What is it?
Molecular Breast Imaging (MBI) is a specialized nuclear medicine breast imaging techniques that requires intravenous injection of a radioactive agent.
Some centers are using MBI for supplemental screening for women with dense breasts and also for problem solving.
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 (MBI, Fig. 15), or between one detector and a compression paddle (Breast Specific Gamma Imaging [BSGI], Fig. 16), for about 10 minutes per view (for a total of 40 minutes for a routine examination), with positioning otherwise similar to mammography.
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. 17). 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. This is comparable to background radiation and is about three- to four-fold the effective radiation dose from standard digital mammography (0.5 mSv) or tomosynthesis (0.6 to 0.7 mSv) .
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 Clinic [2, 3] in screening research trials and now in usual clinical practice. Similar results have been observed in community practice [4, 5].
MBI 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.
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. In 2017, the ACR Practice Parameter  included MBI as a potential option in supplemental screening for high-risk women and those with dense breasts who cannot undergo MRI, but cautioned that the technique involves ionizing radiation to the whole body. Chart 1 below compares the radiation dose to other common imaging studies. Most of the radiation dose from sestamibi is deposited in the gastrointestinal track (gall bladder, colon) and bladder, and not the breasts. Data from atomic bomb survivors and other historical data have failed to show any increased risk of cancer from exposure to these low doses of radiation. As such, many national and international organizations recommend against associating any risk with the radiation exposure from such medical testing.
MBI is 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.
Note that the Mayo Clinic and several of its investigators receive royalties through licensing agreements for MBI.
1. Hruska CB. Molecular breast imaging for screening in dense breasts: State of the art and future directions. AJR Am J Roentgenol 2017; 208:275-283
2. Rhodes DJ, Hruska CB, Phillips SW, Whaley DH, O’Connor MK. Dedicated dual-head gamma imaging for breast cancer screening in women with mammographically dense breasts. Radiology 2011; 258:106-118
3. Rhodes DJ, Hruska CB, Conners AL, et al. JOURNAL CLUB: Molecular breast imaging at reduced radiation dose for supplemental screening in mammographically dense breasts. AJR Am J Roentgenol 2015; 204:241-251
4. Shermis 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 2016:1-8
5. Hruska 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; 12:1103-1105
6. American College Of Radiology. ACR practice parameter for the performance of molecular breast imaging (MBI) using a dedicated gamma camera. 2017; https://www.acr.org/~/media/ACR/Documents/PGTS/guidelines/MBI.pdf Accessed April 15, 2020