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Image Quality
Chapter 4
77
Medical
Post-Graduate
06/13/2016

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Term
Spatial resolution
Definition
Describes the level of detail that can be seen on an image.
Term
Point Spread Function (PSF)
Definition
Response of the imaging system to a point input.
Term
Any imaging system with the same PSF at all locations in the field of view is refered to as
Definition
stationary or shift invariant
Term
Imaging system that has PSF's that depend on the position in the field of view is called
Definition
nonstationary
Term
The PSF describes the extent of __________ that is introduced by an imaging system.
Definition
blurring
Term
When evaluating a LSF, the profile of the line is measured __________ to the line.
Definition
perpendicular
Term
The Edge Spread Function, ESF, is evaluated by imaging a
Definition
sharp edge.
Term
Three basic types of spread functions:
Definition

PSF - point spread funciton

LSF - line spread function

ESF - edge spread function

Term
The blurring process is described mathematically by the _________ procedure.
Definition
convolution
Term
The discrete implementation of convolution can be done by
Definition
shifting the kernel, multipling the data by the convolution kernel and summing the results (shift, multiple and add)
Term
What type of kernel is used for a running average?
Definition
a RECT function
Term
What happens when negative values are used in a kernel?
Definition
Edge enchancement will result but so will an increase in noise.
Term
In the spatial frequency domain, smaller objects correspond to ______ frequencies and larger objects correspond to _______ frequencies.
Definition
smaller objects - higher frequencies, larger objects - lower frequencies
Term
If an object is of size Δ mm, the spatial frequency, F, is given by
Definition
F = 1/(2*Δ) mm-1
Term
A sine wave is characterized by what three parameters and is given by what equation?
Definition

a - amplitude, f - frequency, Ψ - phase

g(x) = a sin(2πfx + Ψ)

Term
A Fourier transform does what?
Definition
It converts a temporal or spatial signal into the frequency domain by decomposing the signal into a series of sine waves.
Term
Conceptually, the Modulation Transfer Function shows
Definition
The reduction in signal amplitude as a function of spatial frequency.
Term
Mathematically, the MTF is given by
Definition
the Fourier transform of the line spread function.
Term
In terms of the MTF, limiting spatial resolution is considered to occur when?
Definition
The spatial frequency at which the MTF crosses the 10% level.
Term
Nyquist frequency
Definition

The maximum frequency that can be detected by an imaging system that has a detector pitch of Δ without aliasing.  FN = 1/(2Δ)

 

Term
What happens when an input signal with a frequency greater than the Nyquist frequency is imaged?
Definition
Aliasing will occur.  The frequency on the image will wrap around the Nyquist frequency.
Term
detector pitch
Definition
The center-to-center distance between adjacent detector elements.
Term
The fundamental limit on spatial resolution is determined by
Definition
the width of the detector element.
Term
The Fourier transform of the RECT function (width a) is given by
Definition
The Sinc function; it is centered at 0 with a value of 1 and goes to 0 at a frequency of 1/a.
Term
Contrast resolution is the ability to
Definition
detect subtle changes in gray scale and distinguish them from noise in the image.
Term
Accuracy describes
Definition
How close a measurment is to the actual value.
Term
Precision refers to
Definition
The repeatability or variation in a measurement.
Term
What are the sources of image noise?
Definition
Grain noise, electronic noise, structure noise, anatomical noise, quantum noise
Term
What is structure noise caused by?
Definition
The differences in the offset and gain of individual detector elements.
Term
offset image
Definition
an image measured when there is no radiation incident on the detector.
Term
gain image
Definition
An image measured with radiation incident on the detector but without an object in the field.
Term
A normal or Gaussian distribution is determined by
Definition
the mean and standard deviation
Term
variance is given by
Definition
the square of the standard deviation
Term
1, 2, and 3 sigma represent
Definition
68%, 95%, and 99% of the observations
Term
A Poisson distribution can be approximated by a Normal distribution when we stipulate
Definition
σ = sqrt(mean)
Term
Noise per pixel is given by
Definition

σ = SQRT(N)

N - average number of photons recorded in each detector

Term
Relative Noise is given by
Definition
Relative noise = coefficient of variation (COV) = σ/N = 1/SQRT(N)
Term
NPS
Definition
Noise Power Spectrum - It gives the frequency dependence of the noise variance.  It is a frequency dependent measure of how an imaging system operates on the noise input into the system.
Term
Uncorrelated noise is called ________ noise and the NPS for it is  ________.
Definition
white noise, The NPS is a flat line.
Term
Subject Contrast
Definition
Differences in the x-ray beam fluence that occur as the beam passes through the patient but before it arrives at the detector.
Term

Subject Contrast is defined as

The range of values is

Definition

Cs = (A-B)/A where A > B

0-1 (0-100%)

Term

What causes detector contrast?

Give a linear and non-linear example.

Definition

Detector contrast is due to the response of the detector.

Screen film has a non-linear response while Digital X-ray detectors have a linear response.

