Resident Physics Lectures Christensen, Chapter 6Filters George David Associate Professor Department of Radiology Medical College of Georgia

Energy Spectrum X-ray beams from tubes Polychromatic Polychromatic 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 tissue lowest energy photons selectively removed energy of low energy photons contributes to dose does not contribute to image photons don’t reach film Patient film

Ideal Filtration absorption characteristics absorbs all low energy radiation absorbs no high energy radiation high atomic number desirable increases photoelectric absorption of low energy photons

Filter’s Function shape beam’s energy spectrum selectively attenuate low energy photons less low energy radiation incident on patient energy deposited in filter, not in patient Filter Film

Filtration Locations x-ray tube and housing inherent filtration inherent filtration metal sheets placed in beam path placed between tube and collimator or in collimator Usually aluminum added filtration added filtration collimator mirror table (for under-table tube fluoro) Filter Tabletop X-Rays Tabletop Light Lamp

Inherent Filtration Absorption of x-rays by tube glass insert insulating oil housing window Insert Insulating Oil Glass Window

Inherent Filtration Typical Inherent Filtration mm aluminum equivalent mostly due to glass insert envelope Insert Insulating Oil Window glass insert envelope

Added Filtration Filtration intentionally added by placing metal plate in collimator Added Filter

Filtration Effects decreases beam intensity increases mean beam energy slightly decreases contrast at usual kVp range lowest energy photons cannot penetrate patient to reach film Only clinically important for mammography (<30 kVp)

Practical Filters Aluminum Most common atomic # 13 inexpensive copper good for high kVp sometimes used in combination with aluminum aluminum absorbs copper’s 8 keV characteristic radiation Aluminum Filter

Filters: The Good & Bad Disadvantages reduces beam intensity increases tube loading exposure time patient motion Advantage gross reduction in patient dose 80% typical

Filtration vs. HVL Filtration equivalent absorber present in beam whatever is actually in the beam filters as if it is ?? mm of aluminum Half Value Layer HVL (HVL) amount of absorber (aluminum) which reduces beam intensity by exactly 50% Easy to measure

Half Value Layer (HVL) Depends upon kVp waveform (single/three phase) inherent & added filtration Minimum HVL regulated by law Maximum HVL regulated only in mammography kVp HVL (mm Al) Georgia State Rules & Regulations for X-Ray

Radiographic HVL Setup

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) Marginal HVL = 2.5 mm filter mR (mm Al) Acceptable HVL > 2.5 mm OK! Must add Al to reduce beam to exactly 50% filter mR (mm Al) Unacceptable HVL < 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 filter mR (mm Al) kVp Measurements