DR Digital Imaging Flashcards
Digital
A device or system that represents information as continuously variable numerical values.
Example - digital watch
CR and DR are digital
Analog
A device or system that represents information as continuously variable physical quantities.
Example - mechanical watch
X-ray film
FPD Flat Panel Detector (DR Imaging Plate)
Takes the place of cassettes and readers; 3 different types:
Tethered (w/cord)
Fixed (mounted in Bucky)
Wireless (uses radio signal sent from FPD to base)
Capture element
Remnant X-ray energy is captured.
Indirect DR - amorphous Sillicon
- converts X-ray to visible light
Direct DR - amorphous selenium
- converts X-ray directly to electrons
Coupling element
Transfers X-ray generated signal to collection element
Indirect DR - amorphous silicon
- light sent to photoconductive material
Direct DR - amorphous selenium
-electrons sent to TFT
Collection element
Collects photons or electrons to be quantified - assigned a digital value
Indirect DR -TFT - electrons from photoconductive material collected by TFT
Direct DR - TFT - electrons from amorphous selenium collected by TFT
Algorithm
A finite sequence of instructions, an explicit step by step procedure for solving a problem, often used for calculation and data processing. A computers mathematical “recipe”, not one size fits all
Sampling
The process used to digitize the spatial information in an image, typically achieved by dividing an image into a square or rectanglar array of sampling points.
Spatial resolution
The ability to see really small structures, the field of view, matrix size
Pixel size
Measured from side to side of the pixel, a less accurate measure, does not account for dead zones
Pixel pitch
Measures center of pixel to center of other; takes into consideration dead zones, a more accurate assessment.
Pixel density
/mm
the number of pixels per given area
Amorphous
having no shape or organization
Indirect Conversion Flat Panel Detectors
Detector that uses thin layers of amorphous silicon with an array of photodiodes - coated w/Photostimulable phosphor that emits light - converted to an electrical charge, signal sent to ADC to convert to digital signal. High QDE - lower patient dose; loss of resolution due to divergence of light emitted.
Amorphous Silicon (a-Si)
Phosphor material - emits light when stimulated
Direct Conversion Flat Panel Detectors
Uses a amorphous selenium coated TFT that directly converts X-ray to electrical signals. Absorbs remnant radiation in the selenium layers that are collected by capacitors, then sent to the ADC to convert to digital.
No light conversion = no light divergence
Amorphous Selenium (a-Se)
Absorbs X-ray - converts directly to electrons (no light divergence)
TFT - Thin Film Transistor
Collect electric charges, positioned in a matrix, detects charges on a pixel by pixel basis.
Capable of very high spatial resolution (greater than 20lp/mm)
AMA - Active Matrix Array
Amorphous silicon, capacitor, and TFT assembly
DEL - Detector Element
Small square capable of recording a certain number of electrons.
The smallest possible dot of information.
Capacitor
Temporarily collects electrical signal
Matrix
The layout of cells in rows and columns containing pixels/voxels. The larger the matrix, the better the resolution.
Hounsfeld Units/CT Numbers
a quantitive scale for describing radio density calculated by comparing the linear attenuation coefficient of each pixel
Dot/matrix image
A 2 dimensional patterned array, used to represent characters, symbols and images
LP/mm - Line Pairs per Millimeter
The measure of image sharpness. The ability of the human eye to see the number of high contrast pairs of lines appearing in the space of a single mm.
Pixel
the smallest component of a picture, 2 dimensional - length x width.
Each pixel corresponds to a shade of gray.
Voxel
3 dimensional, volume element (CT scan) each voxel corresponds to a shade of gray representing a volume within a patient.
Dynamic Range/Bit Range/Bit Depth/Pixel Depth
The range of gray shades that can be assigned to a pixel, the higher the range, the more gray shades available to display the image.
scintillation
xray photons are converted to light within the flat panel detector by a layer of cesium iodide amorphous silicon
The divergent properties of light
Spreads out across all directions.
The goal of ALL digital imaging systems is to convert X-ray energy into electron energy, Why?
Because electrons can be counted, either individually or as a sum negative charge. Counting something allows you to assign it a digital value. Computer can store info held as numbers(digital info held as binary code), but they cannot store analog information.
Digitizing
The process of converting colors, shades, and shapes held as analog info into numbers
Signal
Wanted anatomical information
- the voltage from the CCD/TFT
Noise
Unwanted electrical interference
- the voltage supplying the CCD/TFT creates electron noise
Noise interferes with
The purity of the voltage signal which occurs from therapy exposure
Signal to noise ratio
A high signal to noise ratio indicates little noise
A low signal to noise ratio indicates lots of noise
Quantum mottle
Noise
Too few X-rays in the beam to adequately form an image
Structure mottle
Uneven distribution and/or size of phosphor crystals
Monitors
DAC devices
Contrast resolution
The ability to distinguish adjacent structures that have similar densities.
Rated in Bit depth
Aka pixel depth/bit range/dynamic range
Latitude
Margin for technique error
More grays =
Greater latitude
Windowing
Allows the radiographer to alter the image contrast and density. Achieved by altering window width or window level
Window width
Changes the range of gray tones
Increasing = longer scale of contrast (low contrast)
Decreasing = shorter scale of contrast (high contrast)
Window level
Adjusts the overall optical density (also referred to brightness)
Raising = increase in overall density
Lowering = decrease in overall density
Dose creep
Overexposing the patient only to let auto rescaling take care of making the image optimal
(Bad)
Look up table (LUT)
A table of predetermined luminance values depending on anatomy.
Controls automatic rescaling
Results in image having appropriate brightness and contrast
Veil glare
Glares a great distraction to the eye
Shuttering should never be used
To mask poor collimation
Background removal or shutter border
Underexposed borders around the collimation edges allow excell light to enter eyes
