Tomography

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Fig.1:Basic principle of tomography: superposition free tomographic cross sections S1 and S2 compared with the projected image P
Fig.2:Optical coherence tomography of fingertip

Tomography refers to imaging by sections or sectioning, through the use of any kind of penetrating wave. The method is used in radiology, archaeology, biology, atmospheric science, geophysics, oceanography, plasma physics, materials science, astrophysics, quantum information, and other sciences. In most cases it is based on the mathematical procedure called tomographic reconstruction.

Overview[edit]

Tomography refers to imaging by sections or sectioning, through the use of any kind of penetrating wave. The method is used in radiology, archaeology, biology, atmospheric science, geophysics, oceanography, plasma physics, materials science, astrophysics, quantum information, and other sciences. In most cases it is based on the mathematical procedure called tomographic reconstruction.The word tomography is derived from Ancient Greek τόμος tomos, "slice, section" and γράφω graphō, "to write" (see also Etymology).

In conventional medical X-ray tomography, clinical staff make a sectional image through a body by moving an X-ray source and the film in opposite directions during the exposure. Consequently, structures in the focal plane appear sharper, while structures in other planes appear blurred.[1] By modifying the direction and extent of the movement, operators can select different focal planes which contain the structures of interest. Before the advent of more modern computer-assisted techniques, this technique, developed in the 1930s by the radiologist Alessandro Vallebona, proved useful in reducing the problem of superimposition of structures in projectional (shadow) radiography.

Brief history[edit]

In a 1953 article in the medical journal Chest, B. Pollak of the Fort William Sanatorium described the use of planography, another term for tomography.[2] A chapter in the American Roentgen Ray Society's 1996 book A History of the Radiological Sciences also provides a detailed history of the development of conventional tomography from its inception until being supplanted by computer assisted tomographic techniques starting in the mid to late-1970s.[3]

A device used in tomography is called a tomograph, while the image produced is a tomogram. Tomography as the computed tomographic (CT) scanner was invented by Sir Godfrey Hounsfield, and thereby made an exceptional contribution to medicine.

Modern tomography[edit]

More modern variations of tomography involve gathering projection data from multiple directions and feeding the data into a tomographic reconstruction software algorithm processed by a computer.[4] Different types of signal acquisition can be used in similar calculation algorithms in order to create a tomographic image. Tomograms are derived using several different physical phenomena listed in the following table:[citation needed]

Physical phenomenon Type of tomogram
X-rays CT
gamma rays SPECT
radio-frequency waves MRI
Electrical Resistance ERT
electron-positron annihilation PET
electrons Electron tomography or 3D TEM
muons Muon tomography
ions atom probe
magnetic particles magnetic particle imaging
fluid flow hydraulic tomography

Some recent advances rely on using simultaneously integrated physical phenomena, e.g. X-rays for both CT and angiography, combined CT/MRI and combined CT/PET.

The term volume imaging might describe these technologies more accurately than the term tomography. However, in the majority of cases in clinical routine, staff request output from these procedures as 2-D slice images. As more and more clinical decisions come to depend on more advanced volume visualization techniques, the terms tomography/tomogram may go out of fashion.[citation needed]

Many different reconstruction algorithms exist. Most algorithms fall into one of two categories: filtered back projection (FBP) and iterative reconstruction (IR). These procedures give inexact results: they represent a compromise between accuracy and computation time required. FBP demands fewer computational resources, while IR generally produces fewer artifacts (errors in the reconstruction) at a higher computing cost.[4]

Although MRI and ultrasound are transmission methods, they typically do not require movement of the transmitter to acquire data from different directions. In MRI, both projections and higher spatial harmonics are sampled by applying spatially-varying magnetic fields; no moving parts are necessary to generate an image. On the other hand, since ultrasound uses time-of-flight to spatially encode the received signal, it is not strictly a tomographic method and does not require multiple acquisitions at all.

Schematic Configuration and Basic Principle[edit]

In this section, we will explain the schematic configuration and Basic Principle of Tomography in the case that especially uses tomography utilizing the Parallel beam irradiation optical system.

Basic Principle[edit]

Tomography is a technology that uses Tomographic Optical System to obtain virtual 'slices' (Tomographic image) of specific cross section of a scanned object, allowing the user to see inside the object without cutting. There are several types of Tomographic Optical System including the Parallel beam irradiation optical system. Parallel beam irradiation optical system may be the easiest and most practical example of Tomographic Optical System therefore, we explain "How to obtain the Tomographic image" based on Parallel beam irradiation optical system.

The Fig. 3 is intended to mathematical modeling and explanation of the principle of tomography utilizing the Parallel beam irradiation optical system.

