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The precession frequency of nuclear spins in a magnetic field has been alluded to now a number of times; it is Meff = yBeff. A physical result of this statement is that if Beff, the effective magnetic field at the nucleus, can be made to vary in space, then the resonant frequency will vary in space. This fact is the basis of imaging of nuclei in the human body, and for that matter, in any region of matter. In an imaging experiment, coils of wire are placed about a subject in a magnetic field, and pulsed current is run through these coils to produce gradient magnetic fields that vary with space and time. The nuclear resonance frequencies of nuclei in the subject in turn vary with space and time. Since different tissues (e.g., muscle compared to bone) have different concentrations of nuclei such as protons, as well as differing relaxation times T1 and T2, for these nuclei it is possible to convert a

FIGURE 10 NMR spectra of the spin 2 quadrupolar nucleus 87Rb, taken under (top) static, (middle two) magic angle spinning, and (bottom) conditions of magic angle spinning, and multiple quantum coherence (MQMAS).

three-dimensional net of intensities of nuclear resonance frequencies in various tissues into images representing the tissues themselves. The physician can then call any two-dimensional slice of this information such that sections of the human body can be viewed, appropriately colored, on a television monitor. The information may also be converted to colored photographs that rival photographs of actual organs in their appearance, and are in general of higher resolution than is achievable from X-ray films. The general scheme of a patient in the machine is shown in Fig. 11. One such slice of information taken from a sagittal scan through the eye is shown in Fig. 12.

As a further example of the ability of NMR to form an image of a section through the human body without the use of damaging ionizing radiation, Fig. 13a and b show whole-body scans. Figure 13a is a section through the upper chest region perpendicular to the spinal cord. The patient is prone, and the spinal column is seen at the bottom center of the scan. The two upper arms, including muscle, fat, and bone, are seen on either side of the torso to the right and left. Figure 13b is another section of the same individual, but this time taken parallel to the spinal cord,

FIGURE 11 Arrangement of a magnet and patient for a whole body imaging scan. [Courtesy of Wang NMR, Inc.]

seen as the vertical column in the center. Clearly seen are the lungs, portions of the ribs, and a detailed picture of the spinal column.

Imaging is in its infancy, and given the fact that radio-frequency radiation is nonionizing, it is likely that such a technique will be widely used in lieu of X radiation for specific applications in which sensitivity of the body to X-rays is a problem. Also, since NMR is nucleus specific, whereas X-ray scans see only dense versus nondense matter, the diagnostic potential of NMR imaging is quite promising.

For example, the use of 31P as an NMR tag to detect concentrations of creatine phosphorus in the heart of a patient after a coronary infarction may be used to diagnose the extent of the damage to the heart muscle.

If one examines the statement just made carefully, it may be seen that the entire discussion of the utility of NMR to probe materials lies in the fact that nuclei have a number

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FIGURE 12 Sagittal scan through the eye region of a human. [Courtesy of Dr. John Schenck, General Electric Company.]

FIGURE 12 Sagittal scan through the eye region of a human. [Courtesy of Dr. John Schenck, General Electric Company.]

FIGURE 13 (a) Scan through upper trunk perpendicular to spine. (b) Scan through upper trunk parallel to the spinal cord. [Courtesy of Dr. John Schenck, General Electric Company.]

ation times available to the nucleus. The field of imaging is just beginning to use these fingerprints for enhanced resolution. For example, there now exist "T1 images", and "T2 images" that use the fact that transverse and longitudinal relaxation times of nuclei in a given tissue are characteristic of that tissue. The full range of interactions of nuclear behavior has yet to be exploited for imaging.

Two recent developments which illustrate the burgeoning power of NMR imaging (MRI) in medical, biological, and materials science are (1) noninvasive diagnosis of cancer by so-called chemical-shift imaging; (2) imaging of live silk-butterfly pupae growing inside the cocoon; and (3) imaging with the imaged body (live and human, or inanimate, and a rubber band) outside of the magnetic system, via the so-called NMR "Mouse." Examples of each type of image is shown in Fig. 14.

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