Transcript Rt a wk 3 - El Camino College

RT
A
2007
WK 4
X-RAY TUBE & EXPOSURES
INVERSE SQ LAW
CIRCUITRY
Objectives

X-ray tube review

Inverse Square
Law

Properties of Xrays

X-ray Circuit

“Prime Factors”
X-Ray Properties

Are highly penetrating, invisible rays
which are a form of electromagnetic
radiation.

Are electrically neutral and therefore not
affected by either electric or magnetic
fields
X-Ray Properties

Can be produced over a wide variety of
energies and wavelengths (polyenergetic
& heterogeneous).

Release very small amounts of heat
upon passing through matter.
X-Ray Properties

Travel in straight lines.

Travel at the speed of light, 3 X 108
meters per second in a vacuum.

Can ionize matter.
X-Ray Properties

Cause fluorescence of certain crystals.

Cannot be focused by a lens.

Affects photographic film.
X-Ray Properties

Produce chemical and biological
changes in matter through ionization and
excitation.

Produce secondary and scatter radiation.
PRIME
FACTORS
How does the “technique”
What is it?
How does it affect the “image”
PRIME FACTORS
• KVP
• MAS
• DISTANCE
• The amount of overall blackening on
a radiograph, or of a certain part of
the image, is referred to as density.
• Density results from two things:
• the amount of radiation that reaches a
particular area of the film,
• and the amount of black metallic
silver deposited when the film is
developed.
Producing optimal radiographs
How much of the radiation
received by the patient
Actually reaches the IR ?
About 1%
Creating the IMAGE
• When x-rays pass through a patient's body,
three things can happen:
• (1) the x-ray photon is transmitted, passing
through the body, interacting with the film, and
producing a dark area on the film;
• (2) the x-ray photon is absorbed in an area of
greater tissue density, producing lighter areas on
the film; and
• (3) the x-ray photon is scattered and reaches the
film causing an overall gray fog.
Radiographic Prime factors
• The factors principally responsible for xray quality and quantity.
• These are mAs,
• kVp,
• distance (SID).
PRIME FACTORS
• SID - ADJUSTED IN THE ROOM
• KVP - “intensity of beam”
CONTAST RANGE FOR PART
• MAS – “density of image”
CHANGES WITH PT SIZE
mAs & kVp
Introduction to
Technical Factors to
Create an image
X-ray quantity (mAs)
• is a measure of the number of xray photons in the beam.
• Also called x-ray output, intensity
or exposure.
X-ray quality
(kVp)
• is a measure of the penetrating
ability of the x-ray beam
• The quantity of electron flow,
or current, in the X-ray tube is
described in units of
milliamperes (mA).
• The maximum kinetic energy
of the accelerated electrons is
defined in terms of kilovolts
peak potential (kVp).
IMAGES
• DENSITY = THE AMOUNT OF
BLACKENING “DARKNESS” ON THE
RADIOGRAPH
• CONTRAST – THE DIFFERENCES
BETWEEN THE BLACKS TO THE
WHITES
Film Screen
• Overexposed
– Referring to a radiograph that is too dark
because too much x-radiation reached the
image receptor
• Underexposed
– Referring to a radiograph that is too light
because too little x-radiation reached the
image receptor
Density on Image
• When a radiologist looks at a radiograph,
they looks for any anatomic or pathologic
change that causes a change from the
normal density.
• That could be, a solid mass in the lung
that stops x-rays from reaching the film
and decreases the amount of film
darkening in that area (appears “light or
white on image”
• Or excessive “AIR” – looks BLACK
mAs
mA X s = mAs
Milliamperage
• mA
• One milliampere is equal to one
thousandth of an ampere.
• The amount of current supplied to the
x-ray tube
• Range 10 to 1200 mA
Tube current (mA)
• Tube current is equal to the
number of electrons flowing from
the cathode to the anode per unit
time
• Exposure of the beam for a given
kVp and filtration is proportional to
the tube current
Time
• In seconds
• How long x-rays will be produced
• 0.001 to 6 seconds
mAs Changes
• at least 20 - 30 % mas change
needed to see a visible change in
density
mAs DOUBLED =
DENSITY DOUBLED
+ 25%
+ 50 % mas
Kilovoltage Peak
• kVp
• One kilovolt is = to 1000 volts
• The amount of voltage selected for the
x-ray tube
• Range 45 to 120 kVp (diagnostic range)
• kVp controls contrast
Tube voltage (kVp)
• Determines the maximum energy
in the beam
• spectrum and affects the quality
of the output spectrum
• Efficiency of x-ray production is
directly related to tube voltage
Contrastthe differences
between blacks to whites
• Kilovolts to anode side – kVp (40 -120)
• Kilovolts controls how fast the electrons
are sent across the tube
• kVp – controls CONTRAST on images
• Low kvp – more absorbed – black - white
• High kvp - more grays on image
Influencing factors: kVp
15% rule:
 15% kVp = doubling of exposure to the film
 15% kVp = halving of exposure to the film
15% rule will always change the contrast of the
image because kV is the primary method of
changing image contrast.
Remember :
15% change ( ) KVP has the same effect as
doubling or ½ the MAS on density
+ 15% kvp
- 15% kvp
X-Ray Machine
• Purpose:
– provide a specific current (mA) & voltage
(kV) to the x-ray tube
– convert electrical energy to electromagnetic
energy (x rays) in a controlled manner
• control the energy of the x-ray photons
• control the number of photons
kVp = energy mAs = amount
X-RAY CIRCUITRY
Introduction to Circuitry
Contributions by Mosby, Thompson Publisher, Carlton, Bushberg, and the WWW.
How the current gets to the TUBE
Current from the outlet
Generator+ Tranformers
(where the power comes from)
Review Handouts
•Circuit Board
•Symbols
•Function
high voltage,
low current
low voltage, high current
Bushberg, et al., The Essential Physics
of Medical Imaging, 2nd ed., p. 126.
© UW and Brent K. Stewart PhD, DABMP
54
AUTOTRANSFORMER
• RAISES OR LOWERS THE
VOLTAGE
• KVP CONTROL TAPS
LOCATED
• 220 VOLTS INCOMING
CONVERTED FROM 100
T0 300 VOLTS
TIMER SWITCH
• Timer switch
ends exposure
• Timer – length of
exposure set at
control panel
3 Divisions of Circuit Board
• PRIMARY
(CONTROL PANEL)
yellow
• SECONDARY
(HIGH VOLTAGE)
blue
• FILAMENT
(LOW CURRENT)
purple
TRANSFORMERS
(Step Up or Step Down)
• Increases the VOLTAGE going
to the ANODE side of the tube
OR
• 110 volts to 110,000 volts
• Decreases the CURRENT
going to the CATHODE side
of the tube
• 5 Amps to 50 milliamps
X-Ray Tube Circuit
Functional Position
Control Console
Transformers
Tube
QUESTIONS ?