Unit 1 Flashcards
What is a radiograph?
an image generated through the use of X-rays
What is required to make a radiograph?
X-rays, object and recording device
What colours are what on a radiograph?
Bones are white, soft tissue is gray and everything else is black
What does kVp stand for?
kilovolts peak
Why can x-rays be dangerous?
they are a form of ionizing radiation
What is the correlation between kVp and contrast?
Higher kVp lowers contrast (chest) lower kVp increases contrast (abdomen)W
What factors affect attenuation?
atomic number, density, thickness, and energy of the x-ray photons
What is an IR?
Image receptor, the device that resolves the image
What is isotropic emission?
Release of beams equally in all directions
How do we reduce the effects of beam geometry?
smaller source, collimation and less OID (object to image receptor distance) - less space for electrons to diverge
What is the relationship between intensity and distance?
Intensity=1/distance^2
What is the charge of an electron?
-1.6x10^-19 coulombs
How many electrons in a coulomb?
6.24x10^18
What is the charge of a proton?
1.6x10^-19 coulombs
What are the 4 laws of electrostatics?
- opposites attract, likes repel
- charge is usually distributed evenly around objects
- charges concentrate around sharp corners
- coulomb’s law
What is the formula for Coulomb’s law?
F=k(q1q2/d^2)
What Is the q in coulombs law?
charge in Coulombs
What is the d in coulombs law?
distance in meters
What would happen to the electrostatic force between the cathode and the anode if we increased the distance between them by a factor of 5?
the force would be reduced by a factor of 25
What is electrical potential?
work required to move a positive charge from infinity to its current position, measured in volts (J/C)
What is potential difference?
how much energy is transferred moving a charge from one point to another, the negative point isn’t necessarily negative it is just the smaller of the 2 charges
What is electrodynamics?
the study of charges in motion
What is a conventional current?
what way the +ve charge WOULD move
What is electron current?
What way the -ve charge DOES move
What is an ampere?
it is a rate, and the unit for currents (C/s)
what is mAs?
total amount of charge
(C/s) x s = C
What is the relationship between current and potential difference?
Need potential difference for current to flow, however potential difference can exist without current
What is the potential difference in the X-ray tube?
the kVp - energy of electrons (eV)
What is mA in the tube?
the rate at which charge is flowing across the tube
What is mAs in the tube?
the total amount of charge flowing across the tube
What is the difference between direct an alternating current?
direct is unidirectional while alternating switches direction
What kind of current do we have running across the X-ray tube during an exposure?
Direct current
Why can’t you give an average of an alternating current?
peak symmetry
How do you find the Effective (RMS) of an AC?
Effective (RMS) = Peak/1.414 OR Peak x 0.707
What is the effective value?
The value of a constant DC that would generate the same amount of heat
What does RMS stand for?
Root mean squared
What are conductors?
Copper, Gold, Silver and Aluminum
- conduction band and valence band overlap
What are insulators?
No electron flow in conduction band
- oil, glass, rubber, plastic and ceramics
What are semi-conductors?
- they have small intrinsic currents
- can be doped into conductors
- silicon and germanium
What is resistance?
impedes the current: caused by interactions of current electrons with the conductor or impurities
Unit: ohm - 1 ohm produced when 1V pushes 1A through a resistor
How do we see energy lost due to resistance?
Heat - work must be done to overcome the resistance
What are the factors that affect resistance?
Material, Area, Length and temperature
How does Area affect resistance?
more space to move through less resistance
- inversely proportional
2xArea = 1/2 resistance
How does Length affect resistance?
more atoms to pass by, more resistance
- directly proportional
2xLength = 2xResistance
How does temperature affect resistance?
Affects materials differently
- Increased T of conductor = resistance goes up
- Increased T of insulator/semi-conductor = resistance goes down
What is Ohm’s Law?
The current flowing through a conductor is directly proportional to the potential difference between its ends, so long as all physical conditions remain constant
I=V/R
What do the V I and R stand for in Ohm’s Law?
V is voltage measured in Volts
I is current measured in Amps
R is resistance in Ohms
What is a series circuit?
one current path
CURRENT REMAINS THE SAME
voltage drops across each resistance
Rt = R1 + R2 + R3
It = I1 = I2 = I3
Vt = V1 + V2 + V3
What is a parallel circuit?
VOLTAGE REMAINS THE SAME
Highest I through the lowest R
1/Rt = 1/R1 + 1/R2 + 1/R3
It = I1 + I2 + I3
Vt = V1 = V2 = V3
What is a GFCI?
a ground fault circuit interrupter
- super sensitive detection of faults in outlets
- must be reset
What is power?
the rate at which work is done
Unit: Watt (W) - Joules per second
V=P/I
P=I^2R
P=V^2/R
How does a capacitor work?
similar to a battery
the potential difference decreases as the capacitor charges
- charging stops when there is no more potential difference
- used in timer circuits, electrical storage and waveform smoothing
What are magnetic dipoles?
charges in motion creating magnetism, the direction of the motion affects the direction of the dipole
What is a magnetic field?
perpendicular to the motion of charged particles
What are lines of force?
amount of lines of force is Flux (Wb)
What is flux density?
concentration of lines per area (T)
What is a magnetic domain?
several aligned dipoles - happens spontaneously in some materials
What are diamagnetic materials?
no inherent dipoles
slightly repelled by magnets
water, wood, plastic
What are paramagnetic materials?
some inherent dipoles
can be weakly magnetized by an external magnetic field
gadolinium
What are ferromagnetic materials?
contain many inherent dipoles
can become strong magnets
iron, cobalt
What are the ways to create a magnet?
