MRI SAR Estimator
Specific Absorption Rate (SAR) measures power deposited per unit mass in tissue (W/kg). IEC 60601-2-33 limits whole-body SAR to 2 W/kg (normal mode) and local head SAR to 3.2 W/kg. SAR scales with B₁⁺ amplitude squared, frequency squared, and tissue conductivity.
Equations & Parameters ▸
\(\text{SAR}=\dfrac{\sigma|\mathbf{E}|^2}{2\rho}\approx\dfrac{\omega^2\mu_0^2 B_1^{+\,2}\sigma\,r_{eff}^2}{2\rho}\quad[\text{W/kg}]\quad\mu_0=4\pi\!\times\!10^{-7}\text{ H/m}\)
| B₁⁺ | RF excitation field amplitude at the tissue (µT). Typical body coil: 10–20 µT for a 90° pulse. |
| f | Larmor frequency. SAR scales as f² (or B₀²). |
| σ | Tissue electrical conductivity (S/m). Muscle ≈ 0.7–0.9 S/m at 128 MHz. |
| ρ | Tissue mass density (kg/m³). Muscle ≈ 1050 kg/m³. |
| Duty cycle | Fraction of time RF is on. SAR is time-averaged. |
| SAR formula | SAR = σ·|E|²/(2ρ) ≈ (ω·µ₀·B₁⁺)²·σ/(2ρ·µ₀²) = ω²·(B₁⁺)²·σ/(2ρ). |
Physical constants used
| µ₀ | 4π×10⁻⁷ H/m |
| ¹H γ/2π | 42.577 MHz/T (Larmor frequency per Tesla) |
| ¹H at 1.5 T | 63.87 MHz |
| ¹H at 3 T | 127.74 MHz |
| ¹H at 7 T | 297.7 MHz |
| σ_muscle @ 128 MHz | ≈ 0.77 S/m |
| ε_muscle @ 128 MHz | ≈ 58 |
| IEC SAR limit (WB normal) | 2 W/kg (over 6 min) |
| IEC SAR limit (head) | 3.2 W/kg (over 10 min) |
Inputs
µT
peak RF field at tissueMHz
¹H: 64/128/298 MHz at 1.5/3/7TS/m
enabled for custom tissuekg/m³
enabled for custom tissue(0–1)
time-average fractionResults
SAR
Peak local SAR—
Time-averaged SAR—
IEC whole-body limit (2 W/kg)—
IEC head limit (3.2 W/kg)—
Context
E-field estimate—
ω·B₁⁺ product—
B₁⁺ for 2 W/kg limit—
Safety margin—
⚠ Disclaimer: This is a simplified local SAR estimate for educational purposes only. Clinical SAR compliance requires full electromagnetic simulation with patient-specific models. Always consult IEC 60601-2-33 and vendor-provided SAR models for actual scanner operation.