Fig.3:Considering a parallel beam irradiation optical system where the angle between the object and the transmission light equals θ. Here, The numbers in the figure (see the numbers within the parentheses) respectively indicate: (1) = an object; (2) = the parallel beam light source; (3) = the screen; (4) = transmission beam; (5) = the datum circle; (6) = the origin; and (7) = a fluoroscopic image (a one-dimensional image; pθ(s)). Two datum coordinate systems xy and ts are also imagined in order to explain the positional relations and movements of features (0) - (7) in the figure. In addition, a virtual circle centered at the abovementioned origin (6) is set on the datum plane (it will be called “the datum circle” henceforth). This datum circle (6) will be represents the orbit of the parallel beam irradiation optical system

By using Parallel beam irradiation optical system, we can experimentally obtain the series of fluoroscopic image (a one-dimensional images” pθ(s) of specific cross section of a scanned object for each θ. Here, θ represents angle between the object and the transmission light beam. Here, the beam having a angle θ to will be the collection of lays represented by following {l}_{[\theta,s]}(t) (eq.1).

{l}_{[\theta,s]}(t) =t
\begin{bmatrix}
-\sin \theta \\
\cos \theta \\
\end{bmatrix} +
\begin{bmatrix}
s\cos \theta \\
s\sin \theta \\
\end{bmatrix} (eq.1)

And pθ(s) is defined by following (eq.2), when we consider the mathmatical model such that the Absorption coefficient of the Object at each (x,y) are represented by μ(x,y) and we suppose that "the transmission beam penetrates without diffraction, diffusion, or reflection although it is absorbed by the object and its attenuation is assumed to occur in accordance with the Beer-Lambert law.

p_{\theta}(s) = -{\int}_{-\infty}^{\infty}\mu(s\cos \theta  -t\sin \theta,s\sin \theta + t\cos \theta )\,dt (eq.2)

We can define following function of two variables. In this article, following p(s, θ) is called to be "the collection of fluoroscopic images ".

p (s, θ)=pθ(s)  (eq.3)

That means that, p(s,\theta) is a resultant of Radon transformation of μ(x,y). Therefore, “What we want to know (μ(x,y))” can be reconstructed from “What we majored ( p(s,θ)) ” by using inverse radon transformation .

Schematic Configuration[edit]

Configuration and motions of Parallel beam irradiation optical system, referring the Fig.3. ◆Statements
Numbers (1)-(7) shown in the Fig.3 (see the numbers within the parentheses) respectively indicate: (1) = an object; (2) = the parallel beam light source; (3) = the screen; (4) = transmission beam; (5) = the datum circle; (6) = the origin; and (7) = a fluoroscopic image (a one-dimensional image; p (s, θ)).

Two datum coordinate systems xy and ts are imagined in order to explain the positional relations and movements of features (0) - (7) in the figure. The xy and ts coordinate systems share the origin (6) and they are positioned on the same plane. That is, the xy plane and the ts plane are the same. Henceforth, this virtual plane will be called “the datum plane”. In addition, a virtual circle centered at the abovementioned origin (6) is set on the datum plane (it will be called “the datum circle” henceforth). This datum circle (6) will be represents the orbit of the parallel beam irradiation optical system. Naturally, the origin (6), the datum circle (5), and the datum coordinate systems are virtual features which are imagined for mathematical purposes.

◆Motion of parallel beam irradiation optical system
The parallel beam irradiation optical system, that is the key component of CT scanner, is composed of the parallel beam light source (2) and the screen (3). The parallel beam light source (2) and the screen (3) are positioned so that they face with each other in parallel with the origin (6) in between, both being in contact with the datum circle (6).

These two features ((2) and (3)) can rotate counterclockwise around the origin (6) together with the ts coordinate system while maintaining the relative positional relations between themselves and with the ts coordinate system (so, these two features ((2) and (3)) are always opposed each other). The ts plane is positioned so that the direction from the parallel beam light source (2) to the screen (3) matches the positive direction of the t-axis while the s-axis parallels these two features. Henceforth, the angle between the x- and the s-axes will be indicated as θ. In other words, parallel beam irradiation optical system where the angle between the object and the transmission beam equals θ. This datum circle (6) will be represents the orbit of the parallel beam irradiation optical system.

On the other hand, the object (1) will be scanned by CT scanner is fixed to xy coordination system. Hence. object (1) will not be moved while the parallel beam irradiation optical system are rotated around the object (1). It is obvious that the object (1) shall be smaller than datum circle.

◆Obtaining transmission image‘s’
During the above-mentioned motion (pivoting around the object(1)) of parallel beam irradiation optical system, the parallel beam light source (2) emits transmission beam (4) which can be considered as “parallel rays” in a geometrical optical sense. The traveling direction of each ray of the transmission beam (4) is parallel to the t-axis. The transmission beam (4), which is emitted by the parallel beam light source (2), penetrates the object and reaches the screen (3) after attenuated due to the absorption by the object.

Optical transmission can be presumed to occur ideally. That is, transmission beam penetrates without diffraction, diffusion, or reflection although it is absorbed by the object and its attenuation is assumed to occur in accordance with the Beer-Lambert law.

