Lecture 6- Basic Image Quality Flashcards
what is image quality
it is the usefulness of an image in determining accurate diagnosis and it is highly dependent on the diagnostic task
what affects image quality
source, imaging subject, detection system, processing and or tomographic image reconstruction, display, obesrver
what is spatial resolution
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
factors determining spatial resoluton
focal spot size, magnification factors, detector sampling distance, detector aperture size, reconstruction filter, number of projections
point spread function
going from a point to a larger area
what are the requirements of linear shift invariant
PSF and further spatial frequency domain analysis can fully characterize a system only if the system is linear and shift-invariant
LSI system
a system that is both linear and shift invariant
why is the spatial frequency domain used
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
modulation trasnfer function
defines the ability of a system to reproduce the amplitude of an input signal at different frequencies
what are the methods of measuring MTF
point source method, slit method, edge method
point source method
phantom, small FOV reconstruction, to PSF
what is the slit method
slit represents a line impulse function, the response of the system to a line impulse function is line spread function.
what is the nyquist criteria
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
pre-sampled MTF
describes the system response up to and not including the stage of sampling
what is noise
varaitions in the spatial distribution of image signal that cannot be attributed to anatomic differences
what are the sources of noise
electronic noise, structure noise, quantam noise
what is electronic noise
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.
how do you reduce electronic noise
detector cooling by decreased thermal noise, noise reduction circuitry, electronics shielding
what is structure noise
originated from spatial variation of detector pixels. Dark field signal or gain characteristics
how can you correct structure noise
dark field calibration and gain calibration.
does structure noise change over time
yes so need to perform routine calibration
what is quantum noise
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
for photon counting statistics, noise level of the counted photons increases as the mean number of photons increases- T/F
true
noise measurement
a simple practical measurement of noise is to calculate the standard deviation of a group of image pixels
what is noise as standard deviation
appropirate for simple discussions of image quality
standard deviation is a complete or incomplete measure of image nise
incomplete
what is the noise power spectrum
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.
contrast
relative difference between two signal levels
intrinsic factors for subject contrast
physical properties of the subject relative to background tissues- thickness: Z and P
extrensic factors for subject contrast
how the image acquisition parameters can be optimized to enhance subject contrast.
what is detector contrast
the shapre of the characteristic curve that shows how the detector maps input energy to output signal
what is contrast to noise ratio
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)
what is a contrast detail diagram used for
used to subjectively determine the minimum detectable object size at different contrast levels
what does CNR not consider
no consideration of object size, noise correlation, and spatial resolution. Not good metric for absolute detectability evaluation.
what is better than CNR
SNR defined using statistical decision theory and task based approaches is more reusable metric for quantifying object
