Q&A Flashcards
X-rays and visible light
are both forms of EM
Compared to visible light photos, X ray photons have
- Shorter wavelength, high frequency and higher energy
Some Same properties
- Weight, Speed, Etc
wavelength and frequency relation
inversely related
energy and frequency relation
are the same
During dental radiograph exposure, what is the primary type of interaction happening in the pt between X-ray photons and outer shell electrons
COMPTON
where are X-rays made in machine
filament
how are X-rays made
passing electron current through wire – jump from atom to atom
cloud of electrons collect around wire – thermionic
electrons and light released
between cathode and anode
high voltage
- Accelerated to huge speed due to positive speed and collide with black area target
(Tungsten)
direction of X-ray photons released from Tungsten target
all directions
result of X-ray photons being released in all directions from Tungsten target
Low energy absorbed by Al
Too divergent – hit side of spacer cone
Only right direction and right energy reach patient
continuous and characteristic spectrum interaction
Occur in x ray tube
Electron interacting with atom at target (tungsten)
Aim to lead to production of X-ray photons
photoelectric and Compton effect
Occur in pt
X-ray photons interacting with atom in tissue
Leads to attenuation of X-ray beam
continuous spectrum target interaction
Incoming electron passes close to target nucleus in Tungsten target, causing it to be rapidly slowed down and deflected
- Lost kinetic energy released as photons
- Closeness to nucleus determines degree deflected
Energy of photon can be anything between 0eV and the energy of the original electron
characteristic spectrum target interaction
Incoming electron collides with an inner-shell electron and either displaces it into a more peripheral shell (excitation) or removes it completely (ionisation)
- Reshuffle to fill space in inner most shell
Energy of photon can only be specific values characteristic of that tissue
- Jump difference between higher and lower shell
Characteristic to atom e.g. Tungsten
photoelectric effect
in pt tissues
Involves inner shell electrons
Results in complete absorption of x ray photon
Energy of incoming photon must be equal to or just greater than the binding energy of the electron
Contributes to image contrast and patient dose
compton effect
in pt tissues
Involves outer shell electrons
Results in scatter and partial absorption of photon
Energy of the incoming photon is much greater than the binding energy of electron
Contributes to pt dose
grading of radiographs (3 levels)
1 - excellent
2 - diagnostically acceptable
3 - unacceptable
excellent Grade 1 radiographs
no errors of pt preparation , exposure, positioning, processing or film handling
target - more than or = 70%
diagnostically acceptable Grade 2 radiographs
some errors of pt preparation, positioning, processing or film handling, but which do not detract from the diagnostic utility of the radiograph
target - less than or = 20%
unacceptable Grade 3 radiographs
errors of patient preparation, exposure, positioning, or film handling, which render the radiograph diagnostically unacceptable
target - less than 10%
reject analysis
needs to be carried out as well
why may a radiograph be too dark
overexposed or underdeveloped so grade 3
If a person stands 2m from an X-ray source absorbed 20uSv
how many received at 4m?
5uSv
further away from X ray source means
lower dose
inverse square law for X-ray intensity
Twice as far away quarter dose
Double distance = Quarter dose
How far away should you be from X-ray source and pt when exposing a dental radiograph (< or = 70kV)
1.5m at least
From source and pt (Compton effect scatter)
Never stand in direct line of pt X-ray beam
In a small room treat whole room as controlled area
- Stand out of door
Grays measure
absorbed dose
Sieverts (Sv) measure
effective dose and equivalent dose
absorbed dose (D)
measure of the amount of energy absorbed from the radiation beam per unit mass
medical exposures values often in mGy (x10^-3)
equivalent dose (H)
takes into accoutn the radiobiological effectiveness of different types of radiation
equivalent dose = absorbed dose X radiation weighting factor
radiation weighting Factor
X-ray and gamma photons+
Beta particles=
Alpha particles=
1
1
20
effect dose (E)
takes into account the radio-sensitivities of different types of tissue
- some body parts are more sensitive to radiation than others
this allows for different investigations irradiating different parts of the body to be compared, by converting all doses to the equivalent whole body dose
uses a tissue weighting factor
tissue weighting factors trend
higher the number = the more sensitive the tissue
tissue weighting factors
gonads = red bone marrow = breast = thyroid gland = bone surface = skin =
gonads = 0.20 red bone marrow = 0.12 breast = 0.05 thyroid gland = 0.05 bone surface = 0.01 skin = 0.01
also Brain not that sensitive but thyroid gland is more sensitive
pregnancy and dental radiographs
No problem taking dental radiographs in 1st trimester
May delay for emotional/psychological reasons
effective dose and risk
more likely to die from periapical than win the lottery
- 1 in 10 million compared to 1 in 47 million
Perspectives of risk
1 in 1 million risk
- CBCT impacted molar
- panoramic
<1 in 5 million risk
- maxillary occlusal
- periapical/bitewing
1 in 125,000 risk
- transatlantic flight x 2
- 100g brazil nuts
radiosensitivity and age
younger pts more radiosensitive since cells dividing more rapidly
longer lifespan = more time for effects to develop
age groups multiplication factor for risk
<10
x 3
age groups multiplication factor for risk
10-20
x2
age groups multiplication factor for risk
20-30
x 1.5
age groups multiplication factor for risk
30-50
x 0.5
age groups multiplication factor for risk
50- 80
x 0.3
age groups multiplication factor for risk
> 80
negligible risk
skin focus distance for correct magnification and dose of radiation
needs to be 20cm from source
