Resident Physics Lectures year 1 Christensen Chapter 6

Resident Physics Lectures (year 1) Christensen, Chapter 6 Filters George David Associate Professor Department of Radiology Medical College of Georgia

Energy Spectrum X-ray beams from tubes Polychromatic Brehmstrahlung Characteristic spectrum of energies from 0 – k. Vp set on generator average beam energy 1/3 to 1/2 of peak (k. Vp) k. Vp (as set on generator) Filtered X-ray Beam

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 image receptor Filtered X-ray Beam Patient Image Receptor

Filtration Ideal absorption characteristics absorbs all low energy radiation absorbs no high energy radiation 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 Filtered X-ray Beam

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

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

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

Filter Attributes Most commonly aluminum 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 Half Value Layer (HVL) HVL amount of absorber (aluminum) which reduces beam intensity by exactly 50% Easy to measure in field

Half Value Layer (HVL) Measured HVL depends upon k. Vp waveform (single/three phase) inherent & added filtration Minimum HVL regulated by law k. Vp 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. 5 100 2. 7 110 3. 0 120 3. 2 130 3. 5 140 3. 8 150 4. 1 Georgia State Rules & Regulations for X-Ray