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Diagnostic X-rays
What is it?
• Photons
• Approximately 20 keV to 150 keV
• Up to 300 keV used for superficial therapeutic applications
Why Do We Care?
• Ionizing radiation
• Stochastic effects (cancer)
• Deterministic effects (variety)
Why Do We Care?
• Regulatory
– Occupational Health and Safety Act, administered by Ministry of Labour (Ontario)
– Healing Arts Radiation Protection Act (HARP Act) and associated radiation protection regulations, administered by Ministry of Health and Long Term Care (Ontario)
– Safety Code 35 (replaced Safety Code 20A), (Health Canada)
Physical Principles
Ionization
• The process by which atoms or molecules acquire a net charge due to:
– Loss of electron(s)
– Breakup of a molecule
• Important aspect is there is a generation of charge, usually electrons, which can be collected using an electric circuit
Exposure
• A measurement of ionization in air
• Strict definition is X = dQ/dm, where dQ is “the total charge of the ions of one sign produced in air when all of the electrons liberated by photons in a volume element of air having a mass dm are completely stopped in air”
Exposure
• Exposure is measured in coulombs per kg
• We usually talk about the roentgen (R) when evaluating exposure
1 R = 2.58x10-4 C/kg or 1 C/kg = 3876 R
• To generate 1 C of charge in air, we need to absorb 33.85 J of energy, so 1 R is equivalent to:
2.58x10-4 C/kg * 33.85 J/C = 0.00873 J/kg of air
Exposure
• 1 R of exposure corresponds roughly to 9.5 mSv of dose in water-like media
Dose
• Energy transfer to a medium is a 2 step process
– KERMA (kinetic energy released)
– Absorption of energy (i.e. dose)
• KERMA: photons interact with an atoms, setting electrons into motion
• Absorption: electrons interact with the medium through ionization and excitation
KERMA
hE
hhK
mass
energy
dm
EdK
tr
tr
hdhEh
hd
hdspectralK
h
tr
max
0
Dose
• All of the energy transferred to an electron will not be deposited locally
– Bremsstrahlung
– High energy electrons may deposit dose over several cm’s
mass
energy
dm
EdD ab
Dose
• US uses the unit “rad” for dose, where 1 rad = 100 erg/g
• Proper SI is the “gray”, where 1 Gy = 1 J/kg = 107 erg / 103 g = 100 rad
Equivalent Dose
• Not all types of radiation are equally damaging. To account for this, we define a radiation weighting factor, wR , per ICRP 103:
• Deq (Sv) = Dabs (Gy) * wR
photons, electrons, muons………………………...wR = 1
neutrons (spectral dependence)…..…………….wR = 2.5 to 22
protons and charged pions...………………………wR = 2
alphas, heavier particles, fission fragments..wR = 20
Ex. (a)1 Gy of protons is twice as damaging as 1 Gy of x-rays.
(b) 1 Sv of protons is equally as damaging as 1 Sv of x-rays.
Effective Dose
• Also called Effective Dose Equivalent; we use effective dose to evaluate risk
• Not all tissues are equally sensitive. To account for this, we define a tissue weighting factor, wT , per ICRP 103:
• Heff (Sv) = Deq (Sv) * wT
Ex. A mammogram delivers 3 mSv of equivalent dose to each breast. The effective dose is
Deff = 3 * wT for breast= 3 * .12= .36 mSv
SourceskV range
X-ray Machines
X-ray Machines
• All are variations of the same theme
• All contain an X-ray tube connected to
– Filament circuit
– High voltage circuit
– Timing circuit
– Digital detector (all modern systems)
X-ray source → patient → detector → processing / display
X-ray Tubes
X-ray Tubes
Photon Production
Bremsstrahlung Radiation
• Approximately 0.5% of incident e- are decelerated by attractive force of nucleus
• Energy lost by e- given off as an x-ray photon
Photon Energy
• Coulombic force (1/r2)
• h (1/r2)
• Maximum photon energy results from direct impact of nucleus and e- (rare event)
• Emax= energy of e-
Bremsstrahlung Efficiency
000,820n)(ionizatio lossenergy lCollisiona
hlung)(bremsstra lossEnergy Radiative ZEKE
•EKE are in keV
•Z = atomic number of target material
•For tungsten (Z = 74)-For 100 keV, ~99% goes towards heat-For 6 MeV, ~46% goes towards heat
• Emax determined by x-ray tube kVp
• Filtration from tube glass and housing
• Eave = 1/2 or 1/3 of Emax
Continuous BremsstrahlungSpectrum
Characteristic X-rays
1. e--e- interaction via repulsive forces
2. K-shell e- is ejected (electron energy> binding energy)
3. Electron from a outer energy level fills the void in the k-shell
4. Excess energy is emitted as an x-ray photon (Ephoton=Evacant -Etransition)
Characteristic X-rays
Binding energy dependence on distance and Z
Characteristic X-rays
• Shell capturing e- designates the transition
• α, is the donor is an adjacent shell
• β, if the donor is not an adjacent shell
• Transitions are numbered by the sublevel # of the donor shell
Relative Intensities of Characteristic X-rays
• As electron energy increases, the proportion of characteristic x-ray photons increases relative to those from bremsstrahlung
• for 80 kVp, ~ 5% photons are from characteristic x-rays
• for 100 kVp, ~ 10% photons are from characteristic x-rays
Beam Quality
• Describes penetrating ability of a beam and is specified as
– kVp: peak accelerating voltage
• Eavg ≈ Emax / 3
– HVT: half-value thickness in some medium, typically Al or Cu
• Contrast: 50 kVp with 0.75 mm Al HVT versus100 kVp with 2.15 mm Al HVT
• Higher energy = more penetrating
X-ray Generators
X-ray Generators
• Electrical generators that produce the electricity required to operate an x-ray tube
• 3 principal circuits
1. Filament circuit
2. High voltage circuit
3. Timer circuit (important for diagnostic)
Filament Circuit
Step down
transformer (<10V)
Variable resistor
network (controls
filament current)
Focal spot size
selection switch
Consequences of AC
from transformer?
