Ch. 8 X-ray emission Flashcards
What is the intensity of the x-ray beam of an x-ray imaging system measured in in air?
mGy(subscript a) formerly milliroentgen
What is mGy (air) called?
x-ray quantity
x-ray quantity and radiation exposure are both measured in…
mGy (air) or mR
mGy measures what in the air?
the number of ion pairs produced in the air by a quantity of x-rays.
X-ray quantity is the number of…
x-rays in the useful beam
Most general purpose radiographic tubes, when operated at 70 kVp produce x-ray intensities of about…
50 microgray/mAs or 5 mR/mAs at a 100cm SID
x-ray quantity is directly proportional to…
mAs
formula for relationship of mAs to quantity
I(1)/I(2) = mAs(1)/mAs(2)
x-ray quantity is proportional to the square of the ratio of…
kVp
formula for relationship of kVp to quantity..
I(1)/I(2) = (kVp(1)/kVp(2))^2
to maintain constant exposure of the IR, kVp should increase by ___ and mAs should…
15%, decrease by 50%
DESCRIBE inverse square law.
x-ray intensity varies inversely with the square of the distance from the x-ray tube target
Formula for inverse square law?
I(1)/I(2) = (d (2)/d (1))^2
To maintain constant exposure when SID is increased, mAs should be…
increased by SID^2
What is the formula for the square law?
mAs (1)/mAs (2) = SID(1)^2/SID(2)^2
DESCRIBE the square law
When SID is increased, mAs must be increased by SID^2 to maintain constant exposure to the IR
How much filtration is positioned in the useful beam in a metal filter?
1-5 mm of Al
Adding filtration to the useful beam reduces…
patient dose
As the energy of an x-ray beam is increased, the penetrability is…
also increased
What does penetrability refer to?
the ability of x-rays to penetrate deeper in tissue
Penetrability of a beam is also called…
x-ray quality
What do not affect radiation quality?
Distance and mAs
Attenuation is…
the reduction in x-ray intensity that results from absorption and scattering
How is the quality of x-rays measured?
by the HVL.
What is the range of the HVL in a diagnostic beam?
3-5 mm Al or 3-6 cm of soft tissue
The HVL is the…
thickness of absorbing material necessary to reduce the x-ray intensity to half of its original value.
X-ray beam quality can be identified by kVp or filtration, but what is most appropriate?
HVL
What can increase x-ray quality?
kVp and Filtration
What can increase x-ray quantity?
mAs & kVp
What can reduce x-ray quantity?
distance and filtration
What has no affect at all on x-ray quality?
mAs & Distance
What happens to beam quality and the HVL if kVp is increased?
quality is increased
What are the classifications of filtration?
added, inherent, compensating
How much filtration is in the inherent classification?
0.5 mm of equivalent Al
What is the bow tie filter?
a filter used with CT to compensate the shape of the head or body
