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Transcript of Resident Physics Lectures Christensen, Chapter 6Filters George David Associate Professor Department...
Resident Physics Lectures
Christensen, Chapter 6
FiltersFilters
George DavidAssociate ProfessorDepartment of RadiologyMedical College of Georgia
Energy SpectrumX-ray beams from tubes
PolychromaticPolychromatic Brehmstrahlung Characteristic
spectrum of energies from 0 – kVp set on generator
average beam energy 1/3 to 1/2 of peak (kVp)
kVp(as set on generator)
Unfiltered Beams
most energy deposited in first few centimeters of tissuelowest energy photons
selectively removedenergy of low energy
photonscontributes to dosedoes not contribute to
image photons don’t reach film
Patient
film
Ideal Filtrationabsorption characteristics
absorbs all low energy radiation
absorbs no high energy radiation
high atomic number desirableincreases photoelectric
absorption of low energy photons
Filter’s Functionshape beam’s energy
spectrumselectively attenuate low
energy photonsless low energy radiation
incident on patientenergy deposited in filter,
not in patient
Filter
Film
Filtration Locationsx-ray tube and housing
inherent filtrationinherent filtration
metal sheets placed in beam pathplaced between tube and collimator or in collimatorUsually aluminum added filtrationadded filtration
collimator mirrortable (for under-table tube
fluoro)
Filter
Tabletop
X-Rays
Tabletop
Light
Lamp
Inherent FiltrationAbsorption of x-rays
by tubeglass insertinsulating oilhousing window
Insert
Insulating Oil
Glass Window
Inherent FiltrationTypical Inherent Filtration
.5 - 1.0 mm aluminum equivalentmostly due to glass insert envelope
Insert
Insulating Oil
Windowglass insert envelope
Added FiltrationFiltration intentionally added by placing
metal plate in collimator
Added Filter
Filtration Effectsdecreases beam intensityincreases mean beam energyslightly decreases contrast at usual
kVp rangelowest energy photons cannot penetrate
patient to reach filmOnly clinically important for
mammography (<30 kVp)
Practical FiltersAluminum
Most commonatomic # 13 inexpensive
coppergood for high kVpsometimes used in combination with aluminum
aluminum absorbs copper’s 8 keV characteristic radiation
Aluminum Filter
Filters: The Good & Bad
Disadvantagesreduces beam intensityincreases
tube loading exposure time
patient motion
Advantagegross reduction in patient dose
80% typical
Filtration vs. HVL
Filtrationequivalent absorber present in beamwhatever is actually in the beam filters as if it
is ?? mm of aluminumHalf Value Layer (HVLHVL)
amount of absorber (aluminum) which reduces beam intensity by exactly 50%
Easy to measure
Half Value Layer (HVL)Depends upon
kVpwaveform
(single/three phase)inherent & added
filtrationMinimum HVL
regulated by lawMaximum HVL
regulated only in mammography
kVp HVL (mm Al) 30 0.3 40 0.4 49 0.5 50 1.2 60 1.3 70 1.5 71 2.1 80 2.3 90 2.5100 2.7110 3.0120 3.2130 3.5140 3.8150 4.1
Georgia State Rules & Regulations for X-Ray
Radiographic HVL Setup
R
Filter
Tabletop
Radiographic
Checking HVL Compliance(Radiographic)
How much aluminum must be placed in beam to reduce intensity by exactly 50%?
90 kVp Measurements; 2.5 mm Al minimum HVL
filter mR(mm Al)------------------- 0 2502.5 125
MarginalHVL = 2.5 mm
filter mR(mm Al)------------------- 0 2502.5 133
AcceptableHVL > 2.5 mm
OK! Must add Al to reduce beam to exactly 50%
filter mR(mm Al)------------------- 0 2502.5 111
UnacceptableHVL < 2.5 mm
Not OK! Must remove Al to reduce beam to exactly 50%
Checking HVL Compliance(Radiographic)
Is this machine legal?2.5 mm Al minimum filtration at
90 kVp
R
Filter
Tabletop
Radiographic
filter mR(mm Al)------------------- 0 4502.5 205
90 kVp Measurements