Lecture 5 Flashcards
(31 cards)
Explain what is subject contrast
Difference in some physical aspect of signal due to the object being image
Ie. X-ray attenuation in object
Causes a fundamental difference in image formed
To increase SC = increase attenuation coefficient (mue (E)) (ie. decrease energy causes the increase in AC) or decrease Z
Decrease SC = increase Kev (beam intensity) and contrast changes drastically due to changes in AC
Cs= 1-e^-mueZ
What happens when the energy of the beam drops
Number of low energy photons increase, thus greater absorption
Explain the relationship between contrast and dose
Increase kVp = decrease dose and contrast
Less abosorption occurs
Compromise between dose and image quality
Explain what’s spatial resolution
Describes the ability of an imaging system to accurately resolve objects in the spatial dimensions (x,y) of image
How well can an imaging system distinct two seperate objects as they become smaller and closer
Explain what’s image blur
Causes unsharpness of structure
Due to movement unsharpness and geometric factors
What’re the factors that cause movement unsharpness?
Patient = voultanry or involuntary, inadequate immobilisation, long exposure time
Image receptor = Bucky tray or cassette holder not secured
X-ray tube = brakes not applied
How is an umbra produced?
By a source that’s a pin point
Gives sharp edges of the object
Darkest part of the shadow
How is a penumbra produced?
Real X-ray sources aren’t pinpoints but they’re a focal spot
So it doesn’t give sharp edges, but more like partial illumination or an edge gradient as the penumbra only sees some of the source
Causes edge blur (geometric blur)
How do you reduce geometric blur?
Reduce focal spot size
Large FS = 0.8 mm - 1.2
Small = 0.6-0.8
Explain the variation in focus spot size within a field (image plane)
Due to 15 deg angle of the anode, the anode will be sloped
There’s an actual focal spot size (projected by slope) and a projected focal spot size
You’ll se the focal spot size change along the image plane with anode side having smaller focal spot and increases in length towards the cathode
But the centre is focussed, and the edges are blurred
Explain what the anode heel effect is
Anode has a 15 deg slope
So there’s more material, and so when electrons are shot towards the anode, and photons are emitted from the anode, the photons tend to be absorbed by the excess anode material and so only low intensity beam is emitted as result (hence they are higher energy photons)
And cathode side, the photons don’t get pre absorbed and so it s a high intensity beam (with low photon energy)
Explain the occurrence of scattered radiation
We try to attain a balance between Compton scatter and photoelectric effect when adjusting the beam intensity.
Ie. tissue = 26 keV, bone = 35 keV when both CS and PE roughly equal each other
Compton scatter produces scatter
What’re the factors that effect scatter production?
Decrease kVp = increase backward and side scatter (less scatter reaches film)
Increase kVp= increase forward scatter (more reaches film)
Large field size = increase scatter (vice versa)
Increase thickness = increase scatter
Grids = decrease scatter (Ie. long strips with wide spaces lose less primary photons 10:1
How do you calculate grid ratio?
Height of lead strips divided by space in between
Summarise how noise evaluation is done
Absolute noise (NPS) is the standard deviation Increase in SD = increase noise
Calculate Relative noise (SNR)
Good image has SNR greater than 1
If not, contrast is reduced
Noise comes from decrease photons being detected
Explain how an image is viewed mathematically
Complex mix of sine waves with set frequencies, amplitude, phase (relative position), orientation (angle)
What does each stripe pattern (ie. sine wave) have to have in a FT
Particular spatial frequcy
Amplitude
Orientation (mid of y and x)
Phase
Stripe patterns are more specific when added each time to the image
How are all the wave/stripe patterns stored
Stored in the Fourier spectrum
Fourier spectrum provides a summary of all the differnt spatial frequencies found in the spatial domain (ie. image) - called k-space in mri
Define image detail
RApidly changing intensity (increase in spatial frequency)
Therefore no rapid change in intensity = blurry (no detail)
Distinguish MTFs
Explain what’s modulation
Amount of change in a signal
Contrast is the result of modulation of image intensity and so modulation can measure contrast
How’s contrast measured using the modulation of an image
Modulation of signal (ie. pixel intensity) calculate like
M(f) = Smax - Smin/ Smax + Smin
M depends on frequency (M is a function of f)
And so M(f) gives an idea on level of fluctuation within signal and the amount of modulation that has occurred in that signal for a particular signal stretch wighin system
How is MTF derived and what is it?
Derived from the modulation function (M(f))
MTF tell the modulation of output signal relative to the modulation of input signal at a particular spatial frequency
Which can tell us imperfections of imaging system or parts of the system (so can the system produce high quality images?)
Also tells us how contrast is lost as spatial frequency increases
So. SF increases = decreases MTF
MTF is responsible for converting contrast values of different sized objects (object contrast) into contrast intensity levels (image contrast)
What is spatial frequency?
How often do the components of a sine curve repeat themselves per unit of distance (cycles/m)
In summary, what does MTF describe
Contrast and spatial frequency
Doesn’t describe the effect of noise, so not a complete description of performance (ie. not artefacts either)
