Chapter 8 The X-ray Machine
• The Console is the part of the machine
that the operator controls the operation of
the x-ray machine.
• All machine console are a little different
but there are always similarities. The
console is where we control x-ray tube
current and voltage.
The Console Controls
• The console will have controls for:
• mA and time or mAs
• kVp
• Focal Spot
• Line Voltage Compensation
• Automatic Exposure Control
Symbols Used to Draw Circuits
• We will be using the symbols to define the circuits
in the x-ray machine
Console Circuits
Line Compensation
• At the bottom left is
the controls for line
voltage
compensation.
• Most machine are
designed to operate
at 220 volts while
some will work with
110 volts or 440 volts
Line Compensation
• The power company
often cannot provide
exactly 220 volts at all
times.
• Elevators and Air
Conditioners may
reduce the voltage
available for the x-ray
unit.
Line Compensation
• Older machine have a
meter to monitor the
line voltage attached
to the
autotransformer.
• The operator can
adjust the taps on the
transformer to
account for low or
high incoming
voltage.
Line Compensation
• More modern units
automatically adjusts
for the incoming
power so a meter is
not provided.
• Often over looked by
the operator.
• Results in improper
exposure.
Autotransformer
• The autotransformer
is designed to supply
voltage of varying
magnitude to several
different circuits of the
x-ray machine
including both the
filament circuit and
high voltage circuits.
Autotransformer
• The autotransformer
has only one winding
and one core.
• The single winding
has a number of
connection or electric
taps.
kVp Adjustment
• Most consoles will have one or two knobs that
change the taps on the autotransformer for
major and minor kVp.
• Modern units have a LED readout of kVp.
kVp Adjustment
• Setting the desired
kVp will determine the
voltage applied to the
step-up transformer in
the high voltage
section of the
machine.
kVp Adjustment
• If a meter is provided,
it is placed across the
output terminals of
the autotransformer
and therefore it reads
voltage and not kVp.
The scale will read in
kVp.
mA Control
• The tube current, the
number of electrons
crossing from the
cathode to anode per
second is measured in
milliapmeres (mA).
• The quantity of electrons
is determined by filament
temperature.
mA Control
• The filament normally
operates at currents
between 3 and 6 A.
• The Tube Current is
controlled through a
separate circuit called
the filament circuit
mA Control
• Voltage is provided by
taps of the
autotransformer. This
voltage is reduced
with precise resisters
to a value
corresponding to the
mA stations available.
mA Control
• Tube current is
usually not
continuously variable,
usually only currents
of 50, 100, 150, 200 &
300 mA and higher
are provided.
• Newer units are
continuously variable.
mA Control
• The voltage is then
delivered to the
filament transformer.
The filament
transformer lowers
the voltage so it is
called a step down
transformer.
mA Control
• The selection of the
small or large filament
is connected to the
mA selection or as a
separate control.
Exposure Timers
• For any given radiographic examination,
the number of x-rays reaching the image
receptor is directly related to the tube
current and the time that the tube in
energized.
• The timer circuit is separate from the other
main circuits.
Exposure Timers
• It consists of a
mechanical or
electronic device
whose action is to
make and break the
high voltage across
the tube on the
primary side of the
high voltage section.
Types of Timers
• There are five types of timers:
• Mechanical Timers
• Synchronous Timers
• Electronic Timers
• mAs Timers
• Phototimers
Mechanical Timers
• Very simple device that has a clock
mechanism.
• Operator turns the dial to the desired time.
As it unwinds, the exposure is made.
• Can be used for exposure time longer than
250 milliseconds.
• Very old machine and dental units.
Electronic Timers
• Most sophisticated, complicated and most
accurate timer.
• Consists of complex circuit based upon
the time required to charge a capacitor
through a variable resister.
• Depending upon the incoming power
accurate to 1 ms. Most units have this type
timer.
mAs Timers
• Most modern machine are designed to
accurately control the tube current and
exposure time.
• The product of mA and time (mAs)
determines the number of x-ray photons
emitted and the density on the film.
mAs Timer
• A special type of timer
monitors the product
of mA and terminates
the exposure when
the desired mAs has
been attained.
• This is a mAs timer.
mAs Timer
• Designed to provide
the shortest exposure
and the highest safe
tube current for the
given filament.
• Some have the ability
to change mA
manually.
mAs Timer
• Since it monitors the
actual tube current, it
is on the secondary
side of the H.V.
