector resolution and elimination of the local nonuniformities of the point spread function (PSF) at the CFOV, with three wobble samples. The resulting average resolution improvement fur the central 5 cm of the FOV was approximately 13% in the radial and 19% in the tangential direction, with an associated 50% penalty in the reconstructed image noise.

MicroPET II: an ultra-high resolution small animal PET system

MicroPET II is a second-generation microPET scanner dedicated to high resolution PET imaging of small animals. The system consists of 90 scintillation detector modules arranged in a 3-ring configuration with a radius of 16.0 cm and ail axial extent of 4.9 cm. Each detector module consists of a 14/spl times/14 array of lutetium oxyorthosilicate crystals coupled to a multi-channel photomultiplier tube (Hamamatsu H7546) through a coherent optical fiber bundle. Printed circuit boards with a charge-division readout scheme were used to decode the 196 crystals in each array from 64 anode signals. Electronics from Concorde Microsystems. Inc. was used for signal amplification, digitization, and coincidence processing. Preliminary data showed a system with peak sensitivity of 2.26%. Energy resolution ranges from 28% to 75% with a mean of 42%. Image resolution ranges from 1.07 mm FWHM at the center of field of view (CFOV) to 1.40 mm FWHM in the radial direction and 1.14 mm FWHM in the tangential direction at 1 cm offset from CFOV. Further improvements in image and energy resolution are expected when the system geometry is fully modeled and the crystal lookup tables are improved.

Brain Imaging in Small Animals Using MicroPET

MicroPET is a prototype high-resolution positron emission tomography (PET) scanner designed for imaging small laboratory animals. It is the first PET system to make use of the new scintillator lutetium oxyorthosilicate and also the first system to employ fiber-optic coupling as a means for reading out very small scintillator elements with minimal deadspace. MicroPET consists of a total of thirty, 8 × 8 element detectors in a ring of diameter 17.1 cm. The animal port is 16 cm in diameter, and the useful field of view measures 11 cm transaxially by 1.8 cm axially. There are no interplane septa and data are acquired exclusively in three-dimensional mode. The system also includes a computer-controlled animal bed with built-in wobble motion. MicroPET produces images with a spatial resolution of 2 mm in all three axes and has sufficient sensitivity to realize this resolution for many applications. For studying biological systems that are easy to saturate, improved sensitivity could be realized by the addition of a second or third detector ring, which would also serve to increase the axial coverage of the system. The use of microPET to image the brain of several species of animals is presented. Based on the success of this first prototype system, a second, higher resolution, higher sensitivity system is now being developed exclusively for use in mice and rats.

Evaluation of optical fiber bundles for coupling a small LSO crystal array to a multi-channel PMT

One of the limiting factors in high resolution PET detectors is the readout of scintillation light from arrays of small crystals and the subsequent accurate identification of each array element. Furthermore, if position-sensitive or multichannel photomultiplier tubes (PMTs) are employed for readout, some form of light guide between the scintillator and PMT is required to maintain high packing fraction. The solution to this problem in microPET, a small animal PET scanner, was 1 to 1 coupling of the scintillation light from the crystal array to each channel of a multi-channel PMT using individual optical fibers. In developing a high resolution detector module for the next generation of microPET tomographs, the authors evaluated 8 types of optical fiber bundles with a 12/spl times/12 array of 1/spl times/1/spl times/10 mm LSO crystals. The authors examined the identification of array elements, energy resolution and light transmission. They tested 4 plastic and 4 glass fiber bundles, with various lengths and magnification factors, coupled to a Hamamatsu R5900-M64 multi-channel PMT. Although each fiber bundle resolved all the elements of the array, the best performance in terms of crystal separation (peak-to-valley ratio of 4:1), light transmission (85%) and energy resolution (/spl sim/20%) was not achieved by any single sample. This is due to the trade-off between cross-talk and light transmission in the fiber bundle. The ultimate choice of fiber bundle therefore depends on the relative importance of light transmission (energy and timing resolution) and optical cross-talk (spatial resolution).