CT Basics (1-2) Flashcards
LAC (Radiation Physics)
Linear Attenuation Coeffiecient: fraction of radiation attenuated (absorbed/scattered) per cm
CT numbers (Hounsfield Units)
Unit to which scanner converts LAC of all tissues within scan field
Large CT numbers are generated by…
…materials (e.g., bone/metal) that absorb a large fraction of the x-ray beam (and appear bright)
Small (even negative) CT numbers are generated by…
…materials (e.g., air/fat) that attenuate very little radiation (and appear dark)
3 conditions of x-ray production (which occur within a vacuum-sealed x-ray tube)
Source of Electrons
Means of rapid acceleration of electrons
Means of rapid deceleration of electrons (releasing electron energy)
Source of electrons
Cathode filament
Rotating Anode
Target of electron stream
Rotating anode where accelerated electrons collide and release energy as heat and x-rays
Tungsten target
Means of accomodating significant heat generated by electron collisions within x-ray tube (3)
Thicker anodes
Faster rotation
Unique tungsten alloys with higher heat capacities and faster cooling rates
Photon “quantity” aka “Intensity”
Number of photons in x-ray beam
Directly proportional to radiation dose and detector exposure
X-ray “Quality”
“Energy” of x-ray photons in the beam
A.k.a. “Hardness,” “Penetrability”
Polychromatic
aka Polyenergetic
x-ray beam’s photons have different energies
The unit keV describes:
Average energy of photons in the x-ray beam
3 Prime (adjustable) exposure factors that control the energy & intensity of the x-ray beam
Milliamperage (mA)
Rotation Time (sec)
Kilovoltage peak (kVp)
mA
“milliamperage”
Measurement for tube current
Controls number of electrons flowing through x-ray tube per second
Intensity (quantity), patient dose, detector exposure are all directly proportional to (1)
mA
ADM
Automatic Dose Modulation
mA varies throughout scan as density of tissue varies
Rotation Time
Total time required for one full cycle of x-ray tube around patient
mAs
Rotation time x milliamperes
Carries a reciprocal relationship to time (mAs/s=mA)
kVp (3)
“Kilovoltage peak”
Voltage of the x-ray tube
“tube potential”
Result of increased kVp (5)
Higher energy photons (“beam quality”) More total photons in x-ray beam (“beam quantity”)
More “intense” beam, more Pt dose, more photons to detector
Higher % photons penetrating Pt to reach image receptor
More even penetration of patient loss of image contrast
kVp modification required for increased part thickness/density
Increase
Aluminum filtration effect (3)
Removes mostly low energy photons from beam (they would add dose to patient, but not reach detectors)
Decreases intensity
Increases avg beam energy
Bowtie Filter
Shapes intensity of beam to match shape of patient (reducing intensity on outer edges of Pt)
Bowtie Filter
Shapes intensity of beam to match shape of patient (reducing intensity on outer edges of Pt)
Utility of CT warm-up (3)
Slowly heats anode to avoid:
Cracking anode disk
Becoming unstable in rotation
Rendering tube useless
CT warm-up
Using low-mA, low-kVp to slowly increase anote temp
