TwinTree Insert

05-04 Spectroscopy of Other Nuclei

ver the years, interest in ¹H MRS in­creased rapidly, and it is now more com­­mon than ³¹P MR spec­tro­scopy. Other nuclei like ¹³C and ¹⁹F are becom­ing more readily accessible by standard equipment.

Table 05-02 shows some impor­tant properties of selected nuclei which are of in­te­rest in biological studies.

Table 05-02:
Advantages and disadvantages of selected nuclei for MR spectroscopy.

The spin quantum number, n, is a funda­mental property of the atomic nucleus. Among other things, it is known that the nuclear spins can occupy 2n+1 en­er­gy le­v­els, so nuclei with a spin of 1/2 have two possible energy states, whereas nuclei with a spin of 3/2 have 4 possibilities.

Nuclei with spins larger than 1/2 are said to be quadrupolar. An important prac­tical fea­ture of quadrupolar nuclei is that their relaxation is sensitive to fluctuating elec­tric fields, as well as fluctuating mag­netic fields, so that their T1 and T2 times are much shorter than for nuclei with a spin of 1/2.

Since one wants as much signal as possi­ble, it is desirable for a nucleus to have a high sensitivity, although the natural abun­dance is also important.

The relative sensi­tivities of ³¹P and ¹³C differ by about a fac­tor of 4, but because ³¹P is 100% abundant and ¹³C is only 1.1% abundant (i.e., about 98.9% of carbon nuc­lei are the nonmag­netic ¹²C isotope), the absolute sensitivities differ by about 400 (Table 05-03).

Alternatively, ³⁹K nuclei are about 31 times less sensitive than ¹³C nuclei, but be­cause the potassium isotope is 93% abun­­dant and the carbon isotope is 1.1% abun­­dant, a sample containing potassium would produce a stronger signal than a carbon sample at a similar concentration.

Table 05-03:
Important NMR properties of selected nuclei used for in vivo MR spectroscopy.

05-04-01 Proton Spectroscopy

¹H studies have become increasingly popu­lar as technical difficulties have been over­­come and as interest has switched to areas of metabolism which lack phos­phory­lated meta­bolites (Figure 05-06). ¹H possesses the strongest response of all the atomic nu­clei, and it is found in all biochemicals. Thus, it is a good nucleus for mo­ni­tor­ing metabolism [⇒ Gadian 1990, ⇒ Matson 1999, ⇒ Miller 1991].

Figure 05-06:
Proton spectrum of a normal human brain. PCr: (phospho) creatine; PCho: (phospho) choline; NAA: N-acetylaspartate.

However, there are some major techni­cal problems. Probably the biggest problem with ¹H MRS is the large signal from water in tissue. If we assume a rather con­ser­va­­tive fi­gure for tissue water content of about 65%, then the molar concentration of the water would be about 36 M. Since there are two H nuclei in each molecule of water, this gives a ¹H concentration of over 70 M.

The metabolites which we wish to ob­serve have a maximum concentration of 10 mM or less, which is at least 7,000 times smaller than the water signal. Therefore, spe­cial methods are required for reducing the size of the water signal to a level where it is comparable to that of the metabolites. The simplest method is to use a long selec­tive frequency saturating pulse, but whereas this is very effective in vitro, it can lead to unacceptable heating of tissue in vivo.

Multiple pulse sequences, such as the bi­nomial pulse sequences, can be used to re­­duce the signal from water. If the ¹H nuclei in water are not excited, then they can­not give rise to a signal.

Another method of water suppression exploits the characteristics of a T1 re­la­x­a­tion curve. A selective 180° pulse is used to invert the water magnetization. At first the magnetization will be large and nega­tive, but relaxation processes will start to take it back to its equilibrium values. After 0.69×T1 of water, the water mag­ne­ti­za­tion will be approximately zero. At this point a 90° excitation pulse would pro­duce a rela­tively strong signal from the metabolites and very little signal from the water.

Such methods are also applied in MR imaging (cf. Chapter 11).

If we were using a spin-echo pulse se­quence in association with a par­ti­cu­lar vo­l­u­me se­lec­tion method then we could ar­range for the echo to be formed at the ap­­pro­pri­ate time after the selective inversion pulse so as to produce a minimum wa­ter sig­nal.

Another problem with ¹H MRS is the narrow dispersion of the ¹H peaks. They lie in quite a narrow frequency range, thus there is a lot of overlapping. The problem can be helped by working at higher mag­netic field strengths, but although ultrahigh-field whole-body systems do exist, the ma­jority of MR spectroscopy is performed at 1.5 and 3.0 Tesla on equipment for which MR imaging rather than MR spec­tro­sco­py con­si­de­ra­tions dictate the field strength.

Clinical applications of proton MRS of the human brain include epilepsy, space-oc­cupying lesions, multiple sclerosis, degen­erative diseases, among them Alzheimer’s, Parkinson’s, and Huntington’s, hypoxia, and some metabolic diseases. A fine over­view and review of clinical ¹H MRS in cen­tral nervous system disorders was pub­li­shed by Öz and numerous co-authors from dif­ferent centers in 2014 [⇒ Öz 2014]. There are also other diseases which have been studied using proton MR spectroscopy, particularly diseases of the muscu­lar system.

