X-rays (Diagnostic Radiology)

Question 1

What is Electric Power defined as?

What about Electric Energy?

Answer 1

Electric power = kV mA

Electric Energy = kV mAs

Question 2

General formula for bremsstrahlung X-ray Production Efficiency

Answer 2

Radiative energy loss / collision energy loss

=

E_kZ / 820,000

Question 3

What does the X-ray generator permit the control of? (three things)

Answer 3

Tube Voltage (kVp)

Tube current (mA)

Exposure time (Seconds)

Question 4

Quickly what is the law of transformers? Can say it in text.

Answer 4

Voltage is proportional to the number of turns in the transformer.

V_p/V_s = N_p/N_s

Question 5

What is the general goal of the falling-load generator control circuit?

Answer 5

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

Question 6

Just remember: anode diameter is independent of target size.

Answer 6

Remember it

Question 7

Remember that at voltage <= 40 kV, increases in filament current do not increase the tube current.

Answer 7

Remember it

Question 8

What is target heat capacity proportional to?

Answer 8

Target area

Question 9

What does the anode angle affect? (three things)

Answer 9

Effective focal spot size

X-ray field coverage

tube output intensity (heel effect)

Question 10

Equation for effective focal spot length

Answer 10

EFSL = FSL x sin(theta)

Question 11

What side is the focal spot length longer? Anode side or cathode side?

Answer 11

Cathode side

Remember the geometry calculation from the HW

Question 12

What happens to the focal spot size when

tube current increases,

kVp increased

Answer 12

increases

decreases slightly

Question 13

What is the “maximal power that an x-ray tube focal spot can accept or the generator can deliver”

Answer 13

Power Rating

Question 14

What effect does a longer filament length have on

Power loading

Effective focal spot

field coverage

Answer 14

Power loading - improved power loading

Effective focal spot - increases focal spot size

field coverage - no affect

Question 15

What affect does a smaller anode angle have on the

Power loading

Effective focal spot

field coverage

Answer 15

Power loading - No affect

Effective focal spot - smaller focal spot size

field coverage - smaller field coverage

Question 16

What is the reduction in the x-ray beam intensity toward the anode side called?

How does it occur?

Answer 16

Heel Effect

Greater attenuation of x-rays inside the anode

Question 17

What is the cause of off-focal radiation?

Answer 17

Scattered electrons from anode that are reaccelerated back to the anode outside of the focal spot area

Question 18

What are the effects of off-focal radiation?

Answer 18

Increased patient exposure

Increased random image noise

Increased geometric blurring

Decreased image contrast

Question 19

Effects of adding a filter on,

Patient dose

HVL

X-ray beam intensity

Contrast

Answer 19

Patient dose - reduce

HVL - increase

X-ray beam intensity - reduce

Contrast - reduce

Question 20

What are two major disadvantages of using a filter?

Hint: this compensation needed

Answer 20

Increased tube loading (require more electric energy)

Longer exposure times and possible patient motion

Question 21

Definition of Power Rating

Answer 21

Maximal power that an x-ray tube focal spot can accept or generator can deliver

Question 22

Definition of heat unit (HU)

Answer 22

Unit that expresses the energy deposition on and dissipation from the anode of an x-ray tube

Question 23

Equation for Energy in units of HU

Answer 23

Energy (HU) = kVp x mAs (single-phase waveform)

= kVp x mAs x f (three-phase high frequency)

HU/s = kV x mA (fluoroscopy, continuous)

Question 24

True or false, changing mAs affects the energy spectrum of x-rays?

Answer 24

False

Question 25

Equation for magnification (M)

Answer 25

M = SID / SOD

Or, by definition

M = L_image / L_object

Question 26

What causes blurring around the sides of an object on the image?

Answer 26

Finite size x-ray tube focal spot.

Question 27

Scatter-to-Primary Ratio (SPR) _________ with increased x-ray field size and patient thickness

Answer 27

Increases. More opportunity for scatter

Question 28

What is used to reduce scatter?

Answer 28

Anti-Scatter Grid

(kind of self explanatory)

Question 29

Equation of Grid Ratio

Answer 29

Grid Ratio = H / W

H = height of grid

W = width of grid

Question 30

As grid ratio increases,

transmitted scatter ______

transmitted primary x-ray ________

Contrast resolution ___________

Paient dose ____________

mAs ______________

Answer 30

decrease

decrease

increase

increase

increase

Question 31

What is the written definition of Bucky Factor. What about mathematical?

Answer 31

Bucky factor = Dose with grid / dose without grid

Question 32

What is one method for removing grid lines from an image?

Answer 32

Make the grids move around back and forth or make a single pass (Bucky grid)

Question 33

What happens when the x-ray tube focal spot is misaligned with respect to the focal point of the grid?

Answer 33

Reduction in transmission of primary x-rays

Loss of grid penetration

Grid cutoff

Question 34

Besides using an Anti-scatter grid, what is one way of reducing scatter?

Answer 34

Air gap between patient and detector. Give scattered x-rays room to travel away from detector.