Whereas computed tomography, X-rays or MRI rely on the structural aspects of the patient’s anatomy to detect disease, nuclear medicine, uses radiopharmaceuticals to examine the physiological aspects of organ function to determine pathology. Nuclear medicine is particular important in the diagnosis and staging of cancer. Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) are used in nuclear medicine.

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The US National Cancer Institute’s Patient Information Page provides basic information on how PET and SPECT work to detect abnormalities.