Performance measurements of a depth-encoding PET detector module based on position-sensitive avalanche photodiode read-out.

We are developing a high-resolution, high-efficiency positron emission tomography (PET) detector module with depth of interaction (DOI) capability based on a lutetium oxyorthosilicate (LSO) scintillator array coupled at both ends to position-sensitive avalanche photodiodes (PSAPDs). In this paper we present the DOI resolution, energy resolution and timing resolution results for complete detector modules. The detector module consists of a 7 x 7 matrix of LSO scintillator crystals (1 x 1 x 20 mm3 in dimension) coupled to 8 x 8 mm2 PSAPDs at both ends. Flood histograms were acquired and used to generate crystal look-up tables. The DOI resolution was measured for individual crystals within the array by using the ratio of the signal amplitudes from the two PSAPDs on an event-by-event basis. A measure of the total scintillation light produced was obtained by summing the signal amplitudes from the two PSAPDs. This summed signal was used to measure the energy resolution. The DOI resolution was measured to be 3-4 mm FWHM irrespective of the position of the crystal within the array, or the interaction location along the length of the crystal. The total light signal and energy resolution was almost independent of the depth of interaction. The measured energy resolution averaged 14% FWHM. The coincidence timing resolution measured using a pair of identical detector modules was 4.5 ns FWHM. These results are consistent with the design goals and the performance required of a compact, high-resolution and high-efficiency PET detector module for small animal and breast imaging applications.

Comparison of NaI(Tl), CdTe, and HgI2 surgical probes: physical characterization.

The physical properties of three surgical probes containing different radiation detectors are compared: a NaI(Tl) scintillator with a flexible, fiber-optic light guide, and two semiconductor detectors that operate at room temperature, CdTe and HgI2. Also compared are spectra, energy resolutions, and counting efficiencies measured at a variety of gamma-ray energies between 30 and 1000 keV. The energy resolution of the NaI probe is substantially poorer than that of either semiconductor probe due in part to light losses in coupling the scintillator to the fiber optics. The semiconductor probes have complex spectral response due to charge-carrier trapping and K x-ray escape, and not all photoelectric interactions in these detectors contribute to the useful part of the photopeak. Above 120 keV the counting efficiency for the NaI probe is an order of magnitude higher than for the CdTe and HgI2 probes. Both energy resolution and counting efficiency are slightly better for the HgI2 probe than for the CdTe probe.

Accurate automatic exposure controller for mammography: design and performance.

An automatic exposure controller has been designed that controls the optical film density for film, screen, and radiographic techniques typically used in mammography to within 0.05 over a range of 1.3-6.7-cm thickness of Lucite. This degree of accuracy is better than that reported for presently available controllers. The detector system consists of four cadmium telluride detectors and involves the use of a control algorithm to read the detectors and turn off the mammography unit at the correct time. This algorithm is implemented by a microprocessor, which also provides the means for a convenient calibration.

A computerized rotating laminar radionuclide camera.

We have constructed a radionuclide camera that embodies a unique detector-collimator concept and provides a radically new approach to imaging. The heart of the instrument is a linear array of semiconductor detectors separated by thin tungsten plates that confine the field of view of each detector to one dimension. This collimator design has a higher collection efficiency than the standard parallel-hole collimator but cannot directly produce a two-dimensional image. When multiple measurements are taken as the array rotates through 180 degrees, a computerized image restoration algorithm can then produce two-dimensional images with resolution determined by the width of the detectors. A small prototype camera has produced images with resolution superior to conventional Anger cameras.

Lateral diffusion of visual pigment in photorecptor disk membranes.

Visual pigment molecules are found to move transversely, but not longitudinally, in both rod and cone outer segments of mud puppy and frog. This is consistent with the idea that they are immersed in a two-dimensional fluid disk membrane. The diffusion coefficient for the motion is about 5 x 10(-9) square centimeters per second at 20 degrees C, corresponding to a root-mean-square molecular displacement of 0.3 micrometer in 1 second.

Biradical spin labeling for nerve membranes.

We have explored the behavior of pair interaction in a suitable biradical when it is bound by van der Waals' forces in a nerve membrane. We have concomitantly examined a set of model solvents to represent this situation. It appears that the biradical suffers a restriction of internal motion with a minimal restraint on its external motion in the nerve medium. The biradical is so situated as not to respond to the passage of the action potential.