Sypko W. Andreae

Towards a Practical Implementation of the MLE Algorithm for Positron Emission Tomography

Recognizing that the quality of images obtained by application of the Maximum Likelihood Estimator (MLE) to Positron Emission Tomography (PET) and Single Photon Emission Tomography (SPECT) appears to be substantially better than those obtained by conventional methods, we have started to develop methods that will facilitate the necessary research for a good evaluation of the algorithm and may lead to its practical application for research and routine tomography. We have found that the non-linear MLE algorithm can be used with pixel sizes which are smaller than the sampling distance, without interpolation, obtaining excellent resolution and no noticeable increase in noise. We have studied the role of symmetry in reducing the amount of matrix element storage requirements for full size applications of the algorithm and have used that concept to carry out two reconstructions of the Derenzo phantom with data from the ECAT-III instrument. The results show excellent signal-to-noise (S/N) ratio, particularly for data with low total counts, excellent sharpness, but low contrast at high frequencies when using the Shepp-Vardi model for probability matrices.

On The Applicability Of The Maximum Likelihood Estimator Algorithm For Image Recovery In Accelerated Positron Emitter Beam Injection

The Maximum Likelihood Estimator (MLE) algorithm for tomographic image reconstruction is being investigated in substantial detail by a number of research groups, as it appears to promise images with very low noise and increased sharpness when compared with filtered backprojection techniques. Recently, however, it has been found that the reconstruction of data from uniform activity distributions exhibits strong peaks and valleys when the number of iterations increases toward a maximum in the likelihood function. This problem has now been investigated with our Positron Emitter Beam Analyzer (PEBA) camera, which, because of its small size and favorable geometry, has allowed an analysis with enough detail to find the origin of that apparent instability. The findings can be summarized as follows: 1) The very low noise of the MLE reconstructions comes about by the ability of the Poisson-based MLE algorithm to generate an image which favors the matching of experimental data (detector pairs) containing few counts. 2) The image instability at a high number of iterations is a direct consequence of the above characteristic. 3) The matrix of probability elements needed for the MLE reconstruction provides the link between the two above observed phenomena. It appears that, by proper system design, it is possible to obtain the favorable low noise characteristic without the instability. The applicability of the above findings to true tomography (PEBA does not carry out a true tomographic reconstruction) seems direct, but confirmation should be obtained by further research on the question.

Beam‐Profile Detector

A beam‐profile detector that employs a pair of continuously moving scintillation counters as a detector and a PHA as a multichannel scaler is described. The number of counts per fixed number of incident particles at every 0.1 in. of travel is recorded in consecutive channels of the analyzer. A total of 7 in. is traversed in 42 sec after which the profile may immediately be displayed on the CRT readout of the PHA. The device is useful in its present form at accelerators having high beam repetition rates. Plans for its adaptation for use at pulsed accelerators are also discussed.

An error-correcting data link between small and large computers

The need for a data-link connecting small data-acquisition computers to a central computer with great analysis power arose in a particular context at the Lawrence Radiation Laboratory in Berkeley. Both the type of high-energy physics experiments being performed and the operation of the available large computer, a CDC 6600, posed unusual design problems.

AUTOMATIC DIGITIZATION OF SPARK CHAMBER EVENTS BY VIDICON SCANNER

Author(s): Andreae, Sypko W.; Kirsten, Frederick; Nunamaker, Thomas A.; Perez-Mendez, Victor.

Direct-View Vidicon Scanning System for Spark Chambers

A vidicon camera tube furnishes an ideal means for directly viewing spark-chamber events. By digitizing spark locations initially, one can perform the subsequent circuit logic by digital-circuit techniques. Information stored on magnetic tape can be presented in a format suitable for immediate analysis by a computer. A recent system for scanning two views of 12 spark-chamber gaps is described along with plans for future developments.