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Biomedical Imaging I
X-Ray Imaging,
Instrumentation
Interactions between X-rays and Matter
In the diagnostic range, below 200 keV, three mechanisms
dominate the attenuation:
• Coherent scattering,
• Photoelectric absorbtion,
• Compton Scattering
For photon energies larger that 1 MeV another mechanism
called Pair Production is the dominant interaction
mechanism.
01/30
II.2
Coherent Scattering
Occurs in low energy radiation that is not sufficient to eject
the electrons out of orbit.
It is the deflection of X-ray beams caused by atoms being
excited by the incident radiation and then reemitting waves
at the same wavelength.
Relatively unimportant in the energies used for diagnostic
radiology.
Eg = hn
- -K
M L
01/30
-
Eg ~ hn
-
-
II.3
Photoelectric absorption (t)
The photon knocks an electron out of one of the inner shells of a target atom.
The photon is destroyed in the process. Desirable interaction for imaging.
Eg = hn
- -K
M L
-
-
- Ee
Energy balance:
-
Ee  Eg  EI
-
-
• The electron exits from its shell into the energy
continuum (it leaves the field of the nucleus).
• This process is possible for a given shell only if
Eg  IK, L, M,...
• The process is most likely for Eg  EK, L, M,...
(resonance)
• The cross section decreases with increasing
photon energy
• Increases strongly with Z (Z5), decreases with Eg
(1/E3.5)
01/30
Zero
N
Continuum
E
M
L
K
II.4
The remaining atom becomes a positively charged ion.
Accompanying the ionization there occurs:
1) Characteristic radiation or fluorescent radiation in the
form of X-ray photon will be emitted carrying an energy
equal to the difference in energy between the outer shell
electron and, for example, the L-shell electron.
2) Auger effect (an alternative to characteristic radiation)
Energy released by the outer shell electron is transferred
to another orbital electron.
The orbital electron that acquires enough energy to
escape is called Auger electron.
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II.5
The photoelectric effect always yield three end products:
1)a photoelectron,
2) Characteristic radiation or Auger electrons
3) a positive ion.
The photoelectric absorbtion is the most desirable type
of interaction in X-ray imaging.
X-ray photon is completely absorbed producing little
scattered radiation (scattered radiation is dangerous for
personnel and produce image noise)
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II.6
Compton scattering (s)
Scattering process: Photon “bounces off” atom and “survives”, momentum and
energy are exchanged.
In a Compton scattering process, an x-ray photon interacts with one of the
weakly bound electrons of the atom. This electron can be considered free
because
Ex-ray  1-100 keV >> EI  few eV.
y
Eg
y
Eg’

x
x

Ee
Inelastic scattering process:
Eg '  Eg  Ee 
01/30
Eg
1   (1  cos )
; 
Eg
me c 2
II.7
Compton Scattering
The fractional change in wavelength and photon energy with angle varies
significantly with the initial energy of the photon.
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II.8
Relative Importance of two major type of Interactions
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II.9
Pair production (k)
If the photon energy Eg exceeds the value 2 mec2 = 1.02
MeV, an electron-positron pair can be produced with
destruction of the photon. The kinetic energy of the
resulting particles is given by Ee = Ep = Eg - 2 mec2
This process can take place only in interaction with a
nucleus, to account for conservation of momentum and
energy. The cross section for pair production is
proportional to Z2 and dominates the interaction at very
high energies (>5 MeV).
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II.10
Cross sections for Different Processes / Materials
L EDGE
K EDGE
Compton
scattering
Pair
production
Photoelectric
absorbtion
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II.11
X-Ray Generation
X-ray Tube
Working Principle:
Accelerated charge causes EM radiation
 bombardment of a target material with a beam of fast electrons
A
C
V
-
+
Electrons are emitted thermally from a heated cathode (C) and are accelerated
toward the anode target (A) by the applied voltage (aka potential) V (~kV).
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II.13
Bremsstrahlung
Continuous spectrum of EM radiation is
produced by abrupt deceleration of charged
particles (“Bremsstrahlung” is German for
“braking radiation”).
hn
Deceleration is caused by deflection of electrons K
in the Coulomb field of the nuclei
Nucleus
Most of the energy is converted into heat,
<~1% is x-ray
The energy of the generated x–ray photon is
given by energy conservation:
The maximum energy for the produced photon
is given by:
01/30
K’
hn  K  K '
Emax  hn  K  e  V
II.14
X-Ray Tube Spectrum
Bremsstrahlung creates a continuous spectrum ("white radiation") from E=0 to
Emax with I  1/E.
The efficiency for bremsstrahlung generation increases with Z, therefore heavy
metals are used in x-ray tubes.
absorbed
by tube
material
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II.15
Characteristic radiation
Characteristic narrow lines of intense x-ray are superimposed on the
continuous bremsstrahlung spectrum.
