Molecular breast imaging (MBI) is a nuclear medicine technology that uses dual-head cadmium zinc telluride (CZT) gamma cameras to image functional uptake of a radiotracer, Tc-99m sestamibi, in the breast. An important factor in adoption of MBI in the screening setting is reduction of the necessary administered dose of Tc-99m sestamibi from the typically used dose of 740 MBq to approximately 148 MBq, such that MBI’s whole-body effective dose is comparable to that of screening mammography. Methods that increase MBI count sensitivity may allow a proportional reduction in the necessary administered dose. Our objective was to evaluate the impact of two count sensitivity improvement methods on image quality by evaluating count sensitivity, spatial resolution, and lesion contrast in phantom simulations.Methods:
Two dual-head CZT-based MBI systems were studied: LumaGem and Discovery NM 750b. Two count sensitivity improvement methods were implemented: registered collimators optimized for dedicated breast imaging and widened energy acceptance window optimized for use with CZT. System sensitivity, spatial resolution, and tumor contrast-to-noise ratio (CNR) were measured comparing standard collimation and energy window setting [126–154 keV (+10%, −10%)] with optimal collimation and a wide energy window [110–154 keV (+10%, −21%)].Results:
Compared to the standard collimator designs and energy windows for these two systems, use of registered optimized collimation and wide energy window increased system sensitivity by a factor of 2.8–3.6. Spatial resolution decreased slightly for both systems with new collimation. At 3 cm from the collimator face, LumaGem’s spatial resolution was 4.8 and 5.6 mm with standard and optimized collimation; Discovery NM 750b’s spatial resolution was 4.4 and 4.6 mm with standard and optimized collimation, respectively. For both systems, at tumor depths of 1 and 3 cm, use of optimized collimation and wide energy window significantly improved CNR compared to standard settings for tumors 8.0 and 9.2 mm in diameter. At the closer depth of 1 cm, optimized collimation and wide energy window also significantly improved CNR for 5.9 mm tumors on Discovery NM 750b.Conclusions:
Registered optimized collimation and wide energy window yield a substantial gain in count sensitivity and measurable gain in CNR, with some loss in spatial resolution compared to the standard collimator designs and energy windows used on these two systems. At low-count densities calculated to represent doses of 148 MBq, this tradeoff results in adequate count density and lesion contrast for detection of lesions ≥8 mm in the middle of a typical breast (3 cm deep) and lesions ≥6 mm close to the collimator (1 cm deep).
This research was supported by Mayo Clinic Foundation; National Institutes of Health (R21R33 CA 128407). The Mayo Foundation and authors of this work (C. B. Hruska, A. L. Weinmann, and M. K. O’Connor) obtain royalties from licensing arrangements between the Mayo Foundation and Gamma Medica-Ideas.
II. MATERIALS AND METHODS
II.A. Molecular breast imaging systems
II.B. Methods for increased sensitivity
II.B.1. Collimator optimization
II.B.2. Energy acceptance window
II.C. Evaluation of sensitivity improvement methods in phantoms
II.C.1. Imaging parameters
II.C.2. System sensitivity
II.C.3. Spatial resolution
II.C.4. Lesion contrast-to-noise ratio
II.C.5. Image content in widened energy window
III.A. System sensitivity
III.B. Spatial resolution
III.C. Lesion contrast-to-noise ratio
III.D. Image content in widened energy window
IV. DISCUSSION AND CONCLUSION
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