Physics of Radiation Flashcards
Wilhelm Roentgen
discovered radiation in 1895
How did Roentgen discover radiation?
He:
- created a vacuum tube, electrical current, and screens; noticed the green lights coming from the screen and named those rays “x-rays” (with “x” as the symbol for the unknown)
- demonstrated that shadowed images can be recorded permanently on photographic plates by placing objects between the tube and the plate
- exposed his wife’s hand for 15 minutes and saw the appearance of bones. This image is considered the first radiograph of the human body (you could see her ring as well)
Otto Walkhoff
created the first dental radiograph in 1895
- placed a glass photographic plate in his mouth and exposed himself for 25 minutes
Edmund Kells
created the first practical radiograph in dentistry in 1896
Atom
Smallest component of an element
- Consists of a nucleus and orbiting electron(s)
Molecule
two or more atoms combined
- molecule is the smallest particle of substance that retains the property of the original substance
Nucleus
Positively-charged central core of an atom consisting of protons and neutrons
Contains nearly all of the atom’s mass
Protons
+ charge
Neutrons
no electrical charge
Electrons
(-) charge
Orbiting shells
three-dimensional structures around the nucleus that contain electrons
each shell can contain only a specific number of electrons
Shells are designated to which capital letters?
KLMNOPQ
The closer the orbit is to the nucleus, the — its binding energy (stronger bond)
Higher
What letter shell interaction has the highest binding energy and is the most useful for medical imaging?
K Shell
In order to remove an orbital electron from its shell?
the energy equal to or greater than the binding energy of a specific shell is needed
Radiation
propagation of energy through space in the form of particles or waves
T/F: Any exposure, no matter how small, doesn’t have the potential to cause harm
False, it does
Ionization
process by which an atom gains or loses electrons to become a negatively- or positively-charged atom
Ion
charged particle that is either positive or negative
Ion pair
positive ion (atom missing an electron) and a negative ion (ejected electron)
Ionizing radiation
any type of radiation that is high in energy and is capable of producing ions
What type of radiations are formed because of an ionizing process?
Particulate and electromagnetic radiation
Particulate radiation
involves particles that have a mass and travel in straight lines at high speeds
may have a charge, except for neutrons
include neutrons, protons, electrons, Beta particles, and Gamma particles
Can particulate radiation reach the speed of light?
No
Electromagnetic radiation
electric and magnetic fields of energy that move through space in a wave-like motion
includes radio waves, microwaves, visible light, UV rays, x-rays, and gamma rays (from low to high energy)
Only high energy waves have?
Ionizing capacity
X rays
powerful invisible rays that are capable of penetrating substances
travels at the speed of light and has no mass or charge
Wavelength
distance between the peaks or crests of one wave to another (measured in horizontal length)
determines the energy of the radiation
Longer wavelength
low frequency
has less energy and thus less ability to penetrate objects
Shorter wavelength
high frequency
has more energy and more ability to penetrate objects
X-ray Characteristics
Invisible
No mass or weight
No electrical charge
Travel at the speed of light
Travel in a wave-like motion
Short wavelength of high frequency
Cause ionization
Can produce images
Can cause biological changes in cells
what is first in the sequence of x-ray production?
An electrical current travels through the step-down transformer to the cathode and reduces the voltage from 110/220 to 3-5 volts
what is second in the sequence of x-ray production?
Filament is heated when the switch is activated
what is third in the sequence of x-ray production?
At the cathode, an electron cloud forms around the molybdenum cup by thermionic emission
what is fourth in the sequence of x-ray production?
Electrons are expelled at a high speed toward the anode’s focal spot within the tungsten target
what is fifth in the sequence of x-ray production?
Electrons hit the tungsten target at the anode and transform kinetic energy into x-rays.
When the electrons hit the tungsten target, heat is formed, but it is dissipated through the copper stem.
Bremsstrahlung (braking) radiation
Major source (primary kind) of x-rays produced in dentistry (70%)
Results when high-energy electrons come close to the nuclei of the tungsten atoms. The electrons move towards the nucleus (since they have opposite charges) and then slow down (this is why the name “braking” is used).
When the electrons brake, energy is released (photons)
Characteristic radiation
Electrons from the cathode dislodge electrons from the inner orbital K/L shell of the tungsten atoms.
When the electrons of the inner shells are dislodged, electrons from other orbits fill the empty space left by the ejected electron. During this process, energy is produced.
Not as significant a source of x-rays as braking radiation.
After the x-rays leave the tube, they interact with?
any structure that is in the path of the x-rays
Depending on the interaction between the x-rays and matter, different names are given
Compton scatter
Responsible for most of the interactions with matter in dental x-rays (around 60%)
What is the outcome of a compton scatter?
A photon interacts with an outer shell electron of a matter. The photon loses energy but continues to travel, and the electron is ejected. It is always easier to hit the** outer shells** than it is to hit the inner shells because the outer shell is bigger and contains more electrons
Photoelectric scatter
Responsible for about 30% of the interactions with matter in dental x-rays
what happens during a photelectric scatter?
