In Nuclear Medicine we inject a radioactive isotope in to the body of the subject to get diagnostic information for various diseases. This radionuclide which emits gamma photons or positrons are also called radiopharmaceuticals. When this radionuclide decays, there is an emission of gamma rays or high-energy photons. These high energy photons can exit the body with less scattering or attenuation. A nuclear radiation detector is used to detect photons emitted from radioactive decays, such as a gamma camera and then produce a radioactivity distribution image in the subject’s body. Emission computed tomography is an approach in which multiple cross-sectional images of tissues and organs of the patient body can be produced. This technique thus provides better image contrast and improvised detection of abnormalities. ECT is divided into two modalities: SPECT and PET. SPECT uses radionuclides such as technetium-99 m and measures the gamma ray emissions of radionuclides from within the patient body. PET makes use of carbon-11 or fluorine-18 and measures photons generated following the annihilation of positron and an electron.
Single-photon emission computed tomography (SPECT, or less commonly, SPET) is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera. However, it is able to provide true 3D information. This information is typically presented as cross-sectional slices through the patient, but can be freely reformatted or manipulated as required.
The technique requires delivery of a gamma-emitting radioisotope (a radionuclide) into the patient, normally through injection into the bloodstream. On occasion, the radioisotope is a simple soluble dissolved ion, such as an isotope of gallium (III). Most of the time, though, a marker radioisotope is attached to a specific ligand to create a radio ligand, whose properties bind it to certain types of tissues. This allows the combination of ligand and radiopharmaceutical to be carried and bound to a place of interest in the body, where the ligand concentration is seen by a gamma camera.
2. Radionuclide Generators
A radionuclide generator consists of a parent-daughter radionuclide pair contained in an apparatus that permits separation and extraction of the daughter from the parent. The daughter product activity is replenished continuously by decay of the parent and may be extracted repeatedly. Table 1 lists some radionuclide generators of interest to nuclear medicine. They are an important source of metastable radionuclides.
Table 1. Important Radionuclides used in SPECT
The most important generator is the 99Mo-99mTc system, because of the widespread use of 99mTc for radionuclide imaging. Technetium-99m emits ? rays (140 keV) that are very favorable for use with a gamma camera. It has a reasonable half-life (6 hours), delivers a relatively low radiation dose per emitted ? ray, and can be used to label a wide variety of imaging agents. More than 1850 TBq (50,000 Ci) of 99Mo per week are required to meet the worldwide requirements for nuclear medicine procedures. A 99Mo-99mTc generator is shown in Figure 1. The parent 99Mo activity in the form of molybdate ion, MoO4-2 is bound to an alumina (Al2O3) column. The daughter 99mTc activity, produced in the form of 99mTcO-4, is not as strongly bound to alumina and is eluted from the column with 5 to 25 mL of normal saline. Technetium-99m activity builds up again after an elution and maximum activity is available about 24 hours later; however, usable quantities are available 3 to 6 hours later. Commercially prepared generators are sterilized, well shielded, and largely automated in operation. Typically they are used for approximately 1 week and then discarded because of natural decay of the 99Mo parent. Decay of the 99Mo-99mTc parent-daughter pair is an example of transient equilibrium. Under idealized conditions, and a branching ratio of 0.876, the ratio of 99mTc /99Mo activity in a generator in a state of transient equilibrium would be approximately 0.96, and the time to maximum activity following an elution would be approximately 23 hours. However, these equations do not accurately predict the amount of 99mTc actually obtained in individual elutions, because most generators do not yield 100% of the available activity. Typical generator elution efficiencies are 80% to 90%, depending on the size and type of generator, volume of eluent, and so on. Furthermore, the efficiency can vary from one elution to the next. In practice, efficiency variations of ±10% or more can occur in successive elutions of the same generator. These may be caused by chemical changes in the column, including some that are caused by the intense radiation levels. Failure to keep a