Tomography is imaging by sections or sectioning to obtain images of slices through objects like the human body. Tomography is derived from the Greek words 'to cut or section' (tomos) and 'to write' (graphein). A device used in tomography is called a tomograph, while the image produced is a tomogram.
The first medical applications utilized x-rays for images of tissues based on their x-ray attenuation coefficient. The mathematical basis for tomographic imaging was laid down by Johann Radon. This type of imaging is used in different medical applications as for example computed tomography, ultrasound imaging, positron emission tomography and magnetic resonance imaging (MRI) also called magnetic resonance tomography (MRT).
Conventional x-ray tomographic techniques show organ structures lying in a predetermined plane (the focal plane), while blurring the tissue structures in planes above and below by linear or complex geometrical motion of the x-ray tube and film cassette.
Basically, computed tomography is the reconstruction of an image from its projections. In the strict sense of the word, a projection at a given angle is the integral of the image in the direction specified by that angle. The CT images (slices) are created in the axial plane, while coronal and sagittal images can be rendered by computer reconstruction.

See also Zonography, Computed or Computerized Axial Tomography, Resolution Element, Radiographic Noise, Intravenous Pyelogram.

Cerebral metabolic imaging can be accomplished with positron emission computer tomography (PET), magnetic resonance spectroscopy, and functional magnetic resonance imaging.
PET uses positron-emitting radioisotopes of elements with short half-live such as fluorine-18, oxygen-15, nitrogen-13, and carbon-11 as tracers to image and to measure the cerebral metabolism.

The cerebral blood pool can be imaged with single photon emission computed tomography (SPECT) and positron emission computer tomography (PET) techniques. The used radiopharmaceuticals are technetium-99m labeled red blood cells (RCBs) for SPECT and O-15 labeled water for PET imaging. Measured can be cerebral blood volume, brain perfusion and cerebral metabolism.

See Cerebral Metabolic Imaging, Fluorodeoxyglucose, Regional Cerebral Blood Flow, Coincidence Detection, Annihilation Coincidence Detection and Cyclotron.