Molecular Breast Imaging (MBI) and Breast Specific Gamma Imaging (BSGI):
What is it? Molecular Breast Imaging (MBI) and Breast Specific Gamma Imaging (BSGI) are both specialized nuclear medicine breast imaging techniques that require intravenous injection of a radioactive agent. Due to differences in equipment, BSGI requires a higher radiation dose than MBI.
MBI or BSGI can be useful diagnostic tools in dense breasts. A diagnostic tool is one which helps diagnose cancer once a symptom appears (like a suspicious area on a mammogram or lump that can be felt but not seen on mammography or ultrasound).
How it works: The short-lived radioactive agent 99mTc-sestamibi accumulates in cancer cells more than normal cells, allowing cancer to be seen on the basis of differences in metabolism. Starting about 5 minutes after intravenous injection of the radiotracer, each breast is gently stabilized between two detectors (Figure 14. MBI), or between one detector and a compression paddle (Figure 15. BSGI), for about 10 minutes per view (for a total of 40 minutes for a routine examination), with positioning otherwise similar to mammography.
Figure 14. MBI has 2 detectors Figure 15. BSGI has 1 detector
Sometimes additional images are needed to fully include all the breast tissue. This technology does not look at the anatomy of the breast as a mammogram or breast ultrasound does; rather, it examines the functional behavior of the breast tissue (by showing differences in cellular uptake of the radioactive agent) (see Fig. 16). The radioactive agent emits invisible gamma rays, and a gamma camera is used to detect these gamma rays.
BSGI employs sodium iodide scintillation crystals and requires a higher amount of 99mTc-sestamibi activity to be administered, in the range of 15 to 30 millicuries (mCi), which delivers an effective radiation dose of 4.5 to 9 mSv. New MBI systems make use of a pair of cadmium zinc telluride (CZT) digital detectors with specialized collimators, both of which improve detection of gamma rays, allowing imaging to be performed using a lower amount of 99mTc-sestamibi, typically 6 to 8 mCi (an “off-label” dose), which delivers an effective radiation dose of 1.8 to 2.4 mSv.
Figure 16. Use of molecular breast imaging (MBI) for screening. This 65-year-old woman has heterogeneously dense breasts, with no abnormality seen on mammography (left image, MLO view – meaning image taken from a side angle). MLO MBI image (right) obtained after i.v. injection of 8 mCi (300 MBq) 99mTc-sestamibi shows intense uptake of radiotracer (arrow) in a 1.9 cm grade 2 invasive ductal cancer with negative axillary node biopsy.
Benefits: MBI, performed with a low-radiation-dose protocol, detects an additional 7 to 8 cancers per thousand women screened compared to mammography alone, and is being used at the Mayo Clinici,ii in screening research trials and now in usual clinical practice. One recent study from a community practice showed similar added cancer detection rate from MBI.iii,iv
MBI and BSGI can be helpful for some women who need but cannot tolerate MRI for reasons such as kidney failure, claustrophobia or who have pacemakers or some other metallic implants. MBI or BSGI can be used in women with dense breast tissue who have a suspicious area on a mammogram which cannot be identified on ultrasound. Uncommonly, lumps or areas of scarring remain concerning after mammography and ultrasound but are not able to be biopsied by mammography or ultrasound, and molecular breast imaging can be used for further evaluation in these circumstances.
Considerations: The Mayo Clinic has been using molecular breast imaging at effective radiation doses three- to four-times higher than a mammogram for screening women with dense breasts with excellent results. The radiation from this test is to the whole body (unlike mammography which is a low dose to just the breasts). Uptake of radiotracer in normal breast tissue increases in the luteal phase of the menstrual cycle, which may complicate interpretation: when performed, screening studies are typically scheduled in days 7 to 14 of the cycle in premenopausal women. The ACR Practice Parameter in 2017v suggests MBI is a potential option in supplemental screening for high-risk women and those with dense breasts who cannot undergo MRI, but cautions that the technique involves ionizing radiation to the whole body with attendant risk of potentially inducing cancer. MBI and BSGI are never used in women who are pregnant. Facilities offering MBI should have direct biopsy capability; otherwise, MRI may be needed to clarify or biopsy an abnormality detected on MBI.
Chart 1.Graph compares the effective radiation dose (mSv) to the whole body from common medical exams (CT = computed tomography; PET = positron emission tomography). Annual background radiation is between 2 and 10 mSv (greater at higher elevations such as Denver, CO). The annual limit for radiation workers is 50 mSv, below which it is considered unlikely to observe cancers caused by radiation exposure. Any risk from radiation is greater in younger individuals, especially those under the age of 30, and radiation exposure should always be minimized (except when undergoing treatment of a known cancer).
iRhodes DJ, Hruska, CB, Phillips, SW, et al. Dedicated dual-head gamma imaging for breast cancer screening in women with mammographically dense breasts. Radiology 2011;258:106-118. Retrieved from http://pubs.rsna.org/doi/citedby/10.1148/radiol.10100625
iiRhodes DJ, Hruska CB, Conners AL, et al. JOURNAL CLUB: Molecular Breast Imaging at Reduced Radiation Dose for Supplemental Screening in Mammographically Dense Breasts. AJR American Journal of Roentgenology 2015; 204:241-251
iiiShermis RB, Wilson KD, Doyle MT, et al. Supplemental Breast Cancer Screening With Molecular Breast Imaging for Women With Dense Breast Tissue. AJR AM J Roentgenol 2016; 10.2214/AJR.15.15924;1-8.
ivHruska CB, O'Connor MK. Curies, and Grays, and Sieverts, Oh My: A Guide for Discussing Radiation Dose and Risk of Molecular Breast Imaging. J Am Coll Radiol. 2015;12(10):1103-5.
Note that the Mayo Clinic and several of its investigators receive royalties through licensing agreements for MBI.