Chapter 5 Lecture Notes

X-Ray Equipment

There are two main uses for X-Ray energy. One is for diagnostic reasons and the second is for therapeutic uses.

Diagnostic: Methods used to determine the source or cause of a disease, disorder, or traumatic injury.

Therapeutic: Methods used to treat and rehabilitate a disease, disorder, or traumatic injury.

These are some pictures of different X-Ray equipment.

Once you are working in the field you will get to know your machine inside and out. They might look like some of this, but it might not look like any of these.

As you can see the table can go from having the patient standing straight up (vertical) to having the patient being head down at thirty degrees.

This is a typical X-Ray room.

Power for X-Ray Generation

Incoming Line Current or main - when you plug in a power cord for the X-Ray machine it will usually across more than the normal household socket. It might be twice the normal 120 volts to 240 volts as illustrated below. Then the X-Ray machine will convert it to high voltage and also to high current in different parts of the X-Ray circuit.

Single-Phase Power

When you turn on an electrical device in your house, alternating current is engaged. With alternating current the current goes toward the light then switches away from the light. It does this 60 time a second. However, in the X-Ray circuit we need direct current. So, as illustrated in the diagram the current that is below the horizontal axis must be modified so that it is above the axis.

The symbol for single phase power is .

The above diagram shows that with single phase power there are times when the x-rays are not powerful enough for diagnostic purposes. These are when the current dips to zero, the brown sections, and no photon (x-rays) are produced. Therefore, single phase power does not produce consistently good x-rays.

For part B in that above diagram, this illustrates Peak kilovolts (kVp) and the root mean square kilovolts. The root mean square kilovolts (rms voltage) is similar to the average voltage for the cycle. There is an easy way to estimate the rms voltage, you take the kVp and multiply by 0.707.

Root Mean Square (rms) Voltage of single-phase sinusoidal wave is 70.7% of peak voltage.

Example:

What is the approximate rms voltage of a single-phase sine wave with a peak of 95 kVp?

Three-Phase Power

The type of power that is usually used for X-Ray machines is called three-phase power. In three-phase power three wires with power come into a transformer, with maybe a fourth for a ground, then put together for use in the x-ray machine.

The diagram below shows in part A that all three wires bring voltage in at different times. That is why the peaks are different. The blue one has the first peak, then the yellow and finally the red. As part B shows the voltage ripple is small compared to single phase which has a 100% ripple. Three-phase has much less ripple so it produces better and more consistant x-rays.

The symbol for three-phase power is .

Voltage Ripple: The amount of flucuation in the voltage of the power to an x-ray machine. For x-ray machines, the less ripple the better.

A Basic X-Ray Circuit

Now we are going to look are the X-Ray Circuit. There are two main parts of the circuit, one is the main circuit and the second is the filament circuit.

Main part of X-Ray Circuit: supplies power to the x-ray tube so that x-rays are produced.

Filament Circuit: supplies power to the filament of the x-ray tube so that the filament supplies enough electrons by thermionic emission.

Main Circuit Parts:

Main Switch: The switch that generates the power to the x-ray tube.

Exposure Switch: A remote control device that permits current to flow through the circuit.

Timer: Device used to end the exposure at an accurately measured preset time.

In the diagram below are the important parts of the circuit. The blue part is the main x-ray circuit and the tan part is the filament circuit.

1. main breaker - this is where the alternating current comes from to power the circuit.
2. exposure switch - when you push the button to start an exposure this switch closes to start the exposure.
3. autotransformer - this is where you adjust the kVp for the exposure.
4. timer circuit - this part of the circuit stops the exposure.
5. high-voltage step-up transformer - this transformer bumps the voltage up so that the x-ray tube has very high voltage to make the electrons have enough energy to form x-rays.
6. four-diode rectification circuit - this makes the current only go in one direction through the x-ray tube.
7. filament circuit variable resistor - this variable resistor adjusts the current going to the filament.
8. filament step-down transformer - this transformer steps the voltage down and therefore the current up.
9. x-ray tube - this is where the x-rays are created.
10. rotor stator - this rotates the anode.

Exposure Switch

Timer Circuit

Milliampere-Second Timers: A device used in falling load generators and some capacitor units to monitor the product of mA and time on the secondary side of the high-voltage step-up transformer.

Automatic Exposure Control Timers: A device programmed to terminate the radiographic exposure time.

Filament Circuit

Control Factor Electrical Device and Location in Circuit
kVp Selection kVp Level Autotransformer (between incoming line and exposure switch)
mA Selection Filament Current Variable resistor (in filament circuit between incoming line and step-down transformer)
Time Selection Length of exposure Timer circuit (between exposure switch and step-up transformer)
Rotor Switch Speed of rotating anode Stator (separate circuit from stator of anode motor)
Exposure Switch Moment of exposure Switch (between autotransformer and timer circuit)

Generators

Now lets look at the types of generators that could power the x-ray machine.

Single-Phase Generators: (single-phase, two pulse) Voltage ripple of 100%.

From the diagram the power comes in at 220 V then goes through the step-up transformer to high voltage. The diodes convert it to direct current with 100% ripple and the x-rays coming out also have 100% ripple.

Three-Phase Generator: (three-phase, 6 pulse)
A full-wave rectified, three-phase, six-pulse waveform produces approximately 35 percent more average photon energy than full-wave rectified, single phase. Voltage ripple of 13-25%.

The diagram is similar to the previous one but uses three-phase power. The voltage ripple is about 13% and an x-ray ripple of about 50%.

Three-Phase 12 Pulse:
A full-wave rectified, three-phase, 12-pulse waveform produces approximately 40 percent more average photon energy than full-wave rectified single-pulse. Voltage ripple of 4-10%.

With three-phase, 12 pulse power the voltage ripple is about 3% and the x-ray ripple is about 10%. Remember that the less ripple the better.

For the diagram part A shows the radiation (x-ray) output for the four different types of power. The one that has not been mentioned yet is High Frequency power.

Part B of the diagram shows the ripple for the tube kVP.

Part C of the diagram shows the ripple for the average tube mA current.

You need to know each of the parts of the diagram, that is the ripple values and which power generation is used.

Generator Power

To calculate the power generated by the generators for the different power sources you use the following information.

Power for a generator: P = IV

Power for a generator: P = (0.7)IV

High Frequency

The High Frequency power generators have the least amount of voltage ripple. The circuit is more complicated than the other circuits.

Voltage ripple of 3-4%.

There are also moble x-ray machines that use capacitors for their power.

Capacitor Discharge Mobile Units

In the diagram, the top part shows that capacitor charging then the exposure starts and the capacitor discharges putting power to the tube. The bottom part shows the kVp during the exposure. Notice that the kVp drops as the exposure proceeds, this follows the discharge of the capacitor.

Battery-Operated Mobile Units

Power similar to at least

mA Time in Seconds mAs achieved
600
0.1
60
500
0.2
100
400
0.2
80
Total
0.5
240

Automatic Exposure Controls

Phototimers: Often used to describe all automatic exposure controls; this type of AED is rarely used in modern radiography.

Ionization Chambers: An automatic exposure control device used to terminate the exposure after a desired exposure has been reached.

Minimum Reaction Time: The length of time necessary for the AEC to respond to the radiation and for the generator to terminate the exposure.

Backup Time: Backup times cannot exceed the tube limit and should be set at 150% of the anticipated manual exposure mAs.

The End