Proof of concept for low-dose molecular breast imaging with a dual-head CZT gamma camera. Part I. Evaluation in phantoms.

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 (+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).

Proof of concept for low-dose molecular breast imaging with a dual-head CZT gamma camera. Part II. Evaluation in patients.

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 ± 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.

Design of optimal collimation for dedicated molecular breast imaging systems.

Molecular breast imaging (MBI) is a functional imaging technique that uses specialized small field-of-view gamma cameras to detect the preferential uptake of a radiotracer in breast lesions. MBI has potential to be a useful adjunct method to screening mammography for the detection of occult breast cancer. However, a current limitation of MBI is the high radiation dose (a factor of 7-10 times that of screening mammography) associated with current technology. The purpose of this study was to optimize the gamma camera collimation with the aim of improving sensitivity while retaining adequate resolution for the detection of sub-10-mm lesions. Square-hole collimators with holes matched to the pixilated cadmium zinc telluride detector elements of the MBI system were designed. Data from MBI patient studies and parameters of existing dual-head MBI systems were used to guide the range of desired collimator resolutions, source-to-collimator distances, pixel sizes, and collimator materials that were examined. General equations describing collimator performance for a conventional gamma camera were used in the design process along with several important adjustments to account for the specialized imaging geometry of the MBI system. Both theoretical calculations and a Monte Carlo model were used to measure the geometric efficiency (or sensitivity) and resolution of each designed collimator. Results showed that through optimal collimation, collimator sensitivity could be improved by factors of 1.5-3.2, while maintaining a collimator resolution of either < or =5 or < or = 7.5 mm at a distance of 3 cm from the collimator face. These gains in collimator sensitivity permit an inversely proportional drop in the required dose to perform

Half-time Tc-99m sestamibi imaging with a direct conversion molecular breast imaging system.

BACKGROUND: In an effort to reduce necessary acquisition time to perform molecular breast imaging (MBI), we compared diagnostic performance of MBI performed with standard 10-min-per-view acquisitions and half-time 5-min-per-view acquisitions, with and without wide beam reconstruction (WBR) processing. METHODS: Eighty-two bilateral, two-view MBI studies were reviewed. Studies were performed with 300 MBq Tc-99 m sestamibi and a direct conversion molecular breast imaging (DC-MBI) system. Acquisitions were 10 min-per-view; the first half of each was extracted to create 5-min-per-view datasets, and WBR processing was applied.The 10-min-, 5-min-, and 5-min-per-view WBR studies were independently interpreted in a randomized, blinded fashion by two radiologists. Assessments of 1 to 5 were assigned; 4 and 5 were considered test positive. Background parenchymal uptake, lesion type, distribution of non-mass lesions, lesion intensity, and image quality were described. RESULTS: Considering detection of all malignant and benign lesions, 5 min-per-view MBI had lower sensitivity (mean of 70% vs. 85% (p ≤ 0.04) for two readers) and lower area under curve (AUC) (mean of 92.7 vs. 99.6, p ≤ 0.01) but had similar specificity (p = 1.0). WBR processing did not alter sensitivity, specificity, or AUC obtained at 5 min-per-view.Overall agreement in final assessment between 5-min-per-view and 10-min-per-view acquisition types was near perfect (κ = 0.82 to 0.89); however, fair to moderate agreement was observed for assessment category 3 (probably benign) (κ = 0.24 to 0.48). Of 33 malignant lesions, 6 (18%) were changed from assessment of 4 or 5 with 10-min-per-view MBI to assessment of 3 with 5-min-per-view MBI. Image quality of 5-min-per-view studies was reduced compared to 10-min-per-view studies for both readers (3.24 vs. 3.98, p < 0.0001 and 3.60 vs. 3.91, p < 0.0001). WBR processing improved image quality for one reader (3.85 vs. 3.24, p < 0.0001). CONCLUSIONS: Although similar radiologic interpretations were obtained with 10-min- and 5-min-per-view DC-MBI, resulting in substantial agreement in final assessment, notable exceptions were found: (1) perceived image quality at 5 min-per-view was lower than that for 10-min-per-view studies and (2) in a number of cases, assessment was downgraded from a recommendation of biopsy to that of short interval follow-up.

Collimator design for a dedicated molecular breast imaging-guided biopsy system: proof-of-concept.

PURPOSE: Molecular breast imaging (MBI) is a dedicated nuclear medicine breast imaging modality that employs dual-head cadmium zinc telluride (CZT) gamma cameras to functionally detect breast cancer. MBI has been shown to detect breast cancers otherwise occult on mammography and ultrasound. Currently, a MBI-guided biopsy system does not exist to biopsy such lesions. Our objective was to consider the utility of a novel conical slant-hole (CSH) collimator for rapid (<1 min) and accurate monitoring of lesion position to serve as part of a MBI-guided biopsy system. METHODS: An initial CSH collimator design was derived from the dimensions of a parallel-hole collimator optimized for MBI performed with dual-head CZT gamma cameras. The parameters of the CSH collimator included the collimator height, cone slant angle, thickness of septa and cones of the collimator, and the annular areas exposed at the base of the cones. These parameters were varied within the geometric constraints of the MBI system to create several potential CSH collimator designs. The CSH collimator designs were evaluated using Monte Carlo simulations. The model included a breast compressed to a thickness of 6 cm with a 1-cm diameter lesion located 3 cm from the collimator face. The number of particles simulated was chosen to represent the count density of a low-dose, screening MBI study acquired with the parallel-hole collimator for 10 min after a ∼150 MBq (4 mCi) injection of Tc-99m sestamibi. The same number of particles was used for the CSH collimator simulations. In the resulting simulated images, the count sensitivity, spatial resolution, and accuracy of the lesion depth determined from the lesion profile width were evaluated. RESULTS: The CSH collimator design with default parameters derived from the optimal parallel-hole collimator provided 1-min images with error in the lesion depth estimation of 1.1 ± 0.7 mm and over 21 times the lesion count sensitivity relative to 1-min images acquired with the current parallel-hole collimator. Sensitivity was increased via more vertical cone slant angles, larger annular areas, thinner cone walls, shorter cone heights, and thinner radiating septa. Full width at half maximum trended in the opposite direction as sensitivity for all parameters. There was less error in the depth estimates for less vertical slant angles, smaller annular areas, thinner cone walls, cone heights near 1 cm, and generally thinner radiating septa. CONCLUSIONS: A Monte Carlo model was used to demonstrate the feasibility of a CSH collimator design for rapid biopsy application in molecular breast imaging. Specifically, lesion depth of a 1-cm diameter lesion positioned in the center of a typical breast can be estimated with error of less than 2 mm using circumferential count profiles of images acquired in 1 min.