Introduction to Medical Imaging
Lecture No. 3
Dr. Yousif Mohamed Yousif Abdallah
A bit of History
•
William Roentgen discovered X-rays
in 1895 and determined they had the
following properties
1.
2.
3.
4.
•
•
Travel in straight lines
Are exponentially absorbed in matter
with the exponent proportional to the
mass of the absorbing material
Darken photographic plates
Make shadows of absorbing material on
photosensitive paper
Roentgen was awarded the Nobel
Prize in 1901
Debate over the wave vs. particle
nature of X-rays led the development
of relativity and quantum mechanics
The Electromagnetic Spectrum
Cu-Kα
Generating X-rays for
Diffraction
 To get an accurate picture of the structure of a
crystalline material requires X-radiation that is as
close to monochromatic as possible.
 The function of the x-ray tube and associated
electronics is to produce a limited frequency range
of high-intensity x-rays.
 Filters, monochromators, specially tuned detectors
and software are then used to further refine the
frequency used in the analysis.
The X-ray Tube
• Schematic cross section of an Xray tube as used in our lab
• The anode is a pure metal. Cu,
Mo, Fe, Co and Cr are in
common use in XRD applications.
Cu is used on our Scintag system
• Cu, Co and Mo will be available
on our new systems
• The tube is cooled by water and
housed in a shielding aluminum
tower
X-rays Tube Schematic
HV Power Supply Schematic
• In most systems,
the anode (at top in
8) is kept at ground
• #2 (KV) and #7
(ma) are what are
adjusted on the
power supply with
#1 and #5
• In our lab, we only
routinely adjust
filament current (#5)
from operating (35
ma) to “idle” (10
ma) levels
Characteristics of Common Anode Materials
Material At. # K1 (Å) K2 (Å)
Cr
24
2.290
2.294
Char
Min
(keV)
5.98
Opt
kV
Advantages (Disadvantages)
40
High resolution for large d-spacings,
particularly organics (High attenuation
in air)
Most useful for Fe-rich materials
where Fe fluorescence is a problem
(Strongly fluoresces Cr in specimens)
Fe
26
1.936
1.940
7.10
40
Co
27
1.789
1.793
7.71
40
Useful for Fe-rich materials where Fe
fluorescence is a problem
Cu
29
1.541
1.544
8.86
45
Mo
42
0.709
0.714
20.00
80
Best overall for most inorganic
materials (Fluoresces Fe and Co K
and these elements in specimens can
be problematic)
Short wavelength good for small unit
cells, particularly metal alloys (Poor
resolution of large d-spacings;
optimal kV exceeds capabilities of
most HV power supplies.)
Generation of X-rays

c

• X-rays may be described as
waves and particles, having
both wavelength () and
energy (E)
• In the equations at left:
(1)
E  h
E
E
hc

(2)
(3)
12.398

– E is the energy of the electron flux in
KeV
– h is Planck’s constant (4.135 x 10-15
eVs)
– v is the frequency
– c is the speed of light (3 x 1018 Å/s)
–  is the wavelength in Å
(4)
• Substituting (1) into (2)
yields (3), the relationship
between wavelength and
energy.
• In (4) all constants are
substituted
Continuous Spectrum
• X-rays are produced whenever matter is irradiated
with a beam of high-energy charged particles or
photons
• In an x-ray tube, the interactions are between the
electrons and the target. Since energy must be
conserved, the energy loss from the interaction
results in the release of x-ray photons
• The energy (wavelength) will be equal to the energy
loss (Equation 4).
• This process generates a broad band of continuous
radiation (a.k.a. bremsstrahlung or white radiation)
Continuous Spectrum
hc 12.398
 min 

V
V
• The minimum wavelength (
in angstroms) is dependent
on the accelerating potential
( in KV) of the electrons, by
the equation above.
• The continuum reaches a
maximum intensity at a
wavelength of about 1.5 to 2
times the min as indicated
by the shape of the curve
Generating Characteristic Radiation
• The photoelectric effect is
responsible for generation
of characteristic x-rays.
