X-Ray Tubes and Linear Accelerators

Question 1

an electrode that is held at positive voltage is called

Answer 1

anode

Question 2

an electrode that is held at negative voltage is called

Answer 2

cathode

Question 3

what is the energy gained by one electron traveling through a voltage of 1 volt (V)?

Answer 3

1 electron volt (eV) which is a unit of energy

Question 4

1 eV in joules (J)?

Answer 4

1 eV = 1.6^-19J

Question 5

What is the general energy created by x-ray tubes used in diagnostic imaging?

Answer 5

~ 200 eV

Question 6

In orthovoltage what is the energy of x-rays being produced?

Answer 6

~ 500 keV

Question 7

what is the process of electrons being “boiled” off from the cathode?

Answer 7

thermionic emission

Question 8

describe the structure of x-ray tube

Answer 8

Question 9

how does an electron generated by thermionic emission from the cathode generates x-ray from the andoe?

Answer 9

The high voltage between the anode and the cathode creates an electric field which accelerates the electron toward the anode. The electron gains energy as it travels form the cathode to anode and collides with the metalllic anode which results in producing x-ray photons via “bremsstrahlung process”

Question 10

why is the x-ray tube is in vacuum?

Answer 10

eleminates air which can cause sparking due to the high voltages used and would cause the electron to scatter.

Question 11

what is heel effet?

Answer 11

It is related to differential attenuation of photons by the anode itself.

photons produced on the more distal side have to travel through more anode material before they emerge, resulting in attenuation.

Thus, there is a lower flux of photons on the left side relative to the right side.

Question 12

what needs to be considered to choose the materials for anode?

Answer 12

high-Z material should be used as the energy loss via radiative loss (via bremsstrahlung photons) is much higher in high-Z material

Also, need to use a material with high melting point.

Question 13

The anode in most diagnostic tubes is made from

Answer 13

tungsten (Z=74) which has the highest melting point of any element (3422C)

Question 14

what parameter decides the maximum photon energy?

Answer 14

kVp, voltage (in kV) at the peak of the oscillation

Question 15

what parameter controls the number of the photons?

Answer 15

the electron current in the tube in milli-Amp (mA)

Question 16

describe the depth dose from KV vs MV beams

Answer 16

Question 17

What are the two microwave systems

Answer 17

Magnetron or Klystron

Question 18

explain the pros and cons of magnetron as microwave system

Answer 18

Question 19

What is Klystron?

Answer 19

an amplifier which takes RF input which has a power in watts (W) and amplifies it to a power of megawatts (MW)

Question 20

Two types of waveguide in a linac?

Answer 20

the standing waveguide and traveling waveguide

Question 21

what is the role of waveguide?

Answer 21

• not to be confused with “accelerating waveguide”
• conduit for high-power microwaves to accelerating waveguide
• filled with SF6 which is inert gas and decreases chance of electrical arc

Question 22

what is the accelerating waveguide?

Answer 22

it’s a long cylinidrical chamber where electrons are accelerated to nearly speed of light (length 1-1.5 m long, diameter of 1 ft)

Question 23

why does bending magnet has 270 degree as a default angle?

Answer 23

note that 90 degree has very large spread and 270 refocuses the electrons to a point.

This is important to reduce penumbra blurring.

Question 24

What are the 4 main sections of linac?

Answer 24

1. Microwave generation (magnetron) / amplification (klystron)
2. Waveguide
3. Accelerating waveguide (traveling and standing wave)
4. Treatment head (bending magnet, photon mode, electron mode)