X-ray Dosimetry Tools

Tube output, kVp/mAs adjustment, half-value layer and inverse-square law for medical physicists

kVp / mAs adjustment (15 percent rule, mAs square law)

Original kVp

Original mAs

New kVp

Goal

Required new mAs

—

mAs

15% rule: 15% kVp increase ≈ doubling mAs for image density. Entrance dose ∝ mAs × kVp².

Reading without filter (I₀)

Filter thickness 1 (mm)

Reading at thickness 1 (I₁)

Filter thickness 2 (mm)

Reading at thickness 2 (I₂)

HVL

—

mm Al

Linear attenuation coefficient µ

—

cm⁻¹

HVL = ln(2) / µ. µ derived by linear regression of ln(I) vs thickness from the two filter readings.

Original distance d₁ (cm)

Original dose I₁

New distance d₂ (cm)

Unit

Dose at new distance

—

mSv

I₂ = I₁ × (d₁ / d₂)². Doubling distance → ¼ dose. Useful for radiation safety planning at new working distances.

Tube output (µGy / mAs at 1 m)

Tube voltage (kVp)

mAs

Source-to-skin distance (cm)

Backscatter factor (BSF)

Reference kVp for output

Entrance skin dose (ESD)

—

µGy

Air kerma free-in-air at SSD

—

µGy

K_air = Y(kVp) × mAs × (100 / SSD)² · (kVp/kVp_ref)². ESD = K_air × BSF.

IAEA Human Health Series No. 4 (2007). Comprehensive Clinical Audits of Diagnostic Radiology Practices.
Bushberg JT et al. The Essential Physics of Medical Imaging, 4th ed. (2020). Chapters 6–7 (X-ray production, dosimetry).
IPEM Report 78 (2002). Catalogue of Diagnostic X-ray Spectra and Other Data.
NCRP Report No. 168 (2010) — fluoroscopy dose management.
15% rule (kVp/mAs trade-off) — Carlton & Adler, Principles of Radiographic Imaging, 6th ed.
Backscatter factors: Grosswendt B. Backscatter factors for X-rays generated at voltages between 10 and 100 kV. Phys Med Biol 1990;35:1233-1245.