Production of X-rays

Question 1

Who discovered X-rays and in what year?

Answer 1

X-rays were discovered by Roentgen in 1895.

Question 2

What was Roentgen studying when he discovered X-rays?

Answer 2

He was studying cathode rays in a gas discharge tube.

Question 3

What properties of X-rays were observed by Roentgen?

Answer 3

X-rays could penetrate opaque substances, produce fluorescence, blacken a photographic plate, and ionize a gas.

Question 4

What is the structure of a conventional X-ray tube?

Answer 4

The tube consists of a glass envelope evacuated to high vacuum, with a cathode at one end and an anode at the other.

Question 5

What is the cathode in an X-ray tube made of?

Answer 5

The cathode is a tungsten filament that emits electrons when heated.

Question 6

What is the function of the anode in an X-ray tube?

Answer 6

The anode consists of a thick copper rod with a tungsten target where accelerated electrons strike to produce X-rays.

Question 7

What is thermionic emission?

Answer 7

Thermionic emission is the phenomenon where a heated filament emits electrons.

Question 8

Why is tungsten used as a target material in X-ray tubes?

Answer 8

Tungsten has a high atomic number (Z = 74) and a high melting point (3,370°C), making it suitable for withstanding intense heat.

Question 9

What is the purpose of the oil bath surrounding an X-ray tube?

Answer 9

The oil bath insulates the tube housing from high voltage and absorbs heat from the anode.

Question 10

What is the focal spot in an X-ray tube?

Answer 10

The focal spot is the area from which X-rays are emitted, and it should be as small as possible for sharp images.

Question 11

What is the heel effect in X-ray tubes?

Answer 11

The heel effect is the variation in intensity of the X-ray beam, decreasing from the cathode to the anode direction.

Question 12

What does the cathode assembly in a modern X-ray tube consist of?

Answer 12

It consists of a wire filament, a circuit for filament current, and a negatively charged focusing cup.

Question 13

What is the basic structure of an X-ray circuit?

Answer 13

The circuit includes a high-voltage circuit for accelerating electrons and a low-voltage circuit for heating the filament.

Question 14

What is the role of the step-down transformer in an X-ray circuit?

Answer 14

It provides the filament current, typically 10 V at about 6 A.

Question 15

How is the high voltage supplied to the X-ray tube?

Answer 15

It is supplied by a step-up transformer connected to an autotransformer and a rheostat.

Question 16

What is a self-rectified X-ray unit?

Answer 16

A self-rectified unit generates tube current and X-rays only during the half-cycle when the anode is positive.

Question 17

What is a disadvantage of the self-rectified circuit?

Answer 17

No x-rays are generated during the inverse voltage cycle, resulting in low output.

Question 18

What happens during the inverse voltage cycle in a self-rectified circuit?

Answer 18

Electrons emitted by thermionic emission flow from the anode to the cathode, bombarding the cathode filament.

Question 19

How can the problem of tube conduction during inverse voltage be solved?

Answer 19

By using voltage rectifiers.

Question 20

What is half-wave rectification?

Answer 20

A type of rectification where current flows during the cycle when the anode is positive relative to the cathode.

Question 21

What are the two types of high-voltage rectifiers?

Answer 21

• Valve rectifiers
• Solid state rectifiers

Question 22

What is the principle of a valve rectifier?

Answer 22

It allows current to flow only from the anode to the cathode and does not conduct during the inverse cycle.

Question 23

What materials are commonly used in solid state rectifiers?

Answer 23

• Selenium
• Silicon
• Germanium

Question 24

What does full-wave rectification achieve in an x-ray tube?

Answer 24

The effective tube current is higher because current flows during both half-cycles.

Question 25

What is a smoothing condenser used for?

Answer 25

To keep the voltage across the tube nearly constant.

Question 26

What is the purpose of a three-phase x-ray generator?

Answer 26

To provide high x-ray output in a short time to minimize patient motion blur.

Question 27

How is a three-phase power line structured?

Answer 27

It is supplied through three separate wires with voltage waveforms kept slightly out of phase.

Question 28

What is the voltage ripple percentage for a three-phase, six-pulse system?

Answer 28

13% to 25%

Question 29

What is the advantage of a three-phase, 12-pulse circuit?

Answer 29

It shows much less ripple, ranging from 3% to 10%.

Question 30

What is the function of a constant potential x-ray generator?

Answer 30

It provides nearly constant voltage with a ripple of less than 2%.

Question 31

What are the advantages of high-frequency x-ray generators?

Answer 31

* Reduced weight and size * Low-voltage ripple * Greatest efficiency of x-ray production * Maximum x-ray output per mAs * Shorter exposure times

Question 32

What are the two mechanisms of x-ray production?

Answer 32

* Bremsstrahlung * Characteristic x-rays

Question 33

What does bremsstrahlung mean?

