The figure shows the result obtained on an AMX-500 spectrometer with a BGU (10 A) gradient unit and an inverse z-gradient probe-head. The measurement of the dip is indicated. In this spectrum a gradient of 0.105 T/m was applied.

7. Comments

The experiment as described works only for superconducting magnets in which the direction of the magnetic field B₀ is vertical, i.e., along the axis of the sample tube, and the field gradient is in the same z-direction. The resonance frequency of a proton in the presence of a z-gradient is given by Equation (2).

ω_z = γ B_eff = γ[B₀ + B_z](2)

Thus the resonance frequency of the proton is dependent on its z-position. A field gradient G_z can be defined as in Equation (3).

G_z = B_z1 − B_z2Δz = Δωγ Δz(3)

Using a sample such as the one described above with well-defined points z1 and z2 determined by the thickness of the Teflon disk, one can measure an image of the water distribution in the tube from a gradient spin-echo experiment and calculate from this image the strength of the gradients.

This experiment can be thought of as a normal spin-echo sequence (see Exp. 6.2), where in addition a gradient is switched on after the first 90° pulse. For technical reasons, however, the gradient pulse is divided into two, so that the 180° r.f. pulse can be inserted in between. Each spin is spatially labeled by its resonance frequency, thus giving an image of the water distribution within the sample. The 180° pulse refocuses all magnetizations so that the spin-echo builds up to a maximum at a time d2 after the 180° pulse. To avoid oscillations, acquisition is started only after d3.

8. Own Observations