Chapter 4 - Image Quality Flashcards
Describes the level of detail that can be seen on an image
Spatial Resolution
Spatial resolution relates to how small an object can be seen on a particular imaging system
- a single number value that characterizes the approx. resolution limit of an imaging system
- often considered to be the frequency at which the MTF crosses the 10% level or some other agreed-upon and specified level.
LIMITING Spatial Resolution
Refers to the two dimensions of a single image, or to the three dimensions of a set of tomographic images such as computed tomography (CT) or magnetic resonance imaging (MRI)
Spatial domain
Most basic measure of the resolution properties of an imaging system; the response of the imaging system to a point source (input to the imaging system)
Point Spread Function (PSF)
- also called the impulse response function
An imaging system with the same POINT SPREAD FUNCTION (PSF) at all locations in the fields of view
STATIONARY or SHIFT INVARIANT
A systems that has POINT SPREAD FUNCTIONs (PSFs) that vary depending on the position in the field of view
NONSTATIONARY
When an imaging system is stimulated with a signal in the form of a line, the _____________ can be evaluated.
LINE SPREAD FUNCTION (LSF)
Device required in analog imaging system to measure the optical density as a function of position
Scanning microdensitometer
Particularly useful when the spatial resolution characteristics of glare or scatter phenomenon are the subject of interest
EDGE SPREAD FUNCTION (ESF)
An integral calculus procedure that accurately describes mathematically what the blurring process does physically
Convolution
A process wherein convolution techniques are used to restore (improve) spatial resolution
Deconvolution
Another useful way to express the resolution of an imaging system is to make use of the _________.
SPATIAL FREQUENCY DOMAIN
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An algorithm that decomposes a spatial or time domain signal into a series of sine waves that, when summed, replicate that signal
Fourier Transform
Resulting data after a spatial domain signal is Fourier transformed
Frequency Domain
Perfect line source input
Delta function
Sets the upper bound on the spatial frequency that can be detected for a digital detector
Nyquist Frequency
The center-to-center distance between adjacent deal extortion elements
Detector Pitch
Refers to the ability to detect very subtle changes in gray scale and distinguish them from the noise in the image
-characterized by measurements that pertain to signal-to-noise ratio(SNR) in an image.
Contrast resolution
Image noise…the actual distribution of the silver grains is random and is a function of the manufacturing and development process.
-image is formed by millions of silver grains attached to the clear film substrate
Grain noise
Typically additive noise from various sources that does not scale with the signal level
- worst in areas of low actual signal in the detector
Electronic noise
Image noise that represents a reproducible pattern on the image that reflects differences in the gain of individual detector elements or groups of detector elements
-can be corrected using a flat film algorithm
Structured Noise
Characterized by exposing the detector to radiation in the absence of an object
Gain image
Measured with no radiation incident on the detector
Offset image
Image noise which refers to anatomy in the patient which is not pertinent to the specific imaging system
Anatomical noise
Any number of particles or objects that can be counted, such as electrons, X-Ray photons, optical photons, or even brush strokes on impressionist paintings.
Quanta
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Quantum noise
A)halfway point in the observed data set
B)most frequent observation, the highest point in the histogram
C)width of the distribution
A) median
B) mode
C) range
Most widely used statistical distribution in scientific analysis and other observational settings
Normal distribution(also called the Gaussian Distribution)
Statistical distribution governed by only one parameter
Poisson Distribution
Characterizes frequency dependence of the noise variance
Noise Power Spectrum (NPS)
A type of uncorrelated noise wherein the noise in each pixel of a 2D image is NOT DEPENDENT upon the noise values in any of its surrounding pixels producing a flat horizontal line in the noise power spectrum and no noise correlation
White noise
Fundamental contrast that arises in the signal, after it has interacted with the patient but before it has been detected
Subject contrast
…the incident beam of energy from the imaging system will eventually reach the detector(s)…
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Detector contrast
- screen-film radiography- nonlinear response to X-rays
- digital radiography- linear response
Raw image information is processed into an image that is finally meant for physician viewing
Displayed Contrast
Most commonly used look-up table (LUT) in radiological imaging
Window/level
An object size-independent measure of the signal level in the presence of noise
Contrast-to-Noise ratio (CNR)
Object size DEPENDENT measure of the signal level in the presence of noise
Signal-to-Noise ratio
SNR >or= 5 : an object will almost always be recognized (detected)
Detection performance continuously degrades as SNR approaches zero
ROSE CRITERION
A conceptual visual method for combining the concepts of spatial resolution and contrast resolution
CONTRAST DETAIL DIAGRAM (CD Diagram)
Characterization of an X-ray imaging system which describes the over-all frequency-dependent SNR performance of the system
- combines concepts of spatial and contrast resolution but the methods of analysis are less visual and more quantitative
- defined as the ratio of the SNR2 OUTPUT from the system the SNR2 of the signal INPUT into the system
Detective Quantum Efficiency (DQE)
Refers to the probability that the patient is actually abnormal (TP)
Positive predictive value (PPV)
Refers to the probability that the patient is actually normal (TN)
NEGATIVE PREDICTIVE VALUE