X-ray Generators
• Maximum line voltage: 480 V
• We need 1 – 10 V at the filament, but 20,000 – 150,000 V at the target!
• X-ray generators use transformers to obtain these voltages
• Transformers are based on electromagnetic (EM) induction
Aside: Induction
Changing magnetic field
induces voltage in wire
Strength of magnetic field
determines strength of
induced voltage
Direction of magnetic field
change determines the
direction of electron flow
Process is reciprocal (B)
Magnitude of induced
magnetic field α number of
turns in coil
Aside: Transformers
Common iron core = couples
magnetic field into secondary
Wires are electrically insulated
but not magnetically insulated
Current in must be changing
(AC)
EM Transformers
Law of transformers
Ns > Np, step up transformer
Ns = Np, isolation transformer
Ns < Np, step down
transformer
s
p
s
p
N
N
V
V
Transformers Used for X-ray kV Generators
• 1:500 – 1:1000 turns to get 100 kVp from 100 - 200 V
• Centre of secondary winding is “tapped to ground”
– kVp remains the same between anode and cathode
– Max. circuit voltage is limited to kVp/2
– Reduced electrical insulation requirements
– Improved electrical safety
Generation of DC Voltage for Filament
• AC voltage to generate X-rays just doesn’t make sense, so we rectify the voltage
Single Phase Generator
For 60Hz AC, pulse length =
1/120s = 8 ms
Tube current is nonlinear and
drops off below 40 kV
Capacitors hold and release
charge in a time delayed
manner
3 Phase Generator
• Most modern kV X-ray systems use this type of generator
3 voltage sources out
of phase by 1/3 cycle
Rectified to make a 6
pulse waveform (near
DC)
Timing X-ray Exposure
• Electronic timers– Activates/terminates switch on primary or secondary circuit
(μs accuracy)
– Backup countdown timer
• Switches– Limiting component for accuracy
– EM switch energizer to open
– De-energized to close (i.e. only at zero AC amplitude so accuracy is 8 ms)
– Grid switching used for 3-phase and high frequency generators (+/- 1 - 2 ms)
Interactions with Tissues
Interactions
• Diagnostic energies (< 150 keV)
– Coherent scattering
– Photoelectric effect
– Incoherent (Compton) scattering
• Therapeutic energies (100’s of keV – MeV)
– Interactions have threshold energies
– Photodisintegration/photodissociation (10+ MeV)
– Pair/triplet production (1.022/4.088 MeV)
Interactions
Coherent Scattering
Photoelectric
Compton (Incoherent) Scattering
Pair Production
Triplet Production
Original e-
e+/e- pair
Protective Measures(and shielding calcs!)
How Do We Reduce Exposure?
