Molecular Breast Imaging

Molecular Breast Imaging (MBI) is a specialized nuclear medicine breast imaging technique that requires intravenous injection of a radioactive agent and uses a specialized gamma camera imaging system.

Some centers are using MBI for supplemental screening for women with dense breasts. MBI is used for further evaluation of findings seen on mammography and breast ultrasound. MBI can be used in women when breast MRI is recommended but cannot be performed. The American College of Radiology indicates MBI is usually not appropriate for screening due to relative lack of evidence and higher radiation exposure [1].

The short-lived radioactive tracer 99mTc-sestamibi accumulates in cancer cells more than normal cells, allowing cancer to be seen due to differences in metabolism. Starting about 5 minutes after intravenous injection of the radiotracer, similar to mammographic positioning but with less compression, each breast is gently stabilized between two detectors (MBI, Fig. 16), or between one detector and a compression paddle (Breast Specific Gamma Imaging [BSGI], Fig. 17), for about 7 to 10 minutes per view (for a total of 28 to 40 minutes for a routine examination).

MBI does not look at the anatomy of the breast as a mammogram or breast ultrasound does. This technique examines the functional behavior of the breast tissue because the radiotracer accumulates in areas of rapid cell division such as cancers. The radioactive tracer emits invisible gamma rays, and a gamma camera is used to detect these gamma rays. Areas where there is more intense radiotracer uptake are visible on MBI, even in dense breast tissue, and may represent cancer (see Fig. 18).

MBI, performed with a low-radiation-dose protocol, detects an additional 7 to 8 cancers per thousand women screened compared to mammography alone, as shown in prospective clinical trials conducted at the Mayo Clinic [2, 3]. Similar results have been observed in community practice [4, 5]. A current trial underway, called Density MATTERS, is evaluating two rounds of annual screening MBI compared to tomosynthesis in women with dense breasts.

MBI can be helpful for women who need but cannot tolerate MRI due to kidney failure, claustrophobia, pacemakers, or other metallic implants.

MBI using only one detector has been referred to as BSGI (breast-specific gamma imaging). With improvement of this technology, including use of two detectors, less radiotracer is needed to perform the study. With modern systems, MBI uses about 8 mCi 99mTc-sestamibi, resulting in a radiation dose to the whole body that is comparable to the radiation received in the natural environment in one year (about four times that of a mammogram) [6]. Most of the radiation dose from sestamibi is deposited in the gastrointestinal tract (gall bladder, colon) and bladder, and not the breasts.  The overall radiation exposure from MBI is very small and considered within acceptable limits (Table 1).

Among premenopausal women undergoing MBI, the radiotracer uptake can be higher in normal breast tissue during the latter half of their menstrual cycle, which may complicate interpretation of the test. To avoid this, premenopausal women undergoing MBI may wish to schedule their test earlier in their menstrual cycle (typically days 7 to 14 after the period starts).

Table 1: Comparison of effective radiation doses.

¹Dose to the breast is the radiation dose to glandular breast tissue. For 2D mammography, tomosynthesis, and contrast-enhanced mammography, this dose is the sum from two views acquired during a typical screening examination, such as craniocaudal and mediolateral oblique views.

²Effective radiation dose is calculated by taking into account every organ in the body exposed to radiation by the test and each organ’s sensitivity to radiation. Health Physics Society (HPS) and American Association of Physicists in Medicine (AAPM) guidelines suggest a 100 mSv effective dose threshold, below which there is no expected risk; the limit for annual exposure of radiation workers is 50 mSv. The benefit of these low-dose medical imaging tests in detecting breast cancer is thought to far outweigh any potential risk. Nevertheless, radiation exposure should always be minimized (except when undergoing treatment of a known cancer).

MBI is not 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.