Image Reconstruction Techniques Flashcards
Translate Rotate Scan Geometry
Maths relating the intensity of the signal
- I-I0e-ux
- ln(Io/I) = ux for a slab of single material, x thick
- ln(Io/I) = integral ( u * dxi) for multiple materials
Integrating the signal lost from each bit of the slice, weighted by it’s attenuation coefficent. How much is lost per unit length * length, summed over path length.
Translation Rotation Scan geometry
4 steps of image reconstructing
- Measure a set of projections
- Filter them if using fbp
- Back project them across the image plane
- Repeat for large set of projections
What is wrong with simple back projection?
It produces a very blurred image (1/r PSF that is the same at all points in the image).
i.e. you’r projection is not perfect point representation of your object, when you project that bac you get blurring.
How do you apply filtering to make back projection better?
- 2D or 1D convolutions, or multiplication in fourier space
- Filter projections with a convolution kernel to counteract the 1/r blur, then do your back projecting.
- When projected the negative parts of the filtered projections im to counteract the 1/r blurring. Convultion filter is matched to 1/r blur.
What’s a sinogram?
What is it used for?
Scanner stores transmission profiles as 2D image called sinogram. Each line in the sinogram is a profile of the object at a different angle.
Iterative reconstruction
3 advantages, 1 disadvantages
Advantages
- Potential for lower dose scanning
- Lower noise images
- Potential for artefact reduction
Disadvantages
- Computer intensive
Iterative reconstruction
6 steps
- Accquire projections
- Inital guess made at image (FBP)
- Comparrison of projections from guess and actual data
- Initial guess is corrected to match the measured projections
- Repeat
CT beam geometry
- x-ray tubeo
- main collimator
- secondary collimator
- patient
- post patient collimator
- detector
CT pitch
Single & Multislice
Single slice, Pitch = T/w
- T = table travel per rotation
- w = nominal slice width
Multi-slice, Pitch(det) = T/d, Pitch(w) = T/w
- T = table travel per rotation
- w = veam width
- d = detector width
Electron beam CT scanner
components
- Electron gun
- Deflector coil
- Electron beam
- Target ring
- Detector ring
Getting electrons to go really fast, using magnets to stear the beam around the target ring, producing x-rays.
What does all projection imaging suffer from?
What modality tries to reduce this?
Superimposition of structures which can mimic and or mask pathology
Tomosynthesis
Digital Breast Tomo
How does it work?
- X-ray tube moves in arc about a pivot point
- Breast & detector are stationay
- Acquire many low dose projections
- Computer reconstructs data into slices at different depths in breast
