The MRI Scanner
Magnetic Resonance Imaging
(MRI) takes advantage of the fact that the nucleus of a hydrogen atom (a single
proton) behaves like a weak compass needle. In the presence of a strong magnetic
field, the hydrogen atoms will align themselves, but a radio signal of the
correct resonant frequency will cause them to deflect slightly. When the signal
is removed, the atoms return to their equilibrium state and emit a radio signal
of their own. An MRI scanner can detect these signals and use them to map the
distribution of molecules with lots of hydrogen atoms – ie, water and fat. In
this way, it can create detailed images of the inside of the body.
A. Scanning table
The patient can only be
scanned from inside the magnetic coil, so a motorised table slides them in and
out.
B. RF system
An antenna produces a radio
signal to ‘nudge’ the hydrogen nuclei and listento the answering radio wave they
emit.
C. Liquid helium
Liquid helium is pumped
through an enclosing jacket to cool the superconducting magnets almost to
absolute zero.
D. Main magnet
Superconducting magnetic
coils produce a magnetic field of 1.5 teslas – that’s about 300times stronger
than a fridge magnet.
E. Patient
The high magnetic fields mean
that patients with cochlear implants, pacemakers or embedded shrapnel usually
can’t be scanned.
F. Gradient system
A second coil distorts the
main magnetic field so that the resonant frequency of the protons varies
according to position.
