X-Ray Imaging Fundamentals and Equipment Vocabulary

A comprehensive vocabulary set summarizing key terms related to x-ray production, image quality, digital imaging parameters, radiation physics, and tube components.

Photoelectric Interaction

An x-ray absorption event in which a photon transfers all its energy to an inner-shell electron, most likely in tissues with a high atomic number and when low-energy beams are used.

Tissue Density

The compactness of atomic particles within tissue; higher density increases x-ray attenuation.

Attenuation

The reduction in x-ray beam intensity as it passes through matter due to absorption and scattering.

kVp (Kilovolt Peak)

The peak voltage applied to the x-ray tube; determines beam energy and influences beam penetration and image contrast.

Radiographic Contrast

The visible difference between densities (shades of gray) on an x-ray image resulting from differential absorption.

Subject Contrast

Contrast produced by differences in the patient’s anatomy and beam quality; higher at low kVp.

Gray Scale

The range of densities displayed on a radiograph; a long gray scale indicates low contrast.

Brightness (Digital Imaging)

The overall lightness or darkness of a displayed digital image, controlled electronically rather than by exposure.

Signal-to-Noise Ratio (SNR)

The ratio of useful image signal to background noise; higher SNR equals better image quality.

Quantum Noise (Quantum Mottle)

Grainy image appearance caused by an insufficient number of x-ray photons (underexposure).

Spatial Resolution

The ability to distinguish small, closely spaced structures; measured in line pairs per millimeter (lp/mm).

Line Pairs per Millimeter (lp/mm)

A unit expressing spatial resolution; the number of distinct line pairs that can be imaged within 1 mm.

Contrast Resolution

The ability of an imaging system to differentiate between small differences in x-ray intensity, critical for soft-tissue imaging.

Shape Distortion

Misrepresentation of an object’s shape (elongation or foreshortening) caused by misalignment of tube, part, or receptor.

Size Distortion (Magnification)

Enlargement of the recorded object when it is farther from the image receptor and closer to the x-ray source.

Field of View (FOV)

The physical dimensions of anatomy included in a digital image; increasing FOV for a fixed matrix enlarges pixel size.

Matrix Size

The number of pixels arranged in rows and columns in a digital image; a larger matrix with the same FOV yields smaller pixels.

Pixel Size

The physical dimensions of a single picture element; smaller pixels improve spatial resolution.

Pixel Density

The number of pixels per unit area; higher density implies smaller pixels and better detail.

Pixel Pitch

The distance from the center of one pixel to the center of an adjacent pixel; decreased pitch enhances resolution.

Spatial Frequency

The number of line pairs per millimeter in an image; higher frequency corresponds to better spatial resolution.

Bit Depth

The number of bits used to represent each pixel’s gray level; greater bit depth increases contrast resolution.

Wilhelm Roentgen

Discoverer of x-rays on November 8, 1895; first Nobel laureate in Physics (1901).

Crookes Tube

An early vacuum tube used by Roentgen in the discovery of x-rays.

Electromagnetic Radiation

Energy that travels in waves at the speed of light, including x-rays, with properties of both waves and particles.

Wavelength (λ)

The distance between successive wave crests; for diagnostic x-rays typically 0.1–1 Å.

Frequency (ν)

The number of wave cycles per second, measured in hertz (Hz); inversely proportional to wavelength.

Photon

A discrete packet of electromagnetic energy; the particle aspect of x-rays.

Equation c = λν

Formula relating speed of light (c), wavelength (λ), and frequency (ν) of electromagnetic radiation.

Angstrom (Å)

A unit of length equal to 10⁻¹⁰ m, commonly used to express x-ray wavelengths.

Fluorescence

Immediate emission of light from certain materials (e.g., barium platinocyanide) when exposed to x-rays.

Ionization

The removal of an electron from an atom by x-ray photons, potentially causing biological damage.

ALARA Principle

Radiation protection concept that exposures should be As Low As Reasonably Achievable through time, distance, and shielding.

Cathode

The negative electrode of the x-ray tube containing the filament(s) that emit electrons.

Filament

Tungsten wire within the cathode heated to release electrons via thermionic emission.

Thermionic Emission

The release of electrons from a heated filament in the x-ray tube.

Space Charge Effect

Limiting of additional electron emission due to the buildup of a negative cloud around the filament.

Focusing Cup

Negatively charged nickel enclosure surrounding the filament that directs electrons toward the anode.

Tube Current (mA)

The flow rate of electrons from cathode to anode; higher mA increases x-ray quantity but not energy.

Filament Current

The current that heats the filament to achieve thermionic emission.

Anode

The positive electrode where electrons strike to produce x-rays; contains the target focal track.

Target (Focal Track)

Tungsten area of the anode bombarded by electrons to generate x-ray photons.

Rotating Anode

An anode design that spins (≈3,000–10,000 RPM) to spread heat over a larger surface, increasing tube heat capacity.

Rotor

The internal component that turns the rotating anode via electromagnetic induction.

Stator

The external electric motor that induces rotation of the rotor inside the x-ray tube glass envelope.

Tungsten

High-atomic-number, high-melting-point metal used in filaments and anode targets for efficient x-ray production.

Molybdenum

Lightweight metal backing the tungsten target to reduce anode mass and improve heat dissipation.

Graphite Layer (Anode)

Layer behind molybdenum that further decreases anode weight and enhances thermal conductivity.