Lecture 6- Basic Image Quality

Question 1

what is image quality

Answer 1

it is the usefulness of an image in determining accurate diagnosis and it is highly dependent on the diagnostic task

Question 2

what affects image quality

Answer 2

source, imaging subject, detection system, processing and or tomographic image reconstruction, display, obesrver

Question 3

what is spatial resolution

Answer 3

level of detail that can be seen on an image. human perception of detail is based on edge detection, blurring effect of imaging system softens the edges of objects

Question 4

factors determining spatial resoluton

Answer 4

focal spot size, magnification factors, detector sampling distance, detector aperture size, reconstruction filter, number of projections

Question 5

point spread function

Answer 5

going from a point to a larger area

Question 6

what are the requirements of linear shift invariant

Answer 6

PSF and further spatial frequency domain analysis can fully characterize a system only if the system is linear and shift-invariant

Question 7

LSI system

Answer 7

a system that is both linear and shift invariant

Question 8

why is the spatial frequency domain used

Answer 8

convinient and efficient, all real objects can be decomposed into sine waves of different ammplitudes, frequencies, and phases. A single analysis in spatial frequency domain can be used to predict perfomance of all possible objects. Computation in spatial frequency domain in easier than just the spatial domain

Question 9

modulation trasnfer function

Answer 9

defines the ability of a system to reproduce the amplitude of an input signal at different frequencies

Question 10

what are the methods of measuring MTF

Answer 10

point source method, slit method, edge method

Question 11

point source method

Answer 11

phantom, small FOV reconstruction, to PSF

Question 12

what is the slit method

Answer 12

slit represents a line impulse function, the response of the system to a line impulse function is line spread function.

Question 13

what is the nyquist criteria

Answer 13

it is the sampling frequency needs to be at least twice the band limit of the signal to avoid aliasing or the band limit of the signal has to be less than the nyquist frequency to avoid aliasing

Question 14

pre-sampled MTF

Answer 14

describes the system response up to and not including the stage of sampling

Question 15

what is noise

Answer 15

varaitions in the spatial distribution of image signal that cannot be attributed to anatomic differences

Question 16

what are the sources of noise

Answer 16

electronic noise, structure noise, quantam noise

Question 17

what is electronic noise

Answer 17

originated from electronic components of the detection system. it is additive to the original signal . usually low compared to the original signal. More significant contribution to image quality when detected number of photons is low.

Question 18

how do you reduce electronic noise

Answer 18

detector cooling by decreased thermal noise, noise reduction circuitry, electronics shielding

Question 19

what is structure noise

Answer 19

originated from spatial variation of detector pixels. Dark field signal or gain characteristics

Question 20

how can you correct structure noise

Answer 20

dark field calibration and gain calibration.

Question 21

does structure noise change over time

Answer 21

yes so need to perform routine calibration

Question 22

what is quantum noise

Answer 22

uncertainty in photon counting statistics. Xray coutning statistics obey the Poisson distribution. Standard deviation is the square root of the mean number of photons. When N is large, approaching Gaussian

Question 23

for photon counting statistics, noise level of the counted photons increases as the mean number of photons increases- T/F

Answer 23

true

Question 24

noise measurement

Answer 24

a simple practical measurement of noise is to calculate the standard deviation of a group of image pixels

Question 25

what is noise as standard deviation

Answer 25

appropirate for simple discussions of image quality

Question 26

standard deviation is a complete or incomplete measure of image nise

Answer 26

incomplete

Question 27

what is the noise power spectrum

Answer 27

more complete descriptor of image noise. defines both the magnitude and spatial frequency characteristics of image noise Noise power spectrum can be amount the various frequency components of the image. Integration of NPS over all frequencies yields variance.

Question 28

contrast

Answer 28

relative difference between two signal levels

Question 29

intrinsic factors for subject contrast

Answer 29

physical properties of the subject relative to background tissues- thickness: Z and P

Question 30

extrensic factors for subject contrast

Answer 30

how the image acquisition parameters can be optimized to enhance subject contrast.

Question 31

what is detector contrast

Answer 31

the shapre of the characteristic curve that shows how the detector maps input energy to output signal

Question 32

what is contrast to noise ratio

Answer 32

a simple measure of signal and noise. Useful metric to compare image quality hen other factors are fixed ( noise correlation, spatial resolution, and object size)

Question 33

what is a contrast detail diagram used for

Answer 33

used to subjectively determine the minimum detectable object size at different contrast levels

Question 34

what does CNR not consider

Answer 34

no consideration of object size, noise correlation, and spatial resolution. Not good metric for absolute detectability evaluation.

Question 35

what is better than CNR

Answer 35

SNR defined using statistical decision theory and task based approaches is more reusable metric for quantifying object