14
X-Ray Imaging and
Computed Tomography
14.1 INTRODUCTION AND OVERVIEW
Medical x-ray imaging is an imaging modality operating in the x-ray electromagnetic
spectrum. The difference with other parts of the electromagnetic spectrum lies in the fact
that x-ray operates in very short wavelengths, much shorter than the ultraviolet light of the
visible spectrum. The x-ray photon energies range from 10keV (1.6 × 10 −15 J) to 100keV
(1.6 × 10 −14 J) or, equivalently, the x-ray wavelengths ranging from 0.124 to 0.0124 nm.
The x-ray energy electromagnetic radiation was discovered by the German scientist
Wilhelm Conrad Röntgen (1845–1923) in 1895 while testing a gas discharge tube.
The name x-ray was chosen at the time because it was an unknown type of radiation,
i.e., at the time it was not known that the x-ray was indeed some type of electromagnetic
radiation such as light, only with much shorter wavelengths.
Röntgen’s discovery was made when accelerated electrons traveling through vacuum in a glass tube were hitting both the anode and the glass wall. The interaction of
the accelerated electrons with the anode or the glass wall apparently produced some
form of radiation that made certain materials light up. In the case of Röntgen’s experiment, the material that started glowing was barium platinocyanide. The principle
of x-ray radiation relies on the deceleration of the electrons during the interaction
with the atomic nuclei of the heavy metal anode and the heavy metals in the glass
wall where the electron energy is converted into another form of energy, electric and
magnetic fields.
The potential for imaging the inside of the human body was almost immediately recognized as the main usage of the piercing new form of radiation and is still one of the most
used means of medical imaging with continuously increasing capabilities. In this chapter,
we first describe the physics of x-ray and then discuss some of the major imaging methods
based on x-ray, including regular x-ray imaging and x-ray computed tomography (CT).
14.2 PHYSICS OF X-RAY
We start this section by introducing some of the main concepts that are heavily
used in x-ray imaging. Two of the concepts in x-ray radiography are radiopaque and
radiolucent. Radiopaque refers to characteristics of an object that is impenetrable to
x-ray. Irradiating such an object with x-ray results in low radiation exposure of the
detector on the opposite side of the medium. On the other hand, radiolucent means
transparent to x-ray radiation, i.e., high exposure of the detector on the opposite side.
The reference material used in x-ray imaging is water that is used as a point of reference or comparison to assess the transparency of other materials. Roughly speaking,
water’s transparency to x-ray falls in the middle of the range of x-ray absorption.
261
X-Ray Imaging and
Computed Tomography
14.1 INTRODUCTION AND OVERVIEW
Medical x-ray imaging is an imaging modality operating in the x-ray electromagnetic
spectrum. The difference with other parts of the electromagnetic spectrum lies in the fact
that x-ray operates in very short wavelengths, much shorter than the ultraviolet light of the
visible spectrum. The x-ray photon energies range from 10keV (1.6 × 10 −15 J) to 100keV
(1.6 × 10 −14 J) or, equivalently, the x-ray wavelengths ranging from 0.124 to 0.0124 nm.
The x-ray energy electromagnetic radiation was discovered by the German scientist
Wilhelm Conrad Röntgen (1845–1923) in 1895 while testing a gas discharge tube.
The name x-ray was chosen at the time because it was an unknown type of radiation,
i.e., at the time it was not known that the x-ray was indeed some type of electromagnetic
radiation such as light, only with much shorter wavelengths.
Röntgen’s discovery was made when accelerated electrons traveling through vacuum in a glass tube were hitting both the anode and the glass wall. The interaction of
the accelerated electrons with the anode or the glass wall apparently produced some
form of radiation that made certain materials light up. In the case of Röntgen’s experiment, the material that started glowing was barium platinocyanide. The principle
of x-ray radiation relies on the deceleration of the electrons during the interaction
with the atomic nuclei of the heavy metal anode and the heavy metals in the glass
wall where the electron energy is converted into another form of energy, electric and
magnetic fields.
The potential for imaging the inside of the human body was almost immediately recognized as the main usage of the piercing new form of radiation and is still one of the most
used means of medical imaging with continuously increasing capabilities. In this chapter,
we first describe the physics of x-ray and then discuss some of the major imaging methods
based on x-ray, including regular x-ray imaging and x-ray computed tomography (CT).
14.2 PHYSICS OF X-RAY
We start this section by introducing some of the main concepts that are heavily
used in x-ray imaging. Two of the concepts in x-ray radiography are radiopaque and
radiolucent. Radiopaque refers to characteristics of an object that is impenetrable to
x-ray. Irradiating such an object with x-ray results in low radiation exposure of the
detector on the opposite side of the medium. On the other hand, radiolucent means
transparent to x-ray radiation, i.e., high exposure of the detector on the opposite side.
The reference material used in x-ray imaging is water that is used as a point of reference or comparison to assess the transparency of other materials. Roughly speaking,
water’s transparency to x-ray falls in the middle of the range of x-ray absorption.
261