kV and mA Meters

kV meter and mA meter
radiologic physics

1. kV Meter
and
mA Meter

2. 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.

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

4. 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.

5. 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

6. A high kVp is available
at a relatively low mA
and vice versa.
↑ kVp = ↓ mA
↓ kVp = ↑ mA

7. 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

8. X-rays are attenuated
exponentially.
– Attenuation is the reduction
in x-ray intensity that results
from absorption and
scattering.

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

10. 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.

11. mA Meter
The 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.

12. •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.

13. •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

14. Compensating for a change in SID (Source-
to-Image-Distance) by changing mAs by
the factor of SID
2
is known as the
square law.
mAs
1
= (SID
1
)
2
_______ _________
mAs
2
= (SID
2
)
2

15. X-ray Quantity and kVp
I
1
= (kVp
1
)
2
__ _____
I
2
= (kVp
2
)
2

16. X-ray Quantity and Distance
aka INVERSE SQUARE LAW
I
1
= (d
1
)
2
__ ___
I
2
= (d
2
)
2

17. PREPARED BY GROUP TWO
JULABAR, MARONA YSABEL G.
ALIASUT, GAILRICH
PANGILINAN, RUFFA MAE
DATU, EUNICE LORENZ
MALIWAT, KEANA JAMILLA
RIVERA, JONIE
GAMBOA, DESIREE DAWN
OCOP, TRINA MILISE
PARAS, VANESSA YVRESSE
BALUYUT, PRINCESS PAULINE
PADILLON, GRACYN