Answer 33

Braking radiation resulting from interaction between a high-speed electron and a nucleus.

Question 34

How does the energy loss per atom by electrons depend on atomic number?

Answer 34

It depends on the square of the atomic number (Z2).

Question 35

What is the relationship between efficiency of x-ray production and atomic number?

Answer 35

Efficiency depends on the first power of atomic number and the voltage applied.

Question 36

What is the typical efficiency of x-ray production with tungsten at 100 kV?

Answer 36

Less than 1%.

Question 37

What happens to the majority of input energy in x-ray production?

Answer 37

Approximately 99% appears as heat.

Question 38

What efficiency can be achieved for high-energy x-rays?

Answer 38

30% to 95% for accelerator beams depending upon energy.

Question 39

What are characteristic x-rays?

Answer 39

X-rays produced when an electron ejects an orbital electron from an atom, resulting in the emission of electromagnetic radiation as outer electrons fill the vacancy. ## Footnote Characteristic x-rays are specific to the target material and the electron shells involved in the transitions.

Question 40

What happens to an electron with kinetic energy E0 during interaction with target atoms?

Answer 40

It may eject an orbital electron, becoming ionized, and recedes with energy E0 − ΔE, where ΔE is the energy given to the ejected electron. ## Footnote ΔE includes energy spent overcoming binding energy and energy carried by the ejected electron.

Question 41

What is the energy relationship when an outer orbital electron falls to fill a vacancy?

Answer 41

The energy radiated is in the form of electromagnetic radiation, characterized by the difference in binding energies between the shells involved. ## Footnote If an electron descends from the L shell to the K shell, the emitted photon has energy hν = EK − EL.

Question 42

How do characteristic x-rays differ from bremsstrahlung x-rays?

Answer 42

Characteristic x-rays are emitted at discrete energies, while bremsstrahlung x-rays have a continuous spectrum of energies. ## Footnote Characteristic x-rays are specific to the atomic structure of the target material.

Question 43

What is critical absorption energy?

Answer 43

The minimum energy an incident electron must have to strip an electron from an atom. ## Footnote It varies for different elements, as shown in provided tables.

Question 44

Describe the x-ray energy spectrum produced by an x-ray machine.

Answer 44

It shows a continuous distribution of energies for bremsstrahlung photons combined with discrete energies for characteristic radiation. ## Footnote The spectrum is influenced by factors like voltage and filtration.

Question 45

What is the function of added filtration in x-ray machines?

Answer 45

To enrich the beam with higher-energy photons by absorbing lower-energy components, thus increasing the penetrating power. ## Footnote Filtration modifies the energy spectrum, primarily affecting low-energy photons.

Question 46

What is the one-third rule in x-ray energy?

Answer 46

The average x-ray energy is approximately one-third of the maximum energy or kVp. ## Footnote This is a rough approximation and can vary significantly with filtration.

Question 47

What is exposure in the context of x-ray machines?

Answer 47

A measure of ionization per unit mass of air produced by the x-ray output. ## Footnote It is affected by various factors, including filament current and tube current.

Question 48

How does filament current affect x-ray output?

Answer 48

A small change in filament current produces a large change in relative exposure rate, making its constancy critical. ## Footnote Typical operating conditions show a significant relationship between filament current and exposure.

Question 49

What is the relationship between tube current and exposure rate?

Answer 49

There is a linear relationship; doubling the tube current approximately doubles the output. ## Footnote This linearity is crucial for consistent x-ray output.

Question 50

How does tube voltage affect x-ray output?

Answer 50

The increase in x-ray output with voltage is approximately proportional to the square of the kilovoltage. ## Footnote This relationship varies with filtration and other factors.

Question 51

What is the significance of tungsten in x-ray tubes?

Answer 51

Tungsten is chosen for both the filament and target due to its high melting point (3,370°C) and high atomic number (Z = 74), enhancing x-ray production efficiency. ## Footnote Its properties help manage heat and improve x-ray generation.

Question 52

What is the effect of increasing filtration on the x-ray beam?

Answer 52

It decreases the total intensity of the beam while increasing the average energy and penetrating power. ## Footnote A balance of voltage and filtration is required for optimal beam quality.

Question 53

What are the operating characteristics of an x-ray machine?

Answer 53

They include the relationships between x-ray output, filament current, tube current, and tube voltage. ## Footnote These characteristics are essential for understanding and optimizing x-ray production.

Question 54

What is the role of a high-frequency generator in x-ray production?

Answer 54

It provides nearly constant high-voltage potential, resulting in higher x-ray output per mAs and shorter exposure times. ## Footnote This leads to improved efficiency in x-ray generation.

Question 55

True or False: The efficiency of x-ray production is less than 1% for x-ray tubes operating at 100 kVp.

Answer 55

True. ## Footnote Most input energy is converted into heat rather than x-rays.