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MRI medical imaging
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Radiography
Prof. Defeng Wang
Department of Imaging and Interventional Radiology
The Chinese University of Hong Kong
Email: [email protected]
1
Outline
● I. Introduction
● II. Basic Physics of X-ray
● III. Radiography Equipment
● IV. Image Quality
● V. Mode of Radiography & Application
● VI. Biologic Effects & Safety
2
I. Introduction
● X-rays discovery by Wilhelm Roentgen in 1895
● Roentgen: Nobel laureate in physics (1901)
● 70% of the medical imaging are based on X-ray
● One of the major diagnostic tools in medicine
Wilhelm Conrad Röntgen3
From
“Cathode”
rays to “X-ray”
4
First Publication
First Image
“X”: “Mysterious”
5
Characteristics of X-ray
● Physical effects
� Penetrating – high energy
� Fluoroscopy – emit visible light
� Ionizing
● Chemical effects
� Perceiving light – AgBr.
� Change the color of certain material, like lead glass,
crystals
● Biological effects
� Direct & indirect
6
Outline
● I. Introduction
● II. Basic Physics of X-ray
● III. Radiography Equipment
● IV. Image Quality
● V. Mode of Radiography & Application
● VI. Biologic Effects & Safety
7
Basic Physics of X-ray
1. Physics of X-ray
2. Generation of X-ray
3. X-ray interaction with matter
4. Quantifying interactions
II. Basic Physics of X-ray
8
● Electromagnetic radiation emitted by charged particles
● Photons which can penetrate through matter
● Have no mass or charge
● Travel at the speed of light
● Energy = hν = hc/λ
1. What are X-rays?
II. Basic Physics of X-ray
9
II. Basic Physics of X-ray
10
● X-rays are generated when high energetic electrons interact
with matter
● Happened in X-Ray tube
� Cathode current releases electrons by thermal excitation
� Bremsstrahlung (� “braking radiation”)
� yields a continuous X-ray spectrum
� Characteristic radiation
� yields characteristic peaks
II. Basic Physics of X-ray
2. Generation of X-ray
11
● X-ray production efficiency of bremsstrahlung is influenced by the
target atomic number (Z) and acceleration potential (kVp)
Example:
• Diagnostic:100-keV electrons impinging tungsten (Z = 74)
X-ray production ~ 0.7%
• Therapeutic: 6-MeV electrons, tungsten target
X-ray production ~ 44%
Bremsstrahlung
12
Characteristic radiation
when an electron from the high energy shell (e.g., L-
shell with energy EL) drops into the low energy shell
(e.g., K-shell with energy EK), a photon of energy
E = EL – EK will be emitted
13
Bremsstrahlung & Characteristic radiation
Intensity distribution in the Röntgen spectrum of molybdenum for different voltages. The
excitation potential of the K-series is 20.1 kV. This series appears as characteristic peaks in the
25 kV curve. The peaks Kα and Kβ are due to L-shell and M-shell drops respectively. 14
X-Ray Tube
● How does a X-ray tube work?
15
Energy of X-ray
wavelength
frequency
Planck’s constant
speed of light
(in vacuum)
16
● Photoelectric Effect (PE)
● Compton Scattering
● Pair Production
● Rayleigh scattering
These four major interactions are of importance to
diagnostic radiology and nuclear medicine
3. X-ray interaction with matter
II. Basic Physics of X-ray
17
● Photoelectric Effect
� Interaction of incident photon with inner shell electron
� Results in a photoelectron and characteristic x-ray
II. Basic Physics of X-ray
18
Proportional to
Z3/E3
e.g.,
mammogram
E: photon energy
Z: atom number of the matter
density 0.91 0.91
fat muscle
Z 5.92 7.46
Muscle absorbs 2
times of X-ray than fat
(7.463/5.923)!
About Photoelectric Effect
19
● Compton Scattering
� Interaction of incident photon and outer shell electron
� Results in ionization of the atom, a scattered photon,
and the ejected electron
E0= incident photon energy
Esc= scattered photon energy
Ee= ejected electron energy
II. Basic Physics of X-ray
Major source of radiation scattering:
- Lower the quality of image;
- Needs better protection.
