Molecular Breast Imaging

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

Some centers are using MBI for supplemental screening for women with dense breasts. MBI can also be used for further evaluation of abnormalities seen on mammography or 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], though the Society of Nuclear Medicine does endorse its use for screening women with dense breasts [2].

The short-lived radioactive material 99mTc-sestamibi accumulates in cancer cells more than normal cells, allowing cancer to be seen due to differences in uptake. Starting about 5 minutes after intravenous injection of the radioactive material, similar to mammographic positioning but with less compression, each breast is gently stabilized between two detectors (Fig. MBI-1) 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 radioactive material accumulates in areas of rapid cell division such as cancers. The radioactive material emits invisible gamma rays, and a gamma camera is used to detect these gamma rays. Areas with more intense uptake of radioactive material are visible on MBI, even in dense breast tissue, and may represent cancer (see Fig. MBI-2).

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 [3, 4]. Similar results have been observed in community practice [5, 6]. A current trial underway, called Density MATTERS, is evaluating two rounds of annual screening MBI compared to 3D mammograms (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 [7].

With modern systems, MBI uses about 8 mCi 99mTc-sestamibi, resulting in a radiation dose to the whole body that is less than the radiation received in the natural environment in one year (and about four times that of a mammogram) [8]. 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, uptake of the radioactive material can be higher in normal breast tissue during the latter half of the menstrual cycle, which may complicate interpretation of the test. To minimize this issue, premenopausal women undergoing MBI may wish to schedule their test earlier in their menstrual cycle (typically 7 to 14 days after the period starts).

MBI is more effective at detecting invasive ductal cancer than invasive lobular cancer, with sensitivity of 86% vs. 57% (P<.0001) in the largest published experience from Mayo Clinic [9].

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.