metrics

Question 1

spatial resolution

Answer 1

ability to detect contrast in high spatial frequency details

expressed by the MTF

function of line pairs per mm
(cycles/mm)

Question 2

what spatial resolution depends on

Answer 2

source focal spot
magnification
detector resolution

Question 3

MTF

Answer 3

modulation transfer function

expresses how spatial frequencies are transferred through system

a higher modulation value at higher spatial frequencies :
good for visualising higher frequencies

depends on lens
Some lenses are designed to be able to very accurately resolve lower spatial frequencies, and have a very low cut-off frequency (i.e. they cannot resolve higher spatial frequencies)

Question 4

measuring the MTF

Answer 4

take a bar pattern test object and image it
start from a lower freq, assume mod of 1, measure other mod values and plot them against spatial frequency

disadv:
sampling at given frequencies, not full curve
using square instead of sinusoidal input, slight overestimation

measure IRF and fourier-transform it

Question 5

point spread function

Answer 5

implies subdividing the pixel surface in small squares and expressing the relative efficiency of each

Question 6

line spread function

Answer 6

implies subdividing the pixel surface in small stripes and expressing the relative efficiency of each

Question 7

measuring PSF

Answer 7

one should have a pencil beam of negligible transverse direction and scan the pixel surface with it

Question 8

measuring LSF

Answer 8

a ‘blade’ of radiation scanned in 1D only
impractical so ERF measured

LSF is derivative of ERF

fourier transform of LSF is MTF

Question 9

ERF

Answer 9

edge response function

uses edge target images whose gray values are often affected by noise and other factors, decreasing its accuracy.

The edge response process requires taking the derivative of a well defined edge = LSF

graph is pixel intensity over distance from edge

slanted edge
tilted edge technique is more easily accomplished and gives superior results

/ slanted slit

Question 10

ideal detector LSF

Answer 10

rectangular function

realistically edges will be rounded off
width wider than a single pixel

Question 11

LSF effect on MTF

Answer 11

narrow LSF will generate a large MTF
(extending to high spatial frequencies)

most of the time

Question 12

finding MTF of the entire imaging system

Answer 12

product of the MTFs of the individual components
(convolution in frequency domain)

components being:
focal spot
scintillator
detection layer etc

MTF(f.spot) broadened if object closer to detector
MRF(scint)&(det) are broadened if object is closer to source

Question 13

noise types

Answer 13

quantum noise:
intrinsic, fluctuations, stochastic (random)
always present

electronic noise:
read noise, dark current, detector+readout electronics

film-grain noise
in films

structural noise
overlying and underlying anatomic structures

Question 14

quantum mottle

Answer 14

number of photons contributing to image per cm^2

varies due to random fluctuations

proportional to 1/sqrt(N)

affected by preset exposure factors (mA)

Question 15

comparing noise profiles

Answer 15

variance, mean
frequency dependence -> NPS

Question 16

NPS noise power spectrum

Answer 16

(measure of how similar 2 signals are)

gives the variance (noise) of a system

noise power spectral density, expressing how power in a random signal distributed between frequencies

(indirect)
calculated as the FT of the autocorrelation function
-> spatial autocorrelation

or (direct)
directly calculating the square modulus of the fourier transform of the data itself

for digital detectors, discrete FT algorithms used, integrals replaced with sums

Question 17

autocorrelation function

Answer 17

mathematical tool for finding repeat patterns

Question 18

NPS white noise

Answer 18

white noise has the same power at all frequencies

flat noise profile = noise is uncorrelated

ideal:
noise in each pixel is independent to other pixels

real:
crosstalk

Question 19

power

Answer 19

amplitude of the corresponding Fourier coefficient

Question 20

NPS as a function of exposure

Answer 20

noise depends on number of quanta

higher sample exposure, lower NPS

Question 21

NPS and MTF

Answer 21

both 2D function
pixel isotropy(identical) is assumed

high MTF, system can record noise at a high freq
low MTF, high frequency noise is blurred out

Question 22

detection of xray photons process

(CCD camera)

Answer 22

1. N x-rays impinge on the phosphor
2. each x-ray creates many visible photons
3. not all visible photons are collected by the fiber optics
4. fiber optic transfer not 100% efficient
5. phosphor spectrum and CCD camera sensitivity curve not perfectly matched

Question 23

quantum accounting diagrams

Answer 23

lowest point defines SNR
SNR less than or equal to sqrt(lowest point)

Question 24

detection efficiency

Answer 24

increased efficiency, lower spatial resolution

x-ray interaction in the sensor material for direct detection systems
in the phosphor for indirect ones

efficiency: number of photons stopped as a fraction of N0

Question 25

Broadened MTF effect on blurring

Answer 25

Lower blurring, broadened MTF