1. Longitudinal magnetisation.
Longitudinal magnetisation. For simplicity protons are now shown as vectors. (A) The magnetic moments of protons precessing in the external magnetic field begin to cancel each other out. Opposing protons A and A′ and B and B′ ‘neutralise’ each other leaving a net number of protons lying parallel to B0. Protons precessing parallel to B0 also begin to cancel each other out. C (pointing to the left) and C′ (pointing to the right) oppose and neutralise each other. This occurs in all directions bar one: the direction of the z-axis, along B0. The result is a sum magnetic field that is typically depicted as a vector (B). This figure is only reproduced in colour in the online version.
Stuart Currie et al. Postgrad Med J 2013;89:
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