medical equipment lec 2

1. Medical Equipment III
X-ray Imaging

2. The X-ray tube

3. Filament
 The filament is heated to boil off electrons which are then
accelerated to the anode.
 Tungsten is used in x-ray tube because of its high melting
point at 3370°C.
 The filament is mounted within a negatively charged
focusing cup. Collectively, these elements are termed the
“cathode assembly”.
 At start, a low current flows through the filament to warm it
and prepare it for the big thermal jolt necessary for x-ray
production.
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4. Filament and tube currents
 Filament current is the current flow
through the filament to raise its
temperature and release electrons.
 Tube current is the flow of released
electrons from the filament to the anode
across the x-ray tube.
• A small rise in “filament current” will
result in a large rise in “tube current”.
• Tube currents range: 50-800 mA.
• Filament currents range: 2-5 A
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5. Tube voltage
 A potential difference (voltage) is impressed
across the x-ray tube with the filament (or
cathode) at a negative potential and the target
(or anode) at a positive potential.
 In most x-ray equipment, ac is converted to
direct current (dc), and the voltage between
filament and target is kept at or near its
maximum value. The conversion of ac to dc is
called rectification.
 This potential difference is known as the
“accelerating voltage” or kVp. It ranges from
20 to 150 KV.
5
Full-wave rectified
single-phase and
three-phase tube

6. • The energy of the x-rays (keV) is determined by the
voltage applied (kVp).
 The amount of x rays produced is directly proportional to
the product of tube current in milliamperes and exposure
time in seconds (mA・sec) .
6
X-ray emission spectra
for a 100-kVp tungsten
target operated at 50, 100,
and
150 mA.

7. Filament cup housing
 Metallic shroud containing the two filaments.
 Usually made from nickel.
 Contains a negative charge
 To repel electrons, so condenses electron beam to
a small area on focal track.
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8. Cup and Dual Filament
 Cup Effectiveness
 Charge
 Filament size & shape
 Position of filament w/in
cup
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9.  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.
 At low tube voltages, electrons are
released from the filament more rapidly
than they are accelerated . Therefore, the
space charge accumulates around the
filament.
9
Space-charge limited and emission-limited
tubes

10.  At low tube voltages and high
filament currents,
 The accumulated cloud opposes the
release of additional electrons from
the filament, therefore, limits tube
current.
 The tube current is said to be space-
charge limited.
 At low filament currents, a
saturation voltage is reached above
which the tube current does not
vary with increasing voltage.
 At the saturation voltage, tube
current is limited by the rate at which
electrons are released from the
filament. 10
Space-charge limited and emission-limited
tubes

11. Block Diagram of the X-ray
machine
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12. Detailed Block Diagram
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13. Envelope and Housing
 Tube vacuum:
 To prevent collisions between air molecules as electrons
accelerate between the filament and target.
 The tube is mounted inside a metal housing that is
grounded electrically. A lead sheath inside the metal
housing absorbs radiation emerging from the x-ray tube in
undesired directions.
 Oil surrounds the x-ray tube to:
(a) absorb heat radiated from the anode, (b) insulate the
housing from the high voltage applied to the tube.
 Bellows in the housing permits heated oil to expand when
the tube is used.
 Bellows are connected to a switch that interrupts the operation
of the x-ray tube if oil reaches a temperature exceeding the
heat storage capacity of the tube housing (Thermal overload
detection) 13

14. X-ray Generator
 Modifies incoming line voltage and
current to provide an x-ray tube
with the power needed to produce
an x-ray beam of the desired peak-
kilovoltage (kVp), current (mA)
and duration (Time).
 Control panel: Permits the
selection of technical factors and
initiation of radiographic exposures:
mA, kVp, Time
 Transformer: Transformers
modify the voltage of incoming AC
line voltage to increase or decrease
the voltage in a circuit.
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15.  Step-up transformer: Supplies high voltage to
the x-ray tube (voltage increases and current
decreases)
 Step-down transformer: Supplies power to
heat the filament of the x-ray tube (voltage
decreases and current increases)
 Autotransformer: Supplies voltage for the two
circuits and provide a selector for the desired
KVp and mA
 Rectifiers: Convert AC into DC required by
the x-ray tube. A rectifier restricts current
flow in an x-ray tube in one direction (from
cathode to anode), thereby preventing damage
of the x-ray tube filament. Two types: Half
wave and Full wave.
15
X-ray Generator

16.  High-voltage cable: Special
highly insulated cables that
deliver high voltage to the x-
ray tube.
 Enters the tube housing
through insulated openings.
16
X-ray Generator

17. Anode
 3 to 5 inches in diameter.
 is attached to the rotor of an
induction motor.
 rotates at speeds up to 10,000
rpm.
 The induction motor is energized
for about 1 second before high
voltage is applied.
 The rotating anode, together with
the stator and rotor of the
induction motor, are known
collectively as the anode 17

18. Induction Motor
 Works on the principle of
electromagnetic induction.
 Current flowing in the stator develops
a magnetic field.
 Stator windings are sequentially
energized so that the induced
magnetic field rotates on the axis of
the stator.
 This causes the rotor to rotate.
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19. Target material
 Tungsten-rhenium alloy (2-10%
rhenium) is used as the target
 Tungsten is used for three reasons
 High melting point (3370 oC)
 High thermal conductivity
 High atomic number (74)
 the higher the atomic number of the metal in
the target, the higher the efficiency of X-ray
production.
 Molybdenum
 Surrounds tungsten target area.
 Assists in dissipating heat.
 Graphite
 Serves as mount for molybdenum and tungsten
target. 19

20. Tube cooling
 Only ~1% of the electrons
energy is converted into X-
rays: the remainder is
dissipated in heat.
 Heat transfer
 Radiation
 Conduction
 Convection
20

21. Energy rating charts
 Physical limitations for the values of kVp and tube current
are set by the power rating of the particular X-ray tube,
defined as the maximum power dissipated in an exposure
time of 0.1 s.
 For example, a tube with a power rating of 10 kW can operate at a
kVp of 125 kV and a tube current of 1 A for ~78 ms.
 Rating charts are used to determine whether the target of
an x-ray tube might be damaged by a particular
combination of tube voltage, tube current, and exposure
time.
 Depends on the characteristics of the applied voltage (1ϕ
or 3ϕ) and on the properties of the x-ray tube
 (e.g., distance between filament and target, focal spot size, shape of
the cathode assembly and target, and shape of the glass envelope). 21

22. Energy rating charts
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Medical Equipment II Spring 2015 Inas A.
Yassine

23. Anode cooling curve
 Energy ratings for the anode and the tube housing are
expressed in terms of heat storage capacities.
 The heat storage capacity of a certain tube component
is the total number of heat units that may be absorbed
without component damage.
 For single-phase power:
kVp x mA x s = HU
23

24. Anode cooling curve
 An anode thermal-
characteristics chart
describes the rate at which
energy may be delivered to
an anode without
exceeding its capacity for
storing heat.
 The chart also shows the
rate at which heat is
radiated from the anode to
the insulating oil and
housing.
 Chart helps to determine
time interval between 24
The anode heat-storage capacity is 72,000 HU

25. 25
Rotating targets damaged by excessive loading or
improper rotation of the target.

26. Reference Book:
 William R. Hendee, E. Russell Ritenour,
“Medical Imaging Physics”, Chapter 5,
Fourth Edition.
 N. Smith et. Al., "Introduction to Medical
Imaging Physics, Engineering and
Clinical Applications“, 2011.