Horace E. Cascio

Design Considerations for Positron Emission Tomography

The surge of interest in medical research with positron-emitting nuclides has been accompanied by improvements in the design of positron emission tomographs (scanners). Bismuth germanate has emerged as the scintillation material of choice for high resolution scanners, as its detection efficiency with small crystals is higher than that of NaI. Other detector materials, such as CsF, may be useful when timing accuracy is of great importance, as in high count rate studies or time-of-flight measurements. Circular detector arrays are now generally preferred because they provide the best sensitivity and uniformity of coverage. Multiple rings are used to increase the axial coverage. Other design parameters, such as detector and collimator dimensions, are surveyed and their effect on scanner performance is discussed.

Performance Evaluation and Calibration of the Neuro-PET Scanner

The Neuro-PET is a circular ring seven-slice positron emission tomograph designed for imaging human heads and small animals. The scanner uses 512 bismuth germanate detectors 8.25 mm wide packed tightly together in four layers to achieve high spatial resolution (6-7 mm FWHM) without the use of beam blockers. Because of the small 38 cm ring diameter, the sensitivity is also very high: 70,000 c/s per true slice with medium energy threshold (375 keV) for a 20 cm diameter phantom containing 1 ?Ci/cc of positron-emitting activity, according to a preliminary measurement. There are three switch-selectable thresholds, and the sensitivity will be higher in the low threshold setting. The Neuro-PET is calibrated with a round or elliptical phantom that approximates a patients head; this method eliminates the effects of scatter and self-attenuation to first order. Further software corrections for these artifacts are made in the reconstruction program, which reduce the measured scatter to zero, as determined with a 5 cm cold spot. With a 1 cm cold spot, the apparent activity at the center of the cold spot is 18% of the surrounding activity, which is clearly a consequence of the limits of spatial resolution, rather than scatter. The Neuro-PET has been in clinical operation since June 1982, and approximately 30 patients have been scanned to date.

An automated differential thermal and potentiometric titration apparatus for binding studies

A differential pH-thermal titration apparatus is described which can detect pH differences with a sensitivity of +/- 0.0001 pH units and a thermal sensitivity of +/- 0.00002 degree C at a time constant of 0.1 s. With a reaction which yields 1 kcal mol-1, the current system can detect concentrations as low as 4 X 10(-6) M or, in a 2 ml volume, a total amount of 40 nmol. With a time constant of 0.1 s, the sensitivity is 20 +/- 4 micro degrees C. The experimental protocol is specified by a microprocessor and three modes of operation are possible: titration at constant rate of reagent addition, titration at variable rates of addition so that the contents of both cells are at either constant pH or at a constant temperature and variable rate when a rate of change is specified. Experimental data are collected in files, corrected for heat loss, initial baseline drift, and changes in volume. The final corrected data from the standardized run of 0.01338 M HCl in 0.2 M KCl at 25 degrees C calibrate the pH scale and yield the calorimetric conversion constants and pKw which are calculated and stored for subsequent corrections for the titration of an unknown acid or the measurement of binding constants and heats.

Simple switching circuit to improve vidicon (OMA2) linearity

A simple switching circuit which allows a change of the target scan voltage to a new value at the end of the preparation frames in OMA2 vidicon detectors is described. This voltage change reduces lag significantly and leads to a linear response of the vidicon to pulsed illumination. The circuit is readily incorporated into OMA2 detectors.

Composite video generation for an OMA vidicon detector

A commercial vidicon detector, designed for scientific purposes with user‐controlled target scanning, was modified to produce a computer‐controlled optional composite video signal permitting rapid acquisition of two‐dimensional data. A video preamplifier and a processing amplifier were incorporated in the camera head, and a special function card, providing the composite video signals, was inserted into a spare slot of the control unit; minor wiring changes were required to accommodate these modifications. A resolution in excess of 400 lines is obtained in the horizontal direction, and a 10‐μ reticle is clearly resolved through a microscope.

PERFORMANCE EVALUATION AND CALIBRATION OF THE NEURO-PET SCANNER

The Neuro-PET is a circular ring seven-slice positron emission tomograph designed for imaging human heads and small animals. The scanner uses 512 bismuth germanate detectors 8.25 mm wide packed tightly together in four layers to achieve high spatial resolution (6-7 mm FWHM) without the use of beam blockers. Because of the small 38 cm ring diameter, the sensitivity is also very high: 70,000 c/s per true slice with medium energy threshold (375 keV) for a 20 cm diameter phantom containing 1 ¿Ci/cc of positron-emitting activity, according to a preliminary measurement. There are three switch-selectable thresholds, and the sensitivity will be higher in the low threshold setting. The Neuro-PET is calibrated with a round or elliptical phantom that approximates a patients head; this method eliminates the effects of scatter and self-attenuation to first order. Further software corrections for these artifacts are made in the reconstruction program, which reduce the measured scatter to zero, as determined with a 5 cm cold spot. With a 1 cm cold spot, the apparent activity at the center of the cold spot is 18% of the surrounding activity, which is clearly a consequence of the limits of spatial resolution, rather than scatter. The Neuro-PET has been in clinical operation since June 1982, and approximately 30 patients have been scanned to date.