Interact with a magnet
hammer
run current through (right hand)
What are permanent magnets?
hard ferromagnetic material
aligned domains stay aligned
How do you destroy magnetism?
heat
hammer
reversing the field (left hand)
What are temporary magnets?
soft ferromagnetic material
only stay aligned in magnetic field
electromagnets
What is electromagnetism?
creating magnetism from electricity
running a current through a wire
can be turned on and off
What type of magnetic force in straight wires?
weak
Why do you want coiled wire in electromagnetism?
lines of force add together
increased flux density
What factors affect magnetic field strength?
number of turns in coil
size of current
size of iron core
What are the uses of electromagnetism?
electromagnetic locks
transformers
circuit breakers
motors
What kind of current is used to create movement for the motor principle?
alternating
What magnets does the motor principle use?
1 permanent and 1 temporary
What is the conversion of energy in electromagnetism?
electrical energy - magnetic energy - kinetic energy
What is electromagnetic induction?
using magnetism to create electricity, opposite of electromagnetism
- basis of power generation
- uses a permanent magnet, conductor and movement
- the conductor must cross magnetic lines of force for a charge to occur
What is the conversion of energy in electromagnetic induction?
kinetic energy - magnetic energy - kinetic energy
How is electromagnetic induction force created through movement?
when moving parallel to lines creates 0V, pea voltage when crossing at 90 degrees
What are the factors that can affect induced voltage?
- permanent magnet strength
- armature: # of loops in coil and size of iron core
- speed of movement
- angle armature crosses lines of force
What are the uses of electromagnetic induction?
transformers, induction motors and generators
What is a transformer?
electromagnetism in the primary wire (pulsating DC OR AC) creates electromagnetic induction in the secondary wire
- both wires have the same amount of volts per turn
What is the turns ration and transformer law?
Transformers can increase or decrease their voltage in their secondary wire
- turns ratio: Ns/Np
Pp=Ps
Ratio >1 = step-up - V increase I decrease
Ratio <1 = step-down V decrease I increase
What is the transformer law?
Vp/Np = Vs/Ns
Vs/Vp = Ns/Np
Is/Ip = Np/Ns
What is lamination in transformers?
layered, not a solid core
- increases efficiency
What is an autotransformer?
smaller steps up or down
- this is why you have to choose mA in steps on X-ray machine
What is a generator?
- supply the energy used to create X-rays
- different generators create different shaped waveforms
- different generators produce x-ray beams of different intensities
What is the issue with AC in X-ray?
generators create AC, but the charge is only effective at creating X-rays through half of the cycle, so wasting half the cycle
What is self-rectification?
occurs with no rectifiers in place
- current can only run in one direction creating a HALF WAVE
exposures take 2x as long
What are rectifiers?
the convert AC current to DC
- use 4-6 in X-ray circuitry
what is generator rating?
Generator rating (kW) = kVp x mA / 1000
kVp will always be 100 kV if not given
- this measures maximum power of the generator
- usually use the max mA a generator can support at 100kVp for 100ms
How are generators classified?
based on waveform and rating
What is a single-phase generator?
-1 armature (coil) rotating within magnetic field
- electromagnetic induction creates voltage: usually AC
- 60Hz frequency
What is a 3 phase generator?
- rotate 3 armature in a magnetic field or rotate magnet surrounded by 3 coils
- induction occurs in all 3, creating 3 separate waveforms
- each are 60 Hz, but 120 degrees out of phase with each other
Why are circuits kept separate until after rectification?
do not want them to summate
- 6 rectifiers for 3-phase, 6 pulse: 2 forward bias, 4 reverse bias
What is a delta connection?
each coil end connected to each other in a triangle
- can provide high power and torque
- more prone to distortion and overheating
What is the star connection?
each coil is connected to a common neutral wire and each other, like spokes on a wheel
- less power and torque
- less distortion and overheating
Types of 3 phase generators?
6 pulse and 12 pulse
12 pulse uses more rectifiers
What are high frequency generators?
can be single or 3 phase
0.5-25 kHz
Benefits: small size, very efficient, better beam quality, higher tube ratings
What are the 7 steps of high frequency from source to tube?
- rectification
- smoothing
- inverter
- high tension transformer
- rectification
- smoothing
- create a radiograph
What happens in the first rectification?
change 60 Hx AC to DC
what happens first smoothing?
provide constant voltage
usually through capacitors - sometimes batteries
inverter?
rapidly switches polarity of voltage
from DC back to AC - high frequency
High Tension transformer?
large steppe transformer
V to kV
high voltage and high frequency
2nd rectification?
change AC kV to DC kV
2nd smoothing?
High frequency DC kV to constant kV
More efficient generators have what kind of ripples?
lower
What is a falling load generator?
uses shortest exposure time possible without damaging the tube
can cause focal spot blooming
when is capacitor discharge used?
charge capacitors prior to leaving for portable X-rays
discharge during exposure
What is a modular generator?
one transformer/generator supplies to 2-3 rooms
