Image quality Flashcards
What are the 5 forms of unsharpness and what is it caused by
geometric- size of focal spot
absorption- alignment and shape of object
motion- motion of object and equipment
detector- detector characteristics e.g pixel spacing
magnification- distance of object from detector
Define total unsharpness
function of all forms of unsharpness (geometric, absorption, motion, detector, magnification)
define resolution
ability of system to resolve 2 separate objects
define resolving power
minimum distance between 2 objects that can be resolved
define penumbra
when the electron beam impacts the target over an area causing the edge of the object to be blurred
define noise
random variation in signal due to number of x-rays incident at each part of the detector (film, digital etc)
how does noise affect an image
reduced the visibility of low contrast regions within the body esp small areas
what is the relation between bit image and colours?
the number of colours is 2 to the power of the bit image
e.g
2 colours = 1 bit image
4 colours = 2 bit image
8 colours = 3 bit image (2^3)
What is a median filter?
filtering technique used for noise removal from images and signals by getting rid of pixel overflow
Radiographic tone scale can be optimized for visual perception by mapping equal log-exposure difference in the transmitted radiation field to equal discriminable brightness difference in the displayed image.
A high contrast radiograph will appear mostly black and white with few shades of gray in between. This is called short scale contrast. A low contrast radiograph will display many shades of gray and few blacks and whites
define binning interns of digital post processing
Binning is a technique by which signals arriving from adjacent physical elements of an electronic detector are combined to produce a larger pixel/voxel.
This increases the signal to noise ratio to provide better contrast resolution, with the trade-off being reduced spatial resolution.
( the process of combining adjacent pixels throughout an image, by summing or averaging their values, during or after readout.)
What is geometric distortion
warping” of the image that distorts the spatial relationship among objects in the image
caused by electric field interfering with electron travel
how can you manipulate the x-ray beam for optimisation of image (3 main things)
- increase tube current = more e-
- increase exposure time = more e-
- increase mAs = more e-
- more e- = more x-rays
- increase kV so
- higher e- energy
- higher max energy of spectrum (kVp)
- higher mean energy of spectrum
- more penetration
- increase filtration so
- more absorption of low energy x-rays
- higher mean energy of spectrum
- more penetration
(shape of spectrum is altered)
How can you adjust contrast to optimise the image?
high contrast object = High noise, low dose
low contrast object = low noise, high dose
How might patient factors require you to adjust for optimisation of image production
e.g reduce exposure factors (time) for kids
or
increase exposure factors for a larger adults jaw
