Physics 1 Lecture Flashcards
Filament
- AKA
- Wire used
- Terminal
- How does it work
Cathode
Tungsten wire
Negative terminal
Filament is heated and e- are produced. Focusing cup directs e- towards anode (positive terminal)
Positive terminal is…
-targeted by
Anode
Tungsten target (from cathode). Attracts e- from filament
2 types of anodes
-relationships with large volumes
Stationary
-problematic when large volumes of xrays required
Rotary
-allows for greater heat dissipation over larger area
Line focus principle
- focal spot to get best resolution
- anode angle
Smaller focal spot=better radiographic resolution
Anode is angled 6-20 to help direct/focus beam of e-
-Effective length gets smaller with smaller angles.
Line focus principle helps
Improve resolution
In the line focus principle,
effective focal length equals
Actual focal length X sin (theta)
Define actual focal spot
-Area of the target hit by e-
The smaller the actual focal spot is…
-Smaller the actual focal spot, the more heat produced
Define effective focal spot
The area of the patient/film hit by Xrays
In effective focal spot, the smaller it is…
The sharper the image
Explain x-ray production
Charge is applied to filament (negative terminal) and anode (positive terminal)
Heated filament results in e- cloud
e- focused by focusing cup and directed or attracted towards anode (positive terminal)
Actual focal spot determined by angle of target
99% of e- collide w/anode and dissipate as heat
Actual focal spot is determined by
Angle of the target
What % of xrays hits target
1%
2 types of anode- electron interaction
Bremsstrahlung
Characteristic
Explain Bremsstrahlung
-AKA
“braking radiation”
e- gets close to nucleus, will slow down due to (+) charge of nucleus
When e- ‘brake’ xray photon released
In bremsstrahlung, how can you get a stronger xray made?
Closer to nucleus e- gets, stronger xray made
What accounts for most of xrays made in a diagnostic x-ray machine?
Bremsstrahlung
Explain characteristic
-what is this process known as
e- from inner shell is released
Outer shell e- fills inner shell void and E is given off
-this process is known as e- cascade
In Characteristic, xray E is dependent on
E difference btw shells
Radiographic opacity depends on
density, e-/gram, atomic #
What do less dense organs look like on radiograph?
- photons
- film
- example
Absorb few photons
Greater # of xray photons reaching the film
Blacker on the film
Lung
What do denser organs look like on radiographs?
- photons
- film
- example
Absorb more photons
Less x-ray photons reached film
Whiter on film
Bone or metal
Give top 5 adiographic opacities
Gas-black Fat-Dark grey Soft tissue/fluid- medium grey Bone-light grey Metal-white
kVp
kilovoltage peak (E used in Xray tube)
mA
milliampere (current)
Exposure time
Seconds (time)
mAs
milliampere X exposure time
kVp
- measures
- Increases
- Produces
- Higher kVp
Quality (E/strength)
Increases penetrating power
Produces contrast (black and whie)
Higher kVp=lower contrast
-partially contributes to image darkness
mAs
- measures
- high mAs
- low mAs
- Measures quantity of photons
- -># e- flowing in
High mAs= more black film
–>Doubling mAs doubles images darkness
Low mAs=white image
How can you double darkness
Double mA
kVp + 15%
Contrast vs latitude
Contrast=black/white
Latitude=shades of grey
High kVp
- contrast
- latitude
Low contrast
High latitude
Low Kvp
- contrast
- latitude
Short contrast
Very few shades of grey
If image had:
high contrast/low latitude
Very few shades of grey
Short scale of contrast
what might kVp and mAs be?
low kVp
highmAs
If image had:
high latitude
many shade of grey
Long scale of contrast
What might kVp and mAs be
High kVp
low mAs
2 types of interaction with matter?
Compton scatter
Photoelectric effect
Describe compton scatter
Scater radiation
-Xray photon frees an orbital (low E) e-
- Photon and e- scatter in different directions
- Higher energy photons-increased scatter
When will compton scatter occur?
Increased density
Increased e- density
Increased E (high kVp)
Independent of atomic #
Disadvantages of compton scatter
Scatter
-decreases film contrast and hazard to others
Explain photoelectric effect
xray photon ejects e- leading to an e- cascade
E of xray is COMPLETELY ABSORBED
Produces good image contrast
What is unique in photoelectric effect?
Xray is absorbed NOT SCATTERED
When will photoelectric absorption occur
Using lower E, low kVp
HIGH atomic #
Will increase contrast and patient exposure
Inverse square law
I(1)/I(2) = d(2)^2/d(1)^2
