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Capítulo Cinco
Espectroscopía


05-01 Introducción


he beginning of NMR was spec­troscopy — which, at those times, was not yet called MRS. Today, for biological and medical applications one dis­tin­gu­ishes MRI (imaging) and MRS (spectroscopy). Meanwhile, however, the heyday of me­di­cal whole-body MRS seems to have already passed.

MR spectroscopy has many useful appli­cations, for instance checking if the wine in Figure 05-01 that you want to drink (or, if you rather prefer orange juice) has been adul­­te­rat­ed or con­ta­mi­nated. This can be done with SNIF-NMR spec­tro­sco­py, which tells the specialist whether the juice (or wine) contains the su­gars of a pure fruit juice or of added sugars. Un­for­tu­na­te­ly, we are more in­te­res­ted in me­di­cal ap­pli­ca­tions of MR spectroscopy.


Figura 05-01:
Old wine bottles, covered with spider webs — most likely they contain an excellent red wine, or per­haps not?


MR spectroscopy was one of the major selling points for the introduction of high field MR machines at 1.5 Tesla in the early 1980s. The combination of imaging and spectroscopy was thought to yield greater diagnostic value. For some years, MRS de­ve­lop­ed to a fa­vo­rite field of research at many places. Still, it never became really po­pu­lar in clinical routine.

With the arrival of clinical systems operating at 3 Tesla, the industry expected a new boom of MRS. Yet, although many 3T machines have spec­tro­sco­pic ca­pa­bi­li­ties, about only one in twenty is used routinely to perform clinical or re­search spec­tro­sco­pic studies.


05-02 Chemical Shift


Until now we have assumed that all protons in a human being resonate at the same re­so­nan­ce fre­quen­cy in a given magnetic field. However, ¹H sig­nals do not all come at the same frequency and so, for instance, the fat signal is gener­ally shifted from its "correct" position.

Why does ¹H in water have a different resonance frequency to ¹H in fat?

Even though both protons are within the very large, uniform external mag­ne­tic field, they actually experience slightly different magnetic fields due to their chem­i­cal envi­ronments. Each proton is surrounded by other nuclei and elec­trons, all of which have a small magnetic field associated with them.

spaceholder redIt is the electrons in chemical bonds which are most significant in af­fec­ting the mag­ne­tic field experienced by a nucleus. Thus, a proton in water is most­ly in­flu­­en­ced by elec­trons in H-O bonds, a similar nucleus in fat by electrons in H-C bonds.

These differences in resonance frequency caused by the nuclei ex­peri­en­cing dif­fe­rent chemical bonds are used for MR spec­troscopy. The dif­fe­ren­ces themselves are known as the chemical shift, δ (Figure 05-02).


Figura 05-02:
Chemical shift (δ): A ¹H spectrum of tissues often reveals two clearly distinct peaks. One is assigned to tissue water, the other one to protons in lipids (in this case triglyceride). Data given in Hertz for 1.0 Tesla.


Chemical shift is simply a difference in frequency and is measured in Hz. The dif­­fe­ren­ce in fre­quen­cy varies with magnetic field so that the chemical shift between wa­ter and fat is about 350 Hz at 2.35 Tesla, but about 700 Hz at 4.7 Tesla.

Fortunately, the change in the frequency difference is directly proportional to the change in the external magnetic field. If the chemical shift in Hz is divided by the basic resonance frequency of the nucleus in Hz, one obtains a number for the che­mi­cal shift, e.g., between water and fat, which is identi­cal regardless of the strength of the applied magnetic field.

Chemical shifts are typically in the range of tens to hundreds of Hz, whereas the re­so­nan­ce fre­quen­cies are typically in the range of tens to hundreds of MHz. This makes the values of the chemical shifts rather small, so the numbers are always mul­ti­plied by one million and expressed in a parts-per-million scale, or ppm.

Some important abbreviations used in MR spectroscopy are listed in Table 05-01.


Tabla 05-01:
Some abbreviations frequently used in MRS.