Ch. 3 Digital radiographic image processing and manipulation Flashcards
For PSP and FPD, once x-ray photons are converted into electrical signals, the signals are available for…
processing and manipulation
What do algorithms determine at the control center?
the image histogram
The PSP imaging plate records a….
wide range of x-ray exposures
If the entire range of exposures were digitized in a PSP system, what would result? Why?
a low density resolution would result because values at the extremely high and low ends of the exposure range would also be digitized
The data recognition program in PSP searches for…
anatomy recorded on the imaging plate by finding the collimation edges and then eliminates scatter outside the collimation
If the data recognition program in PSP cannot find the collimation edges, what can happen?
incorrect data collection and images can be too bright or too dark
In addition to collimation, what ensures that the appropriate recorded densities will be located?
putting the anatomy in the center of the imaging plate
What could happen if the part isn’t centered in the middle of the plate?
The image could be too dark or too light.
Define histogram
The data within the collimated area produce a graphic representation of the optimal densities called a histogram and the value of each tone is represented (horizontal axis), as is the number of pixels in each tone (vertical axis).
What do the values at the left, right, and middle of the histogram represent?
black, light, and medium areas, respectively
If an image is too dark, where will the information be on the histogram?
to the left
What is image sampling?
the plate is scanned and the image’s location is determined. The size of the signal is then determined, and a value is placed on each pixel. A histogram is then generated using the image data.
When a histogram is generated, what does it allow the system to find?
the useful signal by locating the minimum (S1) and maximum (S2) signal within the anatomic regions of interest on the image and then plots the intensities of the signal on a histogram.
On a histogram, what do the x-axis and y-axis represent?
x-axis is the amount of exposure read, and the y-axis is the number of pixels for each exposure.
How is kVp represented on a histogram?
low energy (low kVp) gives a wider histogram and high energyy (high kVp) gives a narrower histogram
A histogram shows the …
distribution of pixel values for any given exposure
A histogram shows the ____ of each of the values, as well as the _____ ____ of values.
frequency; actual number
the shape of the historgram is _____ specific.
anatomy (it is constant for each part exposed)
Why is it important to choose the correct anatomic region on the menu before processing the image plate?
Because a histogram for a chest looks very different from a histogram of a knee or foot and the raw data used to for the histogram are compared to other normal histograms of the same part.
Who started the theory of changing analog to digital over telephone lines? what year?
Harry Nyquist in 1928
Who proved Nyquist’s theory ? what year?
Claude Shannon in 1948
What is Nyquist’s theory often referred to?
the sampling theorem
What does the Nyquist theorem state?
That you need twice the number of pixels of the original signal to be as close to the original signal as possible for a good sample.
What happens if you don’t have at least twice the number of pixels to sample?
the result will be a lack of resolution.
Does oversampling during a Nyquist theorem give you more information?
NO because the human eye can only perceive so much information
Why is there always a loss of detail during PSP imaging?
Because there are a number of conversions that occur-electrons to light, light to digital, and digital to analog-that light photons do not travel in one direction so some light will be lost during the light to digital conversion because light photons spread out.
The longer an image is stored…
the more information is lost.
When a fluctuating signal is produced, frequencies above the Nyquist frequency cause…
aliasing
What happens during aliasing?
a wraparound image is produced, which appears as 2 superimposed images that are slightly out of alignment, resulting in a moire’ effect, which can also happen with grid errors.
What is the critical frequency/
When a sampled frequency is exactly at the Nyquist frequency and a zero amplitude signal results. (it results from frequency phase shifts, causing aliasing of the signal)
What does automatic rescaling mean/
that images are produced with uniform density and contrast, regardless of the amount of exposure.
When do problems occur with automatic rescaling?
when too little exposure is used, resulting in quantum mottle, or when too much exposure is used, resulting in loss of contrast and loss of distinct edges because of increased scatter
Rescaling is no substitute for..
appropriate technical factors
What is a LUT?
a histogram of the luminance values derived during imaging acquisition
Why do we use a LUT?
as a reference to evaluate the raw information and correct the luminance values.
