image manipulation and compression Flashcards
what is a quantum sink
a material/ substance that is very effective at absorbing x-rays
what happens when an XR photon interacts with quantum sink
transfers energy to the sink
- absorption allows us to create detailed images of internal structures of body
example of contrast agents/ contrast media
quantum sinks
quanta
refers to discrete packets/ units in which energy is quantized
- quanta = ‘how much’
what does an image histogram represent
- graphical representation of distribution of pixel intensities in image
- visual summary of tonal range and contrast of image
what does the x-axis of histogram represent
pixel intensities
what does the y-axis of histogram represent
frequency/ number of pixels at each intensity level
how do you construct an image histogram
- convert image to greyscale
- count pixel intensities
- plot the histogram
value of 0 represents
black (in 8-bit greyscale image)
value of 255 represents
white ( in 8-bit greyscale image)
what does a high peak in histogram represent
peaks in histogram indicate intensity levels that are more dominant in image
= significant features/ objects
low/ flat regions in histogram
indicate low frequency/ absence of pixels at certain intensity levels
= uniform background/ low contrast
saturated regions
- shows spikes at extreme ends (0 or 255) - could suggest image full black/ white
= underexposed/ overexposed areas
where does histogram have to be placed for conventional radiography
within latitude of the film
where does histogram have to be placed for digital radiography
- placed adjacent to image display
- provides real time information about distribution of pixel intensities in image
how do we know if detector has received sufficient exposure?
typically done by evaluating image’s overall brightness + contrast
- consider histogram of pixel intensities
what are signals
- amount of transmitted beam we receive at image receptor –> converted into our image
- quanta of photons reaching detector
how can the amount of signal the receptor receives be changed
by changing exposure factors
which type of DR detector is more prone to loss of signal
- iDR
- more prone to light scattering = loss of spatial resolution
- light attenuation - conversion of light through scintillator material = loss
- QDE = converting XR quanta to light (QDE) can affect overall signal:noise
what are exposure indices
- numerical values to quantify exposure received by digital detector
EI
value that represents radiation dose received by detector during image acquisition
- allows for monitoring and maintaining consistent image quality
DI
- deviation index compares actual exposure level of acquired image to reference exposure level
- tells us if there is a deviation between two values
- DI = overexposure
- -ve DI = underexposure
SNR
- signal to noise ratio
- measure of quality + clarity of image
- high SNR = better image quality
what is a look up table (LUT)
- data structure commonly used in computer science + digital systems to map input values to corresponding output values
two main components of LUT
- input values
- output values
- when specific input value provided to LUT –> returns with corresponding output value associated with the input value
essential what does LUT provide
fast and efficient way to retrieve output values without having to perform complex calculations
what happens as a result of LUT to a graph
curve applied to linear lined graph
post-processing image manipulations
- windowing
- edge enhancement
- shuttering
- image orientation
- markers
- LUT changes
what does post processing generally do to image quality
degrades
how does windowing affect image quality
- affects contrast of image
- alters brightness
- does not affect density
steep/ shallow line on curve
contrast
position of curve to pixel data
brightness
what is image compression
- minimising file size of image file without degrading quality of image to unnaceptable level
what are bitmaps
- common type of digital representation
- composed of individual grid pixels where each pixel represents a specific colour
- generally large files
RLE
run length encoding
what is RLE
- compression technique used to reduce size of data
eg
AAAABCCD
= 4AB2CD
LZW compression
lemel ziv welch compression
what is a LZW compression
replaces frequently occuring patterns with shorter codes = reduces size of data
- lossy
two types of compression
- lossy
- lossless
RAID
redundant array of inexpensive disks
pros and cons of RAID
- disk failure redundancy
- massive data space
- expensive
- can still fail
