How X-rays are made
It all begins in an X-ray tube. The tube is two electrodes in a vacuum: the cathode, the source of electrons, and the anode, the target the electrons strike. A large voltage is applied between them — typically 20–150 kV in diagnostic radiology.1
This voltage accelerates electrons released from the cathode toward the anode. The electrons gain kinetic energy proportional to the voltage: an electron accelerated through 50 kV reaches 50 keV.1 When they strike the anode, most of this kinetic energy is converted to heat; only a small fraction is emitted as X-ray photons. This is why managing anode heat is critical in an X-ray tube.
Two X-ray production mechanisms
Two different mechanisms at the anode produce X-rays:
Bremsstrahlung (braking radiation). German for "braking radiation." As a fast electron passes near the nucleus of an anode atom, it changes direction and decelerates; the energy it loses is emitted as an X-ray.1 This forms a continuous spectrum — the bulk of the diagnostic beam.
Characteristic radiation. A fast electron knocks an inner-shell electron out of an anode atom. An electron from an outer shell fills the vacancy, and the energy difference is emitted as an X-ray. Its energy is specific to the target material; it is named after the vacated shell (K, L, etc.).1
How do X-rays interact with tissue?
As the beam leaving the tube crosses the patient, the photons interact with tissue. Two interactions matter in the diagnostic energy range:
Photoelectric effect. The photon gives all its energy to an inner-shell electron and ejects it, being completely absorbed (no photon remains). The explanation of this effect earned Einstein the 1921 Nobel Prize in Physics.1
Compton scattering. The photon collides with an outer electron, transfers part of its energy, and continues on a changed direction. Compton is the predominant interaction in soft tissue across the diagnostic energy range (above about 26 keV).1 These scattered photons add noise/fog to the image and lower contrast.
Contrast is born here
The source of the black-and-white differences in the image is precisely these interactions — especially the photoelectric effect. The probability of photoelectric absorption is roughly proportional to the cube of the atomic number and inversely to the cube of the energy (≈ Z³/E³).1
The practical meaning is large: in the standard radiographic display, high atomic-number structures (calcium in bone, iodine in contrast media) absorb X-rays far more and appear white in the image; soft tissue passes most of the beam and appears darker. In digital systems, windowing and image processing can change this appearance; but the underlying contrast difference comes from the tissues' different attenuation — this differential absorption.1
Attenuation
As an X-ray beam passes through tissue, its intensity decreases due to interactions; this is called attenuation. The probability of interaction per unit thickness is expressed by the linear attenuation coefficient. In the diagnostic range this coefficient decreases as energy rises, and it is proportional to the material's density.1
A useful practical measure of the beam's "hardness" is the half-value layer (HVL): the thickness of material that halves the beam's intensity. The larger the HVL, the more penetrating (harder) the beam. This whole chain — production, interaction, attenuation — ultimately determines the pattern of radiation reaching the detector, that is, the image.
References
- Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. Karakteristik ışın (s.32), bremsstrahlung (s.37), Compton (s.39), fotoelektrik ∝ Z³/E³ (s.42), X-ışını üretimi/kV (s.167). Atıflardaki sayfa numaraları bu baskıya aittir.