Radiographic image and quality Flashcards
Radiographic contrast refers to?
The difference in DENSITY between areas in the radiographs
Radiographic contrast depends on what three factors?
- Subject contrast
- Film contrast
- Fog and scatter
The difference in x-ray intensity transmitted through one part of the subject compared to another is…
Subject contrast
Subject contrast depends on?
- Thickness difference
- Density difference
- Atomic number (Z)
- kVp / quality of beam
Of the four things that affect subject contrast what are the most important?
- Density
- Atomic number
- Ex. Muscle (7.4) and fat (5.9) are hard at high kVp but better at low kVp (mammograms)
Subject contrast equation
= Ismall/Ilarge
- Ex. At 50kVp 25 x-rays get through the thick part and 40 get through the thin
Subject contrast is 40/25 = 1.60 meaning 60% more x-rays make it through
Compare that to 100kVp were 80 and 60 xray make it through. 80/60 = 1.33 or only 33% more thus less contrast
Short scale contrast
High contrast at low kVp - everything is either black or white
Long scale contrast
Tons of gray scale so limited contrast
Exposure latitude… what is it and does short scale contrast have more of it or long scale?
Exposure latitude is the ability to create a proper denisty and contrast image out of a wide range of techniques (wider range of mAs will provide the proper density at a certain kVp)
Long scale (high kVp) allows a wider range of mAs (wide exposure latitude)
kVp controls what of the following?
- Subject contrast
- exposure latitude
- Density
Subject contrast and exposure latitude to the LARGEST degree
….though does have some influence on density
What equation governs how much kVp affects density?
(kVp highest)4/(kVp lowest)4
Ex. kVp increase from 50 to 60
604/504= 2.07 — doubles density
THIS EFFECT changes as kVp increases
Amount of scatter increases with what?
- Increase in thickness
- Field of view size
- Energy of x-ray beam (High kVp)
Amount of scatter is decreased with?
- Collimation
- Grid
- Air gap
Scatter, true fog and exposure fog(accidental exposure to light of film) reduce what?
Radiographich contrast by decreasing film contrast
True fog is what?
Film grains that are developed even when not exposed… Produces unwanted density
Image clarity is made up of two parts
Contrast and image quality
What is image quality?
The ability for the film to reproduce perfectly each point on an object to the screen
Image quality is influenced by?
- Radiographic mottle
- Sharpness
- Resolution
Radiographic mottle is made up of Structure mottle, Quantum mottle and film graniness… which one is the only one of importance.
Quantum mottle
Film graininess is the nonhomogenous arrangement of silver grains in gelatin and can only be seen under lens microscope
Structure mottle is imperfections in the intensifying screen and really don’t matter.
Quantum mottle is?
The statistical fluctuation in the number of x-ray (quanta) per unit area absorbed by the intensifier screen
FEWER QUANTA = MORE MOTTLE
The umbrella and rain example
To see quantum mottle you must have a good quality radiograph with good contrast, or a poor one?
Good quality… because if it is shit you won’t see anything anyways
Quantum mottle is also called?
Noise
High contrast images are limited by what?
Sharpness
Low contrast images are limited by?
Noise - don’t have much contrast so noise can really influence the resolution of the image.
How to increase speed withOUT increasing noise?
Increase phosphor thickness
Use phosphor with higher absorption coefficient
What is the only factor determining noise in screens?
The number of photons used by the screen
Faster the intensifier, what happens to sharpness?
Decreases… cuase usually using thicker screens and therefore more diffusion of light causing less sharp images.
Increase speed with increase in noise is done by?
Use of phosphor with high x-ray to light conversion
Faster film (rare earth’s = more mottle)
Why? The intensifier screen “USES” less electrons
Sharpness is?
The ability to define an edge
Types of unsharpness
Geometric
Motion
Absorption
Screen
How do contrast and sharpness affect one another?
If high contrast, then unsharp edges will be seen
If low contrast, then sharp edges will NOT be seen
Resolving power is?
The ability to record separate images of small objects that are close together
Does an 8 line pair/mm film screen system have really good resolution at 6 or 4 line pair/mm objects?
NO.. well depends… have 8 lp/mm does not tell you have well it will work for object lower than that.
What does the line spread function measure?
The amount of light diffusion by an intensifying screen causing blurred images
Faster screens have higher or lower LSF?
