Focus
NMR Imaging Microscopy High-resolution NMR imaging of a single cell should lead to increased use of this technique for chemical applications In the past several years, proton nu clear magnetic resonance (NMR) im aging has become an established tech nique in diagnostic medicine and bio medical research. Although much of the work in this field has been direct ed toward development of whole-body imagers, James Aguayo, Stephen Blackband, and Joseph Schoeninger of the Johns Hopkins University School of Medicine, working with Markus Hintermann and Mark Mattingly of Bruker Medical Instruments, recently developed a small-bore NMR microscope with sufficient resolution to image a single African clawed toad cell (Nature 1986, 322, 190-91). This improved resolution should lead to in creased use of NMR imaging for chemical, as well as biological or phys iological, applications. NMR imagers differ from conven tional NMR spectrometers primarily in the way in which the magnetic field is applied to the sample. The reso nance frequency of protons is propor tional to the strength of the magnetic field, and in conventional NMR this magnetic field is extremely homoge neous so that the only variations in its strength are attributable to differ ences in the chemical environment ex perienced by the protons. In NMR im aging, a relatively strong linear gradi ent is applied to a portion of the sample so that the resonance frequen cy is dependent on the position of the nuclei relative to the gradient. (The signal at any given frequency is still dependent on the concentrations of the nuclei and their relaxation times.) The resonance frequency information is then converted to distance by com puterized spatial encoding to form an image. Stanford Smith, a professor of chemistry and radiology at the Uni versity of Kentucky, explains that res olution of the image is determined by the minimum volume that contains a sufficient number of nuclei to give a detectable signal. In practice, says Smith, this is determined by the sam ple slice thickness, the number of en coding steps, and the number of data points acquired. (For a more detailed description of both the principles of NMR imaging and its instrumenta
tion, see Smith's I N S T R U M E N T A T I O N
article in the April 1985 issue of ANA LYTICAL C H E M I S T R Y . )
The high-resolution instrument, which was developed by engineers at Bruker, is a modified 9.5-T NMR spectrometer equipped with a set of magnetic-field gradient coils with cor responding control units and power supplies. Bruker has been providing NMR microscope attachments for sev eral years, but the new instrument, with an in-plane resolution of 10 X 13 μνα, is a significant improvement over previous versions. Bruker is cur rently the only instrument manufac turer offering NMR microscope at tachments, but several other compa nies are expected to provide some competition soon. As opposed to whole-body imagers, which use relatively low (under 2.0 T) magnetic fields, the NMR microscope uses a very high 9.5-T field. The high field allows increased signal strength, and hence increased sensitivity, but the resultant higher resonance fre quencies may cause radio frequency penetration problems during imaging of large samples. Penetration has not been a problem for the small samples studied so far, says Blackband, but there were indications in the past that it wouldn't be possible to image hu man subjects at high field strengths because of attenuation problems. Al though some fairly large samples re cently have been imaged successfully using a 4.7-T instrument, "nobody is willing to guess anymore whether whole-body imaging will work at high fields," he says. Nor is it known whether such high fields will prove harmful to living systems. But even if this is the case, says Blackband, the high-resolution NMR microscope should prove a valuable tool in many spheres of science. For example, Blackband suggests that the NMR microscope could be used to look at the diffusion of liquids in polymers, such as the microscale movement of water in nylon. And Smith predicts that high-resolution NMR imaging could be used to view very small objects, such as encapsulat ed pharmaceuticals and gel-perme ation resins, as well as to study agri
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cultural seeds and to look at the lay ered structure of chemicals in photo graphic film. It may be possible to view some of these samples using more established techniques, such as electron or X-ray microscopy, but NMR microscopy has some important advantages compared with these other methods. Unlike elec tron microscopy, which requires sec tioning and staining, NMR microsco py can obtain images on intact sam ples, thus avoiding the artifacts associ ated with such destructive sample preparation. And whereas electron mi croscopy views a structure in terms of density only, NMR microscopy can provide chemical information about the sample. Electron microscopy still has the edge in terms of resolution, however, with state-of-the-art instru ments capable of 0.1-0.2-μπι resolu tion. Although the Bruker instrument used at Johns Hopkins (as well as many other commercial NMR imagers) can now detect only protons, Blackband and Aguayo hope eventual ly to move on to multinuclear ( 19 F, 3ip ) 23]\ja) NMR imaging. Multinucle ar instruments probably will have somewhat lower resolution and sensi tivity because of the reduction in the number of nuclei present, but they would nevertheless be extremely use ful. For example, phosphorus imaging could provide a measure of phospho rus-containing energy metabolites, in cluding ATP, ADP, and phosphocreatinine. The future of NMR microscopy, like that of many other newly emerging techniques, is ripe with possibilities. Because of its high cost, however, it is likely to remain primarily a research tool for some time. "It's like having a camera," says Smith. "You've got a way to look at things at very fine lev els, and people are going to find lots of uses for it. But it is a very expensive technique—it costs $100,000 to add imaging capability once you have a high-resolution NMR, which itself is at least a $300,000 instrument. If it can answer even a few questions that can't be answered any other way, though, it may be well worth the cost." M.D.W.
