Nuclear medicine (NM)
Imaging technique where small amounts of radioactive substances (radio-isotopes/radiopharmaceuticals) are injected into a patient to trace disease processes. Sometimes the radioactive substance may concentrate directly in an organ/tissue, but in most cases it has to be attached to a carrier molecule with a "preference" for a certain tissue or disease process. The concentrated radioactivity may be detected by a so-called gamma camera as typical "hot spots". The radiation to the patient is neither larger nor more dangerous than for a routine X-ray examination, and most of the radioactivity is eliminated from the body in a few hours after the examination.
Radiopharmaceutical imaging is very sensitive and we can obtain a satisfactory image if only a small per cent of the injected product binds to the organ or tissue we want to investigate, while the rest of the product is excreted e.g. via the kidneys. For that reason, it is necessary to delay taking an image to allow the product to become attached to the specific tissue and then to allow excess product to be excreted.
This provides an additional advantage in examinations for heart ailments, for example where it is necessary to visualise what happens in the heart during exercise, since that is often when the patient experiences an attack/pain. Stress electrocardiograms (ECG) are normally performed on a treadmill or exercise bicycle and it is impossible to obtain an image while the patient is exercising. If the radiopharmaceutical is injected while the patient is exercising it will be absorbed by the heart at that moment. The image taken later will show the situation as it was during exercise. The image itself can be taken up to 4-5 hours after the exercise.
The same principle can be used for patients with epilepsy. It is impossible to obtain an image while the patient is suffering a convulsion. A radiopharmaceutical injected during the seizure will be absorbed by the areas of the brain affected by the seizure. This is because the seizure affects the blood flow within the brain tissue itself. The image taken some time later shows the effect during the seizure itself.
Images produced with NM are either planar like X-ray images, or tomographic (SPECT, PET), like CT and MR images. Image resolution is not nearly as sharp as X-ray or especially MRI images. However, nuclear imaging has other important virtues. Radiopharmaceutical imaging provides far more information about function as opposed to appearance or anatomy. Products on the market today enable the physician to assess cardiac perfusion as well as to differentiate Parkinsonian syndromes from other conditions with the same symptoms. NM often has the capability of tissue characterisation. Mass lesions can be characterised as inflammatory or neoplastic, and tumour types can be pinpointed. NM studies lend themselves for whole body screening without increase in radiation dose.
GE Healthcare Glossary