Circuit
• Units here have mAs
timers.
mAs Timer
• APR or Anatomically
Programs Timers
have computers that
store the technical
factors in the
machine.
• Select the view and
enter the patient size
and the machine is
ready!!!!
Phototimers
• A phototimer that
measures the quantity of
radiation reaching the
receptor and terminates
the exposure when
sufficient radiation
needed to produce the
correct density on the
film.
• Offered in addition to a
manual timer.
Phototimers
• There are two types
of phototimers:
• 1. Photomultiplier
tube that reads a
fluorescent screen
behind the film.
• 2. Ion chamber
between the grid and
film.
Phototimers
• Ion Chambers is used
on most modern x-ray
units.
• It is flat and
radiolucent so it will
not interfere with the
image. Multiple
chambers can be
used to optimize the
image.
Phototimers
• Commonly referred to
as Automatic
Exposure Control or
AEC.
• Widely used in
Medical Radiography.
• Used at our Benton
Clinic.
AEC Console
• With AEC, the
operator can select:
• Where to read the
radiation.
• The desired film
density
• kVp and backup mAs
AEC Console
• Many operators do
not measure the
patient and set a
arbitrary back up mAs
or time.
• Ideally, the patient is
measured and the
back up mAs is set at
2X the normal mAs.
AEC Console
• This allows the AEC
to adjust exposure for
the patient’s habitus
and area density.
• Radiation is
measured at the
center of the film or
off to the sides of the
film.
AEC Console
• The center is read for
most radiography and
especially for the
spine.
• The sides are read for
PA chest, abdomen
and rib radiography.
Other functions on the Control
Console.
• The console will also
have the exposure
button or buttons.
• The prep button is
depressed to prepare
the tube for exposure.
• The rotor will spin up
to 3400 RPM.
Exposure Button
• A green light will let
you know that the
machine is ready to
make the exposure.
• The exposure button
is then depressed and
the exposure is
initiated.
Exposure Button
• The button must be
held down until the
exposure is complete.
• If your finger slips off
the button, the
exposure is
terminated.
Exposure Button
• The exposure control
buttons are referred
to as a”Dead man
Switch”
• After the buttons are
released, the rotor
motor reverses and
the rotor reduces
speed.
Exposure Button
• During the exposure
you will hear an
audible tone so you
will know that the
exposure is in
progress.
High Voltage Section
• The high voltage section converts low
voltage from incoming power to kilo-
voltage of the correct wave form.
• It is usually enclosed in a large metal
container in the x-ray room.
High Voltage Section
• It consists of three
primary sections:
– High voltage step up
transformer
– Filament Transformer
– Rectifiers ( Diodes)
• All components
immersed in oil.
High Voltage Transformer
• The high voltage transformer is a step-up
transformer.
• There will be more winding on the
secondary side compared to the primary
side.
• The ratio of windings is referred to as the
turns ratio.
High Voltage Transformer
• The only difference between the primary
and secondary waveforms is the
amplitude.
• The turn ratio for most x-ray high voltage
transformers is between 500 and 1000.
• Incoming Volts converted to output:
Kilovolts.
Voltage Rectification
• Transformers operate with alternating
current.
• Remember that x-ray tubes operate on
direct voltage ( electron moving in one
direction).
• To convert AC to DC we use rectifiers.
Half-Wave Rectification
• Sometimes the x-ray tube
alone will work as the
diode this is called self-
rectification.
• When one or two diodes
are placed in the circuit
that stops the negative
flow of electrons it is
called Half Wave
Rectification.
• 60 pulses per second.
Full-Wave Rectification
• Full wave rectified x-ray
machines contain at least
four diodes.
• It changes the polarity of
the negative half of the
wave.
• This allows 120 pulses of
x-ray per second.
• The exposure time can
be cut in half compared to
half-wave systems.
Three-Phase Power
• If three phases of power are combines
with the phase off by one step, the normal
reduction of voltage back to zero is
removed. Commonly called the Ripple.
• Technical factor cut in half due to more
efficient power.
• Too expensive got office use.
High Frequency Generator
• By changing the
frequency from 60 Hz to a
higher frequency of 500
to 1000, the ripple is
reduced to less than 1%.
• Single phase machine
operating on 220 volts
and even 110 volts are
more efficient that
machine operating on
three-phase power.
Types of X-ray Generators
• The type of generator will determine the efficiency
of the machine.
Wave Forms of Different Generator
Types
• As the ripple effect
decreases, the
efficiency increases.
