CT Image Reconstruction Flashcards
what are some parameters of image quality
spatial resolution, image noise, image artifacts.
what is the radiation dose based on
effective usage of x-ray photons
what is FBP
fan beam reconstruction
what are the two types of reconstruction algorithms
analytical and iterative
what is analytical reconstruction
closed form and one step
what is iterative reconstruction
iteratively solve an optimization problem
how does CT attenutation work
as each thing goes through its a summation of the length and density of each block
forward projection
the 1 D map of the person or thing
back projection
is the 1-D maps over each other to make an image
projection slice theorem
Fourier transform of a parallel beam projection yields the Fourier coefficients of the image object along a straight line in 2-D Fourier space
Limitations of Fourier Space Reconstruction
REgrid data from polar to cartersian coordinate sytem, and the interpolation error in k space is non local. Difficulty for region of interest reconstruction
what is the goal of parallel beam filtered back projection
to develop an algorithm that can reconstruct CT images directly from measured projection data
data redundancy
more data than you need so you can halve the dose- ish
filtered back projection
this is the method of taking the 1D images and summating them into a form
parallel beam FBP
aquire data in the slices and in order to capture all of the information, you can’t just do half.because of the difference in filed of view
ramp filter
this is for the frequency domain so that the low frequency signals are not as effective as high frequency signals. So you do it in real space by convolution and made the edges more prominent. Not complete removal of low frequency, but a sharp edge would be more prominent. A super blurry image could be from this so if you don’r ramp filter then there are not sharp edges
kernals
changing the form of the ramp filter
when do you use a sharp kernal
bone
when do you use a soft kernal
for soft tissue
fan beam
Ct is acquired data with a divergent beam
fan parallel rebinning
if you are doing a short scan, all of the angles in the views you want and you have to have more than 180+2 times the fan angle
what is short scan FBP
it is a
the concept of sinograms
it is defined as parallel beam projections, where the key idea is to follow the trace of a spceific data point during a paralllel beam data aquisition procedure. IN fan-beam case, we have already generalized the concept
how to handle non-uniform redundancy
a simple scheme to handle data redundancy is to pick up one of the redundant rays and discard the other one, however this is not the optimal way to use x-ray photons (radiation dose). Our goal is to use all measured data, use every phton delivered to the patient
why do you weight the data
you weight it for the data redundancy, so this is just one scan going around one are, so thsi is not moving scan. This is as its moving around. You get double of some and you weight it for trustworthiness
helical reconstruction
it interpolates the data in between the slices. You guess at what the signal is beteween the two. There is 360 and 180. SO are you going t ointerpolate when you do a full movement arpound or half the way around
Mutli-slice helical
it is a cone beam has artifacts because there are more detectors remove some of the collimation
is 180 or 360
180 is better because there is less interpolation between the points because you are getting double the points because its closer to the true curve
