Transcript A universal way for virtual Fixture implementation

Projection Radiography (X-Ray)
Instructors: Brian Fleming and Ioana Fleming
[email protected], [email protected]
January 7th, 2010
Today
• X-Ray production
• Interaction with matter / tissue
• Instrumentation
• Applications
1. Atomic Structure
Balanced == Neutral -- No Charge!
Missing Electron == ?
Extra Electron == ?
Electrons
• Orbiting in shells
Electron Binding Energy
• Atom’s ground state – lowest energy
configuration
• Basic principle:
bound energy < unbound energy + electron energy
• Binding energy is difference
• Binding energy of hydrogen electron: 13.6 eV
1 eV is the kinetic energy gained by an
electron that is accelerated across a one (1)
volt potential
Ionization and Excitation
• Ionization is “knocking" an electron out of the atom
creates 1 electron and 1 ion (what charge?)
• Excitation is “knocking" an electron to a higher orbit
Characteristic Radiation
What happens to ionized or excited atom?
• Return to ground state by rearrangement of
electrons
• Causes atom to give of energy
Energy given off as radiation
• infrared
• light
• x-rays
Ionizing Radiation
Radiation with energy > 13.6 eV ionizes H
• Energy required to ionize:
– Air: 34 eV
– Lead: 1 keV
– Tungsten: 4 keV
(average binding energies)
• Radiation energies in medical imaging
– 30 keV - 511 keV
• can ionize 10 - 40,000 atoms
Particulate Radiation
• Any subatomic particle (proton, neutron,
electron) can be considered to be ionizing
radiation (nuclear, beta) if it possesses
enough kinetic energy to ionize an atom
• An electron accelerated across 100 kV
potential difference yields a 100 keV
electron
Ion Drives
Ion Engines
Medical Particle Beams
What are X-Rays ?
Electromagnetic EM Radiation
– radio, microwaves,
– infrared, visible light, ultraviolet
– x-rays, gamma rays
• Particle / photon: E = h * ν
– Planck's constant h = 4.14 * 10-15 eV-sec
– f is frequency
• Electromagnetic wave: λ = c / ν
– C = 3 * 108 meters/sec; speed of light
X-Rays vs. light vs. radio waves
2. How are X-Rays produced?
X-rays are produced when
accelerated electrons interact
with a target, usually a metal
absorber, or with a crystalline
structure.
Electron radiative interactions:
• Characteristic x-rays:
–Electron ejects an inner-shell electron
–Reorganization generates x-ray
• Bremsstrahlung x-rays
–Electron “grazes" nucleus, slows down
–Energy loss generates x-ray (primary
source of x-rays from an x-ray tube)
X-Ray Spectrum
EM Radiation Interactions w/ matter
Completely different than particulate radiation
(electron) interactions:
• Photoelectric effect
• Compton scattering
Photoelectric effect
• Atom completely absorbs incident photon
• All energy is transferred
• Atom produces
- characteristic radiation, and/or
- energetic electron(s)
Iodine
Energy levels
K -33.2keV
L -4.3keV
M -0.6keV
• Characteristic radiation might be
- x-ray
- Other light (very important)
Example
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Photon with energy 40keV enters
Photoelectron from K-shell with
energy (40-33.2)=6.8keV exits
Electron from M- to K-shell
Characteristic radiation at (33.2-0.6)=
31.6KeV in a random direction.
The Atom now has positive charge
What if the energy is higher/lower?
KLM
Compton Scattering
Photon collides with outer-shell electron
• Photon is not absorbed, but it loses energy and it changes
direction (angle θ)
E - Energy of incident photon
E’ - Energy of scattered photon
m0 is rest mass of electron
m0c2 = 511 keV
Medical Imaging
• Photoelectric effect
– Responsible for contrast between tissues
• Compton effect
– Undesirable
– How can we control the angle?
• Important concepts
– Attenuation
– Dose
Attenuation
• The process describing the loss of strength of a beam of
electromagnetic radiation.
• Tissue-dependent attenuation is the primary mechanism
behind contrast in radiology.
Beam Strength
– Photon count = number of
photons in the burst
– Energy flow = how much
energy the bust is carrying
Intensity of an x-ray beam =
energy fluence rate (per unit
area per unit time)
Linear Attenuation Coefficient
• Assuming “narrow beam” geometry = same width as the
beam detector
• Homogeneous slab of thickness Δx
• Fundamental photon attenuation law
N = N0 e -μ Δ x
μ = linear attenuation coefficient
• In terms of intensity:
I = I0 e -μ Δ x
This is known as Beer’s Law
Attenuation Coefficient
• The linear attenuation coefficient μ of all materials
depends on the photon energy of the beam and
the atomic numbers of the elements in the
material.
