Transcript phys586-lec05

X-rays
Ouch!
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X-rays
 X-rays are produced when electrons are
accelerated and collide with a target
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Bremsstrahlung x-rays
Characteristic x-rays
 X-rays are sometimes characterized by the
generating voltage
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0.1-20 kV
20-120 kV
120-300 kV
300 kV – 1 MV
> 1MV
soft x-rays
diagnostic x-rays
orthovoltage x-rays
intermediate energy x-rays
megavoltage x-rays
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Bremmstrahlung
Bremsstrahlung x-rays occur when
electrons are (de)accelerated in the
Coulomb field of a nucleus
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Bremsstrahlung
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Bremsstrahlung
 The power radiated from an accelerating
charge is given by Larmor’s equation
2
P
2 e a
3 c
2
3
 In the case of an electron in the Coulomb field
of a nucleus
a
F
m
k
Ze
2
2
~ Z
r m
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Bremsstrahlung
 The probability of bremsstrahlung goes as Z2,
hence high Z targets are more effective than
low Z
 The energy of the x-rays varies from zero to
the maximum kinetic energy of the electron
(x-ray tube kVp)
 The energy spectrum from a thick target goes
as 1/E but inherent (1mm Al eq) plus
additional (few mm Al) filtration removes the
lower energy x-rays
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Here I am referring to diagnostic x-rays
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Bremsstrahlung
 The unfiltered energy spectrum is
approximately given by Kramer’s law which
was an early application of quantum
mechanics
I  E    KZ T e  E 

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Bremsstrahlung
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Characteristic x-rays
 After excitation, ions
with a vacancy in
their inner shell can
de-excite
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Radiatively through
x-ray fluorescence
Non-radiatively
through the emission
of Auger electrons
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Characteristic X-rays
 Thus an x-ray spectrum will also show
characteristic x-rays arising from L to K and M
to K transitions after ionization of a K electron
 Usually transitions to higher shells
absorbed by the filtration or are not x-rays
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Characteristic X-rays
The probability of K shell fluorescence
increases with Z
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Characteristic X-rays
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Characteristic X-rays
 Sometimes the characteristic x-rays are
emphasized using the same material for
target and filter
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Characteristic x-rays from molybdenum are
effective in maximizing contrast in mammography
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Characteristic X-rays
Mo target, filter, and result
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Directionality
For MeV electrons, bremsstrahlung x-
rays are preferentially emitted in the
electron’s direction
For keV electrons, bremsstrahlung xrays are emitted at larger angles
Characteristic x-rays are emitted
isotropically since there is no angular
correlation between the incident
electron that causes the ionization and
the fluorescent photon
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X-ray Tube
A simplified x-ray tube (Coolidge type)
shows the idea behind most x-ray tubes
today
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X-ray Tube
 In addition to bremsstrahlung and
characteristic x-ray production, electrons also
loose energy through collisions
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Collision losses dominate in this energy region
radiation
loss
collision
loss

EZ
820
 E in
MeV

For 100 keV electrons in W
radiation loss
0 . 1  74

 0 . 009
collision loss
820
Thus >99% of the electron energy goes into
heating the target rather than x-rays
Removing heat from the anode in a vacuum is an
issue
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X-ray Tube
 Efficiency of x-ray production depends on the
tube voltage and the target material
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W (Z=74) in this example
kVp
(V)
Pdeposited  VI
Pradiated  0 . 9  10
Efficiency
 
