Principles of Diagnostic

Imaging 1
Lecture 1– The x-ray machine
• Every x-ray
machine has
three principal
parts:
1) The x-ray tube
2) The high voltage section
(generator)
3) Control console.

Note: except dental and

portable machine
these are housed
compactly.
Room design
Most rooms have:
• The head of the x-ray tube in one room.
• The control console in an adjoining room
• A protective barrier (with a window)
• The high-voltage generator is housed in a cubicle
container (3ft) located in the corner.
In newly designed machines they are ceiled above
the room
X-ray tube
• It is rarely seen by radiologic technologist.

• It is contained in a protective housing.

• It has two main parts:
- The cathode
- The anode

Each of these is called an electrode

Any tube with two electrode is called diode
The x-ray tube is a diode.
 Protective housing
• The tube is housed in a lead lines
metal protective housing.
• The x-ray photons are generated
isotropically or in all directions.

• The housing is designed to limit the

beam to window.
• Those emitted from the window 
useful x-rays

• The others that penetrate through

the housing are leakage radiation
• Those result in unnecessary exposure
of pt and technologist
 Protective housing
• The housing also provide mechanical support and protection from damage.

• On some tubes, the housing also contains oil that provides more insulation and
a thermal cushion.

• Some have a cooling fan to air-cool the tube or the oil in which the tube is
bathed.

• Never hold the tube during an exposure.

• Never use the cables or terminals as handles for positioning the tube.
 The X-Ray Tube Glass Envelope
• X-ray tube is an electronic vacuum tube
• The components of the tube are contained within the glass envelope.

• The glass envelope is made of Pyrex to withstand the tremendous heat

produced during x-ray.

• The vacuum allows for more efficient x-ray production and longer life
• If the tube filled with gas  the e- flow from cathode to anode would be
hindered  less x-rays would be produced and more heat would be
created.

• The window is a 5 cm square with a thin section of glass where the useful
beam is emitted.
• Allow maximum x-ray emission and minimum absorption in the glass
envelope.
 The Cathode
• The cathode is the negative side of the tube and contains two primary parts:
• The filaments Actual focal spot size
NEMA XR5-1.2
• The focusing cup
(thermionic
1-2mm emission)
Tungsten
Target Tungsten filaments
Electron
cloud

(copper) ANODE CATHODE

(molybdenum)

Neg potential of the

Quantum/Photon
focusing cup maintains
1) acts as particles rather than waves
a small electron cloud
2) discrete bundles of energy

Apparent or Projected
Focal Spot Size
NEMA XR5-1.3
 The Filaments
• It is a coil of wire
• It is usually about 2mm in diameter and 1 to 2 cm long
• The filament emits electrons.

• When the filament current is sufficiently intense  the

outer shell e- of the filament atoms are boiled off and
ejected from the filament.
• this phenomenon is called thermionic emission.

• Most tubes have two filaments which provide a choice of

quick exposures or high resolution.
• The filaments are made of tungsten
 The Filaments
• Tungsten is used in x-ray tube
Actual focal spot size
because of it’s high melting point NEMA XR5-1 .2
of 3410°C. (thermionic
1-2mm emission)
• X-rays are produced by thermionic Tungsten
Target Electron
Tungsten fila ments
emission when a 4 A or higher cloud

current is applied.
(copper) ANODE CATHODE

(molybdenum )
• Tungsten doesn’t vaporize easily
 if it did  gassy and internal Quantum/Ph oton
Neg potentia l of the
focusing cup maintains
parts are coated with it  tube 1) acts as p articles rather than waves
2) discrete b undles of energy
a small electron cloud
failure.
Apparent or Projected
Focal Spot Size
NEMA XR5-1 .3
• 1%-2% of thorium increases the
efficiency of thermionic emission
and prolongs tube life.
Focusing Cup
• Filament is embedded in metalic cup
• The negative beam tend to spread out
• The focusing cup has a negative charge so that it can condense
the electron beam to a small area of the anode.
• The effectiveness of the focus cup is determined by:
- Its size and shape
- Its charge
- The filament size and shape
- The position of the filament within the focusing cup.
Filament Current
• When the x-ray machine is turned on, a low current
flows through the filament to warm it and prepare
it for the big thermal volt necessary for x-ray
production.
• Low filament current  it is not hot enough for
thermionic emission.
• Once the current is high enough for thermionic
emission a small rise in filament current will result
in a large rise in tube current.
Filament Current & Tube Current
• The x-ray tube current is adjusted by controlling
the filament current.
• The relationship between tube and filament
current is dependent upon the tube voltage.
Space Charge
• When emitted by the filament, the electrons form a
cloud near the filament momentarily before being
accelerated to the anode.
• This is called a space charge.