These lines are caused by photons that are released when an electron is
knocked out of an inner shell and replaced by one “dropping down” from a
higher shell. The photon energy corresponds to the energy difference between
the shells, causing distinct narrow lines in the spectrum.
Lines are named after the lower shell involved.
Dn=1  -transitions, Dn=2  b-transitions, ...
hn
- -K
M L
-
-
-
-
Continuum
0.5
3
Kg
b
K
11
Kb
L-lines
Tungsten (74W)
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E [keV]
M
L
-
Zero
N
b g
K
70
K-lines
II.16
Realization of X-Ray Tube
• Desired: Point source (less blurring)  electrostatic focusing of electrons
(cathode geometry)
• Limit: potential melting of anode  can increase heat dissipation by
imbedding of target (i.e., tungsten) in copper, rotating target, angled target
surface.
• Tubes come with two or more focal spots of different sizes (low power, sharp
 high power, blurry)
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II.17
Spot Size / Heel Effect
Anode angle reduces apparent spot size
Tradeoff: restricted usable area of image plane because of uneven intensity
(“heel effect”)
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II.18
Magnification and Image Blur
Geometric magnification given
by
I A B
M 
O
A
Blurring of edges and fine
structures due to finite source
size causes penumbra of width
ps
,O
B
A
I
01/30
p
II.19
X-Ray Tube Spectra
• Filtering: Selection of lines, suppression of low-energy x-ray: Cu, Al (Mo in
mammography)
• Tube window glass, metal (4Be in mammography)
• Voltage tuning
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II.20
X-ray Tube ratings
Factors effecting X-ray Intensity:
• Filament temperature controlled by the filament current (few amperes ac or
dc)
• The potential difference between anode and cathode (150 kV peak for chest,
30 kV peak for mammography)
• The target material (should have high atomic number)
For a fixed filament current, the intensity
I I irradiated by the X-ray tube is :
I = Z x tube current (mA) x (kVp)2 x F
atomic number
01/30
tube voltage
rectification factor
(1 for DC)
II.21
X-Ray Detection
Radiography
Few high-quality images are made in a study
• Orthopedic
• Chest
• Abdomen
• (Mammography)
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II.23
Photographic Film
• Photographic film can be exposed by x-ray directly.
• Increased sensitivity to light was noted when silver (Ag) is combined with a
halogen element. Such a combination or comund is known as silver halide
(ex: AgCl, AgBr, AgI).
• It is necessary to use a ‘binder’_an inert substance which will envelope the
silver halide crystals_ commonly called grains_ holding them evenly suspended
and attached to the support. Gelatin was found as a binding material with ideal
properties.
• The combination of silver halides suspended in gelatin is known as silver halide
emulsion.
• Film Composition:
• Transparent plastic substrate (acetate, polyester)
• Coated on both sides with light-sensitive emulsion (gelatin, silver halide
crystals (AgBr) 0.1-1 mm).
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Exposure to light splits ions  atomic silver appears black (negative film)
II.24
AgBr Crystal
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II.25
Silver Image formation theory
(M. J. Langford)
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II.26
The role of silver in photography
(M. J. Langford)
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II.27
Film characteristics
• Blackening depending on deposited energy (E = It)
• Optical Density (measure of film blackness):
D = log 10(Ii/It)=log10 opacity
Ii
01/30
Film
• D > 2: “black,” D = 0.25 - 0.3: “transparent” (or “white”) with standard
light box (useful diagnostic range ~0.5 - ~2.5)
It
II.28
Film Characteristic Curve (H and D Curve) I
Gives relationship between film exposure and optical density D
Characteristics:
• Fog: D at zero exposure (higher the fog level faster the photographic mat)
• Sensitivity (speed S ): Reciprocal of X-ray exposure E in Röntgen ( R*)
needed to produce a density D of 1  S = 1/E
• Linear region
E
1R= dose required to produce 2.08x109 ionization in 1 cm3 air (2.5810-4 Coulomb/kg in air
under 760 mm Hg ambient pressure, 0 °C)
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II.29
Film Characteristic Curve II
• Gamma (maximum slope)
g
D2  D1
log E2  log E1
• Latitude (range of exposure creating appreciable values of D [~0.5 - ~2.5])
• Contrast (curve gradient, DD/D log E  Latitude)
Film gamma
01/30
Contrast, latitude
II.30
Film Resolution
Light sensitivity is directly related to the grain size and
the number, thickness of sensitive layers (interaction volume)
Double sided
Single sided
In both cases, increasing sensitivity decreases resolution
 Tradeoff between sensitivity  resolution
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II.31
Grains
(M. J. Langford)
01/30
II.32
Fluorescent Screens
Low x-ray sensitivity of film 
Fluorescent screens (“phosphors”) are used to convert x-ray energy to light.