• Since the mass of the material itself provides the
attenuation, attenuation coefficients are usually
characterized by μ/ρ, where ρ is the material
density.
Attenuation Coefficient
Human Density ~ 1 g/cm3
Δx = 20cm
N0 = 1,000,000,000,000
Exercise 1:
Eγ = 20 KeV
N = 2,000
ΔE = 999,999,998,000 * 20 keV = 2e13 keV
Exercise 2:
Eγ = 100 KeV
N = 33,000,000,000
ΔE = 967,000,000,000 * 100 keV = 9.6e13 keV
EM Radiation Dose
• How many photons? → fluence
• How much energy? → energy fluence
• What does radiation do to matter? → dose
Exposure
= the creation of ions
• How many ions are created?
• Exposure X, the number of ion pairs produced in
a specific volume of air by EM radiation
• SI Units: C/kg (charge per mass)
• Common Units: Roentgen, R
1 C/kg = 3876 R
Dose
As EM radiation passes through a material, it
deposits energy into it by the photoelectric effect
and Compton scattering.
• How much energy is deposited into material?
• Dose D, the energy deposited per unit volume
• SI unit: Gray (Gy)
1 Gy = 1 J/kg (energy per mass)
• Common unit: rad
1 Gy = 100 rads
1 R of exposure yields 1 rad of absorbed dose in
soft tissue.
So did we kill our test subject?
• 2 x 1013 keV = 3.2 x 10-3 J
– Mass = 80 kg
– 3.2e-3 J / 80 kg = 0.00004 rads = 0.04 mRad
• 9.6 x 1013 keV = 1.54 x 10-2 J
– 1.54e-2 J / 80 kg = 0.00019 rads = 0.19 mRad
• Typical chest x-ray dose ~ 0.1 mRad
• 1000 Rad =
Dose Equivalent
Different types of radiation, when delivering the
same dose, can have different effects on the
body.
• Dose equivalent H
H=D*Q
• Q = quality factor,
– Q ≈ 1 for x-rays, gamma rays, electrons, beta,
– Q ≈ 10 for neutrons and protons,
– Q ≈ 20 for alpha particles.
• Since Q ≈ 1, H = D
• SI unit, Sievert (Sv). More common, rems
Effective Dose
= The sum of dose equivalents to different organs or
body tissues, weighted to produce a value
proportional to risk (the body is not irradiated
uniformly)
• Annual effective dose (average) = 100 mrems
• Chest x-ray = 0.1 mrems
• Fluoroscopic study = several rems
Biological Effects of X-Rays
• Injury to living tissue results from the transfer of
energy to atoms and molecules in the cellular
structure.
• Atoms and molecules become ionized or excited.
• These excitations and ionizations can:
– Produce free radicals
– Break chemical bonds
– Damage molecules that regulate vital cell processes
Prompt and Delayed Effects
• Radiation effects can be categorized by when
they appear
• Prompt, acute effects – skin reddening, hair loss
and radiation burns which develop soon after
large doses of radiation are delivered over short
periods of time
• Delayed effects – cataract formation and cancer
induction that may occur months or years after a
radiation exposure.
Prompt Effects
• Will develop within hours,
days or weeks depending on
the size of the dose. The
larger the dose the sooner
the effect will occur
• Limited to the site of the
exposure.
Prompt Effects
• The skin does not have receptors that sense
radiation exposure. No matter how large a
radiation dose a person receives, there is no
sensation at the time the dose is delivered.
• Some people who have received large doses
claim to feel a tingling at the skin, however it is
believed that the tingling is due to static charge at
the skin surface rather than the direct sensation of
radiation exposure.
Delayed Effects
• Cataracts – induced
when a dose
exceeding 500 rems is
delivered to the lens of
the eye. Radiation
induced cataracts may
take months or years
to appear.
• Extremely unlikely to
receive a substantial
dose to the eye
working with todays
units.
Delayed Effects
• Cancer studies of people
exposed to high doses of
radiation have shown
there is a risk of cancer
induction associated with
high doses.
• Studies demonstrate that
cancer risk is linearly
proportional to the dose
• Radiation induced cancers
may take 10-15 years to
appear.
Cancer Risk Estimates
Putting Risk into Perspective
• 1 in a Million chance of death from activities
common in society
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Smoking 1.4 cigarettes in a lifetime (lung cancer)
Eating 40 tablespoons of peanut butter (aflatoxin)
Spending two days in Los Angeles (air pollution)
Driving 40 miles in a car (accident)
Flying 2500 miles in a jet (accident)
Canoeing for 6 minutes (drowning)
Receiving a dose of 10 mrem of radiation (cancer)
Personnel Exposure Limits
• Annual Dose Exposure limits have been established
based on the recommendations of national and
international commissions.