r-s-t
distance – shielding - time
Distance
• Exposure falls off as 1/r²
– Doubling the distance away from a source decreases your exposure by a factor of four
Time
• Exposure is directly proportional to time
– Spend as little time in the radiation field as necessary
– Use imaging techniques that minimize the beam-on time
Shielding
• Personal shielding
– Lead aprons, gloves, lead-glass spectacles
• Room shielding
– Thick/high density concrete or lead-lined walls
– Used to reduce dose to occupants outside of the imaging suite
Shielding
• Shielding calculations in the province of Ontario were based on the formalism of Health Canada Safety Code 20A (appendix II)
• Safety Code 20A was superseded by Safety Code 35 in 2008
– ON legislation has not been updated to reflect this but best practice is to use SC35
Shielding
• Components of a radiation field
– Primary beam
– Leakage from X-ray tube
– Scattered radiation from “object” (i.e. the patient or whatever you happen to be imaging)
Shielding Calculations
Considerations that go into shielding calculations
• Primary versus secondary barrier
• Controlled versus uncontrolled area
• X-ray tube voltage (kVp)– Penetration; high kVp means lower attenuation
coefficients of shielding materials
• Average current (mA)– Inversely related to beam-on time; typically quote
current * time (mAs)
Shielding Calculation Factors
• Distance (d) from “source” to point of interest
• Maximum permissible dose (P)
• Occupancy factor (T)
• Usage factor (U)
• Workload factor (W)
• Maximum X-ray tube output (O)
Shielding Calculations
• P set in SC35 as 0.04 R/wk for controlled areas, 0.002 R/wk for uncontrolled areas
– These correspond to 20 mSv/yr and 1 mSv/yr respectively
– No sane person would design to these specifications
– Usual practice would be to design a controlled area to 1 mSv/yr and a public access area to 0.1 mSv/yr
Bd
OWUTP
2
BOWUT
dPK
2
Practically,
where we compare K against tabulated data in
Safety Code 35 for equivalent thickness of Pb
Barrier Calculations
Occupancy Factors
• Fraction of workweek for which the area in question is normally occupied
• T= …– 1 – control areas, nursing stations, waiting rooms,
darkrooms, restrooms, lounge (all staff areas!)
– 1/4 – corridors, patient rest/lounge rooms, patient dressing rooms
– 1/16 – closets, elevators, stairwells, street
– Smaller number if you can defend it
Usage Factors
• Fraction of machine “on” time during which the useful beam is directed at the barrier in question
• Primary beam– 1 – floor, walls with vertical cassette holders
– 1/4 – doors, walls of x-ray rooms
– 1/16 – ceilings not normally exposed
• Leakage and scatter radiation– U=1 always
Workload and Output Factors
• Workload (W), the integrated X-ray tube current (mA*min/week)
• Output (O), maximum exposure in primary beam (mR*m² / mA*min)
– Often defined at 1m from the target
• Maximum permissible “dose” usually specified as mR / week (really an exposure standard)
– Typically use 1 mGy = 115 mR (1 mR = 8.73 µGy) to translate to dose (Safety Code 35 Appendix VIII)
Area Designation
• Controlled
– Under the supervision of a Radiation Protection Officer (RPO)
– This person is typically a physician in Ontario (may change when the legislation is updated)
– Practical: area restricted to “X-ray workers” and people they accompany
Barrier Designation
• Primary barriers – used wherever the primary beam intersects
• Secondary barriers – no primary beam, only leakage and scatter
Bd
OWUTP
2
BOWUT
dPK
2
Practically,
where we compare K against tabulated data in
Safety Code 35 for equivalent thickness of Pb
Primary Barriers
Example - primary barrier (125 kVp)
P = W•U•T•K , and thusd2
K = P•d2
W•U•T
Take values as follows:
P = 0.0020 R/weekW = 2000 mA-min/weekd = 3.2 mU = 1/4T = 1
Then K = 4.1•10-5, corresponding toa shielding thickness of 2.95 mm Pb.
Safety Code 35
B
Dd
KFaTWOP
22400
BO
FTWa
DdPK
22400
F is maximum field size (factor of 400 is a normalization to
a 20x20 cm² field); d is source-to-”patient” distance; D is
“patient”-to-observation point distance. is a representative
scatter angle, plugged into a scattering probability function a().
Scatter Barriers
Example – scatter barrier (125 kVp)
P = W•T•a(θ)•F •K , and thus400d2•D2
K = 400P•d2•D2
a(θ)•W•T•F
Take values as follows:
P = 0.0020 R/weekW = 2000 mA-min/weekd = 0.8 m, D = 3.2 mT = 1F = 30 x 30 (cm2) = 900θ =120˚ (reasonable max)a(θ) = 0.0023 (table AIII.3 in SC 35)
Then K = 0.0013, corresponding toa shielding thickness of 1.58 mm Pb.
Safety Code 35
2522 dI
BTWP
TW
dPIB
2522
Leakage Barriers
I is tube current in mA. We note that the maximum allowable
leakage from the housing is 0.115 R/hr at 1 m. If the leakage
and scatter barrier thickness are within 1 TVL of each other,
we add 1 HVL to the thicker so that we attenuate both sufficiently.
Example - leakage barrier (125 kVp)
P = W•T•B , and thus522I•d2
B = 522I•P•d2
W•T
Take values as follows:
P = 0.0020 R/weekW = 2000 mA-min/weekd = 3.2 mT = 1I = 5 mA
Then B = 0.027, corresponding toa shielding thickness of 1.46 mm Pb.
