FILTRATION
Chapter 11: Filtration Study Guide
Must-Know Definitions
Filtration
The process of removing low-energy (soft) x-ray photons from the primary beam.
Purpose:
Decreases patient dose
Produces a more useful x-ray beam
Hardens the beam
Filter
Any material that selectively absorbs photons from the x-ray beam.
Half-Value Layer (HVL)
The amount of absorbing material needed to reduce beam intensity by 50%.
Think: HVL = 50% reduction in beam intensity
Aluminum Equivalency (Al/Eq)
Standard method for expressing filtration.
Example:
0.5 mm Al/Eq = filtering ability equal to 0.5 mm aluminum.
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Types of Filtration
1. Inherent Filtration
Filtration built into the x-ray tube.
Sources:
Glass envelope
Insulating oil
Tube housing window
Typical value:
0.5–1.0 mm Al/Eq
Special note:
Mammography uses beryllium windows.
Reduces inherent filtration to about 0.1 mm Al/Eq.
Memory Tip: Inherent = Inside the tube
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2. Added Filtration
Filtration placed outside the tube housing.
Most common material:
Aluminum (Z = 13)
Purpose:
Absorb low-energy photons
Allow useful photons to pass
Important:
Collimator contributes about 1 mm Al/Eq.
Memory Tip: Added = Added after the tube
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3. Compound Filtration (K-Edge Filter)
Uses 2 or more filtering materials.
Arrangement:
Highest atomic number closest to tube
Lowest atomic number closest to patient
Example:
Thoraeus Filter
Order:
1. Tin (Z = 50)
2. Copper (Z = 29)
3. Aluminum (Z = 13)
Exam Favorite: Highest Z → Tube Lowest Z → Patient
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4. Compensating Filtration
Used to equalize exposure when body part thickness varies.
Purpose:
Produce more uniform receptor exposure
Examples:
Wedge Filter
Used for:
Thoracic spine
Foot
Lower extremities
Trough Filter (Double Wedge)
Used for:
Chest imaging
Balancing mediastinum and lungs
Memory Tip: Wedge = One side thick Trough = Thick on both sides
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Total Filtration
Formula:
Total Filtration = Inherent Filtration + Added Filtration
DOES NOT include:
Compound filters
Compensating filters
Exam Question: "What is total filtration?"
Answer: Inherent + Added filtration
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HVL Requirements
Know the pattern:
kVp Minimum HVL
50 0.5 mm Al
70 1.5 mm Al
100 2.7 mm Al
150 4.1 mm Al
Rule: As kVp increases → HVL increases
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Recommended Total Filtration
Below 50 kVp
0.5 mm Al
50–70 kVp
1.5 mm Al
Above 70 kVp
2.5 mm Al
This is a registry favorite.
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Effects of Filtration
On Patient Dose
↓ Decreases patient dose
On Beam Quality
↑ Increases beam quality
On Beam Intensity
↓ Decreases beam intensity
On Exposure Factors
Must increase technique to maintain receptor exposure.
Memory Tip:
Filtration:
Patient Dose ↓
Beam Intensity ↓
Beam Quality ↑
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Registry Pearls
1. Most common filter material? → Aluminum
2. What does HVL measure? → Beam quality/total filtration
3. What filter removes low-energy photons? → Filtration
4. Two most common compensating filters? → Wedge and Trough
5. Total filtration equals? → Inherent + Added
6. What is beam hardening? → Removal of low-energy photons
7. What type of filter uses multiple materials? → Compound filter
8. What is another name for compound filter? → K-edge filter
9. Which side of a wedge filter goes over the thinner anatomy? → Thick side
10. Why is filtration used? → Reduce patient dose without sacrificing image quality
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Quick Memorization Sheet
FILTRATION = Removes soft photons
HVL = 50% beam reduction
INHERENT = Glass + Oil + Housing
ADDED = Aluminum + Collimator
COMPOUND = Multiple materials (Thoraeus)
COMPENSATING = Wedge + Trough
TOTAL FILTRATION = Inherent + Added
PATIENT DOSE ↓
BEAM QUALITY ↑
BEAM INTENSITY ↓
If you're using Clover Learning or RadTechBootCamp, I'd expect several test questions directly from:
HVL
Inherent vs Added filtration
Total filtration formula
Wedge vs Trough filters
Thoraeus filter order (Tin → Copper → Aluminum)