• Chapter 2 • Contents
  The Basics Magnetic Properties of Nuclei The Boltzmann Distribution The Larmor Equation Resonance Magnetization The Rotating Coordinate System The Magnetic Resonance Signal Frequency Analysis: Fourier Transform

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Chapter Two
Nuclear Magnetic Resonance

02-01 The Basics

ften images taken for medical purposes look similar; one easily re­cog­ni­zes that they show the same part of human anatomy. However, the in­for­ma­tion con­tain­ed in them as well as their spatial resolution is dif­fe­rent. Some de­pict only anatomy, others mix anatomy with metabolic in­for­ma­tion, as the four ima­ges of a brain in Figure 02-01 prove.

Figure 02-01:
Somehow similar pictures: transverse slices through the brain. The methods of creation vary, from (a) positron emmission to­mo­gra­phy, (b) an anatomical specimen, (c) x-ray computed to­mo­gra­phy, and (d) magnetic resonance imaging. The main fundamental discrepancy between the dif­fe­rent techniques is explained on Table 02-01.

Table 02-01:
Spectrum of electromagnetic radiation. The radiation used for NMR and MR ima­ging is several times lower than that for γ-rays and x-rays.

Matter consists of atoms (e.g, ¹H, ¹²C, ¹⁶O, ³¹P, etc.). An atom of one ele­ment dif­fers from an atom of another element in its internal structure: the number of pro­tons, neutrons, and elec­trons. Protons and neutrons com­prise the nu­cle­us; protons are po­si­ti­ve­ly char­ged, neutrons possess no charge, and the electrons orbiting around the nucleus are negatively char­ged. Different nuclear com­po­si­tions and num­bers of surrounding elec­trons are reflected in different phy­si­cal properties.

Unlike color or texture some phy­si­cal properties are not easily per­cei­ved. Such is the case with the mag­ne­tic properties of the nuc­lei which are the basis of the nuc­le­ar magnetic re­so­nance phe­no­me­non. Although these pro­per­ties are not easily vi­su­a­li­zed, they are well de­fi­ned and obey certain rules.

This allows us to make analogies to more well-known phenomena so as to aid our understanding.

With the help of electromagnetic (radio) waves, the magnetic properties per­mit the production of images of the human body which can furnish information about mor­pho­lo­gy and function of the human organism. Owing to the stu­pen­dous spread of fre­quen­cies and wavelengths of electromagnetic waves, their interaction with mat­ter is very dis­si­mi­lar in the different parts of the spectrum.

The radiation used for magnetic resonance imaging is quite different from x-ray and γ-radiation (Table 02-01). It stretches from AM frequencies through mobile, ama­teur ra­dio and TV to FM radio frequencies, is approximately nine orders of mag­ni­tu­de smaller than the frequencies corresponding to x- or γ-rays (used for ra­dio­iso­tope exa­mi­na­tions), and is considered biologically safe (more in Chapter 18).

Since Röntgen’s discovery of the x-rays more than a century has passed. He suc­cee­d­ed in generating images of the human body with x-rays which result from in­ter­ac­tions bet­ween these rays and the electron clouds of atoms.

Nuclear magnetic resonance signals stem from the interaction of radiowaves with the atomic nuclei themselves. This is the reason for the completely dif­fe­rent imag­ing equip­ment necessary and the different contrast behavior of mag­ne­tic re­so­nan­ce imaging as com­pa­red with other medical imaging techniques.