test 2 Intro and computed radiography Flashcards
analog computers
handle data composed of continuously varying electrical currents
digital computers
handle data composed of definite quantities of current
Analog to digital converter
ADC
needed to convert analog input into digital data for processing
converts analog signal into sequence of numbers
digital to analog converter
DAC
converts this info back into analog signals so it can be interpreted by analog display device
the computer operates in what system?
binary system
binary system
two symbol alphabet
bit
each binary number or a single binary digit
teleradiology
transfer of images and patient reports to remote sites
digital
an image constructed from numerical data
ex: CT, MRI
film digitizers
used to convert analog image into digital
disadvantages of conventional
image is permanent
increased pt. exposure
difficulty seeing structures within the same image
processing time, storage, etc
advantages of digital
can be obtained, processed, stored, etc in a more timely manner
can be manipulated without re exposing the pt.
dynamic range
the range of exposures a system can retain to create the visible image
refers to the number of shades of gray
greater the range greater the ratio
the greater the dynamic range, the better
contrast resolution
matrix
a combination of rows and columns
pixel
smallest component picture element
the larger the matrix, the better
the quality
increasing the number of pixels will
improve the quality of the image
quantization
assigning of a unique value to each pixel
AKA the value of each pixel
bit depth
the number of bits used to reproduce image gray levels
bit number is always expressed as the exponent of 2
a system that can display a greater number of shades of gray has better image quality
true or false
TRUE
voxel
each pixel value corresponds to a 3 dimensional volume of tissue
the voxel size depends on the thickness of the slice and the matrix size and field of view
In CT the numeric value of each pixel is called a
Hounsifeld unit (HU)
field of view
the diameter of image reconstruction
think of pieces of a puzzle
the more puzzle pieces the better the picture will be
spatial resolution
the degree of geometric sharpness or accuracy of structural lines actually recorded in the image
also called detail, definition, sharpness, or recorded detail
spatial resolution formula
spatial resolution = FOV/Matrix
4 steps of acquiring the image
- radiation exits the pt. and interacts with the IR
- photons absorbed by photostimulable phosphor in plate
- image formed within crystals to produce latent image
- IP is erased for future use
Imaging plate
thin sheet of plastic that records the image
5 layers of the imaging plate
- protective layer
- phosphor layer
- reflectivve layer
- conductive layer
- light shield layer
- protective layer
thin tough clear plastic that protects the phosphor layer
insulates imaging plate from handling trauma
- phosphor layer
active layer (where image is created)
holds photostimulable phosphor (PSP)
traps electrons during exposure
what is the phosphor used to create the picture
barium fluorohalide with europium
photostimulable luminescence
the phosphor emits light when exposed to xrays and then again when when exposed to another light source
releases light twice
europium acts as an activator
without this activator, there would be no
latent image
storage phosphor
the latent image is stored electrons
structured phosphors
needle phosphors lined up and packed together tightly
gives nice resolution
turbid phosphors
a phosphor layer with a random distribution of phosphor crystals
randomly scattered doesn’t give as good resolution
- reflective layer
sends light in a forward direction when released in cassette reader
may be black to reduce spread of light
helps to prevent blur
conductive layer
grounds the plate to eliminate electrostatic problems
color layer
some newer plates contain color layer to absorb light
support layer
base on which to coat other layers
gives the imaging plate some strength
light shielding layer
prevents light from erasing data on the imaging plate or leaking through the backing and decreasing spatial resolution
backing layer
soft polymer that protects the back of the cassette
where the barcode label is found
imaging plate is scanned with a laser beam and releases stored energy as visible light
TRUE
photomultiplier tube
collects, amplifies and converts light to electrical signal
how does an IP get erased
residual electrons are removed by intense light
plates should be erased
every day
plates should be left no more than
48 hours
CR imaging plate is equivalent to a
200 speed system and mAs must be adjusted because film screen was a 400 speed system
estimated life of an IP
10000 readings
image acquisition
- the remnant beam interacts with electron in the barium fluorohalide crystals
- this interaction gives energy to electron in the crystals
- this allows them to enter the conductive layer
- here they are trapped in an area of the crystal known as the phosphor center
- this trapped signal remains there for some time, but does begin to deteriorate
- some of the exposure still remains trapped even after you read it.
the reader
cassette fed into reader
removes imaging plate
scans with laser to release stored electrons
LASER
Light Amplification of Stimulate Emission of Radiation
creates and amplifies a narrow intense beam of light
scans the imaging plate
scans the plate in a raster pattern
beam shaping optics
keep the size, shape, speed and intensity constant
translation
as the imaging plate moves through the reader the laser scans across multiple times
the translation speed must be coordinated with the scan direction of the laser
beam deflector
moves the laser beam rapidly back and forth across the IP to stimulate the phosphors
light collection optics
direct the released phosphor energy to an optical filter and then to the photodetector
T or F
the smaller the laser beam, the better the resolution
True
T or F
the wider the laser beam the more speed
True
photomultiplier tubes and photodiodes
light detectors of choice for CR
photomultiplier amplifies light signal and sends it to a digitizer
pixel pitch
the space from teh center of a pixel to the center of the adjacent pixel
determines the pixel density (pixels per mm)
digitizing the signal
assigning the image a number
quantization
the value of each sample
sampling frequency
the frequency at which the analog sample is taken
how much data the laser is obtaining during a certain amount of time
T or F
as receptor size decreases sampling frequency increases
True
as sampling frequency increases, spatial resolution
increases
nyquist frequency
the highest spatial frequency that can be recorded by a digital detector
determined by the pixel density
nyquist frequency is half the number of pixels/mm
moire effect
occurs when the grid frequency is equal to he nyquist frequency
spatial resolution
the amount of detail present in any image
ex: the thinner the phosphor layer the higher the resolution
image acquisition
exposure
artifacts
imaging plate artifacts
quantum mottle
grain condition as a result of not enough mAs
grid frequency
refers to the number of grid lines per inch
the higher the frequency the finer the grid lines in the image therefore the less interference
grid ratio
the relationship between the height of the lead strips and the space between them
sampling rate
the number of bits of output it provides
the number of times a second that it can sample and digitize the input signal
shuttering
putting a black background around the collimation during postprocessing
DOES NOT TAKE THE PLACE OF COLLIMATION
exposure indicator
the amount of light given off by the IP is a result of the radiation exposure the plate has received
the light is converted to a signal that’s used to calculate the exposure index
Dose Area Product (DAP)
the volume of tissue irradiated (not just the dose)
used to monitor radiation output from radiographic and fluoroscopic imaging systems
factors that produce low contrast
no grid
inadequate grid efficiency
inadequate beam limitation
increased part size or tissue thickness
histogram
graphic representation of numerical tone values on an xray exposure
advantages of CR
compatible with existing film screen xray equipment
useful in mobile imaging
disadvantages of CR
amount of time necessary to process and image readout
more exposure needed to avoid quantum mottle
appropriate kVp used to achieve optimal images
binary notation
used to describe image size, image shades of gray, and image storage capacity
two systems available for digitizers
laser and CCD linear array
