Principles of Imaging – Vocabulary Review

These vocabulary flashcards cover fundamental terms and definitions from the Principles of Imaging lecture, focusing on exposure factors, image quality attributes, physics interactions, and film/detector characteristics required for radiography exam preparation.

Radiographic Technique

The specific combination of control-panel settings (mA, kVp, time, SID, etc.) chosen to create an image.

Exposure Factors

Variables that determine the quantity and quality of x-rays striking the patient; chiefly mA, kVp, exposure time, and SID.

Milliampere (mA)

Current flow in the x-ray tube; controls the number of electrons striking the target per second and therefore x-ray output.

Exposure Time (s)

Duration the x-ray tube produces radiation; together with mA determines total mAs.

Milliampere-Seconds (mAs)

Product of mA × time; the primary factor controlling image receptor exposure and optical density.

Kilovoltage Peak (kVp)

Maximum voltage across the x-ray tube; governs photon energy, beam penetrability, image contrast, and influences exposure.

Source-to-Image Distance (SID)

Distance from x-ray focal spot to the image receptor; affects beam intensity via the inverse-square law and influences magnification.

Focal-Spot Size

Actual area on the anode struck by electrons; smaller sizes improve sharpness but limit heat loading.

Filtration

Material (usually aluminum) placed in the beam to remove low-energy photons, harden the beam, and reduce patient dose.

Beam Restriction (Collimation)

Limiting the x-ray field size with shutters or cones to decrease patient dose and scatter radiation.

X-Ray Quantity

Number of photons in the beam; measured in roentgens or coulombs/kg, primarily controlled by mAs.

X-Ray Quality

Beam penetrability or energy spectrum; best specified by half-value layer and strongly linked to kVp.

Roentgen (R)

Traditional unit of radiation exposure in air, representing charge produced per mass of air.

Half-Value Layer (HVL)

Thickness of absorbing material needed to reduce beam intensity by 50 %; quantifies x-ray quality.

Inverse-Square Law

Beam intensity is inversely proportional to the square of SID (I₁/I₂ = D₂²/D₁²).

15 % kVp Rule

Increasing kVp by 15 % roughly doubles receptor exposure; decreasing by 15 % halves it (with constant mAs).

Reciprocity Law

Optical density depends on total mAs, independent of how mA and time are combined (within equipment limits).

Optical Density (OD)

Degree of blackening on film; logarithmic ratio of incident to transmitted light through the processed film.

Image Receptor Exposure

Amount of radiation reaching the detector; digital analog of film density, often displayed as brightness.

Overexposure

Image receives too much radiation, yielding excessively high OD (film) or low brightness (digital).

Underexposure

Image receives insufficient radiation, producing low OD or excessive brightness and possible quantum mottle.

Anode Heel Effect

Reduction of beam intensity on the anode side due to x-ray absorption within the anode material.

Grid

Device of alternately spaced lead strips and radiolucent interspace material that absorbs scatter before it hits the receptor.

Grid Ratio

Height of lead strips divided by interspace width; higher ratios remove more scatter but demand higher exposure.

Air-Gap Technique

Increasing OID to allow scatter to miss the detector, thereby improving contrast without a grid.

Scatter Radiation

Secondary photons deviated from original paths after interaction with matter; degrades contrast and adds noise.

Differential Attenuation

Variation in x-ray absorption between different tissues, creating image contrast.

Subject Contrast

Contrast arising from patient anatomy due to tissue thickness, density, and atomic number differences.

Image Receptor Contrast

Contrast inherent to the film-screen system or detector, including processing influences.

High Contrast (Short Scale)

Large OD differences with few gray shades; produced by low kVp, narrow window width, or low dynamic range.

Low Contrast (Long Scale)

Small OD differences with many gray shades; produced by high kVp, wide window width, or wide dynamic range.

Dynamic Range

Range of receptor exposures that can be accurately displayed; in digital imaging, corresponds to gray-scale capability.

Window Width

Digital post-processing control that sets the number of gray shades displayed; narrow width = high contrast.

