RAB: Ch. 4 Image Quality

Question 1

• describes the level of detail that can be seen on an image
• relates to how small an object can be seen on a particular imaging system

Answer 1

Spatial resolution

[clearly the size of the picture element (pixel) in an image sets a limit on what can theoretically be resolved in that image. While it is true that one cannot resolve an object that is smaller than the pixel size, it is also true that one may be able to detect a high-contrast object that is smaller than the pixel size if its signal amplitude is large enough to significantly affect the gray scale value of that pixel]

Question 2

It is the most basic measure of the resolution properties of an imaging system, and it is perhaps the most intuitive as well.

Also called impulse response function

Answer 2

Point Spread Function, PSF

Question 3

Describes the extent of blurring that is introduced by an imaging system, and this blurring is the manifestation of physical events during the image acquisition or reconstruction process

Answer 3

PSF (point spread function)

Question 4

When an imaging system is stimulated with a signal in the form of a line. Once the line is produced on the image (e.g., parallel to the y-axis), a profile through that line is then measured perpendicular to the line (i.e., along the x-axis). The profile is a measure of gray scale as a function of position

Answer 4

Line Spread Function, LSF

Question 5

particularly useful when the spatial distribution characteristics of glare or scatter phenomenon are the subject of interest

Answer 5

Edge Spread Function (ESF)

Question 6

An integral calculus procedure that accurately describes mathematically what the blurring process does physically.

Answer 6

Convolution

Question 7

A method for decomposing a function such as this gray scale profile into the sum of a number of sine waves

________ transform is an algorithm that decomposes a spatial or time domain signal into a series of sine waves that, when summed, replicate that signal. Once a spatial domain signal is Fourier transformed, the resulting data are considered to be in the frequency domain

Answer 7

Fourier transform

Question 8

The spatial frequency at which the amplitude of the MTF decreases to some agreed-upon level.

considered to be the frequency at which the MTF crosses the 10% level

Answer 8

Limiting Spatial Resolution (limiting resolution)

Question 9

sets the upper bound on the spatial frequency that can be detected for a digital detector system with detector pitch

Answer 9

Nyquist Frequency

Question 10

___ occurs when frequencies higher than the Nyquist frequency are imaged (Fig. 4-16). The frequency that is recorded is lower than the incident frequency, and indeed the recorded frequency wraps around the Nyquist frequency

Answer 10

Aliasing

Question 11

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 the signal-to-noise ratio (SNR) in an image.

Relates more to anatomical structures that produce small changes in signal intensity (image gray scale)

Answer 11

Contrast resolution

Question 12

In medical images, ______ has to do with the amount of noise in the image

Answer 12

Precision

Question 13

refers to 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

Answer 13

quanta

Question 14

In the radiology department, to reduce radiation dose to the patient from x-rays or gamma rays, imaging modalities based upon ionizing radiation use relatively few quanta to form the image—indeed, the numbers of quanta are so low that for most medical images involving x-rays or gamma rays, appreciable noise in the image results, and this noise is called _____

Answer 14

quantum noise

Question 15

This noise is the pattern on the image that is generated by patient anatomy that is always present but not important for the diagnosis. For example, in abdominal angiography, the vascular system is the anatomy of interest, and other sources of image contrast such as bowel gas and spine just get in the way

Answer 15

Anatomical noise

Question 16

Most pixelated detectors have a number of parallel channels for reading out the array of detector elements, and this reduces readout time. Each channel uses its own amplifier circuits and these circuits cannot be perfectly tuned with respect to each other. As a result, groups of detector elements that are read out may have different offset noise and gain characteristics, and these cause _______ or fixed pattern noise in digital detector systems.

The key to correcting for structured noise is that it is spatially constant for a period of time

Answer 16

Structured noise

Question 17

What happens in electronic noise?

Answer 17

Electronic detector systems can be analog or digital, but the flow of electrons (current) conveys the signal at one or more points in the imaging chain. There are electrons that resulted from actual signal detection events (in CT, MRI, nuclear imaging, etc.), and there are also electrons that are added to the signal that are not the result of signal detection events—this added electronic noise can be from thermal noise, shot noise, and other electronic noise sources

Question 18

This noise results in screen film radiography, where the image is formed by millions of silver grains attached to the clear film substrate.

When radiographic film is exposed and processed, the increase in observed darkness (optical density) in the film corresponds to an increase in the density of reduced silver grains adhering to the film support

Answer 18

Grain noise

Question 19

The normal distribution, also called the______ distribution, is the most widely used statistical distribution in scientific analysis and other observational settings. The well-known bell-shaped curve (Fig. 4-23) is characterized by two parameters—the mean (x) and the standard deviation (s)

Answer 19

gaussian

Question 20

The very important thing about the ______ distribution is that its shape is governed by only one parameter (m), not two parameters as we saw for the normal distribution

Answer 20

Poisson

Question 21

This is the fundamental contrast that arises in the signal, after it has interacted with the patient but before it has been detected.

-the intrinsic component of subject contrast relates to the actual anatomical or functional changes in the patient’s tissues, which give rise to contrast.

-the extrenisc factors of this conrast relate to how the image acquisition protocol can be optimized to enhance subject contrast.

Answer 21

Subject contrast

Question 22

The incident beam of energy from the imaging system will eventually reach the detector(s), and again radiography is used as an example

Answer 22

Detector contrast

Question 23

The detector contrast acts to modulate the subject contrast, but the acquisition procedure ultimately results in the capture of digital image data on a computer’s hard drive

Answer 23

Displayed contrast

Question 24

________ ratio is an object size-independent measure of the signal level in the presence of noise

-a good metric for describing the signal amplitude relative to the ambient noise in an image, and this is particularly useful for simple objects

Answer 24

Contrast-to-Noise Ratio

Question 25

A metric similar to the CNR, except that the size and shape of the object is explicitly included in the computation.

One of the most meaningful metrics that describes the conspicuity of an object—how well it will be seen by the typical observer

• does not require the test object that generates the signal to be homogeneous

Answer 25

Signal-to-Noise ratio

Question 26

refers to the probability that the patient is actually abnormal (TP), when the diagnostician says the patient is abnormal (TP 1 FP)

Answer 26

Positive predictive value

Question 27

refers to the probability that the patient is actually normal (TN), when the diagnostician says the patient is normal (TN 1 FN)

Answer 27

Negative predicitive value