• Exposures at or below these limits should result in no
exposure effects
Whole Body – Radiation Workers
5 rem/year
(5000 mrem/yr)
Extremities – Radiation Workers
50 rem/year
50,000 mrem/yr
General Public
0.1 rem/year
(100 mrem/yr)
Exposure Effects
• 1000 rad – second degree burns
• 2000 rad – intense swelling within a few hours
• 3000 rad – completely destroys tissue
• 400 rad – acute whole body exposure is LD 50/30*
*LD 50/30 – lethal to 50% of population within 30 days if not
treated
Projection Images:
The creation of a two-dimensional
image “shadow” of the three
dimensional body. X-rays are
transmitted through a patient,
creating a radiograph.
• chest x-rays
• mammography
• dental x-rays
• fluoroscopy
• angiography
• computed tomography
The three standard orientations of projection (slice, tomographic)
images
Axial,
Transaxial,
Coronal
Transverse
Frontal
Sagittal
Oblique Slice: an orientation not corresponding to one of the
standard slice orientation.
3. Radiographic System
X-Ray tube
Anode Angle
Anode angles in diagnostic x-ray tubes range from 7 to 20 degrees, with 12to 15-degree angles most common. The smaller the angle, the better the
resolution.
X-Ray Tube Components
• Filament controls tube current
(mA)
– Tungsten - preferred because
of its high melting point
(3370°C)
• Cathode and focusing cup
• Anode is switched to high
potential
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–
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–
30 -150 kVp
Made of tungsten
Bremsstrahlung is 1%
Heat is 99%
Spins at 3,200-3,600 rpm
• Glass housing; vaccum
Exposure Control
• kVp applied for short duration
– Older machines have a fixed “shutter speed”
– Newer machines allow for variable exposure times
• Tube current (mA) controlled by filament current and
anode voltage
mA * exposure time = mAs
• Max energy - controlled by anode voltage V (keV)
• Radiation Dose - controlled by current and time (mAs)
X-Ray Spectrum
Filtration
• Low energy x-ray will be absorbed by the body
(ouch!), without providing diagnostic information
• Filtration: Process of absorbing low-energy x-ray
photons before they enter the patient
• Inherent Filtration
– Within anode
– Glass housing
• Added Filtration
– Aluminum
– Copper/Aluminum
– Measured in mm Al/Eq
Restriction
• Goal: To direct beam toward desired anatomy
Compensation Filters
• Goal: to even out film exposure
Colimators
• Grids
Grids
Effectiveness in scatter reduction?
grid ratio = h / b
6:1 to 16:1 (radiography) or 2:1 (mammography)
Grids
• Radiation is absorbed by grid
– grid conversion factor
– Typical range 3 < GCF < 8
• Grid visible on x-ray film
– move grid during exposure
– linear or circular motion
Intensifying Screen
• Film stops only 1-2% of x-rays
• Film stops light really well
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Phosphor = calcium tungstate
Flash of light lasts 1 x 10-10 second
~1,000 light photons per 50 keV x-ray photon
Reflective layer prevents light from going backwards
Radiographic Cassette
• Cassette holds two screens; makes “sandwich"
• One side is leaded. Why?
Contrast Agents
• Contrast compounds containing barium or iodine, which
are radio opaque, can be ingested in the gastrointestinal
tract (barium) or injected in the artery or veins to highlight
these vessels. The contrast compounds have high atomic
numbered elements in them that (like bone) essentially
block the X-rays and hence the once hollow organ or
vessel can be more readily seen.
• Air
– Does not absorb x-ray
– “opposite” type of contrast
– By Inflating the lungs, air provides contrast for lung
tissues
4. Applications
X-ray Shoe Fitting Device
In the late 1940's and early 1950's, the shoe-fitting x-ray unit was a
common shoe store sales promotion device and nearly all stores had
one. It was estimated that there were 10,000 of these devices in use.
This particular shoe-fitting x-ray unit was produced by the dominant
company in the field, the Adrian X-Ray Company of Milwaukee WI, now
defunct. Brooks Stevens, a noted industrial designer whose works
included the Milwaukee Road Olympian train and an Oscar Meyer
Wienermobile, designed this machine.
The primary component of a shoe-fitting x-ray unit was the fluoroscope which
consisted essentially of an x-ray tube mounted near the floor and wholly or
partially enclosed in a shielded box and a fluorescent screen. The x-rays
penetrated the shoes and feet and then struck the fluorescent screen. This
resulted in an image of the feet within the shoes. The fluorescent image was
reflected to three viewing ports at the top of the cabinet, where the customer, the
salesperson, and a third person (mom) could view the image at the same time.
The radiation hazards associated with shoe fitting x-ray units were recognized as
early as 1950. The machines were often out of adjustment and were constructed
so radiation leaked into the surrounding area.
Chest X-Ray
Dental X-Rays
Angiogram
Mammography
Thank you