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Effect of Positron Range on Spatial Resolution

Washington University School of Medicine, St. Louis, Missouri

Correspondence: For reprints contact: M. E. Phelps, Div. of Radiation Sciences, The Edward Mallinckrodt Institute of Radiology, St. Louis, Mo. 63110.

ABSTRACT

The effect of ß⁺ range on spatial resolution of imaging systems employing the detection of 511-keV annihilation radiation was determined by measuring the variation in the line-spread functions (LSFs) of positron-emitting radionuclides of ⁶⁴Cu, ¹¹C, and ¹⁵O as compared with the 514-keV gamma-ray emitter ⁸⁵Sr. These radionuclides have maximum ß⁺ energies of 0.656, 0.960, and 1.72 MeV, respectively. The LSFs were measured in a tissue-equivalent phantom with high-resolution ( 2.4 mm FWHM) and low-resolution ( 8.8 mm FWHM) straight-bore collimators coupled to a NaI(Tl) detector. Theoretical LSFs for the ß⁺ ranges were also calculated and convolved with the ⁸⁵Sr LSF to yield the predicted LSFs for ¹¹C and ¹⁵O. The high-resolution study showed a 0% and 2.3% increase in the full-width half-maximum (FWHM) and full-width tenth-maximum (FW0.1M) for the low-energy ß⁺ of ⁶⁴Cu and a 37% (FWHM) and 52% (FW0.1M) increase for the high energy ß⁺ of ¹⁵O as compared with ⁸⁵Sr. However, when the system resolution was decreased to 8.8 mm FWHM, the ⁶⁴Cu showed no change at FWHM or FW0.1M and the ¹⁵O showed a 2.3% (FWHM) and 7.8% (FW0.1M) relative to ⁸⁵Sr. The predicted LSFs were in good agreement with the experimental. These data indicate that the effect of ß⁺ range on spatial resolution is minimal unless the ß⁺ energy is 1.5 MeV and the system resolution is on the order of a few millimeters.