Resident Physics Lectures

1
Resident Physics Lectures

• Christensen, Chapter 6
• Filters

George David Associate Professor Department of
Radiology Medical College of Georgia
2
Energy Spectrum

• X-ray beams from tubes
• 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)
3
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 dont reach film

Patient
film
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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

5
Filters Function

• shape beams energy spectrum
• selectively attenuate low energy photons
• less low energy radiation incident on patient
• energy deposited in filter, not in patient

Filter
Film
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Filtration Locations

• x-ray tube and housing
• 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)

Filter
not mentioned in book
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Inherent Filtration

• Absorption of x-rays by tube
• glass insert
• insulating oil
• housing window

Insulating Oil
Insert
Glass Window
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Inherent Filtration

• Typical Inherent Filtration
• .5 - 1.0 mm aluminum equivalent
• mostly due to glass insert envelope

Insert
Insulating Oil
Window
glass insert envelope
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Added Filtration

• Filtration intentionally added by placing metal
  plate in collimator

10
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

11
Filtration Contrast

• Decrease in contrast clinically important at low
  kVp used for mammography (lt30 kVp)
• inherent filtration undesirable in mammography
• beryllium often used for exit portal (window) of
  glass envelope
• beam essentially unfiltered

12
Mammography Beam

• molybdenum target
• 17.5 19.6 keV characteristic radiation
• considerable Bremsstrahlung above 20 keV
• Higher energy Bremsstrahlung reduces contrast
• molybdenum added filter
• .030 mm thick typically
• high attenuation above19.6 keV k-edge
• attenuates Bremsstrahlung
• much lower attenuation for 17.5 19.6
  characteristic radiation

19.6 keV k-edge
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Practical Filters

• Aluminum
• Most common
• atomic 13
• inexpensive
• copper
• good for high kVp
• sometimes used in combination with aluminum
• aluminum absorbs coppers 8 keV characteristic
  radiation

Aluminum Filter
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Filters The Good Bad

• Disadvantages
• reduces beam intensity
• increases
• tube loading
• exposure time
• patient motion
• Advantage
• gross reduction in patient dose
• 80 typical

15
Filter Thickness

• First cms of filter thickness most effective
• each additional cm has less filter effect than
  previous cm

16
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)
• amount of absorber (aluminum) which reduces beam
  intensity by exactly 50
• Easy to measure

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Half Value Layer

• Depends upon
• kVp
• waveform (single/three phase)
• inherent added filtration
• Minimum HVL regulated by law
• Maximum HVL regulated only in mammography

Georgia State Rules Regulations for X-Ray
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Radiographic HVL Setup
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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
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Checking HVL Compliance(Radiographic)

• Is this machine legal?
• 2.5 mm Al minimum filtration at 90 kVp

90 kVp Measurements
filter mR (mm Al) -------------------
0 450 2.5 205
21
Wedge Filters

• thickness varies
• filter shaped like wedge
• application
• used to obtain uniform film density when large
  change in patient thickness over image field
• long-leg angiography

22
Wedge Filters

• chest filters
• contoured for lung fields

23
Heavy Metal Filters

• Not common
• controversial
• Reduces beam intensity
• requires high speed imaging systems and/or high
  capacity tubes
• K-edge matching
• designed to provide many photons near k-edge of
  barium or iodine
• enhances contrast of barium or iodine

24
Heavy Metal Filter Claims

• Narrower spectrum
• reduction of low energies reduces patient dose
• factor of 2 or more theoretically possible
• reduction of high energies (k-edge absorption)
  improves contrast