ABSTRACT
The vast majority of planar/SPECT procedures are carried out with radiopharmaceuticals labelled with Technetium-99m (99mTc), which has a half-life of 6 h and emits gamma photons at 140 keV. It is obtained from an on-site generator and used to prepare a range of imaging agents via kits that contain all the non-radioactive components in quantities optimized to produce >95 per cent incorporation of the label. Other gamma-emitting radionuclides of interest include Iodine-123 (t½ 13 h) and Indium-111 (t½ 67 h), which have half-lives long enough for delivery from a radiopharmaceutical manufacturer.
Radiopharmaceuticals have been developed for imaging many parts of the body. One of the most widely used studies is the bone scan, using a 99mTc-diphosphonate complex, which binds to hydroxyapatite crystals at the surface of bone in proportion to the rate of bone growth via osteoblastic activity. Renal function can be imaged with 99mTc-mertiatide, which is both filtered and actively secreted by the kidney. An alternative agent, 99mTc-pentetate, is filtered and can be used to estimate glomerular filtration rate. The structure of the renal cortex can be imaged with 99mTc-succimer. Regional cerebral blood flow can be imaged with 99mTc-exametazime or 99mTc-bicisate, both of which are efficiently extracted by the brain on first pass then retained showing regional perfusion. The integrity of dopaminergic nerve terminals in the brain can be imaged with 123I-ioflupane, a substrate for the dopamine transporter; this is useful for diagnosis of Parkinson’s disease. The chemistry and biology of additional agents are also discussed.
