Imaging chemical shift

The usual imaging schemes apply a linear gradient G to frequency encode the data. Applying an inverse Fourier transform maps the spin density as a function of frequency linearly to spatial location. The linear relation between frequency (JD and position x is  

This leads to a shift in the image position of the molecule a with respect to that of the protons of water by  

Consequently, the image of molecule a would overlap water in the region of interest and cause an artifact in the image, which might be incorrectly interpreted as actual spatial features. By increasing G the pixel shift due to chemical differences is reduced. However, much valuable information is contained in the 

A particular resonance peak can be selectively excited by r.f. irradiation to the exclusion of others in the chemical shift spectrum. A long, low-power, amplitudeshaped r.f. pulse can be used to excite a narrow range of resonant frequencies distributed about a particular frequency. Such a soft pulse is more frequency-sensitive than a short, square hard pulse. 

Often the spectrum to be resolved is quite simple. For example, we may only be interested in the water and fat 

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Figure Bl.14.7. Chemical shift imaging sequence [23], Bothx- andj -dimensions are phase encoded. Since line-broadening due to acquiring the echo in the presence of a magnetic field gradient is avoided, chemical shift infonnation is retained in tire echo.

Fig.4.5.7 (a) Chemical shift imaging pulse sequence and (b) schematic drawing of CSI data for a given pixel of an oil-in-water emulsion inside the horizontal concentric cylinders geometry. 

Chemical shift imaging (CSI) was used to monitor the oil volume fraction during the mixing process. Figure 4.5.13 shows normalized volume fraction profiles along the vertical center-line (see Figure 4.5.3) at different times. The mixing time is expressed in strain units as y = tV/(R0 - i)> where t is the time. One revolution of the outer cylinder corresponds to 9.83 strain units. The initial condition (y = 0)... 

H. Rumpel, J. M. Pope 1993, (Chemical shift imaging in nudear magnetic resonance a comparison of methods), Cone. Magn. Reson. 5, 43. 

In the following sections (Sections 5.5.2.2 and 5.5.2.3), two approaches to spatially resolving chemical conversion within a reactor are demonstrated (a) n-dimensional Chemical Shift Imaging (CSI), and (b) volume selective spectroscopy. 

Brown, T. R., Kincaid, B. M. and Ugurbil, K. NMR chemical shift imaging in three dimensions. Proc. Natl Acad. Sci. USA 79 3523-3526,1982. 

Fig. 7 A chemical shift imaging pulse sequence. The MR signal is spatially encoded prior to acquiring the spectral signal in the absence of any applied magnetic field gradients. The shaded gradient pulses applied along z either side of the n refocusing pulse are homospoil gradients.

Fig. 13 3-D cutaway image showing the extent of conversion of the esterification occurring within the fixed bed considered in Figs. 11 and 12. The conversion was calculated from the chemical shift of the OH peak in a 4-D chemical shift image. The chemical shift image was acquired with an isotropic spatial resolution of 625 pm. The RARE image of the structure of the bed was acquired at an isotropic spatial resolution of 78 pm. Both datasets have been reinterpolated on to a common array giving an effective isotropic spatial resolution of 156 pm. The direction of flow is in the negative z direction. The grey scale indicates the fractional conversion within the bed.

In 1982 Hall and Sukumar118 demonstrated the ability to select species processing distinct chemical shifts in images, where the chemical shifts do not overlap, using capillaries of water, acetone, benzene and methylene chloride. Since then, volume-localized spectroscopy and chemical-shift imaging have been applied to a number of medical and non-medical problems. Most of these studies, however, are focused on the H and 31P nucleides, especially those investigations which are clinically oriented119,120. 

The method has been extended to include chemical shift imaging, which allows even higher throughput (99). In this embodiment, a 19-capillary through-flow system enables the simultaneous analysis of 19 samples. The enantiomeric purity of 5600 samples can be determined within one day, the precision being +6%. In this... 

H. Mishima, T. Kobayashi, M. Shimizu, Y. Tamaki, M. Baba, In vivo F-19 chemical shift imaging with FTPA and antibody-coupled FMIQ, J. Magn. Reson. Imaging 1 (1991) 705-709. 

T. Sassa, T. Suhara, H. Ikehira, T. Obata, F. Girard, S. Tanada, Y. Okubo, 19F-magnetic resonance spectroscopy and chemical shift imaging for schizophrenic patients using haloperidol decanoate. Psychiatry Clin. Neurosci. 56 (2002) 637-642. 

P.N. Venkatasubramanian, Y.J. Shen, A.M. Wyrwicz, In vivo F one-dimensional chemical shift imaging study of isoflurane uptake in rabbit brain, NMR Biomed. 6 (1993) 377-382. 

Fl.T. Tran, Q. Guo, D.J. Schumacher, R.B. Buxton, R.F. Mattrey, F chemical shift Imaging technique to measure Intracellular p02 in vivo using perflubron, Acad. Radiol. 2 (1995) 756-761. 

J. J. Potwarka, D. J. Drost, P. C. Williamson, T. Carr, G. Canaran, W. J. Rylett and R. W. A. Neufeld, H-decoupled P chemical shift imaging study of medicated schizophrenic patients and healthy controls. Biol. Psychiatry, 1999, 45,687-693. 

T. Fujimoto, T. Nakano, T. Takano, Y. Hokazono, T. Asakura and T. Tsuji, Study of chronic schizophrenics using P magnetic resonance chemical shift imaging. Acta Psychiatr. Scand., 1992, 86,4554 62. 

J. E. Jensen, J. Miller, P. C. Williamson, R. W. Neufeld, R. S. Menon, A. Malla, R. Manchanda, B. Schaefer, M. Densmore and D. J. Drost, Grey and white matter differences in brain energy metabolism in first episode schizophrenia 31P-MRS chemical shift imaging at 4 Tesla. Psychiatry Res., 2006,146,127-135. 

Figure 5. HP 129Xe Chemical Shift Imaging (left and centre) of a phantom consisting of a 7 mm porous Vycor tube filled with NaY zeolite and placed inside an open 9 mm ID glass tube (right). Images from Xe in the three different chemical shift environments can be clearly separated. The NMR spectrum is shown bottom left.

NaY zeolite at 60.1 ppm. The image was obtained for a 3mm slice with full chemical shift imaging (note that for thermally polarised Xe this type of imaging experiment would be far more demanding in terms of experimental time even than chemical shift resolved imaging, as practiced for the Aerogel samples[30]), and was obtained in 30 min. Thus, the improvement in imaging with HP xenon over thermally polarized xenon is impressive, and indicates that there are real prospects for applications in the characterization of materials. 

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