Human whole-body NMR tomographic imaging: normal sections (1981)

In this paper we demonstrate the kind of images that can be produced by a nuclear magnetic resonance (NMR) tomographic section imager from healthy human subjects. Details of the instrumentation and imaging techniques can be found elsewhere (Hutchison et al., 1980; Edelstein et al., 1980), but a brief description of the imaging method is given here. Our imaging machine is based on a four-coil, air-cored electromagnet manufactured by the Oxford Instrument Company. It produces a magnetic field of 0.04 T and a consequent NMR frequency of 1.7 MHz for the hydrogen protons of body tissues. Our machine uses the NMR signals from hydrogen protons in water and in fat. The machine produces two kinds of image. The first uses proton density as the imaging parameter, and the second uses proton spin-lattice relaxation time (T1). T1 in wet tissues depends essentially on the relative proportions of free and bound water within the tissue under examination; tissues with more free water have longer relaxation times (Packer, 1977). T1 values of normal and malignant animal and human tissues have been presented (e.g. Damadian et al., 1973; Mallard et al., 1980; Ling et al., 1980) and their implications for in-vivo clinical NMR imaging have been discussed (Mallard et al., 1980). The data for each section (including both proton density and T1 information) consist of 128 electrical signals, each signal being collected during a one-second interval. Thus the data from an entire section are gathered in just over two minutes. This compares favourably with previous bodysection NMR images, such as that demonstrated by Mansfield et al., (1978) which took 40 minutes. Each electrical signal is a projection (of a thick slice of the body) onto the same single selected axis. The image reconstruction utilizes the phase information contained in the NMR signal as well as its amplitude. This differs from X-ray and radionuclide CT data where only intensity information is available and different projections must be made from different angles. The technique of projections onto a single axis has the advantage that curvature of the projecting lines should produce only a geometric distortion of the final image and not a degradation of spatial information which could occur if a multiple-angle projection method were used. NMR imaging is particularly subject to such curvature distortion because of possible inhomogeneities in the static, gradient or radio-frequency magnetic fields employed. Our tomographic sections have a Gaussian profile with thickness (equivalent rectangular width) of 18.5 mm. Each imaging element is 7.5 mm wide × 7.5 mm high × 18.5 mm thick, composing a volume of 1 cm3. In each imaging element, the uncertainty in proton density for muscle tissue is about 2.5% and the uncertainty in T1 is about 3.5% for T1 about 200 ms, with some worsening as T1 departs from that optimum value. To evaluate our system we have examined 11 healthy volunteers aged 20 to 55 years. We have obtained tomographic sections from head, thorax and abdomen (including pelvis). No adverse effect was experienced by any of the volunteers either during or after the examination.