Proportional to 1/E
20
● Pair Production
� Only occurs if incident photon’s energy exceeds 1.02 MeV
� Directly interaction with the nucleus
� Creating an electron-positron pair
II. Basic Physics of X-ray
Proportional to E
21
● Rayleigh (Coherent) Scattering
� Incident photon interacts with and excites the total atom
� Occurs at very low energies (15 to 30 keV), increases in
probability with decreasing energy
(Detection of scattered photon has negative effect on image quality)
II. Basic Physics of X-ray
22
1 5 10 50 1000.10.050.01
80
100
60
40
20
Ato
m n
um
ber
of th
e m
att
er
Z
Photoelectric
Effect
(PE)
Compton
Scattering
(CS)
Pair
Production
(PP)
Photon energy MeV1. Probability of PE and CS are equal at 35keV
2. E = 0.8~4MeV, CS dominates
3. E > 5MeV, PP starts
4. E > 50MeV, PP dominatesFor diagnosis, E = 10~150keV
23
4. Quantifying interactions
� Attenuation Coefficients-Linear and Mass
II. Basic Physics of X-ray
24
● Attenuation includes
� Absorption – Photoelectric
� Scattering – Rayleith, Compton
II. Basic Physics of X-ray
Attenuation Coefficients
Io
x
I = Ioe (- µ x)
• I is the radiation intensity after traversing a thickness x
• Io
is the original radiation intensity
• µ is the linear attenuation coefficient (cm-1)
• x is the thickness of attenuating material (cm)
25
µ is a function of both the
photon energy and the material,
that is, µ = µ(E, material)
µ(10 keV,H2O) = 5 cm−1
µ(100 keV,H2O) = 0.17 cm−1
µ(10 keV, Ca) = 144 cm−1
µ(100 keV, Ca) = 0.40 cm−1.
26
Mass attenuation coefficient µm (=µ/ρ)
– normalization of linear attenuation coefficient (µ) for the
mass density of the attenuating medium (ρ)
µm
(10 keV,H2O) = 5 cm2/g
µm
(100 keV,H2O) = 0.17 cm2/g
µm
(10 keV, Ca) = 93 cm2/g
µm
(100 keV, Ca) = 0.258 cm2/g.
27
Outline
● I. Introduction
● II. Basic Physics of X-ray
● III. Radiography Equipment
● IV. Image Quality
● V. Mode of Radiography & Application
● VI. Biologic Effects & Safety
28
Equipment
● 1. Components
● 2. Digital Radiography System
29
Equipment
● 1. Components
� 1.1 X-ray tube
� 1.2 X-ray tube controls
� 1.3 Radiographic tables
� 1.4 Collimator
� 1.5 Control console
� 1.6 Image receptors
30
Equipment
● 1.1 The X-ray Tube
� X-rays are produced in a cathode ray tube
� They are produced from a series of energy conversions
31
Equipment
● 1.1 The X-ray Tube
� Has a positive (anode) and negative (cathode) electrode.
� The cathode (filament) serves as the source of electrons
� High voltage is applied (kV) to accelerate the electrons across the tube
� The anode (target) stops the electrons suddenly which results in the production of x-rays
32
Equipment
● 1.1 The X-ray Tube
� Tube housing protects the patient and operator from radiation emanating in all directions.
� The collimator decreases or increases the size of the x-ray field (inc or dec exposure.)
33
Equipment
● 1.2 X-ray tube controls
� Longitudinal lock - locks tube into
position along the length of the table
� Transverse lock-locks tube into
position across the width of the table
� Vertical lock-locks tube vertically to
set the SID*
� Collimator controls
*SID is the distance
between the source (anode)
and the image receptor (IR)
34
Equipment
● 1.2 X-ray tube controls
� Detent lock: locks the tube into the center of the bucky tray
(where IR is) transversely.
� Tube angulation lock: allows angulation of the tube cephalad
(towards the head) and caudad (towards the feet)
� Tube angle indicator: indicates the degrees of tube angulation
35
Equipment
● 1.3 Radiographic tables
� The table is designed to support the
patient in a position that enhances the
radiographic examination
� The table must be uniformly radiolucent
(allows x-rays to easily pass through)
� It must be easily cleaned.
� It must be hard to scratch.
� Some tables are stationary (ie. They
don’t move)
It is not
designed for
comfort!
