Nuclear Medicine

Nuclear medicine depends on the introduction of pharmaceutical molecules bound with radioactive particles into the patient to image physiological functioning for diagnosis (radioactive particles may be also be used for therapeutic purposes). Different molecules are taken up by different bodily systems at different rates, so the molecule used depends on the information desired; nuclear medicine imaging may be used for oncology, cardiology, neurology and bone scans. Nuclear medicine provides comparatively low spatial resolution, but unique physiological functioning information. As the radioactive isotopes decay, gamma rays are emitted which a gamma camera converts to visible light, counts by photomultiplier tubes and transmits as digital information to a computer. Imaging with a gamma camera in this way is the oldest, cheapest, quickest (15-20 seconds/scan) and most flexible manner of nuclear medicine imaging; however, it is limited to producing two-dimensional images at relatively low resolution.

Single photon emission computed tomography (SPECT) uses a gamma camera in the same way, and depends on the same radiopharmaceuticals, but takes multiple images through a full rotation around the patient. These are then analyzed and combined by computer to create a three-dimensional image. A full SPECT scan takes approximately 15-20 minutes.

Positron emission tomography (PET) is the most expensive nuclear medicine imaging technique, in part because it depends on isotopes with a shorter half-life that are more difficult to obtain than those used by gamma cameras and SPECT. These isotopes release positrons as they decay which, when they annihilate on contact with electrons, produce two high-energy photons in opposite directions. The typical PET scan relies on a dedicated scanner with a detector ring that detects the near-simultaneous arrival of the photons (lower-quality PET scans can also be performed with a special dual gamma camera set-up). PET scans create higher resolution three-dimensional images than SPECT. Today, PET scanners are increasingly (but expensively) combined with CT or MRI scanners for superimposing both images to show physiological functioning correlated with anatomical structures.

Like radiography and CT, nuclear medicine depends on ionizing radiation to produce images. The radiation dose received varies greatly from a single gamma camera image to a full SPECT or PET scan, but generally moderate. However, nuclear medicine creates unique constraints because dedicated, secure labs and professionals for preparation, handling and disposal of radiopharmaceuticals must be provided. The radioactive isotopes used must also be acquired from special nuclear reactors or cyclotrons (in rare cases, cyclotrons are being developed to produce isotopes on-site).