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Actual focal spot Area on the anode/target that is exposed to electrons emitted from the filament.
Anode Postive side of the x-ray tube. Often used synonymously with the word target.
Anode angle Angle (in degrees) between the target surface and a verticle line drawn from
down from the center of the anode.
Artifact A misrepresentation of structures or anatomy
Automatic Exposure Control Device that terminates the x-ray exposure when a pre-determined target dose to
the detector has been achieved. Also called the Phototimer
Bit depth Determines the number of gray levels each pixel can display. A 10 bit system can
display 210 =1024 shades of gray.
Blur Spreading of the x-ray signal which results in decreased spatial resolution.
Bremmstreahlung x-ray X-ray emitted when a charged particle (electron) interacts with the positively
charged nucleus causing the electron to de-accelerate. Also called Radiative
Bucky Factor Ratio equal to the patient dose with a grid to the patient dose without a grid.
Cathode Negative side of the x-ray tube that contains the filament and the focusing cup.
Characteristic x-ray X-ray emitted when electrons transition from outer orbital shells to inner orbital
shells. Characteristic x-ray energy is equal to the difference in shell binding
Classical Scatter X-ray photon interacts with the entire atom and results in scattering of the x-ray
with no transfer of photone energy to patient tissues.
Coherent Scatter See Classical Scatter
Collimation Changing of the x-ray beam size through adjustment of the lead collimator
Compton Scatter Occurs when an x-ray photon transfers some of it’s energy to an outer shell
electron which is ejected. The photon direction is changed, and it’s energy is
Contrast The difference in gray scale between two adjacent areas in an image.
Direct DR Type of digital detector that converts x-ray signal directly to charge.
Dynamic Range The change in grayscale value displayed per unit change in dose to the imaging
Effective focal spot The size of the x-ray beam path projected off the actual focal spot onto the image
Excitation Charged particle interaction that produces heat.
Exposure time Duration of x-ray beam emisssion from the target.
Field-of-view Size of the x-ray beam projected onto the image receptor
Filament Thin coil of wire through which a large current is passed, which heats the
filament and boils off electrons. Part of the cathode.
Filtration Thin pieces of metal placed between the x-ray tube and the collimator to absorb
very low x-rays emitted from the target before they reach the patient.
Grid Device located just above the image receptor to absorb scattered x-rays and
improve image contrast.
Grid cutoff Absorption of un-scattered, primary x-rays in the grid.
Grid ratio Ratio equal to the height of the lead septa in the grid to the width of the
interspace material between the lead septa.
Half-value layer Thickness of material (typically given in aluminum) required to reduced the
intensity of the beam to 50% of it’s original intensity. Measure of x-ray beam
Heel effect A decrease intensity of the x-ray beam on the anode side of the image receptor
due to increased attenuation in the anode.
In-direct DR Digital image receptor that contains a phosphor material that converts x-ray
energy to visible light which is subsequently converted to charge.
Ionization The removal of an orbital electron from the atom.
Kerma Kinetic energy released in matter. Unit is the J/kg or Gray (Gy).
kV unit of electrical potential difference applied between the cathode and the anode
Low contrast resolution / visibility Ability to resolve objects that have very little difference in grayscale value from
surrounding structures.
mA Unit of electrical current.
mAs The product of tube current (mA) and exposure time (s). Determines the total
number of electrons that impact the anode target.
Matrix The number of rows or columns of pixels in and image. In digital image
receptors, the matrix size is equal to the number of rows and columns of detector
Noise Random variation in signal to the image receptor.
OID Object-to-image distance.
Phosphor A material that converts energy into visible light.
Photocell An individual radiaton detector that is part of the automatic exposure control
Photoelectric Effect Total absorption of an xray photon followed by the ejection of an orbial electron.
Phototimer See Automatic Exposure Control
Pixel A digtal picture element.
Quality Describes the penetrating ability (energy) of the emitted x-ray beam.
Quantity Describes the number of x-ray photons emitted in the x-ray beam.
Quantum Noise Random distribution of x-ray signal onto the image receptor elements. Inversely
related to the incident number (quanta) of x-ray photons reaching the receptor .
Scintillator A material that converts energy into visible light.
SID Source-to-image receptor distance. Distance of the x-ray tube target to the image
SOD Source-to-object being imaged distance.
Spatial resolution Ability to resolve two high contrast objects as they become smaller or closer
SSD Source-to-skin distance. Distance of the x-ray tube target to the patient’s skin
TFT Thin-film transistor. Controls the readout of signal from direct and indirect
digital radiographic detectors.
Tube current Rate at which electrons flow from the cathode to the anode in an x-ray tube.
Tube voltage Electrical potential difference applied between the cathode and the anode.