Qualitatively here’s what is
happening:
L-shell to K-shell jump produces
a K x-ray
M-shell to K-shell jump produces
a K x-ray
– An incoming high-energy
photoelectron disloges a k-shell
electron in the target, leaving a
vacancy in the shell
– An outer shell electron then
“jumps” to fill the vacancy
– A characteristic x-ray
(equivalent to the energy
change in the “jump”) is
generated
The Copper K Spectrum
 The diagram at left
shows the 5
possible Cu K
transitions
 L to K “jumps:
– K1 (8.045
keV, 1.5406Å)
– K2 (8.025
keV, 1.5444Å)
 M to K
– K1 K3
(8.903 keV,
1.3922Å)
– K 5
Note: The energy of the K transitions is
higher that that of the K transitions,
but because they are much less
frequent, intensity is lower
Continuous and Characteristic Spectrum
Characteristic Wavelength values (in Å) for
Common Anode Materials
Anode
K1 (100)
K2 (50)
K (15)
Cu
1.54060
1.54439
1.39222
Cr
2.28970
2.29361
2.08487
Fe
1.93604
1.93998
1.75661
Co
1.78897
1.79285
1.62079
Mo
0.70930
0.71359
0.63229
* Relative intensities are shown in parentheses
Making Monochromatic X-rays
• X-rays coming out of the tube will include the
continuum, and the characteristic K1, K2,
and K radiations
• A variety of methods may be used to convert
this radiation into something effectively
monochromatic for diffraction analysis:
• Use of a  filter
• Use of proportional detector and pulse height
selection
• Use of a Si(Li) solid-state detector
• Use of a diffracted- or primary-beam
monochromator
 Filters
• There are two types of absorption of x-rays.
– Mass absorption is linear and dependent on
mass
– Photoelectric absorption is based on quantum
interactions and will increase up to a particular
wavelength, then drop abruptly
• By careful selection of the correct absorber,
photoelectric absorption can be used to
select a “filter” to remove most  radiation
while “passing” most  radiation
 Filters for Common Anodes
Target
K (Å)
filter
Cr
2.291
V
11
6.00
58
3
Fe
1.937
Mn
11
7.43
59
3
Co
1.791
Fe
12
7.87
57
3
Cu
1.542
Ni
15
8.90
52
2
Mo
0.710
Zr
81
6.50
44
1
Thickness
(m)
Density
(g/cc)
% K
% K
Note: Thickness is selected for max/min attenuation/transmission Standard practice
is to choose a filter thickness where the  :  is between 25:1 and 50:1
Filtration of the Cu Spectrum by a Ni Filter
 The Ni absorption edge
lies between the K and
K peaks
 Note the jump in the
continuum to the left of the
K peak from Cu selfabsorption
 Note that the Ni filter does
little to remove the highenergy high-intensity
portion of the continuum
Filter Placement:
• In a diffractometer, the filter may be placed on
the tube or detector side.
• In powder cameras (or systems with large 2D
detectors), the filter will be between the tube
and the camera (or specimen).
Discriminating with Detectors
• Pulse-height Discrimination
– Detector electronics are set to limit the energy of xrays seen by the detector to a threshold level
– Effectively removes the most of the continuum and
radiation produced by sample fluorescence
– Particularly effective combined with a crystal
monochromator
• “Tunable” Detectors
– Modern solid state detectors, are capable of
extremely good energy resolution
– Can selectively “see” only K or K energy
– No other filtration is necessary, thus signal to noise
ratios can be extremely high
– Can negatively impact intensity of signal
Monochromators
• Following the Bragg law, each component wavelength of
a polychromatic beam of radiation directed at a single
crystal of known orientation and d-spacing will be
diffracted at a discrete angle
• Monochromators make use of this fact to selectively
remove radiation outside of a tunable energy range, and
pass only the radiation of interest
– A filter selectively attenuates K and has limited effect on other
wavelengths of X-rays
– a monochromator selectively passes the desired wavelength and
attenuates everything else.
• Monochromators may be placed anywhere in the
diffractometer signal path
Pyroliltic Graphite curved-crystal Monochromator
• A planar crystal will
diffract over a very
small angular range and
significantly reduce the
intensity of the x-ray
signal
• Precisely “bent” and
machined synthetic
crystals allow a
divergent x-ray beam to
be focused effectively
with minimal signal loss
The pyrolitic graphite curved crystal
monochromator is the most widely used type
in XRD laboratories
Graphite Monochromator on Scintag Diffractometer
Diffracted-beam parallel geometry
From left: Receiving scatter slit, soller slit assembly, receiving slit,
monochromator (path bends) and scintillation detector
 A Ni filter will
attenuate Cu K
radiation, but
pass almost
everything else
(including highenergy portions
of the
background
spectrum)
• A Si(Li) detector may be tuned to see only K radiation
• A graphite (PG) monochromator will select Cu K, but the acceptance
windows will also admit a few other wavelengths. A tungsten (W) L
line may be present as anode contamination in an “aged” Cu x-ray tube
• Compton scatter will always contribute something to the background
The X-ray Machine
• The x-ray machine is divided into four
major components.
• The Tube
• The Operating Console
• The High Voltage Section
• The Film Holder , Grid Cabinet or Table
Chiropractic X-ray Room
• Floor mounted x-ray
tube stand.
• Wall grid cabinet or
Bucky
• Mobile Table with grid
cabinet (optional)
• Non-Bucky Film
Holder (optional)
Chiropractic X-ray Room
• Control Booth should
contain the operator
console and
technique charts and
space to store
cassettes.
