C.W. Stearns

Cylindrical PET detector design

A cylindrically shaped high-resolution PET (positron-emission-tomography) detector that uses cross-plane coincidence events is being developed. A 2-D analog coded position-sensitive detector is used. It consists of a hexagonal array of photomultiplier tubes and a rectangular array of crystals, eight elements per tube. The optics has been designed to maximize the light collection and to provide uniform spatial resolution. The detector will be 60 cm in diameter by 11.5 cm wide, the crystals are 3 mm*5.7 mm*30 mm. The design, associated electronics, and results of measurements on a sector of the detector are presented. >

Accelerated image reconstruction for a cylindrical positron tomograph using Fourier domain methods

The authors present a method to reconstruct projection images through assembly in the Fourier domain, rather than by backprojection. By bypassing the rate-limiting backprojection step of filtered backprojection, the proposed algorithm operates more quickly than previous methods. Fourier domain operations are also used to form forward-projected views of the imaged object; portions of these views are required to operate a full three-dimensional reconstruction using the cross-plane projection planes. Inaccuracies in images recovery and artifacts in the image are reduced by oversampling in the Fourier domain; this is easily accomplished by zero-padding the projection data prior to entering the Fourier domain. These Fourier domain algorithms form the basis for the reconstruction algorithm in PCR-II, a volumetric positron imager under construction at Massachusetts General Hospital. >

Three Dimensional Image Reconstruction in the Fourier Domain

Filtered backprojection reconstruction algorithms are based upon the relationship between the Fourier transform of the imaged object and the Fourier transforms of its projections. A new reconstruction algorithm has been developed which performs the image assembly operation in Fourier space, rather than in image space by backprojection. This represents a significant decrease in the number of operations required to assemble the image. The new Fourier domain algorithm has resolution comparable to the filtered backprojection algorithm, and, after correction by a pointwise multiplication, demonstrates proper recovery throughout image space. Although originally intended for three-dimensional imaging applications, the Fourier domain algorithm can also be developed for two-dimensional imaging applications such as planar positron imaging systems.

NECR analysis of 3D brain PET scanner designs

A dedicated 3D brain PET scanner has several advantages, most notably increased sensitivity, over a whole body scanner for neurological studies. However, brain scanners have higher scatter fractions, random count-rates and deadtime for the same activity concentration. We have used noise effective count-rate (NECR) analysis to compare brain scanners of 53, 60, and 66 cm diameter with the GE ADVANCE whole body scanner (93 cm diameter). Monte Carlo simulations of a brain-sized phantom (16 cm diameter, 13 cm length) in the ADVANCE geometry were used to develop a model for NECR performance, which was reconciled to results from a decay series measurement. The model was then used to predict the performance of the brain scanner designs. The brain scanners have noise effective sensitivities (the slope of the NECR curve at zero activity) as much as 40% higher than the body scanner. However, their NECR advantage diminishes quickly as the activity concentration increases. The brain scanners NECR equals the body scanner with about 0.7-0.8 mCi in the phantom; the body scanner has superior NECR performance at higher activity levels. An imaging center concentrating on only very low activity imaging tasks would find the efficiency advantage of a smaller detector diameter valuable, while a center performing higher activity studies such as bolus water injections or 5 mCi FDG injections might prefer the count rate performance of a whole body scanner. >

Simulation studies for cylindrical positron tomography

A VMEbus-based microcomputer system has been used to implement a model for simulation of the flux of gamma rays to cylindrical PET (positron-emission-tomography) detector systems. The model is capable of tracing over one million photons per hour, and has been used to explore some of the effects of opening up planar detector geometries into volumetric imagers. Gross single-channel and coincidence rates can be estimated using the model, as well as the distribution patterns of both true and scattered coincidence events in the radial and axial directions. The model demonstrates the increases in scattered coincidences that are encountered in true (i.e. no collimation) three-dimensional PET imaging and underscores the importance of detector energy resolution in managing scattered events. The projection plane view of scattered coincidence events suggests that scatter compensation can be attempted by filtering the projection data by two-dimensional techniques. Combining this model with models of detector properties and reconstruction processes will permit a full simulation of the entire PET imaging session. >

Design of a Cylindrical Shaped Scintillation Camera for Positron Tomographs

The design of a cylindrically shaped scintillation camera for volume imaging of positron emitters is discussed. The design is based on the detector concepts developed for a single ring scintillation camera, i.e., the Massachusetts General Hospital analog ring camera. Detector characteristics derived from both computer modeling and measurements are presented.

Calibration of detector sensitivity in positron cameras

An improved method for calibrating detector sensitivities in a positron camera has been developed. The calibration phantom is a cylinder of activity placed near the center of the camera and fully within the field of view. The calibration data are processed in such a manner that the following two important properties are achieved: (1) the estimate of detector sensitivity is unaffected by the sensitivities of the other detectors; and (2) the estimates are insensitive to displacements of the calibrating phantom from the camera center. Both of these properties produce a more accurate detector calibration. >

Developments in high-resolution positron emission tomography at MGH

The development of high resolution PET systems is important for the wider application of this techniques. The resolution of PET is limited by a number of physical factors such as positron range, small angle deviation, and sampling frequency. The design of the detector array and its sensitivity remain critical factors; designs incorporating analog coding have proven to be useful. PCR‐I, a single plane PET system, has demonstrated the concept and has produced useful images in animal studies. PCR‐II will extend the concept to a two dimensional detector array resulting in a system with high resolution and high sensitivity.

A low-Z PET detector

In order to examine the potential of low-Z detector materials for PET (positron emission tomography), a small field imaging system using plastic detectors has been designed. In this system the site of a photon interaction in the detector is located using light produced by the first Compton electron. This is in contrast to high-Z detectors where multiple interactions occur. The calculated performance of the detector and supporting measurements are presented. >

Quantitative Imaging with the MGH Analog Ring Positron Tomograph

Performance of the MGH positron camera, PCR-I, is evaluated at high count rates. System resolution is maintained throughout the operating range of the instrument. The analysis of dead-time losses and random coincidence events is complicated by the analog coding system implemented in PCR-I. However, the system non-idealities can be analyzed, and corrections applied to compensate for them. An experiment demonstrating the efficacy of the count rate corrections is presented.