Radiography Flashcards
What is the formula for Magnification of an image, in terms of the size of the object and image?
M=(Limage)/(Lobject)
What is the formula for Magnification of an image, in terms of the distance from source to object to the screen?
M=(a+b)/a where a is the dist from the x-ray tube to the object and b is the distance from the object to the image
Would increasing the distance from the object to the image increase of decrease magnification?
increase!
In screen-film radiography, would a thicker screen cause more or less interactions? More or Less blurring?
More Interactions, More Blurring!
What is the chemical process in screen-film?
You have Ag+ ions originally. When interacted with, they produce Ag. Then while developing, you wash away Ag+.
Formula for Optical Density
OD = -log(T), log is base 10 and T=I/Io (intensity after transmitted thru film/intensity with no film in the way
Describe the important features of the Film Characteristic Curve
x-axis is Kerma (log scale). Y-axis is Optical Density
It starts flat - this level is called the base+fog.
There is then a linear region.
Then there is a shoulder
In two overlayed film characteristic curves, how can you tell which one is “more sensitive” or “has a faster speed”?
It will be the one that starts increasing (has a higher optical density) at a lower Kerma
What does the latitude of a screen-film curve talk about?
The latitude is the region over which the film gives a linear response
Small (narrow) latitude means high or low contrast
high contrast! like when we talked about adjusting brightness windows on digital images…
What does FPTFTAD stand for?
Flat Panel Thin Film Transistor Array Detectors. I think he made the up. The TFT is real, though.
Describe the difference between and Direct and an Indirect TFT (thin film transistor) Array.
With indirect, you get photons scattering in the scintillator so you detect signal in a few pixels. With direct, instead of photons you can ion pairs in the scintillator, and an electric field is applied across the scintillator so you get singal in one pixel/
I should maybe add a question about Scintillators and Intensifying Screens?
maybe
What is a typical Absorption Efficiency of Scintillators?
50% of x-rays are absorbed
What is a typical Conversion Efficiency of Scintillators?
10% of energy of each photon is converted to light
What is the formula for scattered to primary ratio?
SPR=S/P
What is the formula for “fraction”?
F=S/(S+P), where S and P are Secondary and Primary
Think about the Scatter-to-Primary Ratio Graph… Would a thicker person result in the curve be shifted up or down?
Shifted up. Thicker=more scatter.
What is the Bucky Factor? What is a typical value?
the relative increase of x-ray intensity you need when you put a grid in the way to get the intensity you would have without a grid. Typical value is 3 - 8.
What is a typical grid ratio value? Grid Ratio=Height of grid (or depth?) / width between bars of the grid.
GR=H/W is typically between 8 and 14. Usually around 10.
What is a Bucky Grid?
Reciprocating Grid. One that moves.
Define Exposure and its units
Electric charge per unit mass produced by ionizing radiation. Units are the Roentgen R=C/kg
Define Dose and its units
energy absorbed per unit mass by ionizing radiation. Units are Gy=J/kg.
Dose is also = (W)(exposure)
Define W and its units
energy deposited per ion pair. J/C or eV/ion pair. 33.97!
What is a Z value for soft tissue?
about 7.4. H=1 and O=8
What energy range is ideal for mammography and Why?
15-20 keV. Think about the Graph of linear attenuation as a function of energy. There is a larger separation between Fat, Glandular, and Infiltrating Ductial Carcinoma (tumor) at lower energies. Anything lower then 15 will just be delivered as dose… bad.
What target is often used for mammography? What are the characteristic photon energies?
Molybdenum. 17.5 and 19.6 keV.
DESCRIBE THE K-EDGE
the energy level of a photon which is enough to knock out k-shell (inner-most shell) electrons. If photon energies are greater than this, there are lots more opportunities to interact, so more attenuation!
Describe the Heel Effect.
the photon intensity is affected on one side by the anode you you end up with a boob-shaped curve. Great for imaging breasts.
Molybdenum. What are the values for Z, k-edge, and two characteristic X-rays
Z=42
K-edge=20 keV
x-rays are 17.9 and 19.5 keV.
Rhodium is sometimes used as a target in mammography. What are the values for Z, k-edge, and two characteristic X-rays
Z=45
K-edge=23.2 keV
x-rays are 20.2 and 22.7 keV.
What can Beryllium be used for in mammograpy?
as a window to minimize absorption. Beam hardening.
What is the Z of Beryllium
4
compressing breasts lowers or increases scatter-to-primary ratio
lowers it.
What are typical focal spot sizes in mammography and radiography?
0.3mm and 1.2 mm.
What are typical values for frames per second, number of images, and amount of radiation compared to a normal x-ray in fluoroscopy?
around 18000 images
30 frames per second
1/1000 of typical dose
What kind of detectors do we need for fluoroscopy?
very sensitive, low noise
What is an image intensifier in fluoroscopy?
basically a photomultiplier tube. curved, large input with an anti-scatter grid and a small, flat output.
describe pincusion distortion
fallout in brightness at the periphery of a fluoroscopic imag
What are typical input and output diameters in fluoroscopy?
Input 15-35 cm
Output 2.5cm
What to things contribute to brightness gain in fluoroscopy?
Electronic Gain ~50 and Minification gain ~50-140
Fluoroscopy? Would a larger field of view result in a bettwe or worse resolution?
worse
