Physics of Diagnostic Nuclear Medicine
Diagnostic nuclear medicine involves the injection of radionuclides and then detection of their activity with an imaging device (usually a scintillating material attached to a photomultiplier tube (gamma camera)).
The most commonly used radioisotope is Technetium-99m, a metastable daughter product following negative beta decay of molybdenum-99. Technetium-99m decays to 99Tc with a half life of 6 hours, releasing a monoenergetic gamma photon of 140 keV. This is then imaged with a gamma camera. 99mTc can be attached to a number of molecules allowing it to image functional activity in a number of organs.
Common imaging studies include:
- Whole Body Bone Scan - 99mTc-MDP is used for whole body bone scans. It localises to osteoblasts and allows imaging of increased bone activity.
- Myocardial perfusion imaging - 99mTc may be combined with several compounds that localise to active myocardial cells. It allows ischaemic areas of the heart to be determined.
- Cardiac ventriculograpy - 99mTc can be used to evaluate heart function (ejection fraction) by imaging the ventricles. This is useful in chemotherapy to assess heart function pre-anthracycline or trastuzumab therapy.
- Functional brain imaging - 99m Tc can be coupled with compounds that localise to areas of high brain activity. This can be used to identify areas of the brain which are involved in seizure activity.
PET utilises the principle of positron annihilation by using radionuclides that decay through positive beta decay. Positrons generated by the decay combine with an electron and annihalate, releasing two photons with energies of 0.51 MeV in the process. The photons are released in opposite directions.
The most commonly used compound for PET Scanning is 18FDG (fluoro-2-deoxyglucose). The molecule is metabolised within the cell initially but is unable to progress on to the citric acid cycle, and is also difficult for the cell to excrete. Therefore, cells that have a high glucose metabolism concentrate 18FDG. Patients must remain still during scanning to prevent artefact in muscles.
18FDG is manufactured in a cyclotron through proton bombardment of 18O ('heavy water'). This causes a proton to enter, and a neutron to leave, the nucleus. This creates 18F. It has a half life of just under 2 hours; it is rapidly incorporated into deoxyglucose. It must be shipped to the PET scanner within hours to perform an adequate scan.
The patient is placed within a circle of detectors. These are capable of measuring both the attenuation of the annihalation photons as well as the time taken for photons to reach opposite sets of detectors.
Use of Diagnostic Nuclear Medicine
All nuclear medicine studies are functional and have poor resolution. This makes them inappropriate for use in planning by themselves, but they can be very useful if fused with a CT image. This allows determination of GTV with more accuracy.
- Localisation of metastatic sites may be more accurate with PET/nuclear medicine. While this is not always helpful for planning, it can help to choose an appropriate treatment.
- Images may be fused with CT to identify important areas for treatment
- Expensive - PET Scans average about $1000, whole body bone scans about $550.
- Exposure to ionising radiation
- Poor image resolution
- Limited access for PET scanning
- No attenuation information