Note: Leakage and scatter thicknesses are within 1 TVL of each other,therefore add 1 HVL to the thicker of the two.
Safety Code 35
CT Scanners
• CT shielding calculations are best done using the “isodose map method”, accepted by NCRP Report No. 147. These calculations are similar to diagnostic x-ray shielding calculations, but there are some differences
– The dominant source of exposure is patient scatter
– Vendors provide isodose maps of radiation exposure around the CT unit at the highest operating kVp
Isodose Map• 140 kVp• open collimators • isocenter plane
X dref = 2.0 m d= 4.7 m
CT Shielding CalculationD = Dref •W•T•fconv • B , and thus
(d/dref)2
B = D• (d/dref)2
Dref •W•T•fconv
Take values as follows:
W = 10000 mA-min/week (75 pt/week*20 rev/pt*400 mAs/rev)
D = .01 mSv/week, Dref = .024 uGy/mAsd = 4.7 m, dref = 2.0 mT = 1fconv = 0.060 mSv-s
uGy-min
Then B = 3.8*10-3, corresponding toa shielding thickness of 1.6 mm Pb.
Note: In practice, 3.2 mm Pb is commonly used in cancer centres. Why?
Personal Protective Equipment
• Staff performing imaging wear lead aprons of 0.5 mm lead equivalent specified at 150 kVp (maximum energy that tends to be used for diagnostic imaging)
• Personal dosimeters are required for “X-ray Workers”
Exposure Standards
What Are We Protecting Against?
• Unnecessary radiation exposure
– Patient over exposure
– Staff weekly and annual dose limits
• ALARA
– As Low As Reasonably Achievable all relevant social and economic factors considered
• We want to
– Prevent deterministic effects
– Limit stochastic effects to a reasonable level
Federal
• Safety Code 35
• Sets out a complete radiation protection standard for keV energies
– Responsibility of personnel
– Procedures for ALARA (staff & patients)
– Facility & equipment requirements
– Image processing systems
– Other equipment (PPE, other stuff)
Federal
– Radiation protection surveys
– Disposal of equipment
– Development of a QA program, including daily, weekly, monthly, quarterly, semi-annual, and annual tests
– Dose limits
– Methodology and tables for calculation of shielding requirements
– Related RED Act sections
Provincial
• Healing Arts Radiation Protection Act (HARP)
• Sets out rules for who can prescribe and deliver X-rays to humans for medical purposes
• Covers all X-rays not regulated by CNSC (i.e. all X-ray sources below 6 MV)
• Also sets out regulations governing entrance exposure and other technical parameters for a wide range of imaging procedures (does not include CT scans, yet)
Provincial
• Occupational Health and Safety Act OReg 861 (X-ray safety)
• Defines dose limits for “X-ray workers” and “other workers”
• Rules for the registration and installation of X-ray devices of all types
Dose Limits (Ontario)Part of body
irradiatedExposure conditions and
commentsDose Equivalent Annual Limit
(mSv)X-ray workers Other workers
Whole body Uniform irradiation 50 5
Partial or non-uniform irradiation
of body
Limit applies to effective dose equivalent (i.e.
tissue weighted)
50 5
Lens of eye Irradiated alone or with other organs
150 50
Skin Mean dose equivalent to basal layer > 1cm²
500 50
Other organs Effective dose equivalent limit applies, with priority given to
equivalent dose limit to the organ as applicable
500 50
Medical Applications
Medical Applications
• Transmission radiography– Physical film cassettes, image intensifiers, silicon
flat panels (current technology)
– Special case: mammography
• Fluoroscopy– Typically used for interventional procedures, so
positioning stuff inside of people under real-time imaging
• Computed tomography (CT)– 3D imaging of the internal anatomy
Transmission Radiography
• Acquisition of 2D image of 3D anatomy
• Optical density on film related to attenuation in the patient at that location (e-µx)
Mammography
• Lower energy X-rays generated using Mo target (~ 20 kVp) to get better soft tissue contrast
• Special screen-film combinations or, newer systems, silicon flat panel detectors
Fluoroscopy
• Often, a portable transmission radiography system is used
• X-ray tube and image intensifier / flat panel mounted on a “C-arm”
• Continuous X-ray operation for real-time visualization of a radio-opaque “thing” (e.g. catheter)
Computed Tomography
• Fan beam and detector array rotate around a patient on a sliding couch (helical imaging)
• 3D reconstruction of internal anatomy– Images are really maps of
attenuation coefficients calibrated to electron and physical density
– Attenuation coefficients calculated using a series of back projections along ray paths
So how are things different at higher (MV)photon energies???
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