Unit 2A-Image Quality & Digital Image Pre/Post Processing Flashcards
Define the following term:
Resolution
The ability to distinguish the individual parts of an object or closely adjacent objects
Define the follow term
Spatial resolution
How small an object can be seen or how close to objects can be and still be recognized as two objects.
Define the follow the term
Contrast resolution
The ability to distinguish between differences in shades of gray (i.e fat or muscle)
T/F
Spatial resolution is an intrinsic property of an imaging system.
True
What are a few examples of intrinic charectristic properties of an imaging system?
- Matrix
- Pixel size
- Focal spot
Others inlude; thickness of detector, Noise suppression, Lazer reader thickness in CR systems, ect.
Define the following term:
Spatial domain
Two dimensions of a single image or three dimensions of avolumentric image
What is this describing?
The generated profile of an image is a measure of a grayscale as a function of position.
Line spread function
What can spatial resolution be measured as?
Full width at Half maximum
What is this describing?
The physical distance in mm between the two points where the line intensity has been reduced to half of the central (maximum value).
Full width at half maximum
Example of an exposure made thrugh a sheet of lead with a slit in it (refer to slideshow if confuzzeld)
How is spatial resolution measured physically and in what unit?
Obtained from a lead foil line pair template
-Unit: LP/mm
What is the relationship between the size of the resolved object and the spatial frequency?
size of the resolved object is inversely proportional to SF
What is the relationship between the smallest absolute size that can be reproduced to the spatial frequency?
Smallest absolute size that can be reproduced is inversely proportional to one-half the spatial frequency
What is the smallest object that can be resolved by an x ray machine with a spatial frequency of 3lp/mm?
MOS=1/2 (1/SF)
MOS=1/2(1/3)
Therefore; MOS=0.167
T/F
The use of large pixels would reduce the SR
True
T/F
Intrinsic sources of blur such as focal spot size can be overcome by reducing pixel size.
False
What modality has the best SR? The worst?
Best: CT
Worst: Mammography
As per SC35, at what frequency must SR of DR, CR and Fluoroscopic systems be tested?
Annually
As per SC 35, at what frequency must SR of CT systems be tested?
Quarterly
Define the following term
Contrast resolution/Low contrast resolution
How well the difference is rendered between an object and its surroundings
(Can you see where one shade of grey ends and another one starts?)
Define the following term:
Modulation
Change
What shades would be seen with 100% modulation?
Black and white
all shades
T/F
Modulation is always reduced as spatial frequencies increases.
True
What percentage of modulation would there be with uniform grey?
0% Modulation
In a processed image, can the modulation ever be 100%?
No :/
Define the following term
Modulation transfer frunction
The curve of modulation amplitude and the ability of the detector to retain the contrast (signal amplitude) of an object.
ratio of the recorded contrast of an image to the real object’s subject contrast – ability of a system to retain the contrast of an object
T/F
Modulation transfer function can also measure how much spatial frequency is transferred from the object to an image?
True
Refer to slideshow (slide 33); it makes more sense
If an image has one half the contrast of the real anatomy, what is the MTF?
MTF=0.5
If an image has 100% of the subject contrast, what is the MTF?
MTF=1
(no imaging system can do this)
T/F
As CR decreases, so does MTF
True
Which system has a higher spatial resolution?
B
T/F
As MTF increases, so does SF
False; As MTF increases, SF decreases
As per SC 35, at what frequency must contrast resolution of DR, CR and Fluoroscopic systems be tested?
Annually
As per SC35, at what frequency must contrat resolution of CT systems be tested?
Quarterly
What term is this describing
All corrections that are made to the raw digital image data due to physical flaws in acquisition that are inhernt in the elements and circuitry of the receptor or the processor
Acquisition processing to Digital image Pre-Processing
T/F
Detector elements can suffer from electronic faults in CR systems.
False; only in DR systems
What effect sould the Anode Heel effect have on feild uniformity?
Results in less x ray intensity reaching the anode end of the image receptor than the cathode end
T/F
The active matrix array in DR systems consits of a maximum 20 DELS
False; Consists of hundreds of detector elements (DELS)
T/F
The longer the wire, the greater the resistance
True
(Keep this in mind when considering DELS)
What is the purpose of a flat feild uniformity test?
To test the overall signal or brightness across the area of the imaging field
What is the purpose of fibre optic light guides in CR readers?
To direct the light emitted from the phosphor to the PMT.
What is done to fix the issue of light intensity variations in CR fibre optic light guides?
Fluctuations are detected and the signals are amplifed over the specific spots on the IR by hardware or software to make it look more uniform
What is this describing?
In DR, individual dels can drop completely out of the image due to malfunction that results in missing informatioin in the data
Del drop out
How is DEL drop out correcrted in the image?
A kernel averagese the pixels surrounding the malfunctioning component and fills this dead space in the image by interpolation
Why does DEL drop out occur?
Normal wear and tear of IRs over long persiods of time
T/F
Severe malfunctioning of DELs can be fixed by interpolation.
False; requires replacing the detector
T/F
If parts or rows of colums (of DELS) fail, algorithms can be configured to interpolate pixel values.
True
What is Segmentation error and what does it result from?
The Darkening of the entire image as as a result of the entire plate being treated as a single exposure
What kind of software makes the distinction between pitch black areas of background density and very dark areas in the image within the real anatomy?
Exposure Feild Recognition software
In a histogram on an x ray machine, what does the x and y axis represent?
x=Amount of exposure
y=Number of pixels
On a histogram, how is the graph read?
Scale of brightness, with the lighter shades on the left side on the graph, and the darker shades on the right side
(remember not to look at the graph anatomically lol)
Describe the process of histogram analysis:
- The technologist selects an algorithm, which tells the computer which historgram to use
- The comuter generates histogram from image and compares it with existing ideal histogram for the selected body part
What determines the range of the histogram data set that should be inluded in the displayed image?
VOI (values of interest) stored in the histogram models
What does (Smin) represent in a histogram?
The minimum useful grey shade value
What does (Smax) represent in a histogram?
The maximum useful grey shade value
How does the computer rescale values for brightness?
Computer finds the difference in the (Smean) values between the image histogram and ideal hisotgram, and makes this adjustment difference to all values in the VOI
How much can a computer rescale for an overexposure?
Overexposure of up to 500%
How much can a computer rescale for an underexposure?
Underexposure of as much as 60%
T/F
The computer is better at fixing brightness issues as opposed to contrast issues.
True
T/F
The steeper the gray scale (or short gray scale) curve, the higher the image contrast.
True
T/F
More shades of gray will produce a shallower slope
True
T/F
If you have a steep curve, you have less shades of grey.
True
What are the 6 causes of histogram analysis errors?
- Incorrect algorithm (body part selection)
- Off centered central ray
- Collimation
- Scatter radiation
- Not clearly defining area of interest (CR only)
- Multiple projections on one IR
(for more info, look on slides)
Histogram errors are less common in DR systems. Why?
In CR systems it processes the entire plate. In DR, it only processes pixel values, therefore, errors are less likely to occur
What is an exposure indicator?
EI are readings that denote the amount of radiation (quantity of photons) that struck the IR
