Topic 7 - Intro to X-Rays Flashcards
CONTRAST SNR, FOM Improving contrast in images
Equation for magnification?
image size/object size or (s1+s2)/s1
What is the penumbra effect?
Focal spot size causes blurring.
(p/F) = (S2/S1)
Name some examples of detectors
- Film
- Film/screen
- CR plates
- Image intensifier
- Amorphous Flat Panel
- CMOS
What do you have to consider when choosing a detector?
- Efficiency (DQE)
- Spatial resolution (MTF)
- Noise (NPS, SNR)
- Dynamic range
Photons interact and are removed from the beam, what does the attenuation depend on?
1) Z
2) Density
3) Thickness
How is contrast calculated?
C = (Int1-Int2)/Int1
Show that contrast does not depend on I, only on µ’s and t’s - the subject’s characteristics
NB: we’ve seen that
C =1− exp[(µ1 − µ2 )t2]
What happened to t1?
These are two uniform absorbers! they only change Intensity!
How does contrast change with x-ray energy?
Away from absorption edges, attenuation coefficients decrease with increasing x-ray energy. Therefore, generally speaking the contrast DECREASES with increasing energy.
what does the contrast depend and not depend on?
It depends on the detail (t, µ) and on the x-ray energy (through µ), but NOT ON THE AMOUNT OF X-RAYS you’ve used
It also does not tell you how well you can see it.
What does the noise in the image determine?
How likely you are to see a detail with a given contrast.
How do you calculate the SNR?
SO HERE’S HOW WELL YOU SEE SOMETHING: it depends on C (-> object, xray energy), and it gets better with the SQUARE ROOT of the number of x-rays
N1 = The signal
Why do we get noise?
The uncertainty principle, quantum mechanics, poisson distribution
What is the variance of the noise equal to?
the mean
standard deviation?
sqrt (variance)
how is the dose and snr related?
SNR proportional to the sqrt dose
what is figure of merit?
the performance of the device = is independent of the dose and is a function of energy
How do you get the maximum FOM?
the contrast decreases with increasing energy (bad) but the dose also increases with increasing energy (good).
The maximum is when you get the best balance between balance between contrast and dose.
You want the maximum to be wide
What are ways to improve contrast in X-rays?
A. Remove scatter
B. Increase inherent contrast
• Contrast agent
C. Reduce kV
– Photoelectric effect α Z3 E-3
D. Remove effects of overlying tissues
E. Process images
Subject contrast? with and without scatter?
How do you remove scatter?
- Field size
- Air gap
- Grid
- Scanning slit
- Tissue thickness
how do you calculate the SNR for multiple pixels?
m = number of pixels
How do you calculate the reduction as a result of the air gap ?
Why does this graph flatten?
The graph flattens because as D2 decreases you decrease the amount of scattered that reaches the receptor.
What does the Signal depends on? (N)
Sample material, thickness, beam energy, and be evaluated precisely only by means of sophisticated MonteCarlo techniques
How do grids reduce noise? and how do you calculate the grid ratio?
By removing scatter as scattered photons have different angles that do not penetrate the grid.
grid ratio = t/d
How do you calculate selectivity?
How do you calculate contrast improvement?
How do you calculate bucky factor?
How do you calculate the contrast improvement?
Grid Selection factors
1) contrast improvement
2) patient exposure
3) x-ray tube loading
How does tissue thickness reduce scatter?
making it thinner reduces scatter
Other methods of contrast improvement?
Digital post-processing
- Contrast agents (Iodine,barium)
- Dual energy (take radiographs at 2 energies dominated by photoelectric effect and compton scatter)
- Digital subtraction
