TWO DETECTOR POSITRON CAMERA WITH HONEYCOMB LEAD CONVERTERS FOR MEDICAL IMAGING: PERFORMANCE AND DEVELOPMENTS

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A Multiwire Proportional Chamber (MWPC) camera for Emission Tomography with positron emitting isotopes has been built. The coincident 511 KeV gammas are detected by their interaction with lead-on-plastic honeybomc converters coupled to planar MWPC detectors with sensitive area 48 x 48 cm{sup 2} placed 50 cm apart with clear space in between for the patient. Each detector box has two MWPC: the innermost MWPC are coupled to two converters, while the outer ones have only 1 converter. This configuration leads to an effective sensitivity of 1600 coincidence cts./min.{mu}Ci for a source placed in the center of the median plane between ... continued below

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Perez-Mendez, V.; Lim, C.B.; Ortendahl, D.; Hattner, R.; Kaufman, L. & Price, D.C. May 1, 1978.

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A Multiwire Proportional Chamber (MWPC) camera for Emission Tomography with positron emitting isotopes has been built. The coincident 511 KeV gammas are detected by their interaction with lead-on-plastic honeybomc converters coupled to planar MWPC detectors with sensitive area 48 x 48 cm{sup 2} placed 50 cm apart with clear space in between for the patient. Each detector box has two MWPC: the innermost MWPC are coupled to two converters, while the outer ones have only 1 converter. This configuration leads to an effective sensitivity of 1600 coincidence cts./min.{mu}Ci for a source placed in the center of the median plane between the detectors. With the present electronics and lead-on-plastic honeycomb converters the spatial resolution is 8 mm FWHM and the event rate at which images are taken with the camera is 30,000-40,000 events/min. The camera can be improved both in maximum event rate and spatial resolution by changing the dimensions of the converters. Decreasing the aperture size of the converters for the same thickness of lead wall leads to an increase in detection efficiency and hence improved count rate, as well as a decrease in the FWHM of the spatial resolution. They are continuing to investigate the use of surface conducting glass tube arrays with high concentrations of lead oxide (50% or more) and with hole size between 1.5 and 2 mm. Figure 2b shows the expected detection efficiency of various converters as a function of lead content and wall thickness. Alternative techniques under investigation are the use of etched hole patterns on stacks of thin lead-antimony or tungsten sheets. All of these gamma converters can be made with hole sizes 1.5-2.0 mm, compared to the 3.5 mm of the present honeycomb converters. Other factors which contribute to image degradation are multiple scattering of the 511 KeV gammas in the converters; effects of this are seen in the images of point sources which have long tails outside the main peak. Electronic techniques which they have developed are capable of rejecting over 95% of these scatter events. Data acquisition at high rates introduces a background distribution in the images due to accidental coincidence events of two 511 KeV gammas not originating from the same positron annihilation. Duplicate electronics, which is being implemented, can map out this accidentals distribution and will allow them to make a properly normalized pixel by pixel correction. These modifications, when implemented in a new camera, are expected to yield a sensitivity and event rate a factor of 10 better than the present camera. The spatial resolution is expected to be between 4-6 mm, depending on the amount of scatter within the object.

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  • Positron Emission tomography, Montreal, Canada, June 2-3, 1978

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  • Report No.: LBL-7599
  • Grant Number: DE-AC02-05CH11231
  • Office of Scientific & Technical Information Report Number: 1007489
  • Archival Resource Key: ark:/67531/metadc832219

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  • May 1, 1978

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  • May 19, 2016, 3:16 p.m.

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  • June 15, 2016, 9:53 p.m.

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Perez-Mendez, V.; Lim, C.B.; Ortendahl, D.; Hattner, R.; Kaufman, L. & Price, D.C. TWO DETECTOR POSITRON CAMERA WITH HONEYCOMB LEAD CONVERTERS FOR MEDICAL IMAGING: PERFORMANCE AND DEVELOPMENTS, article, May 1, 1978; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc832219/: accessed September 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.