Breast imaging technology: Recent advances in imaging endogenous or transferred gene expression utilizing radionuclide technologies in living subjects - applications to breast cancer

Two major technologies that employ radioactive isotopes (Supplementary Table 1) for imaging molecular events currently exist: SPECT (carried out with a gamma camera), employing isotopes in which a single high energy photon is directly emitted [1]; and PET, in which a positron is emitted from the radioactive isotope [2]. For positron imaging, positrons are emitted from nuclei of proton-rich isotopes, and eventually interact with electrons. Annihilation occurs, and the mass of the electron and positron is converted into two gamma rays that travel outward from the site of annihilation at ~ 180° to one another. Scintillation crystals composed of different materials are used to capture the gamma rays for both SPECT and PET. The collection of many events allows reconstruction of the source of the emissions (Supplementary Figure 1). PET data can be corrected for attenuation, which occurs because some of the gamma rays do not traverse through all of the surrounding tissue. SPECT data is more difficult to correct for attenuation effects. PET is about 10-fold more sensitive than SPECT, primarily because the SPECT cameras use collimators that reject many of the counts from the source [3]. There are dedicated breast PET imaging systems under development [4] that may prove to be clinically important for breast and/or axillary imaging with improved resolution and/or sensitivity. We have recently developed microPET technology at UCLA, which allows high resolution (~ 2 mm in each axial direction) imaging of small animals such as mice [5] (Supplementary Figure 2).Although the focus of this review is on radionuclide imaging technologies, it is worth briefly reviewing other imaging modalities and how they contrast to PET and SPECT. Magnetic resonance imaging (MRI) [6] and computed tomography (CT) [7] provide high resolution anatomical imaging that is not available through SPECT and PET. Development of new contrast agents for MRI is allowing the study of molecular events, but these app