92. Magnetic Resonance Imaging

yaboiemil's version from 2016-02-04 12:27

Origin of NMR Signal

1. The NMR signal originates when an external magnetic field is applied to atoms
with an odd amount of nucleons (or odd amount of protons) which means they have
1/2 spin. These atoms will try to align (either towards or opposite) to the magnetic field.
During this, they will absorb electromagnetic radiation, which moves them to a higher
energy level. These atoms will then go through a relaxation period, whereby they will
release their energy - this is NMR signal.

Magnetic Field Gradients in image acquisition

2. If a limb is put into a magnetic field, all the atoms will rotate together, and any image
present will be lost.
3. To gain higher resolution and distinction in an image, you must make a crossover
of the XY plane, obtaining info from only this slice.



4. These extra gradients are created by extra coils which disturb the uniformity of the
Magnetic Field. (1T as opposed to several mT of flux)

Clinical Value of MRI

5. You will experience 0.3-1T of Magnetic Flux in an MRI
6. MRI provides a high contrast resolution of soft tissues, often to tenths of a millimeter as it is a
spatial reconstruction of resonating nuclei density.
7. It can also provide information on the speed of blood flowing through the veins by means of
MAGNETIC RESONANCE ANGIOGRAPHY which can calculate the velocity of blood flow due to
the flow of "prepared" nuclei away from the imaging part, and the flow of "unprepared nuclei" away
from the imaging part.
8. Temperature affect the relaxation time, meaning that temperature changes in tissue can be
examines, e.g. during ultrasound or laser tissue heating.

Safety Problems in MRI

9.MRI magnets best to worst:
      1. Superconductor Magnets 2. Electromagnets 3.Permanent Magnets
10. The loud noise an MRI creates may be disturbing for some patients
11. No ferromagnetic implants/fragments may be present in the body, because the magnetic field
will tear them out, causing damage to the patient and possibly the machine.
12. Feelings of Claustrophobia may arise in the machine due to the small tube a person must enter.