Unit 3 Equipment Operations -- X-ray Production and the Tube

CH 7, 8, AEC

What type of tube support system is the tube suspended from one ceiling on two tracks arranged perpendicular to each other?

Ceiling Mounted 

What tube support system allows the greatest amount of flexibility and tube travel?

Ceiling Mounted 

What type of tube support system is the most expensive and requires the greatest amount of architectural work to the facility?

Ceiling Mounted

What type of tube support system is the tube connected to tracks located on the ceiling and the floor?

Floor to Ceiling Mounted

What type of tube support system is the tube fixed to the floor, and can commonly be attached to the table?

Floor/Table Mounted

What type of tube support system is the least expensive but allows for the least flexibility for manipulation of the tube?

Floor/Table Mounted

The fluoro tube is mounted (over/under) the table in a standard R/F room?

Under

What is an example of a mobile fluoroscopic unit?

C-arm

The tube’s enclosure is made from what two materials?

Glass

Metal 

What tube component keeps leakage radiation at a reasonable level and guards against electric shock?

Protective Housing

What are any xrays emitted from the tube housing other than from the window called?

Leakage Radiation

The federal standard for maximum leakage radiation is how many milligray per hour at 1 meter?

1 mGy

What is the primary function of the tube’s protective housing?

Shielding

What is the only part of the tube housing through which X-rays are emitted?

Window

What two components are considered the internal components of the tube housing?

Glass Envelope

Dielectric Oil

What tube housing component maintains a vacuum and allows for the most efficient xray production and tube life?

Glass Envelope

What tube component maintains physical support for internal tube structures?

Glass Envelope

What sits within the tube housing and surrounds the tube within the glass envelope?

Dielectric Oil

What component inside the glass envelope helps to safeguard against electrical shock from incoming high voltage?

Dielectric Oil

What component protects the operator and patient from burns by dissipating heat generated by the tube?

Dielectric Oil

What component of the tube acts as an electrical insulator?

Dielectric Oil 

What component of the tube is a small coil of wire made of thoriated tungsten?

Filament

The tube current is (indirectly/directly) proportional to the filament current.

Directly

What internal tube component adjusts the filament current to get the correct amount of electrons in the cloud?

Rheostat

What internal tube component heats the filament to the point of thermionic emission?

Rheostat

The cloud of electrons created by thermionic emission of the filament is called?

Space Charge

During which process do electrons in the tungsten filament gain enough thermal energy to escape the wire and form an electron cloud around it?

Thermionic Emission

What material is the filament made of?

Thoriated Tungsten (W) 

The addition of thorium to the tungsten (increases/decreases) the efficiency of thermionic emission and prolongs tube life.

Increases

What is the most common cause of tube failure?

Vaporization of the filament

Which filament material allows high mA currents to be applied without burning out, does not evaporate easily (maintaining the tube vacuum and preventing the tube from becoming gassy), and is preferred for its durability and efficiency?

Thoriated Tungsten

What is a small, metal cup that holds the filament called?

Focusing Cup

The focusing cup holding the filament has a (positive/negative) charge.

Negative

What cathode component repels negative electrons inward to narrow the beam and improve spatial resolution?

Focusing Cup

What is the larger protective cover that surrounds the focusing cup and filament called?

Shroud

Which of the following is a function of the shroud located in the cathode?

1. Mechanical Support

2. Electrical Insulation

3. Heat Shielding

4. Improved Spatial Resolution

1. Mechanical Support

2. Electrical Insulation

3. Heat Shielding

In what kind of xray tube does heating one of the two filaments control the focal spot size?

Dual Focus 

Which x-ray tube component acts as the target for high-speed electrons from the cathode and converts their kinetic energy into x-rays?

Anode

The cathode has a (positive/negative) charge.

Negative

The anode has a (positive/negative) charge.

Positive

How much of all the energy generated by the electrons results in xray production?

1%

How much of all the energy generated by electrons is converted into heat?

99%

What component is required to withstand the heat generated by electrons?

Anode

The anode is most commonly made from what material?

Tungsten

Tungsten has a (high/low) atomic number.

High

Which of the following are positive characteristics of tungsten?

1. Thermal Conductivity

2. High Melting Point

3. Low Atomic Number

4. High Density

1. Thermal Conductivity

2. High Melting Point

What are other materials are commonly used to create the anode?

Molybdenum

Graphite

What is the physical area of the anode target that is struck with projectile electrons when an exposure is made called?

Actual Focal Spot

What is the actual source of xray photons?

Actual Focal Spot

What is the actual size of the xray beam as it is projected down from the anode towards the patient called?

Effective Focal Spot

(T/F) The effective focal spot does not vary with the anode target angle.

False

What describes the relationship between the size of the actual focal spot and the effective focal spot?

Line Focus Principle

What is the angle between the surface of the anode and the vertical axis of the xray tube called?

