Room: AAPM ePoster Library
Purpose: Accurately tracking breast cancer progression with 99mTc-sestamibi dual-headed Molecular Breast Imaging is limited by the lack of absolute tumor uptake quantification. We have simulated a GE NM750b system capable of individually recording photon interactions to develop and evaluate locoregional attenuation- and scatter-correction techniques.
Methods: Our Geant4-based application with a novel MATLAB extension for pixelated CZT-detector charge collection was initially validated with 99mTc point, line, and volumetric phantoms. Subsequently, we simulated (2.5mm)^3 sources (150M histories) at 40 distances above detector within either 10cm 50/50-breast tissue or air. For each acquisition (N=80), we calculated 3 counts-per-1M-decays (cpMd) in each of 12 concentric circular ROIs (r=0-75mm). 1) NC: photopeak-window (127-154keV) counts with no corrections. 2) AC: NC with attenuation-correction (geometric-mean, µ=0.153cm-1). 3) SAC: NC with scatter-correction (dual-energy-window, 113-126keV, k=0.81) and AC. Quantitative accuracy of all cpMd estimations was assessed relative to the average in-air (N=40), photopeak-window, r=75mm ROI NC cpMd (gold-standard).
Results: Simulated and measured data had good agreement in energy spectra and resolution (5.9% vs 6.9% mean FWHM), spatial resolution (0.1mm mean FWHM difference), and sensitivity (567 vs. 573 mean counts/min/µCi). The mean±SD gold-standard cpMd was 255±2. We found an r=15mm ROI was necessary and sufficient to capture all primary photons (-0.2%±0.9% in-air UC ?cpMd) but required SAC for accurate in-tissue quantification (NC/AC/SAC ?cpMd: -41.3±2.39% / 16.3%±2.6% / -4.7%±1.7%). Smaller ROIs in-tissue accepted fewer scatter photons and required only AC for accurate quantification (r=7.5mm ?cpMd: 1.5%±5.3%). Although single-pixel ROIs included the fewest scatter photons, the sharp decrease of total incident photons with distance led to largest overall errors (AC ?cpMd: -86.6%±2.1%).
Conclusion: Initial experiments with our newly-validated GE NM750b simulation program highlighted the need for different source quantification techniques depending on ROI size. With on-going clinically-realistic simulations, we will develop guidelines for accurate and precise tumor uptake quantification in patient imaging.
Funding Support, Disclosures, and Conflict of Interest: Research supported in part by a grant from GE Healthcare.