8. Digital Image Characteristics & Image Processing Flashcards
what are matrices made of
rows and columns
what is the relationship between pixel size and resolution
inverse
greater resolution = smaller pixel size
each pixel has how many colours
the colour is calculated from what
one colour
its the average of the scale in it
what is the relationship between the pixel size and the blurring
inverse
larger pixel size = more blurring
what is a matrix
number of pixels in an image
what is the field of view
what is it measured in
measurement in mm
describes the physical size of the image
digital image quality is improved with a ___ matrix size that includes a ___ number of ___ pixels
larger matrix
greater number of smaller pixels
as pixels get smaller there ___ rows and columns of pixels making the image more ___
more rows and columns
makes image more clear
pixel size equation
pixel size = field of view/matrix size
is there a unit for matrix size, if so what is it
no unit as its the number of pixels
a smaller field of view with the same matrix size has what effect on the pixel size and resolution and amount of data stored
smaller pixels
higher spatial resolution
same amount of data to store
what do pixels impact
what do matrix size impact
pixels = spatial resolution
matrix = amount of data to store/transfer
what is the advantage of larger matrix size
clearer images
what are the 3 disadvantages of having a larger matrix size
processing time
network transmission time
digital storage space
what is the pixel bit depth
what is it determined by
number of bits determines the number of shades of gray that can be displayed
numerical value assigned to a pixel is determined by the relative attenuation of x-rays passing through the volume of tissue
what is a bit and what does it do
the more bits the ___ shades of greys and colors you can store
the 0s and 1s that stores binary info
more
what is contrast
difference in density or difference in degree of grayness between areas of the radiograph image
what is the resolution
clarity and average of the grey scale
what is noise
scatter
what is distortion, what are its 2 types
elongation and foreshortening
what is the pixel bit depth and contrast resolution relationship
what is the logic for it
greater the pixel pit depth the more precise the digitization of the analog signal and the greater number of shades of grey available for image display
increasing the number of shades of grey available to display on a digital image, improving the contrast resolution
what does it mean if there is more contrast resolution in terms of color and greys
more contrast = more colours and greys
the greater the number of pixels in the image matrix what happens to the pixel size
the smaller the pixel size
what is the pixel density
greater number of pixels per unit area
what is the pixel pitch
distance measured from the centre of a pixel to an adjacent pixel
what is the relationship between the pixel density and pitch and spatial resolution
increasing density and decreasing pitch increases the resolution
what is the spatial resolution limited to and what kind of image improves it
limited by size of pixel
image composed of smaller sized pixels has improved spatial resolution
what needs to happen to the pixel density and pitch increase the spatial resolution
increasing pixel density and decreasing pixel pitch
spatial frequency is defined by what
unit of line pairs per millimeters (lp/mm)
small objects have ___ spatial frequency
higher
increasing the number of lp/mm resolved in the imaging system results in ___ spatial resolution
improved
what is the modulation transfer function
measure of the imaging systems ability to display the contrast of anatomic objects varying in size
what is the range of values for the modulation transfer frequency
an imaging system that has a high MTF has what effect on the visibility
0-1.0
high MTF = improved anatomic detail visibility
what is the equation for the MTF
MTF = (max intensity - min intensity)/(max intensity + min intensity)
variation in the MTF comes from what
MTF of 1 is easiest to achieve when
Variation comes from variation in intensity exiting tissue
1 is easiest to achieve when objects have low spatial frequency
point spread function is what
basic measure of resolution of an imaging system
image produced from a single point stimulus to a detector
describes the blurring properties of an imaging system
digital image processing occurs after what and before what
after extraction
before display and diagnostic interpretation
what is a histogram analysis and what 2 things is it used for
image processing technique
used to identify the edges of an image
used to assess raw data prior to image display
what is histogram analysis error
entire imaging plate is scanned to extract the image from the photostimulable phosphor
if at least not 3 edges are not identified an histogram analysis error could occur
histogram analysis errors are less likely to occur with DR IRs or CR IRs
why
DR IRs
because the image data are extracted only from the exposed detectors
what is the exposure indicator
numeric value indicating the level of radiation exposure to the digital IR
what is the deviation index
difference between the desired target exposure to the IR and the actual exposure to the IR
what does >0 and <0 mean for the deviation index
<0 = decreased exposure to the IR >0 = increased exposure to the IR
what are the 3 limitations of the deviation index
collimation
centering
noise
all impacts image quality
what are look up tables and what do they do in terms of contrast and pixel distribution
methods for altering an original pixel values to change the display and contrast of an image
Takes low contrast images and gives more definition to lights and darks and moves the greys in between them
Changes pixel distribution from linear to non-linear
digital IRs have ___ exposure and a ___ dynamic range
linear exposure
wide dynamic range
what are the 4 categories of digital image artefacts
detectors
image data extraction to the ADC
during the ADC
during signal processing by the computer before display
what are the image plate factors that can cause image artefacts
Stains in Imaging plate, particulates, scratches, cracks or fogging of plate
what are the data extraction factors that can cause image artefacts
Artifacts occurring during data extraction on CR systems = laser optics, transport mechanisms, light guide, and laser sampling and stationary grid frequencies issues
what is soft copy viewing
when an image is displayed on a workstation instead of film or other physical medium
display monitors can be of what 2 types
cathode ray tubes
liquid crystal displays
what is the LCD display monitor made of
LCD light passes through liquid crystal and displays image on glass base plate. Additional components include a source for the electrical signal and light
Flat faceplate and dimensions are thinner
Plasma monitors are very similar to LCD, instead of liquid crystal layer, there is a thin layer of pixel
what is the CRT display monitor made of
CRT = accelerating and focusing e- strike fluorescent screen. Image scanned on screen in lines so # of lines affects image quality. Recommended that CRT monitors scan at least 525 lines per 1/30 of a second. typically has a curved faceplate, and its dimensions are thicker
what is the relationship between the window level and the image brightness
direct proportional relationship
increasing window level increases the image brightness
what is the relationship between the window width and the image brightness
narrow decreased window width displays higher radiograph contrast
what does window level change
Window level sets midpoint of the range of brightness visible in the image. Changing it incr/decreases image brightness in the entire range.
what does window width change
the contrast of the image
what is subtraction in postprocessing
Remove superimposed image which allows better visualisation of anatomical region of interest
Edge enhancement improves visibility of small high contrast structures (eg can see blood vessels high contrast)
what is blurring in postprocessing
blurring by digital processing fixed by digital subtraction so results in enhanced visibility of detail
what is smoothing in postprocessing
suppresses image noise/quantum noise
spatial resolution is degraded
