The X-Ray Tube & X-Ray Production

Brehmsstrahlung Radiation

"Braking" radiation. A projectile electron completely avoids the orbital electrons of the tungsten atom and travels close to the nucleus. The electrostatic force of the nucleus slows down the electron. As the electron loses energy it suddenly changes direction, and the energy loss reappears as an x-ray photon.

What is needed for x-ray production?

1. A source of electrons 2. A target made of appropriate material 3. High voltage 4. A vacuum

Why should x-ray exposure be kept as short as possible?

1. It decreases the amount of exposure to the patient. 2. It reduces the risk of patient movement, which can cause fuzzy images.

Why is the target made of tungsten?

1. It has a high atomic number, making it more stable. 2. Thermal conductivity--it dissipates heat. 3. It has a high melting point.

Focusing cup

A metal cup where the filament is embedded. It focuses the electrons toward the target. It is negatively charged, which keeps the electrons in a straight path toward the tungsten target.

Characteristic radiation

A projectile electron interacts with the inner (K) shell of the tungsten atom. When the K-shell electron is ejected from its orbit, an outer-shell electron drops into the open position and creates an energy difference which is emitted as an x-ray photon.

Filament

A small coil of wires made out of tungsten, which melts at 3400 degrees F. Electrons are boiled off and emitted when the wire is heated by applying a current.

Which type of radiation is the most common produced?

Brehmsstrahlung.

What are the two electrodes found in the x-ray tube?

Cathode (-) and anode (+)

What do we call an x-ray tube that has not been used in a while?

Cold.

Why must the x-ray tube be a vacuum?

Electrons can flow from the cathode to the anode without interference from air molecules.

What are the names used for the part of the anode where the electrons hit?

Focal point, focal spot, target

How do we get an x-ray tube ready when it is cold?

Follow the facility's warm up procedures.

In what manner do electrons travel toward the anode?

In a straight line.

What are the dangers of high exposures?

It can decrease the life of the x-ray tube, which is very expensive.

What are the advantage and disadvantage of using a small focal spot?

It gives greater detail on the radiograph, but it creates more heat. This can be minimized by using a smaller target angle which allows the focal spot area on the target to be larger than what the actual effective focal spot size is.

How is an x-ray tube protected from leaking radiation?

It is lined with a lead lining.

What is a dead man switch?

It is the button on the pendant that must be pressed in order to use any of the other buttons. It is a safety feature to prevent accidental activation of the machine.

kVp

Kilovoltage peak. Unit of measurement for x-ray voltages. Determines beam penetrability or quality.

mA

Milliamperes. Unit of measurement for x-ray/electrical current. The amount of electrons that cross the tube from the cathode to the anode. Determines quantity.

What are the two types of anodes?

Stationary and rotating

Cathode

The negative side of the x-ray tube. It is made up of two parts, the filament and the focusing cup. It is the source of electrons for producing x-rays.

Anode

The positive electrode in the x-ray tube.

How is an x-ray produced?

The tungsten filament is heated by an electrical current until it reaches incandescence. When the exposure switch is depressed, a high potential difference occurs across the tube, and electrons are repelled from the cathode and shoot across to the anode. When the electrons hit the target, they react to produce x-rays.

What is the anode heel effect?

The x-ray beam has greater intensity on the cathode side of the tube, with intensity diminishing toward the anode side.

Glass envelope

The x-ray tube which contains the anode and cathode.

How is an x-ray tube insulated from high voltage potential?

There is a space between the housing and the glass/metal envelope that contains a special oil.

What is the rotating part of the anode, the target, made of?

Tungsten

How many filaments to most x-ray tubes have?

Two-- a large and a small.