What is a mapping function in which all pixels (each with its own specific gray value) are changed to a new gray value.
a LUT
There is a LUT for every ____ ____.
atomic part
How is a LUT graphed?
by plotting the original values ranging from 0 to 255 on the horizontal axis and the new values (also 0-255) on the vertical axis.
How is contrast increased or decreased in a LUT?
by changing the slope of the graph
How is the brightness/density increased or decreased in a LUT?
by moving the line up or down the y-axis.
Digital systems have a greater ___ ____ than film/screen imaging.
dynamic range
Why does a digital image appear linear when graphed?
because all shades of gray are visible, giving the image a very wide latitude.
Why do digital systems use various contrast enhancement parameters?
Because if all shades were left in the image, the contrast would be so low as to make adjacent densities difficult to differentiate.
What is contrast manipulation?
it involves converting the digital input data to an image with appropriate brightness and contrast using contrast enhancement parameters.
In contrast manipulation, how is image contrast controlled?
by using a parameter that changes the steepness of the exposure gradient.
In contrast manipulation, how is the brightness controlled?
it can be varied at the toe and shoulder of the curve to remove the extremely low values and the extremely high values.
Detail or sharpness is referred to as…
spatial frequency resolution
In film/screen how is sharpness controlled?
focal spot size, screen and/or film speed, and OID
What affect image sharpness of both digital and film/screen?
focal spot and OID
How can digital images be manipulated more for their sharpness?
by adjusting processing parameters
What happens after the signal is obtained for each pixel?
the signals are averaged to shorten processing time and storage. The more pixels involved in the averaging, the smoother the image appears. The signal strength of one pixel is averaged with the strength of adjacent pixels (neighborhood pixels)
When does edge enhancement occur?
When fewer pixels in the neighborhood are included in the signal average.
During edge enhancement, the smaller the neighborhood…
the greater the enhancement
When can frequencies be amplified and other frequencies suppressed?
When the frequencies of areas of interest are known.
What is high pass filtering?
When the frequencies of areas of interest are known and frequencies can be amplified and other frequencies suppressed.
What does high pass filtering do?
increases contrast and edge enhancement.
What is masking?
suppressing frequencies
What can masking do?
it can result in the loss of small details
When is high pass filtering useful?
for enhancing large structures such as organs and soft tissues
What is the downfall of high pass filtering?
it can be noisy
Other than high pass filtering, what is another type of spatial frequency filtering?
smoothing
What is smoothing also known as?
low pass filtering
How does smoothing work?
by averaging each pixel’s frequency with surrounding pixel values to remove high frequency noise.
What is the result of smoothing?
a reduction of noise and contrast
What is low pass filtering (smoothing) useful for?
viewing small structures such as fine bone tissues
What are the most common image processing parameters used for?
brightness and contrast
What image processing parameter is used to manipulate brightness?
window level
What image processing parameter is used to manipulate contrast?
window width
The higher the window level, the ____ the image.
darker
The wider the window width, the ____ the ___.
lower, contrast
Using a mouse to control width and contrast, one direction controls ____ and the other direction controls ____.
brightness; contrast
What is veil glare?
anytime a radiographic image is viewed, whether film/screen or digital, unexposed borders around the collimation edges allow excess light to enter the eye.
What is the result of veil glare?
the excess light causes oversensitization of a chemical within the eye called rhodopsin and results in temporary white light blindness and is a great distraction that interferes with image reception by the eye.
How do they try to fix veil glare in film/screen?
with black cardboard glare masks or special automatic collimation view boxes
How doe they try to fix veil glare in digital?
with automatic shuttering to blacken out the white collimation borders
What is background removal?
removing the white unexposed borders and results in an overall smaller number of pixels and reduces the amount of information to be stored.
What is image stitching?
when anatomy or the area of interest is too large to fit on one cassette, multiple images can be stitched together using specialized software programs.
What does image stitching eliminate?
the need for large 36 inch cassettes that were used in film/screen
What does image annotation allow for?
a selection of preset terms and/or manual text input and can be particularly useful when such additional information is necessary.
How many types of magnification manipulation are there?
2; one for magnifying a specific area of the radiograph and one for zooming the entire radiograph
