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Session: Radiopharmaceutical Imaging and Dosimetry [Return to Session]

Annihilation Gamma-Ray Interference In Tc-99m Clinical Images: Energy Spectrum Softening Due To Lead Shielding

D Vergara, K Grizzard*, Yale New Haven Hospital Department of Radiology and Biomedical Imaging and Yale University School of Medicine, New Haven, CT


MO-E115-IePD-F5-2 (Monday, 7/11/2022) 1:15 PM - 1:45 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 5

Purpose: to determine if image quality (IQ) degradation in Tc-99m tracer studies has occurred, due to annihilation gamma-rays (AGR) in the environment from an adjacent PET scanner room, and what amount of lead shielding is needed to prevent it.

Methods: SPECT and planar image sets were acquired of a Jaszczak SPECT phantom filled with Tc-99m, using a GE Infinia Hawkeye SPECT scanner. A torso-like water phantom (PET phantom), filled with F-18, was positioned across the hall in the PET scanner room. A clinical protocol and routine imaging parameters were used. Tc-99m and F-18 activities were selected based on typical count rates and dose rates, respectively. Varying amounts of lead shielding were placed directly in front of the PET phantom for each imaging study. A Tc-99m image set without AGR present (control group) was also created, with count rates corresponding to the F-18 image sets. Count rates, region-of-interest mean and standard deviation values were recorded. A contrast degradation factor (CDF) was computed for the spheres in all image sets for quantitative analysis.

Results: Decay corrected count rates and their respective F-18-to-reference-set ratios (0.97-1.0) preliminarily show that the same amount of counts were detected by the SPECT scanner, regardless of the amount of lead shielding. Average CDF is 1.01 when no additional lead shielding is applied, suggesting that CNR is not significantly degraded. Visual and quantitative comparison between the F-18 image sets and the control group shows that IQ is progressively worsened with increasing lead shielding thickness (CDF < 0.6).

Conclusion: IQ of SPECT images is not degraded when 1/16” of lead shielding is used between SPECT and PET scanners, under this particular configuration. Increasing shielding thickness appears to broaden the energy spectrum of incident AGR into the energy window of low energy radionuclides used for SPECT imaging, further degrading image contrast.


Shielding, SPECT, PET


IM- SPECT : Quality Control and Image Quality Assessment

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