• The wall between the
booth and X-ray unit
is shielded.
Chiropractic X-ray Room
• High Voltage Section
or Generator used to
change incoming
power to levels
needed to produce xrays.
The X-Ray Tube Development
• Dr. Roentgen used a
Crookes-Hittorf tube
to make the first x-ray
image.
• There was no
shielding so x-rays
were emitted in all
directions.
The X-Ray Tube Development
• The Coolidge Hot
cathode tube was a
major advancement in
tube Design. The
radiator at the end of
the anode cool the
anode.
The X-Ray Tube Development
• This is the variety of
tube designs
available in 1948.
• The Coolidge tube
was still available.
The X-Ray Tube Development
• Two major hazards
plagued early
radiography.
• Excessive radiation
exposure
• Electric Shock
The X-Ray Tube Development
• This is a modern
rotating anode xray tube.
• It is encased
completely in a
metal protective
housing.
The X-Ray Modern X-ray Tube
• There are two
principle parts:
• The rotating anode
• The cathode
• Any tube that has two
electrodes is called a
diode.
The X-Ray Modern X-ray Tube
• There are two
principle parts:
• The rotating anode
• The cathode
• Any tube that has two
electrodes is called a
diode.
Part of the X-ray Tube
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.
Protective Housing
• The tube can not
have more than 100
mR at 1 m (26 µ
C/kg) / Hour when
operated at it
maximum output.
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.
Protective Housing
• Never hold the tube
during an exposure.
• Never use the cables
or terminals as
handles.
Protective Housing
• The housing
incorporates specially
designed high voltage
receptacles to protect
against electrical
shock.
• Some housing have a
fan for cooling.
The X-Ray Tube Glass
Envelope
• The glass envelope is
made of Pyrex to
withstand the
tremendous heat
produced during xray.
• The window is a 5 cm
square with a thin
section of glass
where the useful
beam is emitted.
The Cathode
• The cathode is the
negative side of the
tube and contains two
primary parts:
• The filaments
• The focusing cup
The Filaments
• Most tube have two
filaments which
provide a choice of
quick exposures or
high resolution.
• The filaments are
made of thoriated
tungsten.
The Filaments
• Tungsten is used in xray tube because of
it’s high melting point
of 3410°C.
• X-rays are produced
by thermionic
emission when a 4 A
or higher current is
applied.
Focusing Cup
• The focusing
cup has a
negative
charge so that
it can
condense the
electron beam
to a small area
of the anode.
Filament Current
• When the x-ray machine is turned on, a
low current flows through the the filament
to warm it and prepare it for the big
thermal jolt necessary for x-ray production.
Filament Current
• The filament 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.
Saturation Current
• When 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.
The Anode
• The anode is the
positive side of the
tube.
• X-ray tubes are
classified by the type
of anode:
– Stationary ( top)
– Rotating (bottom)
The Stationary Anode
• Stationary anodes
are used in dental xray and some
portable x-ray
machine where high
tube current and
power are not
required.
The Rotating Anode
• 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.
– 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 Rotating Anode
• Tungsten-rhenium is
used as the target for
the electron beam.
• Tungsten is used for
three reasons
– High atomic number
– Heat conductivity
– High melting point..
The Rotating Anode
• The rotor is an
electromagnetic
induction motor.
• It spins at 3400 rpm.
• High speed anodes
spin at 10,000 rpm.
The Rotating Anode
• 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.
The Rotating Anode
• When the exposure
button is depressed,
current is applied to
the tube that
produces a magnetic
field that starts the
rotation of the anode.
The Rotating Anode
• 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.
Tube cooling
• The x-ray tube uses all three forms of cooling.
– Radiation
– Conduction
– Convection
Focal Tracks
• With a rotating anode,
the electrons strike a
moving target forming
focal tracks on the
tube.
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.
Line-Focus Principle
• By angling the anode
target, one makes the
the effective focal
spot much smaller
than the actual area
of interaction.
• The angling of the
target is know as the
line focus principle.
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°.
Line-Focus Principle
• The advantage of
Line focus is it
provides the
sharpness of the
small focal spot with
the heat capacity of
the large large focal
spot.
Line-Focus Principle
• 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°
Off Focal Radiation
• Remember that xrays are produced in
all directions. The
electrons can
rebound and interact
with other areas of
the anode.
• This is called OffFocal Radiation.
Control of Off Focal Radiation
• A diaphragm is
placed between the
tube and the
collimator to reduce
off focus rays.
Anode Heel Effect
• One unfortunate
consequence of the
line-focus principle is
that the radiation
intensity on the
cathode side of the
x-ray tube is higher
than the anode side.
Anode Heel Effect
• The x-rays are
emitted isotropically
or in all directions.
• Some of the beam is
absorbed by the
target resulting in a
lower intensity.
