Topic 11 Flashcards
(36 cards)
Spatial resolution definition?
Ability to detect contrast in high (small ) spatial frequency details function of line pairs per mm
Modulation transfer function?
MTF expresses how spatial frequencies are transferred through system
What does the spatial resolution depend on ?
1 ) Source focal spot 2 ) Magnification 3 ) Detector resolution
Which MTF would you choose?
Screen a because it extends into higher frequencies.
How do you calculate the MTF with the bar pattern test object?
amplitude of modulation at u/amplitude of modulation at 0
The point spread function
describes the response of an imaging system to a point source or point object.
The line spread function
is the response when subdividing the pixel surface in ideally infinitely small strips and expressing the relative efficiency of each
To measure the PSF
In principle one should have a pencil beam of negligible transverse direction and scan the pixel surface with it.
To measure the LSF?
A “blade” of radiation scanned in 1D only.
Instead of measuring the LSF what do we measure instead?
The edge response function.
How do you measure the Edge response function?
A sheet of tungsten, stick it front of the detector. Then shift the tungsten while the xrays are on and then for every shift, you look at each pixel response. If its isotropic and the detector responds in the same way, the more you uncover the more you count. This is a mechanism to sample the edge response function. Then all you do is take the derivative of this and get the line spread function.
A shortcut to measuring the edge response function?
Take an image of a slanted edge of a blocker such as tungsten.
The ideal detector LSF is what shape? and what is it in reality?
rectangular. rounded edges
How can you get the MTF from the LSF?
Fourier transform the LSF and take the modulus
What is the assumption made from the slanted edge method of obtaining the ERF?
That all of the pixels respond in the same way.
What are the three types of noise in x-rays?
Quantum noise
- X ray photons distributed randomly in space and time. governed by the poisson statistics. so std = sqrt(counts).
- so you try to bring it close to* poisson limit.
- AlWAYS present in any x-ray image. A statistical process.*
Electronic noise ( due to the detector + readout electronics: read noise, dark current, etc.) No electronic device is perfect. if you have an integrator. the pixel will collect data. the electrical charge is stored. if you switch on the transistor you have a transient. you cannot switch on something and not have parasitic noise all over the place. you have dark noise. when its being switched off.
Film-grain noise (in films) -
Structural noise : overlying & underlying anatomic structures
How can you characterise the noise?
Standard deviation,
Frequency.
What is quantum mottle?
The number of photons contributing to image per cm^2
Varies due to random fluctuations
Proportional to 1/√N
The noise power spectrum
This is the noise power spectral density, expressing how power (= amplitude of the corresponding Fourier coefficient) in a random signal distributed between frequencies
How do you calculate the Noise Power Spectrum?
Calculated as the FT of the autocorrelation function. (basically a mathematical tool for finding repeating patterns). which expresses how correlated noise is as a function o time/distance.
What is the autocorrelation function?
same function convolved with itself. same function shifted in space, integrated over the axis.
if there are hidden frequencies in that function that are not obvious to see, when you shift it, one on top of the other and there is a hidden frequency which is contained in the function with a higher coefficient than other frequencies when you put them in phase with eachother you will have a bigger overlap.
If they overlap it will provide a peak - finding repeating patterns.
The NPS is a function of /…..?
Exposure
What does the MTF and NPS tell you about the system?
The MTF - tells you how contrast is transferred by your system
The NPS - How noise is transferred by the system as a function of frequency.
What is a quantum accounting diagram?
A diagram that shows the number of x-ray photons at each stage of image processing in the xray detector.
(any level smaller than N, would mean a deterioration of the system. This is called the primary quantum sink.)