Consequently, a fluoroscopic image (7) is recorded on the screen as a one-dimensional image (one image is recorded for every θ corresponding to all s values). When the angle between the object and transmission beam is θ and if the intensity of transmission beam (4) having reached a s point on the screen is expressed as p (s, θ), it expresses a fluoroscopic image (7) corresponding to each θ.

Types of tomography[edit]

Name Source of data Abbreviation Year of introduction
Atom probe tomography Atom probe APT
Computed Tomography Imaging Spectrometer[5] Visible light spectral imaging CTIS
Confocal microscopy (Laser scanning confocal microscopy) Laser scanning confocal microscopy LSCM
Cryo-electron tomography Cryo-electron microscopy Cryo-ET
Electrical capacitance tomography Electrical capacitance ECT
Electrical resistivity tomography Electrical resistivity ERT
Electrical impedance tomography Electrical impedance EIT 1984
Electron tomography Electron attenuation/scatter ET
Functional magnetic resonance imaging Magnetic resonance fMRI 1992
Hydraulic tomography fluid flow HT 2000
Laser Ablation Tomography Laser Ablation & Fluorescent Microscopy LAT 2013
Magnetic induction tomography Magnetic induction MIT
Magnetic resonance imaging or nuclear magnetic resonance tomography Nuclear magnetic moment MRI or MRT
Muon tomography muons
Neutron tomography Neutron
Ocean acoustic tomography Sonar
Optical coherence tomography Interferometry OCT
Optical diffusion tomography Absorption of light ODT
Optical projection tomography Optical microscope OPT
Photoacoustic imaging in biomedicine Photoacoustic spectroscopy PAT
Positron emission tomography Positron emission PET
Positron emission tomography - computed tomography Positron emission & X-ray PET-CT
Quantum tomography Quantum state
Single photon emission computed tomography Gamma ray SPECT
Seismic tomography Seismic waves
Thermoacoustic imaging Photoacoustic spectroscopy TAT
Ultrasound-modulated optical tomography Ultrasound UOT
Ultrasound transmission tomography Ultrasound
X-ray tomography X-ray CT, CATScan 1971
Zeeman-Doppler imaging Zeeman effect

Discrete tomography and Geometric tomography, on the other hand, are research areas[citation needed] that deal with the reconstruction of objects that are discrete (such as crystals) or homogeneous. They are concerned with reconstruction methods, and as such they are not restricted to any of the particular (experimental) tomography methods listed above.

Synchrotron X-ray tomographic microscopy[edit]

A new technique called synchrotron X-ray tomographic microscopy (SRXTM) allows for detailed three-dimensional scanning of fossils.[6]

The construction of third-generation synchrotron sources combined with the tremendous improvement of detector technology, data storage and processing capabilities since the 1990s has led to a boost of high-end synchrotron tomography in materials research with a wide range of different applications, e.g. the visualization and quantitative analysis of differently absorbing phases, microp orosities,cracks, precipitates or grains in a specimen. Synchrotron radiation is created by accelerating free particles in high vacuum. By the laws of electrodynamics this acceleration leads to the emission of electromagnetic radiation (Jackson, 1975). Linear particle acceleration is one possibility, but apart from the very high electric fields one would need it is more practical to hold the charged particles on a closed trajectory in order to obtain a source of continuous radiation. Magnetic fields are used to force the particles onto the desired orbit and prevent them from flying in a straight line. The radial acceleration associated with the change of direction then generates radiation.... [7]

See also[edit]

Media related to Tomography at Wikimedia Commons

References[edit]

  1. ^ Tomography at the US National Library of Medicine Medical Subject Headings (MeSH)
  2. ^ Pollak, B. (December 1953). "Experiences with Planography". Chest (American College of Chest Physicians) 24 (6): 663–669. doi:10.1378/chest.24.6.663. ISSN 0012-3692. Retrieved July 10, 2011. 
  3. ^ Littleton, J.T. "Conventional Tomography". A History of the Radiological Sciences (PDF). American Roentgen Ray Society. Retrieved 29 November 2014. 
  4. ^ a b Herman, G. T., Fundamentals of computerized tomography: Image reconstruction from projection, 2nd edition, Springer, 2009
  5. ^ Ralf Habel, Michael Kudenov, Michael Wimmer: Practical Spectral Photography
  6. ^ Donoghue et al. (Aug 10, 2006). "Synchrotron X-ray tomographic microscopy of fossil embryos (letter)". Nature 442 (7103): 680–683. Bibcode:2006Natur.442..680D. doi:10.1038/nature04890. PMID 16900198. 
  7. ^ Banhart, John, ed. Advanced Tomographic Methods in Materials Research and Engineering. Monographs on the Physics and Chemistry of Materials. Oxford ; New York: Oxford University Press, 2008.

External links[edit]