Physics - rad Flashcards
What is the associated voltage ripple for each type of generator?
- Single phase = 100%
- Three phase
- 6 pulses = 14% (13-25% on image)
- 12 pulses = 4% (3-10% on image)
- High frequcncy = 4-15%
- Constant potential = 2%
How do Step up and step down transformers compare?
- Step up: Ns>Np
- increases voltage, decreases amperage
- Step down: Ns
- decreases voltage, increases amperage
This is because:
- ratio of voltage output = the ratio of number of wire turns
- N is the numbr of wire turns
- Input power (voltage x current) = output power
How is power calculated and what are the si units?
- Watt (W) is the si
- W = J/s
- power = voltage (v) x current (i)
Thus
- 1W = 1 volt x 1 ampere (1 ampere = 1 coulomb(C)/s)
- so, V=J/C via algebra
What is the SI unit of energy?
Joule (J) = 1W x 1 sec
How do you calculate heat unit (HU) for three phase and high frequency generators? What is the correction factor multiplier?
HU = kVP x mA x s x fcorr
the correction factor (fcorr) = 1.35-1.4
What is the relationship b/w the joule and the heat unit?
HU = 1.4 x heat input (J)
What is the federal leakage regulation?
< 0.1 R/hour @ 1 meter from focal spot
What is the relationship between the focal spot, focusing cup and filament length?
The focusing cup determind the focal spot width
The filament length determines the filament length
How do you decrease the heel effect?
- Use a larger anode angle
- use a smaller film or collimate more
- Use a larger SID
- position cathode over thicker tissue and anode side over thinner tissue
- (potentially increase kVp according to Watson’s noted)
In regard to the space cloud, what does “space charge limited” and “emission limited” mena?
- <40 kVp = space charge limited - not all electrons pulled to the anode, tube current insufficient to pull all electrons away from the cloud
- >40 kVp = emission limited - all electrons pulled to anode, total # electrons (tube current) is determined by the filament current - further increase in kVp will not change tube current
What increases Bremsstrahlung w-ray production?
increased kVp and increased Z
What influcences x-ray quantity?
- Directly proportional to mAs - mA double, quantity doubles
- increase kVp, incr qauntity (and quality)
Accodring to WarMachine, what things increase quantity? page 40
- Increased Z (more overall x-rays via Bremsstrahlung)
- Increased kVp
- Increased mAs
- Increased voltage ripple (more overall Bremsstrahlung)
What influences quality?
- increased kVp
- increased filtration
- decreased voltage waveform ripple (less ripple in tipple = better quality)
How do you decrease magnification?
- Use a small focal spot
- Use a small anode angle
- Minimize the OFD(OID) → b
- Maximize the FFD(SID) → a+b
How do you decrease the prenumbra?
- use small focal spot
- small object to image distance
- large focus to object distance
- place ROI on anode side
because,
P = ( F x OID)/SOD → F is the focal spot size
How do you calculate true magnification? And when does true mag (M) equal mag (m)?
M = m + (m-1)(f/d)
M= true mag
m=mag
f=focal spot size
d=object size
Note: when f<<<
WHat influences the probability of photoelectric effect occuring?
- increases at energies just above the BE of the k shell (k-edge)
- rapidly decreases at energies much above k-edge
- Inversely proportional to the incident photon energy cubed: 1/E3
- probability directly proportional to the anatomic number cubed: Z3
What is the k-edge for iodine?
33.2
What is tha k-edge for barium?
37.4
Does compton scatter occur with inner or outer shell electrons?
outer shell
How do you calculate wavelength change in compton scatter?
What is the relationship between the angle of deflection and the enegery of the scattered photon?
- change wavelength = 0.024(1- cosø)
- As the angle of defelction decreases, the energy retained by the scattered photon increases
What increases the probability of Compton scatter?
- directly proportional to density
- increases with low binding energy electrons
- Dominates above 30 kEv
- PE= compton at 25 kEv
- proportional to 1/E
- INDEPENDENT of Z
WHat is a major difference between linear attenuation coefficent and mass attenuation coefficient?
LAC is effected by density, MAC is not