Radiation characteristic
3 characteristic -
beam = quality , quantity, intensity
controlled by kilovoltage + filtration
kVp setting is only component that will influence penetrating ability of the beam of radiation
Voltage + kilovoltage
Voltage:
measurement of electrical force that cause electron to move to cathode to anode
determines speed of electron
measure volts or kilovolts (1000)
Lab 60 kV = 60,000
modern machine - 60-70 kV
Kilovoltage peak:
max or peak voltage
regulates speed and energy of electron
determine the penetrating power
High kVp = more penetrating power = shorter wavelengths
Density + contrast
Contrast:
how sharply black = white areas differentiated on an image
Density:
Overall darkness or blackness of an image
KVP + Contrast
A. lower kVp = high contrast + short scale
B. High kVp = low contrast + long scale
Exposure time
Exposure time:
interval of time during which x-ray are produce
longer exposure time = longer the x-ray are emitted
longer exposure = darker the image
Impulses:
x-ray created in burst or pulses
60 pulse every one second
1/60 a pulse
kVp and exposure time are inversely related
x-ray beam quantity + amperage
Quantity; # of x-ray produce
Amperage:
determines amount of electron flow through the cathode
dent radiograph units amperage is small
mA - 1/1000 of amperage
Modern machines → 6-8 mA
Milliamperage
Regulates the temp of the cathode filament
Increase temp results in # of electron produced
Exposure time and mA are inversely related
Milliampere seconds MAS
combination of mA and exposure time
milliamperes x exposure time = mAs
mA + exposure time = total amount of radiation created
Effect of milliamperage on film density
low in mA = decrease in density, image will be lighter
increase in mA = increase in density, image will be darker
X-ray beam intensity
Quantity + quality x-ray photon per unit of exposure time affected by:
miliamperage
kilovoltage
exposure time
distance
Distance + intensity
Distance affect intensity of beam
increase of beam reduce as distance increase
must change in exposure time, kVp, or mA or maintain intensity
Inverse square law
relationship between distance and intensity of radiation
factors inversely proportional
distance is double + no change are made to machine, ¼ intense
Flashlight principle:
lines represent intensity → depends of power of source
Intensity is inversely proportional to the square of distance from the source
Rule of thumb PID from:
8in → 16in reduce intensity to ¼ value at 8in
18in to 8in increase to 4x value at 16in
Half-value layer
reduction of intensity of x-ray beam by one-half by placing a thickness in its path (aluminum)
aluminum filter increase the penetrating ability of the beam while reducing intensity
removes low energy
Radiation Biology
Manmade radiation :
medical procedures- 36%
nuclear medical - 12%
consumer products - 2%
industrials + occupational- 1%
Average person is expose to 3.1 mSv/yr of human-made radiation per year
Biological effect
ionization : energy transferred to the irradiated tissue primarily by photelectric and Compton interaction, change in biological
Free radical formation - photon ionizes the water in the cell, highly reactive and unstable
2 theories of radiation injury
Direct theory - cell damage result when ionizing radiation directly hit critical area w/in cell causing damage (DNA)
Indirect theory ( poison water) - x-ray photon causes damage by cell ionizing in water forming toxic
Threshold theory : No visible change occur until a certain threshold dose is reached, w/ increasing lvl of exposure a threshold will be reached
Nonthreshold theory: No matter how small the dose, biological damage occur, no safe amount
Nonstochastic Effect: have a threshold, increase in severity w. increase absorbed dose, effect occur once threshold is exceeded. Physical effect, significant cell damage
Stomachstic effect: do not have a dose threshold, direct function of dose, probability increase w/ increase absorbed dose, severity of effect does not depend on magnitude of dose - 1 photon could cause cancer
Radiation injury sequence + repair
latent period - time that elapses between exposure → ionizing radiation
Period of injury - different type of injury may occur ranging from cell injury to cell death
Recovery period - not all injuries are permanent, most damage called by low-level radiation is repaired w/in cell of body
Cumulative effect of radiation
Effect of radiation exposure are additive, damage that remain unrepaired accumulates in tissue
Determining factors
total dose → dose rate → amount of tissue irradiated → cell sensitive → age
Radiation effect
Short term effect - seen w/ higher dose over a short period of time (hair loss)
Long term effect - seen w/ smaller dose over a long period of time (cancer, genetic effect)
Somatic effect
Effect that can occur in cells of the body except productive cell
effects are present in person irradiated
cancer
not transmitted to future generation
have a recovery rate of 75% during 1st 24 hour
repair continues at the same rate
Genetic effect
occur only in reproductive cell
not seen in person irradiated
radiation-induced mutation are passed on to future gen
Cell sensitivity to radiation
based on 3 thing;
mitotic activity : increased activity make cell more sensitive
Cell differentiation : immature cells more sensitive
Cell metabolism : higher metabolism more sensitive
Critical organ exposed during dent radio
skin, thyroid gland, lens of the eye, bone marrow ( in mand.)
Radiation measurement
established by ICRU, 2 system used
old system traditional or standard
new system SI system
Measurement term
Exposure: Ionization in air produced by x-ray
Absorbed dose: amount of radiation asorbed
Dose equivalent: compares biological effect of different type of radiation and place them on a common scale, different type of radiation have different effect on tissue.