Guidelines for Quality Control Testing of Molecular Breast Imaging Systems

Molecular breast imaging (MBI) is a nuclear medicine test that uses dedicated γ-cameras designed for imaging of the breast. Despite growing adoption of MBI, there is currently a lack of guidance on appropriate quality control procedures for MBI systems. Tests designed for conventional γ-cameras either do not apply or must be modified for dedicated detectors. Our objective was to provide practical guidance for physics testing of MBI systems by adapting existing quality control procedures for conventional systems. Methods: Quality control tests designed for conventional γ-cameras were attempted on a dedicated MBI system and then modified as necessary to accommodate the pixelated detector, limited space between dual-detector heads, and inability to fully rotate the detector gantry. Results: MBI systems were found to warrant quality control testing of uniformity, spatial resolution, count sensitivity, energy resolution, and lesion contrast. The modified procedures and special considerations needed for these tests were investigated and described. Physics tests of intrinsic uniformity, count rate parameters, and overall system performance for SPECT did not apply to dedicated MBI systems. Conclusion: Routine physics testing of dedicated MBI equipment is important for verifying system specifications and monitoring changes in performance. As adoption of MBI grows, routine testing may be required for obtaining and maintaining accreditation from regulatory bodies.

Molecular Breast Imaging: Administered Activity Does Not Require Adjustment Based on Patient Size

At our institution, molecular breast imaging (MBI) is performed with 300 MBq of ^99mTc-sestamibi for all patients. For some nuclear medicine procedures, administered activity or imaging time is increased for patients of larger size to obtain adequate counts. Our objective was to assess whether uptake of ^99mTc-sestamibi in the breast is influenced by patient size. Methods: Records from patients who underwent a clinical MBI examination between July and November 2016 were reviewed. Those in whom our standard injection and preparation techniques were followed were included in the analysis. Patients were injected with approximately 300 MBq of ^99mTc-sestamibi. Residual activity was measured to allow calculation of exact administered activity for each patient. Breast images were acquired at 10 min/view using a dual-head cadmium zinc telluride-based γ-camera. Breast thickness was measured as the distance between the 2 detectors. Patient height, weight, body surface area, and body mass index were obtained from records. Lean body mass with the James equation (LBM_James) and Janmahasatian correction (LBM_Janma) was calculated. Count density in the breast tissue was measured by drawing a region of interest around the central breast tissue of the right breast mediolateral-oblique view of the lower detector. Count density was expressed as cts/cm²/MBq of administered activity. Spearman correlation coefficient (r_s) was calculated. Results: A total of 200 patients were analyzed. No dose infiltration was suspected at any injection. Average administered activity was 292 MBq (SD, 13.8 MBq; range, 247-326 MBq). Average count density was 7.2 cts/cm²/MBq (SD, 2.7 cts/cm²/MBq; range, 3.1-17.8 cts/cm²/MBq). MBI count density was weakly negatively correlated with height (r_s = -0.18; P = 0.01), weight (r_s = -0.23; p = <0.001), body mass index (r_s = -0.16; P = 0.02), body surface area (r_s = -0.22; P = 0.002), LBM_James (r_s = -0.23; P = 0.001), and LBM_Janma (r_s = -0.23; P = 0.001). No correlation was observed between count density and breast thickness (r_s = 0.06; P = 0.37). Conclusion: Our results suggest a lack of relationship between uptake of ^99mTc-sestamibi in breast tissue and body size or compressed breast thickness. Altering from the standard 300 MBq of administered activity for larger patients is likely unnecessary.

Improved visualization of breast tissue on a dedicated breast PET system through ergonomic redesign of the imaging table

Background

Breast lesions closer than 2 cm to the chest wall are difficult to position in the field of view of dedicated breast PET (db-PET) systems. This inability to detect such lesions is a significant limitation of these systems. The primary objective of this study was to determine if modifications to the design of the imaging table and detector used for a db-PET system would enable improved visualization of breast tissue close to the chest wall. All studies were performed on a commercially available db-PET system (Mammi-PET). A central square section of the imaging table, containing the standard 180-mm circular aperture, was modified such that it could be removed and replaced by thinner sections with a larger aperture. Additional changes were made to the cover plate of the detector array and the patient mattress. A total of 60 patients were studied. After administration of F-18 FDG, 30 patients were imaged with a 220-mm-diameter aperture and the standard aperture, and 30 patients with a 200-mm aperture and the standard aperture. On all scans, the length of breast tissue in the field of view was measured as the greatest extent of tissue from the nipple back to the posterior edge of the breast. Image quality and patient comfort were recorded.

Results

Averaged over both breasts, relative to the standard aperture, the increase in breast length was 12.5 + 7.7 mm with the 220-mm aperture, and 12.3 + 6.5 mm with the 200-mm aperture (p < 0.05 for both apertures). In ~ 5% of cases, the larger apertures resulted in some degradation in image quality due to closer proximity to cardiac/hepatic activity. In 10–20% of cases, movement of the breast tissue was observed as the detector ring was moved to scan the anterior region of the breast. The patient survey indicated no significant difference in the comfort level between the standard aperture and either of the prototype apertures.

Conclusions

Modifications to the image table and system resulted in a significant gain in the volume of breast tissue that could be imaged on the db-PET system and should allow better visualization of lesions close to the chest wall.

Potential Ways to Address Shortage Situations of 99Mo/99mTc

^99mTc, the most common radioisotope used in nuclear medicine, is produced in a nuclear reactor from the decay of ⁹⁹Mo. There are only a few aging nuclear reactors around the world that produce ⁹⁹Mo, and one of the major contributors, the National Research Universal (Canada), ceased production on October 31, 2016. The National Research Universal produced approximately 40% of the world's ⁹⁹Mo supply, so with its shut down, shortages of ⁹⁹Mo/^99mTc are expected. Methods: Nuclear pharmacies and nuclear medicine departments throughout the United States were contacted and asked to provide their strategies for coping with a shortage of ⁹⁹Mo/^99mTc. Each of these strategies was evaluated on the basis of its effectiveness for conserving ^99mTc while still meeting the needs of the patients. Results: From the responses, the following 6 categories of strategies, in order of importance, were compiled: contractual agreements with commercial nuclear pharmacies, alternative imaging protocols, changes in imaging schedules, software use, generator management, and reduction of ordered doses or elimination of backup doses. Conclusion: The supply chain of ⁹⁹Mo/^99mTc is quite fragile; therefore, being aware of the most appropriate coping strategies is crucial. It is essential to build a strong collaboration between the nuclear pharmacy and nuclear medicine department during a shortage situation. With both nuclear medicine departments and nuclear pharmacies implementing viable strategies, such as the ones proposed, the amount of ^99mTc available during a shortage situation can be maximized.