Gradients allow for fast and efficient water suppression. Figure 7.16 shows spectra of a hydrogel in which the water is suppressed with the Watergate 13 sequence. An alternative gradient method is the so-called Dryclean 14 sequence in which a stimulated echo sequence is used to attenuate resonances based on differences in the diffusion rates of the molecules. Figure 7.17 shows spectra of a biological tissue in which the water signal is suppressed using Watergate and Dryclean, respectively.
Figure
7.16. 500 MHz single pulse proton spectrum of an acrylamide hydrogel (top).
The spectrum below is acquired with the Watergate method. 1 ms sine-shaped
gradient pulses are use with a strength of 30 G/cm. (sample courtesy of
Prof. Tanaka, Massachusetts Institute of Technology).
Figure 7.17. 500 MHz proton spectra of human high grade Pleiomorphic Liposarcoma tissue (sample courtesy of Dr. Singer, Brigham and Women's Hospital). MAS 5 kHz. Top: single pulse excitation; center: Watergate; bottom: Dry-clean.
The combination of magnetic field gradients and hr-MAS spectroscopy allows one to measure the diffusion constants of molecules in tissue samples under high resolution conditions.
Figure
7.18 displays a series of proton spectra of a Liposarcoma tissue, obtained
with a Stimulated Echo (STE) sequence. The spectra are acquired with increasing
gradient strengths. Apart from measuring diffusion, the stimulated echo
sequence is used for spectral editing by attenuating the resonances from
mobile components in the sample (see figure 7.19).
Figure 7.18. 500 MHz STE diffusion spectra of a high grade human Pleomorphic Liposarcoma tissue (sample courtesy of Dr. Singer, Brigham and Women's Hospital). MAS 10 kHz; 10 ms gradients,0-40 G/ cm, TE=100 ms.
Figure 7.19. 500 MHz STE proton spectrum (top) and single pulse spectrum (bottom) of a high grade human Pleomorphic Liposarcoma tissue (sample courtesy of Dr. Singer, Brigham and Women's Hospital). MAS 10 kHz; 10 ms gradients,30 G/cm, TE=100 ms.