kV Meter and

mA Meter

The x-ray machine takes energy from an electrical source (usually a 220V outlet) and converts it to two separate voltage streams:

• One stream is a low voltage source which can be varied so that different amounts of current can flow through the high resistance wire that composes the filament in the cathode. The current that flows through the filament is measured in milliamperes (mA).

• The second stream is very high voltage, measured in kilovolts (thousands of volts at peak voltage -- kVp).  This voltage is applied across the gap between the anode and the cathode.

When the kVp is increased, the x-ray photons that are produced have a shorter wavelength, and thus have higher energy.

The number of x-ray photons produced (i.e. the intensity of the beam) depends on the number of electrons that boil off the filament. This depends on the temperature of the heating element which in turns depends on the current flowing through the filament. The amount of current flowing through the filament is controlled by the mA selector.

kV Meter

Kilovolt peak determines the quality of the x-ray beam. • When we say the quality of the x-ray

beam, we mean its energy and penetrability.– Penetrability refers to the ability of x-

rays to penetrate deeper in tissue.• High energy x-rays are able to penetrate

tissue more deeply than low energy x-rays

A high kVp is available at a relatively low mA and vice versa.

↑ kVp = ↓ mA↓ kVp = ↑ mA

A higher kVp will give your image lower contrast [many shades of gray (wide latitude)]. Lower kVp will give your image a higher contrast image [fewer shades of gray (narrow latiude)].

↑ kVp = ↓ contrast = ↑ latitude↓ kVp = ↑ contrast = ↓ latitude

X-rays are attenuated exponentially.

– Attenuation is the reduction in x-ray intensity that results from absorption and scattering.

Distance and mAs DO NOT affect radiation quality, but they do affect radiation quantity.

In radiography, the quality of x-rays is measured by the Half Value Layer (HVL).

The HVL of an x-ray beam is the thickness of absorbing material necessary to reduce the x-ray intensity to half of its original value.

X-ray beam quality can be identified by kVp or filtration, but HVL is most appropriate as it is the best method for specifying x-ray quality.

mA MeterThe number of electrons emitted by the filament is determined by the temperature of the filament.

– The filament temperature is in turn controlled by the filament current, which is measured in amperes (A).

– As the filament current increases, the filament becomes hotter, and more electrons are released by thermionic emission.

↑ F. Current = Hotter F. = ↑ # of electrons

• Filaments normally operate at currents of 3 to 6 A.

• mA measures the x-ray beam’s quantity, the number of x-rays in the useful beam.

• X-ray quantity is also known as :–intensity of the x-ray beam–radiation exposure.

• X-ray quantity is directly proportional to mAs. – When mAs is doubled, the number of

electrons striking the tube target is doubled, therefore the number of x-rays emitted is doubled.

↑ mAs = ↑ # of electrons = ↑ # of x-rays

Compensating for a change in SID (Source-to-Image-Distance) by changing mAs by the factor of SID2 is known as the square law.

mAs1 = (SID1)2

_______ _________

mAs2 = (SID2)2

X-ray Quantity and kVp

I1 = (kVp1)2

__ _____I2 = (kVp2)2

X-ray Quantity and Distanceaka INVERSE SQUARE LAW

I1 = (d1)2

__ ___I2 = (d2)2

PREPARED BY GROUP TWOJULABAR, MARONA YSABEL G.

ALIASUT, GAILRICHPANGILINAN, RUFFA MAEDATU, EUNICE LORENZ

MALIWAT, KEANA JAMILLARIVERA, JONIE

GAMBOA, DESIREE DAWNOCOP, TRINA MILISE

PARAS, VANESSA YVRESSEBALUYUT, PRINCESS PAULINE

PADILLON, GRACYN