Investigation of Minimal Detectable Activity of I-131 for SPECT and Its Clinical Relevance
X Kong1*, M Georgiou2, S Gulec3,4, A Mcgoron3, X Wu1, S Shukla4, (1) JFK Medical Center, Miami, FL, (2) University of Miami School of Medicine, Miami, FL, (3) Florida International University, Miami, FL, (4) Aventura Hospital, Aventura, FL
Presentations
SU-F-202-7 (Sunday, 7/10/2022) 2:00 PM - 3:00 PM [Eastern Time (GMT-4)]
Room 202
Purpose: To investigate the minimal detectable activity (MDA) of I-131 under clinical conditions for SPECT units, and correlate the experimental results into clinical detectability findings.
Methods: Cylindrical phantoms were designed to mimic neck anatomy by using two 13cm diameter plexiglass containers filled with water. Different volumes of liquid I-131 (from 0.2cc to 1cc) were drawn into different sizes of syringes with luer lock (1cc Φ4.76mm; 3cc Φ8.73mm and 5cc Φ12.7mm). For each container three syringes were taped to the inside wall. The concentration ranges from starting activity of 15μCi/cc to 0.11μCi/cc at the end of the study. The whole study last 67 days which is over 8 half-lives of I-131. Total 11 scans were acquired for Siemens Symbia Intevo SPECT/CT and 10 scans were acquired for GE Infinia SPECT. I-131 regions of interest (ROI) were drawn on the SPECT images. The contrast to noise (CNR) ratio was evaluated. RadiAnt DICOM viewer with assessment tools were used to measure signal strength, background noise then calculate CNR. The Curie equation was utilized to evaluate lesion detectability. A CNR of 5 is accepted as the cutoff threshold.
Results: The CNR trend-lines for different volume sizes were plotted against activity. The trend-lines show little to none size dependence when approaching the threshold. The I-131 MDA under optimal conditions for Siemens Symbia is 0.18μCi. For Siemens Non-AC SPECT, MDA is 0.25μCi. For GE Infinia, MDA is 0.40μCi.
Conclusion: For clinical detectability of thyroid remnant and metastatic lymph nodes on Siemens Symbia SPECT/CT, based on 20% uptake of 1mCi injection on 20gram thyroid, the corresponding volume is 0.018cc, 4mm in diameter. For metastatic lymph nodes, RAS-like (40% uptake) require a 5mm in size; BRAF-like (3% uptake) require a 1.2cm in size.
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