MRI-basic principles

Clinical Magnetic Resonance Imaging (MRI) uses the magnetic properties
of hydrogen and its interaction with both a large external magnetic field
and radio waves to produce highly detailed images of the human body. In
this first module, we will discuss some basic principles of magnetism, the
magnetic properties of the hydrogen nucleus, and its interaction with the
externally applied magnetic field (B0).
In its early days, MRI was known as NMR. This stands for Nuclear Magnetic
Resonance. Although the name has changed (primarily due to the
negative connotation of the word “nuclear”), the basic principles are the
same. We derive our images from the magnetic resonance properties of
nuclear particles (specifically hydrogen).
In order to perform MRI, we first need a strong magnetic field. The field
strength of the magnets used for MR is measured in units of Tesla. One
(1) Tesla is equal to 10,000 Gauss. The magnetic field of the earth is
approximately 0.5 Gauss. Given that relationship, a 1.0 T magnet has a
magnetic field approximately 20,000 times stronger than that of the
earth. The type of magnets used for MR imaging usually belongs to one of
three types; permanent, resistive, and superconductive.
A permanent magnet is sometimes referred to as a vertical field magnet.
These magnets are constructed of two magnets (one at each pole).

The patient lies on a scanning table between these two plates.
Advantages of these systems are: 1) Relatively low cost, 2) No electricity
or cryogenic liquids are needed to maintain the magnetic field, 3) Their
more open design may help alleviate some patient anxiety, 4) Nearly nonexistant
fringe field. It should be noted that not all vertical field magnets
are permanent magnets.
Resistive magnets are constructed from a coil of wire. The more turns to
the coil, and the more current in the coil, the higher the magnetic field.
These types of magnets are most often designed to produce a horizontal
field due to their solenoid design (figure 2).
As previously mentioned, some vertical field systems are based on
resistive magnets. The main advantages of these types of magnets are:
1) No liquid cryogen, 2) The ability to “turn off” the magnetic field, 3)
Relatively small fringe field.
resistive system: