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X-Ray Imaging and Computed Tomography
As can be seen from Equation 14.3, frequency of an electromagnetic wave is related
to its wavelength as follows:
C
(14.4)
f = l
Since all the energy of the photon is only kinetic energy, work equals the kinetic
energy, i.e.,
hC
E = W = qU = hf =
(14.5)
l
Equation 14.5 states that the x-ray’s wavelength is primarily identified by the voltage
(potential difference) used to create accelerate electrons. However, Equation 14.5
paints the ideal picture of 100% conversion efficiency from the kinetic energy of
the electron to the photon, which is highly improbable. In most cases, not all energy
from the accelerated electron is released during deceleration when the electron hits
the anode. A large portion of energy is converted into heat. In practical systems,
even with the highest efficiency, only approximately 1% of the electron energy is
converted into electromagnetic radiation; the remaining energy needs to be disposed
off as heat. This makes cooling an important concern in x-ray imaging.
As shown in Figure 14.1, the produced x-ray beams are then formed using a collimator. The biological tissues are irradiated with the collimated beams, and the
attenuated beams after passing through the biological tissues are then detected by
an array of detectors or simply by a sensitive film. This typical setup of biomedical
x-ray imaging systems will be discussed later in this chapter.
Based on the choice of the anode material, certain electrons carrying a particular quota of energy are preferably absorbed more than others. This results in an
x-ray photon spectrum that is characteristic for the anode material, and it is hence
called characteristic radiation. Peaks in the spectrum relate to significantly higher
deceleration probability and resonance energy levels of the anode material based
on atomic configuration and lattice structure. A representative x-ray spectrum is
illustrated in Figure 14.2.
The spectrum emitted by the anode material will range from short wavelengths
at the higher energy levels to long wavelengths at the lower energy levels as is clear
from Figure 14.2. The shorter wavelengths are most desirable for their ability to
make a high level of discrimination capability, which translates to a better image
resolution. Conversely, the longer wavelengths are undesirable because of the low
resolution and shallow penetration. Therefore, longer wavelengths will need to be
eliminated as much as possible since they provide no additional image detail but do
add to the damage created by x-ray radiation on biological tissues. Tissue damage
takes place at the genetic level, since the wavelengths of x-ray radiation are of the
order of the size of the molecular bounds in DNA.
Filtering of electromagnetic radiation at these energy levels can be accomplished
by metal plates of various thicknesses in the path of the photon beam. For instance,
X-Ray Imaging and Computed Tomography
As can be seen from Equation 14.3, frequency of an electromagnetic wave is related
to its wavelength as follows:
C
(14.4)
f = l
Since all the energy of the photon is only kinetic energy, work equals the kinetic
energy, i.e.,
hC
E = W = qU = hf =
(14.5)
l
Equation 14.5 states that the x-ray’s wavelength is primarily identified by the voltage
(potential difference) used to create accelerate electrons. However, Equation 14.5
paints the ideal picture of 100% conversion efficiency from the kinetic energy of
the electron to the photon, which is highly improbable. In most cases, not all energy
from the accelerated electron is released during deceleration when the electron hits
the anode. A large portion of energy is converted into heat. In practical systems,
even with the highest efficiency, only approximately 1% of the electron energy is
converted into electromagnetic radiation; the remaining energy needs to be disposed
off as heat. This makes cooling an important concern in x-ray imaging.
As shown in Figure 14.1, the produced x-ray beams are then formed using a collimator. The biological tissues are irradiated with the collimated beams, and the
attenuated beams after passing through the biological tissues are then detected by
an array of detectors or simply by a sensitive film. This typical setup of biomedical
x-ray imaging systems will be discussed later in this chapter.
Based on the choice of the anode material, certain electrons carrying a particular quota of energy are preferably absorbed more than others. This results in an
x-ray photon spectrum that is characteristic for the anode material, and it is hence
called characteristic radiation. Peaks in the spectrum relate to significantly higher
deceleration probability and resonance energy levels of the anode material based
on atomic configuration and lattice structure. A representative x-ray spectrum is
illustrated in Figure 14.2.
The spectrum emitted by the anode material will range from short wavelengths
at the higher energy levels to long wavelengths at the lower energy levels as is clear
from Figure 14.2. The shorter wavelengths are most desirable for their ability to
make a high level of discrimination capability, which translates to a better image
resolution. Conversely, the longer wavelengths are undesirable because of the low
resolution and shallow penetration. Therefore, longer wavelengths will need to be
eliminated as much as possible since they provide no additional image detail but do
add to the damage created by x-ray radiation on biological tissues. Tissue damage
takes place at the genetic level, since the wavelengths of x-ray radiation are of the
order of the size of the molecular bounds in DNA.
Filtering of electromagnetic radiation at these energy levels can be accomplished
by metal plates of various thicknesses in the path of the photon beam. For instance,