• Fluorescence:
The particles produced by x-ray interaction (electrons, photons) lose part of
their energy by exiting the valence electrons in the medium, which upon
relaxation emit light.
• Conversion efficiency:
Fraction of the absorbed x-ray energy converted to light.
• CaWO2 (Calcium tungstate):20-50%
radiates ultraviolet and blue
• Rare earth phosphors: LaOBr (blue), Gd2O2S (green), Y2O2S:Tb: 12 - 18%
La: Lantanum, Gd: Gadolinium, Tb: Terbium, Yt: Ytrium
• Quantum efficiency:
Fraction of incident x-rays that interact with screen (30 - 60%).
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II.33
Screen / Film Combinations
• Sandwiching phosphor and film in light-tight cassette or a film between two
screens. Major advantage: reduce the exposure required to form an image
• Resolution vs. sensitivity:
• Most x-rays are absorbed close to the entrance surface. Lateral light spread
degrades spatial resolution. The light intensity emitted by screen is linearly
dependent on x-ray intensity.
• Thicker screen increases sensitivity (larger interaction volume) but degrades
resolution due to light scatter / lateral spread
Film emulsion
Light-tight
cassette
Foam
X ray
X-ray photons
Crystals
Phosphor screen
Film
Light spread
 Tradeoff between sensitivity  resolution
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II.34
Characteristics of Fluorescent Screen
Optimizing sensitivity:
• Fluorescence wavelengths are chosen to match spectral sensitivity of film:
CaWO2: 350nm - 580nm, peak @ 430 nm (blue)
Rare earths: green - blue
• Dual-coated film, two screen layers
• Optically reflective layers
cassette
photoreflective layer
fluorescent screen
Tradeoff between
photosensitive layer
film substrate
sensitivity  resolution
dose  image quality
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II.35
Fluoroscopy
Lower x-ray levels are produced continuously and many images must be
presented almost immediately
• Angiography
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II.36
Image Intensifier
Image intensifier tubes convert the x-ray image into a small bright optical
image, which can then be recorded using a TV camera.
1,5-2 cm
15 - 30 cm
• Conversion of x-ray energy to light in the input phosphor screen (CsI)
• Emission of low-energy electrons by photo-emissive layer (Sb : Antimony)
• Acceleration (to enhance brightness) and focusing of electrons on output
phosphor screen (ZnCdS)
• The ratio of image brightness of the two phosphors is called the brightness
gain of the intensifier tube
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II.37
Scintillation Detector
X-ray photon
Scintillation crystal like NaI emit light photons in
proportion to the absorbed x-ray photon energy
The photocathode is coated with a
Scintillation
photoemission material that emits electrons
crystal
when striken by light photons in proportion to the
Photocathode
intensity of the light.
(grounded)
The electrons will be accelerated toward the
first dynode (V1) which is covered by a material
that emits secondary electrons when striken by
an electron.
The number of electrons are multiplied when they
are propagating down the tube.
V1
V2
Vn
Anode
(1200 V)
The output current is proportional to the number
of x-ray photons.
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II.38
Photocathode :
Quantum efficiency of a photocathode = number of photoelectrons emitted
/ number of incident photons
Practical photocathodes show maximum quantum efficiencies of 20-30%
Dynode:
Conventional dynode materials are BeO, MgO, Cs3Sb
The multiplication factor for a single dynode is given by
 = number of secondary electrons emitted
/ primary incident electron
If N stages are provided in the multiplier section, the overall
gain for the PM tube is N.
Conventional dynode materials are characterized by a typical value of
=5. Ten stages will therefore result in an overall tube gain of 510 or 10 7.
01/30
II.39
t
h
e
Limits of Analog Systems
s
i
g
n
a
l
(Screen/film, intensifiers):
d
e
g
r
a
d
a
t
i
o
n
• Film has limited latitude,
• Film acts as detector, storage, display,
t
h
a
t
• Development, storage,
o
c
c
u
r
s
• Many steps involved, loss in image information,
w
i
t
h
• Analog noise
e
a
c
h
01/30
c
o
m
p
o
n
e
n
t
c
II.40
01/30
II.41
Comparison Analog - Digital
© GE Medical Systems
01/30
II.42
Digital Image Detectors (CCD Based, I)
• Charge coupled detector (CCD):
• IC detector comprising a photodiode, a charging circuit, a capacitor and a
charge transfer circuit (MOS capacitor).
Phosphor is optically coupled by lens or fiber taper to 1k×1k CCD array (realtime imaging).
01/30
II.43
CCD must perform 4 tasks to generate an image:
• Generate Charge --> Photoelectric Effect
• Collect Charge --> pixels: an array of electrodes (called gates)
• Transfer Charge --> Apply a differential voltage across gates.
