Tomography

The term resonance imaging ( or tomography) ( from Ancient Greek τομή, tome, "cut" and γράφειν grafëin, "write" ), various imaging techniques are summarized, which determine the internal spatial structure of an object and in the form of cross-sectional images (also tomograms called or tomograms ) can represent. Synonymous terms are cross-sectional imaging or tomography method.

A sectional image shows the internal structures are shown as they were available after slicing the object or after the excision of a thin disk. This is called a superposition-free representation of the corresponding object layer (as opposed to particular projection methods, such as the ordinary X-ray with the overlay for all structures that are within the beam path behind the other ). This difference is in the adjacent figure with two tomographic slice images (S1 and S2 ) and a projection image (P) of the same volume illustrates.

Tomographic methods can accommodate either a single layer or larger volumes, which can then be displayed, for example as a series of parallel cross-sectional images. And methods, each receiving individual layers, can be used for recording of three-dimensional data by the object is scanned in a series of parallel cross-sectional images.

Tomographic methods are particularly in medical imaging is of great importance, but also some method of geosciences, physics, paleontology, or materials sciences can be under the tomographic methods to classify.

Method in medicine

In medicine, particularly following tomographic method are important:

• The "classical" X-ray tomography ( now used rarely )
• Diagnostic ultrasound ( sonography ),
• Computed tomography (CT ),
• Magnetic resonance imaging (MRI, magnetic resonance imaging ),
• Positron emission tomography (PET),
• The single-photon - emission computed tomography ( SPECT),
• Optical coherence tomography ( OCT) and
• Electrical Impedance Tomography (EIT).

The basics of tomography in medicine in the form of the "classical " X-ray tomography was developed by the radiologist Alessandro Vallebona 1930 in Genoa.

The difference between the superposition-free representation in medical imaging and a projection imaging is illustrated in the figures below. In projection methods such as radiography ( conventional X-ray examination ) a shadow picture is taken in the overlay between different structures if they are in the beam path after the other. For example, superimposed on the conventional x-ray image the soft tissues of the anterior and posterior chest wall and the bony structures of the chest, the pulmonary structures. This would complicate the diagnosis of a lung tumor (eg lung cancer ). Each CT - or MRI - sectional image of the thorax, however, shows only a 0.5 to 10 mm thick layer, which is practically without superimposition.

The same tumor in computed tomography ( axial cross-section )

Composed of the individual layers CT 3D image

Reconstructed 3D tooth model from data of CBCT scans

Each pixel in a sectional image corresponding to a volume element (voxel ) of the entire three-dimensional data set. The height of the voxel corresponds to the layer thickness. In the post-processing can be obtained from the voxels, for example, in space as layer arranged images ( multiplanar reconstruction, MPR ), stratified ( subvolume ) maximum intensity projections (MIPs ) or three-dimensional images of the examined object (volume rendering) can be calculated.

Methods in the geosciences

In other areas of non-destructive examination Similar procedures are used which operate according to similar mathematical inversion methods. These are:

• In geophysics, for example, the geoelectric and seismic tomography and the ground radar
• In geodesy, laser scanning and methods of altimetry and gravimetry

Methods in physics

• Quantum state tomography, which allows with similar mathematical methods, such as in medicine, the complete measurement of the quantum state of an object ( eg, its density matrix or its location and momentum distribution)
• Electron- tomography, in which the individual cross-sectional images (projections ) with the transmission electron microscope ( TEM) can be produced.

More tomographic method

• Neutron tomography: applications in paleontology and materials science
• Tomographic atom probe, atom -probe tomography (APT ) or three-dimensional atom probe ( 3DAP ): applications in materials science
• Photoacoustic tomography ( PAT): applications in materials science and biomedical research
• Computed Tomography Imaging Spectrometer ( CTIS ): A process for the spectral absorption of images