Chapter 6 Lecture Notes

The X-Ray Tube

 

Target Area

Target, focus, focal point, focal spot mean the same thing. This is where the high-voltage electrons hit the anode.

Actual focal spot: The physical area of the focal track that is impacted.

Focal Track: The portion of the anode where the high-voltage electron stream will impact. When discussing a rotating anode this describes the circular path that will be impacted by the electron beam.

Effective focal spot: The area of the focal spot that is projected out of the tube toward the object being radiographed.

In the diagram below there are two anode with different size focal spots. The one on the left has a larger actual focal spot than the one on the right. Because of the angle of the anode the effective focal spot is also different for each anode.

focal spot

The angle of the focal spot is one way to control the size of the effective focal spot. The larger the angle the larger the effective focal spot, as illustrated in the next two diagrams.

anode

 

line-focus principle

Line-Focus Principle: Used to reduce the effective area of the focal spot.

The effective focal-spot size is controlled by the size of the actual focal spot and the anode target angle.
The effective focal spot's vertical dimension is the one that is stated as the focal-spot size.

beam field

 

Anode Heel Effect

Anode Heel Effect: Due to the geometry of the angled anode target, the radiation intensity is greater on the cathode side.

As the figure below indicates the intensity of the x-ray beam is greater towards the cathode (filament) end of the tube.

anode heel effect

The reason that the Heel Effect occurs is illustrated below. The letters represent interactions with electrons from the cathode and the lines coming from the interactions are x-rays. More x-rays will be able to get out of the anode on the right side (the angle side) than on the left side because of all the material the x-rays must pass through to get out of the anode.

heel effect

 

The next two graphs show the percent of exposure on the vertical axis and the horizontal axis is the distance from the center of the exposure position towards the anode (left) and the cathode (right). As you can see the percent of exposure goes up the closer you get to the cathode side of the center.

heel effect

 

The picture below shows what can happen to an anode when the anode stops turning. The anode actually melts.

anode melt

 

 

 

The Envelope

The envelope is the glass housing that protects the tube. It is also used to help protect from excessive exposure to x-rays. The envelope is the first part of the filtration system.

tubes

 

Vacuum

The removal of the air permits electrons to flow from cathode to anode without encountering the gas atoms of air.

 

Protective Housing

The housing controls leakage and scatter radiation, isolates the high voltages, and provides a means to cool the tube.

housing

Leakage Radiation: Any photons that escape from the housing except at the port. Leakage radiation must not exceed 100 mR/hr at 1 meter.

 

Off-Focus Radiation

Off-Focus Radiation: Photons that were not produced at the focal spot or extrafocal radiation. These can be produced when an electron from the filament interacts with the tungsten. Then the electron that was given off from the ionization does the same thing to another tungsten atom and creates another x-ray.

off-focus

The image below shows an extreme effect of off-focus radiation. You can see the image of the nose from the off-focus radiation.

off focus

 

 

Rating Charts and Cooling Curves

Radiographic Tube Rating Chart: A guide regarding the most common technical factor combinations that can be used without overloading the tube.

When you use the Radiographic Tube Rating Charts for the X-Ray machine you want to make sure that the kVP setting and the time of the exposure intersect below the mA reading. For example, in the chart below if you set the kVp to 120 kVp and the exposure time to 0.4 seconds you will be below the 125 mA setting but above the 150 mA setting. If you use the 150 mA setting you could damage the x-ray tube.

chart 1

Chart 2

Chart 3chart 4

chart 5chart 6

 

Example 1

You set the kVp to 90 kVp and the exposure time to 0.3 seconds and the mA setting to 500 mA for the chart below. Is the setting safe for the x-ray tube?

example 1

To solve this I drew a line along the 90 kVp setting on the left axis and a vertical line up from 0.3 seconds until they meet. In this example the lines meet above the 500 mA setting therfore, this is not a safe exposure for the tube.

 

Example 2

This time you set the kVp to 110 kVp and the exposure time to 0.2 seconds with a mA setting of 400 mA. Will this be safe for the x-ray tube?

example 2

This time I draw a line along the 110 kVp and up from 0.2 seconds to where they meet. In this case it is below the 400 mA, therefore safe for the tube.