- A photon interacts with an inner shell electron
- The photon is absorbed and the electron is ejected
- Ionization occurs
less common
coherent (thompson) scatter
Responsible for about 8% of the interactions with matter in dental x-rays
what happens during a coherent scatter?
- Photons interact with an outer shell electron but the matter is not altered
- Photons continue to travel with no loss of energy.
- Ionization does not occur
No interaction (with object)
- Photons pass through the atom unchanged (dodging the bullets).
- Produces densities in dental radiographs. If all photons passed through matter (patient) and reach the receptor, the image will be fully exposed (dark)
Primary radiation (X-rays)
Photons (bundle of energy) leave the x-ray tube as primary radiation (photons have not interacted with the patient’s body or any other objects)
Secondary (scatter) radiation
Created because primary beams are interacting with an object (matter) such as the patient’s oral structures (i.e., the x-rays that contacts the tooth creates secondary radiation)
- Not useful for producing diagnostic quality images
- creates fog
Is secondary radiation more or less penetrating than primary?
less, because it usually has longer wavelengths
Radiolucent areas
dark areas
Produced by less dense structures that permit the passage of x-rays
e.g., cysts, air spaces, soft tissues, tooth decay, dental pulp
Radiopaque areas
light areas
Produced by denser structures that obstruct the passage of x-rays
e.g., lamina dura (part of the bone around the roots that is dense), restorations, enamel, implants
Quality
- Describes the energy or penetrating ability of the x-ray beam
- Controlled by kilovoltage
- Higher kVp produces more penetrating x-rays than lower kVp
- Higher kVp (80-100) is used for denser areas such as molars
Quantity
- Describes the number of x-rays produced
- Controlled by ampere (A): the number of electrons that flow through the cathode
- 1 milliampere (mA) = 1/1000 ampere. A dental radiograph operates in very small amounts of amperes, and therefore the unit used in dentistry is mA
- Generally, 7-15 mA is required. Settings above 15 mA are not recommended as this can cause excessive heating
Intensity
Total energy contained in the beam
Affected by mA, kVp, distance, and exposure time
To increase the intenstity of the beam:
Increase mA, kVp, and time
Decrease the distance
To decrease the intenstity of the beam:
Decrease mA, kVp, and time
Increase the distance
Half-value layer
Thickness of aluminum filters that reduces the intensity of the beam by half
The filters remove less penetrating, longer wavelengths
Inverse square law
theory that radiation intensity is inversely proportional to the square of the distance from the source of radiation
To calculate the radiation density:
- Step 1: note how much shorter or longer the distance is (2 times farther → write 2, 3 times shorter → write 3)
- Step 2: square the number (2 times farther or shorter → 2 squared = 4; 3 times farther or shorter → 3 squared = 9)
- **Invert the number only if the source is farther **(2 times farther → 4 → ¼, this means the beam is ¼ intense) (3 times shorter → 9 → do not invert, this means the beam is 9 times more intense)
- If cone length changed from 8 to 16 inches, how did the intensity change? 2 times further → 2, 2 squared = 4, therefore further invert = ¼. Reduced by 4 times, or ¼ the original intensity.
Density
Overall darkness of a radiographic image
Proportional to mA, kVp, and exposure time
Milliampere-seconds (mA-s)
combination of milliamperes and seconds
To maintain similar density:
If time is increased, mA must be decreased
If time is decreased, mA must be increased
15mA, 90kVp, 10 sec is changed to 10mA, 90 kVp.
To keep the same density, how long should the exposure be?
15 seconds
When is density decreased?
if the target structure is dense
Contrast
the difference between the lighter and darker shades of gray on a radiograph
dependent on kVp
What results in many shades of gray?
increased kVp
(long-scale contrast, low contrast)
- Best for detecting bone abnormalities
What results in more black and white areas?
decreased kVp
(short-scale contrast, high contrast)
- Best for detecting caries
Sharpness
Capacity to produce details and distinct outlines
Penumbra
blurring at the edges of a structure on a radiograph
Sharpness can be increased when:
- Tungsten target of anode (focal spot) is small
- Object to receptor distance is short
- Tube (target) to receptor distance is long
What are the most important reasons for fuzziness?
Patient and tube head movements
Distortion
Variation of size and shape of the actual object
Magnification
enlargement of the object
Increased tube (target) to receptor distance allows?
more parallel rays and produces less magnification
Object to receptor distance:
shorter distance produces less magnification
Overlap
caused by improper horizontal angulation
Elongation and foreshortening are caused by?
improper vertical angulation
Elongation
caused by underangulation
- Increase angulation to correct the error
Foreshortening
caused by overangulation
* Decrease angulation to correct the error
In paralleling technique, the receptor and long axis of the tooth must be?
parallel to each other to prevent distortion
