Image Acquisition & Technical Evaluation Part 1

Vocabulary flashcards covering body habitus classifications, image quality factors, technical exposure factors, grid physics, computer imaging terminology, and automatic exposure controls for radiography.

Body Habitus

The general size and shape of a patient.

Sthenic

The strong, active, average patient type characterized by a moderately transverse heart and moderately high diaphragm.

Asthenic

A small, frail patient type where the heart is nearly vertical and the lungs are long with apices above the clavicles.

Hyposthenic

A thin but healthy slender person situating between sthenic and asthenic types, where the stomach and gallbladder are fairly high.

Hypersthenic

A big-frame, often overweight (Bariatric) patient with a high, transverse stomach and a heart axis that is almost transverse.

Radiographic Technique Influences

Pathology type, size, and composition.

Brightness

The amount of light transmitted by the monitor and the light reflected off the monitor affecting image appearance.

Contrast

The ability of a digital system to display changes in grayscale values to differentiate small differences in densities.

Subject Contrast

Refers to various body tissue densities and thickness, also known as differential absorption.

Differential Absorption Interactions

Composed of Photoelectric interactions, Compton Interactions, and Transmitted interactions.

Spatial Resolution

The ability to render small objects on the image, also known as Image Detail.

Spatial Frequency

The quantity used to describe spatial resolution, measured in $lp/mm$.

DR Spatial Resolution Determinant

Spatial resolution in Digital Radiography is determined principally by pixel size.

PSP Spatial Resolution Determinants

In Phosphor Plate systems (CR), resolution is determined by phosphor layer thickness and pixel size.

Image Distortion

The misrepresentation of object size and shape on an image.

Elongation

A form of distortion where the object or part of interest appears bigger or longer than normal.

Foreshortening

A form of distortion where the anatomic part appears smaller or shorter than normal due to poor alignment.

Exposure Factors

Another name for technical factors like mA, exposure time, kVp, and SID.

mA (Milliamperage)

The controlling factor of x-ray intensity that determines the number of electrons boiled off the filament via thermionic emission.

Reciprocity Law

The law stating that any combination of mA and exposure time will produce identical receptor exposure.

Exposure Time

A factor usually kept as short as possible to reduce patient motion blur.

kVp (Kilovoltage Peak)

Controls x-ray beam energy, quality, or penetrability by increasing the kinetic energy of electrons.

15% Rule

To maintain exposure: increase kVp by $15\%$ to cut mAs in half, or decrease kVp by $15\%$ to double mAs.

Exposure Indicator (EI)

A numeric value that indicates how much radiation reaches the image detector.

Deviation Index (DI)

Indicates how far the actual exposure was from the ideal exposure for a specific exam.

Dose Creep

The gradual increase in radiation exposure over time because digital images look diagnostic even when overexposed.

SID (Source-to-Image Distance)

The distance from the tube to the IR which has no effect on radiation energy but follows the Inverse Square Law.

Scattered Radiation

Radiation emitted from the patient in all directions that reduces radiographic contrast and provides no useful information.

Factors Affecting Scatter

kVp, field size/beam restriction, and patient/part thickness.

Grid

A device positioned between the patient and the IR to improve contrast by cleaning up scatter radiation.

Grid Ratio

The height of the grid strip divided by the interspace width ($h/D$).

Grid Frequency

The number of grid strips or grid lines per centimeter.

Grid Interspace Material

Commonly made of aluminum or plastic fiber.

Grid Strip Material

Lead strips.

Grid Cutoff

The undesirable absorption of primary or useful x-rays by the grid, often due to inadequate SID.

Parallel Grid

Also known as a linear grid, it cleans up scatter in only one direction.

Crossed Grid

Also known as a cross-hatch grid, it consists of two linear grids placed perpendicular to each other.

Focused Grid

A grid designed with lead strips angled to match the divergent x-ray beam to minimize grid cutoff.

Moving Grid

Also known as the Potter-Bucky diaphragm, it moves during exposure to blur and remove grid lines.

Stationary Grid

Low-ratio grids used primarily in mobile radiography and cross-table radiography.

Virtual Grids

Digitally reconstructed radiographs produced with fewer scatter interactions to reduce dose and improve contrast.

Off-Level Grid

A grid error caused by the tube being improperly positioned, resulting in cutoff across the image.

Off-Centered Grid

Also known as lateral decentering, where any lateral shift from the center of the grid results in cutoff.

Off-Focus Grid

A grid error occurring when the SID is outside the specified range, most critical for high-ratio grids.

Upside Down Grid

An error that causes severe grid cutoff toward the edges of the image.

Total Filtration

The sum of inherent and added filtration, which must be at least $2.5\,mm\,Al$ equivalent.

Compensating Filters

Filters shaped to provide more uniform density on radiographs of irregular anatomy.

Beam Restriction

Functions to reduce patient dose and improve image contrast.

Anode Heel Effect

A phenomenon where radiation intensity is higher on the cathode side and lower on the anode side.

Bit

A single unit of data and the smallest increment of data on a computer.

Byte

A computer data unit made up of $8$ bits.

Pixel

Also known as a picture element, the smallest element in a digital image.

Pixel Pitch

The distance from the center of one pixel to the center of the next.

Bit Depth

The number of bits stored per pixel, defining the shades of gray available ($2^n$).

Matrix

A square arrangement of numbers in columns and rows that correspond to discrete pixel values.

Postprocessing

The ability to manipulate the image after the exposure has been made.

Window Level

A post-processing term referring to the function that controls how light or dark an image is.

Window Width

A post-processing function that controls the ratio of black to white, or contrast.

Resolution

The amount of detail an image shows; high resolution produces a sharper image.

Visibility

How structures in an image are perceived and distinguished clearly.

Quantum Mottle

Graininess in an image caused by too little exposure (underexposure).

Automatic Rescaling

The computer's effort to fix exposure errors to produce uniform density and contrast.

AEC (Automatic Exposure Control)

A system used to regulate the amount of ionizing radiation delivered to the IR.

AEC Device Types

Phototimer and Ionization Chamber.

Variable Technique Chart

A chart where mAs is fixed and kV is increased as part thickness increases.

Fixed Technique Chart

Specifies a particular kV for each part while mAs varies to compensate for patient thickness.

Anatomically Programmed Technique (APR)

A system where the radiographer selects a part and size on a console, and the unit chooses a preprogrammed mAs and kV algorithm.

Extended Dynamic Range

Digital imaging's ability to compensate for approximately $80\%$ underexposure and $500\%$ overexposure.

Sthenic Colon

Colon is spread evenly with a slight dip in the transverse colon.

Asthenic Diaphragm

The diaphragm is situated low in the body.

Sthenic Lungs

Lungs are moderate in length.

Hypersthenic Diaphragm

The diaphragm is high.

Hypersthenic Colon

The colon is located around the periphery of the abdomen.

Thinner Phosphor Layer

Results in higher resolution in PSP systems.

High Frequency Generator

Equipment type that improves image quality and decreases patient dose through constant voltage potential.

Grid Ratio Cleanup

High-ratio grids are more effective at reducing scatter radiation than low-ratio grids.

Reciprocating Grid

The most common type of moving grid, motor-driven back and forth during exposure.

Anode Angle and Heel Effect

A smaller anode angle results in a larger or greater heel effect.

Quantum Mottle and Rescaling

Rescaling should not be relied on to fix too little exposure because it leads to quantum mottle.

Positioning for AEC

Exact positioning and centering are critical, as the view of interest must be directly over the photocell.