X-rays (Diagnostic Radiology) Flashcards Preview

Medical Physics 2nd Semester > X-rays (Diagnostic Radiology) > Flashcards

Flashcards in X-rays (Diagnostic Radiology) Deck (34)
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1

What is Electric Power defined as?

What about Electric Energy?

Electric power = kV mA

Electric Energy = kV mAs

2

General formula for bremsstrahlung X-ray Production Efficiency

Radiative energy loss / collision energy loss

=

EkZ / 820,000

3

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

Tube Voltage (kVp)

Tube current (mA)

Exposure time (Seconds)

4

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

5

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

6

Just remember: anode diameter is independent of target size.

Remember it

7

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

Remember it

8

What is target heat capacity proportional to?

Target area

9

What does the anode angle affect? (three things)

Effective focal spot size

X-ray field coverage

tube output intensity (heel effect)

10

Equation for effective focal spot length

EFSL = FSL x sin(theta)

11

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

Cathode side

Remember the geometry calculation from the HW

12

What happens to the focal spot size when

tube current increases,

kVp increased

increases

decreases slightly

13

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

Power Rating

14

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

15

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

16

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

17

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

18

What are the effects of off-focal radiation?

Increased patient exposure

Increased random image noise

Increased geometric blurring

Decreased image contrast

19

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

20

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

21

Definition of Power Rating

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

22

Definition of heat unit (HU)

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

23

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)

24

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

False

25

Equation for magnification (M)

M = SID / SOD

 

Or, by definition

 

M = Limage / Lobject

26

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

Finite size x-ray tube focal spot.

27

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

Increases. More opportunity for scatter

28

What is used to reduce scatter?

Anti-Scatter Grid

 

(kind of self explanatory)

29

Equation of Grid Ratio

Grid Ratio = H / W

 

H = height of grid

W = width of grid

30

As grid ratio increases,

transmitted scatter ______

transmitted primary x-ray ________

Contrast resolution ___________

Paient dose ____________

mAs ______________

decrease

decrease

increase

increase

increase