B - Factors affecting scatter radiation fog Flashcards
What are The Two Diagnostic Attenuation Processes?
Photoelectric Effect/Photoelectric Absorption
Compton Effect/Compton Scatter
What is Compton Effect/Scatter and Scatter Fog?
Type of secondary radiation
The patient is the most significant source of scatter radiation
Most occupational exposure comes from scatter radiation
Scatter radiation can drastically affect the contrast (difference in degree of greyness of structures) of an image
When there is a significant reduction of contrast this is referred to as ‘scatter fog’
There are 4 main factors affecting how much scatter radiation we will get. What are they?
kVp
Volume of Tissue
Field Size
Density of Tissue
There are 4 main factors affecting how much scatter radiation we will get. What are they?
kVp
Volume of Tissue
Field Size
Density of Tissue
What is Parameter Changing - kVp?
As kVp increases, number of photoelectric absorption interactions decreases therefore percentage of compton scatter interactions increases in relation
(So you may read that as kVp increases, scatter decreases, which is true, but the percentage of Compton scatter interaction IN RELATION to absorption increases, therefore we say scatter increases)
This decreases image contrast
The overall energy level of the scattered photons will be higher as kVp increases
INCREASING kVp = INCREASED SCATTER = DECREASED QUALITY
What is involved in Parameter Changing – Volume of Tissue?
The volume of tissue irradiated is determined by the thickness of the subject and the size of the radiation field
When the subject is more than 10 to 12cm in thickness, the amount of fog becomes objectionable unless the field size is very small
As volume increases, scatter radiation increases
When there is a greater quantity of tissue in the path of the x-ray beam, there will be more opportunity for interactions that produce scatter
Patient positioning is important!
INCREASING THICKNESS= INCREASED SCATTER = DECREASED QUALITY
What is involved in Parameter Changing – Field Size?
The field size is the area set by the collimators on the x-ray tube, it determines the area of the patient that is exposed to the radiation
As field size increases, scatter radiation increases
The less of patient that is exposed to radiation, the less opportunity for scatter interactions to occur
INCREASING FIELD SIZE= INCREASED SCATTER = DECREASED QUALITY
What is involved in Parameter Changing – Density of Material?
The density of the material is determined by the number of electrons (density of electrons) present in the outer shell of the atom
The more dense the material (more electrons in outermost shell), the greater the number of “electron-scattering” opportunity there will be per given unit of the material
As density of material increases, scatter radiation increases
INCREASING DENSITY OF MATERIAL = INCREASED SCATTER = DECREASED QUALITY
What does scatter not change with?
Atomic number of material (except for materials with significant proportion of hydrogen)
What are the 4 Factors Affecting Scatter Radiation Fog?
↑ Kilovoltage= ↑ scatter = ↑ fog
↑ Volume of Tissue= ↑ scatter = ↑ fog
↑ Field Size= ↑ scatter = ↑ fog
↑ Density of Matter = ↑ scatter = ↑ fog
What factors affect X-ray photon quantity?
X-ray photon quantityrefers to the number of photons produced during an exposure.
Factors influencing x-ray quantity includes:
peak voltage (kVp): beam quantity is approximately proportional to the square of the tube potential
generatortype/voltage waveform: reducing ripple increases beam quantity
beamfiltration: increasing filtration reduces beam quantity
distance from the beam:inverse square law
current (mA): beam quantity is directly proportional to current
exposure time (seconds): beam quantity is directly proportional to exposure time
anode material: beam quantity is directly proportional to the atomic number (Z) of theanodematerial
What factors affect x-ray photon quality?
X-ray photons qualityrelates to the x-ray spectrum changes and the effective photon energy. The effective photon energy is approximately equal to between one third to one half of the maximum photon energy.
Factors influencing x-ray quality include:
peak voltage (kVp)
voltage waveform: reducing ripple increases quality
beam filtration: increasing filtration increases quality through beam hardening
anode material: photon energy depends on the binding energies of shells in the anode material
What is inverse square law?
The strength of the X-ray beam is inversely proportional to the square of distance from the source (X).
Standing back by double the distance from a source of radiation (d to 2d) will quarter the dose to the radiologist or radiographer.
