The diagnostic breast imaging tool Positron Emission Mammography uses short-lived positron isotopes to detect breast cancer.
Positron emission mammography imaging – PEM
The diagnostic breast imaging tool Positron Emission Mammography (PEM) uses short-lived positron-emitting isotopes to generate high-resolution tomographic images of cancer within the breast.
PEM works by using an intravenous injection of the radiopharmaceutical 2-deoxy-2-(18F)fluoro-D-glucose abbreviated as 18F-FDG. Thereby, the radiopharmaceutical is based on the design of the radioisotope fluorine-18 (half-life = 109.8 mins) which is attached to the delivery compound deoxyglucose to produce 18F-FDG. This modified radiolabelled glucose is then absorbed by the cancer cells via the glucose transporter 1 system. The fundamental principle of PEM technology works on the premise that cancer cells display a high uptake of glucose. This imaging agent is effective in patients with dense breast tissue that may present with multiple lesions.
The radiopharmaceutical (18F-FDG) enters the cancer cell undergoing phosphorylation and therefore cannot be transported back out of the cell. This process then leads to the accumulation of the imaging agent and because the fluorine-18 nucleus is unstable it undergoes a decay process continually emitting positrons. The positron collides with an electron in the tumour tissue which results in annihilation to produce two 511 keV gamma rays emitted in opposite directions. During the PEM scan, the gamma rays are detected when they strike a pair of detectors that are placed between the breasts. The detected gamma rays are then amplified by photon-sensitive photomultipliers which translate into an electrical signal that then becomes an image.
PEM breast imaging
Positron Emission Mammography is currently part of the diagnostic toolkit to help assess patients that had detectable abnormalities in their mammogram. Both PEM and PET (positron emission tomography) are able to provide functional imaging by using the radiotracer 18F-FDG. However, PEM is primarily used for small body parts and utilises gentle immobilisation of the breast to attain higher spatial resolution: 1-2 mm for PEM and 4-6 mm for PET. The crystal detectors in PEM are constructed to provide this improved spatial resolution including 1.5 mm in-plane and 5 mm between planes. Also, the combination of PET and CT (computed tomography) scanners using 18F-FDG are beneficial for the staging and restaging of advanced breast carcinoma.
In addition, PEM was approved by the US Food and Drug Administration and has been introduced into the clinical setting as a diagnostic aid to mammography and breast ultrasonography. Furthermore, PEM is currently under clinical investigation to improve the sensitivity of breast cancer screening programmes. The indications for PEM include:
- the initial staging evaluation of patients with diagnosed cancer;
- distinguishing recurrent carcinoma from scar tissue;
- monitoring response to chemotherapy treatment
PEM has high imaging sensitivity for breast lesions. However, its clinical utility requires further investigation. Nevertheless, PEM cannot provide the anatomical detail that is provided by magnetic resonance imaging (MRI).