Small Angle Gamma Ray Scatter: What Is The Impact On Image Quality

Abstract

Not unlike the potential small translocation errors associated with the angle of variation from perpendicular defined by the collimator 'hole' length and diameter, limitations of energy discrimination in scintigraphy might also result in a small degradation of resolution and/or contrast. Methods: A series of 99mTc line sources were acquired with multiple energy windows to provide simultaneous acquisition of data. Each energy window combination was acquired with the line source on the collimator surface, against 4.0cm of water attenuation and against 8.0cm of water attenuation. A series of 99mTc thyroid and brain phantoms were also acquired using dual energy windows positioned to capture a 10% window below 140keV and a 10% window above 140keV. The thyroid and the brain phantoms were each positioned on the collimator surface, against 4.0cm of water attenuation and against 8.0cm of water attenuation. Results: The mean improvement in FWHM for 140keV + 10% over 140keV – 10% was 0.26mm (95% CI: 0.16 – 0.36) or 4.0% (95% CI: 2.5 – 5.5) which showed a statistically significant variation from the hypothetical zero difference (P < 0.001). While a 15% window centred on 144keV provided superior image contrast compared to both 15% and 20% centred on 140keV, the improvement is associated with a decrease in accumulated counts. The increase in sampling error associated with these decreased counts, however, is 0.1% to 0.2%. Conclusion: Energy discrimination with a symmetric window about the photopeak is ineffective in eliminating photons scattered by as much as 45 to 60 degrees. While the direct impact on image resolution is perhaps negligible, low angle scatter is a major source of potential artifact and contrast degradation. A 15% energy window centered on 144keV might provide an optimized trade-off between contrast and total counts.