05-04-02 Carbon Spectroscopy

Unlike ¹H and ³¹P, the magnetically ac­tive isotope of carbon, ¹³C, is not the most ab­un­dant form of the nucleus. ¹³C can be found in all biochemicals and the signals have a wide dispersion, i.e., they occur over quite a large frequency range, which re­­duces the likelihood of overlapping peaks.

The biggest drawback of ¹³C is the exceed­ingly weak signal and problems with ¹³C-¹H coupling. In a ¹H-coupled ¹³C spectrum, most of the peaks will be split into two or more smaller peaks, which complicate the spec­trum and reduces the signal-to-noise ratio, but the coupling effect can be removed by decoupling techniques. Direct irradiation at the proton resonance frequency is the sim­plest option, al­though this can lead to heat­ing of tissue in vivo.

Multiple pulse techniques are available which are just as effective as direct ir­ra­di­a­­tion but use a fraction of the power. The need for decoupling means that for ¹³C MR spectroscopy, the instrument must be capa­ble of operating two channels si­mul­ta­ne­­ous­ly, which increases complexity and also cost.

spaceholder redOne advantage of ¹³C MR spectroscopy is that we can perform labeling studies. By giv­­ing carbon-labeled compounds to an animal or patient, we can follow the large and distinct­ive signals from the labeled compound and try to work out how it is me­ta­bo­liz­ed in the body. Because each carbon position in a molecule will produce a cha­rac­te­ris­tic sig­nal, the labeling experiment can be used to follow not only which mo­le­cu­les end up with the label but also the exact position in which they end up.

This information is ex­tremely useful in working out which bio­chemical pathways were used in the con­version of one molecule to another. Similar labeling studies are not possible with ³¹P and ¹H because they are already 100% abundant. Un­for­tu­na­te­ly, ¹³C MR spec­troscopy labeling is fairly costly.

¹³C MRS can detect sig­nals from sugars, lipids, and glycogen in the liver and in muscle. It can obtain infor­mation about the carbon balance of energy metabolism, which is complementary to the information obtainable by ³¹P MR spec­troscopy about energy metabolism [⇒ Matson 1999, ⇒ Shulman 1990].

A promising application for ¹³C spec­troscopy is the analysis of body fluids such as blood and urine. This can be done rou­tinely with very high field analytical NMR spec­tro­me­ters. For the same purpose, pro­ton spectroscopy can be used.

05-04-03 Fluorine Spectroscopy

¹⁹F has a strong NMR signal and is 100% abundant. Fluorine studies have been per­­form­ed on the metabolism of fluorine-con­taining drugs; since there are no naturally oc­cur­ring fluorine signals in the body, all fluorine signals must come from the drug or its metabolites.

The drawback of fluorine MRS is that despite the strong signal, we still need drug con­cen­tra­tions in the order of 1-10 mM in the tissue, which is a rather high con­cen­tra­­tion for many drugs.

The resonance fre­quency of fluorine is quite close to that of ¹H at the same mag­ne­tic field, and it is of­ten possible to perform fluorine studies on the ¹H channel of MRS equipment without the need for major modifications.

05-04-04 Sodium and Potassium Spectroscopy

²³Na and ³⁹K differ from the other nuclei mentioned because they do not have spins of 1/2. They both have a spin of 3/2 and are therefore quadrupolar nuclei. Both are high-abundance isotopes (²³Na is 100% abundant and ³⁹K is 93.1% abun­dant).

²³Na has quite high extracellular concen­trations, whereas ³⁹K has quite high in­tra­­cel­lu­lar con­cen­tra­tions, and both have an important role to play in ion balance. A big difference between the two is that sodium has a reasonably strong signal, com­par­able to phosphorous, whereas potassium has a weak signal. The absolute sen­si­ti­vi­ty of ³⁹K is about three times that of ¹³C, but the potassium signal is much broa­der than the carbon sig­nal because the T2 of potassium is very short. This is due to the effect of quadrupo­lar relaxation, which reduces the signal-to-noise ratio of the peaks.

³⁹K also has a very low resonance fre­quency, which adds to the tech­ni­cal dif­fi­­cul­ties of the experiment. Animal studies with potassium have been performed at 4.7 Tesla, but, to our knowledge, the nucleus has not been studied at 1.5 Tesla. Al­though sodium also has a broad signal due to quadrupolar effects, the much grea­ter sig­nal strength means that reasonable spectra can be obtained.

Unfortunately, ²³Na and ³⁹K have no natu­ral chemical-shift dispersion. In other words, the entire signal from an in vivo sample comes at the same frequency. Methods do exist for separating the chemical shift of intracellular ²³Na and ³⁹K from ex­tra­cel­lu­lar sodium and potassium using chemical-shift reagents (rather like the re­la­xa­tion contrast agents used in imag­ing), but studies are currently restricted to cells and animals [⇒ Kohler 1991, ⇒ Matson 1999, ⇒ Rashid 1991].