Radiofrequency Excitation

The image, or measured spin density in an MRI experiment, is not necessarily equivalent to the physical spin density of the object being imaged, even when relaxation and Fourier transform effects are taken into account. The image is actually a picture of the signal received by the RF receive coil or RF probe. It is, in fact, proportional to the product of the field (which would be produced by the receive coil) and the transverse magnetization (which itself depends on the transmit coil’s field). This has been discussed in UNIT B2.1.