Basic Physics of MR Signal and Image Generation - Advance Image Processing in Magnetic Resonance Imaging - page 5

Images are extremely rich in information content. The image pixel value is
in general dependent on a lot of intrinsic parameters, including the nuclear spin
density ( , the spin-lattice relaxation time (T1), the spin-spin relaxation time

(T 2), molecular motions (such as diffusion and perfusion), susceptibility effects,
and chemical shift differences. The imaging effects of these parameters can be
suppressed or enhanced in a specific experiment by another set of operator-
selectable parameters, such as repetition time (TR), echo time (TE), and flip angle
(

ρ )

). Therefore, an MR image obtained from the same anatomical site can look
drastically different with different data acquisition protocols.
In the present chapter, the basic physical principles of MRI are presented.
The objective is to allow readers to understand and interpret the MR signal and
image generation in order to introduce them to processing analysis: why and how
signal processing theories and methods, described in the following chapters of
the topic, can be applied on MR signals and images.
For a deeper study of MRI physical principles, extensive literature exists
[6-13], which the reader is encouraged to consult; part of the material of this
chapter has been extracted from these texts.

α

1.2 NUCLEAR SPIN

The basis of NMR lies in a property possessed by certain nuclei, called the spin

). The spin angular momentum of the nucleus can be con-
sidered as an outcome of the rotational or spinning motion of the nucleus about
its own axis. For this reason, nuclei having spin angular momentum are often
referred to as nuclear spins. The spin angular momentum of a nucleus is defined
by the nuclear

angular momentum ( p

spin quantum number I, and is given by the relationship

−

| phII1 | =⋅ [ ( + ) ] (1.1)

where h

and h is the Planck's constant. The value of spin quantum number
depends on the structure of the nucleus — the number of protons and neutrons —

= h/2 π

and can be an integer, half-integer, or zero. In Table 1.1 , the spin quantum number

of some selected nuclei is given. Hydrogen ( 1/2), the most abundant

element in nature and in the body, is most receptive to NMR experiments. On
the other hand, the most common isotopes of carbon (

1

H, with I =

12

C) and oxygen (

16

O) have

nuclei with I = 0 and hence cannot be observed by magnetic resonance experi-

ments.
Because the nucleus is a charged particle, spin angular momentum is accom-
panied by a magnetic moment ( µ ) given by

µ = γ p (1.2)

where γ is called the gyromagnetic ratio .

are vector quantities having magnitude and direction.
The gyromagnetic ratio is characteristic of a particular nucleus (see Table 1.1),
and it is proportional to the charge-to-mass ratio of the nucleus.

Note that both µ and p