large / strong (constant) magnetic field B1
nuclei rotate about direction of field / precess (1)
radio frequency / r.f. pulse B1
causes resonance in nuclei , nuclei absorb energy (1)
(pulse) is at the Larmor frequency (1)
on relaxation / nuclei de-excite emit (pulse of) r.f. B1
detected and processed B1
non-uniform field (superimposed) B1
allows for position of nuclei to be determined B1
and for location of detection to be changed (1)
(B6 plus any two extra details, 1 each, max 2) B2
This is the mark scheme to a question that asks to explain the principles behind MRI. My question is, what is the non-uniform field superimposed to? Is it the same field as the first magnetic field or the RF pulse that puts the atoms in resonance? Where does the relaxation time of the atom after the RF pulse is stopped come in to all of this?