Inherent filtration is built into the tube; added filtration is what comes after it
Key takeaways
- Inherent: about 0.5 mm Al equivalent from the tube.
- Added: about 1 mm Al sheet, plus about 1 mm Al equivalent from the collimator mirror in Bushong.
- NCRP-based total minimums: 2.5 mm Al above 70 kVp, 1.5 mm from 50 to 70, 0.5 mm below 50. Bontrager puts 70 kV itself at 2.5 mm.
- 21 CFR 1020.30 sets minimum half-value layer, not a total filtration number.
- More filtration hardens the beam and lowers quantity and receptor exposure, so holding exposure steady takes more technique.
Inherent filtration is what the tube itself does to the beam. Added filtration is what sits after the tube. Total filtration is the two together.
What counts as inherent, and what counts as added?
| Layer | Type | About how much |
|---|---|---|
| Glass or metal enclosure at the window | Inherent | 0.5 mm Al equivalent, with the items below |
| Insulating oil, housing window | Inherent in Sherer; Bushong names only the enclosure | Included in the 0.5 mm |
| Aluminum sheet between housing and collimator | Added | 1 mm Al |
| Collimator mirror | Added in Bushong | 1 mm Al equivalent in Bushong, almost 1 mm in Sherer |
| Total | 2.5 mm Al equivalent |
Inherent filtration tends to rise as a tube ages, because vaporized tungsten deposits on the window (Bushong 11th ed., PDF p.162). That same tungsten vaporization is what thins the filament.
Is the collimator mirror added filtration?
In Bushong, yes. Sherer lists the mirror on its own, after inherent filtration and apart from its added sheets. Bushong says 1 mm and Sherer says almost 1 mm, both books reach a 2.5 mm total, and neither calls the mirror inherent. Their labels differ, and ARRT® doesn’t publish a ruling on it. Its specification says it avoids content where resources conflict, so we won’t call this an exam answer. Our judgment, for registry study: don’t file the mirror under inherent.
What is the required minimum?
Two different numbers get mixed up here.
Total filtration is an NCRP figure, not the federal rule. NCRP Report 102, Table 3.1, shows 0.5 mm Al under its 30 kVp column, 1.5 mm under 50 and 70 kVp, and 2.5 mm under 90 to 150 kVp. Modified from that report in Sherer’s table:
- Above 70 kVp: 2.5 mm Al equivalent
- 50 to 70 kVp: 1.5 mm Al equivalent
- Below 50 kVp: 0.5 mm Al equivalent
- Mobile and fluoroscopic units: 2.5 mm Al equivalent
ARRT’s outline cites NCRP #102 for minimum filtration. The mobile and fluoroscopic line is Sherer’s: we found no separate figure for those units in the copy of Report 102 we read.
The sources differ at exactly 70 kVp. Report 102’s table and Sherer put it in the 1.5 mm group, while Bontrager says 2.5 mm for equipment operating at 70 kV or higher (PDF p.83). Report 102’s table has no column between 70 and 90 kVp. If a question sits on that boundary, follow the source it or your program names.
Half-value layer is what the federal rule sets. 21 CFR 1020.30(m) gives a minimum HVL for each kV and requires enough filtration to reach it. It states no 2.5 mm filtration figure. At 80 kV, the minimum HVL is 2.9 mm Al for systems made on or after June 10, 2006, and 2.3 mm Al for older ones. The Image Production guide has more rows.
What does filtration do to the beam?
It removes low-energy photons that would stop in the patient and never reach the receptor. Quality and HVL go up, and quantity goes down. So skin dose falls. Receptor exposure falls too (Bushong, Table 8-1, PDF p.156), so holding it steady takes more technique. Bushong says a harder beam can also lower image contrast with screen-film (PDF p.159), and doesn’t state this for digital. ARRT’s factor table from March 1, 2027 lists tube filtration under both image contrast and subject contrast (p.6). See our filtration lesson.
On ARRT’s outlines, tube filtration is listed under Image Production (p.7). Minimum filtration and its effect on skin and organ exposure are listed under Safety, Radiation Protection (p.5), which is 29 of the 200 scored questions today and 25 from March 1, 2027.
Compensating filters are on the 2022 outline and gone from the 2027 outline, which applies from March 1, 2027.
Practice this with the equipment questions, or read the Equipment Operation and QA chapter.
Frequently asked questions
Is the collimator mirror inherent or added filtration?
Bushong calls it added filtration, about 1 mm Al equivalent. Sherer lists the mirror separately from both its inherent and added lists. Neither book calls it inherent.
What is the minimum total filtration above 70 kVp?
2.5 mm Al equivalent, per the table in Sherer modified from NCRP Report 102. Fixed units from 50 to 70 kVp need 1.5 mm, and below 50 kVp need 0.5 mm. Bontrager instead puts 70 kV itself in the 2.5 mm group.
Does 21 CFR 1020.30 require 2.5 mm of aluminum?
No. It sets a minimum half-value layer for each kV and requires enough filtration to reach it. On systems made on or after June 10, 2006, the minimum HVL at 80 kV is 2.9 mm Al.
What does adding filtration do to the beam?
It removes more low-energy photons, so beam quality and half-value layer rise and beam quantity falls. Bushong's Table 8-1 shows receptor exposure falling too, so keeping it the same takes more technique.
Sources
Found an error? Email [email protected] with the page and the source.
- 21 CFR 1020.30(m), Beam quality, Table 1 (Cornell LII copy and repo copy) Regulation
- NCRP Report No. 102, Medical X-Ray, Electron Beam and Gamma-Ray Protection for Energies Up to 50 MeV, 1989, Table 3.1, report p.13 Guidance
- ARRT Radiography Examination Content Specifications, 2022 and 2027, pp.1, 5 and 7 (question counts; filtration; NCRP #102); 2027 p.6 (factor table) Official
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