9
2
ZV I
Pradiated
Pdeposited
 0 . 9  10
9
ZV
Heat X(%) rays
(%)
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99.7 0.3
200 99
1
6000 65
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X-ray Tube
X-ray tubes
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X-ray Tube
More detail
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X-ray Tube
Housing for shielding (Pb) and cooling
(oil)
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X-ray Tube
More detail
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X-ray Tube
The main parts of the x-ray tube are
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Cathode/filament
 Typical electron current is 0.1-1.0 A for short
exposures (< 100 ms)
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Anode/target
Glass/metal envelope
Accelerating voltage
 Typical voltage is 20-150 kVp
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Cathode
 Cathode consists of
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Low R tungsten wire for thermionic emission
 Tungsten has a high melting point (3370C) and minimum
deposit on the glass tube
 Tube current is controlled by varying the filament current
which is a few amps
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A focusing cup
 Uses electric field lines to focus the electrons
 Typically there are two filaments
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Long one: higher current, lower resolution
 Large focal spot
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Short one: lower current, higher resolution
 Small focal spot
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Cathode
Dual focus filament is common
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Anode
 Usually made of tungsten in copper because
of high Z and high melting point
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Molybdenum and rhodium used for soft tissue
imaging
 Large rotating surface for heat distribution
and radiative heat loss
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Rotation of 3k-10k revolutions/minute
Resides in a vacuum (~10-6 torr)
Thermally decoupled from motor to avoid
overheating of the shaft
 Target is at an tilted angle with respect to
axis
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Bremsstrahlung is emitted at ~ right angles for
low energy electrons
Determines focal spot size
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Anode
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Anode
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Anode
 The heating of the anode limits the voltage,
current, and exposure time
 An exposure rating chart gives these limits
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Anode
Power = V x I (watts)
Energy = Power x time = V x I x s
(joules)
HU (Heating Unit) ~ J
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Damaged anodes
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Anode
 The angle determines the projected focal spot
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The smaller the angle the better the resolution
Typically 7-20 degrees
 Angle
Incident electron
beam width
‘ Angle
Actual focal
spot size Incident electron
beam width
Apparent focal spot size
Film
Actual focal
spot size
Increased
apparent
focal spot size
Film
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X-rays
The energy of the photons depends on
the electron energy (kVp) and the
target atomic number Z
The number of photons depends on the
the electron energy (kVp), Z, and the
beam current (mA)
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A typical number / area is ~ 1013 / m2
About 1% will hit the film ~ 1011 / m2
Absorption and detection efficiency will
further reduce this number
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Automatic Exposure Control
X Ray tube
Collimator
Beam
Soft
Air tissue
Bone
Patient
Table
Grid
AEC detectors
Cassette
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AEC detectors can ionization chambers or solidstate detectors
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Automatic Exposure Control
 Most modern x-rays machines are equipped
with automatic exposure control also called a
phototime
 The AEC sets the technical parameters of the
machine (kV, mA, time, …) in order to avoid
repeated exposures
 AEC is used to keep the radiographic quality
(film density) equal on all patients
 AEC detectors can be ionization chambers or
solid state detectors
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Grid
To reduce the number of secondary
scattered photons making it to the film,
a grid between the patient and film is
used
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Details
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Grid
Grid bars are usually lead whereas the grid
openings are usually made of aluminum or
carbon
Grid thickness is typically 3 mm
Grid ratio is H/W and 10/1 is typical
Grid frequency of 60 lines / cm is typical
B/W/H on the figure might be 0.045,
0.120, 1.20 in mm
The Bucky factor is the entrance exposure
w/wo the grid while achieving the same
film density – 4 is average
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Accelerating Voltage
The potential difference between
cathode and anode must be generated
by 60 Hz 220V AC power
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High voltages are produced using a
transformer
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Accelerating Voltage
Electrons are accelerated when the
filament is at a negative potential with
respect to the target
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Diode circuits can be used to provide
rectification (AC to DC voltage)
Three phase power (6 pulse or 12 pulse)
can be used to reduce ripple
Constant potential operation can be
achieved by using constant potential
(voltage regulations) or high frequency xray generators
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Half-wave Rectifier
Not very efficient
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Full-wave Bridge Rectifier
This circuit allows the entire input
waveform to be used
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Accelerating Voltage
Single phase single pulse
kV ripple (%)
100%
Single phase 2-pulse
13%
Three phase 6-pulse
4%
Three phase 12-pulse
Line voltage
0.01 s
0.02 s
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Images
 Analog radiography
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Film based – still widely used
Fluorescent screens are used to convert x-rays into
visible light that is then recorded on film
Screens are more efficient at stopping x-rays than
the film (CaWO4 or Gd2O2S:Tb or other rare earth)
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Analog Radiography
The film itself has excellent spatial
resolution but
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Film detects 0.65% of incident x-ray
energy
Gd2O2S detects 29.5% of incident x-ray
energy
Thus using phosphor screens greatly
reduces the radiation dose to the
patient
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And also reduces load on the x-ray tube
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Analog Radiography
There are two efficiency considerations
Absorption efficiency or QDE
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Fraction of incident x-rays that interact
with the screen
Depends on kVp and screen thickness
 Gd2O2S has a QDE of ~ 60% for 80 kVp beam,
20 cm patient, 120 mg/cm2 screen thickness
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Analog Radiography
Conversion efficiency
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Fraction of absorbed x-ray energy that is
emitted as light
5% for CaWO4
15% for Gd2O2S
 50,000 eV x 0.15 = 7500 eV
 7500 eV / 2.7 eV = 2800 photons produced per
absorbed x-ray
 50-90% reduction in photon diffusion to film
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Analog Radiography
Film is an emulsion containing silver-
halide grains (AgBr and AgI) coated on
Film
mylar
Body
X-Ray
source
Dark
Light
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Analog Radiography
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Film Badge
 A film badge consists of a photographic film
with various filters
 The film is a gelatin emulsion containing silverhalide grains (95% AgBr and 5% AgI) on a
supporting material
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Grain diameter is ~ 1mm
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Film Badge
 The film is exposed by light by
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An electron is released from Br- and moves about
the 1m diameter crystal
The electron may be captured by a trap such as a
crystal imperfection or AgS speck
The trapped electron attracts mobile Ag+ ions
where it is subsequently neutralized
Additional Ag atoms are formed by repeated
trapping and neutralization
These Ag atoms are called a latent image center
The developing process effectively amplifies this
process turning the grains with latent image
centers into a visible silver deposit
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Film Badge
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Film Badge
 Silver atoms at latent image centers
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Images
 Digital radiography
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Detector based
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Digitial Radiography
CCD systems
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CCD systems use a scintillator like
gadolinium disulphide to convert x-rays to
visible light
Light is collected by optics to demagnify
the 35x45cm2 film to 2-4 cm2 CCD
We’ll talk about CCD’s much later in the
course but essentially visible light is
converted into charge that is amplified and
readout
A negative is the thickness of the detector
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system because of the optical system
Digital Radiography
Indirect or direct conversion thin-film
transistor (TFT) arrays
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Also called FPD (flat panel detectors)
We’ll cover these later in the course as well
– probably through a student talk
The idea is that charge proportional to the
x-rays received is stored on a capacitor
The charges are conducted out by
transistors one row at a time and
subsequently amplified, multiplexed, and
digitized
The readout is very fast
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Digital Radiography
Indirect or direct conversion thin-film
transistor (TFT) arrays
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Indirect conversion uses a scintillator layer
(like CsI:Tl) to convert x-rays to visible
light and amorphous silicon photodiodes to
convert visible light into charge
Direct conversion uses an x-ray
photoconductor layer (usually amorphous
selenium) to convert x-rays to charge
An applied electric field directs the charges
to the charge collection electrodes
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Digital Radiography
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Digital Radiography
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Digital Radiography
Readout
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Images
Digital radiography
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“The battle over image quality, however,
may be incomprehensible to anyone
without a background in high-energy
physics.”
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X-rays
For bone tissue, the linear attenuation
coefficient is much greater than that for
soft body tissue
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