X-rays (Diagnostic Radiology) Flashcards
What is Electric Power defined as?
What about Electric Energy?
Electric power = kV mA
Electric Energy = kV mAs
General formula for bremsstrahlung X-ray Production Efficiency
Radiative energy loss / collision energy loss
=
EkZ / 820,000
What does the X-ray generator permit the control of? (three things)
Tube Voltage (kVp)
Tube current (mA)
Exposure time (Seconds)
Quickly what is the law of transformers? Can say it in text.
Voltage is proportional to the number of turns in the transformer.
Vp/Vs = Np/Ns
What is the general goal of the falling-load generator control circuit?
Provide shortest exposure time possible for a given kVp and mAs technique
Continuosuly decreasing mA with time, in order to deliver maximal continuous x-ray output within power deposition capabilities of focal spot
Just remember: anode diameter is independent of target size.
Remember it
Remember that at voltage <= 40 kV, increases in filament current do not increase the tube current.
Remember it
What is target heat capacity proportional to?
Target area
What does the anode angle affect? (three things)
Effective focal spot size
X-ray field coverage
tube output intensity (heel effect)
Equation for effective focal spot length
EFSL = FSL x sin(theta)
What side is the focal spot length longer? Anode side or cathode side?
Cathode side
Remember the geometry calculation from the HW
What happens to the focal spot size when
tube current increases,
kVp increased
increases
decreases slightly
What is the “maximal power that an x-ray tube focal spot can accept or the generator can deliver”
Power Rating
What effect does a longer filament length have on
Power loading
Effective focal spot
field coverage
Power loading - improved power loading
Effective focal spot - increases focal spot size
field coverage - no affect
What affect does a smaller anode angle have on the
Power loading
Effective focal spot
field coverage
Power loading - No affect
Effective focal spot - smaller focal spot size
field coverage - smaller field coverage
What is the reduction in the x-ray beam intensity toward the anode side called?
How does it occur?
Heel Effect
Greater attenuation of x-rays inside the anode
What is the cause of off-focal radiation?
Scattered electrons from anode that are reaccelerated back to the anode outside of the focal spot area
What are the effects of off-focal radiation?
Increased patient exposure
Increased random image noise
Increased geometric blurring
Decreased image contrast
Effects of adding a filter on,
Patient dose
HVL
X-ray beam intensity
Contrast
Patient dose - reduce
HVL - increase
X-ray beam intensity - reduce
Contrast - reduce
What are two major disadvantages of using a filter?
Hint: this compensation needed
Increased tube loading (require more electric energy)
Longer exposure times and possible patient motion
Definition of Power Rating
Maximal power that an x-ray tube focal spot can accept or generator can deliver
Definition of heat unit (HU)
Unit that expresses the energy deposition on and dissipation from the anode of an x-ray tube
Equation for Energy in units of HU
Energy (HU) = kVp x mAs (single-phase waveform)
= kVp x mAs x f (three-phase high frequency)
HU/s = kV x mA (fluoroscopy, continuous)
True or false, changing mAs affects the energy spectrum of x-rays?
False
Equation for magnification (M)
M = SID / SOD
Or, by definition
M = Limage / Lobject
What causes blurring around the sides of an object on the image?
Finite size x-ray tube focal spot.
Scatter-to-Primary Ratio (SPR) _________ with increased x-ray field size and patient thickness
Increases. More opportunity for scatter
What is used to reduce scatter?
Anti-Scatter Grid
(kind of self explanatory)
Equation of Grid Ratio
Grid Ratio = H / W
H = height of grid
W = width of grid
As grid ratio increases,
transmitted scatter ______
transmitted primary x-ray ________
Contrast resolution ___________
Paient dose ____________
mAs ______________
decrease
decrease
increase
increase
increase
What is the written definition of Bucky Factor. What about mathematical?
Bucky factor = Dose with grid / dose without grid
What is one method for removing grid lines from an image?
Make the grids move around back and forth or make a single pass (Bucky grid)
What happens when the x-ray tube focal spot is misaligned with respect to the focal point of the grid?
Reduction in transmission of primary x-rays
Loss of grid penetration
Grid cutoff
Besides using an Anti-scatter grid, what is one way of reducing scatter?
Air gap between patient and detector. Give scattered x-rays room to travel away from detector.
