Chapter 3 The Xray Beam

The production of xrays requires what?

A rapidly moving stream of electrons that are suddenly decelerated or stopped

The source of electrons is what?

The cathode, or negative electrode

The negative electrode is heated and electrons are what?

Emitted through the process of thermionic emission

Electrons are attracted to the positively charged anode and do what?

move rapidly toward the anode, where they are stopped or decelerated

What is Bremsstrahlung?

a German word meaning braking or slowing down radiation

Bremsstrahlung interactions occur when?

a projectile (incident) electron completely avoids the orbital electrons of a tungsten atom and travels very close to its nucleus

Bremsstrahlung is about what?

85% of the xray beam

The very strong negative electrostatic force surrounding the positively charged nucleus causes what?

the incident electron to suddenly slow down

As the electron looses energy does what?

Changes direction, and the energy loss then reappears as an xray photon

The closer the projectile electron travels to the nucleus, the stronger the what?

the attraction

Xray energy is measured in what?

kilo electron volts (keV)

1 keV is equivalent to what?

1,000 electron volts

Below 70kVp, 100% of the xray results from?

Bremsstrahlung interactions

At what kVp does about 85% of the beam results from bremsstrahlung interactions?

above 70kVp

Characteristic interactions are produced when?

a projectile electron interacts with an electron from the inner shell of a tungsten atom

K-shell electrons in tungsten have the strongest binding energy at what keV?

69.5

For a projectile electron to remove this orbital electron, it must possess what?

energy equal to or greater then 69.5 keV

What is electron transition?

When the k-shell electron is ejected from its orbit, an outer-shell electron drops into the open position and creates an energy difference

Electron transition generally consists of what?

an M to K, or L to K transition

Photons created from characteristic interactions have what?

Very low energy, and may not even reach the patient

K-shell characteristic xrays in the xray beam have an energy range of approximately 57 to 69 keV, therefore they do what?

contribute a small percentage to the useful xray beams

Below 70 kVp, there are no what?

characteristic xrays present in the beam

Above 70 kVp, characteristic beams make up about what of the kVp?

15%

Why are certain xray interactions termed characteristic xrays?

because their energies are characteristic of the tungsten target element and it binding energy values

Most xray interactions result from bremsstrahlung interactions in what department?

Those produced in Diagnostic Radiology

The xray beam is what?

polyenergetic (has many energies) and consists of a wide range of energies

What is the xray emission spectrum?

a wide range of energies

The lowest energies are always approximately what?

15 to 20 keV, and the highest energies can't exceed the kVp set

The xray emission spectrum, or the range and intensity of xrays emitted, changes with what?

different exposure technique settings on the control panel

What are deadman switches?

the switches that are used to make an xray exposure

Deadman switches require what?

positive pressure to be applied during the entire xray exposure process

The first deadman switch is usually called the what?

The rotor or prep button

The second deadman switch is usually called the what?

exposure or xray button

Pushing the rotor or prep button causes what?

an electrical current to be induced across the filament in the cathode

The filament current during rotor is approximately what?

3 to 5 A and operates at approximately 10 V

Thermionic emission refers to what?

the boiling off of electrons from the filament

The electrons emitted from the filament during thermionic emission form what?

a thermionic cloud around the filament called space charge

The space charge effects refers to what?

the tendency of the space charge to prevent more electrons to be boiled off of the filament

The focusing cup (with its own negative charge) does what?

Forces the electrons in the space charge to to remain together

By pushing the rotor or prep button, the radiographer also activates what?

The stator that drives the rotor and rotating target

While thermionic emission is occurring and the space charge is forming, the stator starts what?

The anode, accelerating it to top speed in preparation for xray production

When the radiographer pushes the exposure or xray button, what begins?

xray exposure

Tube filament refers to what?

the flow of electrons from cathode to anode

Tube filament is measured in units called what?

Milliamperes

What happens on the cathode side of the xray tube during prep?

- the filament heats up

- the heat boils electrons off the filament

- the electrons gather in a cloud

- the focusing cup keeps the electrons together

- number of electrons in space charge is limited

What happens on the anode side of the xray tube during prep?

the rotating target begins to turn rapidly, quickly reaching top speed

What happens on the cathode side of the xray tube during exposure?

- high negative charge strongly repels electrons

- electrons stream away from the cathode and toward the anode

What happens on the anode side of the xray tube during exposure?

- high positive charge strongly attracts electrons in the tube current

- the electrons strike the anode

- xrays and heat are produced

As these electrons strike the anode target, their kinetic energy is converted into what?

Either electromagnetic energy (xrays) or thermal energy (heat)

Most of the electron kinetic energy in the tube current is converted to what?

Heat

Less than ___ of electron kinetic energy is converted to xrays?