Signal electrons move down vertical registers (columns) to
horizontal register. Each line is serially read out by an on-chip
amplifier.
• Detect Charge --> individual charge packets are converted to an output
voltage and then digitally encoded
01/30
II.44
Digital Image Detectors (CCD Based, II)
01/30
II.45
Digital Image Detectors (non-CCD)
• CsI layer deposited directly on array of photodiodes with switching matrix
[GE 2000, first FDA approved fully digital system (11 yrs, $130 million)]
• Direct conversion of x-ray into charge (lead iodide, selenium, zinc cadmium
telluride, thallium bromide)
01/30
II.46
Putting it all together: Mammography
Used for detection and diagnosis (symptomatic
and screening) of breast cancer, pre-surgical
localization of suspicious areas, and guidance of
needle biopsies. Breast cancer is detected on the
basis of four types of signs on the mammogram:
• Characteristic morphology of a tumor mass
• Presentation of mineral deposits called
microcalcifications
• Architectural distortions of normal tissue
patterns
• Asymmetry between corresponding regions
of images on the left and right breast
 Need for good image contrast of various
tissue types.
Simple x-ray shadowgram from a quasi-point
source.
Structures are magnified depending on distance
to breast-image receptor.
01/30
II.47
Mammography Contrast
Image contrast is due to varying linear attenuation coefficient of different types of tissue
in the breast (adipose tissue (fat), fibroglandular, tumor).
Contrast decreases toward higher energies  the recommended optimum for
mammography is in the region 18 - 23 keV depending on tissue thickness and
composition.
01/30
II.48
X-ray Projection Angiography
Concerned with diseases of the circulatory system. Contrast material is used to opacify
vascular structures of interest. Contrast agent is an iodine-containing compound with
maximum iodine (Z=53) concentration of ~350 mg/cm3.
Important application is monitoring of therapeutic manipulations (angioplasty,
atherectomy, intraluminal stents, catheter placement).
Source produces short, intense pulses to produce clear images of moving vessels. Pulse
duration ranges from 100-200 ms (for cerebral studies) to 5-10 ms (for cardiac studies).
01/30
II.49
Biological Effects of X-Ray
Units
Intensity [W/cm2]: Power per unit area
= number of photons [n]  photon energy [hn] / time [t] / area [A]
A
hn
n
Roentgen [R]:
Measure of energy (It): the amount of radiation that produces 2.5810-4
Coulomb [C] of charge separation in air @ standard conditions.
01/30
II.51
Two different materials, if subjected to the same exposure, will in general
absorb different amounts of energy.
Because many important phenomena, including changes in physical
properties or induced chemical reactions, would be expected to scale as the
energy absorbed per unit mass of the material, a unit that measures this
quantity is of fundamental interest.
Absorbed Radiation Dose [rad]:
Defines the absorbed energy (dependent on target medium):
1 rad = 0.01 joule absorbed by 1 kg of material.
1 Gray [Gy] = 100 rad.
01/30
II.52
Determinants of Biological Effects
• Damage depends on deposited (= absorbed) energy (intensity  time) per tissue
volume.
• Threshold: No known minimum level below which no damage occurs.
• Exposure time directly effects
• Exposed area: The larger the exposed area the greater the damage (collimators,
shields!).
• Variation in Species / Individuals:
• Variation in cell sensitivity: Most sensitive are nonspecialized, rapidly dividing cells
(Most sensitive: White blood cells, red blood cells, epithelial cells. Less sensitive:
Muscle, nerve cells)
• Short/long term effects: Short-term effects for unusually large (> 100 rad) doses
(nausea, vomiting, fever, shock, death). Long-term effects (carcinogenic/genetic
effects) even for diagnostic levels  maximum allowable dose 5 R/yr or 0.2 R/working
day [Nat. Counc. on Rad. Prot. and Meas.]
01/30
II.53
Radiation Dose for Various X-Ray Procedures
X-ray procedure/exposure
01/30
Exposure [mR]
Chest
20
Brain
250
Abdomen
550
Dental
650
Breast
54
Xeromammography
200
CT/slice
1000
II.54
Effects of ionizing radiation on the living tissue
Direct effects:
Indirect effects:
Effects on the
macromolecules (for
example, protein, RNA,
DNA) of cells. The effects
on the proteins can be
repaired by the cell.
However, effects on DNA
can not be repaired yielding
genetic mutation and death
of the cell.
Effects on the water
molecules. 80% of human
body is made up of water.
Water molecules are
converted to other
molecules (H and free
radical OH ) with incoming
radiation. The excess
energy of these molecules
may affect the other
molecules and break their
molecular bonds yielding
toxic molecules (H2O2).
01/30
II.55