Introduction - Correction Techniques in Emission Tomography - page 4

Tomographic images are usually reconstructed during a mathematical process
from many projection images acquired at certain angles around the biological
object. As image reconstruction is essential for advanced emission tomography,
a chapter of the principle of image reconstruction is included in this topic.
1.3 Electromagnetic spectrum
Emission tomography is an imaging technique based on electromagnetic
radiation, a phenomenon of oscillating self-propagating waves comprising elec-
tric and magnetic field components. Electromagnetic radiation can be classi-
fied into several subtypes according to the frequency or wavelength of its wave.
Examples are radio waves, infrared radiation, visible light, ultraviolet radia-
tion, X-rays and gamma rays. The human eye is sensitive to only a small band
of the electromagnetic spectrum called the visible spectrum. The basic \unit"
(quantum) of all electromagnetic waves is the photon.
Photons traveling through biological tissue may be absorbed and scattered
by water, suspended particles, and dissolved matter. The attenuation coe-
cient is a measure of the conversion of energy to heat and chemical energy;
thus, the higher the attenuation coecient is, the higher the probability of
photons to be absorbed or scattered by the medium.
Analyzing the attenuation coecients at different wavelengths (Figure
1.2), it becomes obvious that gamma radiation has good properties for emis-
sion imaging since it is only mildly attenuated by water or biological tissue.
The emitted gamma photons can be detected and localized outside the biolog-
ical sample without being completely absorbed. Photons out of the visual light
spectrum have also ideal properties in water, but the absorption becomes sig-
nificant if hemoglobin is on the pathway of these lower energy photons. Since
hemoglobin is always part of organs or biological tissue, optical imaging is
feasible only at the level of a few centimeters rather than at higher ranges as
compared to imaging with gamma rays.
1.4 Need for correction techniques
The generation of tomographic images involves a number of processing
steps until image quality and accuracy is reached that is sucient for vi-
sual inspection or further image-based analysis. In general, the workflow for
emission tomographic imaging can be described by five processing units: data