Purpose: 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 (þ10%, À10%)] with optimal collimation and a wide energy window [110-154 keV (þ10%, À21%)]. 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).

Purpose: Molecular breast imaging (MBI) has shown promise as an adjunct screening technique to mammography for women with dense breasts. The demonstration of reliable lesion detection with MBI performed at low administered doses of Tc-99 m sestamibi, comparable in effective radiation dose to screening mammography, is essential to adoption of MBI for screening. The concept of performing low-dose MBI with dual-head cadmium zinc telluride (CZT) gamma cameras has been investigated in phantoms in Part I. In this work, the objectives were to evaluate the impact of the count sensitivity improvement methods on image quality in patient MBI exams and to determine if adequate lesion detection could be achieved at reduced doses. Methods: Following the implementation of two count sensitivity improvement methods, registered collimation optimized for near-field imaging and energy acceptance window optimized for CZT, MBI exams were performed in the course of clinical care. Clinical image count density (counts/ cm 2 ) was compared between standard MBI [740 MBq (20 mCi) Tc-99 m sestamibi, standard collimation, standard energy window] and low-dose MBI [296 MBq (8 mCi) Tc-99 m sestamibi, optimized collimation, wide energy window] in a cohort of 50 patients who had both types of MBI exams performed. Lesion detection at low doses was evaluated in a separate cohort of 32 patients, in which low-dose MBI was performed following 296 MBq injection and acquired in dynamic mode, allowing the generation of images acquired for 2.5, 5, 7.5, and 10 min/breast view with proportionately reduced count densities. Diagnostic accuracy at each count density level was compared and kappa statistic was used to assess intrareader agreement between 10 min acquisitions and those at shorter acquisition durations. Results: In patient studies, low-dose MBI performed with 296 MBq Tc-99 m sestamibi and new optimal collimation/wide energy window resulted in an average relative gain in count density of 4.2 6 1.3 compared to standard MBI performed with 740 MBq. Interpretation of low-dose 296 MBq images with count densities corresponding to acquisitions of 2.5, 5, 7.5, and 10 min/view and median lesion size of 1.4 cm resulted in similar diagnostic accuracy across count densities and substantial to near-perfect intrareader agreement between full 10 min-views and lower count density views. Conclusions: Review of patient studies showed that registered optimized collimation and wide energy window resulted in a substantial gain in count sensitivity as previously indicated by phantom results. This proof of concept work indicates that MBI performed at administered doses of 296 MBq Tc-99 m sestamibi with the applied count sensitivity improvements permits the detection of small breast lesions in patients. Findings suggest that further reductions in acquisition duration or administered dose may be achievable.