1%

The radiographer initiates and controls what?

The production of xrays

Manipulating the prime exposure factors on the control panel allows what?

both the quantity and and the quality of the xray beam to be altered

The quantity of the xray beam indicates what?

the number of xray photons in the primary beam

The quality of the xray beam indicates what?

its penetrating power

The kilovoltage that is set by the radiographer and applied across the xray tube determines what?

the speed at which the electrons in the tube current move

Selecting a higher voltage results in greater what?

Repulsion of electrons from the cathode and greater attraction of electrons toward the anode

The higher the energy of the photons, the greater their what?

penetrability, or ease with which they move through tissue

kVp directly affects what?

The energy or quality of the xrays produced

- is not a proportional relationship

kVp affects the quantity or number of what?

Xray photons produced since higher kVps produce more bremsstrahlung interactions

Characteristic peak is always where?

at 70

Increasing the kVp from 72 to 82 shows what?

An increase in the quantity of xrays (amplitude) and the xray emission shifts toward the right, indicating an increase in the quality

Three basics types of xray generations are what?

- single-phase

- three-phase

- high frequency

Voltage ripples describes what?

Voltage waveforms in terms of how much the voltage varies during xray production

Decreasing the kVp by 15% does what?

half the image exposure

Small changes to kVp equal what?

large beam changes

Milliamperage is selected on what?

The xray control panel to operate the tube current

Tube current is what?

The number of electrons flowing per unit time between the cathode and the anode

The mA set by the radiographer determines what?

The number of electrons flowing in the tube and the quantity of xrays produced

The quantity of electrons in the tube current is directly proportional to what?

The mA

If the mA increases, the quantity of electrons and xrays proportionally do what?

Increases, and if it decreases the quantity decreases

The mA does not affect what?

The quality or energy of the xrays produced

mA and Xray quantity

- higher mA results in more electrons moving in the tube current from the cathode to the anode

- the more electrons in the tube current, the more xrays produced

- the number of xrays produced is directly proportional to mA

Exposure time determines what?

The length of time over which the xray tube produces xrays

The exposure time determines what?

The length of time for which the tube current is allowed to flow from cathode to anode

The longer the exposure time, the greater the what?

quantity of electrons that flow from the cathode to the anode, greater quantity of xrays produced

Exposure time and Xray quantity

- a longer exposure time results in more electrons moving in the tube current from cathode to anode

- the more electrons in the tube current, the more xrays produced

- the number of xrays produced is directly proportional to the exposure time

When mA is multiplied by exposure time, the result is know as what?

mAs, milliamperage-seconds

Mathematically, mAs is simply expressed as what?

mA x S = mAs

- s represents the exposure time in fractions of a seconds

The quantity of electrons flowing from the cathode to the anode is what?

Directly proportional to mAs

The quantity of xray photons produced is directly proportional to what?

the quantity of electrons

The line focus principle describes what?

The relationship between the actual and the effective focal spots in an xray tube

The effective focal spot size refers to what?

the projected focal spot size as measured directly under the anode target

A tubes focal spot is an important factor because of what?

A large focal spot can withstand heat produced by large exposures, whereas a small one produces better image quality

The line-focus principle demonstrates how ? ( by angling the face of the anode target)

the actual focal spot can remain relatively large while the effect focal spot is reduced in size

When manufactured, every tube has a specific anode target angle, typically ranging from what?

5 to 20 degrees

Based on the line-focus principle, the amount of the target angle determines what?

The size of the effective focal spot

A larger target angle does what?

Produce a larger effective focal spot and vice versa

The anode heel effect occurs because of what?

the angle of the target

The heel effect describes what?

How the xray beam has greater intensity on the cathode side of the tube

As xrays are produced, they do what?

leave the anode in all directions

The xrays that are emitted toward the anode side of the tube have farther to travel, and what happens?

Some get absorbed by the anode, and they are reduced in number compared with the photons that are emitted in the direction of the cathode

The difference in the intensities between the cathode and anode can be what?

45%

The heel effect can be used to advantage in radiography because of what?

The cathode end of the tube can be placed over a thicker body part, resulting in a more even exposure to the image receptor

What is the remnant beam?

The xray beam that eventually records the body part onto the image receptor

The xrays that exit the tube are what?

Polyenergetic; the consist of low-energy, medium-energy, and high-energy photons

The low energy photons cannot do what?

Penetrate art of the anatomy and do not contribute to image formation

- the only contribute to patient dose

Reduction of the low- energy photons requires what?

That filtration be added to the xray beam to attenuate or absorb the photons

Added filtration describes what?

The filtration that is added anywhere below the port of the xray tube

Aluminum is the material primarily used for filtration because it does what?

absorbs more low energy photons while the useful high energy photons can penetrate the aluminum and exit