• There is one more
type of generator. It
uses is called stored
energy.
Stored Energy Generators
• If 220 volt power is
not available, the
operator may choose
a stored energy
machine.
• A battery charger is
powered by typical
house hold current.
• If produces direct
current.
Stored energy or Capacitor
Discharge Generators
• There is a short
charging time before
the exposure can be
made.
• The disadvantage to
the design is a drop in
power at the end of
the exposure of about
1 kV/mAs. This is
called a falling load
generator.
Generator Types Pros & Cons
• Single phase half or self rectified: Cheap
but not efficient. Full wave rectified better.
• Three phase: Expensive to install but
cheaper to maintain. Too costly for most
offices. 6 pulse less costly than 12 pulse
• High Frequency: very efficient and works
with single or three phase power.
• Stored energy: works on conventional 110
volt power but batteries must be replaced.
The Basic X-ray Circuits
• Circuits
that make
up the
basic x-
ray
machine.
Other Parts of the X-ray Room
• The tube is
suspended on the
tube stand.
• The tube stand may
be wall and floor
mounted or ceiling
suspended.Locks are
provided for
horizontal and vertical
movement.
Other Parts of the X-ray Room
• When the tube is
angled toward the
wall grid holder, the
horizontal lock allow
us to set the distance
between the tube and
the film (SID).
Other Parts of the X-ray Room
• When the tube is
aimed at the table,
the vertical lock
allows us to set the
SID.
• Hanging on the wall
grid cabinet is the
non-Bucky film
holder.
• It allows erect non-
grid films.
Other Parts of the X-ray Room
• X-ray tables may be
bolted to the floor or
mobile. The table will
also have a grid
cabinet for grid
radiography.
• We will discuss grids
in greater detail next
week.
Collimator and Angle Indicator
• The tube stand also
has an angle indicator
attached parallel to
the tube.
• There are views that
will require tube
angles.
Collimator and Angle Indicator
• The Collimator is
attached to the x-ray
tube below the glass
window where the
useful beam is
emitted.
• Lead shutters are
used to restrict the
beam.
Collimator and Angle Indicator
• A mirror and light
source allows us to
restrict the beam to
the area of interest.
• Collimation is our
greatest tool in
keeping patient
exposure as low as
possible.
Other items that may be in a
x-ray room.
• Fluoroscopy Equipment: Allows dynamic
imaging of the body.
• Consists of:
• Image intensifier with television camera
and monitor.
• Spot-film device for making radiographs or
• Motion picture camera or digital imaging.
Image Intensifier & Fluoroscopy
• Thomas Edison invented the fluoroscope
in 1896. Early units consisted of a
fluorescent hand held viewer that the
doctor held in from of the patient during
continuous exposure.
• This resulted in the first x-ray death.
• Dose is still relatively high compared to
plain film radiography.
Image Intensifier & Fluoroscopy
• Plain film radiography uses up to several
hundred mA and fractions of seconds.
• Fluoroscopy tubes operate at less than 5
mA but for minutes. 2 to 4 mA is normal.
• In California Fluoroscopy requires
additional training and license or permits
for technologist and doctors.
Image Intensifier & Fluoroscopy
• Shortly after WW2, Bell Laboratories
invented the photomultiplier tube. This was
developed into the modern image
intensifier.
• The multiplication of the light emitted by a
input fluorescent screen is picked up by a
cesium photocathode and converted into
electrons.
Image Intensifier & Fluoroscopy
• A potential of about 25,000 volts is
maintained between the photocathode and
the anode.
• There are electronic optics and
electrostatic focusing lenses between the
photocathode and output phosphor.
Image Intensifier & Fluoroscopy
• The output phosphor can be viewed via
mirror optics or a video monitoring
system.
• A Videotape recorded can be placed into
the video chain.
• Fluoroscopy allows the evaluation of the
internal structures in motion. Normal uses
include:
Uses of Fluoroscopy
• Dynamic spinal imaging of range of motion
and with contrast called myelograms.
• Dynamic studies of joints with or without
contrast media.
• Studies of the digestive system.
• Studies of arteries and blood flow called
angiography.
Uses of Fluoroscopy
• When connected to a computer, for digital
fluoroscopy and spot films.
• With digital fluoroscopy, digital
angiography is possible.
• By over-lapping an image without contrast,
digital subtraction angiography is
performed where the bone is removed.
End of Lecture
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