Digital imaging week 3 cont.II Flashcards
Different frequencies built into the system to allow for more spatial resolution
sampling frequency
to match the optical and electronic resolution of system, there should be a sufficient amount of _ to represent the original signal presented
samples per horizontal line
Sampling frequency is expressed in
pixels/mm
the major factor that limits the recorded detail for indirect digital systems is the
size of the detector element
The major factor that limits the recorded detail for direct systems is the
size of the detector element
what uses a fine laser beam of variable spot sizes down to 50 um (micrometers)
laser scanners
a matrix of data that manipulates the values of gray levels, converting an image input value to a different output value
Grayscale rendition or look up table (LUT)
An image processing, lookup tables are often called _
LUT’s
LUT’s link _ to output values
index #
One common LUT is called the
colormap or palette
The color map or palette is used to determine the _
colors and intensity values with which a particular image will be displayed
what is a graphic display of the distribution of pixel values
Histogram
Each image has its own histogram and is evaluated to determine the
adequacy of the image receptor exposure
what is used to identify the edges of the image and assess the raw data prior to image display
Histogram
uses processing algorithms and compares it with prestablished histogram for the part
Histogram analysis
what is generated with the pixel values on the x-axis and the # of pixels on the y-axis
histogram graph
the peeks and valleys represent the
subject contrast
the voi is identified with S1 representing the _ and S2 representing the
min. useful signal
max. useful signal
computer system automatically adjusts the brightness by comparing the histogram with the LUT
automatic rescaling
Overexposures of _ and underexposures of _ can be rescaled without losing image quality
12%
6%
For production of an optimal histogram you need a correct choose center collimate control cover
technique selection correct body part CR to VOI closely, leaving min background scatter at least 3% of IR
grid error that occurs with digital image receptor systems when the grid lines are captured and scanned parallel to the scan lines in the IP reader
Moire Effect
Digital imaging requires that the spatial resolution frequency signal be sampled twice from each cycle, this is the
Nyquist criterion
what occurs when the Nyquist criterion is violated; incoming data is sampled less than twice per cycle
Aliasing
Aliasing appears as a
Moire pattern
for CR the cassette used is a
Photostimulable storage phosphor imaging plate
PSP or IP
CR is also called
indirect acquistion
Two part process
- Scintillator
2. Photodetector
converts photons to light
scintillator
converts light into an electrical signal
photodetector (charged couple device)
Layers of an IP
Protective layer Phosphor layer conductor layer support layer light shield layer backing layer
what are the layer's function Protective layer- Phosphor layer- conductor layer- support layer- backing layer-
insulates from trauma
- active component of IP
- grounds plate to avoid static
- support base for other layers
- prevents light from erasing data
- attaches to cassette
most common IP phosphor is
barium flourohalide bromides with europium activators
a min. of _ of the IR must be exposed for an accurate exposure index to be obtained
3%
the latent image will lose _ of its energy in _ hrs.
2%
8 hrs.
latent image is processed by loading the cassette into an image reader device where the plate is scanned by a
helium-neon laser beam
the beam scans the _ and emits the latent image in the form of _, which are then converted into an electrical signal, then converted to a _ for that level of luminescence
phosphors
light photons
digital value
reading the IP requires a finely focused red laser beam hat frees the trapped electrons, allowing them to return to a lower energy state called
photostimulated luminescence (PSL)
4 CR artifacts
- Phantom images
- Dropout artifacts
- Quantum
- heat blur
Phantom images may appear from
incomplete IP erasure
reduction in resolution, result of dust
dropout artifacts
caused from inadequate exposure (mAs)
Quantum
IR exposure to extreme heat
heat blur
4 factors that produce low contrast and are prone to cause IP fog
- high kV
- No grid or inadequate grid efficiency
- Inadequate beam limitation
- Increased part size or tissue density
DR is also called
direct acquisition
for a DR what is not needed
cassette
direct acquisition directly converts
photons to an electrical signal
DR uses
amorphus selenium or amorphus silicon
material lacking crystalline structure
Amorphous
CR’s advantages vs DR
less expensive
can be used portably
compatible w/existing radiographic tables
DR’s advantages
immediate readout
cassette free
can be re-exposed immediately
CR’s disadvantages
delayed readout
cassettes needed
plate must be erased before use
DR’s disadvantages
more expensive
not compatible w/existing equipment
what is the greatest advantage of digital imaging over film/screen
post-processing
