Exposure factors

Question 1

Kilovoltage

Answer 1

During exposure the anode is maintained at a high positive electrical potential
Electrons attracted to anode (negative electrical potential)
When potential energy higher than binding energy the electron cloud leaves the filament and accelerates towards the anode in a narrow beam
This potential difference in energy between anode and cathode is measured in kilovolts and is called kilovoltage (kV) or kilovoltage peak (kVp)
Peak – indicates the maximum energy available at that particular setting (radiation is a wave)
Kilovoltage determines how hard x-rays will penetrate patient
Quality factor
Increased kilovoltage = shorter wavelength x-rays = increased penetration penetration
Higher kVp settings allow for lower mAs settings = shorter exposure time
Inverse relationship between kVp and mAs

Question 2

Santes rule

Answer 2

(2x thickness in cm) + SID in inches = kVp
Take measurements with caliper
Measure SID (should be 100 cm which is 40 inches)
This equation is for tabletop only
If film placed in grid a grid quality factor must be added (later discussion)

Question 3

Milliamperage

Answer 3

Electrons are produced by heating the cathode filament – “boils off” electrons from atomic orbits = Thermionic Emission
Free electrons form cloud around filament
Number of electrons is directly proportional to temperature of filament
Ie: higher temperature = more electrons produced and vice versa
Electrical current heats the filament – measured in milliamperage (mA)
As mA is increased, the higher the temperature of the filament and more electrons boiled off
mA controls the number of x rays produced
Quantity factor

Question 4

Time for exposure factors

Answer 4

Total number x-rays being produced also depends on how long filament heated for
Direct relationship between mA and time
Quantity of x-rays required for given exposure best expressed as product of mA and time = milliamperage per second (mAs)
mAs can be calculated by following equation:
mA x time(sec) = mAs

Question 5

Exposure times

Answer 5

Exposure time – period during which x-rays are permitted to leave the x-ray tube
Higher mAs settings allow for shorter exposure times while producing same quantity of x-rays
Shorter exposure times reduce possibility of motion showing on radiographs
Motion = most common artifact in veterinary radiography
Need to have exposure time of 1/30 second or faster
Also reduces amount of exposure to personnel within x-ray suite

Question 6

Distance for exposures

Answer 6

Distance between radiation source (tube) and image receptor (film) is called SID (Source-Image Distance)
As SID decreased, intensity of x-ray beam is increased – inversely proportional
SID is also a quality factor
SID follows inverse square law: (I/d2)
If you double distance then intensity or radiation is 1/22 or ¼
When SID changed must compensate by adjusting mAs (quantity x-rays produced) using this calculation:
Old mAs x (new SID)2= new mAs
(old SID)2
Whenever possible the SID should be kept constant
Should be checked prior to every procedure

Question 7

mA and time

Answer 7

control current supplied to cathode which heats filament
Heating of filament supplies electron cloud which becomes electron beam
Hotter the filament/longer heated = more electrons in cloud = more electrons in beam
QUANTITY FACTOR
Primarily affects density
Once electron beam hits target it interacts with target producing 99% heat and 1% radiation
Radiation directed down to patient due to angle of target
Intensity of x-ray beam defined as number of x-rays travelling from tube towards image receptor per second

Question 8

kVp adjusts

Answer 8

kVp adjusts current applied to field – gives speed to electron beam
Faster speed = short wavelength = higher penetration
Penetrating power determines how hard electrons hit target of anode and x-ray film
QUALITY FACTOR
Primarily affects contrast

Question 9

SID

Answer 9

distance between x-ray tube and film
Greater SID = fewer x-rays able to penetrate film
QUALITY FACTOR