36
Equipment
● 1.4 Radiographic tables
� Bucky tray in the table is to hold image receptors (IR) and a
radiographic grid
37
Equipment
● 1.5 Collimator
� Attaches directly below the x-ray tube
� Serves as a x-ray beam limiting device
� Control the size and shape of x-ray field
As field size increases,
intensity of scatter radiation
also increases rapidly.
Especially during
fluoroscopy
38
Equipment
● 1.6 Control console
� The control console is device that allows the technologist to set
technical factors (mAs & kVp) and to make an exposure.
kVp
The Higher kVp – more penetrating
Ranges is 50 -110 in Diagnostic x-ray
mA is the current in combination with the
time
Determines HOW LONG the beam will
stay on
Controls the density on the film/image
39
Equipment
● 1.7 Image receptors
� film cassettes
�CR (computerized radiography) imaging plates (IP).
� photostimulable phosphor
�Digital “flat” detector
40
● 2. Digital Radiography System
� DR is cassetteless
� In DR detectors, the materials used for detecting the x-ray
signal and the sensors are permanently enclosed inside a rigid
protective housing
� Thin-film transistor (TFT) detector arrays may be used in both
direct- and indirect-conversion detectors
Equipment
41
Digital Radiography Equipment
42
● Direct conversion
�x-ray photons are absorbed by
the coating material and
immediately converted into an
electrical signal.
● Indirect conversion
� Indirect conversion is a two-step
process: x-ray photons are
converted to light, and then the
light photons are converted to an
electrical signal.
Equipment
43
Outline
● I. Introduction
● II. Basic Physics of X-ray
● III. Radiography Equipment
● IV. Image Quality
● V. Mode of Radiography & Application
● VI. Biologic Effects & Safety
44
Image quality
● 1. Characteristics
● 2. Quantitative Measurements
45
Image quality
● 1. Image quality characteristics
�1.1 Contrast resolution
�1.2 Spatial resolution
�1.3 Image noise
�1.4 Uniformity/Artifacts
46
Image quality
● 1.1 Contrast
� the ability of distinguishing
between similar tissues, e.g.
liver vs. spleen
� high contrast presented as white
and black on a radiograph
� Plain film radiography have
lower contrast resolution than
CT
47
Image quality
● Contrast of Image
�Determined by tissue contrast + imaging condition
�Tissues contrast: atomic number (Z), mass density, etc
● Lead (atomic number 82, 11340 kg per cubic meter)
● Oxygen (atomic number 8, 1.492kg per cubic meter)
�Imaging condition: scatter radiation, kV, mA, dynamic range,
etc
● Scatter increased → noise increased, contrast decreased
● kV increased → contrast decreased
● Larger dynamic range → less contrast
● mA increased → contrast increased
48
Image quality
● Example:
Scatter increased
→ noise increased, contrast decreased
49
Image quality
● Example:
mA decreased
→ noise increased, contrast decreased
50
Image quality
● 1.2 Spatial Resolution
�The ability to faithfully reproduce small objects under
sufficient subject contrast (contrast resolution)
�Described subjectively by the degree of blurring
�System of higher spatial resolution can distinguish
objects with higher spatial frequency
51
Line pair phantoms
Higher resolution in
system “blue”
Image quality
52
Image quality
● Spatial Resolution of Image
�Determined by
�Reconstruction
● Smaller pixel size, smaller FOV, larger matrix size →
higher resolution
�Devices
● Smaller detector size → higher resolution
● Smaller tube focal spot size → higher resolution
● Narrower predetector collimation → higher resolution
53
Image quality
● 1.3 Image Noise
�Noise is the random (stochastic)
component in the image
�Radiographic noise = random
fluctuation on the optical density of
the image
54
Image quality
● 1.3 Image Noise
�Determinators:
� Scatter radiation↑ → noise↑
� mA↑ → noise↓
� pixel size↑ → noise↓
� Considering patient safety:
� Children – high kV, low mAs
� Mammography – low kV, high mAs
55
● Example:
noise affect contrast resolution
Image quality
56
Image quality
● 1.4 Artifacts
�Any irregularity on an image which is not caused by
proper shadowing of tissue by the primary x-ray beam
