Topic 20: CT lecture slides Flashcards
What kind of attenuation dominates in CT?
Compton scatter, which relates to the electron density
How does CT work?
- images are taken all around a patient 2. Data is reconstructed 3. A tomographic slice through the patient is formed
Why does the X-ray source need high loading?
operate at high energies (12-140kV
Detectors need to be what?
Fast- because gantry rotates High absorption efficiency Small and compact High stability Large dynamic range
How does a Xenon ionisation chamber detect?
- X-rays ionise gas - Electric field attracts ions - Charge collected proportional to x-ray intensity
Problem with xenon ionisation chamber detector?
Quantum efficiency of gas low. Uses high Z gas Gas at high pressures Make detector long Final efficiency 60%-70%
What is used instead of Xenon ionisation chamber detectors?
Solid state detectors, which consists of a scintillator and photodiode which has a higher absorption efficiency however may have dead space between pixels.
Weigh up the first generation CT scanners.
Con: Slow and therefore motion artifacts Pros: easy calibration low cost high scatter rejection due to beam collimation true parallel-beam image geometry
why did people like the first CT scanner?
- it had loads of contrast resolution! 2. It has decreased structural noise. you know what feature and tissue type is where! high diagnostic value
what did they do to make CT faster? and what was the problem that arose in the third generation?
made the aperture wider and rotated, but ring artifacts started to appear, because different pixel recieve responses (errors) were back-projected and rotate around.
how did they solve the problem with the third generation?
source was rotated and pixels were remained still.
how can you characterise the spiral CT?
pitch = table travel per rotation/slice width
what is the trade off when deciding the pitch?
higher pitch means lower radiation dose but at the expense of partial volume effect (loss of detail)
No single axial slice is entirely irradiated so you are missing information so you need to solve this with……
interpolation
What is a multi-slice CT?
Replacing array with a matrix which means you can aquire data faster
How much faster is a multi-slice ct?
by the number of rows in the array
what is the problem with multislice CT?
Apart from the central one, slices are seen from a angle, which causes artifacts when these are reconstructed
thick vs thin slices?
thick slices have lower noise, and thin slices reduce partial volume effects and allow off axis image creation with isotropic resolution.
mu for each pixel is converted to a CT number. how do you calculate the ct numbers?
if k = 1000 it is in hounsfield units.
what is the hounsfield unit for air and water?
air = -1000
water = 0
how does the window and level affect the image?
level - is like your base white 0
window is your max white and min
What affects image quality
Image constrast
Spatial resolution
Noise - which is affected by tube current (how many photons), scan time and slice thickness?
Artifacts
What do we look at in quality assurance? (not necessary to remember all)
noise,
spatial res
contrast scale
sensitivity
dose
alignment
slice thickness
Types of artifacts?
- Ring
- Motion artifacts
- Spectral
- Streak
- Partial volume
- Cone beam
- Noise.
How do you remove a ring artifact?
removed by averaging or by appropriate filters
Spectral artifacts
Because beam is polychromatic
beam is hardened across patient
average energy higher in the centre and attenuation is higher for higher energies
CT numbers higher at edge
Cupping edge
Correction of spectral artifacts ?
Use bow tie filter
Streaking
High density objects
cause streaks
bullet, surgical pin, hip replacement
Partial volume effects
brings about blurring
3-d voxel
occupied by more than one tissue type
Cone beam artifacts
When the cone beam artifact is not angled right,
problem in multislice CT
Noise as an artifact …what type of noise dominates?
statistical noise dominates
how can you reduce noise in CT?
increase current
increase rotation time
increase slice thickness
(more photons)
Why is the dose in CT higher than DRadiography?
CT reconstructing image from huge number of angular projections.
CT dose index
Dose to a depth in a scanned volume for a complete series of slices
measure with a long ionisation chamber inserted into a phantom
large contribution from scatter
Dose reduction
- Expose less (reduce mA or shorten scan time)
- > but this lowers constrast-to-noise ratio
- To keep CNR constant - e.g. expose less and increase slice thickness –>may affect spatial resolution
- More sophisticated: adaptively change exposure as patient anatomy changes