Anode Angle

Between how many degrees is a typical anode angle set within the tube?

6-20 degrees

(T/F) Because of the anode angle, the effective focal spot will always be smaller than the actual focal spot.

True

The smaller the anode angle, the (smaller/larger) the effective focal spot will be.

Smaller

The (smaller/larger) the anode angle, the more heat the anode can handle.

Larger

The smaller the anode angle, the (more/less) heat the anode can handle.

Less

What will be more condensed with a smaller anode angle?

Heat

(T/F) A larger anode angle allows for heat to spread out.

True

What is the phenomenon in which the angled anode obstructs part of the x-ray beam, causing the intensity to vary across the x-ray field?

Anode Heel Effect

During the anode heel effect, are (more/fewer) x-ray photons emitted from the anode side of the tube?

Fewer

The (smaller/larger) the anode angle, the larger the heel effect.

Smaller

A decreased anode angle, (increases/decreases) the anode heel effect

Increases

What are two of the influencing factors of the anode heel effect?

SID

Field Size

A (longer/shorter) SID will increase the anode heel effect.

Shorter

Decreased SID, (increases/decreased) the anode heel effect.

Increases

When observing the anode heel effect, shorter distances cause the image receptor to be exposed to more of the (low/high) intensity part of the xray beam?

Low

A (smaller/larger) field size exposes the receptor to more of the low intensity beam on the anode side and high intensity beam on the cathode side.

Larger

A smaller field size will limit the beam intensity and (increase/decrease) the difference in intensity across the xray field.

Decrease

An increased field size will (increase/decrease) the anode heel effect.

Increase

(T/F) Positioning larger and denser areas on the cathode side of the tube results in more even exposure to the receptor.

True

What are the two types of anodes?

Rotating

Stationary

Which type of anode has a higher heat capacity and can withstand a higher mA and thus a lower exposure time?

Rotating

Which type of anode is more commonly used in xray tubes?

Rotating

A rotating anode uses what kind of motor to turn?

Electromagnetic Induction

What creates a rotating magnetic field that causes the rotor (and thus the anode) to spin, allowing for better heat dissipation and enabling higher tube currents?

Stator

Which type of anode has a lower heat capacity, is less expensive, and is used in dental clinics, portable machines, and other special purpose units?

Stationary

What process occurs when heat is created within the xray tube, especially within the anode?

Tube Loading

What equation is used to find the heat units or the tube loading?

Heat Units = kVp x mAs x W (waveform factor)  

Different currents or waveforms produce different amounts of heat and are represented by?

Waveform Factor

What is the waveform factor for a single phase generator?

1

What is the waveform factor for a three phase, six pulse generator?

1.35

What is the waveform factor for a three phase, twelve pulse generator?

1.41

What is the waveform factor for a high frequency generator?

1.45

Calculate the heat units created from a single exposure using 120 kVp, 20 mAs on a high frequency xray system.

120 kVp x 20 mAs x 1.45 (W) = 3,480 HU  

Calculate the heat units created from two exposures using 120 kVp, 20 mAs on a three phase, six pulse xray system.

120 kVp x 20 mAs x 1.35 (W) = 3,240 HU x 2 exposures = 6,480 HU

Calculate the total heat units created from two exposures, the first exposure was taken at 120 kVp at 20 mAs and the second exposure was taken at 90 kVp at 20 mAs. Both exposures were made on a single phase xray system.

120 kVp x 20 mAs x 1.0 (W) = 2,400 HU

90 kVp x 20 mAs x 1.0 (W) = 1,800 HU

2,400 HU + 1,800 HU = 4,200 HU

A smooth, shiny appearance of a rotating anode indicates that the anode is old or new?

New

Slow rotation caused by bearing damage, repeated overload, and the exceeding of maximum heat storage capacity are all examples of what?

Anode Separation

Using a higher exposure time and a lower mA (increases/decreases) the heat capacity of the tube.

Increases

An increased exposure time and decreased mA will increase the heat capacity of the tube, but this is not ideal because of _.

Motion

Using a (smaller/larger) filament size will increase the heat capacity of the tube?

Larger

An increase in filament size results in an increased focal spot size and will (increase/decrease) the heat capacity of the xray tube?

Increase

Using a larger filament size to increase the heat capacity of the tube is not ideal because it will decrease the _.

Spatial Resolution 

Anodes with a larger angle will (increase/decrease) the heat capacity of the tube.

Increase

An increased anode angle will increase the focal spot size and therefore will (increase/decrease) the heat capacity of the tube.

Increase

Anodes with a faster rotation speed will allow the generated heat to be (more/less) concentrated.

Less

An increased anode rotation speed will (increase/decrease) the heat concentration on the anode and therefore increase the heat capacity of the tube.

Decrease

(T/F) Modern tubes have a microprocessor that does not allow for unsafe exposures.

True