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In both photostimulable phosphor (PSP) and flat-panel detector systems (FPD), after the x-ray photons have been converted into ___________, these signals are available for processing and manipulation.
electrical
Preproccessing
Takes place in the computer where the algorithms determine the image histogram
Postprocessing
Done by the technologist through various user functions
In image formation, using values at the extreme high and low ends of the exposure range would result in a........
low-density resolution
*To avoid this, exposure data recognition processes only the optimal density exposure range.
Data recognition program searches for anatomy recorded on the imaging plate such as
1. Finding collimator edges
2. Eliminating scatter outside collimation
Information within the collimated area is the signal used for image data. This is the source for a __________-____________ exposure data indicator.
vendor-specific
Failure of the system to find the collimation edges can result in.......
incorrect data collection
Equally important is centering anatomy to the center of the imaging plate. Ensures that appropriate recorded densities are located. Failure to do so could result in an image that is..............
too bright or too dark
Histogram
A graphical representation of exposure values collected from the imaging plate
The value of each tone is represented on the....
horizontal axis (of histogram)
The number of pixels in each tone is represented on the.....
vertical axis (of histogram)
Values to the left are _______; as tones vary towards the right, they get ___________. Middle=medium tones
darker;brighter (histogram)
A dark image will show the majority of its data on the?
left side. (histogram)
a light image will show the majority of its data on the?
right side (histogram)
Image Sampling
(order of)
1. Plate is scanned.
2. Image location and orientation are determined.
3. Size of the signal is determined.
4. Value is placed on each pixel.
5. A histogram is generated from the image data
Histogram is generated that allows system to find useful signal by locating the ___________ (S1) and ____________ (S2) signal within the anatomic regions of interest in the image.
minimum;maximum
Histogram identifies all densities on the imaging plate in the form of a graph:
X-axis is related to ____________ ____ ___________.
Y-axis displays the ___________ ___ ___________ for each exposure
amount of exposure; number of pixels
Histogram:
Graphic representation appears as a series of ___________ ____ __________ and has a pattern that varies for each body part.
peaks and valleys
Low energy (kVp) gives a _________ histogram.
wider
High energy (kVp) gives a _______ histogram.
narrow
A Histogram shows...
the distribution of pixel values for any given exposure.
For example:
Pixels have a value of 1, 2, 3, and 4 for a specific exposure.
Histogram shows the frequency of each of those values and actual number of values.
Histogram sets the minimum (S1) and maximum (S2) "useful" pixel values.
*Histogram Analysis
Analysis is complex.
Shape of the histogram stays fairly constant for each part exposed (anatomy specific).
Histogram analysis: Image correction
Raw data used to form the histogram are compared with a "normal" histogram of the same body part by the computer.
The Nyquist Theorem
when sampling a signal, the sampling frequency must be greater than twice the bandwidth of the input signal so that the reconstruction of the original image will be as close to the original signal as possible.
Many scientists have revised the Nyquist Theorem. It is often referred to simply as the _____________ _______________.
Sampling theorem
In the Nyquist theorem, at least _______ the number of pixels needed to form the image must be ____________.
twice;sampled
In the Nyquist theorem, if too few pixels are sampled, the result is a .........
lack of resolution
Does oversampling result in additional information according to the Nyquist theorem?
No
*The number of conversions in PSP (Nyquist Theorem)
1. electron to light
2. light to digital information
3. digital to analog
*results in lack of detail
Some light is lost during the light-to-digital conversion because of the........
spreading of light photons. (Nyquist theorem)
The longer the electrons are stored, the more energy they gain or lose?
lose
Aliasing occurs in digital imaging when.....
the spatial frequency is greater than the Nyquist frequency.
When sampling occurs less than twice per cycle, what happens?
Information is lost
Fluctuating signal is produced
also know as Foldover or biasing
Aliasing results in a ________ effect.
moire
Automatic Rescaling
Images are produced that have uniform brightness and contrast regardless of the amount of exposure
Problems occur with rescaling:
Too little exposure results in quantum mottle.
Too much exposure results in a loss of contrast and loss of distinct edges because of detector saturation.
Look-Up Table
All pixels are changed to a new gray value.
Image will have appropriate appearance in brightness and contrast.
(LUT) Contrast can be increased or decreased by changing the _________ of the graph.
slope
(LUT) Brightness (density) can be increased or decreased by moving the line.....
up or down the y-axis
Digital systems have greater ______________ _____________ than film/screen imaging.
dynamic range (latitude)
Spatial frequency resolution
Refers to detail or sharpness of the image
Edge Enhancement
Smooths images by averaging pixels
Amplification, also known as high-pass filtering
results in an increase of contrast and edge enhancement
Suppression of frequencies (masking)
, can result in loss of small details.
This technique is useful for enhancing large structures, such as organs and soft tissues, but can be noisy.
Smoothing (low-pass filtering)
Results from averaging of the frequency of each pixel with surrounding pixel values to remove high-frequency noise
Result is a reduction of noise and contrast
Low-pass filtering is useful for viewing small structures such as fine bone tissuesImage Manipulation
Image Manipulation
-Window level and window width
-Background removal or shutter
-Image orientation
-Image stitching
-Image annotation
-Magnification
Window level
Controls how light or dark the image is
Window width
Controls the ratio of black to white, or contrast
Background Removal or Shutter