57
Image quality
Causes of artifact in X-ray
● Scratches in the detector
● dead pixels
● unread scan lines
● inhomogeneous X-ray beam intensity
● afterglow
58
�But sometimes mimic a foreign object
Image quality
59
�Exposure artifacts caused by patient’s motion during
exposure
Image quality
60
● 2. Quantitative Measurements
�2.1 Modulation Transfer Function (MTF)
�2.2 Signal-to-Noise Ratio (SNR)
�2.3 Detective Quantum Efficiency (DQE)
Image quality
61
● 2.1 Modulation Transfer Function (MTF)
�Point spread function (PSF): describing the
spot distribution of a point (light source) on a
image by a imaging system
Image quality
Point
(light source)
Point’s spot
on image
Ideal imaging system real imaging system
62
● 2.1 Modulation Transfer Function (MTF)
�The Fourier transform (FT): converting “intensity –
distance” relationship into “intensity - 1/distance”
(spatial frequency) relationship
�The FT of a PSF results in the MTF
Image quality
63
● 2.1 Modulation Transfer Function (MTF)
�Smaller object, higher spatial frequency, lower MTF;
�As MTF value is reduced, image blur increases;
�MTF = 1.0: absolutely perfect image;
�Usually, the spatial frequency at the .1 (10%) MTF is
identified as the limiting resolution of the system
Image quality
64
● 2.2 Signal-to-Noise Ratio (SNR)
�SNR is the inverse of the relative noise;
�signal (N) increases, the SNR increases
Image quality
N NSNR NNσ
= = =
photon-detecting process is essentially a Poisson
process (the variance is equal to the mean)
65
● 2.3 Detective Quantum Efficiency (DQE)
�MTF mainly reflect signal, DQE mainly reflect
signal-to-noise performance
Image quality
( )
( )
2
2
out
in
SNRDQE
SNR=
66
Outline
● I. Introduction
● II. Basic Physics of X-ray
● III. Radiography Equipment
● IV. Image Quality
● V. Mode of Radiography & Application
● VI. Biologic Effects & Safety
67
Mode of Radiography and Application
● 1. Routine examination
�Fluoroscopy
�Plain film radiography
● 2. Special examination
�Mammography
● 3. Contrast media examination
68
Mode of Radiography and Application
1.1 Fluoroscopy
�dynamic motion of internal
structures in real time
�Specialized x-ray tube
� Image receptor
�Fluoroscopic screen
�Mirrors
�Image intensification
● Video camera and monitor
69
Mode of Radiography and Application
1.1 Fluoroscopy
� Old fluoroscopy “imaging” chain
70
Mode of Radiography and Application
1.1 Fluoroscopy
� New fluoroscopy “imaging” chain
71
● Image intensifier
72
Mode of Radiography and Application
● Fluoroscopy application
� Used to visualize motion of
internal fluid and structures
� GI tract studies
� Angiograms
� Orthopedics surgery
73
Mode of Radiography and Application
1.2 Plain film radiography
�Plain film radiographs display
the shadow of the body part
on the film.
� an important tool for the
diagnosis of many disorders
�The X-rays are absorbed by
the material they pass
through in differing amounts
depending on
the density and composition
of the material. 74
Mode of Radiography and Application
● Plain film radiography application
�Chest
�Abdomen
�Spine
�Extremities & Joints
�Skull
Plain film radiography’s
application in different
organ/system 75
Mode of Radiography and Application
2. Mammography
� A mammography machine is an X-ray machine dedicated to
breast imaging.
� Mammograms are obtained with much lower energy X-rays
� “Soft X-ray”
� 30~45 kV; E=17~19keV
76
Mode of Radiography and Application
● Mammography
77
Mode of Radiography and Application
● 3. Contrast media examination
� Contrast is chemical substance which is introduced in human
body via enteral/parenteral route to visualize certain structures
not seen in plain radiography.
� Types of contrast
�Positive- produce opaque image, e.g. barium sulphate, iodine
containing contrasts such as urografin, omnipaque, iopamiro.
�Negative- produce radiolucent image, e.g. air.
78
Mode of Radiography and Application
● 3. Contrast media examination
�Routes of contrast
�Enteral contrast is given by oral route.
�Anal orifice contrast study is called barium
enema/gastrografin enema
�Intravenous.
