B1 mapping refers to measuring the spatial distribution of the RF transmit field (B1⁺) inside the subject. At 3 T and above, wavelength effects cause significant B1⁺ inhomogeneity, which manifests as flip angle variations across the image. B1 maps are used for quantitative MRI, SAR estimation, and parallel transmit calibration.

Why B1 Mapping Matters at High Field

At 3 T (128 MHz), the wavelength in tissue (~25 cm) becomes comparable to body dimensions, causing constructive and destructive interference of the RF field. A nominal 90° pulse may deliver 70° in some regions and 110° in others. Quantitative MRI techniques (T1 mapping, T2 mapping, MR spectroscopy) require accurate flip angles for valid results.

Double Angle Method (DAM)

Two spin-echo images are acquired with the same TR but with nominal flip angles α and 2α. The signal ratio gives: \(|S(2\alpha)/S(\alpha)| = 2\cos(\alpha_{actual})\), from which the actual flip angle can be calculated pixel-by-pixel. Simple but slow (requires long TR for T1 recovery).

Actual Flip Angle Imaging (AFI)

Two interleaved acquisitions with different TRs (TR1 ≪ TR2) but the same flip angle. The signal ratio depends only on flip angle and the TR ratio (not T1 for reasonable conditions), allowing fast B1 mapping.

Parallel Transmit (pTx)

At 7 T, multiple transmit channels (typically 8) each drive a separate coil element. By controlling the amplitude and phase of each channel, the B1⁺ distribution can be shaped (RF shimming) to improve uniformity or focus power. B1 maps for each channel are a prerequisite for pTx calibration.