Image Quality Metrics
Fundamental Image Metrics
Noise (Quantum Mottle)
Image noise is the fluctuation in pixel intensity due to statistical uncertainty. Noise is typically measured as the standard deviation (σ) of pixel intensity over a uniform area. Reducing image noise is important as noise limits the ability to distinguish low contrast objects within an image. Image noise can arrive from the random fluctuations in photon distribution (photon statistics), digitization and electron noise sources such as dark currents.

 σ is the standard deviation
 x_{i} is an event
 x̅ is the average number of events
 N is the number of measurements
Statistics of Image Noise
Image noise may be statistically modeled as a Gaussian Distribution which, for the large number of information carriers collected, may be approximated as a Poisson Distribution. The probability of observing x events over a given interval for a Poisson distribution may be computed using the below equation where m is the average number of events collected:
Standard deviation of a Poisson distribution may be simplified as:
This allows simple computation of signaltonoise ratio (SNR) for Poisson distributions as:
Rose Criteria
The Rose Criteria states that a signaltonoise ratio (SNR) of 5 or greater is required for reliable detection of an object within an image.
Contrast
Contrast is the difference in intensity, I, between two objects in an image. Contrast allows differentiation between objects:
Factors influencing contrast
 Inherent contrast between subject and background
 Display contrast
 Window
 Level
 Physical perturbations
 Fogging
 Scatter
 Nonuniform detector efficiency
Spatial Resolution
Spatial resolution (R) is the minimum separation between two high contrast objects such that they appear as two separate objects, often measured in linepairs per millimeter (lp/mm).
Factors influencing spatial resolution
 Geometric penumbra
 Unsharpness originating from finite source size in radiographic systems.
 Subject unsharpness
 Rather than a sharp differentiation between subject and background the edges may blur together.
 Detector unsharpness
 Unsharpness arising from scatter or diffusion in the detector
 Motion (subject or detector)
Modality  Spatial Resolution (mm) 

Screen Film Radiography  0.08 
Digital Radiography  0.2 
Fluoroscopy  0.125 
Screen Film Mammography  0.03 
Digital Mammography  0.050.1 
CT  0.3 
SPECT  7 
PET  5 
MRI  1 
Ultrasound (5MHz)  0.3 
Nyquist Frequency (F_{N})
The Nyquist Frequency is the minimum sampling frequency required for an object to be distinguished. The concept of the Nyquist frequency is taken from waveform analysis. For a waveform, the Nyquist frequency is twice the frequency of the waveform.
When applied to image analysis, the Nyquist frequency is half the size of the object.
Aliasing
When an object is sampled at a lower frequency than its Nyquist Frequency, its appearance will be inaccurate even in a system with zero noise and high contrast. This inaccuracy, resulting from insufficient sampling, is known as aliasing.
Quantitative Image Analysis
ContrasttoNoise Ratio (CNR)
Contrasttonoise ratio (CNR) is a metric that relates how easily low contrast objects may be distinguished from their surroundings. Higher CNR indicates a more readily distinguishable object.
Noise Power Spectrum (NPS)
Noise power spectrum (NPS) is used to describe how the magnitude of image noise varies with spatial frequency.
There are two types of NPS noise:

 Uncorrelated (white) noise: Noise which does not vary as a function of spatial frequency
 Correlated noise: Noise which changes as a function of spatial frequency
Modular Transfer Function (MTF)
Modular Transfer Function (MTF) is a measure of a detector system’s ability to render contrast as a function of spatial resolution. MTF = 0.1 is generally taken to be the limit of detectability for humans.
Detective Quantum Efficiency (DQE)
Detector Quantum Efficiency (DQE) is a measure of the imaging system’s efficiency in converting data carriers into an image. DQE is influenced by every component of the imaging system and is a good metric of overall dose efficiency for an imaging system.
Quantum Detection Efficiency (QDE)
Quantum Detection Efficiency (QDE) is a measure of how efficient the detection system is at collecting information carriers. DQE is impacted by a number of factors including detector construction and geometry.
Receiver Operating Characteristic (ROC) Curves
ROC curves plot the probability of correct detection (true positive) as a function of the probability of false detection (false positive). ROC curves are used to determine a detection method’s usefulness.
Actually Abnormal  Actually Normal  

Diagnosed as Abnormal  True Positive (TP)  False Positive (FP) 
Diagnosed as Normal  False Negative (FN)  True Negative (TN) 
Test Quality Metrics
Area under the curve (AUC): Higher is better; ideal is 1
Sensitivity: True Positive Fraction (TPF)
Specificity: Fraction of normal cases diagnosed correctly
False Positive Fraction (FPF)
Accuracy: Percent of cases correctly diagnosed
Positive Predictive Value (PPV): fraction of positive predictions which are accurate
Negative Predictive Value (NPV): Fraction of negative predictions which are accurate
Knowledge Test
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