Intro Flashcards
Which side of the X-ray tube do X-rays come out of?
Anode
How are X-rays created within the X-ray tube?
the cathode creates a lot of free electrons, when they hit the anode they become X-ray photons
What % of electrons become X-rays?
1-2% the rest turns into heat
X-rays move isotropically, what does this mean?
They move in every direction
What is primary radiation (PR)?
All the radiation between the source and the patient
Incident radiation or useful beam
What is Remnant Radiation?
Radiation which leaves the patient and is going to hit the image receptor
RR=PR-(Absorbed+Scattered)
What is secondary radiation?
radiation coming from a place other than the source or target
What is scattered radiation?
primary radiation which has been deflected or changed direction
What is CR?
Central ray - not to be confused with computed radiography
centre of the Xray beam
What is FS/T/S?
Focal spot, target or source - where the X-rays originate
What is SID?
Source to image receptor distance
What is SGD?
Source grid distance
What is SOD?
Source to object distance - the primary beam
also known as TOD (target to object distance), AOD (anode to object distance), and FOD (focal to object distance)
What is OID?
object to image receptor distance
also known as OFD - object film distance
What is mA?
milliamperes
- quantity or intensity
- one ampere is defined as the current that flows with electric charge of one Coulomb per second
- values of mA influence amount of radiation
- rate of radiation production
What is s?
value of time selected
- controls the duration of the exposure
What is mAs?
Milliampere seconds
- total quantity of radiation used for a particular exposure
What is kVp?
kilovolts peak (quality of radiation)
- determines penetrating ability of the radiation
What will happen to the wavelength of the X-ray photons if kVp is increased?
The wavelength will decrease, but the photon will have increased energy
What is attenuation?
any process that decreases/reduces the intensity of the beam
What factors affect attenuation?
Thickness, atomic number and physical density
Why does increased atomic number affect attenuation?
increased amount of outer shell electrons
What is the atomic number of bone, muscle and water?
bone - 13.8
muscle - 7.4
water - 5.9
What are the physical densities of bone, soft tissue and air?
bone - 1.8 g/cm3
soft tissue - 1.0 g/cm3
air - 0.001 g/cm3
How does kVp affect absorption?
by increasing the kVp you are giving the X-ray photons more energy and a shorter wavelength this decreases absorption
Does mAs affect absorption?
no
What is high contrast?
lots of dark shades and brighter shades, few shades of grey
Brightness differences between structures is large
What is low contrast?
many shades of grey, brightness differences between structures are small
What is mA?
current (I)
What is mAs?
total amount of e flow over entire exposure
What is reciprocity?
variety of mA and time settings that could produce the same mAs
What are the advantages to reciprocity?
- control motion
- focal spot size
- breathing techniques
Why would you want to use a smaller focal spot?
better spacial resolution - crisper edges
What is the small focal spot limit by?
mA NOT mAs
What is the inverse square law?
I1/I2=d2^2/d1^2
What is the intensity maintenance law?
also known as the square law
mAs2/mAs1=d2^2/d1^2
What is subject contrast?
a measurement of intensity of the remnant beam caused by differential absorption
What does image contrast depend on?
subject contrast, scatter radiation, algorithms, processing, post processing
kVp and intensity formula?
I1/I2 = (kVp1/kVp2)^2
kVp, mAs and intensity?
I.e. solving for I2
I2 = I1 x (kVp1/kVp2)^2 x (mAs1/mAs2)
What are grids?
Devices that reduce the amount of scattered radiation reaching the image receptor
What factors affect scatter?
kVp, field size, thickness, composition of irradiated material
How is a basic grid constructed?
strips of lead interspaced with radiolucent material
What is the grid ratio?
GR=h/D
D is the width of the interspace material
h is the height of the grid
If you increase the grid ratio, what happens to the amount of scatter reaching the IR?
decreases
Different frequencies effect on grid?
higher frequency, thinner Pb strips, less visible
lower frequency, thicker Pb strips, more visible
What is contrast improvement factor?
K = image contrast with grid/image contrast without grid
1.5-2.5
depends on kVp, patient thickness and field size
What are the Bucky Factors?
How much of an increase in technique is needed compared to non-grid
No Grid - 1
5:1 - 2
6:1 - 3
8:1 - 4
12:1 - 5
16:1 - 6
Who invented moving grids?
Dr. Hollis E Potter in 1920
moves to blur out grid lines
What kind of movement do moving grids do?
either reciprocating (side to side 2-3cm) or oscillating (circular)
Will the movement of the grid cause more grid cut-off?
a little bit
What are the 4 types of grid errors?
off center
off level - angle mismatch
off focus - out of proper SID
upside down
What is the air gap technique?
alternative to using a grid
increasing OID - scatter misses IR
1 foot of SID per inch of OID
Magnification Factor
MF = SID/SOD
MF = Image size/object Size
object size= Image size/MF