• Space charge effect is produced due to the

electrostatic repulsion (e- are difficult to be emitted
by the filament).
Saturation Current
• With very high mA and very low kVp, the
thermionic emission can be space charge limited.

• With high mA the cloud makes it difficult for

subsequent electrons to be emitted.

• Above 1000 mA space charge limited exposure can

be a major problem.
Dual-focus tube
• Most diagnostic tubes have two focal spots

• Two filaments are in the focal spot

• Large focal spot (1 to 2.5 mm) large filament
large details  when techniques that produce high
heat are required.
• Small focal spot (0.3- 1 mm)  small filament
fine details are required.

• The selection is made with mA selector on the

control console
The anode
• Two types : stationary and rotating
• Stationary : portable x-ray machine  where low
mA and power are the only needed

• Rotating : produce high intensity x-ray beams in a

short time.
• The rotating anode allows the electron beam to
interact with a much larger target area.
• The heat is not confined to a small area.
The Rotating Anode
• The anode serves three functions:
• Receives the electrons emitted from the cathode.
• It is a electrical conductor (e- from the tube to cables
and back to the high-voltage section of the x-ray
machine).
• Mechanical support for the target.
The Rotating Anode
• The Anode must also be a good thermal conductor.
• When the electron beam strikes the anode more
than 99% of the kinetic energy is converted to heat.
• The heat must be conducted before melting the
anode.
• Copper is the most common anode material
 Anode target
• Target is the area of the anode struck by the e-
• In rotating anode the target is a rotating disc.
• Tungsten-rhenium is used as the target for the electron beam.
• Tungsten is used for three reasons

• High atomic number

- Higher efficiency x-ray production and x-ray energy.

• Heat conductivity

• High melting point

- During exposure anode is raised to 2000 C
The Rotating Anode
• The rotor is an electromagnetic induction motor.
• It spins at 3400 rpm.
• High speed anodes spin at 10,000 rpm.
• Even with the anode rotating, some melting occurs. The
heat must be rapidly dissipated.
• Molybdenum and copper are used to rapidly transfer the
heat from the target.
• When the exposure button is depressed, current is applied to the tube that
produces a magnetic field by the stator outside the glass envelope

• The magnetic field interacts with the rotor and starts the rotation of the anode.
• That’s why we have to wait a sec before taking the exposure to allow the rotor
accelerating to its designed revolution per minute.

When the anode is spinning at the correct speed, the exposure can be made.
• After the exposure is completed, it slows by reversing the motor.
• This is what can happen
to an anode when the
anode stops turning .
• The anode actually
melts .
Target Area
• Target, focus, focal point, focal spot mean the
same thing. This is where the high-voltage
electrons hit the anode.

• Actual focal spot: The physical area of the focal

track that is impacted.

• Effective focal spot: The area of the focal spot that

is projected out of the tube toward the object
being radiographed.
Line-Focus Principle
• The focal spot is the area of the anode from which the x-
rays are emitted.
• The focal spot impacts the geometric resolution of the x-ray
image.
• Size decreases  the heat of the target is concentrated to
smaller area.
• Here comes the line focus principle
• By angling the anode target, one makes the effective focal
spot much smaller than the actual area of interaction.
• The angling of the target is know as the line focus principle.
• in the diagram below there are two anodes with different size
focal spots .
• The one on the left has a larger actual focal spot than the one
on the right
The angle of the focal spot is one way to control the size of the
effective focal spot.
The larger the angle the larger the effective focal spot, as
illustrated
Line-Focus Principle
• Used to reduce the effective area of the focal spot .
• The effective focal-spot size is controlled by the size
of the actual focal spot and the anode target angle.
• The effective focal spot's vertical dimension is the
one that is stated as the focal-spot size.
Line-Focus Principle
• The Effective Focal Spot is the beam projected onto the patient.
• As the anode angle decreases, the effective focal spot decreases.
• Diagnostic tube target angles range from 5 to 15°.
• The advantage of Line focus is it provides the sharpness of the
small focal spot with the heat capacity of the large focal spot.
• Smaller target angles will produce smaller effective focal spots
and sharper images.
• To cover a 17” the angle must be 12°
• To cover 36” the angle must be 14°