Chapter 9 Flashcards
What determines the overall quality of an image?
Visibility of anatomic structures and the accuracy of their structural lines (sharpness)
Brightness and contrast are used to describe an images:
Visibility
How is image sharpness acheived?
Maximizing spatial resolution and minimizing distortion
How is visibility acheived?
Properly balancing brightness and contrast
What is brightness?
The amount of luminance of a display monitor
Brightness can easily be adjusted in digtial imaging because the images are formed by:
Numerical data
How is brightness adjusted in digital imaging?
Adjusting technique or adjusting window during post processing
Why should a radiographer repeat the image if it is way too dark or too bright?
Windowing can correct brightness but it cannot correct over or under penetrated
What does a receptor need to properly utilize as much of the remnant beam as possible?
Wide dynamic range
What should the radiographer evaluate before sending an image to PACS?
Proper exposure, not just brightness
What is saturation?
Receptor is over exposed, resulting in an image that is burnt out and cannot be corrected with windowing
What causes an image to look grainy?
Receptor is underexposed
What are exposure indicators?
Numeric values that indicate the amount of radiation exposure received
What is image contrast?
The exhibited difference in brightness levels within an image
What causes an image to have a washed out appearance?
Too little contrast
What causes an image to have a black and white appearance
High contrast
When do we want high contrast vs low contrast?
High - bony structures
Low - Soft tissue
What is radiographic contrast?
Combined results of dynamic range, radiation quality, and the characteristics of tissues being imaged
What is subject contrast?
Refers to the absorption characteristics of the tissues being imaged
What tissue characteristics affect contrast?
Thickness, density, and effective atomic number
How does high radiation quality alter subject contrast?
The stronger the beam, the more likely it is to scatter or pass through the body
Less chance of absorption results in fewer brightness differences - lower contrast
How does high absorption affect subject contrast?
More brightness differences in the image, but can also degrade image quality of IR is not properly exposed
Contrast is a complex image attribute, it is more _______ than brightness
Subjective
What is grayscale?
The number of different shades of gray diplayed
What is used to describe an imaging systems ability to differentiate between small structures that attenuate radiation similarly?
Contrast Resolution
What is high contrast resolution?
Able to differentiate between small, similar structures
Benefit of increased pixel depth?
Provides a larger amount of grays = higher contrast resolution
How is contrast adjusted once an image is processed?
Adjusting window width
What is spatial resolution?
Used to evaluate the accuracy of anatomic structural lines
How does pixel size impact spatial resolution?
The larger the pixels, the less spatial resolution
Pixel size formula:
FOV/matrix size
If displayed FOV is increased for a fixed matrix size, then pixel size is:
Increased
If matrix size is increased for a fixed FOV, then pixel size is:
Decreased
The relationship between FOV and matrix size is:
Inversely proportional
Resolution in digital imaging is limited by pixel size and:
spatial frequency
What is spatial frequency?
Unit of line pairs per millimeter (lp/mm)
Relationship between objects and spatial frequency?
Smaller objects have greater spatial frequency
Larger objects have lower spatial frequency
What makes up a line pair?
A line and a space; a white and black line
Relationship between spatial frequency and resolution?
Directly proportional - the more line pairs, the greater the resolution
What is MTF?
Modulation Transfer Function - measure of an imaging systems ability to display the contrast of anatomic objects in varying size
What are the values for MTE?
0 to 1
0 - no difference in brightness
1- maximum difference in brightness
What is DQE?
Detective Quantum Efficiency - measurement of how efficiently an image receptor is in converting x-ray exposure it receives into a quality radiographic image
What does 100% DQE represent?
IR would be able to convert all of the radiation it receives into an image with zero information lost
What is distortion?
The misrepresentation of the anatomic part in imaging
What is size distortion?
Refers to the increased size of the anatomic structure
What has a strong effect on size distortion?
SID
OID
Increased OID, results in ______ magnification
Increased
Increased SID, results in ________ magnification
Decreased
What are the two forms of shape distortion?
Elongation and foreshortening
Because digital receptors can detect lower levels of radiation, they are _____ sensitive to scatter
More
What is quantum noise (mottle)?
Caused by too little radiation reaching the IR - gives a grainy appearance
Method of describing the strength of radiation exposure compared with the amount of noise present in a digital image
Signal-to-noise Ratio (SNR)
What is signal?
Amount of radiation exposure captured by the IR
What causes noise?
Not enough radiation hits the IR
How can noise be decreased?
Increasing signal, however, this also increases patient dose
What is the method of describing contrast resoltuion compared with the amont of noise present in an image?
Contrast-to-noise ratio (CTN)
How does noise affect CNR and SNR?
Negatively
What are artifacts?
Unwanted darkness or brightness level
