TwinTree Insert

05-05 Localized In Vivo Spectroscopy

n traditional chemical/analytical NMR spectroscopy, the sample is placed in­side the coil and the signal from the whole sam­ple is observed.

This is fine as long as we have a homogeneous sample. However, if we want to ob­tain signals from living ani­mals or humans, this approach would lead to signals from a mix­ture of the different tis­sues within the detecting region of the coil.

Therefore, to obtain in vivo spectra from a particular type of tissue, it is ne­ces­sa­ry to limit or localize the volume from which we actually detect the signal.

The simplest localization technique is to use a surface coil. The size of the vo­lume from which the signal is received is deter­mined by the size and con­fi­gu­ra­tion of this coil. Whereas this technique is very simple and gives a good signal-to-noise ra­tio, it has a number of disadvantages, in­cluding contamination from surface tissue, a de­crease in signal from tissues at increas­ing depth, and large variations of the flip angle across the excited volume. The latter problem can be partly reduced if a stand­ard RF coil is used for exciting the whole sam­ple and a surface coil is used to de­tect the signal from the localized volume, or if spe­cialized (adiabatic) RF pulses are applied.

The other techniques discussed here al­low a volume to be defined on pre­vi­ous­ly ac­quir­ed positioning images, thus reducing the problem of con­ta­mi­nation from other tissues.

The quality of the magnetic field within the volume of in­te­rest is optimized by means of localized shimming. Smaller volumes allow for better shimming, but as the sig­nal in the spectrum is proportional to the volume size, very small volumes are im­prac­ti­cal except for ¹H spectroscopy.

For lo­ca­lized spec­tro­scopy of nuclei other than ¹H we must be able to obtain a lo­ca­liz­ed proton signal from the same volume; this is required for lo­ca­liz­ed shim­ming since the sensitivity of the other nuclei is too poor.

A number of localization schemes have been proposed. Four of the most widely used schemes will be discussed here. An extended discussion of spectroscopy tech­ni­ques can be found in the papers by Matson and Weiner as well as Sauter and col­la­bo­ra­tors [⇒ Matson 1999, ⇒ Sauter 1992].

05-05-01 Stimulated Echo Spec­troscopy

The stimulated echo is one of five echoes generated by a three 90° pulse RF se­­quen­ce. If these three RF pulses are made selective in orthogonal planes a sti­mu­lat­ed echo will only be obtained from the volume located at the intersection of the three planes.

The signal is attenuated by T1 de­cay between the second and third pulse and by T2 decay in the other two intervals. The problem of T2 decay makes the scheme best suited to ¹H rather than ³¹P spec­troscopy, due to the short T2 values of phos­pho­rus.

A number of acronyms are used to de­scribe pulse sequences of this type, including STEAM [⇒ Frahm 1988], STEV [⇒ Kimmich 1987], and VOSY (Figure 05-07).

Figure 05-07:
STEAM proton spectra of (a) a normal human brain; (b) hypoxia; (c) hepatic encephalopathy; (d) Alzheimer’s disease. The abbreviations are explained in Table 05-01.

05-05-02 Point-Resolved Spec­troscopy

The point resolved spectroscopy localiza­tion technique (PRESS) consists of a spin-echo train with two 180° pulses. Each pulse is applied in the presence of an or­tho­go­nal gra­dient which allows the selection of a volume element within the sample to be ex­amined. PRESS needs relatively long echo times.

05-05-03 Image-Selected In Vivo Spec­troscopy

The ISIS (Image Selected In Vivo Spec­troscopy) sequence consists of eight steps [⇒ Ordidge 1986]. In each step, a sequence of selective inversion pulses is applied, fol­­low­ed by a nonselective excitation pulse.

The combination of inversion pulses and receiver phases is unique for each of the steps. By combining the resulting eight signals in the correct manner, the signals from the volume of interest will add up, while those from the rest of the sample will cancel each other.

Since the FID rather than an echo is observed, there is no T2 decay. This makes the sequence well-suited to ³¹P spectroscopy.

For ¹H spectroscopy, the sequence should be modified so that the very large water sig­nal from the outer volume (i.e., the rest of the sample) is suppressed to avoid dy­na­mic range problems.

This is done in an OSIRIS sequence, a modified form of the ISIS sequence in which the signal from the outer volume is sup­pres­sed with a noise pulse. Because of the eight steps involved, the sequence is rather susceptible to motion artifacts, since any movement will lead to in­com­plete cancellation in the outer volume.

For the ISIS scheme to work efficiently, the magnetization must come close to full re­co­very before the next pulse is applied, so rather long TR values are required.