Class Notes

advertisement
BME 560
Medical Imaging: X-ray, CT, and
Nuclear Methods
X-ray Instrumentation Part 1
Today
• X-ray Systems
• The X-ray tube
– Principle
– Components
• X-ray Spectra
• Filtering
Discovery of X-rays
• Wilhelm Conrad Roentgen
(Röntgen) (1845-1923)
• Observed X-ray
fluorescence from a
vacuum tube in 1895.
• Coined the term “X-ray”
• First Nobel prize in
Physics (1901)
X-ray System
Source
Collimator
Filter
Subject
Anti-scatter
Detector
X-ray System
Produces X-rays
from electrical
energy
Tailors X-ray
spectrum
Converts X-rays
to light and
records
Source
Restrictor
(Collimator)
Determines size
and shape of
beam
Filter
Subject
Anti-scatter
Selectively
removes scattered
photons
Detector
X-ray Sources
• Generate energetic electrons (particulate
radiation)
– Generate electrons
– Accelerate electrons (kinetic energy) toward target
– Focus electrons on target
• Energetic electrons on target generate X-rays
(EM radiation)
– Bremsstrahlung
– Characteristic radiation
X-ray Tube
This basic design dates to
a 1913 tube developed by
William Coolidge (GE)
Cathode: generates
electrons (hot cathode)
Anode (or target): converts
electrons to X-rays
electrons
Rotor: Rotates anode to
dissipate heat
Glass enclosure: Maintains
vacuum
X-rays
X-ray Tube operation
1. Heat the cathode
filament to “boil” electrons
off.
2. Set up a high electric
field (with high-voltage
source) from cathode to
anode.
3. Electrons removed from
the filament travel from
cathode to anode and have
uniform energy equal to the
potential difference.
100 kVp potential results in 100 keV electrons hitting anode
X-ray Tube Operation
• Why do you think this process must be under
vacuum?
Cathode Operation
• The hot cathode works by thermionic emission
– The process of using thermal energy to overcome
electron binding energy and free electrons (also
called Edison effect)
– The filament is generally made of tungsten
because of its high melting point and other
features.
• There are also cold-cathode devices using
other methods to generate free electrons.
Cathode Operation
The focusing cup helps to direct
the electron flow to a particular
spot on the anode.
Focal spot: The region on the
anode struck by the electron
beam.
When might a small focal spot be
good?
When might a large focal spot be
good?
The focal track is the region on
the rotating anode impacted by
the electron beam. It wears out
first.
Anode Operation
•
The anode has two functions:
1. Convert energetic electrons to useful X-rays
2. Step 1 generates a lot of heat – must dissipate heat
Material choice is important
High Z
High melting point
Good thermal properties
Characteristic X-rays
Clean vacuum
Bremsstrahlung production
Efficiency = 9 x 10-10 Z (atomic number)V(voltage)
(This is an approximation.)
Efficiency: the ratio of the Bremsstrahlung x-ray energy to the incident
electron energy.
The remaining portion of the electron energy (1 - Efficiency) is converted into
heat in the x-ray target. Anode heating is a major issue in x-ray tubes.
Exercise:
Calculate the efficiency for x-ray production for 100keV electron beams on
tungsten (Z = 74).
< 1% efficiency!
Anode Operation
• Common anode surface materials
– Tungsten (Z = 74): most common for routine
usage; sometimes alloyed with other materials
(Rhenium)
– Molybdenum (Z = 42): has nice characteristic Xrays at 17.5 ad 19.6 keV, good for mammography
– Rhodium (Z = 45): similar to Mo, expensive
Anode Operation
• Anode base material is only for structure and
thermal properties:
– Molybdenum
– graphite
Surface
Base
X-ray Housing
• The tube is encased
in a housing for
physical integrity.
• The housing also
provides shielding
in all directions
except the window.
• The housing
provides thermal
properties and may
enclose an oil
envelope or other
coolant.
Window: blocks visible light, but permits Xrays to exit – beryllium is good.
Heating and Cooling
• Heating is most pronounced at the focal spot.
• The anode base has to carry heat away from
the focal spot as it rotates.
• Ultimately, heat is dissipated in the housing.
Anode damaged by
local overheating
From Sprawls
Anode Angle
Anode angle determines apparent focal spot size and focal track size
e-
X
Typical anode angles
range from 7 to 20
degrees
e-
X
Heel Effect
The effect of beam intensity decreasing
severely toward the anode side of the beam
The X-rays nearly parallel to the face of the
anode are attenuated more because of the
penetration depth of the electron beam.
How does anode angle affect this?
X-ray Tube Operation
Be able to name all the
parts and explain their
function (except maybe
B)
Other Technologies
• Transmission Target (Thin target)
e-beam
X-ray
• Field emission (cold cathode)
High local electric field removes
electrons by tunneling
Carbon nanotube-based X-ray
devices
X-ray Operation
• Key parameters
– Voltage (kVp): Determines the highest energy of X-ray
produced (penetration)
– Current (mA): Determines the X-ray flux
(photons/area/time) produced
– Time (s)
• Often refer to mAs
– 100 mA for 1 second is same number of photons as 50 mA
for 2 seconds
– Heat loading is different, though.
Spectral Effects
Current
Photons per mAs per mm^2 at 750 mm
12000000
10000000
8000000
6000000
10 mA s
20 mA s
4000000
50 mA s
2000000
0
0
10
20
30
40
50
Energy (keV)
60
70
80
90
100
Spectral Effects
Voltage
Photons per mAs per mm^2 at 750 mm
800000
700000
600000
500000
400000
50 kVp
100 kVp
300000
150 kVp
200000
100000
0
0
10
20
30
40
50
Energy (keV)
Spectral Effects
Target material
Photons per mAs per mm^2 at 750 mm
10000000
1000000
100000
10000
Mo (Z = 42)
1000
Rh (Z = 45)
W (Z = 74)
100
10
1
0
2.5
5
7.5
10
12.5
15
17.5
Energy (keV)
20
22.5
25
27.5
30
Spectral Effects
Anode Angle
Photons per mAs per mAs per mm^2 at 750 mm
250000
200000
150000
6 degree
15 degree
100000
22 degree
50000
0
0
10
20
30
40
50
Energy (keV)
60
70
80
90
100
X-ray System
Produces X-rays
from electrical
energy
Tailors X-ray
spectrum
Converts X-rays
to light and
records
Source
Restrictor
(Collimator)
Determines size
and shape of
beam
Filter
Subject
Anti-scatter
Selectively
removes scattered
photons
Detector
Restriction
• Absorb the beam except in desired direction
– Reduce dose
– Reduce scatter
• Plates – preformed shapes
• Collimators - adjustable
Filtration
• Low-energy X-rays do
not penetrate soft tissue
well
– Contribute to dose
– Not imaged
• Use a material with high
attenuation of the lowenergy, but relatively
low attenuation of highenergy X-rays. No Kedges!
– Aluminum is the
standard.
Filtration
• Refer to NIST database
– Aluminum
– Copper
– Cerium
• Total filtration is referenced to equivalent
thickness of Al (Al-eq)
Filtration
All parts provide some
filtering
Filtration
Effective Energy
• The effective energy is
the equivalent
monoenergetic beam
with same HVL as the
polychromatic beam
• A weighted “average”
energy
• “Quality”,
“penetration”
Compensation Filters
• Filters of nonuniform thickness may be used to
compensate for beam nonuniformity or subject
thickness.
Download