Term
What does LUT stand for and what is it used for?
Definition
Look-Up Table.  It converts a stored image gray scale value to a displayed gray scale value on the monitor.
Term
Most commonly used LUT in radiology.
Definition

Window and Level

 

Term
Formula for contrast-to-noise ratio
Definition

CNR = (Signal from an object - background signal) / (background noise)

 

CNR = (Xs - Xbg) / σbg

Term
What is the Rose Criterion?
Definition
If the SNR ≥ 5, then the signal will be detected.
Term
Signal-to-Noise Ratio
Definition

Integrated signal over an ROI that encapsulates the object of interest, divided by the noise of the background.

Σ(xi-xbg)/σbg

Term
CD Diagram
Definition
Contrast-Detail Diagram - a visual tool that combines the concepts of spatial resolution and contrast resolution.
Term
CD Diagram: Detail is on what axis?  Contrast is on what axis?
Definition

Detail is on the X-axis.  Contrast is on the Y-axis.

 

This was based on Bushberg.  There are other CD phantoms out there.

Term
CD diagram: Smaller disks (higher resolution) are to the _______ and disks with less contrast are to the ________.
Definition
smaller disks are to the left, disks with less contrast are towards the bottom.
Term
Smoothing will ________ detail (spatial resolution) but will _________ contrast resolution.
Definition
reduce, increase
Term
Deconvolution can be used to improve _________, but at what cost?
Definition
improve spatial resolution but it also amplifies noise levels in the image.
Term
Sampling Pitch
Definition
The center-to-center distance between adjacent detector elements.
Term
Flat Field correction is given by
Definition

Icorrected = g * (Iraw - Ioffset) / (Igain - Ioffset)

g is the mean gray scale of the denominator

Term
Contrast-to-noise ratio is a measure of
Definition
the signal level in the presence of noise.
Term
CD Diagram - The hardest disk to see is located
Definition
at the lower left corner of the diagram.  It has the highest spatial resolution and the lowest contrast.
Term
MTF describes how well an imaging system processes ________ and NPS describes how well an imaging system processes __________.
Definition
MTF - signal, NPS - noise
Term
What does DQE stand for and what does it describe?
Definition

Detective Quantum Efficiency - overall frequency-dependent SNR performance of a system.

It is a good descriptor of the dose efficiency of a detector system - how well the imaging system converts SNR2 incident on a detetector to SNR2 in an image.

Term
DQE is defined as
Definition
DQE(f) = SNR2out / SNR2in
Term
The formula for actually calculating DQE is given by
Definition
DQE = k*[MTF(f)]2 / (N * NPS(f))
Term
What does ROC stand for.  What does it show.
Definition

Receiver Operating Characteristic (ROC) Curve.

It is a plot of True Positive Fraction (sensitivity) vs False Positive Fraction (1-specificity).

Term
True Positive Fraction is called  _______ and is given by
Definition
sensitivity, TPF = TP / (TP + FN)
Term
True Negative Fraction is known as _________ and is given by?
Definition
specificity - TNF = TN / (TN + FP)
Term
The False Positive Fraction is given by
Definition
FPF = 1 - Specificity = FP / (FP + TN)
Term
Pure guessing on a ROC curve results in
Definition
a 45° line from the bottom-left to the top-right
Term
The overall performance shown on a ROC curve is given by
Definition
the area under the curve (AUC).  Maximum value is 1.0
Term
Describe histogram equilization of an image.  What is it? Why would you do it?
Definition
A histogram shows the distribution of pixel values.  If they are all bunched together, then there won't be much contrast.  If the distribution is equalized out, then contrast will be improved.
Term
What is a line profile.
Definition
A line profile shows the variation in intensity along a line passing through an image.  Line profiles can be useful for examing an image.  For example, determining where boundaries occur.
Term
What is ictal and interictal?  What kind of image processing is used for this?
Definition

ictal - during the seizure

interictal - seizure free

Using image subraction between ictal and interictal images one can identify the epiloptogenic zone.

Term
What is the opposite of blurring?  What does each do to high frequency content.
Definition

Opposite of blurring is sharpening.

Blurring reduces high frequency noise.

Sharpening increases high frequency noise.

Term
Describe the mathmatical process used to apply a filter to the image data.
Definition

Take the Fourrier transform of both the filter and the image data.  Multiply the two together.  Take the inverse Fourrier transform.  

Note: This results in the same outcome as convolving the filter with the image data.

Term
Describe salt and pepper noise
Definition
A small number of pixels are corrupted.  The bad pixels take on random values of black or white.
Term
Image Segmentation
Definition
Classifying voxels of an image into a set of distinct classes.
Term
What two steps are used in filtered backprojection.
Definition

1. filter the projections with the ramp filter

2. backproject the filtered projection data

Term

In ROCs, accuracy is defined as?

Why is it a bad measure of diagnostic performance.

Definition

Accuracy = (TP + TN) / ( TP + TN + FP + FN)

 

It is highly influenced by the incidence of the disease.

Term
What is the Positive predictive value?
Definition

The probability that the patient is actually positive (abnormal) when the test result is positive.

 

PPV = TP / (TP + FP)

Term
What is the negative predictive value?
Definition

The probability that the patient is actually negative (normal) when the test result says the patient is negative.

 

NPV = TN / (TN + FN)

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