 

Example 3

With a new chart you set the kVp at 100 kVp, the exposure time at 0.8 seconds and the mA at 150 mA. Is this a safe exposure for the tube?

example 3

Drawing the lines from 100 kVp and 0.8 seconds the lines intersect above the 150 mA setting so this is not safe for the tube.

 

Example 4

This last exposure is set at 80 kVp, 0.2 seconds and 200 mA. Is it safe for the tube?

example 4

In this case when I connect the lines along the 80 kVp and o.2 seconds they intersect right at 200 mA. This would be marginally safe. I would say yes this is safe for the tube.

 

X-Ray Tube Heating

There are three main types of heat generation in an x-ray tube. The first is by convection.

Convection: The transfer of thermal energy by actual physical movement from one location to another of a substance in which thermal energy is stored. Also known as thermal convection. The air is what is moving the heat energy around in the tube. If the tube has oil in it, then the oil is the convection material.

The second is by conduction.

Conduction:The flow of thermal energy through a substance from a higher-to a lower-temperature region. This is from the stator axel to the anode.

The third is by radiation.

Radiation: The energy radiated by solids, liquids, and gases in the form of electromagnetic waves as a result of their temperature. Also known as thermal radiation. This occurs when the electrons hit the anode target and heat up the anode. The x-ray radiation is given off which heats up the tube.

heat

 

Anode Cooling Charts: Permits the calculation of the time necessary for the anode to cool enough for additional exposure to be taken.

cooling

cooling 2

Heat Unit: a quantity related to the heat storage capacity of an X-ray tube.

heat unit

Generator Type Constant
Single-phase
1.00
Three-Phase, Six Pulse
1.35
Three-Phase, Twelve Pulse
1.41
High-Frequency
1.45

 

Example 5

Calculate the heat units generated for the following exposures.
Single-phase, rectified unit: 250 mA, 0.7 seconds, and 200 kVp.

example 5

 

Example 6

Calculate the heat units generated for the following exposures.
Three-phase, six pulse, rectified unit: 300 mA, 0.5 seconds, and 110 kVp.

example 6

 

Example 7

Calculate the heat units generated for the following exposures.
High Frequency unit: 500 mA, 0.9 seconds, and 300 kVp.

example 7

 

When you need to find out how long it will take to cool the anode so that you can make more exposures you need to follow the instructions below.

To use the Anode Cooling Chart:
1. Find the total heat units applied on the vertical scale.
2. Read from the heat units over to the cooling curve and then down to read the corresponding time.
3. Calculate the time necessary for the anode to cool to any desired level and subtract the corresponding time of the initial exposure.

 

Example 8

For a High Frequency unit, calculate the length of time necessary for the anode to cool to 50000 HU after 5 exposures of 500 mA for 0.7 seconds at 120 kVp.

First find the heat units for one exposure.

example 8a

Then find the heat units for 5 exposures.

example 8b

Now use the chart and find the times.

example 8c

From the chart I went to just over 300000 HU which is about 1 minute. Then I went to 50000 HU and this is about 6.25 minutes. The difference between these times is the amount of time to cool.

It will take about 5.25 minutes for the anode to cool so that we can take more exposures.

 

Another cooling chart the we need to be familiar with is the Housing Cooling Chart.

Housing Cooling Chart: Permits the calculation of the time necessary for the housing to cool enough for additional exposures to be made.

housing cooling

 

Recommendations for Extending Tube Life

1. Warm up the anode following manufacturer's recommendations.

2. Do not hold the rotor switch unnecessarily. Double-press switchs should be completely depressed in one motion. Dual switches should have exposure switch depressed first, followed by the rotor switch.

3. Use lower mA stations when possible.

4. Use lower-speed rotor when possible.

5. Do not make repeated exposures near tube loading limits.

6. Do not rotate the tube housing rapidly from one position to another.

7. Do not use a tube when you can hear loud rotor bearings.

 

The End