2 X-ray Modality
2.1 Brief History
On Friday evening, November 8, 1895 Wilhelm Conrad Röntgen discovered a “new kind of ray” that penetrated matter. Röntgen, a 50-year old professor of physics at Julius
Maximilian University of Wurzburg, named the new kind of ray X-strahlen, or “X-rays”
(“X” for unknown). Röntgen was looking for the “invisible high-frequency rays” that
Hermann Ludwig Ferdinand von Helmholtz had predicted from the Maxwell’s theory of electromagnetic radiation. Röntgen’s discovery was submitted for publication on
December 28, 1895 and was published on January 5, 1896.
Röntgen developed the first X-ray pictures on photographic plates, and one of the first materials tested was human tissue. The most famous picture was an image of his wife’s hand with a ring on her finger (Figure 2-1).
Figure 2-1 The first reported image of human tissue. Mrs. Röntgen’s hand with a ring, taken in
In 1901 Röntgen received the Nobel Prize for Physics, which was the first Nobel Prize in physics ever awarded.
The first medical use of the X-ray was on January 13, 1896 by Drs. Ratcliffe and HallEdwards, in which they showed the location of a small needle in a woman’s hand. As a
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Ch. 2 consequence, Dr. J.H. Clayton performed the first X-ray guided surgery nine days after the publication of the existence of X-rays.
2.2 X-ray Physics
An X-ray is electromagnetic (EM) radiation similar to light, radio waves, and TV waves.
Electromagnetic radiation has energy, which is often measured in units of electron volts.
An electron volt (eV) is the energy required to move a quantum of charge through 1 volt of potential energy. A quantum of charge is 1.60x10-19 coulombs, or the charge of one single electron. Table 2-1 shows some of the components of the EM spectrum, their frequency, wavelength, energy, and use.
Table 2-1 Electromagnetic Wave Spectrum (from [Enderle et al.])
Figure 2-2 graphically shows the electromagnetic spectrum and the corresponding energy of each component.
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Figure 2-2 The Electromagnetic Spectrum. The photon energies are given in electron volts (eV).
Planck showed that the relationship between energy and frequency for EM waves is given by
E = hf
where E is energy (keV), h is Planck’s constant (4.13x10-18 keV s, or 6.63x10-34 J s), and f is the frequency (Hz). (1 eV=1.6x10-19 joules)
At least in a vacuum, all EM waves propagate at the same speed, which is known as the speed of light (c = 3.0x108 m s-1). An important relationship between the speed of light and the frequency of the EM radiation is given by c=λf (2.2)
where λ is the wavelength (m).
X-rays are also characterized as particles. This is a wonderful example of the duality of nature – energy can be viewed simultaneously as both a wave and a particle. Viewed as a particle, an X-ray particle with velocity v (m s-1) and mass m (kg) has a momentum p (kg m s-1) given by
These X-ray particles are called photons, and these photons are delivered in packets called quanta. If the particle energy is greater than the binding energy of the electron, then the photons are capable of ionizing atoms. For example, when a hydrogen atom with its electron in the lowest energy configuration gets hit by a photon (light wave) and is boosted to the next lowest energy level, the energy levels are given by
En = −
13.6 eV n2
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Ch. 2 where n is the energy band of the atom. The energy difference between the lowest (n=1) and second lowest (n=2) levels corresponds to a photon with wavelength 1216 angstroms
(1 angstrom=0.1 nanometers). Thus, to ionize hydrogen, one would need a photon of
1216 angstroms. Of course, other atoms have different binding energies. There is no unique energy that defines the threshold for ionizing human tissue because human tissue is not composed of only one element.
Diagnostic radiation is typically in the range of 100 nm to about