Higher.. more spread.. thus less sharpness and less resolution
What is modulation transfer function?
objective measurement of the combined effects of sharpness and resolution
Other terms… how much information is transferred from x-ray image to detecting system
information recorded/information received
As the demand for higher spatial resolution is put on the system.. what happens to MTF?
It goes down or decrease.. getting shitter and shitter until it can’t distinquinsh two objects and therefore all information is lost by one.
Lost information is “modulation of the transfer of image”
Why use MTF?
Compare systmes
How to get the MTF of the whole x-ray system?
Multiply the MTF of the individual parts.
MTF can never be more than what number?
1.0
What is the wiener spectrum?
The measure of the total noise recorded by the film (grain noise and quantum noise)
How well a screen can record noise at a given spatial frequency.
The ability of a screen system to record noise decreases with spatial frequency (resolution) increasing or decreasing
INCREASING.. same as MTF
Spatial resolution is?
The ability to differentiate between small objects
What usually limits spatial resolution in a digital system?
Pixel size (Nyquist theory)
Spatial Doman vs frequency domain
Spatial Domain
- What we see on the computer screen and what is taken in by the detector as information
- mm and seconds
Frequency Domain
- Transformed to this so computers can compute raw data faster
- cycles/mm and cycles/second
Point spread function is what?
Basic measurement of resolution properties of a system
- How a system responds to a point input
- 2-D
- Degree of spreading is what is measured - BLURRING
- Small hole in attenuating metal is used to cover the detector
IF all the PSF of each dexel is the same then it is called stationary or shift invariant
If all the PSF of each dexel are different - nonstationary
Line spread function?
Another measurement to see how your system is doing
Practical assessment of spatial resolution
Measure the width of the line and see how it blurrs - thinner the better
Edge spread Function?
Same as PSF and LSF but uses a sharp edge
Meaures scatter and glare
What is convolution?
Math that is used to reconstruct images so that they are smooth or edge enhancing
Basically you avg out the outlyers
Kernel?
The type of convolution that is performed
ie bone vs ST vs lung
Another word for convolution or kernel is?
boxcar averaging or running average
If a kernel does nothing to change the raw data and the data stays the same after the kernel is run.. it is called?
Delta function
What happens to spatial frequency when RECT increases?
Spatial frequency decreases… think of RECT as the size of the detector. The bigger the size the less it can differentiate.
Fourier transform is what?
A way to take spatial domain information and change it into frequency domain.
It also takes a single spatial domain input and breaks it down to the different frequencies that make it up.
This allows you to compare where singals are coming from, which ones are weak and which are strong.
Higher frequencies or small objects have a higher or lower rate of resolution loss in imaging systems when compared to low frequencies or large objects?
Resolution loss in an imaging system is larger in small objects or high freq
Nyquist Freq.
The highest incident freq that can be detected by a system
Nf = 1/2 (sampling rate)
Ex. If a detector pitch is 0.05mm what is your Nf?
0.05/1 x 1/2 = 1/0.1 = 10
Aliasing?
If an incident frequency is higher than the nyquist freq.
This incident freq is then recorded as a lower freq that does not break the nyquist freq
Contrast resolution is made of?
Grain Noise
Electronic noise
Structure noise - noise based on how the detector reads out the image
Anatomical noise
Quantum noise
Precision is limited by what?
quantum… more information or signal.. the more precise you will be
Normal distribution vs Poisson?
ND has a mean and standard deviation
Poisson only has a mean and the SD is derived from that mean by square rooting the mean
They are almost identical
In normal distribution.. how much (percentage) of the data is in each standard deviations?
SD 1 = 68%
SD 2 = 95%
SD 3 = 99%
Contrast is made up of what types?
- Subject
- Density
- thickness
- kVp
- Z
- Detector
- Think of the dynamic range curve and how well a system can handle differences in contrast
- Display
- Display units can only produce 8-10 bit of information while the image is transmitter in 8-14 bit. Therefore you will gain contrast and loose gray scale when returning to display
What is a LUT?
Look-up table
This is window and leveling. Basically the display cannot display the entire gray scale of the image. Therefore you use a LUT to decide what range of grayscale you want. This all depends on what you are looking for or at.
Are contrast and spatial resolution dependent on eachtoher>?
No….. the are not dependent on eachother but they do affect an image together.
Think of the Contrast detail diagrams (circles of different contrast and detail)
Detective quantum efficiency?
How well a system transmits your SNR from input to screen.. what it adds from it and what it takes away.
What the fuck does gestalt mean?