�Intraarterial.
�External opening on body surface study is called sinogram /
fistulogram/loopogram
79
Mode of Radiography and Application
3. Contrast media examination application
�System related contrast studies
�GIT
● Barium swallow
● Barium meal
● Barium enema
�hepatobiliary system
�Urinogenital system
�Breast
● mammary ductography
80
Mode of Radiography and Application
Barium meal examination: radiographs of the oesophagus are taken
after barium sulfate is swallowed by a patient 81
Mode of Radiography and Application
● Contrast media examination
Ulcer
Barium meal82
Mode of Radiography and Application
● Contrast media examination
Intravenous urography (IVU): the contrast is injected into a vein ('intravenous'
injection), travels in the bloodstream, concentrates in the kidneys, and is passed
out into the ureters with urine made by the kidneys. The structure of the kidneys,
ureters and bladder shows up clearly as white on X-ray pictures 83
Mode of Radiography and Application
● Contrast media examination
ovarian
ductmammary
duct
hysterosalpingography mammary ductography
84
Outline
● I. Introduction
● II. Basic Physics of X-ray
● III. Radiography Equipment
● IV. Image Quality
● V. Mode of Radiography & Application
● VI. Biologic Effects & Safety
85
Biologic Effects & Safety
VI. Biologic Effects & Safety
● 1. Direct and indirect
● 2. Different dose and tissues
● 3. Types of effect
● 4. Quantification
● 5. Principles of Radiation Safety
86
Biologic Effects & Safety
● 1. Direct and indirect biologic effects
�Direct effect
�Radiolysis of DNA
� Indirect Effect
�Free radicals by radiolysis of water.
�2H20 H
2O+ + H
20-
�H2O+ OH. + H+
�Hydroxyl radicals react with other molecules (such as DNA)
damaging them.
dissociation of molecules by
nuclear radiation
87
Biologic Effects & Safety
88
Biologic Effects & Safety
● 2.1 Effects by different dose
�At lower doses cells are able to repair damage
without cell death (shoulder region)
89
Biologic Effects & Safety
● 2.2 Effect on different tissue
�Tissue Type (Law of Bergonne and Tribondeau)
�Rapidly dividing tissue is more radiosensitive
�Rapidly growing cells are more radiosensitive
�Younger and more immature cells are more radiosensitive
�Mature cells are less radiosensitive
�Dividing cells are more sensitive in G2 and G1 parts of the
cell cycle
90
Biologic Effects & Safety
● 3. Types of biologic effect on humans
� Stochastic effects
�Threshold after which there is an all or nothing effect
�e.g. Cancer or genetic effects
� Deterministic Effects
�Vary with Dose
�e.g. lens opacification, blood changes
� Total body irradiation
�Highly unlikely that an individual would survive a total exposure of
more than 3 Gray without intensive medical treatment
� Partial body irradiation
�Cataracts are formed if eyes are exposed to more than 2 Gray
�Hair loss occurs at exposures over 3 Gray
91
Biologic Effects & Safety
92
Biologic Effects & Safety
● 4.1 Units of Radiation Exposure and Dose
�Absorbed dose (Gray)
�Dose absorbed by the 1 kg irradiated material with 1 joule of
energy.
�Therefore the absorbed dose is a useful measure and is
applicable to any type or energy of ionizing radiation
93
Biologic Effects & Safety
● 4.1 Units of Radiation Exposure and Dose
�Dose Equivalence (Sievert)-- Relative biological
effectiveness of different types of ionizing radiation
�Dose is multiplied by a radiation weighting factor
(WR)
�Dose Equivalence = D x WR
94
Biologic Effects & Safety
● 4.2 Dose limit (ICRP Prescribed Limits per annum)
�Members of public
�1 mSv per annum above background
�5 mSv to eye
�20 mSv to hands
�Radiation workers
�20 mSv per annum above background
�150 mSv to eye
�500 mSv to hands
�Pregnant women must receive no more than 2mSv
per annum95
Biologic Effects & Safety
● 5. Principles of Radiation Safety:
�Minimise Exposure Time
�Maximise Distance from Source
�Use Correct Shielding
�Follow Manufacturers Instructions
�Keep dosage As Low As Reasonably Achievable
96
