Patent GB-1461077-A (GB)
Title: Nuclear magnetic resonance techniques
IPC: H01F7/20
Filed: 1973-11-02
Abstract: 1461077 NMR analysis of local areas of a body K NIWA 2 Nov 1973 [2 Feb 1973] 51127/73 Heading G1N To obtain information about a region within a body, nuclear magnetic resonance signals from that region alone are analyzed. The signals from that region are distinguished from those of surrounding areas by subjecting the region to a magnetic field which differs from that applied to the other areas; the resonant frequency of, e.g. protons in the region is thus different from its value in the other areas and can be extracted by a suitable narrowband detector. The invention is applicable to analysis of internal regions of the human body. For this purpose a uniform main magnetic field Ho is applied to the body and is modified in the selected region by a field # Hs from one or more coils or a permanent magnet or from currents produced by pairs of electrodes on the body. It is also possible to arrange # Hs to be zero in the selected region, but to have a real value elsewhere. The coils or magnets can be external or internal of the body; when coils or electrodes are used their currents and phase can be adjusted to define the size and shape of the region. Alternatively the modification in the region can be provided by ferrite powder, paramagnetic ions or ultrasonic waves. A super-conducting magnet may be used. When ferromagnetic devices are used to provide the magnetic field they may be provided with a short circuit turn to prevent entry of the NMR radio frequency field. The main magnetic field or the detected frequency or the field applied to the region can be scanned through a small value to ease the problem of detecting the very sharp resonances involved. Blood measurements: analysis of the NMR signals allows blood flow velocity to be measured; the size of a blood vessel can also be found and used to find blood flow-rates. Blood flow pulses and respiratory movement also modulate the NMR signals so that pulse rate and respiratory condition can be monitored. In Fig. 7 blood flows along a vessel shown in dotted lines at speed v; the region Op has a magnetic field different from the other areas, and a pulse in coil Cp produces resonance in the flowing blood of that region. The field in Op is scanned along the vessel so that resonance does not immediately cease in the otherwise different field outside Op, and coils C 1 , C 2  pick up signals indicating the magnetization of the flowing blood. The coil signals can be combined to give the velocity of flow, the volume of the magnetized flowing region (and hence the vessel size), and the blood flow-rate. One coil can be used if its coupling with the vessel can be calculated. In another embodiment, Fig. 10, a main field is applied in region 3, and a local field in cylinder 12; the tested body is moved relative to cylinder 1 until modulation of the NMR signal at the blood pulse rate indicates that a blood vessel 2 lies wholly within the cylinder. The cylinder size is reduced until the modulated signal decreases, at which point the cylinder cross-section is the same as that of the blood vessel. Having thus found the vessel size the flow velocity can be found by measuring the blood pulse modulated NMR signals a known distance downstream and seeing how long a modulation pattern takes to occur at the downstream detector after it has occurred at the Fig. 10 detector. Two or more pick-up coils can be used in the NMR signal detector to compensate for signals  from unwanted protons or for signals induced by the RF fields.