Rad T 110

Electromagnetic Radiation

The Wave Equation

• Velocity = frequency x wavelength– Remember, photons all travel at the speed of

light, 186,000 miles/second so velocity is in effect a constant

• Consequently, any changes to either frequency or wavelength have to be accompanied by a change in the other.

• So, if frequency increases then wavelength decreases.

• Conversely, if frequency decreases then wavelength increases.

What is frequency?

• Frequency is defined as the number of wave forms per second.

• In the case of x-ray photons about 1019 cycles per second.

• Frequency is a representation of energy.

• As beam energy is increased, frequency increases.

Beam Energy

• Beam energy is determined by kVp.

• Therefore, the higher the kVp the higher the photon frequency.

• The higher the kVp– The higher the beam quality– The harder the beam– The higher the penetrability of the beam

Electromagnetic Spectrum

Wave-particle Duality

• Higher energy photons act like particles.

• Low energy photons act like waves.

Inverse Square Law

• Addresses the issue of the divergent beam.

• As the beam travels it diverges or spreads out. The further it travels the more spread out it becomes.

• However, given that the same number of photons are in the beam their concentration or intensity will be diminished.

Single Phase Electric Current

• Full

• Half

Voltage ripple

Three phase Electric Current

• 6 pulse

• 12 pulseVoltage ripple

Electromagnetic Induction

• Faraday’s Law– An electric current is induced in a circuit if part

of the circuit is in a changing (alternating) magnetic field.

– Also, a magnetic field can be generated by alternating magnetic fields.

Transformers and Mutual Induction

• Transformers work with AC current through the application of Faraday’s Law.– An AC electric current induces a varying magnetic

field in a piece of iron (electromagnet).– Another piece of wire is wrapped around the iron. In

this wire an electric current is induced. Why, because the magnet field is moving throughout the iron.

– So, without an AC current you would not induce a varying magnetic field.

Step-down transformers

• These transformers also work on the principle of mutual induction; however, their goal is to decrease the incoming voltage.

• So, the secondary side has fewer windings than the primary side.

• Any device you have at home that runs on batteries and AC has a step-down transformer. It is usually the little black box that gets hot.

Step Up Transformers

• These transformers are intended to increase voltage.

• Remember, incoming voltage is relatively low (<480 V) is in order to create kVp you are going to have to dramatically increase the voltage.

• This is done by having more wires on the secondary side of the transformer than the primary side.

• The primary side of the transformer is were the voltage comes in.

• The step-up transformer can also be called a high-voltage or high-tension transformer.

Closed core transformer

Types of Transformers in X-ray Circuits

• Autotransformer– Has only one winding and works on the

principle of self-induction (because of the single winding).

• Step-up transformer, high voltage or high tension

• Step-down or filament transformer– Both of these are generally shell type

transformers and work using mutual induction.