Gray Scale of Contrast

Range of densities from white to black visible on the image.

Brightness (Digital Density)

Luminance of an area in a digital display; inversely related to exposure received by the detector.

Recorded Detail (Spatial Resolution)

Sharpness with which small structures are depicted; influenced by focal spot, SID, OID, and detector factors.

Sharpness of Detail

Clarity of structural borders; primarily controlled by focal-spot size and geometric setup.

Magnification Factor (MF)

SID ÷ SOD; quantifies enlargement of the object on the image.

Quantum Mottle

Random noise produced by an insufficient number of x-ray photons interacting with the detector.

Film Graininess

Random distribution of silver-halide crystal sizes in film emulsion contributing to noise.

Structure Mottle

Noise from the phosphor structure of intensifying screens.

Radiographic Noise

Total random fluctuations in optical density or brightness, including film grain, structure mottle, quantum mottle, and scatter.

Characteristic (H&D) Curve

Plot of optical density versus log relative exposure that describes film response to radiation.

Toe of Curve

Low-exposure region of the characteristic curve where OD rises slowly.

Straight-Line Portion

Linear mid-section of the characteristic curve representing normally exposed densities.

Shoulder of Curve

High-exposure region where OD levels off toward maximum blackening.

Base Density

Optical density (~0.1) from the film base and tint present even without exposure.

Fog Density

Undesirable density from storage, scatter, or chemical causes; should not exceed ~0.1 OD.

Base-Plus-Fog

Combined inherent densities (≈0.1–0.3 OD) representing the minimum density on a processed film.

Average Gradient

Slope between 0.25 and 2.0 OD above base-plus-fog; quantifies film contrast.

Image Receptor Speed

Sensitivity of film/screen or detector; defined as 1 ÷ exposure (R) to produce OD 1.0 plus base-fog.

Latitude

Range of exposures that yield diagnostic ODs; wide latitude gives forgiving technique, but lower contrast.

Destructive Pathology

Disease processes that decrease tissue density (e.g., osteoporosis) and increase receptor exposure.

Additive Pathology

Conditions that increase tissue density or thickness (e.g., pleural effusion, osteopetrosis), reducing receptor exposure.

Attenuation

Reduction in x-ray intensity via absorption or scatter as it passes through matter.

Photoelectric Effect

Interaction where an x-ray photon is completely absorbed, ejecting an inner-shell electron; enhances contrast, prominent at low kVp/high Z.

Compton Scatter

Inelastic interaction where photon loses energy and changes direction; main source of scatter degrading image quality.

Pair Production

High-energy photon interaction (>1.02 MeV) producing an electron-positron pair; not encountered in diagnostic imaging.

Half-Value Layer Test

Quality-control measurement that verifies filtration by determining material thickness needed to halve beam intensity.

Beam Hardening

Process of increasing average photon energy by removing low-energy photons through filtration.

Grid Conversion Factor

Multiplier applied to mAs when adding or changing a grid to maintain exposure.

Exposure Maintenance Formula

mAs₁/mAs₂ = SID₁²/SID₂²; adjusts mAs for changes in SID to keep receptor exposure constant.

Optimum kVp

kVp that provides sufficient penetration and desired contrast while minimizing patient dose.

Five-Percent kVp Rule

For finer contrast adjustments: ±5 % change in kVp requires reciprocal ±30 % change in mAs to keep exposure constant.

Focal Spot Blur (Penumbra)

Unsharpness at image edges due to finite focal-spot size; increases with larger focal spots, short SID, or long OID.

Motion Blur

Image unsharpness caused by voluntary or involuntary movement of patient or equipment during exposure.

Sensitometry

Study of film response to exposure and processing, using characteristic curves for quality control.

Scatter-to-Primary Ratio

Amount of scatter reaching the receptor divided by primary radiation; lower ratios yield higher contrast.

Contrast Resolution

Ability to distinguish tissues of similar x-ray attenuation; improved by reducing noise and scatter.

Spatial Frequency

Number of line pairs per millimeter that an imaging system can resolve; describes spatial resolution.