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