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We are designing an instrument which will perform correlated emission-transmission image
acquisition, but which departs from previous systems by incorporating a low-power x-ray tube and
generator, rather than a radionuclide source, for the transmission image. The system uses an array
of high-purity germanium (HPGe) detectors and detector electronics with energy discrimination
circuitry to separate x-rays (at 100 or 120 kVp) from higher energy gamma rays from the 99mTc
or 123j radiopharmaceutical injected into the patient. The data acquisition electronics have time
constants matching the charge collection time (50 ns) of the HPGe detectors to maximize count-rate
capabilities (up to 1 million cps per detector element), while maintaining adequate energy resolution
(approximately 10% FWHM). Each detector channel has two energy windows for simultaneous
transmission-emission imaging or for dual-energy x-ray studies. A host computer provides system
control as well as data acquisition, data correction, tomographic image reconstruction, image
display, and data analysis.
As a radionuclide imaging system, this instrument will function as a single-slice SPECT
scanner with high-count rate capabilities and excellent energy resolution for imaging short-lived
radionuclides, improved photopeak discrimination and scatter rejection, and simultaneous imaging
of multiple radionuclides. The system also will generate radiographic images in either a
tomographic or projection scanning mode, while dual-energy x-ray CT will provide material specific
imaging. However, the novel and potentially powerful capabilities of this instrument
would derive from its inherent correlation of functional information from SPECT with precise
anatomic information from CT or the material-specific morphologic information from dual-energy
x-ray CT. The simultaneously acquired radiographic images should relieve the deficiencies of poor
statistics and limited spatial resolution commonly associated with SPECT systems. Dual-energy xray
CT also can provide an energy-corrected and anatomically-correlated map of attenuation
coefficients for more accurate quantitation of emission radionuclide data.
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