Anode Heel Effect
• The difference in the
intensity can vary by
as much as 45%.
• If the center is 100%
the anode side of the
beam can as low as
75% and the cathode
as much as 120%.
Anode Heel Effect
• The heel effect should
be considered when
positioning areas of
the body with different
thickness or density.
• The cathode side
should be over the
area of greatest
density.
Anode Heel Effect
• As the angle of the
anode decreases, the
anode heel effect
increases.
• This can result in
incomplete coverage
of the film with the
beam.
Anode Heel Effect on
Resolution
• The sharpness of the
image can be
dependent upon
which area of the
beam coverage you
are looking at.
• Similar to the shape
distortion when the
tube is not centered.
Anode Heel Effect
• The anode should be
up and the cathode
down for the full spine
x-ray.
• The patient is less
dense at the c-spine
and more dense at
the pelvis.
X-ray Tube Rating Charts
• With careful use, the xray tube can provide long
periods of service.
• Inconsiderate or careless
operation can lead to
shortened life or abrupt
failure.
• X-ray tubes are very
expensive. Cost varies
from $2,000 to $20,000.
X-ray Tube Rating Charts
• Tube life is extended by :
• Use of minimum mAs & kVp appropriate
for the exam.
• Use of faster images receptors require
lower mAs and kVp. They extend tube life.
Causes of X-ray Tube Failure
• All causes of tube failure relate to the
thermal characteristics of the tube.
• When the temperature of the anode during
a single exposure is excessive, localized
melting and pitting occurs.
• These surface irregularities lead to
variable and reduced radiation output.
Causes of X-ray Tube Failure
• If the melting is severe, the tungsten
vaporizes and can plate the port. This can
cause added filtering or interference with
the flow of electrons.
• If the temperature of the anode increases
to rapidly,the anode can crack and then
become unstable in rotation.
Causes of X-ray Tube Failure
• Maximum
radiographic
techniques must
never be applied to
a cold anode.
• These images show
damage to the anode,
Causes of X-ray Tube Failure
• During long
exposures (1 to 3
seconds) the anode
may actually glow like
a light bulb.
• The heat may cause
a failure of the
bearing for the anode
or a crack in the glass
envelope.
Filament failure
• Because of the high heat of the filament,
tungsten atoms are slowly vaporized and
plate the inside of the glass envelope. This
will eventually lead to arcing and tube
failure.
• Continuous high mA radiography will
actually lead to the filament breakage.
Tube Warm-up Procedures
• By warming the anode through a series of
exposures and increasing kVp settings,
the anode will build up heat that is needed
to avoid fracture of the anode.
• This process takes a little over one minute
put will add to the life of the tube.
• Close shutters of collimator.
Tube Warm-up Procedures
•
•
•
•
•
•
Make exposure of 12 mAs @ 70 kVp
Wait 15 seconds
Make exposure of 12 mAs @ 80 kVp
Wait 15 seconds
Make exposure of 12 mAs @ 90 kVp
Tube warm up is now complete.
X-ray Tube Rating Charts
• It is essential for the x-ray operator to
understand how to use tube rating charts.
• There are three types of charts:
– Radiographic Rating Chart
– Anode Cooling Chart
– Tube Housing Cooling Chart
Radiographic Rating Charts
• A tube may be used
in many ways with
many variables. Such
as:
– Large or Small Focal
spot
– 10,000 RPM or 3,400
RPM Rotor Speed
– Single-phase or high
frequency power.
Radiographic Rating Charts
• Even the angle of the
anode is important.
• Always look at the
correct chart.
• The x-axis and y-axis
are graduated in kVp
and time.
Radiographic Rating Charts
• The mA is graphed as
a curved line.
• Any combination of
kVp and Time below
the line should be
safe for a single
exposure.
Radiographic Rating Charts
• Most machine has
built-in protection to
help you avoid tube
overload.
• Microprocessor
controlled generator
way display the
percent of tube load.
Anode Cooling Chart
• The anode has a
limited capacity for
storing heat.
• Heat is continuously
dissipated to the oil
bath and tube
housing by
conduction and
radiation.
Anode Cooling Chart
• It’s possible through
prolonged use of
multiple exposures to
exceed the heat
storage capacity of
the anode.
• Thermal energy in xray is measured in
Heat Units (HU)
Anode Cooling Chart
• HU= kVp x mA x time (s)
• This chart is not dependent
upon the filament size or
speed of anode rotation
• The cooling is rapid at first
but slows as the anode
cools. It is not uncommon
for it to take 15 minutes to
cool the tube.
Housing Cooling Charts
• The tube housing cooling chart is very
similar to the anode cooling chart.
• The tube housing will generally have a
capacity of about 1 to 1.5 million HU.
• Complete cooling may take 1 to 2 hours.
The End of Lecture
Any question?